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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2020	May 06, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Trading Symbol	ALLK
Entity Registrant Name	Allakos Inc.
Entity Central Index Key	0001564824
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		48,766,351
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity File Number	001-38582
Entity Tax Identification Number	45-4798831
Entity Address, Address Line One	975 Island Drive
Entity Address, Address Line Two	Suite 201
Entity Address, City or Town	Redwood City
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94065
City Area Code	650
Local Phone Number	597-5002
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, par value $0.001
Security Exchange Name	NASDAQ
Document Quarterly Report	true
Document Transition Report	false
Entity Incorporation, State or Country Code	DE

BALANCE SHEETS (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 135,942	$ 38,367
Investments in marketable securities	343,863	457,534
Prepaid expenses and other current assets	3,953	3,969
Total current assets	483,758	499,870
Property and equipment, net	8,072	8,410
Operating lease right-of-use assets	5,705	5,775
Other long-term assets	2,839	2,839
Total assets	500,374	516,894
Current liabilities:
Accounts payable	9,062	5,963
Accrued expenses and other current liabilities	5,273	7,098
Total current liabilities	14,335	13,061
Other long-term liabilities	7,995	8,112
Total liabilities	22,330	21,173
Commitments and contingencies (Note 6)
Stockholders’ equity:
Preferred stock, $0.001 par value per share; 20,000 shares authorized as of March 31, 2020 and December 31, 2019; no shares issued and outstanding as of March 31, 2020 and December 31, 2019	0	0
Common stock, $0.001 par value per share; 200,000 shares authorized as of March 31, 2020 and December 31, 2019; 48,765 and 48,668 shares issued and outstanding as of March 31, 2020 and December 31, 2019, respectively	48	48
Additional paid-in capital	693,298	685,020
Accumulated other comprehensive gain	2,006	137
Accumulated deficit	(217,308)	(189,484)
Total stockholders’ equity	478,044	495,721
Total liabilities and stockholders’ equity	$ 500,374	$ 516,894

STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating expenses
Research and development	$ 18,285	$ 15,098
General and administrative	11,588	5,829
Total operating expenses	29,873	20,927
Loss from operations	(29,873)	(20,927)
Interest income, net	1,989	1,030
Other income (expense), net	60	(56)
Net loss	(27,824)	(19,953)
Unrealized gain on marketable securities, net of tax	1,869	45
Comprehensive loss	$ (25,955)	$ (19,908)
Net loss per common share:
Basic and diluted	$ (0.57)	$ (0.47)
Weighted-average number of common shares outstanding:
Basic and diluted	48,691	42,620

STATEMENTS OF STOCKHOLDERS' EQUITY (unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Balance at Dec. 31, 2018	$ 183,994	$ 42	$ 288,079	$ (15)	$ (104,112)
Balance, Shares at Dec. 31, 2018		42,117
Stock-based compensation expense	2,834	$ 0	2,834	0	0
Issuance of common stock upon exercise of stock options	368	$ 1	367	0	0
Issuance of common stock upon exercise of stock options, Shares		968
Issuance of common stock upon 2018 ESPP purchase	595	$ 0	595	0	0
Issuance of common stock upon 2018 ESPP purchase, Shares		39
Vesting of restricted common stock	6	$ 0	6	0	0
Unrealized gain on marketable securities	45	0	0	45	0
Net loss	(19,953)	0	0	0	(19,953)
Balance at Mar. 31, 2019	167,889	$ 43	291,881	30	(124,065)
Balance, Shares at Mar. 31, 2019		43,124
Balance at Dec. 31, 2019	495,721	$ 48	685,020	137	(189,484)
Balance, Shares at Dec. 31, 2019		48,668
Stock-based compensation expense	7,489	$ 0	7,489	0	0
Issuance of common stock upon exercise of stock options	$ 33	$ 0	33	0	0
Issuance of common stock upon exercise of stock options, Shares	57	57
Issuance of common stock upon 2018 ESPP purchase	$ 756	$ 0	756	0	0
Issuance of common stock upon 2018 ESPP purchase, Shares		40
Unrealized gain on marketable securities	1,869	$ 0	0	1,869	0
Net loss	(27,824)	0	0	0	(27,824)
Balance at Mar. 31, 2020	$ 478,044	$ 48	$ 693,298	$ 2,006	$ (217,308)
Balance, Shares at Mar. 31, 2020		48,765

STATEMENTS OF CASH FLOWS (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities
Net loss	$ (27,824)	$ (19,953)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	393	357
Stock-based compensation	7,489	2,834
Net amortization of premiums and discounts on marketable securities	221	(802)
Noncash lease expense	70	73
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(113)	1,320
Accounts payable	3,092	1,278
Accrued expenses and other current liabilities	(1,942)	1,408
Other long-term liabilities	0	206
Net cash used in operating activities	(18,614)	(13,279)
Cash flows from investing activities
Purchases of marketable securities	(15,052)	(70,759)
Proceeds from maturities of marketable securities	130,500	81,000
Purchases of property and equipment	(48)	(121)
Net cash provided by investing activities	115,400	10,120
Cash flows from financing activities
Proceeds from exercise of stock options	33	368
Proceeds from issuance of common stock under the 2018 ESPP	756	595
Net cash provided by financing activities	789	963
Net increase (decrease) in cash, cash equivalents and restricted cash	97,575	(2,196)
Cash, cash equivalents and restricted cash, beginning of period	40,642	34,462
Cash, cash equivalents and restricted cash, end of period	138,217	32,266
Noncash investing and financing items:
Right-of-use assets obtained in exchange for lease obligations	0	6,050
Property and equipment purchased in accounts payable	7	419
Vesting of restricted common stock subject to repurchase	$ 0	$ 6

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2020

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 March 31, 2020, the statements of operations and comprehensive loss, statements stockholders’ equity and statements of cash flows for the three months ended March 31, 2020 and 2019 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 March 31, 2020 and its results of operations and comprehensive loss for the three months ended March 31, 2020 and 2019 and its cash flows for the three months ended March 31, 2020 and 2019. 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, 2019 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 Annual Report on Form 10-K, which was filed with the Securities and Exchange Commission (the “SEC”) on February 25, 2020.

