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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2019	May 02, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Trading Symbol	KZR
Entity Registrant Name	Kezar Life Sciences, Inc.
Entity Central Index Key	0001645666
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Emerging Growth Company	true
Entity Small Business	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		19,119,421

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 18,470	$ 24,182
Marketable securities	82,664	83,250
Prepaid expenses	1,664	1,884
Other current assets	991	489
Total current assets	103,789	109,805
Property and equipment, net	4,846	4,595
Other assets	282	282
Total assets	113,056	114,682
Operating lease right-of-use asset	4,139
Current liabilities:
Accounts payable	1,448	193
Accrued liabilities	2,406	2,678
Operating lease liabilities, current	804
Deferred rent, current		354
Other liabilities, current	100	112
Total current liabilities	4,758	3,337
Operating lease liabilities, noncurrent	6,153
Deferred rent		2,548
Total liabilities	10,911	5,885
Common stock, $0.001 par value, 125,000,000 shares authorized as of March 31, 2019 and December 31, 2018; 19,118,421 and 19,114,421 shares issued and outstanding as of March 31, 2019 and December 31, 2018, respectively	19	19
Preferred stock, $0.001 par value, 10,000,000 shares authorized as of March 31, 2019 and December 31, 2018; zero shares issued and outstanding as of March 31, 2019 and December 31, 2018
Additional paid-in capital	159,093	158,176
Accumulated other comprehensive loss	(130)	(203)
Accumulated deficit	(56,837)	(49,195)
Total stockholders' equity	102,145	108,797
Total liabilities and stockholders' equity	$ 113,056	$ 114,682

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	19,118,421	19,114,421
Common stock, shares outstanding	19,118,421	19,114,421
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0

Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating expenses:
Research and development	$ 5,927	$ 3,572
General and administrative	2,382	1,514
Total operating expenses	8,309	5,086
Loss from operations	(8,309)	(5,086)
Interest income	667	139
Net loss	$ (7,642)	$ (4,947)
Net loss per common share, basic and diluted	$ (0.40)	$ (6.53)
Weighted-average shares used to compute net loss per common share, basic and diluted	19,042,524	757,399

- Definition

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

+ References + Details

Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (7,642)	$ (4,947)
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments	8	(23)
Unrealized gain on marketable securities	65
Total other comprehensive income (loss), net of tax	73	(23)
Comprehensive loss	$ (7,569)	$ (4,970)

- Definition

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

+ References + Details

Condensed Consolidated Statement of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at Dec. 31, 2017	$ (25,687)		$ 1	$ 451	$ (111)	$ (26,028)
Balance, shares at Dec. 31, 2017			948,578
Balance at Dec. 31, 2017		$ 77,931
Balance, shares at Dec. 31, 2017		12,263,126
Issuance of common stock upon exercise of stock options, net of amount related to early exercised options	25			25
Issuance of common stock upon exercise of stock options, net of amount related to early exercised options, shares			109,818
Vesting related to common shares issued pursuant to early exercises	17			17
Stock-based compensation expense	126			126
Other comprehensive loss	(23)				(23)
Net loss	(4,947)					(4,947)
Balance at Mar. 31, 2018	(30,489)		$ 1	619	(134)	(30,975)
Balance, shares at Mar. 31, 2018			1,058,396
Balance at Mar. 31, 2018		$ 77,931
Balance, shares at Mar. 31, 2018		12,263,126
Balance at Dec. 31, 2018	108,797		$ 19	158,176	(203)	(49,195)
Balance, shares at Dec. 31, 2018			19,114,421
Balance at Dec. 31, 2018		$ 0
Balance, shares at Dec. 31, 2018		0
Issuance of common stock upon exercise of stock options, net of amount related to early exercised options	$ 4			4
Issuance of common stock upon exercise of stock options, net of amount related to early exercised options, shares	4,000		4,000
Vesting related to common shares issued pursuant to early exercises	$ 12			12
Stock-based compensation expense	901			901
Other comprehensive loss	73				73
Net loss	(7,642)					(7,642)
Balance at Mar. 31, 2019	$ 102,145		$ 19	$ 159,093	$ (130)	$ (56,837)
Balance, shares at Mar. 31, 2019			19,118,421
Balance at Mar. 31, 2019		$ 0
Balance, shares at Mar. 31, 2019		0

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (7,642)	$ (4,947)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	303	87
Stock-based compensation	901	126
Amortization of premiums and discounts on marketable securities	(359)
Loss on disposal of property and equipment		97
Changes in operating assets and liabilities
Prepaid expenses and other current assets	(282)	(641)
Other assets		39
Accounts payable and accrued liabilities	560	1,028
Other liabilities, current		149
Deferred rent		296
Operating lease liabilities	(181)
Net cash used in operating activities	(6,700)	(3,766)
Cash flows from investing activities:
Purchases of property and equipment	(34)	(195)
Purchases of marketable securities	(35,936)
Maturities of marketable securities	36,946
Net cash provided by (used in) investing activities	976	(195)
Cash flows from financing activities:
Proceeds from the exercise of stock options	4	25
Net cash provided by financing activities	4	25
Effect of exchange rate changes on cash and cash equivalents and restricted cash	8	(12)
Net decrease in cash, cash equivalents and restricted cash	(5,712)	(3,948)
Cash, cash equivalents and restricted cash at the beginning of period	24,182	51,046
Cash, cash equivalents and restricted cash at the end of period	18,470	47,098
Supplemental disclosures of noncash investing and financing information:
Reclassification of employee stock liability to equity upon vesting	12	17
Addition of tenant improvement paid by landlord		2,054
Purchase of property and equipment in accounts payable	$ 423	21
Deferred offering costs in accrued liabilities		$ 1,300

Organization and Description of the Business

3 Months Ended

Mar. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization and Description of the Business

1. Organization and Description of the Business

Description of Business

Kezar Life Sciences, Inc. (the “Company,” “we,” “us,” or “our”) was incorporated in the state of Delaware in February 2015 and commenced operations in June 2015. The Company is a clinical-stage biotechnology company, discovering and developing novel small molecule therapeutics to treat unmet needs in autoimmunity and cancer. The Company’s principal operations are in South San Francisco, California, and it operates in one segment.

