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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2018	Nov. 01, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q3
Trading Symbol	ALLK
Entity Registrant Name	Allakos Inc.
Entity Central Index Key	0001564824
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		42,113,059

BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 51,635	$ 85,207
Investments in marketable securities	141,903	0
Prepaid expenses and other current assets	3,647	1,037
Total current assets	197,185	86,244
Property and equipment, net	7,624	445
Other long-term assets	952	340
Total assets	205,761	87,029
Current liabilities:
Accounts payable	4,333	1,703
Accrued expenses and other current liabilities	3,742	1,089
Total current liabilities	8,075	2,792
Other long-term liabilities	1,731	36
Total liabilities	9,806	2,828
Commitments and contingencies (Note 6)
Stockholders' equity (deficit):
Preferred stock, $0.001 par value per share; 20,000 and no shares authorized as of September 30, 2018 and December 31, 2017, respectively; no shares issued and outstanding as of September 30, 2018 and December 2017	0	0
Common stock, $0.001 par value per share; 200,000 and 55,000 shares authorized as of September 30, 2018 and December 31, 2017, respectively; 42,113 and 2,114 shares issued and outstanding as of September 30, 2018 and December 31, 2017, respectively	42	3
Additional paid-in capital	285,530	1,803
Accumulated other comprehensive loss	(33)	0
Accumulated deficit	(89,584)	(60,574)
Total stockholders’ equity (deficit)	195,955	(58,768)
Total liabilities, convertible preferred stock and stockholders’ equity (deficit)	205,761	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

BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 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	42,113,059	2,114,000
Common stock, shares outstanding	42,113,059	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

STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating expenses
Research and development	$ 8,706	$ 5,333	$ 22,256	$ 13,455
General and administrative	3,269	900	7,952	2,353
Total operating expenses	11,975	6,233	30,208	15,808
Loss from operations	(11,975)	(6,233)	(30,208)	(15,808)
Interest income (expense), net	836	(579)	1,352	(716)
Other expense, net	(9)	(74)	(154)	(110)
Loss before benefit from income taxes	(11,148)	(6,886)	(29,010)	(16,634)
Provision for (benefit from) income taxes	0	(966)	0	(966)
Net loss	(11,148)	(5,920)	(29,010)	(15,668)
Unrealized loss on marketable securities, net of tax	(36)	0	(33)	0
Comprehensive loss	$ (11,184)	$ (5,920)	$ (29,043)	$ (15,668)
Net loss per common share:
Basic and diluted	$ (0.34)	$ (3.60)	$ (2.34)	$ (10.35)
Weighted-average number of common shares outstanding:
Basic and diluted	32,609	1,643	12,406	1,514

STATEMENTS OF CASH FLOWS (unaudited) - USD ($)	9 Months Ended
STATEMENTS OF CASH FLOWS (unaudited) - USD ($)	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities
Net loss	$ (29,010,000)	$ (15,668,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	183,000	175,000
Accretion of tenant improvement allowance	(52,000)	0
Stock-based compensation	2,031,000	266,000
Net amortization of premiums and discounts on marketable securities	(634,000)	0
Noncash interest related to debt facility	0	76,000
Noncash interest related to convertible promissory notes payable to related parties	0	80,000
Amortization of beneficial conversion feature related to convertible promissory notes payable to related parties	0	424,000
Benefit from deferred income taxes	0	(966,000)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,706,000)	(273,000)
Other long-term assets	440,000	(250,000)
Accounts payable	915,000	27,000
Accrued expenses and other current liabilities	2,637,000	1,059,000
Other long-term liabilities	395,000	0
Net cash used in operating activities	(25,801,000)	(15,050,000)
Cash flows from investing activities
Purchases of marketable securities	(159,456,000)	0
Purchases of property and equipment	(4,261,000)	(251,000)
Proceeds from maturities of marketable securities	18,000,000	0
Net cash used in investing activities	(145,717,000)	(251,000)
Cash flows from financing activities
Proceeds from issuance of common stock, net of issuance costs	138,357,000	0
Proceeds from issuance of convertible promissory notes, net of issuance costs	0	7,414,000
Proceeds from exercise of stock options, net of repurchases	341,000	109,000
Proceeds from repayment of recourse promissory note	50,000	0
Net cash provided by financing activities	138,748,000	7,523,000
Net decrease in cash, cash equivalents and restricted cash	(32,770,000)	(7,778,000)
Cash, cash equivalents and restricted cash, beginning of period	85,207,000	13,416,000
Cash, cash equivalents and restricted cash, end of period	52,437,000	5,638,000
Supplemental disclosures
Cash paid for interest	0	153,000
Noncash investing and financing items:
Recognition of beneficial conversion feature related to convertible promissory notes to related parties, net of benefit for income taxes	0	1,867,000
Property and equipment purchased in accounts payable	1,715,000	8,000
Lessor funded lease incentives included in property and equipment	1,386,000	0
Vesting of restricted common stock subject to repurchase	$ 18,000	$ 22,000

