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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2018	Mar. 07, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	ALLK
Entity Registrant Name	Allakos Inc.
Entity Central Index Key	0001564824
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Public Float			$ 0
Entity Common Stock, Shares Outstanding		43,101,999

BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Preferred Stock, Par value	$ 0.001	$ 0.001
Preferred Stock, Shares Authorized	20,000,000	0
Preferred Stock, Shares Issued	0	0
Preferred Stock, Shares Outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	55,000,000
Common stock, shares issued	42,116,641	2,114,000
Common stock, shares outstanding	42,116,641	2,114,000
Series A Convertible Preferred Stock
Convertible preferred stock, par value	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized	0	26,083,000
Convertible preferred stock, shares issued	0	20,866,000
Convertible preferred stock, shares outstanding	0	20,866,000
Convertible preferred stock, aggregate liquidation preference	$ 0	$ 46,950
Series B Convertible Preferred Stock
Convertible preferred stock, par value	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized	0	12,632,000
Convertible preferred stock, shares issued	0	10,105,000
Convertible preferred stock, shares outstanding	0	10,105,000
Convertible preferred stock, aggregate liquidation preference	$ 0	$ 100,141

BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 33,660	$ 85,207
Investments in marketable securities	145,246	0
Prepaid expenses and other current assets	2,703	1,037
Total current assets	181,609	86,244
Property and equipment, net	8,848	445
Other long-term assets	802	340
Total assets	191,259	87,029
Current liabilities:
Accounts payable	2,092	1,703
Accrued expenses and other current liabilities	3,164	1,089
Total current liabilities	5,256	2,792
Other long-term liabilities	2,009	36
Total liabilities	7,265	2,828
Commitments and contingencies (Note 7)
Stockholders' equity (deficit):
Preferred stock, $0.001 par value per share; 20,000 and no shares authorized as of December 31, 2018 and 2017, respectively; no shares issued and outstanding as of December 31, 2018 and 2017	0	0
Common stock, $0.001 par value per share; 200,000 and 55,000 shares authorized as of December 31, 2018 and 2017, respectively; 42,117 and 2,114 shares issued and outstanding as of December 31, 2018 and 2017, respectively	42	3
Additional paid-in capital	288,079	1,803
Accumulated other comprehensive loss	(15)	0
Accumulated deficit	(104,112)	(60,574)
Total stockholders’ equity (deficit)	183,994	(58,768)
Total liabilities, convertible preferred stock and stockholders’ equity (deficit)	191,259	87,029
Series A Convertible Preferred Stock
Current liabilities:
Convertible preferred stock, value	0	42,996
Series B Convertible Preferred Stock
Current liabilities:
Convertible preferred stock, value	$ 0	$ 99,973

STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (unaudited) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses
Research and development	$ 33,287	$ 18,506	$ 14,672
General and administrative	12,434	3,748	2,388
Total operating expenses	45,721	22,254	17,060
Loss from operations	(45,721)	(22,254)	(17,060)
Interest income (expense), net	2,375	(1,302)	(51)
Other income (expense), net	(192)	(287)	11
Loss before benefit from income taxes	(43,538)	(23,843)	(17,100)
Provision for (benefit from) income taxes	0	(291)	0
Net loss	(43,538)	(23,552)	(17,100)
Unrealized loss on marketable securities, net of tax	(15)	0	0
Comprehensive loss	$ (43,553)	$ (23,552)	$ (17,100)
Net loss per common share:
Basic and diluted	$ (2.20)	$ (14.54)	$ (13.03)
Weighted-average number of common shares outstanding:
Basic and diluted	19,833	1,620	1,312

STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) shares in Thousands, $ in Thousands	Total	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock	Preferred Stock	Preferred Stock Series A Convertible Preferred Stock	Preferred Stock Series B Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Balance, Shares at Dec. 31, 2015							1,565
Balance at Dec. 31, 2015	$ (19,598)						$ 2	$ 322		$ (19,922)
Temporary Equity, Balance, Shares at Dec. 31, 2015				12,422
Temporary Equity, Balance at Dec. 31, 2015				$ 22,210
Issuance of convertible preferred stock for cash, net of issuance costs					$ 18,991
Issuance of convertible preferred stock for cash, net of issuance costs, Shares					8,444
Reclassification of preferred stock tranche liability upon issuance of Series A convertible preferred stock					$ 1,795
Stock-based compensation expense	182							182
Issuance of common stock warrants in connection with debt facility	24							24
Issuance of common stock upon exercise of stock options	20							20
Issuance of common stock upon exercise of stock options, Shares							40
Vesting of restricted common stock	36							36
Unrealized loss on marketable securities, net of tax	0
Net loss	(17,100)									(17,100)
Balance, Shares at Dec. 31, 2016							1,605
Balance at Dec. 31, 2016	(36,436)						$ 2	584		(37,022)
Temporary Equity, Balance, Shares at Dec. 31, 2016				20,866
Temporary Equity, Balance at Dec. 31, 2016				$ 42,996
Issuance of convertible preferred stock for cash, net of issuance costs						$ 92,331
Issuance of convertible preferred stock for cash, net of issuance costs, Shares						9,334
Issuance of Series B convertible preferred stock upon conversion of convertible promissory notes						$ 7,642
Issuance of Series B convertible preferred stock upon conversion of convertible promissory notes, Shares						771
Stock-based compensation expense	402							402
Repurchase of unvested restricted common stock, Shares							(34)
Issuance of common stock upon exercise of stock options	228						$ 1	227
Issuance of common stock upon exercise of stock options, Shares							543
Vesting of restricted common stock	28							28
Unrealized loss on marketable securities, net of tax	0
Recognition of beneficial conversionfeature related to convertible promissory notes payable to related parties, net of tax benefit of $966	1,867							1,867
Reclassification of beneficial conversion feature related to convertible promissory notes payable to related parties, net of tax expense of $675	(1,305)							(1,305)
Net loss	(23,552)									(23,552)
Balance, Shares at Dec. 31, 2017							2,114
Balance at Dec. 31, 2017	(58,768)						$ 3	1,803		(60,574)
Temporary Equity, Balance, Shares at Dec. 31, 2017		20,866	10,105	30,971
Temporary Equity, Balance at Dec. 31, 2017		$ 42,996	$ 99,973	$ 142,969
Proceeds from repayment of recourse promissory note	50							50
Conversion of preferred stock upon initial public offering	142,969			$ (142,969)			$ 30	142,939
Conversion of preferred stock upon initial public offering, Shares				(30,971)			30,972
Issuance of common stock upon initial public offering, net ofoffering costs of $3,466	138,357						$ 8	138,349
Issuance of common stock upon initial public offering, net of offering costs of $3,466, Shares							8,453
Stock-based compensation expense	4,570							4,570
Issuance of common stock upon exercise of stock options	345						$ 1	344
Issuance of common stock upon exercise of stock options, Shares							531
Issuance of common stock upon exercise of warrants, Shares							47
Vesting of restricted common stock	24							24
Unrealized loss on marketable securities, net of tax	(15)								$ (15)
Net loss	(43,538)									(43,538)
Balance, Shares at Dec. 31, 2018							42,117
Balance at Dec. 31, 2018	$ 183,994						$ 42	$ 288,079	$ (15)	$ (104,112)
Temporary Equity, Balance, Shares at Dec. 31, 2018		0	0
Temporary Equity, Balance at Dec. 31, 2018		$ 0	$ 0

STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Recognition of beneficial conversion feature related to convertible promissory notes payable to related parties, tax benefit		$ 966
Reclassification of beneficial conversion feature related to convertible promissory notes payable to related parties, tax expense		675
Series A Convertible Preferred Stock
Stock issuance costs			$ 8
Series B Convertible Preferred Stock
Stock issuance costs		$ 168
Common Stock
Stock issuance costs	$ 3,466

STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net loss	$ (43,538)	$ (23,552)	$ (17,100)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	4,570	402	182
Net amortization of premiums and discounts on marketable securities	(1,310)
Amortization of beneficial conversion feature related to convertible promissory notes payable to related parties		853
Benefit from deferred income taxes		(291)
Depreciation and amortization	242	241	148
Noncash interest related to convertible promissory notes payable to related parties		228
Loss on extinguishment of debt facility		159
Noncash interest related to debt facility		101	29
Accretion of tenant improvement allowance	(82)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,489)	(637)	275
Other long-term assets	313	(150)	(250)
Accounts payable	76	510	(1,141)
Accrued expenses and other current liabilities	2,063	(432)	279
Other long-term liabilities	705
Net cash used in operating activities	(38,450)	(22,568)	(17,578)
Cash flows from investing activities
Purchases of marketable securities	(236,601)
Proceeds from maturities of marketable securities	92,500
Purchases of property and equipment	(6,946)	(264)	(234)
Net cash used in investing activities	(151,047)	(264)	(234)
Cash flows from financing activities
Proceeds from issuance of common stock, net of issuance costs	138,357
Proceeds from issuance of convertible preferred stock, net of issuance costs		92,331	18,991
Proceeds from issuance of convertible promissory notes, net of issuance costs		7,414
Repayment of debt facility		(5,250)
Proceeds from debt facility, net of issuance costs			4,985
Proceeds from exercise of stock options, net of repurchases	345	228	36
Payments for deferred financing costs		(100)
Proceeds from repayment of recourse promissory note	50
Net cash provided by financing activities	138,752	94,623	24,012
Net increase (decrease) in cash, cash equivalents and restricted cash	(50,745)	71,791	6,200
Cash, cash equivalents and restricted cash, beginning of period	85,207	13,416	7,216
Cash, cash equivalents and restricted cash, end of period	34,462	85,207	13,416
Supplemental disclosures
Cash paid for interest		228	39
Noncash investing and financing items:
Conversion of convertible promissory notes payable to related parties		7,642
Recognition of beneficial conversion feature related to convertible promissory notes to related parties, net of benefit for income taxes		1,867
Reclassification of preferred stock tranche liability upon settlement			1,795
Lessor funded lease incentives included in property and equipment	1,386
Reclassification of beneficial conversion feature related to convertible promissory notes payable to related parties, net of tax expense		1,305
Property and equipment purchased in accounts payable	313	89
Deferred initial public offering costs in accounts payable		63
Vesting of restricted common stock subject to repurchase	$ 24	$ 28	20
Issuance of common stock warrants in connection with debt facility			$ 24

Organization and Business

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization and Business

1. Organization and Business

Allakos Inc. (“Allakos” or the “Company”) was incorporated in the state of Delaware in March 2012. Allakos is a clinical stage biopharmaceutical company focused on the development of AK002 for the treatment of eosinophil and mast cell related diseases. The Company’s primary activities to date have included establishing its facilities, recruiting personnel, conducting research and development of its product candidates and raising capital. The Company’s operations are located in Redwood City, California.

Liquidity Matters

Since inception, the Company has incurred net losses and negative cash flows from operations. During the year ended December 31, 2018, the Company incurred a net loss of $43.5 million and used $38.5 million of cash in operations. As of December 31, 2018, the Company had an accumulated deficit of $104.1 million and does not expect to experience positive cash flows from operating activities in the foreseeable future. The Company has financed its operations to date primarily through the sale of common stock and issuance of convertible preferred stock. Management expects to incur additional operating losses in the future as the Company continues to further develop, seek regulatory approval for and, if approved, commence commercialization of its product candidates. The Company had $178.9 million of cash, cash equivalents and marketable securities at December 31, 2018. Management believes that this amount is sufficient to fund the Company’s operations for at least the next 12 months from the issuance date of these financial statements.

Initial Public Offering and Related Transactions

On July 23, 2018, the Company completed an initial public offering (“IPO”), selling 8,203,332 shares of common stock at an offering price of $18.00 per share. Proceeds from the IPO, net of underwriting discounts and commissions, were $137.3 million. Concurrently with the IPO, the Company completed a private placement of 250,000 shares of common stock at the IPO offering price of $18.00 per share to an existing stockholder. Proceeds from this private placement were $4.5 million.

In connection with the completion of the IPO on July 23, 2018, all then outstanding shares of convertible preferred stock were converted into 30,971,627 shares of common stock.

Upon the completion of the IPO, the Company’s certificate of incorporation was amended and restated. Under the amended and restated certificate of incorporation, the Company’s authorized capital stock consists of 200,000,000 shares of common stock with a par value $0.001 per share and 20,000,000 shares of convertible preferred stock with a par value $0.001 per share.

Reverse Stock Split

On July 6, 2018, the Company amended its certificate of incorporation to effect a 1-for-1.25 reverse stock split of every outstanding share of its convertible preferred stock and common stock. The Company’s audited financial statements and accompanying notes for the years ended December 31, 2017 and 2016 on the Company’s Second Amendment to its Registration Statement No. 333-225836, filed with the Securities and Exchange Commission (“SEC”) on July 17, 2018, have been retroactively restated to reflect the reverse stock split.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The financial statements have been prepared in accordance with United States generally accepted accounting principles (“U.S. GAAP”). The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts and disclosures reported in the financial statements and accompanying notes.

Use of Estimates

Management uses significant judgment when making estimates related to common stock valuation and related stock-based compensation expense, accrued expenses related to clinical trials and deferred tax valuation allowances. Management bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Actual results could differ from those estimates under different assumptions or conditions, and those differences could be material to the financial position and results of operations.