Liquidity

Since commencing operations in mid-2015, substantially all of the Company’s efforts have been focused on research, development, and the advancement of the Company’s lead product candidate, KZR-616. The Company’s ultimate success depends on the outcome of the ongoing research and development activities. The Company has not yet generated product sales and as a result has experienced operating losses since inception and had an accumulated deficit of $56.8 million as of March 31, 2019. The Company expects to incur additional losses in the future to conduct research and development and will need to raise additional capital to fully implement management’s business plan. The Company intends to raise such capital through the issuance of additional equity, and potentially through borrowings, strategic alliances with partner companies and other licensing transactions. However, if such financing is not available at adequate levels, the Company may need to reevaluate its operating plans. Management believes that its existing cash, cash equivalents and marketable securities will be sufficient to fund the Company’s cash requirements for at least 12 months following the issuance of these financial statements.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Significant Accounting Policies

The Company’s significant accounting policies are disclosed in the audited consolidated financial statements for the year ended December 31, 2018 and the notes thereto, which are included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018, filed with the U.S. Securities and Exchange Commission (“SEC”) on March 26, 2019 and except for the adoption of the Financial Accounting Standards Board (“FASB”) Accounting Standards Update (“ASU”) No. 2016-02, Leases (Topic 842), there have been no material changes during the three months ended March 31, 2019.

Basis of Presentation and Consolidation

The condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and include the Company’s accounts and those of its wholly owned Australian subsidiary, Kezar Life Sciences Australia Pty Ltd, which is a proprietary company limited by shares. All intercompany balances and transactions have been eliminated upon consolidation.

The condensed consolidated balance sheet at December 31, 2018 has been derived from the audited consolidated financial statements at that date, but does not include all information and footnotes required by GAAP for complete financial statements. These unaudited condensed consolidated financial statements should be read in conjunction with the audited financial statements included in the Company's Annual Report on Form 10-K for the fiscal year ended December 31, 2018.

Unaudited Interim Condensed Consolidated Financial Statements

The accompanying financial information as of March 31, 2019 is unaudited. The condensed consolidated financial statements included in this report reflect all adjustments (consisting only of normal recurring adjustments) that our management considers necessary for the fair statement of the results of operations for the interim periods covered and of our financial condition at the date of the interim balance sheet. The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with GAAP for interim financial information. Accordingly, they do not include all of the information and notes required by GAAP for complete financial statements. The results for interim periods are not necessarily indicative of the results for the entire year or any other interim period. The accompanying condensed consolidated financial statements and related financial information should be read in conjunction with the audited financial statements and the related notes thereto included in our Annual Report on Form 10-K for the fiscal year ended December 31, 2018, filed with the SEC on March 26, 2019.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting period. Significant items subject to such estimates and assumptions include the useful lives of fixed assets, stock-based compensation, and accrued research and development costs. Management bases its estimates on historical experience and on various other market-specific relevant assumptions that management believes to be reasonable under the circumstances. Actual results may differ from those estimates.

Cash and Cash Equivalents and Restricted Cash

The Company considers all highly liquid investments with original maturities of three months or less at the time of purchase to be cash equivalents. Cash equivalents consist of corporate debt securities and highly liquid money market funds.

Restricted cash at January 1, 2018 consisted of approximately a $13,000 deposit at the bank held as collateral for the Company’s credit card program. The collateral requirement was removed and released in May 2018.

Marketable Securities

All marketable securities have been classified as “available-for-sale” in accordance with the Company’s investment policy and cash management strategy. Short-term marketable securities mature within one-year from the balance sheet date. Investments in marketable securities are recorded at fair value, with any unrealized gains and losses reported within accumulated other comprehensive income as a separate component of stockholders’ deficit until realized or until a determination is made that an other-than-temporary decline in market value has occurred. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization and accretion, together with interest on securities, are included in interest income on the Company’s condensed consolidated statements of operations.

Leases

At lease commencement, the Company records a lease liability based on the present value of lease payments over the expected lease term. The Company calculates the present value of lease payments using the discount rate implicit in the lease, unless that rate cannot be readily determined. In that case, the Company uses its incremental borrowing rate, which is the rate of interest that the Company would have to pay to borrow on a collateralized basis an amount equal to the lease payments over the expected lease term. The Company records a corresponding right-of-use (“ROU”) lease asset based on the lease liability, adjusted for any lease incentives received and any initial direct costs paid to the lessor prior to the lease commencement date.

After lease commencement, the Company measures its leases as follows: (i) the lease liability based on the present value of the remaining lease payments using the discount rate determined at lease commencement; and (ii) the ROU lease asset based on the remeasured lease liability, adjusted for any unamortized lease incentives received, any unamortized initial direct costs and the cumulative difference between rent expense and amounts paid under the lease agreement. Any lease incentives received and any initial direct costs are amortized on a straight-line basis over the expected lease term. Rent expense is recorded on a straight-line basis over the expected lease term.

Accounting Pronouncements Adopted in 2019

Leases (Topic 842)

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842). ASU 2016-02 increases transparency and comparability among organizations by recognizing lease assets and lease liabilities on the balance sheet and requires disclosing key information about leasing arrangements. In July 2018, the FASB issued ASU 2018-11, Leases (Topic 842): Targeted Improvements, which provides entities the option to initially apply the transition provisions of ASU 2016-02 at its adoption date and recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption.

The Company adopted the new standard on January 1, 2019 and used the effective date as its date of initial application. Consequently, financial information will not be restated and the disclosures required under the new standard will not be provided for dates and periods before January 1, 2019. The Company elected the package of transition practical expedients that allows the Company not to reassess the lease classification for any expired or existing contracts, whether initial direct costs were incurred, or whether any expired or existing contracts contained leases.

This standard had a material effect on the Company’s financial statements. The most significant effects relate to (1) the recognition of new ROU asset and lease liabilities on the balance sheet for the operating lease for its headquarters at 4000 Shoreline Court, South San Francisco; and (2) providing significant new disclosures about its leasing activities.