Organization and Business	9 Months Ended
Organization and Business	Sep. 30, 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 San Carlos, California. Liquidity Matters Since inception, the Company has incurred net losses and negative cash flows from operations. During the nine months ended September 30, 2018, the Company incurred a net loss of $29.0 million and used $25.8 million of cash in operations. At September 30, 2018, the Company had an accumulated deficit of $89.6 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 $193.5 million of cash, cash equivalents and marketable securities at September 30, 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 financial statements and accompanying notes have been retroactively restated to reflect the reverse stock split.

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The unaudited interim 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.

The interim balance sheet as of September 30, 2018, the statements of operations and comprehensive loss, and statements of cash flows for the nine months ended September 30, 2017 and 2018 are unaudited. The unaudited interim financial statements have been prepared on the same basis as the audited annual financial statements and reflect, in the opinion of management, all adjustments of a normal and recurring nature that are necessary for the fair presentation of the Company’s financial position as of September 30, 2018 and its results of operations and comprehensive loss for the three and nine months ended September 30, 2018 and 2017 and its cash flows for the nine months ended September 30, 2018 and 2017. Certain information and note disclosures normally included in annual audited financial statements prepared in accordance with U.S. GAAP have been omitted. The financial data and the other financial information disclosed in these notes to the interim financial statements are also unaudited. The results of operations for any interim period are not necessarily indicative of the results to be expected for the entire year or for any other future annual or interim period. The balance sheet as of December 31, 2017 included herein was derived from the audited financial statements as of that date. These interim financial statements should be read in conjunction with the Company’s audited financial statements included in the Company’s Prospectus that forms a part of the Company’s Registration Statement on Form S-1, which was filed with the Securities and Exchange Commission (the “SEC”) pursuant to Rule 424(b) on July 19, 2018.

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 a single financial institution that management believes possesses high credit quality. Amounts on deposit with this financial institution 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. Restricted cash as of September 30, 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. 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 accompanying statements of cash flows (in thousands):

	September 30,		December 31,
	2018		2017
Cash and cash equivalents	$	51,635	$	85,207
Restricted cash in long term assets, deposit for lease facility		802		—
Total cash, cash equivalents and restricted cash	$	52,437	$	85,207

The Company had no restricted cash at September 30, 2017 and December 31, 2016.

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 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 and other long-term assets 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. There were no deferred IPO costs at September 30, 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 three months and nine months ended September 30, 2017, the Company recognized noncash interest expense of $25,000 and $76,000, respectively, 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 is included in interest income (expense), net, within the Company’s statements of operations and comprehensive loss.

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.

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

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, lowers 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 as of 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 is provisional and is subject to revision as the Company completes its analysis of the Tax Act, collects and prepares necessary data and interprets any additional guidance issued by standard-setting bodies. As of September 30, 2018, the Company has not yet completed its accounting for all the tax effects of the Tax Act. Detailed analyses of the Company’s deferred tax assets and liabilities will be required through the measurement period ending December 22, 2018 and may result in additional adjustments. Future adjustments, if any, are not expected to impact the Company’s results of operations due to its loss position and full valuation allowance.

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 and consists primarily of unrealized gains and losses on the Company’s investments in marketable securities.

