Principia Biopharma (PRNB) 10-Q 6 Aug 20 2020 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2020	Jul. 31, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q2
Trading Symbol	PRNB
Entity Registrant Name	Principia Biopharma Inc.
Entity Central Index Key	0001510487
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		33,208,537
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Title of 12(b) Security	Common Stock, Par Value $0.0001 Per Share
Security Exchange Name	NASDAQ
Entity File Number	001-38653
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-3487603
Entity Address, Address Line One	220 East Grand Avenue
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	416-7700
Document Quarterly Report	true
Document Transition Report	false

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 170,486	$ 39,537
Short-term marketable securities	146,047	276,043
Prepaid expenses and other current assets	52,889	4,165
Total current assets	369,422	319,745
Property and equipment, net	9,002	9,687
Operating lease right-of-use asset	6,483	0
Restricted cash	567	567
Long-term marketable securities	0	52,257
Other long-term assets	480	480
Total assets	385,954	382,736
Current liabilities:
Accounts payable	1,240	1,850
Deferred rent, current portion	0	1,181
Operating lease liability, current portion	1,896	0
Accrued research and development liabilities	12,744	5,791
Accrued other liabilities	936	602
Accrued compensation	4,897	6,715
Total current liabilities	21,713	16,139
Long-term deferred rent	0	7,619
Long-term operating lease liability	12,801	0
Commitments and contingencies (Note 7)
Stockholders' equity
Common stock, $0.0001 par value, 500,000,000 shares authorized at June 30, 2020 and December 31, 2019; 33,161,289 and 32,950,836 shares issued and outstanding at June 30, 2020 and December 31, 2019, respectively	3	3
Additional paid-in capital	558,538	544,709
Accumulated other comprehensive income	331	9
Accumulated deficit	(207,432)	(185,743)
Total stockholders' equity	351,440	358,978
Total liabilities and stockholders’ equity	$ 385,954	$ 382,736

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Jun. 30, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	33,161,289	32,950,836
Common stock, shares outstanding	33,161,289	32,950,836

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Revenue	$ 50,000	$ 30,000	$ 50,000	$ 35,160
Operating expenses:
Research and development	30,888	18,718	57,630	34,241
General and administrative	9,231	5,233	16,600	9,740
Total operating expenses	40,119	23,951	74,230	43,981
Income (loss) from operations	9,881	6,049	(24,230)	(8,821)
Other income (expense), net	199	(42)	199	(41)
Interest income	730	1,108	2,342	2,290
Net income (loss)	$ 10,810	$ 7,115	$ (21,689)	$ (6,572)
Net income (loss) per share
Basic	$ 0.33	$ 0.30	$ (0.66)	$ (0.28)
Diluted	$ 0.31	$ 0.28	$ (0.66)	$ (0.28)
Weighted-average shares used to calculate net income (loss) per share
Basic	33,087,340	23,927,172	33,040,547	23,896,788
Diluted	35,311,938	25,792,101	33,040,547	23,896,788

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 10,810	$ 7,115	$ (21,689)	$ (6,572)
Other comprehensive income:
Net unrealized gain on available-for-sale securities	266	90	322	239
Comprehensive income (loss)	$ 11,076	$ 7,205	$ (21,367)	$ (6,333)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	ASC 606	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Accumulated DeficitASC 606
Balance at Dec. 31, 2018	$ 169,860	$ 456	$ 2	$ 302,393	$ (128)	$ (132,407)	$ 456
Balance, Shares at Dec. 31, 2018			23,865,451
Stock-based compensation expense	2,205			2,205
Exercise of stock options and vesting of early exercise shares	7			7
Exercise of stock options and vesting of early exercise shares, Shares			1,230
Unrealized gain on available-for-sale securities	$ 149				149
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201409Member
Net income (loss)	$ (13,687)					(13,687)
Balance at Mar. 31, 2019	158,990		$ 2	304,605	21	(145,638)
Balance, Shares at Mar. 31, 2019			23,866,681
Balance at Dec. 31, 2018	169,860	$ 456	$ 2	302,393	(128)	(132,407)	$ 456
Balance, Shares at Dec. 31, 2018			23,865,451
Unrealized gain on available-for-sale securities	239
Net income (loss)	(6,572)
Balance at Jun. 30, 2019	170,678		$ 2	309,088	111	(138,523)
Balance, Shares at Jun. 30, 2019			23,966,418
Balance at Mar. 31, 2019	158,990		$ 2	304,605	21	(145,638)
Balance, Shares at Mar. 31, 2019			23,866,681
Stock-based compensation expense	3,476			3,476
Exercise of stock options and vesting of early exercise shares	135			135
Exercise of stock options and vesting of early exercise shares, Shares			20,057
Issuance of common stock upon exercise of warrants, Shares			20,860
Issuance of shares under the Employee Stock Purchase Plan	872			872
Issuance of shares under the Employee Stock Purchase Plan, Shares			58,820
Unrealized gain on available-for-sale securities	90				90
Net income (loss)	7,115					7,115
Balance at Jun. 30, 2019	170,678		$ 2	309,088	111	(138,523)
Balance, Shares at Jun. 30, 2019			23,966,418
Balance at Dec. 31, 2019	$ 358,978		$ 3	544,709	9	(185,743)
Balance, Shares at Dec. 31, 2019	32,950,836		32,950,836
Stock-based compensation expense	$ 4,389			4,389
Exercise of stock options and vesting of early exercise shares	524			524
Exercise of stock options and vesting of early exercise shares, Shares			75,989
Unrealized gain on available-for-sale securities	56				56
Net income (loss)	(32,499)					(32,499)
Balance at Mar. 31, 2020	331,448		$ 3	549,622	65	(218,242)
Balance, Shares at Mar. 31, 2020			33,026,825
Balance at Dec. 31, 2019	$ 358,978		$ 3	544,709	9	(185,743)
Balance, Shares at Dec. 31, 2019	32,950,836		32,950,836
Unrealized gain on available-for-sale securities	$ 322
Net income (loss)	(21,689)
Balance at Jun. 30, 2020	$ 351,440		$ 3	558,538	331	(207,432)
Balance, Shares at Jun. 30, 2020	33,161,289		33,161,289
Balance at Mar. 31, 2020	$ 331,448		$ 3	549,622	65	(218,242)
Balance, Shares at Mar. 31, 2020			33,026,825
Stock-based compensation expense	6,947			6,947
Exercise of stock options and vesting of early exercise shares	1,181			1,181
Exercise of stock options and vesting of early exercise shares, Shares			102,605
Issuance of shares under the Employee Stock Purchase Plan	788			788
Issuance of shares under the Employee Stock Purchase Plan, Shares			31,859
Unrealized gain on available-for-sale securities	266				266
Net income (loss)	10,810					10,810
Balance at Jun. 30, 2020	$ 351,440		$ 3	$ 558,538	$ 331	$ (207,432)
Balance, Shares at Jun. 30, 2020	33,161,289		33,161,289

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Stockholders' Equity (Parenthetical) - $ / shares	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Statement Of Stockholders Equity [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001

Condensed Consolidated Statem_5

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Operating activities:
Net loss	$ (21,689)	$ (6,572)
Adjustments to reconcile net loss to net cash used in operating activities:
Amortization of discount on marketable securities	(433)	(962)
Realized gain on sales of marketable securities	(199)	0
Depreciation	1,010	835
Stock-based compensation	11,336	5,681
Deferred rent	0	220
Non-cash lease expense	272	0
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(48,724)	1,608
Deferred revenue	0	(5,160)
Accounts payable	(610)	(52)
Accrued liabilities	5,469	1,918
Lease liability	(858)	0
Net cash used in operating activities	(54,426)	(2,484)
Investing activities:
Purchases of property and equipment	(325)	(1,913)
Maturities of marketable securities	169,033	103,535
Sales of marketable securities	101,876	0
Purchases of marketable securities	(87,702)	(93,650)
Net cash provided by investing activities	182,882	7,972
Financing activities:
Proceeds from issuances of common stock upon exercise of options and participation in employee stock purchase plan	2,493	1,014
Net cash provided by financing activities	2,493	1,014
Net increase in cash, cash equivalents and restricted cash	130,949	6,502
Cash, cash equivalents and restricted cash at beginning of period	40,104	35,138
Cash, cash equivalents and restricted cash, at end of period	171,053	41,640
Supplemental disclosures of cash flow information
Non cash tenant improvement allowance used for leasehold improvements	0	8,324
Purchases of property and equipment accrued but not yet paid	$ 0	$ 172

Organization

Organization	6 Months Ended
	Jun. 30, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	1. Description of Business We, Principia Biopharma Inc. (“Principia”), are a late-stage biopharmaceutical company dedicated to bringing transformative therapies to patients with significant unmet medical needs in immune mediated diseases. We were incorporated on October 6, 2008, began operations in February 2011, and are headquartered in South San Francisco, California.