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, calculation of right-of-use assets and lease liabilities, 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 or backed 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 March 31, 2020 represents $2.3 million in deposits for the lease of the Company’s facilities in Redwood City, California and San Carlos, California. Both security deposits are in the form of letters of credit secured by restricted cash. Restricted cash amounts are included within other long-term assets on the Company’s balance sheets.

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):

	March 31,		December 31,
	2020		2019
Cash and cash equivalents	$	135,942	$	38,367
Restricted cash in other long-term assets, deposit for lease facility		2,275		2,275
Total cash, cash equivalents and restricted cash	$	138,217	$	40,642

	March 31,		December 31,
	2019		2018
Cash and cash equivalents	$	31,464	$	33,660
Restricted cash in other long-term assets, deposit for lease facility		802		802
Total cash, cash equivalents and restricted cash	$	32,266	$	34,462

Marketable Securities

The Company invests in marketable securities, primarily securities issued by the U.S. government and its agencies. The Company’s marketable securities are considered available-for-sale and are classified as current assets even when the stated maturities of the underlying securities exceed one year from the date of the current balance sheet being reported. This classification reflects management’s ability and intent to utilize proceeds from the sale of such investments to fund ongoing operations. 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, net, on the statements of operations and comprehensive loss. Realized gains and losses and declines in the fair value of an available-for-sale security due to a credit loss are included in other expense, net, on the statements of operations and comprehensive loss to the extent the Company does not expect to recover the amortized cost basis. Any portion of a decline in fair value that is due to factors other than a credit loss is excluded from earnings and reported as a component of accumulated comprehensive income (loss).

Leases

Effective January 1, 2019, the Company accounts for its leases in accordance with Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) 842, “Leases” (“ASC 842”). Prior period amounts continue to be reported in accordance with the Company’s historic accounting under previous lease guidance. Additionally, the Company elected a number of optional practical expedients made available under the ASC 842 transition guidance. Such elections include (i) carrying forward the Company’s 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 accounts for its leases by recording right-of-use assets and lease liabilities on the Balance Sheet. Right-of-use assets represent the Company’s right to use an underlying asset over the lease term and include any lease payments made prior to the lease commencement date and exclude lease incentives. Lease liabilities represent the present value of the total lease payments over the lease term, calculated using the Company’s incremental borrowing rate. In determining the Company’s incremental borrowing rate, consideration is given to the term of the lease and the Company’s credit risk. The Company recognizes options to extend or terminate a lease when it is reasonably certain that the Company will exercise any such options.

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.

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
	March 31,
	2020			2019
Numerator:
Net loss	$	(27,824	)	$	(19,953	)
Denominator:
Weighted-average shares of common stock outstanding, basic and diluted		48,691			42,620
Net loss per share, basic and diluted	$	(0.57	)	$	(0.47	)

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 Months Ended
	March 31,
	2020		2019
Options to purchase common stock		7,105		6,845
Unvested restricted stock units		572		—
Unvested restricted common stock		—		33
Shares issuable under employee stock purchase plans		8		10
Total		7,685		6,888

Recently Adopted Accounting Pronouncements

In June 2016 the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The FASB subsequently issued amendments to ASU 2016-13, which have the same effective date and transition date of January 1, 2020. This guidance requires companies to recognize an allowance for credit losses on available-for-sale debt securities rather than the current approach of recording a reduction to the carrying value of the asset. The Company adopted ASU 2016-13 effective January 1, 2020.

The guidance limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which carrying value exceeds fair value and requires the reversal of previously recognized credit losses if fair value increases. Based on the composition of the Company’s investment portfolio and other financial assets, current economic conditions and historical credit loss activity, the adoption of the guidance did not have a material impact on the Company’s financial statements and related disclosures. During the three months ended March 31, 2020, the Company’s current investment portfolio was comprised of only US Treasury securities which are not in unrealized loss positions. Upon adoption, the Company did not record an allowance for credit losses on its available-for-sale debt securities.

In August 2018, the FASB issued 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 adopted this ASU effective January 1, 2020. The Company’s adoption of this ASU did not have a material impact on its financial statements and related disclosures.

In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. This ASU affects general principles within Topic 740 and are meant to simplify the accounting for income taxes by removing certain exceptions to the general framework. The ASU further adds guidance to reduce complexity in certain areas, including recognizing a franchise (or similar) tax that is partially based on income as an income-based tax and incremental amounts incurred as a non-income-based tax and recognizing deferred taxes for tax goodwill. ASU 2019-12 also created an exception to the incremental approach for intraperiod tax allocation when there is a loss from continuing operations and income or a gain from other items (for example, other comprehensive income). Under the historical guidance, in this situation, an entity would have recorded an income tax provision for unrealized gains on available-for-sale securities reported in other comprehensive income, with an offsetting income tax benefit recorded in continuing operations. Per ASU 2019-12, under the new guidance, an entity would record no income tax provision in the interim period.