Concentration of Credit Risk and Other Risks and Uncertainties

Financial instruments that potentially subject the Company to credit risk principally consist of cash, cash equivalents and marketable securities. These financial instruments are held in accounts at select financial institutions that management believes possess high credit quality. Amounts on deposit with these financial institutions have and will continue to exceed federally-insured limits. The Company has not experienced any losses on its cash deposits. Additionally, the Company’s investment policy limits its investments to certain types of securities issued by the U.S. government and its agencies.

The Company is subject to a number of risks similar to that of other early-stage biopharmaceutical companies, including, but not limited to, the need to obtain adequate additional funding, possible failure of current or future clinical trials, its reliance on third-parties to conduct its clinical trials, the need to obtain regulatory and marketing approvals for its product candidates, competitive developments, the need to successfully commercialize and gain market acceptance of the Company’s product candidates, its right to develop and commercialize its product candidates pursuant to the terms and conditions of the licenses granted to the Company, protection of proprietary technology, the ability to make milestone, royalty or other payments due under licensing agreements, and the need to secure and maintain adequate manufacturing arrangements with third-parties. If the Company does not successfully commercialize or partner its product candidates, it will be unable to generate product revenue or achieve profitability.

Cash, Cash Equivalents and Restricted Cash

The Company considers all highly liquid investments with original maturities of three months or less from the date of purchase to be cash equivalents. The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the Company’s balance sheets and which, in aggregate, represent the amounts reported in the statements of cash flows (in thousands):

	December 31,
	2018		2017		2016
Cash and cash equivalents	$	33,660	$	85,207	$	13,416
Restricted cash		802		—		—
Total	$	34,462	$	85,207	$	13,416

Restricted cash at December 31, 2018 represents $0.8 million in deposits restricted from withdrawal and held by a bank in the form of collateral for an irrevocable standby letter of credit held as security for the lease of the Company’s facility in Redwood City, California. Restricted cash amounts are included within long term assets on the Company’s balance sheets.

Marketable Securities

The Company invests in marketable securities, primarily securities issued by the U.S. government and its agencies. Investments with contractual maturities greater than 90 days that mature less than one year from the balance sheet date are classified as short-term investments. Those investments with a contractual maturity date greater than one year are considered long-term investments. Unrealized gains and losses are excluded from earnings and are reported as a component of accumulated comprehensive income (loss). The cost of securities sold is determined using the specific-identification method. Interest earned and adjustments for the amortization of premiums and discounts on investments are included in interest income (expense), net, on the statements of operations and comprehensive loss. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on investments in marketable securities are included in other income (expense), net, on the statements of operations and comprehensive loss.

Fair Value Measurements

The Company accounts for fair value of its financial instruments in accordance with Financial Accounting Standards Board (the “FASB”) Accounting Standards Codification (“ASC”) Topic No. 820, Fair Value Measurements (“ASC 820”). ASC 820 establishes a common definition for fair value, establishes a framework for measuring fair value and expands disclosures about such fair value measurements. Additionally, ASC 820 defines fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date.

The Company measures fair value based on a three-level hierarchy of inputs, of which the first two are considered observable and the last unobservable. Unobservable inputs reflect the Company’s own assumptions about current market conditions. The three-level hierarchy of inputs is as follows:

Level 1 – Quoted prices in active markets for identical assets or liabilities.

Level 2 – Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

Level 3 – Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of assets or liabilities.

To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.

The carrying amounts reflected in the Company’s balance sheets for cash and cash equivalents, prepaid expenses and other current assets, other long-term assets, accounts payable, and accrued expenses and other current liabilities approximate fair value, due to their short-term nature.

Deferred Initial Public Offering Costs

Costs incurred in connection with the IPO primarily consist of direct incremental legal, printing and accounting fees. IPO costs are capitalized as incurred and will be offset against proceeds upon consummation of the offering. In the event the offering had been terminated or abandoned, deferred IPO costs would have been expensed in the period such determination had been made. As of December 31, 2017, there was $0.2 million of deferred IPO costs included in other long-term assets on the Company’s balance sheets. The Company completed its IPO in July 2018. Accordingly, there were no deferred IPO offering costs at December 31, 2018.

Lease Liabilities

The Company classifies the agreements for its office and laboratory facilities as an operating lease. Rent expense is recorded on a straight-line basis over the term of the lease. Differences that exist between cash rent payments and the recognition of rent expense, such as those resulting from rent abatements or contractual escalations of minimum lease payments, are recorded as a deferred rent liability and recognized as adjustments to rental expense on a straight-line basis over the term of the lease. The current portion of the deferred rent liability is included within accrued expenses and other current liabilities on the Company’s balance sheets. Noncurrent portion of deferred rent liability is classified as other long-term liabilities. Tenant improvement allowances received are recorded as lease incentive obligations included in accrued expenses and other current liabilities and other long-term liabilities on the Company’s balance sheets and are amortized to rent expense over the term of the lease.

Term Loan Financing Costs

During the year ended December 31, 2017, the Company recognized noncash interest expense of $101,000, related to its then outstanding debt facility. Noncash interest included the amortization and accretion of various costs incurred in connection with the issuance of the associated debt instruments and calculated using the effective interest rate method over the expected term of the debt. In December 2017, the Company repaid all outstanding debt. Noncash interest expense was included in interest income (expense), net, within the Company’s statements of operations and comprehensive loss.

Property and Equipment, Net

Property and equipment are stated at cost, net of accumulated depreciation. Depreciation is computed using the straight-line method over the estimated useful lives of the assets. The useful lives of property and equipment are as follows:

Laboratory equipment – 3 to 5 years

Leasehold improvements – Shorter of remaining lease term or estimated life of the assets

Upon retirement or sale, the cost of disposed assets and their related accumulated depreciation are removed from the balance sheet. Any resulting gains or losses on dispositions of property and equipment are included as a component of other income (expense), net, within the Company’s statements of operations and comprehensive loss. Repair and maintenance costs that do not significantly add value to the property and equipment, or prolong its life, are charged to operating expense as incurred.

Research and Development Expense

Research and development costs are expensed as incurred. Research and development costs include, among others, consulting costs, salaries, benefits, travel, stock-based compensation, laboratory supplies and other non-capital equipment utilized for in-house research, allocation of facilities and overhead costs and external costs paid to third-parties that conduct research and development activities on the Company’s behalf. Amounts incurred in connection with license agreements are also included in research and development expense.

Advance payments for goods or services to be rendered in the future for use in research and development activities are deferred and included in prepaid expenses and other current assets. The deferred amounts are expensed as the related goods are delivered or the services are performed.

Accrued Research and Development Costs

Service agreements with contract research organizations (“CROs”) and contract development and manufacturing organizations (“CDMOs”) comprise a significant component of the Company’s research and development activities. External costs for CROs and CDMOs are recognized as the services are incurred. The Company accrues for expenses resulting from obligations under agreements with its third-parties for which the timing of payments does not match the periods over which the materials or services are provided to the Company. Accruals are recorded based on estimates of services received and efforts expended pursuant to agreements established with CROs, CDMOs and other outside service providers. These estimates are typically based on contracted amounts applied to the proportion of work performed and determined through analysis with internal personnel and external service providers as to the progress or stage of completion of the services.

The Company makes judgements and estimates in determining the accrual balance in each reporting period. In the event advance payments are made to a CRO, CDMO or other outside service provider, the payments are recorded within prepaid expenses and other current assets and subsequently recognized as research and development expense when the associated services have been performed. As actual costs become known, the Company adjusts its liabilities and assets. Inputs, such as the extent of services received and the duration of services to be performed, may vary from the Company’s estimates, which will result in adjustments to research and development expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations. The Company’s historical estimates have not been materially different from actual amounts recorded.

Convertible Debt Features

Beneficial conversion features embedded within the Company’s convertible debt instruments are recognized at their intrinsic value at the commitment date. Intrinsic value is calculated as the difference between the effective conversion price and the fair value of the preferred stock into which the debt is convertible, multiplied by the number of shares of preferred stock into which the debt is convertible. The Company allocates a portion of the proceeds from issuance of the convertible debt to the beneficial conversion feature as a reduction to the carrying value of the debt, with the offset to additional paid-in capital. The resulting debt discount is amortized to interest expense through the stated maturity date of the convertible debt instrument using the effective interest method. Conversion of the debt to convertible preferred stock is accounted for as an extinguishment. Upon conversion, all unamortized discounts at the conversion date are recognized immediately as interest expense. The Company then allocates a portion of the reacquisition price to the repurchase of the beneficial conversion feature, as calculated by the intrinsic value of the conversion option at the extinguishment date. The residual amount, if any, is allocated to the convertible debt instrument. The gain or loss on extinguishment of the convertible debt instrument is measured as the difference between the retired debt’s reacquisition price and carrying amount prior to extinguishment. Gains or losses resulting from convertible debt instruments issued to related parties are classified as capital contributions or distributions.

Preferred Stock Tranche Rights

Convertible preferred stock that includes features the Company has determined are not clearly and closely related to the equity host are bifurcated and accounted for separately as freestanding derivative assets or liabilities on the balance sheet at their estimated fair value. The Company historically recorded preferred stock derivative liabilities resulting from certain investors’ rights to purchase from the Company, on the same terms as the Series A Preferred Stock Purchase Agreement executed in December 2012, additional shares of Series A convertible preferred stock in a second and third tranche. At initial recognition, the Company recorded these derivatives as an asset or liability on the balance sheets at their estimated fair value. The derivatives were subject to remeasurement at each balance sheet date, with changes in fair value recognized as a component of other income (expense), net on the Company’s statements of operations and comprehensive loss. At the time of each tranche funding, the Company remeasured the derivative asset or liability, with the change in fair value recognized as a component of other income (expense), net and then reclassified the remaining value associated with the preferred stock derivative to Series A convertible preferred stock.

Convertible Preferred Stock

The Company recorded all shares of convertible preferred stock net of offering costs at their respective fair values on the dates of issuance. The convertible preferred stock was recorded outside of stockholders’ deficit because, in the event of certain deemed liquidation events considered not solely within the Company’s control, such as a merger, acquisition or sale of all or substantially all of the Company’s assets, the convertible preferred stock will become redeemable at the option of the holders. In the event of a change of control of the Company, proceeds received from the sale of such shares will be distributed in accordance with the liquidation preferences set forth in the Company’s Second Amended and Restated Certificate of Incorporation unless the holders of convertible preferred stock had previously converted their shares of convertible preferred stock into shares of common stock. In connection with the completion of the Company’s IPO in July 2018, all then outstanding shares of convertible preferred stock were converted into 30,971,627 shares of common stock.

Segments

Operating segments are defined as components of an entity about which separate discrete information is available for evaluation by the chief operating decision maker or decision-making group, in deciding how to allocate resources and in assessing performance. The Company’s chief operating decision maker, its Chief Executive Officer, views its operations and manages its business in one operating segment operating exclusively in the United States.

Patent Costs

The Company expenses patent application and related legal costs as incurred and classifies such costs as general and administrative expenses in the statements of operations and comprehensive loss

Stock-Based Compensation

The Company accounts for its stock-based compensation in accordance with FASB ASC Topic 718, Compensation—Stock Compensation (“ASC 718”). ASC 718 requires all stock-based payments to employees and directors to be recognized as expense in the statements of operations and comprehensive loss based on their grant date fair values. For purposes of determining the estimated fair value of stock options granted to employees and directors, the Company uses the Black-Scholes option pricing model. The Black-Scholes option pricing model requires the input of certain assumptions that involve judgment, for which changes can materially affect the resulting estimates of fair value. The assumptions used to determine the fair value of stock options granted were as follows:

Expected volatility – As there is insufficient trading history for the Company’s common stock, the Company has based its computation of expected volatility on the historical volatility of a representative group of public companies with similar characteristics to the Company, including stage of product development and life science industry focus. The historical volatility is calculated based on a period of time commensurate with the expected term assumption.

Expected term – The Company determines the expected term in accordance with the “simplified method” described by SEC Staff Accounting Bulletin No. 107, Share-Based Payment, as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term.

Risk-free interest rate – The Company bases the risk-free interest rate on United States Treasury securities with terms consistent to the expected term of the stock option being valued.

Expected dividends – The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock.

The Company uses historical data to estimate pre-vesting forfeitures and records stock-based compensation expense only for those awards expected to vest. To the extent that actual forfeitures differ from estimates, the difference is recorded as a cumulative adjustment in the period the estimate are revised. The Company expenses the fair value of its stock-based compensation awards to employees on a straight-line basis over the requisite service period, which is generally the vesting period.

Income Taxes

In December 2017, the Tax Cuts and Jobs Act of 2017 (the “Tax Act”) was signed into law. The Tax Act, among other changes, lowered the Company’s federal tax rate from 34% to 21%. Based on provisions of the Tax Act, the Company remeasured its deferred tax assets and liabilities at December 31, 2017 to reflect the lower statutory tax rate, however, since the Company established a full valuation allowance to offset its deferred tax assets, there was no impact to the effective tax rate. The deferred tax remeasurement was provisional and represented our reasonable estimate within the meaning of Staff Accounting Board 118, which provided a measurement period that should not extend beyond one year from the Tax Act’s enactment date for companies to complete the accounting under ASC 740. As of December 31, 2018, the Company has completed its analysis of the income tax effects of the Tax Act. The results of this analysis have been reflected in the Company’s financial statements and related footnotes.

Comprehensive Loss

Comprehensive loss is defined as the change in stockholders’ equity (deficit) during a period from transactions and other events and circumstances from non-owner sources. The difference between net loss and comprehensive loss for the year ended December 31, 2018 is a result of unrealized losses on the Company’s investments in marketable securities included in current assets on the balance sheets.