Significant Accounting Policies

Significant Accounting Policies	6 Months Ended
	Jun. 30, 2020
Accounting Policies [Abstract]
Significant Accounting Policies	2. Significant Accounting Policies Basis of Presentation The accompanying interim condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) and include the accounts of Principia and our wholly-owned Australian subsidiary. All intercompany accounts, transactions and balances have been eliminated. Certain information and note disclosures normally included in the financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to the applicable rules and regulations of the Securities and Exchange Commission (“SEC”). These interim condensed consolidated financial statements and related disclosures are unaudited, have been prepared on the same basis as the annual consolidated financial statements and, in the opinion of management, reflect all adjustments of a normal and recurring nature that are necessary for the fair presentation of our financial position and results of operations for the periods presented. The condensed consolidated balance sheets as of December 31, 2019 included herein were derived from audited consolidated financial statements as of that date. This quarterly report should be read in conjunction with our audited consolidated financial statements included in our Annual Report on Form 10-K filed with the SEC on March 10, 2020 (“2019 Annual Report”). Use of Estimates The preparation of our financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, as well as related disclosure of contingent assets and liabilities. Significant estimates include amounts to determine the fair value of common stock-based awards, warrants, and other issuances, embedded derivatives, accruals for research and development costs and uncertain tax positions, and the estimated periods of performance used in the determination of collaboration revenues. We base our estimates on historical experience and on various other market specific and relevant assumptions that our management believes to be reasonable under the circumstances. Actual results could differ materially from our estimates. Concentration of Credit Risk and Other Risks and Uncertainties Financial instruments that potentially subject us to a significant concentration of credit risk consist primarily of cash, cash equivalents and marketable securities. Our cash and cash equivalents are maintained with a limited number of financial institutions in the United States. Cash amounts on deposit have exceeded and will continue to exceed federally insured limits Additionally, we have established guidelines regarding the diversification of our investments in approved instruments, their credit quality ratings and maturities. The guidelines are designed to preserve principal balances and provide liquidity. We are subject to a number of risks similar to other biopharmaceutical companies with development and pre-commercial operations, including, but not limited to, the need to obtain adequate additional funding, possible failure of current or future preclinical studies or clinical trials or delays in the progress of such trials, our reliance on third parties or partners to conduct our clinical trials, the need to obtain regulatory and marketing approvals for our drug candidates or to rely on partners to do so, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of our drug candidates, delays or problems in the supply of our drug candidates, loss of single source suppliers or failure to comply with manufacturing regulations, identifying other drug candidates, product development and the inherent uncertainty of clinical success, our right to develop and commercialize our drug candidates pursuant to the terms and conditions of the licenses granted to us, protection of proprietary technology, the ability to make or collect milestone, royalty or other payments due, or due to us, under any license or collaboration agreements, and the need to secure and maintain adequate manufacturing arrangements with third parties. If we do not successfully commercialize or partner any of our drug candidates, we will be unable to generate product revenue or achieve profitability. Cash and Cash Equivalents We consider all highly liquid financial instruments with original maturities of 90 days or less at the date of purchase to be cash equivalents. Cash equivalents are stated at fair value. Marketable Securities We carry marketable securities consisting primarily of money market funds, U.S. Treasury securities and obligations of government-sponsored enterprises and corporate bonds and commercial paper. Marketable securities with maturities greater than 90 days at the time of purchase and that mature less than one year from the consolidated balance sheet date are classified as short-term. Marketable securities with a maturity date greater than one year at each balance sheet date are classified as long-term. All of our marketable securities are considered available-for-sale and carried at estimated fair values on the condensed consolidated balance sheets. Unrealized gains or losses, when such losses are not deemed to be credit-related, are excluded from net income and reported in accumulated other comprehensive income (loss) as a separate component of stockholders’ equity on the condensed consolidated balance sheets. Credit-related losses, if any, are recognized in earnings and limited to the difference between the security's fair value and its amortized cost basis. Realized gains and losses from sales of marketable securities are included in other income (expense), net. The cost of securities sold is based on the specific-identification method. Interest earned on marketable securities is included in interest income. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts and such amortization and accretion are included as a component of interest income. Restricted Cash As of June 30, 2020 and December 31, 2019, we had $0.6 million in long-term restricted cash for a lease security deposit. This amount is separated from cash and cash equivalents on the condensed consolidated balance sheets. Segments We have one operating segment. Our chief operating decision maker, our President and Chief Executive Officer, manages our operations on a consolidated basis in assessing performance and allocating resources. Leases We enter into operating lease agreements for our laboratory and office facilities. Effective January 1, 2020, we adopted Accounting Standards Update (“ASU”) No. 2016-02, Leases (Topic 842) as of the adoption date We determine whether an arrangement is or contains a lease at inception. For operating leases, we recognize a ROU asset and operating lease liability in our condensed consolidated balance sheets. ROU assets represent our right to use an underlying asset during the lease term and operating lease liabilities represent our obligation to make lease payments arising from the lease. We recognize ROU assets and lease liabilities at lease commencement based on the present value of lease payments due over the lease term, and adjust ROU assets for any lease prepayments made or lease incentives received. In determining the present value of lease payments, we use the rate implicit in the lease, if known, or we use an estimated incremental borrowing rate based on the information available at the lease commencement date. We recognize lease expense on a straight-line basis over the lease term. Leases with an initial term of 12 months or less are not recorded on the balance sheet. We do not assume renewals in our determination of the lease term unless the renewals are deemed to be reasonably assured at lease inception. Revenue Recognition Effective January 1, 2019, we adopted ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) , or ASC 606, using the modified retrospective approach. Under this approach, we recorded a cumulative adjustment to decrease accumulated deficit and deferred revenue by $0.4 million as of the adoption date. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. We only apply the five-step model to contracts when it is probable that we will collect the consideration we are entitled to in exchange for the goods and services we transfer to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, we assess the goods or services promised within each contract, determine those that are performance obligations and assess whether each promised good or service is distinct. We then recognize as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied . We have entered into licensing and collaboration agreements that are within the scope of ASC 606. Licenses of Intellectual Property: If the license to our intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, we recognize revenues from non-refundable, upfront fees allocated to the license when the license is transferred to the licensee and the licensee is able to use and benefit from the license. For licenses that are bundled with other promised goods or services, we utilize judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue from non-refundable, upfront fees. We evaluate the measure of progress each reporting period and, if necessary, adjust the measure of performance and related revenue recognition . Milestone Payments: At the inception of each arrangement that includes development, regulatory or commercial milestone payments, we evaluate whether the milestones are considered probable of being reached and estimate the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant reversal of cumulative revenue would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within our control or that of our licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received or the underlying activity has been completed. The transaction price is then allocated to each performance obligation on a relative stand-alone selling price basis, for which we recognize revenue as or when the performance obligations under the contract are satisfied. At the end of each subsequent reporting period, we re-evaluate the probability of achievement of such development milestones and any related constraint, and if necessary, adjust our estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect collaboration revenue in the period of adjustment. Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, we recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, we have not recognized any royalty revenue resulting from any of our licensing arrangements . Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in stockholders’ equity (deficit) of a business enterprise during a period, resulting from transactions from non-owner sources, and consists primarily of unrealized gains or losses related to our available-for-sale marketable securities, which are carried at estimated fair values on the consolidated balance sheets . Net Income (Loss) per Share Basic net income (loss) per share is calculated by dividing our net income (loss) by the weighted-average number of shares of common stock outstanding for the period . Diluted net income (loss) per share includes the effect of potentially dilutive securities, which include outstanding warrants and stock options if the effect of their inclusion would be dilutive. In periods of net loss, diluted net loss per share is the same as basic net loss per share as the inclusion of potentially dilutive securities in the calculation would be anti-dilutive . Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board, or FASB, or other standard setting bodies and adopted by us as of the specified effective date. Unless otherwise discussed, the impact of recently issued standards that are not yet effective will not have a material impact on our condensed consolidated financial statements upon adoption. Under the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”), we meet the definition of an emerging growth company, and have elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act . We will remain an emerging growth company until December 31, 2020 . Recently Adopted Accounting Standards or Updates In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases (Topic 842): Targeted Improvements transition as a adjustment In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The updated accounting guidance requires changes to the recognition of credit losses on financial instruments not accounted for at fair value through net income. Specifically, the ASU requires companies to recognize an allowance for credit losses on available-for-sale debt securities that are impaired as a result of credit-related factors and replaces the prior other-than-temporary impairment model. We adopted this ASU effective January 1, 2020 and the adoption did not have a material impact on our condensed consolidated financial statements and related disclosures. In June 2018, the FASB issued ASU No. 2018-07, Compensation–Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting . This ASU simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. We adopted this ASU effective January 1, 2020 and the adoption did not have a material impact on our condensed consolidated financial statements and related disclosures. Our share-based payments related to nonemployees are insignificant. 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. We adopted this ASU effective January 1, 2020 and the adoption did not have a material impact on our condensed consolidated financial statements and related disclosures