The following table discloses the impact on our balance sheet upon adoption (in thousands):

	January 1, 2019		December 31, 2018
Operating lease ROU asset	$	4,236	$	—
Operating lease liabilities, current		774		—
Operating lease liabilities, noncurrent		6,364		—
Deferred rent, current		—		354
Deferred rent, noncurrent		—		2,548

The new standard also provides practical expedients for an entity’s ongoing accounting. The Company elected the short-term lease recognition exemption for all leases that qualify. This means, for those leases that qualify, the Company will not recognize ROU assets or lease liabilities, and this includes not recognizing ROU assets or lease liabilities for existing short-term leases of those assets in transition. The Company did not elect the practical expedient to not separate lease and non-lease components for all of its leases.

Stock Compensation (Topic 718)

In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718). ASU 2018-07 simplifies the accounting for nonemployee share-based payment transactions. This ASU is effective for public entities for interim and annual reporting periods beginning after December 15, 2018. The Company adopted the new standard on January 1, 2019 and determined that it did not have a material impact on the Company’s financial statements.

New Accounting Pronouncements Not Yet Adopted

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326). ASU 2016-13 requires measurement and recognition of expected credit losses for financial assets. This guidance will become effective for us beginning in the first quarter of 2020 and must be adopted using a modified retrospective approach, with certain exceptions. Early adoption is permitted beginning in the first quarter of 2019. The Company is currently evaluating the potential impact of the adoption of this guidance on its financial statements.

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, which modifies the disclosure requirements on fair value measurements. The amendment of ASU No. 2018-13 removes disclosure requirements from Topic 820 in the areas of (1) the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy; (2) the policy for timing of transfers between levels, and (3) the valuation processes for Level 3 fair value measurements. This guidance is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company is currently evaluating the effect the new guidance will have on its consolidated financial statements.

Fair Value Measurements

3 Months Ended

Mar. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

3. Fair Value Measurements

Financial assets and liabilities are recorded at fair value. The carrying amount of certain financial instruments, including cash and cash equivalents, restricted cash, accounts payable and accrued liabilities, approximate fair value due to their relatively short maturities. Assets and liabilities recorded at fair value on a recurring basis in the condensed consolidated balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows:

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

Level 2: Observable inputs other than Level 1 prices, 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 the assets or liabilities.

Assets and liabilities measured at fair value are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. The Company’s assessment of the significance of a particular input to the fair value measurement in its entirety requires management to make judgments and consider factors specific to the asset or liability.

We determine the fair value of Level 1 assets using quoted prices in active markets for identical assets. We review trading activity and pricing for Level 2 investments as of each measurement date. Level 2 inputs, which are obtained from various third-party data providers, represent quoted prices for similar assets in active markets and were derived from observable market data, or, if not directly observable, were derived from or corroborated by other observable market data. There were no transfers between Level 1 and Level 2 securities in the periods presented.

In certain cases, where there is limited activity or less transparency around inputs to valuation, securities are classified as Level 3 within the valuation hierarchy. The Company does not have any assets or liabilities measured using Level 3 inputs as of March 31, 2019 or December 31, 2018.

The following table summarizes our financial assets measured at fair value on a recurring basis and classified under the appropriate level of the fair value hierarchy as described above (in thousands):

	March 31, 2019
	Total		Level 1		Level 2		Level 3
Financial Assets:
Money market funds	$	18,349	$	18,349	$	—	$	—
U.S. Treasury securities		13,695		13,695		—		—
Commercial paper		19,357		—		19,357		—
Corporate debt securities		20,538		—		20,538		—
Government agency bonds		29,075		—		29,075		—
Total	$	101,014	$	32,044	$	68,970	$	—

	December 31, 2018
	Total		Level 1		Level 2		Level 3
Financial Assets:
Money market funds	$	23,156	$	23,156	$	—	$	—
U.S. Treasury securities		25,692		25,692		—		—
Commercial paper		19,190		—		19,190		—
Corporate debt securities		20,451		—		20,451		—
Government agency bonds		17,917		—		17,917		—
Total	$	106,406	$	48,848	$	57,558	$	—

- 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

Available-for-Sale Securities

3 Months Ended

Mar. 31, 2019

Investments Debt And Equity Securities [Abstract]

Available-for-Sale Securities

4. Available-for-Sale Securities

The following table is a summary of available-for-sale securities recorded in cash and cash equivalents or marketable securities in our condensed consolidated balance sheets (in thousands):

	March 31, 2019
	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value
Financial Assets
Money market funds		18,349		—		—		18,349
U.S. Treasury securities		13,691		4		—		13,695
Commercial paper		19,342		15		—		19,357
Corporate debt securities		20,521		17		—		20,538
Government agency bonds		29,066		9		—		29,075
Total	$	100,969	$	45	$	—	$	101,014

	December 31, 2018
	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Financial Assets
Money market funds	$	23,156	$	—	$	—		$	23,156
U.S. Treasury securities		25,698		—		(6	)		25,692
Commercial paper		19,203		—		(13	)		19,190
Corporate debt securities		20,449		4		(2	)		20,451
Government agency bonds		17,921		—		(4	)		17,917
Total	$	106,427	$	4	$	(25	)	$	106,406

As of March 31, 2019, the amortized cost and estimated fair value of the Company’s available-for-sale securities by contractual maturity are shown below (in thousands):

	Amortized		Estimated
	Cost		Fair Value
Marketable securities maturing:
In one year or less	$	82,619	$	82,664
Total marketable securities	$	82,619	$	82,664

Balance Sheet Components

3 Months Ended

Mar. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Balance Sheet Components

5. Balance Sheet Components

Property and Equipment, Net

Property and equipment consists of the following (in thousands):

	March 31, 2019			December 31, 2018
Leasehold improvements	$	3,268		$	3,268
Furniture, laboratory and office equipment		2,489			2,043
Computer equipment		230			219
Total property and equipment		5,987			5,530
Less accumulated depreciation and amortization		(1,141	)		(935	)
Property and equipment, net	$	4,846		$	4,595

Depreciation expense was $0.2 million and $0.1 million for the three months ended March 31, 2019 and 2018, respectively.

Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	March 31, 2019		December 31, 2018
Accrued preclinical and research costs	$	1,128	$	1,146
Accrued clinical costs		528		434
Accrued employee-related costs		554		736
Accrued professional services		90		138
Other		106		224
Total accrued liabilities	$	2,406	$	2,678

Operating Lease

3 Months Ended

Mar. 31, 2019

Leases [Abstract]

Operating Lease

6. Operating Lease

In August 2017, the Company entered into a lease agreement to lease 24,357 square feet of combination laboratory and office space in South San Francisco, California. This lease expires in February 2025.

The contractually specified minimum rent and annual rent increases for the operating lease are included in the measurement of the ROU asset and related lease liabilities. Under the lease arrangement, we may be required to pay directly, or reimburse the lessor for real estate taxes, insurance, utilities, maintenance and other operating costs. Such amounts are generally variable and therefore not included in the measurement of the ROU asset and related lease liability but are instead recognized as variable lease expense in our Condensed Consolidated Statements of Operations when they are incurred. The option to extend the lease was not recognized as part of the Company’s lease liability and ROU asset.

Because the rate implicit in the lease is not readily determinable, the Company uses its incremental borrowing rate to determine the present value of the lease payments. The Company recorded lease liabilities and ROU asset for its office lease based on the present value of lease payments over the expected lease term.

Information related to the Company’s ROU asset and related lease liabilities were as follows (in thousands):

	For three months ended March 31, 2019
Cash paid for operating lease liabilities	$	181
Remaining lease term	5.92 years
Discount rate		10	%

Maturities of lease liabilities as of March 31, 2019 were as follows:
2019	$	1,095
2020		1,497
2021		1,542
2022		1,588
2023		1,635
2024		1,684
Thereafter		282
Total undiscounted lease payments		9,323
Less: imputed interest		(2,366	)
Total lease liabilities	$	6,957

Operating lease liabilities, current		804
Operating lease liabilities, noncurrent		6,153
Total operating lease liabilities	$	6,957

Future minimum lease payments are as follows as of December 31, 2018 (in thousands):

Year ending December 31:
2019	$	1,453
2020		1,497
2021		1,542
2022		1,588
2023		1,635
Thereafter		1,966
Total future minimum lease payments	$	9,681

Stock-Based Compensation

3 Months Ended

Mar. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

7. Stock-Based Compensation

Stock Incentive Plans

2018 Equity Incentive Plan

In June 2018, the Company’s board of directors adopted and its stockholders approved the 2018 Equity Incentive Plan (the “2018 Plan”), which became effective as of June 20, 2018, at which point no further grants will be made under the 2015 Equity Incentive Plan (the “2015 Plan”) described below. Under the 2018 Plan, the Company may grant incentive stock options (“ISOs”), nonstatutory stock options (“NSOs”), stock appreciation rights, restricted stock awards, restricted stock units (“RSUs”) and other stock-based awards for the purchase of that number of shares of common stock. As of March 31, 2019, options to purchase 663,621 shares of common stock and 10,189 RSUs have been granted and 2,179,866 shares were available for future issuance under the 2018 Plan.

Initially, subject to adjustment as provided in the 2018 Plan, the aggregate number of shares of the Company’s common stock that may be issued pursuant to stock awards under the 2018 Plan will not exceed 4,000,000 shares, which is the sum of (i) 1,600,692 shares plus (ii) the number of shares reserved, and remaining available for issuance, under the 2015 Plan at the time the 2018 Plan became effective and (iii) the number of shares subject to stock options or other stock awards granted under the 2015 Plan that expire, terminate are forfeited or otherwise not issued, or are withheld to satisfy a tax withholding obligation in connection with an award or to satisfy a purchase or exercise price of an award (such as upon the expiration or termination of a stock award prior to vesting). The number of shares of the Company’s common stock reserved for issuance under the 2018 Plan will automatically increase on January 1 of each year, beginning on January 1, 2019 and continuing through and including January 1, 2028, by 5% of the total number of shares of capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the Company’s board of directors. The maximum number of shares that may be issued upon the exercise of ISOs under the 2018 Plan is 12,500,000 shares.

2015 Equity Incentive Plan

The Company’s 2015 Equity Incentive Plan provided for the granting of ISOs and NSOs to employees, directors and consultants at the discretion of the board of directors. The 2015 Plan was terminated as to future awards in June 2018, although it continues to govern the terms of options that remain outstanding under the 2015 Plan.

No additional stock awards will be granted under the 2015 Plan, and all outstanding stock awards granted under the 2015 Plan that are repurchased, forfeited, expire or are cancelled will become available for grant under the 2018 Plan in accordance with its terms.

Options granted under the 2015 Plan expire no later than 10 years from the date of grant. Options granted under the 2015 Plan vest over periods determined by the board of directors, generally over four years. The 2015 Plan allows for early exercise of certain options prior to vesting. Upon termination of employment, the unvested shares are subject to repurchase at the original exercise price. As of March 31, 2019, options to purchase 2,091,845 shares of common stock were outstanding under the 2015 Plan.

2018 Employee Stock Purchase Plan

In June 2018, the Company’s board of directors and its stockholders approved the 2018 Employee Stock Purchase Plan (the “ESPP”), which became effective as of June 20, 2018. The ESPP is intended to qualify as an “employee stock purchase plan” within the meaning of Section 423 of the U.S. Internal Revenue Code of 1986, as amended. The number of shares of common stock initially reserved for issuance under the ESPP was 200,000 shares. The ESPP provides for an annual increase on the first day of each year beginning in 2019 and ending in 2028, in each case subject to the approval of the board of directors, equal to the lesser of (i) 1% of the shares of common stock outstanding on the last day of the prior fiscal year or (ii) 375,000 shares; provided, that prior to the date of any such increase, the board of directors may determine that such increase will be less than the amount set forth in clauses (i) and (ii). As of March 31, 2019, no shares of common stock had been issued under the ESPP and 391,144 shares remained available for future issuance under the ESPP. The option price per share of common stock to be paid by a participant upon exercise of the participant’s option on the applicable exercise date for an offering period shall be equal to 85% of the lesser of the fair market value of a share of common stock on (a) the applicable grant date or (b) the applicable exercise date. The Board authorized an initial offering beginning on November 16, 2018 and ending on May 15, 2019. Following the end of the initial offering, a new offering will automatically begin on the day that immediately follows the conclusion of the preceding offering.