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):

	Three Months Ended						Nine Months Ended
	September 30,						September 30,
	2018			2017			2018			2017
Numerator:
Net loss	$	(11,148	)	$	(5,920	)	$	(29,010	)	$	(15,668	)
Denominator:
Weighted-average shares of common stock outstanding, basic and diluted		32,609			1,643			12,406			1,514
Net loss per share, basic and diluted	$	(0.34	)	$	(3.60	)	$	(2.34	)	$	(10.35	)

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):

	Three and Nine Months Ended
	September 30,
	2018		2017
Series A convertible preferred stock		—		20,866
Options to purchase common stock		6,779		4,579
Warrants to purchase common stock		—		48
Unvested restricted common stock		62		119
Shares issuable under 2018 Employee Stock Purchase Plan		12		—
Total		6,853		25,612

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 (“ASU 2016-02”). ASU 2016-02 requires an entity to recognize assets and liabilities arising from a lease for both financing and operating leases. ASU 2016-02 will also require new qualitative and quantitative disclosures to help investors and other financial statement users better understand the amount, timing and uncertainty of cash flows arising from leases. For public entities, ASU 2016-02 is effective for fiscal years beginning after December 15, 2018. Early adoption is permitted. In July 2018, the FASB issued ASU 2018-11, related to another transition method in lease accounting. If elected, the transition method allows entities to initially apply the new leases standard at the adoption date and recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. The Company is evaluating the impact of the adoption of this standard on its financial statements but expects that it will have a material impact to its balance sheets from the recognition of right-of-use assets and corresponding lease liabilities related to its operating leases.

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 adopted ASU 2016-18 retrospectively as of 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

9 Months Ended

Sep. 30, 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):

	September 30, 2018
	Level 1		Level 2		Level 3		Total
Cash equivalents
Money market funds	$	11,059	$	—	$	—	$	11,059
Reverse repurchase agreements (1)		—		38,500		—		38,500
Total cash equivalents		11,059		38,500		—		49,559
Marketable securities
U.S. treasuries		141,903		—		—		141,903
Total cash equivalents and marketable securities	$	152,962	$	38,500	$	—	$	191,462

(1)

The Company, as part of its cash management strategy, may invest in reverse repurchase agreements. Such reverse repurchase agreements are tri-party repurchase agreements and have maturities of three months or less at the time of investment and are collateralized by U.S. treasury and agency securities at 102% of the principal amount. In a tri-party repurchase agreement, a third-party custodian bank functions as an independent intermediary to facilitate transfer of cash and holding the collateral on behalf of the underlying investor for the term of the agreement thereby minimizing risk and exposure to both parties. The reverse repurchase agreements are included within cash equivalents due to their high liquidity and relatively low risk.

	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 three and nine months ended September 30, 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

9 Months Ended

Sep. 30, 2018

Investments Debt And Equity Securities [Abstract]

Marketable Securities

4. Marketable Securities

All marketable securities were considered available-for-sale at September 30, 2018. The amortized cost, gross unrealized holding gains or losses, and fair value of the Company’s marketable securities by major security type at September 30, 2018 are summarized in the table below (in thousands):

	September 30, 2018
	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
Short-term marketable securities
U.S. treasuries	$	141,936	$	—	$	(33	)	$	141,903
Total short-term marketable securities	$	141,936	$	—	$	(33	)	$	141,903

The Company had no other-than-temporary impairments on its marketable securities during the three and nine months ended September 30, 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

9 Months Ended

Sep. 30, 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):

	September 30,			December 31,
	2018			2017
Laboratory equipment	$	2,878		$	949
Furniture and office equipment		1,539			—
Leasehold improvements		55			55
Construction in progress		3,893			—
		8,365			1,004
Less accumulated depreciation		(741	)		(559	)
Property and equipment, net	$	7,624		$	445

Depreciation and amortization expense for the three months ended September 30, 2018 and 2017 was $63,000 and $62,000, respectively. Depreciation and amortization expense for the nine months ended September 30, 2018 and 2017 was $183,000 and $175,000, respectively.

Construction in progress represents direct costs related to the construction of leasehold improvements and assembly of new laboratory equipment associated with the Company’s new Redwood City facility. Costs of assets under construction are capitalized but are not depreciated until the construction is substantially complete and the assets being constructed are ready for their intended use. Depreciation is recorded using the straight-line method over the lesser of the lease term or the estimated useful lives of the leasehold improvements.