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements Financial assets and liabilities are recorded at fair value. We determine fair value using the fair value hierarchy, which establishes three levels of inputs that may be used to measure fair value, as follows : Level 1 inputs include quoted prices in active markets for identical assets or liabilities . Level 2 inputs include observable inputs other than Level 1 inputs, such as quoted prices for similar assets or liabilities in active markets; quoted prices for identical or similar assets or liabilities in markets that are not active; and other inputs that are observable or can be corroborated by observable market data for substantially the full term of the asset or liability . Level 3 inputs include unobservable inputs that are supported by little or no market activity and are significant to the fair value of the underlying asset or liability. Such inputs reflect our best estimate of what market participants would use in pricing the asset or liability at the reporting date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model . 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. Our assessment of the significance of a particular input to the fair value measurement in its entirety requires our management to make judgments and consider factors specific to the asset or liability . Assets and liabilities measured at fair value on a recurring basis as of June 30, 2020 and December 31, 2019 were as follows (in thousands): June 30, 2020 Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 167,679 $ — $ — $ 167,679 Short-term marketable securities Corporate commercial paper — 20,239 — 20,239 Corporate debt securities — 66,742 — 66,742 Government‑sponsored enterprise securities — 9,106 — 9,106 U.S. Treasury securities — 49,960 — 49,960 Total $ 167,679 $ 146,047 $ — $ 313,726 December 31, 2019 Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 17,664 $ — $ — $ 17,664 Corporate commercial paper — 6,983 — 6,983 Corporate debt securities — 10,204 — 10,204 Short-term marketable securities Corporate commercial paper — 95,088 — 95,088 Corporate debt securities — 154,690 — 154,690 Government‑sponsored enterprise securities — 26,265 — 26,265 Long-term marketable securities Corporate debt securities — 14,202 — 14,202 Government‑sponsored enterprise securities — 38,055 — 38,055 Total $ 17,664 $ 345,487 $ — $ 363,151 The carrying amounts of accounts payable and accrued liabilities approximate their fair values due to their short-term maturities. Our Level 2 securities are valued using third-party pricing sources. The pricing services utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly. There were no transfers of assets or liabilities between the fair value measurement levels during the six months ended June 30, 2020 and 2019.

Cash Equivalents and Marketable

Cash Equivalents and Marketable Securities	6 Months Ended
	Jun. 30, 2020
Investments Debt And Equity Securities [Abstract]
Cash Equivalents and Marketable Securities	4. Cash Equivalents and Marketable Securities Cash equivalents and marketable securities consisted of the following as of June 30, 2020 and December 31, 2019 (in thousands): June 30, 2020 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents Money market funds $ 167,679 $ — $ — $ 167,679 Total cash equivalents 167,679 — — 167,679 Short-term marketable securities Corporate commercial paper 20,171 68 — 20,239 Corporate debt securities 66,468 274 — 66,742 Government‑sponsored enterprise securities 9,027 79 — 9,106 U.S. Treasury securities 49,960 1 (1 ) 49,960 Total short-term marketable securities 145,626 422 (1 ) 146,047 Total $ 313,305 $ 422 $ (1 ) $ 313,726 December 31, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents Money market funds $ 17,664 $ — $ — $ 17,664 Corporate commercial paper 6,982 1 — 6,983 Corporate debt securities 10,205 — (1 ) 10,204 Total cash equivalents 34,851 1 (1 ) 34,851 Short-term marketable securities Corporate commercial paper 95,061 36 (9 ) 95,088 Corporate debt securities 154,678 54 (42 ) 154,690 Government‑sponsored enterprise securities 26,231 34 — 26,265 Total short-term marketable securities 275,970 124 (51 ) 276,043 Long-term marketable securities Corporate debt securities 14,194 8 — 14,202 Government‑sponsored enterprise securities 38,037 18 — 38,055 Total long-term marketable securities 52,231 26 — 52,257 Total $ 363,052 $ 151 $ (52 ) $ 363,151 All our marketable securities are considered available-for-sale. In April 2020, we decided to sell certain of our short-term and long-term marketable securities and As of June 30, 2020, we did not have the intent, and management had not made any decisions, to sell our investments that were in an unrealized loss position and it is not more likely than not that we will be required to sell these investments before recovery of their amortized cost basis. We anticipate that we will recover the entire amortized cost basis of such securities and have determined that a credit loss did not exist as of June 30, 2020. In July 2020, we decided to sell certain of our short-term marketable securities and received approximately $17.0 million in gross proceeds, realizing a net gain of $45,000.

Balance Sheet Components

Balance Sheet Components	6 Months Ended
	Jun. 30, 2020
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	5. Balance Sheet Components The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheets that sum to the total of the amounts shown in the condensed consolidated statements of cash flows (in thousands): June 30, 2020 December 31, 2019 June 30, 2019 December 31, 2018 Cash and cash equivalents $ 170,486 $ 39,537 $ 41,073 $ 34,489 Restricted cash, current — — — 82 Restricted cash, non-current 567 567 567 567 Total $ 171,053 $ 40,104 $ 41,640 $ 35,138 Property and equipment as of June 30, 2020 and December 31, 2019 consisted of the following (in thousands): June 30, 2020 December 31, 2019 Laboratory equipment $ 2,313 $ 2,007 Computer equipment 297 289 Furniture and fixtures 1,560 1,549 Leasehold improvements 8,324 8,324 12,494 12,169 Less accumulated depreciation and amortization (3,492 ) (2,482 ) Total $ 9,002 $ 9,687 Prepaid expenses and other current assets as of June 30, 2020 and December 31, 2019 consisted of the following (in thousands): June 30, 2020 December 31, 2019 Collaboration receivable from Sanofi $ 50,000 $ — Other accounts receivable 492 1,272 Prepaid expenses 2,397 2,893 Total $ 52,889 $ 4,165 The collaboration receivable from Sanofi relates to a $50.0 million clinical development milestone that was achieved in June 2020.