The amendments in ASU 2019-12 are effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2020. Early adoption of the amendments is permitted, including adoption in any interim period for which financial statements have not yet been issued. An entity that elects to early adopt the amendments in an interim period should reflect any adjustments as of the beginning of the annual period that includes that interim period. Additionally, an entity that elects early adoption must adopt all the amendments in the same period. The Company elected to early adopt ASU 2019-12 effective January 1, 2020 on a prospective basis. As a result of this election, no benefit from income tax was recorded in continuing operations and no tax provision was recorded in other comprehensive income for the three months ended March 31, 2020 related to the Company’s loss from continuing operations and unrealized gain on available-for-sale securities during the same period. Further, the Company’s adoption had no impact to its effective tax rate.

Impact of Recent Legislation

to have a material impact on its income tax positions; however, it will continue to monitor the provisions of the CARES Act in relation to its operations.

Fair Value Measurements

3 Months Ended

Mar. 31, 2020

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. Financial instruments that are highly liquid with original maturities of three months or less from the date of purchase are included within cash equivalents. Financial instruments that are not highly liquid, or for which their original maturities are greater than three months, are classified as investments in marketable securities. The Company’s financial assets measured at fair value on a recurring basis were as follows (in thousands):

	March 31, 2020
	Level 1		Level 2		Level 3		Total
Cash equivalents
Money market funds	$	134,392	$	—	$	—	$	134,392
Total cash equivalents		134,392		—		—		134,392
Short-term marketable securities
U.S. treasuries		343,863		—		—		343,863
Total short-term marketable securities		343,863		—		—		343,863
Total cash equivalents and short-term marketable securities	$	478,255	$	—	$	—	$	478,255

	December 31, 2019
	Level 1		Level 2		Level 3		Total
Cash equivalents
Money market funds	$	35,935	$	—	$	—	$	35,935
Total cash equivalents		35,935		—		—		35,935
Marketable securities
U.S. treasuries		457,534		—		—		457,534
Total marketable securities		457,534		—		—		457,534
Total cash equivalents and marketable securities	$	493,469	$	—	$	—	$	493,469

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 months ended March 31, 2020 and 2019.

- 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

3 Months Ended

Mar. 31, 2020

Investments Debt And Equity Securities [Abstract]

Marketable Securities

4. Marketable Securities

All marketable securities were considered available-for-sale at March 31, 2020. The amortized cost, gross unrealized holding gains or losses, and fair value of the Company’s marketable securities by major security type at March 31, 2020 and December 31, 2019 are summarized in the table below (in thousands):

	March 31, 2020
	Amortized Cost Basis		Unrealized Gains		Unrealized Losses		Fair Value
Available-for-sale securities
U.S. treasuries classified as investments	$	341,857	$	2,006	$	—	$	343,863
Total available-for-sale securities	$	341,857	$	2,006	$	—	$	343,863

	December 31, 2019
	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
Available-for-sale securities
U.S. treasuries classified as investments	$	457,397	$	161	$	(24	)	$	457,534
Total available-for-sale securities	$	457,397	$	161	$	(24	)	$	457,534

The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. As of March 31, 2020, the Company had no available-for-sale securities in an unrealized loss position. The aggregate fair value of securities held by the Company at December 31, 2019 in an unrealized loss position was $187.4 million, for which all of these securities had remaining maturities of less than one year as of the balance sheet date. The Company has the intent and ability to hold such securities until recovery and has determined that there has been no material change to their credit risk. As a result, the Company determined it did not hold any investments with a credit loss as of March 31, 2020 and December 31, 2019.

There were no material realized gains or losses recognized on the sale or maturity of available-for-sale securities during the years ended December 31, 2019 or 2018, and as a result, there were no material reclassifications out of accumulated other comprehensive gain (loss) for the same periods.

Balance Sheet Components and Supplemental Disclosures

3 Months Ended

Mar. 31, 2020

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):

	March 31,			December 31,
	2020			2019
Laboratory equipment	$	4,189		$	4,170
Furniture and office equipment		1,731			1,695
Leasehold improvements		4,581			4,581
		10,501			10,446
Less accumulated depreciation		(2,429	)		(2,036	)
Property and equipment, net	$	8,072		$	8,410

Depreciation and amortization expense for each of the three months ended March 31, 2020 and 2019 was $0.4 million.

Accrued Expenses and Other Current Liabilities

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

	March 31,		December 31,
	2020		2019
Accrued contract research and development expense	$	1,982	$	4,990
Accrued compensation and benefits expense		2,619		1,608
Lease liability, current		430		410
Other current liabilities		242		90
Total	$	5,273	$	7,098

Commitments and Contingencies

3 Months Ended

Mar. 31, 2020

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

6. Commitments and Contingencies

Lease Obligations

The Company’s lease obligations primarily relate to leased office and laboratory space under noncancelable operating leases.

In January 2018, the Company entered into a lease agreement for approximately 25,000 square feet of office and laboratory space in Redwood City, California (the “2018 Redwood City Lease”). The 2018 Redwood City Lease includes a contractual lease term commencing upon substantial completion and delivery of the premises, which occurred in November 2018. The Company subsequently terminated its previous lease agreement for office and laboratory space in San Carlos, California (the “2015 San Carlos Lease”).