Accrued Expenses and Other Current Liabilities

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

	September 30,		December 31,
	2018		2017
Accrued outside professional services	$	1,461	$	787
Accrued compensation		1,921		265
Other current liabilities		360		37
Total	$	3,742	$	1,089

Commitments and Contingencies

9 Months Ended

Sep. 30, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

6. Commitments and Contingencies

Operating Lease Obligations

The Company’s operating lease obligations primarily relate to its leased office and laboratory space under two separate noncancelable operating leases that begin to expire in June 2019. The Company’s San Carlos lease agreement, which was amended in August 2015, includes two renewal provisions allowing the Company to extend the lease for an additional period of one year each. The amended lease agreement includes a rent abatement and escalation clauses for increased rent over the lease term.

The Company’s Redwood City lease agreement was entered into January 2018, with a contractual lease term commencing upon substantial completion and delivery of the premises. The base term of the lease is 10.75 years with 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 entered into in January 2018 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 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. As of September 30, 2018, lease incentive obligations totaled $1.3 million.

In addition to the minimum future lease commitments presented below, both leases require 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,
2018 (remaining 3 months)	$	105
2019		613
2020		1,233
2021		1,270
2022		1,308
Thereafter		9,652
Total minimum future lease payments	$	14,181

In November 2015, the Company entered into a sublease agreement with a third-party for a portion of the Company’s facilities in San Carlos, California. The sublease has a month-to-month term and can be terminated by either party with a thirty-day written notice. Sublease payments owed are recorded as an offset to the Company’s rent expense.

Net rent expense was $0.4 million and $0.1 million for the three months ended September 30, 2018 and 2017, respectively. Net rent expense was $0.8 million and $0.4 million for the nine months ended September 30, 2018 and 2017, 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. At September 30, 2018, the Company had $6.3 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 of milestone expense for the nine months ended September 30, 2018. The Company did not recognize any milestone expense during the three months ended September 30, 2018 and the three and nine months ended September 30, 2017. Milestone payments are not creditable against royalties. As of September 30, 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 September 30, 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 September 30, 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 September 30, 2018.

Convertible Promissory Notes Payable to Related Parties, Net	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Convertible Promissory Notes Payable to Related Parties, Net	7. 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 nine months ended September 30, 2017, the Company recognized noncash interest expense of $0.5 million associated with the amortization of the discounts, issuance costs and BCF. 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.

Stockholders’ Equity	9 Months Ended
Stockholders’ Equity	Sep. 30, 2018
Stockholders Equity Note [Abstract]
Stockholders’ Equity	8. Stockholders’ Equity As of September 30, 2018, the Company is authorized to issue 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 42,113,059 shares of common stock issued and outstanding at September 30, 2018. 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 September 30, 2018, no dividends on common stock had been declared by the Board of Directors. Common Stock Warrant Exercises During the three months ended September 30, 2018, warrants for the purchase of 47,616 shares of common stock were exercised by an accredited investor by net exercise, resulting in the Company’s issuance of 46,893 shares of common stock.

Stock-Based Compensation

9 Months Ended

Sep. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

9. Stock-Based Compensation

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

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2018		2017		2018		2017
Research and development	$	446	$	41	$	781	$	110
General and administrative		533		70		1,250		156
Total	$	979	$	111	$	2,031	$	266

No income tax benefits for stock-based compensation expense have been recognized for the three and nine months ended September 30, 2018 and 2017 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 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 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.

Following the IPO and upon the effectiveness of the 2018 Plan, the 2012 Equity Incentive Plan, as amended, (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 September 30, 2018, the maximum number of shares that may be added to the 2018 Plan pursuant to the preceding clause is 6,592,658 shares.

Prior to its termination, the 2012 Plan provided for the grant 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.

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:

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2018			2017	2018			2017
Risk-free interest rate		2.77	%	*		2.68	%		1.78	%
Expected volatility		69.68	%	*		75.68	%		78.00	%
Expected dividend yield		—		*		—			—
Expected term (in years)		5.94		*		5.98			6.04

*	There were no grants of stock options during the three months ended September 30, 2017.

The Company’s stock option activity is summarized as follows (number of shares in thousands):

				Weighted-
				Average
	Options			Exercise
	Outstanding			Price
Balance at December 31, 2017		4,884		$	0.67
Granted		2,649		$	7.50
Exercised		(527	)	$	0.65
Forfeited		(227	)	$	0.64
Balance at September 30, 2018		6,779		$	3.34
Options exercisable		3,401		$	0.84
Options vested and expected to vest		6,741		$	3.33

During the three and nine months ended September 30, 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 September 30, 2018, total unrecognized stock-based compensation expense relating to unvested stock options was $12.5 million. This amount is expected to be recognized over a weighted-average period of 3.4 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 three and nine months ended September 30, 2018, stock-based compensation related to the 2018 ESPP was $85,000.