Stock Option Activity

The following table summarizes activity under the Company’s stock option plan and related information (in thousands, except share and per share amounts):

	Number of Options Outstanding			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding at December 31, 2018		2,181,466		$	3.80		8.5	$	43,189
Options granted		578,000		$	22.50
Options exercised		(4,000	)	$	0.90			$	78
Options cancelled		—		$	—
Outstanding at March 31, 2019		2,755,466		$	7.73		8.6	$	30,395
Vested and exercisable at March 31, 2019		1,104,476		$	2.93		7.8	$	16,468

The weighted average grant date fair value of options granted during the three months ended March 31, 2019 was $15.79 per share. The 2015 Plan allows for early exercisable option grants, which permit the grantee to exercise a stock option in exchange for stock before the requisite service is provided (e.g., before the award is vested under its original terms); however, such arrangements permit the Company to subsequently repurchase such shares at the exercise price if the vesting conditions are not satisfied. To date, the Company has made such grants only to non-employee board members. The total intrinsic value of exercised stock options during the three months ended March 31, 2019 was $78,000. The aggregate intrinsic value is calculated as the difference between the exercise price and the estimated fair value of the Company’s common stock at the date of exercise.

Early Exercise Stock Purchase Agreements

As of March 31, 2019 and December 31, 2018, there were 56,038 and 68,349, respectively, of nonvested common shares outstanding that were exercised early and subject to repurchase by the Company at the original issuance price upon termination of the stockholder’s services. The right to repurchase these shares generally lapses with respect to 25% of the shares underlying the option after the applicable vesting commencement date and 1/48 of the shares underlying the original grant per month for 36 months thereafter. The shares purchased pursuant to the early exercise of stock options are not deemed, for accounting purposes, to be issued until those shares vest. The cash received in exchange for exercised and nonvested shares related to stock options granted is recorded as a liability for the early exercise of stock options on the balance sheets with the corresponding par value in common stock and an offset in additional paid-in capital.

Restricted Stock

In addition to the nonvested common shares outstanding described above at “Early Exercise Stock Purchase Agreements,” the Company issued restricted stock to its founders. The fair value of restricted stock on the issuance date is deemed equal to the cash consideration paid by the founders. Restricted stock vests over a four-year period from the applicable vesting commencement date. The following summarizes the activity of nonvested restricted stock:

	Number of Shares
Nonvested—December 31, 2018		20,908
Vested		(15,681	)
Nonvested—March 31, 2019		5,227

Stock-Based Compensation Expense

Total stock-based compensation recognized by function was as follows (in thousands):

	Three Months Ended March 31,
	2019		2018
Research and development	$	447	$	59
General and administrative		454		67
Total stock-based compensation expense	$	901	$	126

As of March 31, 2019, the unrecognized stock-based compensation cost and the estimated weighted average amortization period, using the straight-line attribution method, was as follows (dollars in thousands):

	Unrecognized Compensation Cost		Weighted Average Remaining Amortization Period (Years)
Employee options	$	12,243		2.9
Nonemployee options		2		0.2
Total unrecognized stock-based compensation expense	$	12,245

The fair value of the employee stock options granted is calculated using the Black-Scholes option-pricing model with the following weighted-average assumptions:

	Three Months Ended March 31,
	2019		2018
Expected term (years)		5.9		6.0
Expected volatility	79.9 - 81.9%			82.0	%
Risk-free interest rate	2.4 - 2.6%			2.6	%
Expected dividend yield		—		—

Income Taxes

3 Months Ended

Mar. 31, 2019

Income Tax Disclosure [Abstract]

Income Taxes

8. Income Taxes

No provision for income taxes was recorded for the three months ended March 31, 2019 and 2018, respectively. Deferred tax assets generated from the Company’s net operating losses have been fully reserved, as the Company believes it is not more likely than not that the benefit will be realized.

In December 2015, the Protecting Americans from Tax Hikes (PATH) Act of 2015 was signed into law, which created several new research and development (“R&D”) tax credit provisions, including allowing qualified small businesses to utilize the R&D credit against the employer’s portion of payroll tax up to a maximum of $250,000 per year. The Company qualified as a small business under PATH for both 2016 and 2017. The Company has not yet filed its federal tax return for 2018 in which the Company will determine whether it qualifies as a small business under the PATH Act. During the three months ended March 31, 2019, the Company utilized $93,000 of R&D tax credits as a reduction of payroll expenses to offset its payroll tax liabilities. The remaining R&D tax credits available for future payroll tax liabilities have been recorded as deferred tax assets with a full valuation allowance.

Net Loss per Share

3 Months Ended

Mar. 31, 2019

Earnings Per Share [Abstract]

Net Loss Per Share

9. Net Loss Per Share

Net Loss Per Share

The following table sets forth the calculation of basic and diluted net loss per share during the periods presented (in thousands, except share and per share data):

	Three Months Ended March 31,
	2019			2018
Numerator:
Net loss	$	(7,642	)	$	(4,947	)
Denominator:
Weighted-average shares of common stock outstanding		19,042,524			757,399
Net loss per share, basic and diluted	$	(0.40	)	$	(6.53	)

The following outstanding shares of common stock equivalents were excluded from the computation of the diluted net loss per share for the periods presented because their effect would have been anti-dilutive:

	Three Months Ended March 31,
	2019		2018
Redeemable convertible preferred stock on an if converted basis		—		12,263,126
Stock options to purchase common stock		2,755,466		1,354,965
Common stock subject to future vesting		61,265		252,303
Total		2,816,731		13,870,394

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2019

Accounting Policies [Abstract]

Basis of presentation and consolidation

Basis of Presentation and Consolidation

Unaudited Interim Condensed Consolidated Financial Statements

Use of Estimates

Cash and Cash Equivalents and Restricted Cash

Marketable Securities

Leases

Accounting Pronouncements Adopted in 2019 and New Accounting Pronouncements Not Yet Adopted

Accounting Pronouncements Adopted in 2019

Leases (Topic 842)

The following table discloses the impact on our balance sheet upon adoption (in thousands):

	January 1, 2019		December 31, 2018
Operating lease ROU asset	$	4,236	$	—
Operating lease liabilities, current		774		—
Operating lease liabilities, noncurrent		6,364		—
Deferred rent, current		—		354
Deferred rent, noncurrent		—		2,548