License and Collaboration Agree

License and Collaboration Agreements	6 Months Ended
	Jun. 30, 2020
Research And Development [Abstract]
License and Collaboration Agreements	6. License and Collaboration Agreements Sanofi In November 2017, we entered into a strategic collaboration agreement with Sanofi, or the Sanofi Agreement, for an exclusive license to PRN2246/SAR442168 and backup molecules for development in multiple sclerosis (“MS”) and other central nervous system (“CNS”) diseases. Under the Sanofi Agreement, we have completed the Phase 1 trials and Sanofi is taking on all further development activities. We and Sanofi each have been responsible for certain early development costs, and Sanofi is responsible for all further development and commercialization costs, subject to our Phase 3 option described below . Sanofi has an exclusive license for PRN2246/SAR442168 and its backups for the CNS field, which includes indications of the CNS, retina and ophthalmic nerve. We have agreed not to develop other Bruton Tyrosine Kinase (“BTK”) inhibitors within the CNS field, and Sanofi has agreed not to develop PRN2246/SAR442168 or its backups for any indications outside the CNS field. In the event we cease all development and commercialization of our other BTK inhibitors or unilaterally decide to offer Sanofi a field expansion, Sanofi could expand its field upon a field expansion payment to us as well as potential milestone payments and royalties within the expanded field . In December 2017, we received a $40.0 million upfront payment from Sanofi. In May 2018, we amended the Sanofi Agreement to include additional activities under the early development plan and to modify the definition of one of the milestone payments. Pursuant to the amendment, we received a $10.0 million payment in July 2018 for the completion of a major part of the Phase 1 trial. In August 2018, we received a $5.0 million payment for the successful completion of preclinical toxicology studies. In November 2018, we received $10.0 million in additional payments from Sanofi for successful development activities of PRN2246/SAR442168 related to the early development plan. In June 2019, we received a $30.0 million milestone payment from Sanofi for the initiation of the P hase 2b clinical trial of PRN2246/ SAR442168. In June 2020, we achieved a $ 50.0 million clinical development milestone upon dosing of the first patient in Sanofi’s Phase 3 clinical trial of PRN2246/SAR442168 in relapsing MS and recognized $ 50.0 million of revenue. Payment of this milestone was received in August 2020. Under the amended Sanofi Agreement, we may receive development, regulatory and commercial milestone payments of up to an aggregate of $765.0 million, as well as royalties up to the mid-teens. We have an option to fund a portion of Phase 3 development costs in return for, at our option, either a profit and loss sharing arrangement within the United States, or an additional worldwide royalty that would result in royalties up to the high-teens. The additional royalty option would only be available if we develop rilzabrutinib for major enumerated indications overseen by the Division of Pulmonary, Allergy and Rheumatology Products of the U.S. Food and Drug Administration (the “FDA”) or if we experience a change in control involving certain Sanofi competitors . Royalties are subject to specified reductions and are payable, on a product-by-product and country-by-country basis until the later of the date that all of our patent rights that claim a composition of matter of such product expire in such country, the date of expiration of regulatory exclusivity for such product in such country, or the date that is ten years from the first commercial sale of such product in such country . We identified the following performance obligations under the Sanofi Agreement: (i) granting a license of rights to PRN2246/SAR442168, (ii) transferring of technology (know-how) related to PRN2246/SAR442168, and (iii) providing research and development services related to our responsibilities under the early development plan. We concluded that the delivered license is not distinct at inception of the arrangement due to our proprietary expertise with respect to the licensed compound and related developmental participation under the agreement, which is required for Sanofi to fully realize the value from the delivered license . Therefore, we combined these performance obligations as one unit of accounting and recognized the $40.0 million upfront payment and an aggregate of $25.0 million in milestone payments over the performance period under the Sanofi Agreement, which ended in December 2018 . On January 1, 2019, we adopted ASC 606 using the modified retrospective approach . Upon adoption, we concluded that there was no adjustment necessary to revenue recognized through December 31, 2018 under ASC 606 for the Sanofi Agreement as all deliverables under ASC 605 and all performance obligations under ASC 606 had been completed as of December 31, 2018 . The transaction price under ASC 606 was determined to be $65.0 million, which included the $40.0 million upfront payment we received from Sanofi in December 2017 and an aggregate of $25.0 million in milestone payments received in 2018. All potential future milestones and other payments were considered constrained at the inception of the Sanofi Agreement since we could not conclude it is probable that a significant reversal in the amount recognized will not occur. Upon adoption of ASC 606, we also determined that variable considerations related to certain milestones not previously recognized were constrained because they were not probable, due to the inherent uncertainty related to the achievement of these milestones . In May 2019, we achieved a clinical development milestone upon Sanofi’s initiation of its Phase 2b clinical trial of PRN2246/SAR442168 and Sanofi was obligated to make a $30.0 million milestone payment to us. As the amount due for the clinical development milestone was no longer constrained, we increased the transaction price under ASC 606 by $30.0 million from inception through June 30, 2019. As of June 30, 2019, we fully recognized the $30.0 million milestone payment to us as revenue as all performance obligations had been satisfied in previous periods . In June 2020, we achieved a clinical development milestone upon dosing of the first patient in Sanofi’s Phase 3 clinical trial of PRN2246/SAR442168 in relapsing MS and Sanofi was obligated to make a $50.0 million milestone payment to us. As the amount due for the clinical development milestone was no longer constrained, we increased the transaction price under ASC 606 by $50.0 million from inception through June 30, 2020. As of June 30, 2020, we fully recognized the $50.0 million milestone payment to us as revenue as all performance obligations had been satisfied in previous periods. We will re-evaluate the transaction price in each reporting period relating to potential future milestones as uncertain circumstances are resolved or other changes in events occur. For the three and six months ended June 30, 2020, we recognized $50.0 million in revenue related to the Sanofi Agreement and recorded a corresponding collaboration receivable included within prepaid expenses and other current assets on our condensed consolidated balance sheet as of June 30, 2020. Payment from Sanofi was received in August 2020. For the three and six months ended June 30, 2019, we recognized $30.0 million in revenue related to the Sanofi Agreement. There was no deferred revenue related to the Sanofi Agreement at June 30, 2020 and December 31, 2019. AbbVie In June 2017, we entered into a collaboration agreement with AbbVie, or the AbbVie Agreement, to research and develop oral immunoproteasome inhibitors and received an upfront payment of $15.0 million. Prior to the adoption of ASC 606, we concluded under ASC 605 that the $15.0 million received related to a combined unit of accounting and was required to be recognized as revenue ratably over the performance period, which was estimated to continue through the fourth quarter of 2019. recognized based on a measurement of progress toward the completion of the performance obligation of providing research services for two years, subject to an extension for up to six months. Based on this methodology, we concluded that under ASC 606, approximately $9.8 million in revenue should be recognized through December 31, 2018, as compared to $ 9.4 million under ASC 605. We recorded a cumulative adjustment to decrease accumulated deficit and deferred revenue by $ 0.4 million as of the adoption date . In March 2019, we and AbbVie agreed to conclude the collaboration and as of the date of termination, there were no further financial obligations between us. We recognized the remaining balance of the transaction price of $5.2 million in revenue during the three months ended March 31, 2019, upon termination of the AbbVie Agreement. No revenue was recognized from the AbbVie Agreement in subsequent periods as the agreement had been terminated . University of California License Agreements In November 2009 and September 2011, we entered into license agreements with the Regents of the University of California, or the Regents, which were subsequently amended and restated at various dates (the “UC Agreements”). Under the UC Agreements, the Regents have granted to us exclusive, worldwide licenses, with the right to grant sublicenses, under the Regents’ patent rights in certain patent applications to make, use, sell, offer for sale, and import products and services and practice methods covered by such patent applications in all fields of use. We have paid the Regents license fees of $40,000 in total under the UC Agreements and are required to pay annual license maintenance fees totaling $40,000 prior to launching any licensed product. We may be obligated to make one-time regulatory and development milestone payments under the UC Agreements for future drug candidates and are obligated to pay tiered royalty payments in the low single digits on net sales of the licensed products. Additionally, we are obligated to pay the Regents payments on non-royalty licensing revenue we receive from our sub-licensees for products covered by UC patents. Pursuant to the UC Agreements, we also made IPO milestone payments to the Regents totaling approximately $140,000 in October 2018. Under the UC Agreements, we are required to diligently proceed with the development, manufacture, regulatory approval, and sale of licensed products which include obligations to meet certain development-stage milestones within specified periods of time and to market the resulting licensed products in sufficient quantity to meet market demand. We have the right and option to extend the date by which we must meet any milestone in one year extensions by paying an extension fee for second and subsequent extension, provided we can demonstrate we made diligent efforts to meet the milestone.

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies Leases Our corporate headquarters are located in South San Francisco, California . In April 2018, we signed a lease (the “Lease Agreement”) for approximately 47,500 square feet of office, research and development and laboratory space with occupancy commencing on February 1, 2019 for a seven-year seven-year Pursuant to the Lease Agreement, we provided a letter of credit to the landlord for $ 0.6 million which was recorded as long-term restricted cash at June 30, 2020 and December 31, 2019 . The Lease Agreement allows for a landlord-provided tenant improvement allowance of up to $7.1 million to be applied to the cost of construction of tenant improvements to the new leased premises . The Lease Agreement also provides, and we have utilized, an additional tenant improvement allowance of up to $1.2 million to be applied to the costs of tenant improvements . We utilized the aggregate amount of allowances available to us and have recorded $8.3 million of tenant improvement costs as leasehold improvements in our condensed consolidated balance sheets, with a corresponding lease incentive obligation which was classified as deferred rent as of December 31, 2019. On January 1, 2020, we adopted ASC 842 and recorded an operating lease ROU asset and lease liability in our condensed consolidated balance sheet related to the Lease Agreement. Upon adoption, we recorded an operating lease ROU asset of $6.8 million, an operating lease liability of $15.6 million, and eliminated our existing deferred rent balance of $8.8 million. There was no impact to accumulated deficit upon adoption. We recognize ROU assets and lease liabilities based on the present value of lease payments due over the lease term, and adjust ROU assets for any lease prepayments made or lease incentives received. In determining the present value of remaining lease payments for our corporate headquarters lease , we used an estimated incremental borrowing rate based on the information available at the adoption date. As of June 30, 2020, the remaining lease term related to the Lease Agreement was 5.6 years 9 %. Cash paid for amounts included in the measurement of our lease liability for the six months ended June 30, 2020 was $ 1.5 We recorded rent expense of $0.7 million and $1.3 million for the three and six months ended June 30, 2020, respectively. We recorded rent expense of $0.7 million and $1.4 million for the three and six months ended June 30, 2019, respectively. We recognize lease expense on a straight-line basis over the lease term . Where leases contain escalation clauses, rent abatements, and/or concessions such as rent holidays and landlord or tenant incentives or allowances, we apply them in the determination of straight-line rent expense over the lease term. We amortize l easehold improvements over the shorter of their estimated useful life or the term of the lease. We do not assume renewals in our determination of the lease term unless the renewals are deemed by management to be reasonably assured at lease inception . We have determined that our lease related to the Lease Agreement commenced on August 1, 2018 , when we had right to use or control physical access to the new leased premises . The following table reconciles future lease payments for the periods presented to our operating lease liability as of June 30, 2020 (in thousands): Year ended December 31, 2020 (remaining 6 months) $ 1,550 2021 3,193 2022 3,296 2023 3,403 2024 3,514 2025 and beyond 3,934 Total lease payments 18,890 Present value adjustment (4,193 ) Operating lease liability at June 30, 2020 $ 14,697 Indemnifications We are required to recognize a liability for the fair value of any obligations we assume upon the issuance of a guarantee. We have certain agreements with licensors, licensees, collaborators and service providers that contain indemnification provisions. In such provisions, we typically agree to indemnify the licensor, licensee collaborator or service provider against certain types of third party claims. The maximum amount of the indemnifications is usually not limited. We accrue for known indemnification issues when a loss is probable and can be reasonably estimated. There were no accruals for expenses related to indemnifications for any periods presented . We indemnify each of our officers and directors, as well as certain employees, for certain events or occurrences, subject to certain limitations, while the officer, director or employee is or was serving at our request in such capacity, as permitted under Delaware law and in accordance with our certificate of incorporation, our bylaws and certain indemnification agreements between us and each of our directors, officers and applicable employees. The term of the indemnification period lasts as long as an officer, a director or applicable employee may be subject to any proceeding arising out of acts or omissions of such officer, director or applicable employee in such capacity. The maximum amount of potential future indemnification is unlimited; however, we currently hold director and officer liability insurance. This insurance allows the transfer of risk associated with our exposure and may enable us to recover a portion of any future amounts paid with regards to our officers and directors. We believe that the fair value of these indemnification obligations is minimal. Accordingly, we have not recognized any liabilities relating to these obligations for any period presented .