Defined Contribution Plans	9 Months Ended
Defined Contribution Plans	Sep. 30, 2018
Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]
Defined Contribution Plans	10. 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 three and nine months ended September 30, 2017, the Company made contributions to the SIMPLE IRA plan of $24,000 and $75,000, 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 three and nine months ended September 30, 2018, the Company made contributions to the 401(k) plan of $58,000 and $176,000, respectively.

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 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

	September 30,		December 31,
	2018		2017
Cash and cash equivalents	$	51,635	$	85,207
Restricted cash in long term assets, deposit for lease facility		802		—
Total cash, cash equivalents and restricted cash	$	52,437	$	85,207

The Company had no restricted cash at September 30, 2017 and December 31, 2016.

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

Research and Development Expense

Accrued Research and Development Costs

Convertible Debt Features

Convertible Preferred 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):

	Three Months Ended						Nine Months Ended
	September 30,						September 30,
	2018			2017			2018			2017
Numerator:
Net loss	$	(11,148	)	$	(5,920	)	$	(29,010	)	$	(15,668	)
Denominator:
Weighted-average shares of common stock outstanding, basic and diluted		32,609			1,643			12,406			1,514
Net loss per share, basic and diluted	$	(0.34	)	$	(3.60	)	$	(2.34	)	$	(10.35	)

	Three and Nine Months Ended
	September 30,
	2018		2017
Series A convertible preferred stock		—		20,866
Options to purchase common stock		6,779		4,579
Warrants to purchase common stock		—		48
Unvested restricted common stock		62		119
Shares issuable under 2018 Employee Stock Purchase Plan		12		—
Total		6,853		25,612

Recent Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

9 Months Ended

Sep. 30, 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 accompanying statements of cash flows (in thousands):

	September 30,		December 31,
	2018		2017
Cash and cash equivalents	$	51,635	$	85,207
Restricted cash in long term assets, deposit for lease facility		802		—
Total cash, cash equivalents and restricted cash	$	52,437	$	85,207

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):

	Three Months Ended						Nine Months Ended
	September 30,						September 30,
	2018			2017			2018			2017
Numerator:
Net loss	$	(11,148	)	$	(5,920	)	$	(29,010	)	$	(15,668	)
Denominator:
Weighted-average shares of common stock outstanding, basic and diluted		32,609			1,643			12,406			1,514
Net loss per share, basic and diluted	$	(0.34	)	$	(3.60	)	$	(2.34	)	$	(10.35	)

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

	Three and Nine Months Ended
	September 30,
	2018		2017
Series A convertible preferred stock		—		20,866
Options to purchase common stock		6,779		4,579
Warrants to purchase common stock		—		48
Unvested restricted common stock		62		119
Shares issuable under 2018 Employee Stock Purchase Plan		12		—
Total		6,853		25,612

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2018

Fair Value Disclosures [Abstract]

Summary of Financial Assets Measured at Fair Value on Recurring Basis

	September 30, 2018
	Level 1		Level 2		Level 3		Total
Cash equivalents
Money market funds	$	11,059	$	—	$	—	$	11,059
Reverse repurchase agreements (1)		—		38,500		—		38,500
Total cash equivalents		11,059		38,500		—		49,559
Marketable securities
U.S. treasuries		141,903		—		—		141,903
Total cash equivalents and marketable securities	$	152,962	$	38,500	$	—	$	191,462

(1)

	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)

9 Months Ended

Sep. 30, 2018

Investments Debt And Equity Securities [Abstract]

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

	September 30, 2018
	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
Short-term marketable securities
U.S. treasuries	$	141,936	$	—	$	(33	)	$	141,903
Total short-term marketable securities	$	141,936	$	—	$	(33	)	$	141,903

Balance Sheet Components and Supplemental Disclosures (Tables)

9 Months Ended

Sep. 30, 2018

Balance Sheet Related Disclosures [Abstract]