Stock Compensation (Topic 718)

New Accounting Pronouncements Not Yet Adopted

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2019

Accounting Policies [Abstract]

Impact of Accounting Pronouncements Adoption on Balance Sheet

The following table discloses the impact on our balance sheet upon adoption (in thousands):

	January 1, 2019		December 31, 2018
Operating lease ROU asset	$	4,236	$	—
Operating lease liabilities, current		774		—
Operating lease liabilities, noncurrent		6,364		—
Deferred rent, current		—		354
Deferred rent, noncurrent		—		2,548

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2019

Fair Value Disclosures [Abstract]

Summary of Financial Assets Measured at Fair Value on a Recurring Basis

	March 31, 2019
	Total		Level 1		Level 2		Level 3
Financial Assets:
Money market funds	$	18,349	$	18,349	$	—	$	—
U.S. Treasury securities		13,695		13,695		—		—
Commercial paper		19,357		—		19,357		—
Corporate debt securities		20,538		—		20,538		—
Government agency bonds		29,075		—		29,075		—
Total	$	101,014	$	32,044	$	68,970	$	—

	December 31, 2018
	Total		Level 1		Level 2		Level 3
Financial Assets:
Money market funds	$	23,156	$	23,156	$	—	$	—
U.S. Treasury securities		25,692		25,692		—		—
Commercial paper		19,190		—		19,190		—
Corporate debt securities		20,451		—		20,451		—
Government agency bonds		17,917		—		17,917		—
Total	$	106,406	$	48,848	$	57,558	$	—

Available-for-Sale Securities (Tables)

3 Months Ended

Mar. 31, 2019

Investments Debt And Equity Securities [Abstract]

Summary of Available-For-Sale Securities Recorded in Cash and Cash Equivalents or Marketable Securities

The following table is a summary of available-for-sale securities recorded in cash and cash equivalents or marketable securities in our condensed consolidated balance sheets (in thousands):

	March 31, 2019
	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value
Financial Assets
Money market funds		18,349		—		—		18,349
U.S. Treasury securities		13,691		4		—		13,695
Commercial paper		19,342		15		—		19,357
Corporate debt securities		20,521		17		—		20,538
Government agency bonds		29,066		9		—		29,075
Total	$	100,969	$	45	$	—	$	101,014

	December 31, 2018
	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Financial Assets
Money market funds	$	23,156	$	—	$	—		$	23,156
U.S. Treasury securities		25,698		—		(6	)		25,692
Commercial paper		19,203		—		(13	)		19,190
Corporate debt securities		20,449		4		(2	)		20,451
Government agency bonds		17,921		—		(4	)		17,917
Total	$	106,427	$	4	$	(25	)	$	106,406

Summary of Amortized Cost and Estimated Fair Value of Available-for-sale Securities by Contractual Maturity

As of March 31, 2019, the amortized cost and estimated fair value of the Company’s available-for-sale securities by contractual maturity are shown below (in thousands):

	Amortized		Estimated
	Cost		Fair Value
Marketable securities maturing:
In one year or less	$	82,619	$	82,664
Total marketable securities	$	82,619	$	82,664

Balance Sheet Components (Tables)

3 Months Ended

Mar. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Property and Equipment

Property and equipment consists of the following (in thousands):

	March 31, 2019			December 31, 2018
Leasehold improvements	$	3,268		$	3,268
Furniture, laboratory and office equipment		2,489			2,043
Computer equipment		230			219
Total property and equipment		5,987			5,530
Less accumulated depreciation and amortization		(1,141	)		(935	)
Property and equipment, net	$	4,846		$	4,595

Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	March 31, 2019		December 31, 2018
Accrued preclinical and research costs	$	1,128	$	1,146
Accrued clinical costs		528		434
Accrued employee-related costs		554		736
Accrued professional services		90		138
Other		106		224
Total accrued liabilities	$	2,406	$	2,678

Operating Lease (Tables)

3 Months Ended

Mar. 31, 2019

Leases [Abstract]

Details of Right-of-Use Assets and Lease Liabilities

Information related to the Company’s ROU asset and related lease liabilities were as follows (in thousands):

	For three months ended March 31, 2019
Cash paid for operating lease liabilities	$	181
Remaining lease term	5.92 years
Discount rate		10	%

Maturities of lease liabilities as of March 31, 2019 were as follows:
2019	$	1,095
2020		1,497
2021		1,542
2022		1,588
2023		1,635
2024		1,684
Thereafter		282
Total undiscounted lease payments		9,323
Less: imputed interest		(2,366	)
Total lease liabilities	$	6,957

Operating lease liabilities, current		804
Operating lease liabilities, noncurrent		6,153
Total operating lease liabilities	$	6,957

Schedule of Future Minimum Lease Payments

Future minimum lease payments are as follows as of December 31, 2018 (in thousands):

Year ending December 31:
2019	$	1,453
2020		1,497
2021		1,542
2022		1,588
2023		1,635
Thereafter		1,966
Total future minimum lease payments	$	9,681

Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Activity under Stock Option Plan and Related Information

The following table summarizes activity under the Company’s stock option plan and related information (in thousands, except share and per share amounts):

	Number of Options Outstanding			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding at December 31, 2018		2,181,466		$	3.80		8.5	$	43,189
Options granted		578,000		$	22.50
Options exercised		(4,000	)	$	0.90			$	78
Options cancelled		—		$	—
Outstanding at March 31, 2019		2,755,466		$	7.73		8.6	$	30,395
Vested and exercisable at March 31, 2019		1,104,476		$	2.93		7.8	$	16,468

Summarizes of Activity of Nonvested Restricted Stock

The following summarizes the activity of nonvested restricted stock:

	Number of Shares
Nonvested—December 31, 2018		20,908
Vested		(15,681	)
Nonvested—March 31, 2019		5,227

Stock-Based Compensation Recognized

Total stock-based compensation recognized by function was as follows (in thousands):

	Three Months Ended March 31,
	2019		2018
Research and development	$	447	$	59
General and administrative		454		67
Total stock-based compensation expense	$	901	$	126