Net Loss per Share

The Company calculates basic net loss per share by dividing the net loss attributable to common stockholders by the weighted-average shares of common stock outstanding during the period. The Company calculates diluted net loss per share after giving consideration to all potentially dilutive securities outstanding during the period using the treasury-stock and if-converted methods, except where the effect of including such securities would be anti-dilutive. Because the Company has reported net losses since inception, the effect from potentially dilutive securities would have been anti-dilutive and therefore has been excluded from the calculation of diluted net loss per share.

Basic and diluted net loss per share was calculated as follows (in thousands, except per share data):


	Year Ended December 31,
	2018			2017			2016
Numerator:
Net loss	$	(43,538	)	$	(23,552	)	$	(17,100	)
Denominator:
Weighted-average shares of common stock outstanding, basic and diluted		19,833			1,620			1,312
Net loss per share, basic and diluted	$	(2.20	)	$	(14.54	)	$	(13.03	)

The following table sets forth the potentially dilutive securities that have been excluded from the calculation of diluted net loss per share due to their anti-dilutive effect for the periods indicated (in thousands):


	Year Ended December 31,
	2018		2017		2016
Series A convertible preferred stock		—		20,866		20,866
Series B convertible preferred stock		—		10,105		—
Options to purchase common stock		7,811		4,884		2,722
Warrants to purchase common stock		—		48		48
Unvested restricted common stock		47		104		205
Shares issuable under 2018 Employee Stock Purchase Plan		29		—		—
Total		7,887		36,007		23,841

Foreign Currency Transactions

The Company is party to multiple contract manufacturing and clinical research agreements for which services to be performed are denominated in foreign currencies other than the United States Dollar. The Company records gains and losses attributable to fluctuations in foreign currencies as a component of other income (expense), net, on the statements of operations and comprehensive loss.

Recent Accounting Pronouncements

In August 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement. This ASU eliminates, modifies and adds disclosure requirements for fair value measurements. The amendments in this ASU are effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. The Company is currently evaluating the effects of this ASU on its financial statements and related disclosures.

In August 2018, the SEC adopted amendments to certain disclosure requirements in the Securities Act Release No. 33-10532, Disclosure Update and Simplification. Under the amendments, the Company must provide an analysis of changes in each caption of stockholders' equity presented in the balance sheet in a note or separate statement. The Company will adopt the amendments beginning with the first quarter of fiscal year 2019. The Company does not expect the adoption of this standard to have a material impact on its financial statements.

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), which generally requires lessees to recognize operating and financing lease liabilities and corresponding right-of-use assets on the balance sheet along with enhanced disclosures surrounding the amount, timing and uncertainty of cash flows arising from such leasing arrangements. Subsequently, in July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842): Targeted Improvements, which includes a transition method for companies adopting ASU 2016-02 through the recognition of a cumulative-effect adjustment to the opening balance of accumulated deficit or retained earnings in the period of adoption. The Company plans to adopt both standards effective January 1, 2019 using a modified retrospective approach, taking advantage of certain practical expedients permitted under the transition guidance. Practical expedients the Company plans to elect include (i) carrying forward historical lease classifications, (ii) foregoing a re-evaluation of historical contracts to identify embedded leases, (iii) foregoing a re-assessment of initial direct costs related to leases that existed prior to adoption, (iv) combining lease and non-lease components, and (v) recognizing lease expense for all contracts with an initial term of 12 months or less within the statements of operations and comprehensive loss on a straight-line basis over the requisite lease term. The Company has not yet completed an assessment of the total impact such adoption will have on the Company’s financial statements and related disclosures but expects to record a right-of-use asset and a corresponding liability for the lease of its corporate headquarters during the period of adoption.

In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows: Restricted Cash (“ASU 2016-18”). ASU 2016-18 amends the classification and presentation of changes in restricted cash or restricted cash equivalents in the statement of cash flows. ASU 2016-18 is effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. The Company retrospectively adopted ASU 2016-18 effective January 1, 2018. There was no significant impact to the Company’s financial statements as there were no restricted cash or restricted cash equivalent balances in the prior periods.

Fair Value Measurements

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

3. Fair Value Measurements

The Company measures and reports certain financial instruments as assets and liabilities at fair value on a recurring basis. The Company’s financial assets measured at fair value on a recurring basis were as follows (in thousands):

	December 31, 2018
	Level 1		Level 2		Level 3		Total
Cash equivalents
Money market funds	$	31,555	$	—	$	—	$	31,555
Total cash equivalents	$	31,555	$	—	$	—	$	31,555
Marketable securities
U.S. treasuries	$	145,246	$	—	$	—	$	145,246
Total marketable securities	$	145,246	$	—	$	—	$	145,246
Total cash equivalents and marketable securities	$	176,801	$	—	$	—	$	176,801

	December 31, 2017
	Level 1		Level 2		Level 3		Total
Cash equivalents
Money market funds	$	82,526	$	—	$	—	$	82,526
Total cash equivalents	$	82,526	$	—	$	—	$	82,526

The Company evaluates transfers between levels at the end of each reporting period. There were no transfers of assets or liabilities between levels during the years ended December 31, 2018 and 2017.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Marketable Securities

12 Months Ended

Dec. 31, 2018

Investments Debt And Equity Securities [Abstract]

Marketable Securities

4. Marketable Securities

All marketable securities were considered available-for-sale at December 31, 2018. The amortized cost, gross unrealized holding gains or losses, and fair value of the Company’s marketable securities by major security type at December 31, 2018 are summarized in the table below (in thousands):

	December 31, 2018
	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
Marketable securities
U.S. treasuries	$	145,261	$	—	$	(15	)	$	145,246
Total	$	145,261	$	—	$	(15	)	$	145,246

The Company had no other-than-temporary impairments on its marketable securities during the year ended December 31, 2018. The Company has the intent and ability to hold all marketable securities until their maturities.

The Company held no marketable securities at December 31, 2017.

Balance Sheet Components and Supplemental Disclosures

12 Months Ended

Dec. 31, 2018

Balance Sheet Related Disclosures [Abstract]

Balance Sheet Components and Supplemental Disclosures

5. Balance Sheet Components and Supplemental Disclosures

Property and Equipment, Net

Property and equipment, net, consisted of the following (in thousands):

	December 31,
	2018			2017
Laboratory equipment	$	3,272		$	949
Furniture and office equipment		1,666			—
Leasehold improvements		4,545			55
		9,483			1,004
Less accumulated depreciation		(635	)		(559	)
Property and equipment, net	$	8,848		$	445

Depreciation and amortization expense for the years ended December 31, 2018, 2017 and 2016 was $0.2 million, $0.2 million and $0.1 million, respectively.

Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):

	December 31,
	2018		2017
Accrued outside professional services	$	1,964	$	787
Accrued compensation and benefits		1,041		265
Lease incentive obligation, current		123		—
Other current liabilities		36		37
Total	$	3,164	$	1,089

Debt Facility

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Debt Facility

6. Debt Facility

In June 2016, the Company entered into a Loan and Security Agreement with a financial institution, providing for term loans to the Company, in two tranches for an aggregate principal amount of $5.0 million. Interest on the term loans was calculated at a floating per annum rate equal to the prime rate reported in The Wall Street Journal plus one quarter of one percent (0.25%). Upon execution of the agreement, the Company had immediate access to borrow up to $2.5 million in principal. The remaining $2.5 million in principal would be made available to the Company on or prior to December 31, 2016, subject to the satisfaction of certain borrowing conditions including the achievement of certain pre-defined clinical development milestones. In July 2016, the Company drew down an initial term loan of $2.5 million. Subsequently, in December 2016, the Company drew down the remaining $2.5 million term loan. During 2017 this debt facility was fully repaid and terminated.

The Company incurred upfront costs of $15,000 to issue the debt facility which were classified as a discount to the carrying value of the term loans included on the balance sheet at December 31, 2016. Final payments due to the lender for facility fees of $0.2 million were treated as deferred issuance costs and accreted to interest expense over the term of the loans. Amortization of the upfront issuance costs and accretion of the deferred issuance costs was calculated using the effective interest method.

Common Stock Warrants

As part of the Loan and Security Agreement, the Company also agreed to issue the financial institution warrants to purchase shares of its common stock upon each draw of the term loans. During the year ended December 31, 2016, the Company issued the financial institution warrants to purchase a total of 47,616 shares of common stock at a weighted average exercise price of $0.61 per share. The common stock warrants were immediately exercisable upon issuance and shall remain outstanding for a period of ten years from the date of issuance. Fair value of the common stock warrants totaling $24,000 was recorded as a reduction to the carrying value of the loans and amortized to interest expense over the remaining term of the loans using the effective interest method. The initial fair value of the warrants was determined using the Black-Scholes option pricing model including weighted average assumptions for expected volatility of 80.0%, an expected life equal to the contractual term of the warrants of 10 years and a risk-free interest rate of 2.0%. During the year ended December 31, 2018, all outstanding warrants were exercised by cashless exercise, resulting in the Company’s issuance of 46,893 shares of common stock.

Loss on Extinguishment

In December 2017, the Company repaid the term loans prior to the stated maturity date, recognizing a loss on extinguishment of debt totaling $0.2 million. This amount consisted of a $50,000 prepayment penalty, a write-off of $17,000 of unamortized discount and a write-off of $92,000 of unamortized debt issuance costs. The loss on extinguishment of debt was recorded as other income (expense), net on the statements of operations and comprehensive loss. The write-offs of unamortized discount and unamortized debt issuance costs represent a non-cash adjustment to reconcile net income to net cash used in operating activities on the statement of cash flows.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

7. Commitments and Contingencies

Operating Lease Obligations

The Company’s operating lease obligations primarily relate to leased office and laboratory space under noncancelable operating leases. During the year ended December 31, 2018, the Company terminated the lease agreement for its San Carlos, California office. As of December 31, 2018, the Company’s remaining lease agreement pertains to its corporate headquarters located in Redwood City, California. The lease agreement was entered into during January 2018 and includes a contractual lease term commencing upon substantial completion and delivery of the premises which occurred in November 2018. The base term of the lease is 10.75 years and includes an option to extend an additional term of 5 years. The lease agreement required a security deposit of $0.8 million, which the Company satisfied by establishing a letter of credit secured by restricted cash. Restricted cash is recorded in other long-term assets on the Company’s balance sheet.

The lease agreement allows for a tenant improvement allowance of up to $1.4 million to be applied to the total cost of tenant improvements to the leased premises. Tenant improvement allowances received are recorded as leasehold improvements with corresponding lease incentive obligations included in accrued expenses and other current liabilities and other long-term liabilities on the Company’s balance sheets. Leasehold improvements are depreciated and lease incentive obligations are amortized to rent expense over the term of the lease. As of December 31, 2018, unamortized lease incentive obligations totaled $1.3 million.

In addition to the minimum future lease commitments presented below, the lease requires the Company to pay property taxes, insurance, maintenance and repair costs. Rent expense is recognized using the straight-line method over the respective terms. The Company records a deferred rent liability calculated as the difference between rent expense and cash rental payments. The current portion of the liability is included within accrued expenses and other current liabilities on the Company’s balance sheets. The remaining non-current portion is classified in other long-term liabilities.

Future minimum lease payments required under operating leases are as follows (in thousands):

Fiscal Year Ending December 31,
2019	$	403
2020		1,233
2021		1,271
2022		1,308
2023		1,348
Thereafter		8,304
Total minimum future lease payments	$	13,867

Net rent expense was $1.0 million, $0.5 million and $0.4 for the years ended December 31, 2018, 2017 and 2016, respectively.

Purchase Obligations

The Company has entered into contractual agreements with various research and development organizations and suppliers in the normal course of its business. All contracts are terminable, with varying provisions regarding termination. If a contract were to be terminated, the Company would only be obligated for the products or services that the Company had received through the time of termination as well as any non-cancelable minimum payments contractually agreed upon prior to the effective date of termination. In the case of terminating a clinical trial agreement with an investigational site conducting clinical activities on behalf of the Company, the Company would also be obligated to provide continued support for appropriate safety procedures through completion or termination of the associated study. As of December 31, 2018, the Company had $15.6 million of non-cancelable purchase obligations under these agreements.

In-Licensing Agreements

The Company has entered into exclusive and non-exclusive, royalty bearing license agreements with third-parties for certain intellectual property. Under the terms of the license agreements, the Company is obligated to pay milestone payments upon the achievement of specified clinical, regulatory and commercial milestones. Actual amounts due under the license agreements will vary depending on factors including, but not limited to, the number of products developed and the Company’s ability to further develop and commercialize the licensed products. The Company is also subject to future royalty payments based on sales of the licensed products. In-licensing payments to third-parties for milestones are recognized as research and development expense in the period of achievement.

The Company recognized $0.3 million, zero and $0.3 million of milestone expense for the years ended December 31, 2018, 2017 and 2016, respectively. Milestone payments are not creditable against royalties. As of December 31, 2018, the Company has not incurred any royalty liabilities related to its license agreements, as product sales have not yet commenced.

Exclusive License Agreement with The Johns Hopkins University

In December 2013, the Company entered into a license agreement with The Johns Hopkins University (“JHU”) for a worldwide exclusive license to develop, use, manufacture and commercialize covered product candidates including AK001 and AK002, which was amended in September 30, 2016. Under the terms of the agreement, the Company has made upfront and milestone payments of $0.3 million through December 31, 2018 and may be required to make aggregate additional milestone payments of up to $4.0 million. The Company also issued 88,887 shares of common stock as consideration under the JHU license agreement. In addition to milestone payments, the Company is also subject to single-digit royalties to JHU based on future net sales of each licensed therapeutic product candidate by the Company and its affiliates and sublicensees, with up to a low six-digit dollar minimum annual royalty payment.