The base term of the 2018 Redwood City Lease is 10.75 years and contains a one-time option to extend the lease term for five years. This option to extend the lease term has not been included in the Company’s calculations under ASC 842 as the exercise of the option is highly uncertain and therefore deemed not probable.

The 2018 Redwood City Lease also included a $1.4 million tenant improvement allowance that has been applied to the total cost of tenant improvements made to the leased premises. Tenant improvement allowances received under the 2018 Redwood City Lease

were recorded as leasehold improvements with an offsetting adjustment included in the Company’s calculation of its right-of-use asset under ASC 842. Leasehold improvements are depreciated over the term of the lease.

In December 2019, the Company entered into an additional lease agreement for approximately 98,000 square feet of office and laboratory space in San Carlos, California (the “2019 San Carlos Lease”). The 2019 San Carlos Lease provides for certain limited rent abatement and annual scheduled rent increases over the contractual lease term. The contractual lease term is expected to commence in November 2020 and terminate in August 2031. Future minimum rental payments under the 2019 San Carlos Lease total $77.6 million, which does not include rental payments related to the Company’s one-time option to extend for an additional five years. There was no right-of-use asset or lease liability reflected on the balance sheet as of March 31, 2020 for the 2019 San Carlos Lease as the Company has not yet obtained possession of the space.

The 2018 Redwood City Lease and the 2019 San Carlos Lease required security deposits of $0.8 million and $1.5 million, respectively, which the Company satisfied by establishing letters of credit secured by restricted cash. Restricted cash related to the Company’s lease agreements are recorded in other long-term assets on the Company’s balance sheets.

As described in Note 2, the Company adopted ASC 842 effective January 1, 2019. In accordance with ASC 842, the Company has performed an evaluation of its other contracts with vendors in accordance with ASC 842 and has determined that, except for the leases described above, none of its other contracts contain a lease.

Classification of the Company’s lease liabilities included on the its balance sheets at March 31, 2020 was as follows (in thousands):

	March 31,		December 31,
	2020		2019
Operating lease liabilities
Current portion included in accrued expenses and other current liabilities	$	430	$	410
Non-current portion included in other long-term liabilities		7,995		8,112
Total operating lease liabilities	$	8,425	$	8,522

The components of lease costs, which were included in operating expenses in the Company’s statements of operations and comprehensive loss were as follows (in thousands):

	Three Months Ended March 31,
	2020		2019
Operating lease cost	$	279	$	280
Variable cost		87		87
Total lease costs	$	366	$	367

Future minimum lease payments required under operating leases include on the Company’s balance sheets at March 31, 2020 are as follows (in thousands):

Fiscal Year Ending December 31,
2020 (remaining 9 months)	$	927
2021		1,270
2022		1,308
2023		1,348
2024		1,388
Thereafter		6,916
Total minimum future lease payments ⁽¹⁾		13,157
Less:
Present value adjustment		4,732
Operating lease liabilities	$	8,425

⁽¹⁾	^{Excludes minimum future lease payments of $77.6 million related to the Company’s 2019 San Carlos Lease.}

In addition to the minimum future lease commitments presented above, the lease requires the Company to pay property taxes, insurance, maintenance and repair costs.

Net rent expense was $0.3 million for the each of three months ended March 31, 2020 and 2019.

Operating lease liabilities are based on the net present value of the remaining lease payments over the remaining lease term. In determining the present value of lease payments, the Company used its incremental borrowing rate based on the information available at the lease commencement date. As of March 31, 2020, the remaining lease term is 9.3 years and the discount rate used to determine the operating lease liability was 10.0%.

As of March 31, 2020, the Company is not party to any lease agreements containing material residual value guarantees or material restrictive covenants.

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 March 31, 2020, the Company had $22.6 million of non-cancelable purchase obligations under these agreements.

In April 2020, the Company entered into a Commercial Supply Agreement with a CDMO for the future manufacture and supply of antolimab (AK002) as part of the Company's commercialization efforts. These amounts have not been reflected in the amount of purchase obligations at March 31, 2020 above. See Note 9 "Subsequent Events" for additional information.

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 did not incur any milestone expense for the three months ended March 31, 2020 and 2019. As of March 31, 2020, 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 antolimab (AK002), which was amended in September 2016. Under the terms of the agreement, the Company has made upfront and milestone payments of $0.3 million through March 31, 2020 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 antolimab (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 March 31, 2020 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 March 31, 2020.

Legal Contingencies

On March 10, 2020, a putative securities class action complaint captioned Kim v. Allakos et al., No. 20-cv-01720 (N.D. Cal.) was filed in the United States District Court for the Northern District of California against the Company, its Chief Executive Officer, Dr. Robert Alexander, and its Chief Financial Officer, Mr. Leo Redmond. The complaint asserts claims for violations of Sections 10(b) and 20(a) of the Securities Exchange Act of 1934 and Rule 10b-5 promulgated thereunder and seeks damages based on alleged material misrepresentations and omissions concerning its Phase 2 clinical trials of antolimab (AK002). The proposed class period is August 5, 2019, through December 17, 2019, inclusive. Given the early stage of this litigation matter, the Company cannot reasonably estimate a potential future loss or a range of potential future losses and has not recorded a contingent liability accrual as of March 31, 2020.