Stock-Based Compensation

Stock-Based Compensation	6 Months Ended
	Jun. 30, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	8. Stock-Based Compensation The following table summarizes our stock option activity under our stock plans and related information: Options Outstanding Weighted- Average Exercise Price Balance at December 31, 2019 3,887,780 $ 18.38 Options granted 1,498,120 $ 66.42 Options forfeited (26,201 ) $ 39.70 Options exercised (173,633 ) $ 9.57 Balance at June 30, 2020 5,186,066 $ 32.45 Vested and expected to vest 5,186,066 $ 32.45 Exercisable as of June 30, 2020 2,523,720 $ 12.20 At June 30, 2020 and December 31, 2019, 18,770 shares and 25,081 shares, respectively, related to the early exercise of stock options, The following table summarizes total stock-based compensation expense related to our 2018 Equity Incentive Plan, 2008 Equity Incentive Plan and 2018 Employee Stock Purchase Plan (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2020 2019 2020 2019 Research and development $ 3,850 $ 1,776 $ 6,193 $ 2,926 General and administrative 3,097 1,700 5,143 2,755 Total $ 6,947 $ 3,476 $ 11,336 $ 5,681 As of June 30, 2020, there was approximately $83.1 million of unamortized compensation cost related to stock option awards that is expected to be recognized as expense over a weighted-average period of approximately 3.2 years. The weighted-average remaining contractual term of options outstanding at June 30, 2020 was 8.0 years.

Net Income (Loss) per Share

Net Income (Loss) per Share	6 Months Ended
	Jun. 30, 2020
Earnings Per Share [Abstract]
Net Loss per Share	9. Net Income (Loss) per Share Basic net income (loss) per share is calculated by dividing our net income (loss) by the weighted-average number of shares of common stock outstanding for the period. In periods in which we have net income, diluted net income per share is calculated using the weighted-average number of shares of common stock outstanding and other dilutive securities. In periods of net loss, diluted net loss per share is the same as basic net loss per share as the inclusion of potentially dilutive securities in the calculation would be anti-dilutive . The following table presents a reconciliation of the numerators and denominators of the basic and diluted net income (loss) per share computations and the calculation of basic and diluted net income (loss) per share (in thousands, except share and per share data): Three Months Ended June 30, Six Months Ended June 30, 2020 2019 2020 2019 Numerator: Net income (loss) $ 10,810 $ 7,115 $ (21,689 ) $ (6,572 ) Denominator: Weighted-average common shares outstanding, basic 33,087,340 23,927,172 33,040,547 23,896,788 Effect of dilutive shares 2,224,598 1,864,929 — — Weighted-average shares outstanding used in per share calculation, diluted 35,311,938 25,792,101 33,040,547 23,896,788 Net income (loss) per share, basic $ 0.33 $ 0.30 $ (0.66 ) $ (0.28 ) Net income (loss) per share, diluted $ 0.31 $ 0.28 $ (0.66 ) $ (0.28 ) The following outstanding shares of common stock equivalents were excluded in the computation of diluted net income (loss) per share, because their effect would have been antidilutive for the periods presented : Three Months Ended June 30, Six Months Ended June 30, 2020 2019 2020 2019 Employee stock purchase plan 8,919 11,326 34,966 43,745 Warrants to purchase common stock 11,933 48,406 81,966 168,046 Common stock options issued and outstanding 3,041,438 2,390,056 5,186,066 4,124,358 Early exercised common stock subject to future vesting 18,770 29,460 18,770 29,460 Total 3,081,060 2,479,248 5,321,768 4,365,609

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2020
Income Tax Disclosure [Abstract]
Income Taxes	10. Income Taxes We did not record a provision for income taxes for the three months ended June 30, 2020 and 2019, because all of our taxable income will be fully offset by net operating losses generated in prior years. In addition, the deferred tax assets continue to be subject to a full valuation allowance.

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying interim condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) and include the accounts of Principia and our wholly-owned Australian subsidiary. All intercompany accounts, transactions and balances have been eliminated. Certain information and note disclosures normally included in the financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to the applicable rules and regulations of the Securities and Exchange Commission (“SEC”). These interim condensed consolidated financial statements and related disclosures are unaudited, have been prepared on the same basis as the annual consolidated financial statements and, in the opinion of management, reflect all adjustments of a normal and recurring nature that are necessary for the fair presentation of our financial position and results of operations for the periods presented. The condensed consolidated balance sheets as of December 31, 2019 included herein were derived from audited consolidated financial statements as of that date. This quarterly report should be read in conjunction with our audited consolidated financial statements included in our Annual Report on Form 10-K filed with the SEC on March 10, 2020 (“2019 Annual Report”).
Use of Estimates	Use of Estimates The preparation of our financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, as well as related disclosure of contingent assets and liabilities. Significant estimates include amounts to determine the fair value of common stock-based awards, warrants, and other issuances, embedded derivatives, accruals for research and development costs and uncertain tax positions, and the estimated periods of performance used in the determination of collaboration revenues. We base our estimates on historical experience and on various other market specific and relevant assumptions that our management believes to be reasonable under the circumstances. Actual results could differ materially from our estimates.
Concentration of Credit Risk and Other Risks and Uncertainties	Concentration of Credit Risk and Other Risks and Uncertainties Financial instruments that potentially subject us to a significant concentration of credit risk consist primarily of cash, cash equivalents and marketable securities. Our cash and cash equivalents are maintained with a limited number of financial institutions in the United States. Cash amounts on deposit have exceeded and will continue to exceed federally insured limits Additionally, we have established guidelines regarding the diversification of our investments in approved instruments, their credit quality ratings and maturities. The guidelines are designed to preserve principal balances and provide liquidity. We are subject to a number of risks similar to other biopharmaceutical companies with development and pre-commercial operations, including, but not limited to, the need to obtain adequate additional funding, possible failure of current or future preclinical studies or clinical trials or delays in the progress of such trials, our reliance on third parties or partners to conduct our clinical trials, the need to obtain regulatory and marketing approvals for our drug candidates or to rely on partners to do so, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of our drug candidates, delays or problems in the supply of our drug candidates, loss of single source suppliers or failure to comply with manufacturing regulations, identifying other drug candidates, product development and the inherent uncertainty of clinical success, our right to develop and commercialize our drug candidates pursuant to the terms and conditions of the licenses granted to us, protection of proprietary technology, the ability to make or collect milestone, royalty or other payments due, or due to us, under any license or collaboration agreements, and the need to secure and maintain adequate manufacturing arrangements with third parties. If we do not successfully commercialize or partner any of our drug candidates, we will be unable to generate product revenue or achieve profitability.
Cash and Cash Equivalents	Cash and Cash Equivalents We consider all highly liquid financial instruments with original maturities of 90 days or less at the date of purchase to be cash equivalents. Cash equivalents are stated at fair value.
Marketable Securities	Marketable Securities We carry marketable securities consisting primarily of money market funds, U.S. Treasury securities and obligations of government-sponsored enterprises and corporate bonds and commercial paper. Marketable securities with maturities greater than 90 days at the time of purchase and that mature less than one year from the consolidated balance sheet date are classified as short-term. Marketable securities with a maturity date greater than one year at each balance sheet date are classified as long-term. All of our marketable securities are considered available-for-sale and carried at estimated fair values on the condensed consolidated balance sheets. Unrealized gains or losses, when such losses are not deemed to be credit-related, are excluded from net income and reported in accumulated other comprehensive income (loss) as a separate component of stockholders’ equity on the condensed consolidated balance sheets. Credit-related losses, if any, are recognized in earnings and limited to the difference between the security's fair value and its amortized cost basis. Realized gains and losses from sales of marketable securities are included in other income (expense), net. The cost of securities sold is based on the specific-identification method. Interest earned on marketable securities is included in interest income. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts and such amortization and accretion are included as a component of interest income.
Restricted Cash	Restricted Cash As of June 30, 2020 and December 31, 2019, we had $0.6 million in long-term restricted cash for a lease security deposit. This amount is separated from cash and cash equivalents on the condensed consolidated balance sheets.
Segments	Segments We have one operating segment. Our chief operating decision maker, our President and Chief Executive Officer, manages our operations on a consolidated basis in assessing performance and allocating resources.
Leases	Leases We enter into operating lease agreements for our laboratory and office facilities. Effective January 1, 2020, we adopted Accounting Standards Update (“ASU”) No. 2016-02, Leases (Topic 842) as of the adoption date We determine whether an arrangement is or contains a lease at inception. For operating leases, we recognize a ROU asset and operating lease liability in our condensed consolidated balance sheets. ROU assets represent our right to use an underlying asset during the lease term and operating lease liabilities represent our obligation to make lease payments arising from the lease. We recognize ROU assets and lease liabilities at lease commencement based on the present value of lease payments due over the lease term, and adjust ROU assets for any lease prepayments made or lease incentives received. In determining the present value of lease payments, we use the rate implicit in the lease, if known, or we use an estimated incremental borrowing rate based on the information available at the lease commencement date. We recognize lease expense on a straight-line basis over the lease term. Leases with an initial term of 12 months or less are not recorded on the balance sheet. We do not assume renewals in our determination of the lease term unless the renewals are deemed to be reasonably assured at lease inception.
Revenue Recognition	Revenue Recognition Effective January 1, 2019, we adopted ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) , or ASC 606, using the modified retrospective approach. Under this approach, we recorded a cumulative adjustment to decrease accumulated deficit and deferred revenue by $0.4 million as of the adoption date. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. We only apply the five-step model to contracts when it is probable that we will collect the consideration we are entitled to in exchange for the goods and services we transfer to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, we assess the goods or services promised within each contract, determine those that are performance obligations and assess whether each promised good or service is distinct. We then recognize as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied . We have entered into licensing and collaboration agreements that are within the scope of ASC 606. Licenses of Intellectual Property: If the license to our intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, we recognize revenues from non-refundable, upfront fees allocated to the license when the license is transferred to the licensee and the licensee is able to use and benefit from the license. For licenses that are bundled with other promised goods or services, we utilize judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue from non-refundable, upfront fees. We evaluate the measure of progress each reporting period and, if necessary, adjust the measure of performance and related revenue recognition . Milestone Payments: At the inception of each arrangement that includes development, regulatory or commercial milestone payments, we evaluate whether the milestones are considered probable of being reached and estimate the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant reversal of cumulative revenue would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within our control or that of our licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received or the underlying activity has been completed. The transaction price is then allocated to each performance obligation on a relative stand-alone selling price basis, for which we recognize revenue as or when the performance obligations under the contract are satisfied. At the end of each subsequent reporting period, we re-evaluate the probability of achievement of such development milestones and any related constraint, and if necessary, adjust our estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect collaboration revenue in the period of adjustment. Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, we recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, we have not recognized any royalty revenue resulting from any of our licensing arrangements .
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in stockholders’ equity (deficit) of a business enterprise during a period, resulting from transactions from non-owner sources, and consists primarily of unrealized gains or losses related to our available-for-sale marketable securities, which are carried at estimated fair values on the consolidated balance sheets .
Net Income (Loss) per Share	Net Income (Loss) per Share Basic net income (loss) per share is calculated by dividing our net income (loss) by the weighted-average number of shares of common stock outstanding for the period . Diluted net income (loss) per share includes the effect of potentially dilutive securities, which include outstanding warrants and stock options if the effect of their inclusion would be dilutive. In periods of net loss, diluted net loss per share is the same as basic net loss per share as the inclusion of potentially dilutive securities in the calculation would be anti-dilutive .
Recent Accounting Pronouncements	Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board, or FASB, or other standard setting bodies and adopted by us as of the specified effective date. Unless otherwise discussed, the impact of recently issued standards that are not yet effective will not have a material impact on our condensed consolidated financial statements upon adoption. Under the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”), we meet the definition of an emerging growth company, and have elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act . We will remain an emerging growth company until December 31, 2020 . Recently Adopted Accounting Standards or Updates In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases (Topic 842): Targeted Improvements transition as a adjustment In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The updated accounting guidance requires changes to the recognition of credit losses on financial instruments not accounted for at fair value through net income. Specifically, the ASU requires companies to recognize an allowance for credit losses on available-for-sale debt securities that are impaired as a result of credit-related factors and replaces the prior other-than-temporary impairment model. We adopted this ASU effective January 1, 2020 and the adoption did not have a material impact on our condensed consolidated financial statements and related disclosures. In June 2018, the FASB issued ASU No. 2018-07, Compensation–Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting . This ASU simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. We adopted this ASU effective January 1, 2020 and the adoption did not have a material impact on our condensed consolidated financial statements and related disclosures. Our share-based payments related to nonemployees are insignificant. 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. We adopted this ASU effective January 1, 2020 and the adoption did not have a material impact on our condensed consolidated financial statements and related disclosures
Fair Value Measurements	Financial assets and liabilities are recorded at fair value. We determine fair value using the fair value hierarchy, which establishes three levels of inputs that may be used to measure fair value, as follows : Level 1 inputs include quoted prices in active markets for identical assets or liabilities . Level 2 inputs include observable inputs other than Level 1 inputs, such as quoted prices for similar assets or liabilities in active markets; quoted prices for identical or similar assets or liabilities in markets that are not active; and other inputs that are observable or can be corroborated by observable market data for substantially the full term of the asset or liability . Level 3 inputs include unobservable inputs that are supported by little or no market activity and are significant to the fair value of the underlying asset or liability. Such inputs reflect our best estimate of what market participants would use in pricing the asset or liability at the reporting date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model . 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. Our assessment of the significance of a particular input to the fair value measurement in its entirety requires our management to make judgments and consider factors specific to the asset or liability .