Non-exclusive License Agreement with BioWa Inc. and Lonza Sales AG

In October 2013, the Company entered into a tripartite agreement with BioWa Inc. (“BioWa”), and Lonza Sales AG (“Lonza”), for the non-exclusive worldwide license to develop and commercialize product candidates including AK002 that are manufactured using a technology jointly developed and owned by BioWa and Lonza. Under the terms of the agreement, the Company has made milestone payments of $0.4 million through December 31, 2018 and may be required to make aggregate additional milestone payments of up to $41.0 million. In addition to milestone payments, the Company is also subject to minimum annual commercial license fees of $40,000 per year to BioWa until such time as BioWa receives royalty payments, as well as low single-digit royalties to BioWa and to Lonza. Royalties are based on future net sales by the Company and its affiliates and sublicensees and vary dependent on Lonza’s participation as sole manufacturer for commercial production.

Indemnification Agreements

The Company has entered into indemnification agreements with certain directors and officers that require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. To date, no such matters have arisen and the Company does not believe that the outcome of any claims under indemnification arrangements will have a material adverse effect on its financial positions, results of operations or cash flows. Accordingly, the Company has not recorded a liability related to such indemnifications at December 31, 2018.

Convertible Promissory Notes Payable to Related Parties, Net

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Convertible Promissory Notes Payable to Related Parties, Net

8. Convertible Promissory Notes Payable to Related Parties, Net

In August 2017, the Company entered into a note purchase agreement with existing investors as related parties to raise proceeds of up to $15.0 million via the issuance of convertible promissory notes (the “Notes”). The Notes bore interest at 6% per annum and were subject to automatic conversion upon a subsequent qualified financing event. Additional terms included within the note purchase agreement included an option at the election of the holder, upon maturity, to convert all outstanding principal and accrued interest into Series A convertible preferred stock at a fixed price per share, subject to appropriate adjustment in the event of any stock dividend, stock split, combination or other similar recapitalization. The Company determined this option represented a beneficial conversion feature (“BCF”) at the date of issuance as the fair value of the securities into which the Notes were convertible upon maturity was greater than the effective conversion price on the date of issuance.

During the year ended December 31, 2017, the Company issued $7.5 million in Notes. The Company recorded the Notes at the principal amount received, net of transaction costs of $86,000, with a portion of the proceeds being allocated to the BCF relative to its intrinsic value of $2.8 million. The Company calculated the intrinsic value of the BCF as the difference between the fair value of the underlying Series A convertible preferred stock and the effective conversion price embedded in the Notes. The BCF was initially recorded as an increase to additional paid-in capital with the offset recorded as a discount on the Notes.

During the year ended December 31, 2017, in addition to stated interest of $0.1 million, the Company recognized non-cash interest expense of $0.9 million associated with the amortization of the discounts, issuance costs and BCF. The amortization schedule was calculated using the effective interest method through August 2018, the contractual maturity of the Notes.

In November 2017, the Notes were redeemed contemporaneously with the Company’s Series B convertible preferred stock financing. The aggregate of the outstanding principal and accrued interest balance of $7.6 million was converted into 771,083 shares of Series B convertible preferred stock based on the Series B convertible preferred stock fair value. The redemption of the Notes was accounted for as a debt extinguishment with a portion of the reacquisition price allocated to the BCF and total unamortized debt discount of $60,000 written off to interest expense. The amount allocated to reacquire the BCF was measured using the intrinsic value of the conversion option at the extinguishment date and reflected as a reduction to equity. As a result, the amount allocated to reacquire the Notes was less than the carrying value of the Notes which resulted in a deemed capital contribution received from related parties of $0.9 million.

Benefit from Income Taxes Related to Intra-Period Tax Allocations

The BCF associated with the convertible promissory notes to related parties resulted in a temporary difference between the carrying amount and tax basis of the debt instruments. Upon issuance, the Company recognized the temporary difference as a deferred tax liability of $1.0 million with an offsetting adjustment to additional paid in capital. Recognition of the deferred tax liability resulted in a reduction to the Company’s net deferred tax assets. Accordingly, the Company reduced its existing valuation allowance by $1.0 million and recognized a corresponding income tax benefit of $1.0 million in accordance with ASC 740-10, Accounting for Uncertainty in Income Taxes. During the year ended December 31, 2017, the deferred tax liability was reduced in relation to the amortization of the BCF. Upon extinguishment of the Notes in November 2017, the Company wrote down the remaining $0.7 million of deferred tax liability resulting in a net benefit from income taxes of $0.3 million for the year then ended.

Convertible Preferred Stock and Stockholders' Equity (Deficit)

12 Months Ended

Dec. 31, 2018

Stockholders Equity Note [Abstract]

Convertible Preferred Stock and Stockholders' Equity (Deficit)

9. Convertible Preferred Stock and Stockholders’ Equity (Deficit)

The Company’s amended and restated certificate of incorporation filed on July 23, 2018 authorizes the issuance of a total of 220,000,000 shares of stock. Of these shares, 200,000,000 are designated as common stock and 20,000,000 are designated as preferred stock. There were 42,116,641 shares of common stock issued and outstanding and no shares of preferred stock issued and outstanding at December 31, 2018.

Convertible Preferred Stock

In December 2012, the Company entered into a Series A Preferred Stock Purchase Agreement (the “Series A Purchase Agreement”) with investors under which the Company agreed to sell and investors agreed to purchase up to 14,222,218 shares of Series A convertible preferred stock at a purchase price of $2.25 per share. Upon execution of the Series A Purchase Agreement, the Company issued 4,444,441 shares of Series A convertible preferred stock for net cash proceeds of $9.8 million (the “Initial Closing”).

In August 2014, the Company and its investors amended the Series A Purchase Agreement, pursuant to which the Company agreed to sell and investors agreed to purchase up to an additional 4,444,446 shares of Series A convertible preferred stock under the same terms as the original agreement. From August 2014 through September 2014, the Company issued 6,666,662 shares of Series A convertible preferred stock at a purchase price of $2.25 per share for net cash proceeds of $14.9 million (the “Second Closings”).

In March 2015, the Company and its investors amended the Series A Purchase Agreement a second time, pursuant to which the Company agreed to sell and investors agreed to purchase up to an additional 2,222,229 shares of Series A convertible preferred stock under the same terms as the original agreement. Concurrent with the second amendment, the Company issued 1,310,906 shares of Series A convertible preferred stock at a purchase price of $2.25 per share for net cash proceeds of $2.9 million (the “Additional Second Closings”).

In January 2016, the Company issued 8,444,440 shares of Series A convertible preferred stock at a purchase price of $2.25 per share for net cash proceeds of $19.0 million (the “Third Closings”).

In November 2017, the Company entered into a Series B Preferred Stock Purchase Agreement (the “Series B Purchase Agreement”) with existing as well as new investors for the issuance of up to 10,105,181 shares of Series B convertible preferred stock at a purchase price of $9.91 per share. Upon the execution of the Series B Purchase Agreement, the Company issued 9,334,098 shares of Series B convertible preferred stock for net cash proceeds of $92.3 million. In addition, the Company issued 771,083 shares of Series B convertible preferred stock upon the conversion of outstanding convertible promissory notes to related parties, including accrued interest, in the amount of $7.6 million. See Note 8 — Convertible Promissory Notes Payable to Related Parties, Net.

As of December 31, 2017, convertible preferred stock consisted of the following (in thousands):

`			Shares Issued				Aggregate
	Shares		and		Net Carrying		Liquidation
	Authorized		Outstanding		Value		Preference
Series A convertible preferred stock		26,083		20,866	$	42,996	$	46,950
Series B convertible preferred stock		12,632		10,105		99,973		100,141
Total		38,715		30,971	$	142,969	$	147,091

Prior to the conversion of the Company’s convertible stock upon the Company’s IPO, the significant provisions of each series of convertible preferred stock were as follows:

Dividends – Holders of shares of convertible preferred stock were entitled to receive noncumulative dividends prior to, and in preference to any declaration or payment of any dividend on the common stock at the rate of 8% of the original issue price of the applicable series of convertible preferred stock, when and if declared by the Company’s Board of Directors. After payment of dividends to the holders of shares of convertible preferred stock, any additional dividends were to be paid equally among the holders of convertible preferred stock and common stock on an as converted basis. As of December 31, 2017 and 2016, and through the date of conversion, no dividends had been declared.

Liquidation Preference – In the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company, the holders of Series B convertible preferred stock were entitled to receive, prior and in preference to any distribution from the assets of the Company to the holders of Series A convertible preferred stock or common stock, a per-share amount equal to the sum of the original issue price of Series B convertible preferred stock plus all accrued but unpaid dividends. After the payment of the full Series B liquidation preference, holders of the Series A convertible preferred stock were entitled to receive, prior and in preference to any distribution from the assets of the Company to the holders of common stock, a per-share amount equal to the sum of the original issue price of Series A convertible preferred stock plus all accrued but unpaid dividends. After the payment of all preferential amounts required to be paid upon liquidation to the holders of the convertible preferred stock, the remaining assets would have been distributed to holders of the common stock on a pro-rata basis.

Conversion – Shares of convertible preferred stock were convertible at the holder’s option into shares of common stock, on a share-for-share basis, using a conversion rate determined by dividing the original issue price by the conversion price. The conversion rate could be adjusted upon certain events and for certain dilutive issuances, splits and combinations. The initial conversion price for the Series A and Series B convertible preferred stock was $2.25 and $9.91, respectively. Each share of convertible preferred stock would have been automatically converted into common stock, at its then applicable conversion rate, upon (i) the closing of an underwritten public offering of the Company’s common stock that provides not less than $50 million of gross proceeds at an offering price of not less than $9.91 per share of common stock, as adjusted for recapitalization or (ii) the written request for conversion by the holders of at least a majority of the convertible preferred stock, voting together on an as converted basis.

Voting Rights – Each share of convertible preferred stock had the same voting rights as the number of shares of common stock into which it was convertible and voted together with the holders of common stock as a single class.

Protective Provisions – The holders of convertible preferred stock had certain protective provisions. As long as one million shares of convertible preferred stock remained outstanding, the Company could not, without the approval of the holders of more than 50% of the then-outstanding shares of convertible preferred stock, voting as a single class on an as-converted basis, (i) authorize or create any new class or series of equity security that is senior to or on parity with the convertible preferred stock, (ii) increase or decrease the authorized number of shares under the Company’s equity incentive plans, (iii) consummate a liquidation, dissolution or winding up of the Company, or any deemed liquidation event, (iv) redeem, purchase or otherwise acquire shares of common stock, subject to certain exceptions, (v) change the authorized number of directors, (vi) pay or declare dividends, or (vii) alter or change the rights, preferences or privileges of the convertible preferred stock in a manner that adversely affects their rights, preferences or privileges. In addition, the holders of Series B convertible preferred stock had certain incremental protective provisions. As long as one million shares of Series B convertible preferred stock remained outstanding, the Company could not, without the approval of the holders of at least 60% of the then-outstanding shares of Series B convertible preferred stock, voting as a single class on an as-converted basis, (i) increase or decrease the authorized number of Series B convertible preferred stock, or (ii) amend the Company’s certificate of incorporation in a manner that adversely affects the rights, powers, preferences and other terms of the Series B convertible preferred stock, but does not so affect the Series A convertible preferred stock.

In connection with the completion of the Company’s IPO in July 2018, all the then-outstanding shares of convertible preferred stock were converted into 30,971,627 shares of common stock and the related carrying value was reclassified to common stock and additional paid-in capital.

Common Stock

In April 2014, the Company issued to JHU 88,887 shares of common stock as consideration for intellectual property rights received by the Company under an exclusive license agreement with JHU executed in December 2013. The Company determined the fair value of the common stock was more readily determinable than the fair value of the services rendered. The fair value of the underlying common stock on the date of issuance was $0.39 per share, resulting in total stock compensation expense of $34,000 recognized immediately in accordance with the terms of the agreement, which provided that the shares were fully vested and nonforfeitable at the time of issuance. See Note 7 — Commitments and Contingencies.

Common shares reserved for future issuance upon the exercise, issuance or conversion of the respective equity instruments are as follows (in thousands):

	December 31,
	2018		2017
Series A convertible preferred stock		—		26,083
Series B convertible preferred stock		—		12,632
Stock options issued and outstanding		7,811		4,884
Stock options available for future grant		2,785		2,346
Conversion of common stock warrants		—		48
Shares issuable under 2018 Employee Stock Purchase Plan		500		—
Total		11,096		45,993

Common stockholders are entitled to dividends if and when declared by the Board of Directors subject to the prior rights of preferred stockholders. As of December 31, 2018 and 2017, no dividends on common stock had been declared by the Board of Directors.

Stock-Based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

10. Stock-Based Compensation

Total stock-based compensation expense recognized is as follows (in thousands):


	Year Ended December 31,
	2018		2017		2016
Research and development	$	1,792	$	175	$	108
General and administrative		2,778		227		74
Total	$	4,570	$	402	$	182

No income tax benefits for stock-based compensation expense have been recognized for the years ended December 31, 2018, 2017 and 2016 as a result of the Company’s full valuation allowance applied to net deferred tax assets and net operating loss carryforwards.