Stock-Based Compensation

3 Months Ended

Mar. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

7. Stock-Based Compensation

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

	Three Months Ended
	March 31,
	2020		2019
Research and development	$	2,443	$	1,053
General and administrative		5,046		1,781
Total	$	7,489	$	2,834

No income tax benefits for stock-based compensation expense have been recognized for the three months ended March 31, 2020 and 2019 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 (“RSUs”), 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 Company’s 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 March 31, 2020, the maximum number of shares that may be added to the 2018 Plan pursuant to the preceding clause is 5,212,568 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.

Stock Options

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

	Three Months Ended
	March 31,
	2020			2019
Risk-free interest rate		1.10	%		2.55	%
Expected volatility		68.17	%		69.02	%
Expected dividend yield		—			—
Expected term (in years)		6.08			6.08

The Company’s stock option activity during the three months ended March 31, 2020 is summarized as follows (number of shares in thousands):

				Weighted-
				Average
	Options			Exercise
	Outstanding			Price
Balance at December 31, 2019		7,148		$	12.96
Granted		24		$	58.21
Exercised		(57	)	$	0.58
Forfeited		(10	)	$	13.70
Balance at March 31, 2020		7,105		$	13.21
Options exercisable		4,233		$	5.79
Options vested and expected to vest		7,082		$	13.17

During the three months ended March 31, 2020 and 2019, the Company did not grant any stock options with performance-based or market-based vesting conditions.

As of March 31, 2020, total unrecognized stock-based compensation expense relating to unvested stock options was $38.3 million. This amount is expected to be recognized over a weighted-average period of 2.8 years.

Restricted Stock Units

RSU activity under the 2018 Plan during the three months ended March 31, 2020 is summarized as follows (in thousands, except per share data):

				Weighted-
				Average
				Grant Date
	Shares			Fair Value
Balance at December 31, 2019		542		$	93.67
Granted		31		$	67.57
Forfeited		(1	)	$	95.98
Balance at March 31, 2020		572		$	92.25

The weighted-average fair value of RSUs granted during the three months ended March 31, 2020 was $67.57. No RSUs were granted during the three months ended March 31, 2019.

As of March 31, 2020, total unrecognized stock-based compensation expense relating to unvested RSUs was $47.6 million and the weighted-average remaining vesting period was 3.7 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 four 6-month purchase periods. The Company’s first offering period under the 2018 ESPP commenced on July 18, 2018. During each of the three months ended March 31, 2020 and 2019, stock-based compensation expense related to the 2018 ESPP was $0.2 million.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2020

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Concentration of Credit Risk and Other Risks and Uncertainties

Cash, Cash Equivalents and Restricted Cash

	March 31,		December 31,
	2020		2019
Cash and cash equivalents	$	135,942	$	38,367
Restricted cash in other long-term assets, deposit for lease facility		2,275		2,275
Total cash, cash equivalents and restricted cash	$	138,217	$	40,642

	March 31,		December 31,
	2019		2018
Cash and cash equivalents	$	31,464	$	33,660
Restricted cash in other long-term assets, deposit for lease facility		802		802
Total cash, cash equivalents and restricted cash	$	32,266	$	34,462

Marketable Securities

Leases

Research and Development Expense

Accrued Research and Development Costs

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
	March 31,
	2020			2019
Numerator:
Net loss	$	(27,824	)	$	(19,953	)
Denominator:
Weighted-average shares of common stock outstanding, basic and diluted		48,691			42,620
Net loss per share, basic and diluted	$	(0.57	)	$	(0.47	)

	Three Months Ended
	March 31,
	2020		2019
Options to purchase common stock		7,105		6,845
Unvested restricted stock units		572		—
Unvested restricted common stock		—		33
Shares issuable under employee stock purchase plans		8		10
Total		7,685		6,888

Recent Accounting Pronouncements

Recently Adopted Accounting Pronouncements

Impact of Recent Legislation

In March 2020, the Coronavirus Aid, Relief and Economic Security Act (“the CARES Act”) was signed into law. The CARES Act includes provisions relating to several aspects of corporate income taxes. The Company does not currently expect the CARES Act to have a material impact on its income tax positions; however, it will continue to monitor the provisions of the CARES Act in relation to its operations.

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2020

Accounting Policies [Abstract]

Reconciliation of Cash, Cash Equivalents and Restricted Cash

	March 31,		December 31,
	2020		2019
Cash and cash equivalents	$	135,942	$	38,367
Restricted cash in other long-term assets, deposit for lease facility		2,275		2,275
Total cash, cash equivalents and restricted cash	$	138,217	$	40,642

	March 31,		December 31,
	2019		2018
Cash and cash equivalents	$	31,464	$	33,660
Restricted cash in other long-term assets, deposit for lease facility		802		802
Total cash, cash equivalents and restricted cash	$	32,266	$	34,462

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
	March 31,
	2020			2019
Numerator:
Net loss	$	(27,824	)	$	(19,953	)
Denominator:
Weighted-average shares of common stock outstanding, basic and diluted		48,691			42,620
Net loss per share, basic and diluted	$	(0.57	)	$	(0.47	)

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

	Three Months Ended
	March 31,
	2020		2019
Options to purchase common stock		7,105		6,845
Unvested restricted stock units		572		—
Unvested restricted common stock		—		33
Shares issuable under employee stock purchase plans		8		10
Total		7,685		6,888

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2020

Fair Value Disclosures [Abstract]

Summary of Financial Assets Measured at Fair Value on Recurring Basis

The Company’s financial assets measured at fair value on a recurring basis were as follows (in thousands):