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2020
Fair Value Disclosures [Abstract]
Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis	Assets and liabilities measured at fair value on a recurring basis as of June 30, 2020 and December 31, 2019 were as follows (in thousands): June 30, 2020 Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 167,679 $ — $ — $ 167,679 Short-term marketable securities Corporate commercial paper — 20,239 — 20,239 Corporate debt securities — 66,742 — 66,742 Government‑sponsored enterprise securities — 9,106 — 9,106 U.S. Treasury securities — 49,960 — 49,960 Total $ 167,679 $ 146,047 $ — $ 313,726 December 31, 2019 Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 17,664 $ — $ — $ 17,664 Corporate commercial paper — 6,983 — 6,983 Corporate debt securities — 10,204 — 10,204 Short-term marketable securities Corporate commercial paper — 95,088 — 95,088 Corporate debt securities — 154,690 — 154,690 Government‑sponsored enterprise securities — 26,265 — 26,265 Long-term marketable securities Corporate debt securities — 14,202 — 14,202 Government‑sponsored enterprise securities — 38,055 — 38,055 Total $ 17,664 $ 345,487 $ — $ 363,151

Cash Equivalents and Marketab_2

Cash Equivalents and Marketable Securities (Tables)	6 Months Ended
	Jun. 30, 2020
Investments Debt And Equity Securities [Abstract]
Summary of Cash Equivalents and Marketable Securities	Cash equivalents and marketable securities consisted of the following as of June 30, 2020 and December 31, 2019 (in thousands): June 30, 2020 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents Money market funds $ 167,679 $ — $ — $ 167,679 Total cash equivalents 167,679 — — 167,679 Short-term marketable securities Corporate commercial paper 20,171 68 — 20,239 Corporate debt securities 66,468 274 — 66,742 Government‑sponsored enterprise securities 9,027 79 — 9,106 U.S. Treasury securities 49,960 1 (1 ) 49,960 Total short-term marketable securities 145,626 422 (1 ) 146,047 Total $ 313,305 $ 422 $ (1 ) $ 313,726 December 31, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents Money market funds $ 17,664 $ — $ — $ 17,664 Corporate commercial paper 6,982 1 — 6,983 Corporate debt securities 10,205 — (1 ) 10,204 Total cash equivalents 34,851 1 (1 ) 34,851 Short-term marketable securities Corporate commercial paper 95,061 36 (9 ) 95,088 Corporate debt securities 154,678 54 (42 ) 154,690 Government‑sponsored enterprise securities 26,231 34 — 26,265 Total short-term marketable securities 275,970 124 (51 ) 276,043 Long-term marketable securities Corporate debt securities 14,194 8 — 14,202 Government‑sponsored enterprise securities 38,037 18 — 38,055 Total long-term marketable securities 52,231 26 — 52,257 Total $ 363,052 $ 151 $ (52 ) $ 363,151

Balance Sheet Components (Table

Balance Sheet Components (Tables)	6 Months Ended
	Jun. 30, 2020
Balance Sheet Related Disclosures [Abstract]
Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheets that sum to the total of the amounts shown in the condensed consolidated statements of cash flows (in thousands): June 30, 2020 December 31, 2019 June 30, 2019 December 31, 2018 Cash and cash equivalents $ 170,486 $ 39,537 $ 41,073 $ 34,489 Restricted cash, current — — — 82 Restricted cash, non-current 567 567 567 567 Total $ 171,053 $ 40,104 $ 41,640 $ 35,138
Summary of Property and Equipment	Property and equipment as of June 30, 2020 and December 31, 2019 consisted of the following (in thousands): June 30, 2020 December 31, 2019 Laboratory equipment $ 2,313 $ 2,007 Computer equipment 297 289 Furniture and fixtures 1,560 1,549 Leasehold improvements 8,324 8,324 12,494 12,169 Less accumulated depreciation and amortization (3,492 ) (2,482 ) Total $ 9,002 $ 9,687
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets as of June 30, 2020 and December 31, 2019 consisted of the following (in thousands): June 30, 2020 December 31, 2019 Collaboration receivable from Sanofi $ 50,000 $ — Other accounts receivable 492 1,272 Prepaid expenses 2,397 2,893 Total $ 52,889 $ 4,165

Commitments and Contingencies (

Commitments and Contingencies (Tables)	6 Months Ended
	Jun. 30, 2020
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Lease Payments for Periods Presented to Operating Lease Liability	The following table reconciles future lease payments for the periods presented to our operating lease liability as of June 30, 2020 (in thousands): Year ended December 31, 2020 (remaining 6 months) $ 1,550 2021 3,193 2022 3,296 2023 3,403 2024 3,514 2025 and beyond 3,934 Total lease payments 18,890 Present value adjustment (4,193 ) Operating lease liability at June 30, 2020 $ 14,697

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	6 Months Ended
	Jun. 30, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Activity	The following table summarizes our stock option activity under our stock plans and related information: Options Outstanding Weighted- Average Exercise Price Balance at December 31, 2019 3,887,780 $ 18.38 Options granted 1,498,120 $ 66.42 Options forfeited (26,201 ) $ 39.70 Options exercised (173,633 ) $ 9.57 Balance at June 30, 2020 5,186,066 $ 32.45 Vested and expected to vest 5,186,066 $ 32.45 Exercisable as of June 30, 2020 2,523,720 $ 12.20
Summary of Stock-based Compensation Expense	The following table summarizes total stock-based compensation expense related to our 2018 Equity Incentive Plan, 2008 Equity Incentive Plan and 2018 Employee Stock Purchase Plan (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2020 2019 2020 2019 Research and development $ 3,850 $ 1,776 $ 6,193 $ 2,926 General and administrative 3,097 1,700 5,143 2,755 Total $ 6,947 $ 3,476 $ 11,336 $ 5,681