Equity Incentive Plans

In December 2012, the Company adopted the 2012 Equity Incentive Plan (the “2012 Plan”), as amended and restated, under which it reserved 8,455,045 shares of common stock for the issuance of stock options, stock appreciation rights, restricted stock and restricted stock units to employees, directors and consultants. Stock options granted under the 2012 Plan generally vest over four years and expire no more than 10 years from the date of grant. As of December 31, 2017, there were 2,345,748 shares available for the Company to grant under the 2012 Plan.

In July 2018, the Board of Directors adopted the 2018 Equity Incentive Plan (the “2018 Plan”). The 2018 Plan provides for the grant of incentive stock options, non-statutory stock options, restricted stock awards, restricted stock units, stock appreciation rights and other stock-based awards. The 2018 Plan became effective on the business day immediately prior to the effective date of the registration statement. The Company initially reserved 4,000,000 shares of common stock for issuance under the 2018 Plan. The number of shares of common stock that may be issued under the 2018 Plan will automatically increase on each January 1, beginning with the fiscal year ending December 31, 2019, equal to the least of (i) 5,000,000 shares, (ii) 5% of the outstanding shares of common stock as of the last day of the preceding fiscal year and (iii) such other amount as the Board of Directors may determine. Stock options granted under the 2018 Plan generally vest over four years and expire no more than 10 years from the date of grant.

Following the IPO and upon the effectiveness of the 2018 Plan, the 2012 Plan terminated and no further awards will be granted thereunder. All outstanding awards under the 2012 Plan will continue to be governed by their existing terms. Any shares subject to awards granted under the 2012 Plan that, on or after the termination of the 2012 Plan, expire or terminate and shares previously issued pursuant to awards granted under the 2012 Plan that, on or after the termination of the 2012 Plan, are forfeited or repurchased by the Company will be transferred into the 2018 Plan. As of December 31, 2018, there were 1,325,900 shares available for the Company to grant under the 2018 Plan and the maximum number of shares that may be added to the 2018 Plan pursuant to the preceding clause is 6,485,177 shares.

The following weighted-average assumptions were used to calculate the fair value of stock-based awards granted to employees and directors during the periods indicated:

	Year Ended December 31,
	2018			2017			2016
Risk-free interest rate		2.79	%		1.83	%		1.64	%
Expected volatility		73.47	%		77.59	%		73.22	%
Expected dividend yield		—			—			—
Expected term (in years)		6.01			6.08			6.02

Activity under the 2018 Plan and the 2012 Plan is summarized as follows (in thousands, except per share data):

				Weighted-		Weighted-
				Average		Average		Aggregate
	Options			Exercise		Remaining		Intrinsic
	Outstanding			Price		Years		Value
Balance at December 31, 2017		4,884		$	0.67		8.7	$	16,331
Granted		3,789			17.19
Exercised		(531	)		0.65
Forfeited		(331	)		2.62
Balance at December 31, 2018		7,811		$	8.60		8.2	$	342,292
Options exercisable		3,684		$	1.02		7.0	$	188,812
Options vested and expected to vest		7,770		$	8.57		8.2	$	340,773

The weighted-average fair value of options granted to employees and directors during the years ended December 31, 2018, 2017 and 2016 was $11.05, $0.54 and $0.34 per share, respectively.

The aggregate fair value of stock options that vested during the years ended December 31, 2018, 2017 and 2016 was $1.8 million, $0.2 million and $0.2 million, respectively.

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. Following the IPO, the aggregate intrinsic value represents the value of the Company’s closing stock price on the last trading day of the year in excess of the weighted-average exercise price multiplied by the number of options outstanding or exercisable. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2018, 2017 and 2016 was $ 2.0 million, $0.1 million and $12,000, respectively.

The weighted-average remaining contractual life of options outstanding was 8.2 years and 8.7 years at December 31, 2018 and 2017, respectively. As of December 31, 2018, the weighted-average remaining contractual life was 7.0 years and 8.2 years for exercisable options and vested and expected to vest options, respectively.

During the years ended December 31, 2018 and 2017, the Company did not grant any stock options with performance-based or market-based vesting conditions, nor did the Company grant any stock options to non-employees in exchange for services.

As of December 31, 2018, total unrecognized stock-based compensation expense relating to unvested stock options was $38.2 million. This amount is expected to be recognized over a weighted-average period of 3.6 years.

2018 Employee Stock Purchase Plan

In July 2018, the Company’s Board of Directors and stockholders approved the 2018 Employee Stock Purchase Plan (the “2018 ESPP”). There are 500,000 shares of common stock initially reserved for issuance under the 2018 ESPP. The number of shares of common stock that may be issued under the 2018 ESPP will automatically increase on each January 1, beginning with the fiscal year ending December 31, 2019, equal to the least of (i) 1,000,000 shares, (ii) 1% of the outstanding shares of common stock as of the last day of the immediately preceding fiscal year and (iii) such other amount determined by the 2018 ESPP administrator. Under the 2018 ESPP, employees may purchase shares of the Company’s common stock at a price per share equal to 85% of the lower of the fair market value of the common stock on the first trading day of the offering period or on the exercise date. The 2018 ESPP provides for consecutive, overlapping 24-month offering periods, each of which will include purchase periods. The first offering period commenced on July 18, 2018 and will end on the first trading day on or before August 15, 2020. The second offering period will commence on the first trading day on or after February 15, 2019. During the year ended December 31, 2018, stock-based compensation related to the 2018 ESPP was $0.2 million. As of December 31, 2018, total unrecognized compensation expense relating to shares to be purchased under ESPP was $0.8 million over a weighted-average period of 0.9 years.

The following weighted-average assumptions were used to calculate the fair value of ESPP shares during the year ended December 31, 2018:

Risk-free interest rate	2.42	%
Expected volatility	65.92	%
Expected dividend yield	—
Expected term (in years)	1.24

Restricted Common Stock

The 2012 Plan allows for the issuance of restricted common stock and early exercise of unvested stock options in exchange for restricted common stock. Unvested shares of restricted common stock are subject to repurchase by the Company at the original issuance price in the event of the employee’s termination, either voluntarily or involuntarily. Consideration received for unvested stock-based awards is initially recorded as a liability and subsequently reclassified into stockholders’ deficit as the related awards vest.

A summary of the restricted common stock activity during the year ended December 31, 2018 is as follows (in thousands, except per share data):

				Weighted-
				Average
	Number			Grant Date
	of Shares			Fair Value
Balance at December 31, 2017		104		$	0.43
Vested		(57	)	$	0.43
Repurchased		—		$	—
Balance at December 31, 2018		47		$	0.43

The fair value of restricted common stock that vested during the years ended December 31, 2018 and 2017 was $24,000 and $28,000, respectively.

As of December 31, 2018, total liabilities related to unvested shares of restricted common stock was $20,000. This amount is expected to be recognized over a weighted-average period of 0.8 years.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

11. Income Taxes

The Company’s deferred income tax assets include operating losses and tax credit carryforwards, as well as certain temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Total deferred income tax assets, net of valuation allowance, at December 31, 2018 and 2017 were as follows (in thousands):

	December 31,
	2018			2017
Deferred tax assets
Net operating loss carryforwards	$	24,091		$	15,769
Research and development credits		3,995			2,242
Accruals and reserves		871			50
Fixed and intangible assets		—			51
Total deferred tax assets	$	28,957		$	18,112
Deferred tax liabilities
Fixed and intangible assets	$	65		$	—
Total deferred tax liabilities	$	65		$	—
Net deferred tax assets before valuation allowance		28,892			18,112
Less: Valuation allowance		(28,892	)		(18,112	)
Net deferred tax assets	$	—		$	—

Management has evaluated the positive and negative evidence surrounding the realizability of its deferred tax assets and has determined that it is more likely than not that the Company will not recognize the benefits of its federal and state deferred tax assets, and as a result, a valuation allowance of $28.9 million and $18.1 million has been established at December 31, 2018 and 2017, respectively. The change in the valuation allowance was $10.8 million and $0.7 million for the years ended December 31, 2018 and 2017, respectively. The Company has incurred net operating losses (“NOL”) since inception. As of December 31, 2018, the Company had federal and state NOL carryforwards of $101.2 million and $39.5 million, respectively, which expire beginning in 2032. As of December 31, 2018, the Company had federal and California research and other tax credit carryforwards of $3.8 million and $2.5 million, respectively. The federal tax credits expire beginning in 2033. The California tax credits can be carried forward indefinitely.

The Internal Revenue Code of 1986, as amended (the “Code”), provides for a limitation of the annual use of net operating losses and other tax attributes (such as research and development tax credit carryforwards) following certain ownership changes defined by the Code that could limit the Company’s ability to utilize these carryforwards in the future. At this time, the Company has not completed a study to assess whether an ownership change under Section 382 of the Code has occurred, or whether there have been multiple ownership changes since the Company’s formation. The Company may have experienced ownership changes, as defined by the Code, as a result of past financing transactions and may not be able to take full advantage of these carryforwards for federal or state income tax purposes.

The effective tax rate for the years ended December 31, 2018 and 2017 is different from the federal statutory rate primarily due to the valuation allowance against deferred tax assets as a result of insufficient income. The Company’s effective tax rate differs from the federal statutory tax rate as follows:

	Year Ended December 31,
	2018			2017
Federal statutory tax rate		21.0	%		34.0	%
Change in deferred tax asset valuation allowance		(24.8	)%		(3.0	)%
State taxes, net of federal benefit		0.9	%		0.6	%
Research and development tax credits		3.3	%		1.4	%
Remeasurement of deferreds		—			(31.2	)%
Beneficial conversion feature		—			1.2	%
Other		(0.4	)%		(1.8	)%
Effective tax rate		—			1.2	%

Uncertain Tax Positions

The Company accounts for its uncertain tax positions in accordance with FASB ASC Topic No. 740-10, Accounting for Uncertainty in Income Taxes (“ASC 740-10”). Per ASC 740-10, the Company determines its uncertain tax positions based on a determination of whether and how much of a tax benefit taken by the Company in its tax filings is more likely than not to be sustained upon examination by the relevant income tax authorities.

A reconciliation of the beginning and ending amount of unrecognized benefits is as follows (in thousands):

	Year Ended December 31,
	2018		2017
Balance at the beginning of the year	$	1,149	$	758
Increase related to current year tax positions		678		359
Increase related to prior year tax positions		—		32
Balance at the end of the year	$	1,827	$	1,149

If recognized, gross unrecognized tax benefits would not have an impact on the Company’s effective tax rate due to the Company’s full valuation allowance position. While it is often difficult to predict the final outcome of any particular uncertain tax position, the Company does not believe that the amount of gross unrecognized tax benefits will change significantly in the next twelve months.

It is the Company’s policy to include penalties and interest expense related to income taxes as a component of the income tax provision as necessary. Management determined that no accrual for interest and penalties was required at December 31, 2018 or 2017. Since the Company is in a loss carryforward position, it is generally subject to examination by the U.S. federal, state and local income tax authorities for all tax years in which a loss carryforward is available.

Benefit from Income Taxes Related to Intra-Period Tax Allocations

The beneficial conversion feature associated with the Company’s August 2017 issuance of convertible promissory notes to related parties resulted in a temporary difference between the carrying amount and tax basis of the debt instruments. Upon issuance, the Company recognized the temporary difference as a deferred tax liability of $1.0 million with an offsetting adjustment to additional paid in capital. Recognition of the deferred tax liability resulted in a reduction to the Company’s net deferred tax assets. Accordingly, the Company reduced its existing valuation allowance by $1.0 million and recognized a corresponding income tax benefit of $1.0 million in accordance with ASC 740-10. During the year ended December 31, 2017, the deferred tax liability was reduced in relation to the amortization of the beneficial conversion feature. Upon extinguishment of the notes in November 2017, the Company wrote down the remaining $0.7 million of deferred tax liability resulting in a net benefit from income taxes of $0.3 million for the year ended December 31, 2017.

Recent Changes to U.S. Tax Law

In December 2017, the 2017 Tax Cuts and Jobs Act (the “2017 Tax Act”) was enacted and includes a broad range of provisions, many of which differ significantly from those contained in previous U.S. tax law. The Company accounts for changes in tax law in accordance with ASC 740 which requires companies to recognize the effect of such changes in the period of enactment. However, the SEC staff issued Staff Accounting Bulletin 118 which will allow companies to record provisional amounts during a measurement period that is similar to the measurement period used when accounting for business combinations. Accordingly, the Company adjusted its deferred taxes and related valuation allowances on a provisional basis to reflect the reduction in U.S. federal corporate tax rate from 35% to 21%, based on current understanding of the new law. As of December 31, 2018, the Company has completed its analysis of the income effects of the 2017 Tax Act. There was no material impact on the Company’s financial statements as a result of the analysis.

Defined Contribution Plans

12 Months Ended

Dec. 31, 2018

Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]

Defined Contribution Plans

12. Defined Contribution Plans

In July 2013, the Company established a Savings Incentive Match Plan (the “SIMPLE IRA plan”) for its employees, allowing for both employee and employer contributions for those employees who meet defined minimum age and service requirements. The SIMPLE IRA plan allows participants to defer a portion of their annual compensation on a pretax basis. During the years ended December 31, 2017 and 2016, the Company made contributions to the SIMPLE IRA plan of $0.1 million and $59,000, respectively.