	March 31, 2020
	Level 1		Level 2		Level 3		Total
Cash equivalents
Money market funds	$	134,392	$	—	$	—	$	134,392
Total cash equivalents		134,392		—		—		134,392
Short-term marketable securities
U.S. treasuries		343,863		—		—		343,863
Total short-term marketable securities		343,863		—		—		343,863
Total cash equivalents and short-term marketable securities	$	478,255	$	—	$	—	$	478,255

	December 31, 2019
	Level 1		Level 2		Level 3		Total
Cash equivalents
Money market funds	$	35,935	$	—	$	—	$	35,935
Total cash equivalents		35,935		—		—		35,935
Marketable securities
U.S. treasuries		457,534		—		—		457,534
Total marketable securities		457,534		—		—		457,534
Total cash equivalents and marketable securities	$	493,469	$	—	$	—	$	493,469

Marketable Securities (Tables)

3 Months Ended

Mar. 31, 2020

Investments Debt And Equity Securities [Abstract]

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

	March 31, 2020
	Amortized Cost Basis		Unrealized Gains		Unrealized Losses		Fair Value
Available-for-sale securities
U.S. treasuries classified as investments	$	341,857	$	2,006	$	—	$	343,863
Total available-for-sale securities	$	341,857	$	2,006	$	—	$	343,863

	December 31, 2019
	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
Available-for-sale securities
U.S. treasuries classified as investments	$	457,397	$	161	$	(24	)	$	457,534
Total available-for-sale securities	$	457,397	$	161	$	(24	)	$	457,534

Balance Sheet Components and Supplemental Disclosures (Tables)

3 Months Ended

Mar. 31, 2020

Balance Sheet Related Disclosures [Abstract]

Schedule of Property and Equipment, Net

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

	March 31,			December 31,
	2020			2019
Laboratory equipment	$	4,189		$	4,170
Furniture and office equipment		1,731			1,695
Leasehold improvements		4,581			4,581
		10,501			10,446
Less accumulated depreciation		(2,429	)		(2,036	)
Property and equipment, net	$	8,072		$	8,410

Schedule of Accrued Expenses and Other Current Liabilities

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

	March 31,		December 31,
	2020		2019
Accrued contract research and development expense	$	1,982	$	4,990
Accrued compensation and benefits expense		2,619		1,608
Lease liability, current		430		410
Other current liabilities		242		90
Total	$	5,273	$	7,098

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2020

Commitments And Contingencies Disclosure [Abstract]

Summary of Classification of Company's Lease Liabilities

Classification of the Company’s lease liabilities included on the its balance sheets at March 31, 2020 was as follows (in thousands):

	March 31,		December 31,
	2020		2019
Operating lease liabilities
Current portion included in accrued expenses and other current liabilities	$	430	$	410
Non-current portion included in other long-term liabilities		7,995		8,112
Total operating lease liabilities	$	8,425	$	8,522

Summary of Components of Lease Costs

The components of lease costs, which were included in operating expenses in the Company’s statements of operations and comprehensive loss were as follows (in thousands):

	Three Months Ended March 31,
	2020		2019
Operating lease cost	$	279	$	280
Variable cost		87		87
Total lease costs	$	366	$	367

Summary of Future Minimum Lease Payments Required Under Operating Leases

Future minimum lease payments required under operating leases include on the Company’s balance sheets at March 31, 2020 are as follows (in thousands):

Fiscal Year Ending December 31,
2020 (remaining 9 months)	$	927
2021		1,270
2022		1,308
2023		1,348
2024		1,388
Thereafter		6,916
Total minimum future lease payments ⁽¹⁾		13,157
Less:
Present value adjustment		4,732
Operating lease liabilities	$	8,425

⁽¹⁾	^{Excludes minimum future lease payments of $77.6 million related to the Company’s 2019 San Carlos Lease.}

Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2020

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
	March 31,
	2020		2019
Research and development	$	2,443	$	1,053
General and administrative		5,046		1,781
Total	$	7,489	$	2,834

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

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

	Three Months Ended
	March 31,
	2020			2019
Risk-free interest rate		1.10	%		2.55	%
Expected volatility		68.17	%		69.02	%
Expected dividend yield		—			—
Expected term (in years)		6.08			6.08

Summary of Stock Option Activity

The Company’s stock option activity during the three months ended March 31, 2020 is summarized as follows (number of shares in thousands):

				Weighted-
				Average
	Options			Exercise
	Outstanding			Price
Balance at December 31, 2019		7,148		$	12.96
Granted		24		$	58.21
Exercised		(57	)	$	0.58
Forfeited		(10	)	$	13.70
Balance at March 31, 2020		7,105		$	13.21
Options exercisable		4,233		$	5.79
Options vested and expected to vest		7,082		$	13.17

Summary of Restricted Stock Awards and Restricted Stock Units

RSU activity under the 2018 Plan during the three months ended March 31, 2020 is summarized as follows (in thousands, except per share data):

				Weighted-
				Average
				Grant Date
	Shares			Fair Value
Balance at December 31, 2019		542		$	93.67
Granted		31		$	67.57
Forfeited		(1	)	$	95.98
Balance at March 31, 2020		572		$	92.25