Net Income (Loss) per Share (Ta

Net Income (Loss) per Share (Tables)	6 Months Ended
	Jun. 30, 2020
Earnings Per Share [Abstract]
Schedule of Reconciliation of Numerators and Denominators Used In Computations and Calculation of Basic and Diluted Net Income (Loss) Per Share	The following table presents a reconciliation of the numerators and denominators of the basic and diluted net income (loss) per share computations and the calculation of basic and diluted net income (loss) per share (in thousands, except share and per share data): Three Months Ended June 30, Six Months Ended June 30, 2020 2019 2020 2019 Numerator: Net income (loss) $ 10,810 $ 7,115 $ (21,689 ) $ (6,572 ) Denominator: Weighted-average common shares outstanding, basic 33,087,340 23,927,172 33,040,547 23,896,788 Effect of dilutive shares 2,224,598 1,864,929 — — Weighted-average shares outstanding used in per share calculation, diluted 35,311,938 25,792,101 33,040,547 23,896,788 Net income (loss) per share, basic $ 0.33 $ 0.30 $ (0.66 ) $ (0.28 ) Net income (loss) per share, diluted $ 0.31 $ 0.28 $ (0.66 ) $ (0.28 )
Summary of Common Stock Equivalents Excluded Used in Computation of Diluted Net Income (Loss) Per Share	The following outstanding shares of common stock equivalents were excluded in the computation of diluted net income (loss) per share, because their effect would have been antidilutive for the periods presented : Three Months Ended June 30, Six Months Ended June 30, 2020 2019 2020 2019 Employee stock purchase plan 8,919 11,326 34,966 43,745 Warrants to purchase common stock 11,933 48,406 81,966 168,046 Common stock options issued and outstanding 3,041,438 2,390,056 5,186,066 4,124,358 Early exercised common stock subject to future vesting 18,770 29,460 18,770 29,460 Total 3,081,060 2,479,248 5,321,768 4,365,609

Organization - Additional Infor

Organization - Additional Information (Details)	6 Months Ended
	Jun. 30, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Date of incorporation	Oct. 6, 2008

Significant Accounting Polici_3

Significant Accounting Policies - Additional Information (Details) $ in Thousands	Jan. 01, 2019USD ($)	Jun. 30, 2020USD ($)Segment	Jan. 01, 2020USD ($)	Dec. 31, 2019USD ($)
Summary Of Significant Accounting Policies [Line Items]
Cash cash equivalents and marketable securities		$ 316,500		$ 367,800
Restricted cash		$ 600		600
Number of operating segment | Segment		1
Operating lease right-of-use asset		$ 6,483	$ 6,800	0
Operating lease liability		$ 14,697	15,600
Deferred rent				8,800
ASU 842
Summary Of Significant Accounting Policies [Line Items]
Operating lease right-of-use asset			6,800
Operating lease liability			$ 15,600
Deferred rent				$ 8,800
ASC 606 | Adjustments
Summary Of Significant Accounting Policies [Line Items]
Cumulative adjustment to decrease accumulated deficit and deferred revenue	$ 400

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets
Assets	$ 313,726	$ 363,151
Level 1
Assets
Assets	167,679	17,664
Level 2
Assets
Assets	146,047	345,487
Level 3
Assets
Assets	0	0
Cash Equivalents | Money Market Funds
Assets
Assets	167,679	17,664
Cash Equivalents | Corporate Commercial Paper
Assets
Assets		6,983
Cash Equivalents | Corporate Debt Securities
Assets
Assets		10,204
Cash Equivalents | Level 1 | Money Market Funds
Assets
Assets	167,679	17,664
Cash Equivalents | Level 1 | Corporate Commercial Paper
Assets
Assets		0
Cash Equivalents | Level 1 | Corporate Debt Securities
Assets
Assets		0
Cash Equivalents | Level 2 | Money Market Funds
Assets
Assets	0	0
Cash Equivalents | Level 2 | Corporate Commercial Paper
Assets
Assets		6,983
Cash Equivalents | Level 2 | Corporate Debt Securities
Assets
Assets		10,204
Cash Equivalents | Level 3 | Money Market Funds
Assets
Assets	0	0
Cash Equivalents | Level 3 | Corporate Commercial Paper
Assets
Assets		0
Cash Equivalents | Level 3 | Corporate Debt Securities
Assets
Assets		0
Short-term marketable securities | Corporate Commercial Paper
Assets
Assets	20,239	95,088
Short-term marketable securities | Corporate Debt Securities
Assets
Assets	66,742	154,690
Short-term marketable securities | Government- sponsored Enterprise Securities
Assets
Assets	9,106	26,265
Short-term marketable securities | Level 1 | Corporate Commercial Paper
Assets
Assets	0	0
Short-term marketable securities | Level 1 | Corporate Debt Securities
Assets
Assets	0	0
Short-term marketable securities | Level 1 | Government- sponsored Enterprise Securities
Assets
Assets	0	0
Short-term marketable securities | Level 2 | Corporate Commercial Paper
Assets
Assets	20,239	95,088
Short-term marketable securities | Level 2 | Corporate Debt Securities
Assets
Assets	66,742	154,690
Short-term marketable securities | Level 2 | Government- sponsored Enterprise Securities
Assets
Assets	9,106	26,265
Short-term marketable securities | Level 3 | Corporate Commercial Paper
Assets
Assets	0	0
Short-term marketable securities | Level 3 | Corporate Debt Securities
Assets
Assets	0	0
Short-term marketable securities | Level 3 | Government- sponsored Enterprise Securities
Assets
Assets	0	0
Long-term marketable securities | Corporate Debt Securities
Assets
Assets		14,202
Long-term marketable securities | Government- sponsored Enterprise Securities
Assets
Assets		38,055
Long-term marketable securities | Level 1 | Corporate Debt Securities
Assets
Assets		0
Long-term marketable securities | Level 1 | Government- sponsored Enterprise Securities
Assets
Assets		0
Long-term marketable securities | Level 2 | Corporate Debt Securities
Assets
Assets		14,202
Long-term marketable securities | Level 2 | Government- sponsored Enterprise Securities
Assets
Assets		38,055
Long-term marketable securities | Level 3 | Corporate Debt Securities
Assets
Assets		0
Long-term marketable securities | Level 3 | Government- sponsored Enterprise Securities
Assets
Assets		$ 0
Short Term Marketable Securities | U.S. treasury securities
Assets
Assets	49,960
Short Term Marketable Securities | Level 1 | U.S. treasury securities
Assets
Assets	0
Short Term Marketable Securities | Level 2 | U.S. treasury securities
Assets
Assets	49,960
Short Term Marketable Securities | Level 3 | U.S. treasury securities
Assets
Assets	$ 0

Cash Equivalents and Marketab_3

Cash Equivalents and Marketable Securities - Summary of Short-Term Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	$ 313,305	$ 363,052
Unrealized Gains, Total	422	151
Unrealized Losses, Total	(1)	(52)
Fair Value, Total	313,726	363,151
Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	167,679	34,851
Unrealized Gains, Total	0	1
Unrealized Losses, Total	0	(1)
Fair Value, Total	167,679	34,851
Cash Equivalents | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total		10,205
Unrealized Gains, Total		0
Unrealized Losses, Total		(1)
Fair Value, Total		10,204
Cash Equivalents | Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	167,679	17,664
Unrealized Gains, Total	0	0
Unrealized Losses, Total	0	0
Fair Value, Total	167,679	17,664
Cash Equivalents | Corporate Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total		6,982
Unrealized Gains, Total		1
Unrealized Losses, Total		0
Fair Value, Total		6,983
Short-term marketable securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	145,626	275,970
Unrealized Gains, Total	422	124
Unrealized Losses, Total	(1)	(51)
Fair Value, Total	146,047	276,043
Short-term marketable securities | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	66,468	154,678
Unrealized Gains, Total	274	54
Unrealized Losses, Total	0	(42)
Fair Value, Total	66,742	154,690
Short-term marketable securities | Corporate Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	20,171	95,061
Unrealized Gains, Total	68	36
Unrealized Losses, Total	0	(9)
Fair Value, Total	20,239	95,088
Short-term marketable securities | U.S. treasury securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	49,960
Unrealized Gains, Total	1
Unrealized Losses, Total	(1)
Fair Value, Total	49,960
Short-term marketable securities | Government- sponsored Enterprise Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	9,027	26,231
Unrealized Gains, Total	79	34
Unrealized Losses, Total	0	0
Fair Value, Total	$ 9,106	26,265
Long-term marketable securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total		52,231
Unrealized Gains, Total		26
Unrealized Losses, Total		0
Fair Value, Total		52,257
Long-term marketable securities | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total		14,194
Unrealized Gains, Total		8
Unrealized Losses, Total		0
Fair Value, Total		14,202
Long-term marketable securities | Government- sponsored Enterprise Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total		38,037
Unrealized Gains, Total		18
Unrealized Losses, Total		0
Fair Value, Total		$ 38,055