In January 2018, the Company terminated and replaced the SIMPLE IRA with a defined contribution plan under Section 401(k) of the Internal Revenue Code (the “401(k) plan”). The 401(k) plan covers all employees who meet defined minimum age and service requirements. Employee contributions are voluntary and are determined on an individual basis, limited to the maximum amount allowable under U.S. federal tax regulations. The Company makes matching contributions of up to 4% of the eligible employees’ compensation to the 401(k) plan. During the year ended December 31, 2018, the Company made contributions to the 401(k) plan of $0.3 million.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

13. Selected Quarterly Financial Data (Unaudited)

The following tables summarize the Company’s quarterly results for the years ended December 31, 2018 and 2017 (in thousands, except per share data):

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
2018	2018			2018			2018			2018
Loss from operations	$	(8,709	)	$	(9,524	)	$	(11,975	)	$	(15,513	)
Net loss	$	(8,485	)	$	(9,377	)	$	(11,148	)	$	(14,528	)
Net loss per common share, basic and diluted	$	(4.19	)	$	(4.17	)	$	(0.34	)	$	(0.35	)

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
2017	2017			2017			2017			2017
Loss from operations	$	(4,977	)	$	(4,598	)	$	(6,233	)	$	(6,446	)
Net loss	$	(5,056	)	$	(4,692	)	$	(5,920	)	$	(7,884	)
Net loss per common share, basic and diluted	$	(3.56	)	$	(3.18	)	$	(3.60	)	$	(4.08	)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Concentration of Credit Risk and Other Risks and Uncertainties

Cash, Cash Equivalents and Restricted Cash

	December 31,
	2018		2017		2016
Cash and cash equivalents	$	33,660	$	85,207	$	13,416
Restricted cash		802		—		—
Total	$	34,462	$	85,207	$	13,416

Marketable Securities

Fair Value Measurements

Level 1 – Quoted prices in active markets for identical assets or liabilities.

Level 3 – Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of assets or liabilities.

Deferred Initial Public Offering Costs

Lease Liabilities

Term Loan Financing Costs

Property and Equipment, Net

Laboratory equipment – 3 to 5 years

Leasehold improvements – Shorter of remaining lease term or estimated life of the assets

Research and Development Expense

Accrued Research and Development Costs

Convertible Debt Features

Preferred Stock Tranche Rights

Convertible Preferred Stock

Segments

Patent Costs

The Company expenses patent application and related legal costs as incurred and classifies such costs as general and administrative expenses in the statements of operations and comprehensive loss

Stock-Based Compensation

Risk-free interest rate – The Company bases the risk-free interest rate on United States Treasury securities with terms consistent to the expected term of the stock option being valued.

Expected dividends – The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock.

Income Taxes

Comprehensive Loss

Net Loss per Share

Basic and diluted net loss per share was calculated as follows (in thousands, except per share data):


	Year Ended December 31,
	2018			2017			2016
Numerator:
Net loss	$	(43,538	)	$	(23,552	)	$	(17,100	)
Denominator:
Weighted-average shares of common stock outstanding, basic and diluted		19,833			1,620			1,312
Net loss per share, basic and diluted	$	(2.20	)	$	(14.54	)	$	(13.03	)


	Year Ended December 31,
	2018		2017		2016
Series A convertible preferred stock		—		20,866		20,866
Series B convertible preferred stock		—		10,105		—
Options to purchase common stock		7,811		4,884		2,722
Warrants to purchase common stock		—		48		48
Unvested restricted common stock		47		104		205
Shares issuable under 2018 Employee Stock Purchase Plan		29		—		—
Total		7,887		36,007		23,841

Foreign Currency Transactions

Recent Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Reconciliation of Cash, Cash Equivalents and Restricted Cash

The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the Company’s balance sheets and which, in aggregate, represent the amounts reported in the statements of cash flows (in thousands):

	December 31,
	2018		2017		2016
Cash and cash equivalents	$	33,660	$	85,207	$	13,416
Restricted cash		802		—		—
Total	$	34,462	$	85,207	$	13,416

Calculation of Basic and Diluted Net Loss per Share

Basic and diluted net loss per share was calculated as follows (in thousands, except per share data):


	Year Ended December 31,
	2018			2017			2016
Numerator:
Net loss	$	(43,538	)	$	(23,552	)	$	(17,100	)
Denominator:
Weighted-average shares of common stock outstanding, basic and diluted		19,833			1,620			1,312
Net loss per share, basic and diluted	$	(2.20	)	$	(14.54	)	$	(13.03	)

Summary of Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss per Share


	Year Ended December 31,
	2018		2017		2016
Series A convertible preferred stock		—		20,866		20,866
Series B convertible preferred stock		—		10,105		—
Options to purchase common stock		7,811		4,884		2,722
Warrants to purchase common stock		—		48		48
Unvested restricted common stock		47		104		205
Shares issuable under 2018 Employee Stock Purchase Plan		29		—		—
Total		7,887		36,007		23,841

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Summary of Financial Assets Measured at Fair Value on Recurring Basis

	December 31, 2018
	Level 1		Level 2		Level 3		Total
Cash equivalents
Money market funds	$	31,555	$	—	$	—	$	31,555
Total cash equivalents	$	31,555	$	—	$	—	$	31,555
Marketable securities
U.S. treasuries	$	145,246	$	—	$	—	$	145,246
Total marketable securities	$	145,246	$	—	$	—	$	145,246
Total cash equivalents and marketable securities	$	176,801	$	—	$	—	$	176,801

	December 31, 2017
	Level 1		Level 2		Level 3		Total
Cash equivalents
Money market funds	$	82,526	$	—	$	—	$	82,526
Total cash equivalents	$	82,526	$	—	$	—	$	82,526

Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2018

Investments Debt And Equity Securities [Abstract]

Summary of Amortized Cost, Gross Unrealized Holding Gains or Losses, and Fair Value of Marketable Securities

	December 31, 2018
	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
Marketable securities
U.S. treasuries	$	145,261	$	—	$	(15	)	$	145,246
Total	$	145,261	$	—	$	(15	)	$	145,246

Balance Sheet Components and Supplemental Disclosures (Tables)

12 Months Ended

Dec. 31, 2018

Balance Sheet Related Disclosures [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net, consisted of the following (in thousands):

	December 31,
	2018			2017
Laboratory equipment	$	3,272		$	949
Furniture and office equipment		1,666			—
Leasehold improvements		4,545			55
		9,483			1,004
Less accumulated depreciation		(635	)		(559	)
Property and equipment, net	$	8,848		$	445

Schedule of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):

	December 31,
	2018		2017
Accrued outside professional services	$	1,964	$	787
Accrued compensation and benefits		1,041		265
Lease incentive obligation, current		123		—
Other current liabilities		36		37
Total	$	3,164	$	1,089

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Summary of Future Minimum Lease Payments Required Under Operating Leases

Future minimum lease payments required under operating leases are as follows (in thousands):

Fiscal Year Ending December 31,
2019	$	403
2020		1,233
2021		1,271
2022		1,308
2023		1,348
Thereafter		8,304
Total minimum future lease payments	$	13,867

Convertible Preferred Stock and Stockholders' Equity (Deficit) (Tables)

12 Months Ended

Dec. 31, 2018

Stockholders Equity Note [Abstract]

Schedule of Convertible Preferred Stock

As of December 31, 2017, convertible preferred stock consisted of the following (in thousands):

`			Shares Issued				Aggregate
	Shares		and		Net Carrying		Liquidation
	Authorized		Outstanding		Value		Preference
Series A convertible preferred stock		26,083		20,866	$	42,996	$	46,950
Series B convertible preferred stock		12,632		10,105		99,973		100,141
Total		38,715		30,971	$	142,969	$	147,091

Summary of Common Stock Shares Reserved for Future Issuance

Common shares reserved for future issuance upon the exercise, issuance or conversion of the respective equity instruments are as follows (in thousands):

	December 31,
	2018		2017
Series A convertible preferred stock		—		26,083
Series B convertible preferred stock		—		12,632
Stock options issued and outstanding		7,811		4,884
Stock options available for future grant		2,785		2,346
Conversion of common stock warrants		—		48
Shares issuable under 2018 Employee Stock Purchase Plan		500		—
Total		11,096		45,993

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

+ References

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+ Details

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Total Stock-Based Compensation Expense Recognized

Total stock-based compensation expense recognized is as follows (in thousands):


	Year Ended December 31,
	2018		2017		2016
Research and development	$	1,792	$	175	$	108
General and administrative		2,778		227		74
Total	$	4,570	$	402	$	182

Summary of Weighted-Average Assumptions Used to Calculate Fair Value of Stock-based Awards Granted

The following weighted-average assumptions were used to calculate the fair value of stock-based awards granted to employees and directors during the periods indicated:

	Year Ended December 31,
	2018			2017			2016
Risk-free interest rate		2.79	%		1.83	%		1.64	%
Expected volatility		73.47	%		77.59	%		73.22	%
Expected dividend yield		—			—			—
Expected term (in years)		6.01			6.08			6.02

Summary of Stock Option Activity

Activity under the 2018 Plan and the 2012 Plan is summarized as follows (in thousands, except per share data):

				Weighted-		Weighted-
				Average		Average		Aggregate
	Options			Exercise		Remaining		Intrinsic
	Outstanding			Price		Years		Value
Balance at December 31, 2017		4,884		$	0.67		8.7	$	16,331
Granted		3,789			17.19
Exercised		(531	)		0.65
Forfeited		(331	)		2.62
Balance at December 31, 2018		7,811		$	8.60		8.2	$	342,292
Options exercisable		3,684		$	1.02		7.0	$	188,812
Options vested and expected to vest		7,770		$	8.57		8.2	$	340,773

Summary of Weighted-Average Assumptions Used to Calculate Fair Value of ESPP Shares

The following weighted-average assumptions were used to calculate the fair value of ESPP shares during the year ended December 31, 2018:

Risk-free interest rate	2.42	%
Expected volatility	65.92	%
Expected dividend yield	—
Expected term (in years)	1.24

Summary of Restricted Common Stock Activity

A summary of the restricted common stock activity during the year ended December 31, 2018 is as follows (in thousands, except per share data):

				Weighted-
				Average
	Number			Grant Date
	of Shares			Fair Value
Balance at December 31, 2017		104		$	0.43
Vested		(57	)	$	0.43
Repurchased		—		$	—
Balance at December 31, 2018		47		$	0.43

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Summary of Total Deferred Income Tax Assets, Net of Valuation Allowance

Total deferred income tax assets, net of valuation allowance, at December 31, 2018 and 2017 were as follows (in thousands):

	December 31,
	2018			2017
Deferred tax assets
Net operating loss carryforwards	$	24,091		$	15,769
Research and development credits		3,995			2,242
Accruals and reserves		871			50
Fixed and intangible assets		—			51
Total deferred tax assets	$	28,957		$	18,112
Deferred tax liabilities
Fixed and intangible assets	$	65		$	—
Total deferred tax liabilities	$	65		$	—
Net deferred tax assets before valuation allowance		28,892			18,112
Less: Valuation allowance		(28,892	)		(18,112	)
Net deferred tax assets	$	—		$	—

Summary of Effective Tax Rate Differs From Federal Statutory Tax Rate

The Company’s effective tax rate differs from the federal statutory tax rate as follows:

	Year Ended December 31,
	2018			2017
Federal statutory tax rate		21.0	%		34.0	%
Change in deferred tax asset valuation allowance		(24.8	)%		(3.0	)%
State taxes, net of federal benefit		0.9	%		0.6	%
Research and development tax credits		3.3	%		1.4	%
Remeasurement of deferreds		—			(31.2	)%
Beneficial conversion feature		—			1.2	%
Other		(0.4	)%		(1.8	)%
Effective tax rate		—			1.2	%

Reconciliation of Beginning and Ending Amount of Unrecognized Benefits

A reconciliation of the beginning and ending amount of unrecognized benefits is as follows (in thousands):

	Year Ended December 31,
	2018		2017
Balance at the beginning of the year	$	1,149	$	758
Increase related to current year tax positions		678		359
Increase related to prior year tax positions		—		32
Balance at the end of the year	$	1,827	$	1,149

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Summary of Quarterly Results

The following tables summarize the Company’s quarterly results for the years ended December 31, 2018 and 2017 (in thousands, except per share data):

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
2018	2018			2018			2018			2018
Loss from operations	$	(8,709	)	$	(9,524	)	$	(11,975	)	$	(15,513	)
Net loss	$	(8,485	)	$	(9,377	)	$	(11,148	)	$	(14,528	)
Net loss per common share, basic and diluted	$	(4.19	)	$	(4.17	)	$	(0.34	)	$	(0.35	)

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
2017	2017			2017			2017			2017
Loss from operations	$	(4,977	)	$	(4,598	)	$	(6,233	)	$	(6,446	)
Net loss	$	(5,056	)	$	(4,692	)	$	(5,920	)	$	(7,884	)
Net loss per common share, basic and diluted	$	(3.56	)	$	(3.18	)	$	(3.60	)	$	(4.08	)