Schedule of Property and Equipment, Net

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

	September 30,			December 31,
	2018			2017
Laboratory equipment	$	2,878		$	949
Furniture and office equipment		1,539			—
Leasehold improvements		55			55
Construction in progress		3,893			—
		8,365			1,004
Less accumulated depreciation		(741	)		(559	)
Property and equipment, net	$	7,624		$	445

Schedule of Accrued Expenses and Other Current Liabilities

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

	September 30,		December 31,
	2018		2017
Accrued outside professional services	$	1,461	$	787
Accrued compensation		1,921		265
Other current liabilities		360		37
Total	$	3,742	$	1,089

Commitments and Contingencies (Tables)

9 Months Ended

Sep. 30, 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,
2018 (remaining 3 months)	$	105
2019		613
2020		1,233
2021		1,270
2022		1,308
Thereafter		9,652
Total minimum future lease payments	$	14,181

Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 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):

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2018		2017		2018		2017
Research and development	$	446	$	41	$	781	$	110
General and administrative		533		70		1,250		156
Total	$	979	$	111	$	2,031	$	266

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:

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2018			2017	2018			2017
Risk-free interest rate		2.77	%	*		2.68	%		1.78	%
Expected volatility		69.68	%	*		75.68	%		78.00	%
Expected dividend yield		—		*		—			—
Expected term (in years)		5.94		*		5.98			6.04

*	There were no grants of stock options during the three months ended September 30, 2017.

Summary of Stock Option Activity

The Company’s stock option activity is summarized as follows (number of shares in thousands):

				Weighted-
				Average
	Options			Exercise
	Outstanding			Price
Balance at December 31, 2017		4,884		$	0.67
Granted		2,649		$	7.50
Exercised		(527	)	$	0.65
Forfeited		(227	)	$	0.64
Balance at September 30, 2018		6,779		$	3.34
Options exercisable		3,401		$	0.84
Options vested and expected to vest		6,741		$	3.33

Organization and Business - Additional Information (Details) $ / shares in Units, $ in Thousands			3 Months Ended		9 Months Ended
	Jul. 23, 2018 USD ($) $ / shares shares	Jul. 06, 2018	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) $ / shares shares
Organization and Business [Line Items]
Net loss \| $			$ 11,148	$ 5,920	$ 29,010	$ 15,668
Cash used in operations \| $					25,801	$ 15,050
Accumulated deficit \| $			89,584		89,584		$ 60,574
Cash, cash equivalents and marketable securities \| $			$ 193,500		$ 193,500
Common stock, shares authorized \| shares	200,000,000		200,000,000		200,000,000		55,000,000
Common stock, par value \| $ / shares	$ 0.001		$ 0.001		$ 0.001		$ 0.001
Preferred stock, shares authorized \| shares	20,000,000		20,000,000		20,000,000		0
Preferred stock, par value \| $ / shares	$ 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			46,893
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 - Additional Information (Details) - USD ($)	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jul. 23, 2018	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Restricted cash in long term assets, deposit for lease facility		$ 802,000		$ 0
Restricted cash	$ 0		$ 0			$ 0
Noncash interest related to debt facility	$ 25,000	0	$ 76,000
Income tax rate				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%
Initial Public Offering
Summary Of Significant Accounting Policies [Line Items]
Outstanding convertible preferred stock converted to common stock					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 - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents	$ 51,635	$ 85,207
Restricted cash in long term assets, deposit for lease facility	802	0
Total cash, cash equivalents and restricted cash	$ 52,437	$ 85,207	$ 5,638	$ 13,416

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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net loss	$ (11,148)	$ (5,920)	$ (29,010)	$ (15,668)
Weighted-average number of common shares outstanding:
Weighted-average shares of common stock outstanding, basic and diluted	32,609	1,643	12,406	1,514
Net loss per share, basic and diluted	$ (0.34)	$ (3.60)	$ (2.34)	$ (10.35)

- 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	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
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	6,853	25,612	6,853	25,612
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	0	20,866	0	20,866
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	6,779	4,579	6,779	4,579
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	0	48	0	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	62	119	62	119
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	12	0	12	0