Commitments and Contingencies - Additional Information (Details)	1 Months Ended		3 Months Ended		76 Months Ended	78 Months Ended
	Dec. 31, 2019 USD ($) ft² shares	Jan. 31, 2018 USD ($) ft²	Mar. 31, 2020 USD ($) shares	Mar. 31, 2019 USD ($)	Mar. 31, 2020 USD ($) shares	Mar. 31, 2020 USD ($) shares	Dec. 31, 2013 shares
Commitments And Contingencies [Line Items]
Operating lease right-of-use assets	$ 5,775,000		$ 5,705,000		$ 5,705,000	$ 5,705,000
Operating lease liabilities	$ 8,522,000		8,425,000		$ 8,425,000	$ 8,425,000
Net rent expense			$ 300,000	$ 300,000
Remaining lease term			9 years 3 months 18 days
Operating lease liability discount rate			10.00%		10.00%	10.00%
Non-cancelable purchase obligations			$ 22,600,000		$ 22,600,000	$ 22,600,000
Common stock, shares issued \| shares	48,668,000		48,765,000		48,765,000	48,765,000
In-Licensing Agreements
Commitments And Contingencies [Line Items]
Milestone expense			$ 0	$ 0
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 \| shares							88,887
Exclusive License Agreement with The Johns Hopkins University \| Maximum
Commitments And Contingencies [Line Items]
Aggregate additional milestone payments					4,000,000.0
Non-exclusive License Agreement with BioWa Inc. and Lonza Sales AG
Commitments And Contingencies [Line Items]
Milestone payments						400,000
Non-exclusive License Agreement with BioWa Inc. and Lonza Sales AG \| Minimum
Commitments And Contingencies [Line Items]
Annual commercial license fees						40,000
Non-exclusive License Agreement with BioWa Inc. and Lonza Sales AG \| Maximum
Commitments And Contingencies [Line Items]
Aggregate additional milestone payments						41,000,000.0
2019 San Carlos Lease Agreement
Commitments And Contingencies [Line Items]
Lease agreement initiation period	2019-12
Lease term extension period	5 years
Area of office and laboratory space \| ft²	98,000
Lessee operating lease contractual lease expected commencement period	2020-11
Lessee operating lease expected termination Period	2031-08
Operating lease, future minimum rental payments	$ 77,600,000
Operating lease right-of-use assets			0		0	0
Operating lease liabilities			0		0	0
Security deposit in the form of letter of credit secured by restricted cash			1,500,000		1,500,000	1,500,000
2018 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
Area of office and laboratory space \| ft²		25,000
Tenant improvement allowance		$ 1,400,000
Security deposit in the form of letter of credit secured by restricted cash			$ 800,000		$ 800,000	$ 800,000

Stock-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended
	Jul. 31, 2018	Mar. 31, 2020	Mar. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Tax benefits for stock-based compensation expense recognized		$ 0	$ 0
Stock options granted		24,000
Total unrecognized stock-based compensation expense relating to unvested stock options		$ 38,300,000
Total stock-based compensation expense		$ 7,489,000	$ 2,834,000
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
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average recognition period		2 years 9 months 18 days
Unvested Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average recognition period		3 years 8 months 12 days
Weighted-average fair value of RSUs granted		$ 67.57
Number of RSUs granted		31,000	0
Total unrecognized stock-based compensation expense		$ 47,600,000
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
Total stock-based compensation expense		$ 200,000	$ 200,000
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		5,212,568
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 Stock Option Activity (Details) shares in Thousands	3 Months Ended
	Mar. 31, 2020 $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Options Outstanding, Beginning balance \| shares	7,148
Options Outstanding, Granted \| shares	24
Options Outstanding, Exercised \| shares	(57)
Options Outstanding, forfeited \| shares	(10)
Options Outstanding, Ending balance \| shares	7,105
Options Outstanding, Options exercisable \| shares	4,233
Options Outstanding, Options vested and expected to vest \| shares	7,082
Weighted Average Exercise Price, Beginning balance \| $ / shares	$ 12.96
Weighted Average Exercise Price, Granted \| $ / shares	58.21
Weighted Average Exercise Price, Exercised \| $ / shares	0.58
Weighted Average Exercise Price, Forfeited \| $ / shares	13.70
Weighted Average Exercise Price, Ending balance \| $ / shares	13.21
Weighted Average Exercise Price, Options exercisable \| $ / shares	5.79
Weighted Average Exercise Price, Options vested and expected to vest \| $ / shares	$ 13.17

BALANCE SHEETS (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	20,000,000	20,000,000
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	200,000,000
Common stock, shares issued	48,765,000	48,668,000
Common stock, shares outstanding	48,765,000	48,668,000

Organization and Business - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Net loss	$ 27,824	$ 19,953
Cash used in operations	18,614	$ 13,279
Accumulated deficit	217,308		$ 189,484
Cash, cash equivalents and marketable securities	$ 479,800

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Restricted cash in other long term assets, deposit for lease facility	$ 2,275,000	$ 2,275,000	$ 802,000	$ 802,000
ASU 2016-13
Summary Of Significant Accounting Policies [Line Items]
Debt Securities, available-for-sale, allowance for credit loss	0
ASU 2019-12
Summary Of Significant Accounting Policies [Line Items]
Income tax benefit, continuing operations	0
Income tax provision, other comprehensive Income	$ 0

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
	Mar. 31, 2020	Mar. 31, 2019
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	7,685	6,888
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	7,105	6,845
Unvested Restricted Stock Units
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	572	0
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	0	33
Shares Issuable under Employee Stock Purchase Plans
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	8	10

Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
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