Organisation and Business - Additional Information (Details) $ / shares in Units, $ in Thousands			3 Months Ended								12 Months Ended
	Jul. 23, 2018 USD ($) $ / shares shares	Jul. 06, 2018	Dec. 31, 2018 USD ($) $ / shares shares	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) $ / shares shares	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($)
Organization and Business [Line Items]
Net loss \| $			$ 14,528	$ 11,148	$ 9,377	$ 8,485	$ 7,884	$ 5,920	$ 4,692	$ 5,056	$ 43,538	$ 23,552	$ 17,100
Cash used in operations \| $											38,450	22,568	$ 17,578
Accumulated deficit \| $			104,112				$ 60,574				104,112	$ 60,574
Cash, cash equivalents and marketable securities \| $			$ 178,900								$ 178,900
Common stock, shares authorized \| shares	200,000,000		200,000,000				55,000,000				200,000,000	55,000,000
Common stock, par value \| $ / shares	$ 0.001		$ 0.001				$ 0.001				$ 0.001	$ 0.001
Preferred stock, shares authorized \| shares	20,000,000		20,000,000				0				20,000,000	0
Preferred stock, par value \| $ / shares	$ 0.001		$ 0.001				$ 0.001				$ 0.001	$ 0.001
Reverse stock split		1-for-1.25
Reverse stock split ratio		0.8
Common Stock
Organization and Business [Line Items]
Common stock shares sold \| shares											8,453,000
Proceeds from IPO, net of underwriting discounts and commissions \| $	$ 137,300
Proceeds from private placement \| $	$ 4,500
Initial Public Offering \| Common Stock
Organization and Business [Line Items]
Common stock shares sold \| shares	8,203,332
Common stock offering price per share \| $ / shares	$ 18.00
Convertible preferred stock converted into shares of common stock \| shares	30,971,627
Private Placement \| Common Stock
Organization and Business [Line Items]
Common stock shares sold \| shares	250,000
Common stock offering price per share \| $ / shares	$ 18.00

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents	$ 33,660	$ 85,207	$ 13,416
Restricted cash	802
Total	$ 34,462	$ 85,207	$ 13,416	$ 7,216

Summary of Significant Accounting Policies - Additional Information (Details)	1 Months Ended	12 Months Ended
	Jul. 31, 2018 shares	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Restricted cash		$ 802,000
Noncash interest expense related to outstanding debt facility			$ 101,000	$ 29,000
Number of operating segments \| Segment		1
Expected dividend yield		0.00%
Income tax rate		21.00%	34.00%
Change in federal tax rate, amount		$ 0
Tax Cut And Jobs Act
Summary Of Significant Accounting Policies [Line Items]
Income tax rate		21.00%
Maximum
Summary Of Significant Accounting Policies [Line Items]
Income tax rate			35.00%
Laboratory Equipment \| Minimum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, Useful life		3 years
Laboratory Equipment \| Maximum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, Useful life		5 years
Leasehold Improvements
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, Useful life		Shorter of remaining lease term or estimated life of the assets
Noncash Interest Expense
Summary Of Significant Accounting Policies [Line Items]
Noncash interest expense related to outstanding debt facility			$ 101,000
Initial Public Offering
Summary Of Significant Accounting Policies [Line Items]
Conversion of preferred stock upon initial public offering, Shares \| shares	30,971,627
Initial Public Offering \| Other Long-Term Assets
Summary Of Significant Accounting Policies [Line Items]
Deferred IPO costs		$ 0	$ 200,000

Summary of Significant Accounting Policies - Calculation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ (14,528)	$ (11,148)	$ (9,377)	$ (8,485)	$ (7,884)	$ (5,920)	$ (4,692)	$ (5,056)	$ (43,538)	$ (23,552)	$ (17,100)
Weighted-average number of common shares outstanding:
Weighted-average shares of common stock outstanding, basic and diluted									19,833	1,620	1,312
Net loss per share, basic and diluted	$ (0.35)	$ (0.34)	$ (4.17)	$ (4.19)	$ (4.08)	$ (3.60)	$ (3.18)	$ (3.56)	$ (2.20)	$ (14.54)	$ (13.03)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Summary of Significant Accounting Policies - Summary of Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss per Share (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted net loss per share due to anti-dilutive effect	7,887	36,007	23,841
Series A Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted net loss per share due to anti-dilutive effect		20,866	20,866
Series B Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted net loss per share due to anti-dilutive effect		10,105
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted net loss per share due to anti-dilutive effect	7,811	4,884	2,722
Warrants to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted net loss per share due to anti-dilutive effect		48	48
Unvested Restricted Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted net loss per share due to anti-dilutive effect	47	104	205
Shares Issuable under 2018 Employee Stock Purchase Plan
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted net loss per share due to anti-dilutive effect	29	0

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	$ 145,246	$ 0
Fair Value, Measurements, Recurring Basis
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	31,555	82,526
Marketable securities	145,246
Total cash equivalents and marketable securities	176,801
Fair Value, Measurements, Recurring Basis \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	31,555	82,526
Marketable securities	145,246
Total cash equivalents and marketable securities	176,801
Fair Value, Measurements, Recurring Basis \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Marketable securities	0
Total cash equivalents and marketable securities	0
Fair Value, Measurements, Recurring Basis \| Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Marketable securities	0
Total cash equivalents and marketable securities	0
Money Market Funds \| Fair Value, Measurements, Recurring Basis
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	31,555	82,526
Money Market Funds \| Fair Value, Measurements, Recurring Basis \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	31,555	82,526
Money Market Funds \| Fair Value, Measurements, Recurring Basis \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Money Market Funds \| Fair Value, Measurements, Recurring Basis \| Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	$ 0
U.S. Treasuries \| Fair Value, Measurements, Recurring Basis
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	145,246
U.S. Treasuries \| Fair Value, Measurements, Recurring Basis \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	145,246
U.S. Treasuries \| Fair Value, Measurements, Recurring Basis \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	0
U.S. Treasuries \| Fair Value, Measurements, Recurring Basis \| Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	$ 0

Fair Value Measurements - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Transfers of assets between level 1 to level 2	$ 0	$ 0
Transfers of assets between level 2 to level 1	0	0
Transfers of liabilities between level 1 to level 2	0	0
Transfers of liabilities between level 2 to level 1	0	0
Transfers of assets into level 3	0	0
Transfers of assets out of level 3	0	0
Transfers of liabilities into level 3	0	0
Transfers of liabilities out of level 3	$ 0	$ 0

Marketable Securities - Marketable Securities - Summary of Amortized Cost, Gross Unrealized Holding Gains or Losses, and Fair Value of Marketable Securities (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	$ 145,261
Unrealized Gains	0
Unrealized Losses	(15)
Fair Value	145,246
U.S. Treasuries
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	145,261
Unrealized Gains	0
Unrealized Losses	(15)
Fair Value	$ 145,246

Marketable Securities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Other-than-temporary impairments on marketable securities	$ 0
Marketable securities		$ 0

Balance Sheet Components and Supplemental Disclosures - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 9,483	$ 1,004
Less accumulated depreciation	(635)	(559)
Property and equipment, net	8,848	445
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	3,272	949
Furniture And Office Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,666	0
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 4,545	$ 55

Balance Sheet Components and Supplemental Disclosures - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Balance Sheet Related Disclosures [Abstract]
Depreciation and amortization	$ 242	$ 241	$ 148

Balance Sheet Components and Supplemental Disclosures - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued outside professional services	$ 1,964	$ 787
Accrued compensation and benefits	1,041	265
Lease incentive obligation, current	123
Other current liabilities	36	37
Total	$ 3,164	$ 1,089

Debt Facility - Additional Information (Details)	1 Months Ended				12 Months Ended
Debt Facility - Additional Information (Details)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) $ / shares shares	Jul. 31, 2016 USD ($)	Jun. 30, 2016 USD ($) Tranche	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2018 shares
Debt Instrument [Line Items]
Loss on extinguishment of debt					$ (159,000)
Common Stock
Debt Instrument [Line Items]
Fair value of warrants in reduction to the carrying value of loans and amortized to interest expense		$ 24,000				$ 24,000
Issuance of common stock \| shares							46,893
Common Stock \| Expected Volatility
Debt Instrument [Line Items]
Fair value of warrants input		0.80				0.80
Common Stock \| Expected Life
Debt Instrument [Line Items]
Fair value of warrants, expected contractual erm		10 years				10 years
Common Stock \| Risk-Free Interest Rate
Debt Instrument [Line Items]
Fair value of warrants input		0.02				0.02
Term Loan
Debt Instrument [Line Items]
Number of Tranches \| Tranche				2
Principal amount				$ 5,000,000
Maximum borrowing capacity				2,500,000
Term loan available for draw down				$ 2,500,000
Upfront debt issuance costs		$ 15,000				$ 15,000
Deferred issuance costs		$ 200,000				$ 200,000
Common stock warrants expiration period						10 years
Loss on extinguishment of debt	$ (200,000)
Prepayment penalty	50,000
Write-off of unamortized discount	17,000
Write-off of unamortized debt issuance costs	$ 92,000
Term Loan \| Common Stock
Debt Instrument [Line Items]
Warrants issued to purchase shares of common stock \| shares		47,616				47,616
Weighted average exercise price per share \| $ / shares		$ 0.61				$ 0.61
Term Loan \| The Wall Street Journal Prime Rate
Debt Instrument [Line Items]
Marginal interest rate				0.25%
Term Loan \| First Tranche
Debt Instrument [Line Items]
Term loan drawn			$ 2,500,000
Term Loan \| Second Tranche
Debt Instrument [Line Items]
Term loan drawn		$ 2,500,000

Commitments and Contingencies - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended			61 Months Ended	63 Months Ended
	Jan. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2013
Commitments And Contingencies [Line Items]
Restricted cash in long term assets, deposit for lease facility		$ 800,000			$ 800,000	$ 800,000
Net rent expense		1,000,000	$ 500,000	$ 400,000
Non-cancelable purchase obligations		$ 15,600,000			$ 15,600,000	$ 15,600,000
Common stock, shares issued		42,116,641	2,114,000		42,116,641	42,116,641
In-Licensing Agreements
Commitments And Contingencies [Line Items]
Milestone expense		$ 300,000	$ 0	$ 300,000
Accrued Royalties		0			$ 0	$ 0
Exclusive License Agreement with The Johns Hopkins University
Commitments And Contingencies [Line Items]
Upfront and milestone payments					300,000
Common stock, shares issued							88,887
Non-exclusive License Agreement with BioWa Inc. and Lonza Sales AG
Commitments And Contingencies [Line Items]
Milestone payments						400,000
Maximum \| Exclusive License Agreement with The Johns Hopkins University
Commitments And Contingencies [Line Items]
Aggregate additional milestone payments					4,000,000
Maximum \| Non-exclusive License Agreement with BioWa Inc. and Lonza Sales AG
Commitments And Contingencies [Line Items]
Aggregate additional milestone payments						41,000,000
Minimum \| Non-exclusive License Agreement with BioWa Inc. and Lonza Sales AG
Commitments And Contingencies [Line Items]
Annual commercial license fees						40,000
Redwood City Lease Agreement
Commitments And Contingencies [Line Items]
Lease agreement initiation period	2018-01
Base term of lease	10 years 9 months
Lease term extension period	5 years
Unamortized lease incentive obligations		$ 1,300,000			$ 1,300,000	$ 1,300,000
Redwood City Lease Agreement \| Maximum
Commitments And Contingencies [Line Items]
Tenant improvement allowance	$ 1,400,000

Commitments and Contingencies - Summary of Future Minimum Lease Payments Required Under Operating Leases (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 403
2020	1,233
2021	1,271
2022	1,308
2023	1,348
Thereafter	8,304
Total minimum future lease payments	$ 13,867

Convertible Promissory Notes Payable to Related Parties, Net - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	Nov. 30, 2017	Aug. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short Term Debt [Line Items]
Portion of proceeds allocated to BCF				$ 1,305,000
Noncash interest expense associated with amortization of discounts, issuance costs and BCF				101,000	$ 29,000
Deferred tax liabilities				700,000
Decrease in valuation allowance			$ (10,800,000)	(700,000)
Income tax expense (benefit)			0	(291,000)	$ 0
Convertible Promissory Notes
Short Term Debt [Line Items]
Interest rate per annum		6.00%
Principal amount				7,500,000
Transaction costs				86,000
Portion of proceeds allocated to BCF				2,800,000
Debt, stated interest amount				100,000
Noncash interest expense associated with amortization of discounts, issuance costs and BCF				900,000
Debt instrument, outstanding principal and accrued interest	$ 7,600,000
Unamortized debt discount	60,000
Deemed capital contribution received from related parties	$ 900,000
Deferred tax liabilities			1,000,000
Deferred tax liability write down				700,000
Decrease in valuation allowance			1,000,000
Income tax expense (benefit)			$ (1,000,000)	$ (300,000)
Convertible Promissory Notes \| Series B Convertible Preferred Stock
Short Term Debt [Line Items]
Debt instrument, converted to preferred stock	771,083
Maximum \| Convertible Promissory Notes
Short Term Debt [Line Items]
Value of convertible promissory notes to be issued		$ 15,000,000