Cash Equivalents and Marketab_4

Cash Equivalents and Marketable Securities - Additional Information (Details)	1 Months Ended		6 Months Ended		12 Months Ended
	Jul. 31, 2020USD ($)	Apr. 30, 2020USD ($)	Jun. 30, 2020USD ($)Position	Jun. 30, 2019USD ($)	Dec. 31, 2019USD ($)
Schedule Of Available For Sale Securities [Line Items]
Marketable securities in unrealized loss positions			$ 22,500,000
Number of investment positions of marketable securities | Position			1
Aggregate net unrealized gains of marketable securities			$ 400,000		$ 100,000
Gross proceeds from sales of short-term and long-term marketable securities		$ 101,900,000	101,876,000	$ 0
Gross proceeds from sales of short-term and long-term marketable securities			169,033,000	103,535,000
Realized net gain of marketable securities		$ 200,000	$ 199,000	$ 0
Subsequent Event
Schedule Of Available For Sale Securities [Line Items]
Gross proceeds from sales of short-term and long-term marketable securities	$ 17,000,000
Realized net gain of marketable securities	$ 45,000
Maximum
Schedule Of Available For Sale Securities [Line Items]
Remaining contractual maturities of short-term marketable securities			1 year

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 170,486	$ 39,537	$ 41,073	$ 34,489
Restricted cash, current	0	0	0	82
Restricted cash, non-current	567	567	567	567
Total	$ 171,053	$ 40,104	$ 41,640	$ 35,138

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 12,494	$ 12,169
Less accumulated depreciation and amortization	(3,492)	(2,482)
Total	9,002	9,687
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,313	2,007
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	297	289
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,560	1,549
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 8,324	$ 8,324

Balance Sheet Components - Su_3

Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Prepaid Expense And Other Assets Current [Abstract]
Collaboration receivable from Sanofi	$ 50,000	$ 0
Other accounts receivable	492	1,272
Prepaid expenses	2,397	2,893
Total	$ 52,889	$ 4,165

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) $ in Millions	1 Months Ended
	Jun. 30, 2020USD ($)
Sanofi Agreement
Balance Sheet Components [Line Items]
Clinical development milestone payment receivable	$ 50

License and Collaboration Agr_2

License and Collaboration Agreements - Additional Information (Details) - USD ($)

Jan. 01, 2019

Jun. 30, 2020

Jun. 30, 2019

Nov. 30, 2018

Aug. 31, 2018

Jul. 31, 2018

Dec. 31, 2017

Nov. 30, 2017

Jun. 30, 2017

Jun. 30, 2020

Jun. 30, 2019

Mar. 31, 2019

Jun. 30, 2020

Jun. 30, 2019

Dec. 31, 2018

Dec. 31, 2017

Dec. 31, 2019

Oct. 31, 2018

University Of California San Francisco

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Licensing fees

$ 40,000

Annual licenses maintenance fee

40,000

University Of California San Francisco | IPO

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Milestone payments

$ 140,000

Sanofi Agreement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Payment received

$ 30,000,000

$ 10,000,000

$ 5,000,000

$ 10,000,000

$ 40,000,000

Revenue recognized

$ 50,000,000

$ 30,000,000

50,000,000

$ 30,000,000

Royalty expiration period

10 years

Upfront payment recognized

$ 40,000,000

Aggregate milestone payments

25,000,000

Transaction price determined

65,000,000

Deferred revenue

$ 0

$ 0

0

$ 0

Sanofi Agreement | Maximum

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

License agreement milestone payment receive regulatory milestones and commercial milestones

$ 765,000,000

Sanofi Agreement | Clinical Development Milestones

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Milestone payments to be received

50,000,000

Revenue recognized

$ 50,000,000

50,000,000

30,000,000

Milestone payments received

$ 30,000,000

Increase in transaction price

50,000,000

$ 30,000,000

AbbVie Agreement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Payment received

$ 15,000,000

$ 15,000,000

Revenue recognized

$ 5,200,000

$ 0

Research and development period

2 years

AbbVie Agreement | ASC 606

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenue recognized

9,800,000

AbbVie Agreement | ASC 606 | ASC 605

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenue recognized

$ 9,400,000

AbbVie Agreement | ASC 606 | Adjustments

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Decrease in accumulated deficit

$ 400,000

Decrease in deferred revenue

$ 400,000

AbbVie Agreement | Maximum

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Extension of research and development period

6 months

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Apr. 30, 2018USD ($)ft²	Jun. 30, 2020USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2020USD ($)	Jun. 30, 2019USD ($)	Jan. 01, 2020USD ($)	Dec. 31, 2019USD ($)
Commitments And Contingencies [Line Items]
Operating lease ROU asset		$ 6,483		$ 6,483		$ 6,800	$ 0
Operating lease liability		$ 14,697		$ 14,697		$ 15,600
Deferred rent							8,800
Remaining lease terms		5 years 7 months 6 days		5 years 7 months 6 days
Present value of remaining lease payments discount rate		9.00%		9.00%
Cash paid for amounts included in measurement of lease liability included in net cash used in operating activities				$ 1,500
Rent expense		$ 700	$ 700	1,300	$ 1,400
Indemnification Guarantee
Commitments And Contingencies [Line Items]
Accruals for expenses related to indemnification issues		$ 0		$ 0
April 2018 Lease Agreement
Commitments And Contingencies [Line Items]
Area under lease | ft²	47,500
Commencement date				Feb. 1, 2019
Term of lease agreement		7 years		7 years
Option to extend period		7 years		7 years
Letter of credit		$ 600		$ 600			$ 600
Tenant improvement		8,300		$ 8,300
Lease agreement commencement date				Aug. 1, 2018
April 2018 Lease Agreement | Maximum
Commitments And Contingencies [Line Items]
Tenant improvement		$ 7,100		$ 7,100
Additional tenant improvement allowance	$ 1,200

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Future Lease Payments for Periods Presented to Operating Lease Liability (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Jan. 01, 2020
Commitments And Contingencies Disclosure [Abstract]
2020 (remaining 6 months)	$ 1,550
2021	3,193
2022	3,296
2023	3,403
2024	3,514
2025 and beyond	3,934
Total lease payments	18,890
Present value adjustment	(4,193)
Operating lease liability at June 30, 2020	$ 14,697	$ 15,600

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details)	6 Months Ended
	Jun. 30, 2020$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Options Outstanding, Beginning Balance | shares	3,887,780
Options Outstanding, Options granted | shares	1,498,120
Options Outstanding, Options forfeited | shares	(26,201)
Options Outstanding, Options exercised | shares	(173,633)
Options Outstanding, Ending Balance | shares	5,186,066
Options Outstanding, Vested and expected to vest | shares	5,186,066
Options Outstanding, Exercisable as of June 30, 2020 | shares	2,523,720
Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 18.38
Weighted Average Exercise Price, Options granted | $ / shares	66.42
Weighted Average Exercise Price, Options forfeited | $ / shares	39.70
Weighted Average Exercise Price, Options exercised | $ / shares	9.57
Weighted Average Exercise Price, Ending Balance | $ / shares	32.45
Weighted Average Exercise Price, Vested and expected to vest | $ / shares	32.45
Weighted Average Exercise Price, Exercisable as of June 30, 2020 | $ / shares	$ 12.20

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period	48 months
Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unamortized compensation cost related to stock awards	$ 83.1
Expected weighted average period	3 years 2 months 12 days
Weighted-average remaining contractual term options outstanding	8 years
Early Exercised Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares repurchase in the event of employment termination	18,770	25,081
Unvested stock options liabilities	$ 0.1	$ 0.2

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Stock-based Compensation Expense (Details) - Employee Stock Purchase Plan - 2018 Equity Incentive Plan/2008 Equity Incentive Plan - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 6,947	$ 3,476	$ 11,336	$ 5,681
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	3,850	1,776	6,193	2,926
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 3,097	$ 1,700	$ 5,143	$ 2,755

Net Income (Loss) per Share - S

Net Income (Loss) per Share - Schedule of Reconciliation of Numerators and Denominators Used In Computations and Calculation of Basic and Diluted Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net income (loss)	$ 10,810	$ (32,499)	$ 7,115	$ (13,687)	$ (21,689)	$ (6,572)
Denominator:
Weighted-average common shares outstanding, basic	33,087,340		23,927,172		33,040,547	23,896,788
Effect of dilutive shares	2,224,598		1,864,929		0	0
Weighted-average shares outstanding used in per share calculation, diluted	35,311,938		25,792,101		33,040,547	23,896,788
Net income (loss) per share, basic	$ 0.33		$ 0.30		$ (0.66)	$ (0.28)
Net income (loss) per share, diluted	$ 0.31		$ 0.28		$ (0.66)	$ (0.28)

Net Income (Loss) per Share -_2

Net Income (Loss) per Share - Summary of Common Stock Equivalents Excluded Used in Computation of Diluted Net Income (Loss) Per Share (Details) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	3,081,060	2,479,248	5,321,768	4,365,609
Employee Stock Purchase Plan
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	8,919	11,326	34,966	43,745
Warrants 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	11,933	48,406	81,966	168,046
Common Stock Options Issued and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	3,041,438	2,390,056	5,186,066	4,124,358
Early Exercised Common Stock Subject To Future Vesting
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	18,770	29,460	18,770	29,460

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Income Tax Disclosure [Abstract]
Provision for income taxes	$ 0	$ 0