Unrecognized Stock-based Compensation Cost and Estimated Weighted Average Amortization Period

	Unrecognized Compensation Cost		Weighted Average Remaining Amortization Period (Years)
Employee options	$	12,243		2.9
Nonemployee options		2		0.2
Total unrecognized stock-based compensation expense	$	12,245

Fair Value of Employee Stock Options Granted is Calculated Using Black Scholes Option Pricing Model with Weighted Average Assumptions

The fair value of the employee stock options granted is calculated using the Black-Scholes option-pricing model with the following weighted-average assumptions:

	Three Months Ended March 31,
	2019		2018
Expected term (years)		5.9		6.0
Expected volatility	79.9 - 81.9%			82.0	%
Risk-free interest rate	2.4 - 2.6%			2.6	%
Expected dividend yield		—		—

Net Loss per Share (Tables)

3 Months Ended

Mar. 31, 2019

Earnings Per Share [Abstract]

Calculation of Basic and Diluted Net Loss per Share

The following table sets forth the calculation of basic and diluted net loss per share during the periods presented (in thousands, except share and per share data):

	Three Months Ended March 31,
	2019			2018
Numerator:
Net loss	$	(7,642	)	$	(4,947	)
Denominator:
Weighted-average shares of common stock outstanding		19,042,524			757,399
Net loss per share, basic and diluted	$	(0.40	)	$	(6.53	)

Anti-dilutive Outstanding Shares of Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share

	Three Months Ended March 31,
	2019		2018
Redeemable convertible preferred stock on an if converted basis		—		12,263,126
Stock options to purchase common stock		2,755,466		1,354,965
Common stock subject to future vesting		61,265		252,303
Total		2,816,731		13,870,394

Organization and Description of the Business - Additional Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) Segment	Dec. 31, 2018 USD ($)
Organization And Description Of Business [Line Items]
Number of operating segments \| Segment	1
Accumulated deficit \| $	$ 56,837	$ 49,195
Minimum
Organization And Description Of Business [Line Items]
Sufficient cash and cash equivalents available period term	12 months

Summary of Significant Accounting Policies - Additional Information (Details)	Jan. 01, 2018 USD ($)
Accounting Policies [Abstract]
Restricted cash amount deposited	$ 13,000

Summary of Significant Accounting Policies - Impact of Accounting Pronouncements Adoption on Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Operating lease ROU asset	$ 4,139	$ 4,236
Operating lease liabilities, current	804	774
Operating lease liabilities, noncurrent	$ 6,153	$ 6,364
Deferred rent, current			$ 354
Deferred rent, noncurrent			$ 2,548

Fair Value Measurements - Additional Information (Detail)	Mar. 31, 2019 USD ($)
Fair Value Disclosures [Abstract]
Transfers from Level 1 to Level 2	$ 0
Transfers from Level 2 to Level 1	$ 0

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	$ 101,014	$ 106,406
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	32,044	48,848
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	68,970	57,558
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	18,349	23,156
Money Market Funds \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	18,349	23,156
U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	13,695	25,692
U.S. Treasury Securities \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	13,695	25,692
Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	19,357	19,190
Commercial paper \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	19,357	19,190
Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	20,538	20,451
Corporate debt securities \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	20,538	20,451
Government agency bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	29,075	17,917
Government agency bonds \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial Assets	$ 29,075	$ 17,917

Available-for-Sale Securities - Summary of Available-For-Sale Securities Recorded in Cash and Cash Equivalents or Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 82,619
Fair Value	82,664
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	100,969	$ 106,427
Unrealized Gains	45	4
Unrealized Losses		(25)
Fair Value	101,014	106,406
Fair Value, Measurements, Recurring \| Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	18,349	23,156
Fair Value	18,349	23,156
Fair Value, Measurements, Recurring \| U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	13,691	25,698
Unrealized Gains	4
Unrealized Losses		(6)
Fair Value	13,695	25,692
Fair Value, Measurements, Recurring \| Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	19,342	19,203
Unrealized Gains	15
Unrealized Losses		(13)
Fair Value	19,357	19,190
Fair Value, Measurements, Recurring \| Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	20,521	20,449
Unrealized Gains	17	4
Unrealized Losses		(2)
Fair Value	20,538	20,451
Fair Value, Measurements, Recurring \| Government agency bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	29,066	17,921
Unrealized Gains	9
Unrealized Losses		(4)
Fair Value	$ 29,075	$ 17,917

Available-for-Sale Securities - Summary of Amortized Cost and Estimated Fair Value of Available-for-sale Securities by Contractual Maturity (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Marketable securities maturing, Amortized Cost:
In one year or less, Amortized Cost	$ 82,619
Total marketable securities, Amortized Cost	82,619
Marketable securities maturing, Estimated Fair Value:
In one year or less, Estimated Fair Value	82,664
Total marketable securities, Estimated Fair Value	$ 82,664

Balance Sheet Components - Property and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Total property and equipment	$ 5,987	$ 5,530
Less accumulated depreciation and amortization	(1,141)	(935)
Property and equipment, net	4,846	4,595
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	3,268	3,268
Furniture, Laboratory and Office Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	2,489	2,043
Computer Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 230	$ 219

Balance Sheet Components - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Depreciation expense	$ 0.2	$ 0.1

Balance Sheet Components - Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accrued preclinical and research costs	$ 1,128	$ 1,146
Accrued clinical costs	528	434
Accrued employee-related costs	554	736
Accrued professional services	90	138
Other	106	224
Total accrued liabilities	$ 2,406	$ 2,678

Operating Lease - Additional Information (Details) - South San Francisco, California [Member]	3 Months Ended
	Mar. 31, 2019 ft²
Area of lease	24,357
Lease Expiration Date	Feb. 28, 2025

Operating Lease - Details of Right-of-Use Assets and Lease Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Jan. 01, 2019
Lessee Disclosure [Abstract]
Cash paid for operating lease liabilities	$ 181
Remaining lease term	5 years 11 months 1 day
Discount rate	10.00%
2019	$ 1,095
2020	1,497
2021	1,542
2022	1,588
2023	1,635
2024	1,684
Thereafter	282
Total undiscounted lease payments	9,323
Less: imputed interest	(2,366)
Total lease liabilities	6,957
Operating lease liabilities, current	804	$ 774
Operating lease liabilities, noncurrent	6,153	$ 6,364
Total operating lease liabilities	$ 6,957