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	$ 141,903	$ 0
Fair Value, Measurements, Recurring Basis
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	49,559	82,526
Total cash equivalents and marketable securities	191,462
Fair Value, Measurements, Recurring Basis \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	11,059	82,526
Total cash equivalents and marketable securities	152,962
Fair Value, Measurements, Recurring Basis \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	38,500	0
Total cash equivalents and marketable securities	38,500
Fair Value, Measurements, Recurring Basis \| Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Total cash equivalents and marketable securities	0
Fair Value, Measurements, Recurring Basis \| Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	11,059	82,526
Fair Value, Measurements, Recurring Basis \| Money Market Funds \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	11,059	82,526
Fair Value, Measurements, Recurring Basis \| Money Market Funds \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Fair Value, Measurements, Recurring Basis \| Money Market Funds \| Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	$ 0
Fair Value, Measurements, Recurring Basis \| Reverse Repurchase Agreements
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	38,500
Fair Value, Measurements, Recurring Basis \| Reverse Repurchase Agreements \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0
Fair Value, Measurements, Recurring Basis \| Reverse Repurchase Agreements \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	38,500
Fair Value, Measurements, Recurring Basis \| Reverse Repurchase Agreements \| Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0
Fair Value, Measurements, Recurring Basis \| U.S. Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	141,903
Fair Value, Measurements, Recurring Basis \| U.S. Treasuries \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	141,903
Fair Value, Measurements, Recurring Basis \| U.S. Treasuries \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	0
Fair Value, Measurements, Recurring Basis \| U.S. Treasuries \| Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	$ 0

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details)(Parenthetical)	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Percentage at which reverse repurchase agreements are collateralized	102.00%

Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Transfers of assets between level 1 to level 2	$ 0	$ 0	$ 0	$ 0
Transfers of assets between level 2 to level 1	0	0	0	0
Transfers of liabilities between level 1 to level 2	0	0	0	0
Transfers of liabilities between level 2 to level 1	0	0	0	0
Transfers of assets into level 3	0	0	0	0
Transfers of assets out of level 3	0	0	0	0
Transfers of liabilities into level 3	0	0	0	0
Transfers of liabilities out of level 3	$ 0	$ 0	$ 0	$ 0

Marketable Securities - Summary of Amortized Cost, Gross Unrealized Holding Gains or Losses, and Fair Value of Marketable Securities (Details) $ in Thousands	Sep. 30, 2018 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	$ 141,936
Unrealized Gains	0
Unrealized Losses	(33)
Fair Value	141,903
U.S. Treasuries
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	141,936
Unrealized Gains	0
Unrealized Losses	(33)
Fair Value	$ 141,903

Marketable Securities - Additional Information (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Other-than-temporary impairments on marketable securities	$ 0	$ 0
Marketable securities			$ 0

Balance Sheet Components and Supplemental Disclosures - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 8,365	$ 1,004
Less accumulated depreciation	(741)	(559)
Property and equipment, net	7,624	445
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,878	949
Furniture And Office Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,539	0
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	55	55
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 3,893	$ 0

Balance Sheet Components and Supplemental Disclosures - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Balance Sheet Related Disclosures [Abstract]
Depreciation and amortization	$ 63,000	$ 62,000	$ 183,000	$ 175,000

Balance Sheet Components and Supplemental Disclosures - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued outside professional services	$ 1,461	$ 787
Accrued compensation	1,921	265
Other current liabilities	360	37
Total	$ 3,742	$ 1,089

Commitments and Contingencies - Additional Information (Details)	1 Months Ended		3 Months Ended		9 Months Ended		58 Months Ended	60 Months Ended
	Jan. 31, 2018 USD ($)	Aug. 31, 2015 Provision	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Lease shares	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) shares	Sep. 30, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2013 shares
Commitments And Contingencies [Line Items]
Number of noncancelable operating leases \| Lease					2
Noncancelable operating leases expiration beginning period					2019-06
Number of lease renewal provisions \| Provision		2
Lease term additional extension period		1 year
Restricted cash in long term assets, deposit for lease facility			$ 802,000		$ 802,000		$ 802,000	$ 802,000	$ 0
Net rent expense			400,000	$ 100,000	800,000	$ 400,000
Non-cancelable purchase obligations			$ 6,300,000		$ 6,300,000		$ 6,300,000	$ 6,300,000
Common stock, shares issued \| shares			42,113,059		42,113,059		42,113,059	42,113,059	2,114,000
In-Licensing Agreements
Commitments And Contingencies [Line Items]
Milestone expense			$ 0	$ 0	$ 300,000	$ 0
Accrued Royalties			0		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 \| shares										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 term additional extension period	5 years
Lease agreement initiation period	2018-01
Base term of lease	10 years 9 months
Lease incentive obligations			$ 1,300,000		$ 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	Sep. 30, 2018 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2018 (remaining 3 months)	$ 105
2019	613
2020	1,233
2021	1,270
2022	1,308
Thereafter	9,652
Total minimum future lease payments	$ 14,181