Convertible Preferred Stock and Stockholders' Equity (Deficit) - Additional Information (Details) - USD ($)		1 Months Ended					12 Months Ended
	Apr. 30, 2014	Nov. 30, 2017	Jan. 31, 2016	Mar. 31, 2015	Aug. 31, 2014	Dec. 31, 2012	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jul. 23, 2018
Class Of Stock [Line Items]
Total number of shares authorized										220,000,000
Common stock, shares authorized							200,000,000	55,000,000		200,000,000
Preferred stock, shares authorized							20,000,000	0		20,000,000
Common stock, shares issued							42,116,641	2,114,000
Common stock, shares outstanding							42,116,641	2,114,000
Preferred stock, shares issued							0	0
Preferred stock, shares outstanding							0	0
Proceeds from issuance of convertible preferred stock, net of issuance costs								$ 92,331,000	$ 18,991,000
Proceeds from issuance of common stock							$ 138,357,000
Common stock, dividends declared							0	$ 0
Johns Hopkins University
Class Of Stock [Line Items]
Stock issued, price per share	$ 0.39
Common stock shares issued as a consideration for intellectual property rights	88,887
Value of common stock shares issued as a consideration for intellectual property rights	$ 34,000
Minimum
Class Of Stock [Line Items]
Proceeds from issuance of common stock							$ 50,000,000
Common Stock \| Minimum
Class Of Stock [Line Items]
Stock issued, price per share							$ 9.91
Initial Public Offering \| Common Stock
Class Of Stock [Line Items]
Convertible preferred stock converted into shares of common stock										30,971,627
Series A Convertible Preferred Stock
Class Of Stock [Line Items]
Initial conversion price							2.25
Series A Convertible Preferred Stock \| Initial Closing
Class Of Stock [Line Items]
Convertible preferred stock, shares issuable						14,222,218
Convertible preferred stock, price per share						$ 2.25
Convertible preferred stock, shares issued						4,444,441
Proceeds from issuance of convertible preferred stock, net of issuance costs						$ 9,800,000
Series A Convertible Preferred Stock \| Second Closings
Class Of Stock [Line Items]
Convertible preferred stock, shares issuable					4,444,446
Convertible preferred stock, price per share					$ 2.25
Convertible preferred stock, shares issued					6,666,662
Proceeds from issuance of convertible preferred stock, net of issuance costs					$ 14,900,000
Series A Convertible Preferred Stock \| Additional Second Closings
Class Of Stock [Line Items]
Convertible preferred stock, shares issuable				2,222,229
Convertible preferred stock, price per share				$ 2.25
Convertible preferred stock, shares issued				1,310,906
Proceeds from issuance of convertible preferred stock, net of issuance costs				$ 2,900,000
Series A Convertible Preferred Stock \| Third Closings
Class Of Stock [Line Items]
Convertible preferred stock, price per share			$ 2.25
Convertible preferred stock, shares issued			8,444,440
Proceeds from issuance of convertible preferred stock, net of issuance costs			$ 19,000,000
Series B Convertible Preferred Stock
Class Of Stock [Line Items]
Convertible preferred stock, shares issuable		10,105,181
Convertible preferred stock, price per share		$ 9.91
Convertible preferred stock, shares issued		9,334,098
Proceeds from issuance of convertible preferred stock, net of issuance costs		$ 92,300,000
Shares issued upon conversion of outstanding convertible promissory notes		771,083
Issuance of Series B convertible preferred stock upon conversion of convertible promissory notes		$ 7,600,000
Initial conversion price							$ 9.91
Outstanding shares required for protective provision							1,000,000
Percentage of outstanding shares required for conversion approval							60.00%
Convertible Preferred Stock
Class Of Stock [Line Items]
Dividend percetnage								8.00%
Dividends declared								$ 0	$ 0
Outstanding shares required for protective provision							1,000,000
Percentage of outstanding shares required for conversion approval							50.00%

Convertible Preferred Stock and Stockholders' Equity (Deficit) - Schedule of Convertible Preferred Stock (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Series A Convertible Preferred Stock
Class Of Stock [Line Items]
Convertible preferred stock, shares authorized	0	26,083,000
Convertible preferred stock, shares issued	0	20,866,000
Convertible preferred stock, shares outstanding	0	20,866,000
Convertible prefered stock, net carrying value	$ 0	$ 42,996
Convertible preferred stock, aggregate liquidation preference	$ 0	$ 46,950
Series B Convertible Preferred Stock
Class Of Stock [Line Items]
Convertible preferred stock, shares authorized	0	12,632,000
Convertible preferred stock, shares issued	0	10,105,000
Convertible preferred stock, shares outstanding	0	10,105,000
Convertible prefered stock, net carrying value	$ 0	$ 99,973
Convertible preferred stock, aggregate liquidation preference	$ 0	$ 100,141
Convertible Preferred Stock
Class Of Stock [Line Items]
Convertible preferred stock, shares authorized		38,715,000
Convertible preferred stock, shares issued		30,971,000
Convertible preferred stock, shares outstanding		30,971,000
Convertible prefered stock, net carrying value		$ 142,969
Convertible preferred stock, aggregate liquidation preference		$ 147,091

Convertible Preferred Stock and Stockholders' Equity (Deficit) - Summary of Common Stock Reserved for Future Issuance (Details) - shares shares in Thousands	Dec. 31, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Common shares reserved for future issuance	11,096	45,993
Series A Convertible Preferred Stock
Class Of Stock [Line Items]
Common shares reserved for future issuance	0	26,083
Series B Convertible Preferred Stock
Class Of Stock [Line Items]
Common shares reserved for future issuance	0	12,632
Options to Purchase Common Stock
Class Of Stock [Line Items]
Common shares reserved for future issuance	7,811	4,884
Stock Options Available for Future Grant
Class Of Stock [Line Items]
Common shares reserved for future issuance	2,785	2,346
Shares Issuable Under 2018 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Common shares reserved for future issuance	500	0
Conversion of Common Stock Warrants
Class Of Stock [Line Items]
Common shares reserved for future issuance	0	48

Stock-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jul. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Tax benefits for stock-based compensation expense recognized		$ 0	$ 0	$ 0
Common stock shares reserved for issuance		11,096,000	45,993,000
Aggregate fair value of stock options vested		$ 1,800,000	$ 200,000	200,000
Aggregate intrinsic value of stock options exercised		$ 2,000,000	$ 100,000	12,000
Weighted-average remaining contractual life of options outstanding		8 years 2 months 12 days	8 years 8 months 12 days
Weighted-average remaining contractual life for options exercisable		7 years
Weighted-average remaining contractual life for options vested and expected to vest		8 years 2 months 12 days
Total unrecognized stock-based compensation expense relating to unvested stock options		$ 38,200,000
Total stock-based compensation expense		$ 4,570,000	$ 402,000	$ 182,000
Performance-Based Vesting
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted		0	0
Market-Based Vesting
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted		0	0
Options to Purchase Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance		7,811,000	4,884,000
Weighted-average recognition period		3 years 7 months 6 days
2018 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance	500,000
Increase in number of common stock shares reserved for issuance per year	1,000,000
Percentage increase in shares reserved for issuance per year	1.00%
Weighted-average recognition period		10 months 24 days
Price as a percentage of fair market value of common stock	85.00%
Consecutive overlapping offering period	24 months
First offering period commence date		Jul. 18, 2018
First offering period end date		Aug. 15, 2020
Second offering period commence date		Feb. 15, 2019
Total stock-based compensation expense		$ 200,000
Total unrecognized compensation expense		$ 800,000
Restricted Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average recognition period		9 months 18 days
Total unrecognized compensation expense		$ 20,000
Fair value of restricted common stock vested		$ 24,000	$ 28,000
2012 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance					8,455,045
Award vesting period					4 years
Shares available for grant			2,345,748
2012 Equity Incentive Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option expiration period					10 years
2018 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance	4,000,000
Award vesting period	4 years
Shares available for grant		1,325,900
Increase in number of common stock shares reserved for issuance per year	5,000,000
Percentage increase in shares reserved for issuance per year	5.00%
Maximum number of additional shares authorized to be added to the Plan		6,485,177
2018 Equity Incentive Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option expiration period	10 years
Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average fair value of options granted		$ 11.05	$ 0.54	$ 0.34
Non-Employees \| Service-Based Vesting
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted		0	0

Stock-Based Compensation - Summary of Total Stock-Based Compensation Expense Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 4,570	$ 402	$ 182
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	1,792	175	108
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 2,778	$ 227	$ 74

Stock-Based Compensation - Summary of Weighted-Average Assumptions Used to Calculate Fair Value of Stock-based Awards Granted and ESPP Shares (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected dividend yield	0.00%
ESPP Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	2.42%
Expected volatility	65.92%
Expected term (in years)	1 year 2 months 26 days
Employees and Directors \| Equity Incentive Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	2.79%	1.83%	1.64%
Expected volatility	73.47%	77.59%	73.22%
Expected term (in years)	6 years 3 days	6 years 29 days	6 years 7 days

Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted Average Remaining Years	8 years 2 months 12 days	8 years 8 months 12 days
Weighted Average Remaining Years, Options exercisable	7 years
Weighted Average Remaining Years, Options vested and expected to vest	8 years 2 months 12 days
Aggregate Intrinsic Value, Exercised	$ 2,000,000	$ 100,000	$ 12,000
2018 and 2012 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options Outstanding, Beginning balance	4,884
Options Outstanding, Granted	3,789
Options Outstanding, Exercised	(531)
Options Outstanding, Forfeited	(331)
Options Outstanding, Ending balance	7,811	4,884
Options Outstanding, Options exercisable	3,684
Options Outstanding, Options vested and expected to vest	7,770
Weighted Average Exercise Price, Beginning balance	$ 0.67
Weighted Average Exercise Price, Granted	17.19
Weighted Average Exercise Price, Exercised	0.65
Weighted Average Exercise Price, Forfeited	2.62
Weighted Average Exercise Price, Ending balance	8.60	$ 0.67
Weighted Average Exercise Price, Options exercisable	1.02
Weighted Average Exercise Price, Options vested and expected to vest	$ 8.57
Weighted Average Remaining Years	8 years 2 months 12 days	8 years 8 months 12 days
Weighted Average Remaining Years, Options exercisable	7 years
Weighted Average Remaining Years, Options vested and expected to vest	8 years 2 months 12 days
Aggregate Intrinsic Value, Outstanding	$ 342,292,000	$ 16,331,000
Aggregate Intrinsic Value, Options exercisable	188,812,000
Aggregate Intrinsic Value, Options vested and expected to vest	$ 340,773,000

Stock-Based Compensation - Summary of Restricted Common Stock Activity (Details) - Restricted Common Stock shares in Thousands	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Number of Shares
Balance at December 31, 2017 \| shares	104
Vested \| shares	(57)
Balance at December 31, 2018 \| shares	47
Weighted-Average Grant Date Fair Value
Balance at December 31, 2017 \| $ / shares	$ 0.43
Vested \| $ / shares	0.43
Balance at December 31, 2018 \| $ / shares	$ 0.43

Income Taxes - Summary of Total Deferred Income Tax Assets, Net of Valuation Allowance (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets
Net operating loss carryforwards	$ 24,091	$ 15,769
Research and development credits	3,995	2,242
Accruals and reserves	871	50
Fixed and intangible assets	0	51
Total deferred tax assets	28,957	18,112
Deferred tax liabilities
Fixed and intangible assets	65	0
Total deferred tax liabilities	65	0
Net deferred tax assets before valuation allowance	28,892	18,112
Less: Valuation allowance	(28,892)	(18,112)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Line Items]
Deferred tax assets, valuation allowance	$ 28,892,000	$ 18,112,000
Decrease in valuation allowance	10,800,000	700,000
Federal NOL carryforwards	101,200,000
State NOL carryforwards	39,500,000
Research and development credits	3,995,000	2,242,000
Accrual for interest and penalties	0	0
Deferred tax liabilities		700,000
Income tax benefit	$ 0	$ 291,000	$ 0
Federal statutory tax rate	21.00%	34.00%
Tax Cut And Jobs Act
Income Tax Disclosure [Line Items]
Federal statutory tax rate		21.00%
Maximum
Income Tax Disclosure [Line Items]
Federal statutory tax rate		35.00%
Convertible Promissory Notes
Income Tax Disclosure [Line Items]
Decrease in valuation allowance	$ (1,000,000)
Deferred tax liabilities	1,000,000
Income tax benefit	$ 1,000,000	$ 300,000
Federal
Income Tax Disclosure [Line Items]
NOL carryforwards, expiration year	2032
Research and development credits	$ 3,800,000
Research and development tax credit carryforwards, expiration year	2033
State
Income Tax Disclosure [Line Items]
NOL carryforwards, expiration year	2032
Research and development credits	$ 2,500,000
Research and development tax credit carryforwards, expiration year	2033

Income Taxes - Summary of Effective Tax Rate Differs From Federal Statutory Tax Rate (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	34.00%
Change in deferred tax asset valuation allowance	(24.80%)	(3.00%)
State taxes, net of federal benefit	0.90%	0.60%
Research and development tax credits	3.30%	1.40%
Remeasurement of deferreds		(31.20%)
Beneficial conversion feature		1.20%
Other	(0.40%)	(1.80%)
Effective tax rate		1.20%

Income Taxes - Reconciliation of Beginning and Ending Amount of Unrecognized Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Balance at the beginning of the year	$ 1,149	$ 758
Increase related to current year tax positions	678	359
Increase related to prior year tax positions	0	32
Balance at the end of the year	$ 1,827	$ 1,149

Defined Contribution Plans - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
401(K) Plan
Defined Contribution Plan Disclosure [Line Items]
Contributions made by employer	$ 300,000
Employer contribution matching percentage	4.00%
SIMPLE IRA Plan
Defined Contribution Plan Disclosure [Line Items]
Contributions made by employer		$ 100,000	$ 59,000

Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Results (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Loss from operations	$ (15,513)	$ (11,975)	$ (9,524)	$ (8,709)	$ (6,446)	$ (6,233)	$ (4,598)	$ (4,977)	$ (45,721)	$ (22,254)	$ (17,060)
Net loss	$ (14,528)	$ (11,148)	$ (9,377)	$ (8,485)	$ (7,884)	$ (5,920)	$ (4,692)	$ (5,056)	$ (43,538)	$ (23,552)	$ (17,100)
Net loss per share, basic and diluted	$ (0.35)	$ (0.34)	$ (4.17)	$ (4.19)	$ (4.08)	$ (3.60)	$ (3.18)	$ (3.56)	$ (2.20)	$ (14.54)	$ (13.03)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details