Operating Lease - Schedule of Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Year ending December 31:
2019	$ 1,453
2020	1,497
2021	1,542
2022	1,588
2023	1,635
Thereafter	1,966
Total future minimum lease payments	$ 9,681

Stock-Based Compensation - Additional Information (Details) - USD ($)	3 Months Ended	10 Months Ended
	Mar. 31, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of options granted	578,000
Maximum number of shares issued upon exercise of incentive stock options	1,104,476	1,104,476
Common stock, shares issued	19,118,421	19,118,421	19,114,421
Weighted average grant date fair value of options granted	$ 22.50
Total intrinsic value of exercised stock options	$ 78,000
Early Exercise Stock Purchase Agreements
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	36 months
Nonvested common shares outstanding	56,038	56,038	68,349
Percentage of underlying option shares whose right to purchase lapses after applicable vesting commencement date	25.00%
Right to repurchase shares underlying shares	0.0208
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	4 years
Nonvested common shares outstanding	5,227	5,227	20,908
2018 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for future issuance	2,179,866	2,179,866
Number of shares will increase automatically through every year on specified date	--01-01
Number of shares increase automatically continuing through maximum period	beginning on January 1, 2019 and continuing through and including January 1, 2028
2018 Equity Incentive Plan \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum number of shares available for issuance	1,600,692	1,600,692
Increase in number of shares reserved for issuance as percentage of total number of shares of capital stock outstanding on last date of preceding calendar year	5.00%
2018 Equity Incentive Plan \| Maximum [Member] \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum number of shares available for issuance	4,000,000	4,000,000
2018 Equity Incentive Plan \| Employee options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of options granted		663,621
2018 Equity Incentive Plan \| Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of RSU granted		10,189
2018 Equity Incentive Plan \| Incentive Stock Options \| Maximum [Member] \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum number of shares issued upon exercise of incentive stock options	12,500,000	12,500,000
2015 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	4 years
Options to purchase of common stock outstanding	2,091,845	2,091,845
Weighted average grant date fair value of options granted	$ 15.79
Total intrinsic value of exercised stock options	$ 78,000
2015 Equity Incentive Plan \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted expiry period	10 years
2018 Employee Stock Purchase Plan \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for future issuance	391,144	391,144		200,000
Common stock, shares issued	0	0
Authorized an offering beginning period	Nov. 16, 2018
Authorized an offering ending period	May 15, 2019
2018 Employee Stock Purchase Plan \| Maximum [Member] \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of annual increase in number of common stock shares	1.00%
Annual increase in number of common stock shares	375,000
Option price per share of common stock at fair market value	85.00%

Stock-Based Compensation - Summary of Activity under Stock Option Plan and Related Information (Details) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of options outstanding, Outstanding, Beginning balance \| shares	2,181,466
Number of options outstanding, Options granted \| shares	578,000
Number of options outstanding, Options exercised \| shares	(4,000)
Number of options outstanding, Outstanding, Ending balance \| shares	2,755,466	2,181,466
Number of options outstanding, options vested and exercisable \| shares	1,104,476
Weighted average exercise price, Outstanding, Beginning balance \| $ / shares	$ 3.80
Weighted average exercise price, Options granted \| $ / shares	22.50
Weighted average exercise price, Options exercised \| $ / shares	0.90
Weighted average exercise price, Outstanding, Ending balance \| $ / shares	7.73	$ 3.80
Weighted average exercise price, options vested and exercisable \| $ / shares	$ 2.93
Weighted average remaining contractual term, Outstanding, Beginning balance	8 years 8 months 12 days	8 years 8 months 12 days
Weighted average remaining contractual term,options vested and exercisable	7 years 10 months 24 days
Aggregate intrinsic value, Outstanding, Beginning balance \| $	$ 43,189
Aggregate intrinsic value, Options exercised \| $	78
Aggregate intrinsic value, Outstanding, Ending balance \| $	30,395	$ 43,189
Aggregate intrinsic value, options vested and exercisable \| $	$ 16,468

Stock-Based Compensation - Summarizes of Activity of Nonvested Restricted Stock (Details) - Restricted Stock	3 Months Ended
	Mar. 31, 2019 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Nonvested beginning balance	20,908
Number of Shares Outstanding, Vested	(15,681)
Nonvested ending balance	5,227

Stock-Based Compensation - Stock-Based Compensation Recognized (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 901	$ 126
Research and Development Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	447	59
General and Administrative Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 454	$ 67

Stock-Based Compensation - Unrecognized Stock-based Compensation Cost and Estimated Weighted Average Amortization Period (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Unrecognized compensation cost	$ 12,245
Employee options
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Unrecognized compensation cost	$ 12,243
Weighted average remaining amortization period (years)	2 years 10 months 24 days
Nonemployee options
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Unrecognized costs of unvested options awarded	$ 2
Weighted average remaining amortization period (years)	2 months 12 days

Stock-Based Compensation - Fair Value of Employee Stock Options Granted is Calculated Using Black Scholes Option Pricing Model with Weighted Average Assumptions (Details)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Expected term (years)	5 years 10 months 24 days	6 years 1 month 6 days
Expected volatility		82.00%
Expected volatility, Minimum	79.90%
Expected volatility, Maximum	81.90%
Risk-free interest rate		2.60%
Risk-free interest rate, Minimum	2.40%
Risk-free interest rate, Maximum	2.60%

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income tax provision	$ 0	$ 0
Research and development tax credit as a reduction of payroll expenses	$ 93,000

Net Loss Per Share - Calculation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net loss	$ (7,642)	$ (4,947)
Denominator:
Weighted-average shares of common stock outstanding	19,042,524	757,399
Net loss per common share, basic and diluted	$ (0.40)	$ (6.53)

- Definition

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

+ References + Details

Net Loss Per Share - Anti-dilutive Outstanding Shares of Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share (Details) - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	2,816,731	13,870,394
Redeemable Convertible Preferred Stock on an if Converted Basis
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share		12,263,126
Stock Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	2,755,466	1,354,965
Common stock subject to future vesting [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	61,265	252,303