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents	$ 135,942	$ 38,367	$ 31,464	$ 33,660
Restricted cash in other long-term assets, deposit for lease facility	2,275	2,275	802	802
Total cash, cash equivalents and restricted cash	$ 138,217	$ 40,642	$ 32,266	$ 34,462

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
	Mar. 31, 2020	Mar. 31, 2019
Numerator:
Net loss	$ (27,824)	$ (19,953)
Weighted-average number of common shares outstanding:
Weighted-average shares of common stock outstanding, basic and diluted	48,691	42,620
Net loss per share, basic and diluted	$ (0.57)	$ (0.47)

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	$ 343,863	$ 457,534
Fair Value, Measurements, Recurring Basis
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	134,392	35,935
Short-term marketable securities	343,863	457,534
Total cash equivalents and short-term marketable securities	478,255	493,469
Fair Value, Measurements, Recurring Basis \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	134,392	35,935
Short-term marketable securities	343,863	457,534
Total cash equivalents and short-term marketable securities	478,255	493,469
Fair Value, Measurements, Recurring Basis \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Short-term marketable securities	0	0
Total cash equivalents and short-term marketable securities	0	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
Short-term marketable securities	0	0
Total cash equivalents and short-term marketable securities	0	0
Fair Value, Measurements, Recurring Basis \| Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	134,392	35,935
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	134,392	35,935
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 \| U.S. Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	343,863	457,534
Fair Value, Measurements, Recurring Basis \| U.S. Treasuries \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	343,863	457,534
Fair Value, Measurements, Recurring Basis \| U.S. Treasuries \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	0	0
Fair Value, Measurements, Recurring Basis \| U.S. Treasuries \| Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	$ 0	$ 0

Marketable Securities - Marketable Securities - Summary of Amortized Cost, Gross Unrealized Holding Gains or Losses, and Fair Value of Marketable Securities (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	$ 341,857,000	$ 457,397,000
Unrealized Gains	2,006,000	161,000
Unrealized Losses	0	(24,000)
Fair Value	343,863,000	457,534,000
U.S. Treasuries \| Investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	341,857,000	457,397,000
Unrealized Gains	2,006,000	161,000
Unrealized Losses	0	(24,000)
Fair Value	$ 343,863,000	$ 457,534,000

Balance Sheet Components and Supplemental Disclosures - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 10,501	$ 10,446
Less accumulated depreciation	(2,429)	(2,036)
Property and equipment, net	8,072	8,410
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	4,189	4,170
Furniture And Office Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,731	1,695
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 4,581	$ 4,581

Balance Sheet Components and Supplemental Disclosures - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Payables And Accruals [Abstract]
Accrued contract research and development expense	$ 1,982	$ 4,990
Accrued compensation and benefits expense	2,619	1,608
Lease liability, current	430	410
Other current liabilities	242	90
Total	$ 5,273	$ 7,098

Commitments and Contingencies - Summary of Classification of Company's Lease Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Operating lease liabilities
Current portion included in accrued expenses and other current liabilities	$ 430	$ 410
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:AccruedLiabilitiesCurrent	us-gaap:AccruedLiabilitiesCurrent
Non-current portion included in other long-term liabilities	$ 7,995	$ 8,112
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent	us-gaap:OtherLiabilitiesNoncurrent
Total operating lease liabilities	$ 8,425	$ 8,522

Commitments and Contingencies - Summary of Components of Lease Costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Leases [Abstract]
Operating lease cost	$ 279	$ 280
Variable cost	87	87
Total lease costs	$ 366	$ 367

Commitments and Contingencies - Summary of Future Minimum Lease Payments Required Under Operating Leases (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Operating Lease Liabilities Payments Due [Abstract]
2020 (remaining 9 months)	$ 927
2021	1,270
2022	1,308
2023	1,348
2024	1,388
Thereafter	6,916
Total minimum future lease payments	13,157
Present value adjustment	4,732
Operating lease liabilities	$ 8,425	$ 8,522

Commitments and Contingencies - Summary of Future Minimum Lease Payments Required Under Operating Leases (Parenthetical) (Details) $ in Thousands	Mar. 31, 2020 USD ($)
Commitments And Contingencies [Line Items]
Minimum future lease payments	$ 13,157
2019 San Carlos Lease Agreement
Commitments And Contingencies [Line Items]
Minimum future lease payments	$ 77,600

Stock-Based Compensation - Summary of Total Stock-Based Compensation Expense Recognized (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 7,489	$ 2,834
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	2,443	1,053
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 5,046	$ 1,781

Stock-Based Compensation - Summary of RSU Activity Under 2018 Plan (Details) - Unvested Restricted Stock Units - $ / shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Number of Shares
Balance at December 31, 2019	542,000
Granted	31,000	0
Forfeited	(1,000)
Balance at March 31, 2020	572,000
Weighted-Average Grant Date Fair Value
Balance at December 31, 2019	$ 93.67
Granted	67.57
Forfeited	95.98
Balance at March 31, 2020	$ 92.25

Defined Contribution Plans - Additional Information (Details) - 401(K) Plan - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Contribution Plan Disclosure [Line Items]
Employer contribution matching percentage	4.00%	4.00%
Contributions made by employer	$ 0.2	$ 0.1

Subsequent Event - Additional Information (Details) - Commercial Supply Agreement with Lonza Sales AG - Subsequent Event $ in Millions	1 Months Ended
	Apr. 30, 2020 USD ($)
Subsequent Event [Line Items]
Term of agreement	10 years
Commitments to purchase obligation	$ 28.9