Convertible Promissory Notes Payable to Related Parties, Net - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Aug. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Short Term Debt [Line Items]
Income tax expense (benefit)		$ 0	$ 966	$ 0	$ 966
Convertible Promissory Notes
Short Term Debt [Line Items]
Interest rate per annum	6.00%
Noncash interest expense					$ 500
Deferred tax liabilities		$ 1,000		1,000
Decrease in valuation allowance				1,000
Income tax expense (benefit)				$ 1,000
Maximum \| Convertible Promissory Notes
Short Term Debt [Line Items]
Value of convertible promissory notes to be issued	$ 15,000

Stockholders Equity - Additional Information (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Jul. 23, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Total number of shares authorized	220,000,000	220,000,000
Common stock, shares authorized	200,000,000	200,000,000	200,000,000	55,000,000
Preferred stock, shares authorized	20,000,000	20,000,000	20,000,000	0
Common stock, shares issued	42,113,059	42,113,059		2,114,000
Common stock, shares outstanding	42,113,059	42,113,059		2,114,000
Common stock, dividends declared		$ 0
Warrants for purchase of common stock shares exercised by accredited investor	47,616
Common Stock
Class Of Stock [Line Items]
Shares issued	46,893

Stock-Based Compensation - Summary of Total Stock-Based Compensation Expense Recognized (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 979	$ 111	$ 2,031	$ 266
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	446	41	781	110
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 533	$ 70	$ 1,250	$ 156

Stock-Based Compensation - Additional Information (Details) - USD ($)		1 Months Ended	3 Months Ended		9 Months Ended
	Jul. 23, 2018	Jul. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Tax benefits for stock-based compensation expense recognized			$ 0	$ 0	$ 0	$ 0
Stock options granted					2,649,000
Total unrecognized stock-based compensation expense relating to unvested stock options			12,500,000		$ 12,500,000
Total stock-based compensation expense			$ 979,000	$ 111,000	$ 2,031,000	$ 266,000
Performance-Based Vesting
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted			0	0	0	0
Market-Based Vesting
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted			0	0	0	0
Service-Based Vesting \| Non-Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted			0	0	0	0
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average recognition period					3 years 4 months 24 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%
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			$ 85,000		$ 85,000
2018 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance		4,000,000
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	6,592,658
2012 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period	4 years
2012 Equity Incentive Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option expiration period	10 years

Stock-Based Compensation - Summary of Weighted-Average Assumptions Used to Calculate Fair Value of Stock-based Awards Granted (Details) - Employees and Directors	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	2.77%	2.68%	1.78%
Expected volatility	69.68%	75.68%	78.00%
Expected dividend yield	0.00%	0.00%	0.00%
Expected term (in years)	5 years 11 months 8 days	5 years 11 months 23 days	6 years 14 days

Stock-Based Compensation - Summary of Stock Option Activity (Details) - $ / shares shares in Thousands	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Options Outstanding, Beginning balance	4,884
Options Outstanding, Granted	2,649
Options Outstanding, Exercised	(527)
Options Outstanding, forfeited	(227)
Options Outstanding, Ending balance	6,779
Options Outstanding, Options exercisable	3,401
Options Outstanding, Options vested and expected to vest	6,741
Weighted Average Exercise Price, Beginning balance	$ 0.67
Weighted Average Exercise Price, Granted	7.50
Weighted Average Exercise Price, Exercised	0.65
Weighted Average Exercise Price, Forfeited	0.64
Weighted Average Exercise Price, Ending balance	3.34
Weighted Average Exercise Price, Options exercisable	0.84
Weighted Average Exercise Price, Options vested and expected to vest	$ 3.33

Defined Contribution Plans - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
401(K) Plan
Defined Contribution Plan Disclosure [Line Items]
Contributions made by employer	$ 58,000		$ 176,000
Employer contribution matching percentage			4.00%
SIMPLE IRA Plan
Defined Contribution Plan Disclosure [Line Items]
Contributions made by employer		$ 24,000		$ 75,000