Principia Biopharma (PRNB) 10-Q 8 Aug 19 2019 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Jul. 31, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
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	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		23,970,822
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity File Number	001-38653
Entity Tax Identification Number	263487603
Entity Address, Address Line One	220 East Grand Avenue
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	California
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	416-7700

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 41,073	$ 34,489
Short-term marketable securities	128,492	142,436
Restricted cash	0	82
Prepaid expenses and other current assets	2,157	3,765
Total current assets	171,722	180,772
Property and equipment, net	10,598	1,666
Long-term restricted cash	567	567
Long-term marketable securities	8,972	3,712
Other long-term assets	480	8,804
Total assets	192,339	195,521
Current liabilities:
Accounts payable	4,052	4,439
Deferred rent, current portion	1,132	344
Deferred revenue	0	5,616
Accrued research and development liabilities	4,823	1,520
Accrued other liabilities	591	649
Accrued compensation	2,850	4,312
Total current liabilities	13,448	16,880
Long-term deferred rent	8,213	8,781
Commitments and contingencies (Note 7)
Stockholders' equity (deficit)
Common stock, $0.0001 par value, 500,000,000 authorized at June 30, 2019 and December 31, 2018; 23,966,418 and 23,865,451 shares issued and outstanding at June 30, 2019 and December 31, 2018, respectively	2	2
Additional paid-in-capital	309,088	302,393
Accumulated other comprehensive income (loss)	111	(128)
Accumulated deficit	(138,523)	(132,407)
Total stockholders' equity	170,678	169,860
Total liabilities and stockholders’ equity	$ 192,339	$ 195,521

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
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	23,966,418	23,865,451
Common stock, shares outstanding	23,966,418	23,865,451

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Revenue	$ 30,000	$ 12,987	$ 35,160	$ 24,436
Operating expenses:
Research and development	18,718	8,894	34,241	17,655
General and administrative	5,233	2,222	9,740	4,378
Total operating expenses	23,951	11,116	43,981	22,033
Income (loss) from operations	6,049	1,871	(8,821)	2,403
Other income (expense), net	(42)	(186)	(41)	(523)
Interest income	1,108	112	2,290	227
Net income (loss)	7,115	1,797	(6,572)	2,107
Net income (loss) attributable to common stockholders	$ 7,115	$ 0	$ (6,572)	$ 0
Net income (loss) per share attributable to common stockholders
Basic	$ 0.30	$ 0	$ (0.28)	$ 0
Diluted	$ 0.28	$ 0	$ (0.28)	$ 0
Weighted-average shares used to calculate net income (loss) per share attributable to common stockholders
Basic	23,927,172	681,616	23,896,788	656,129
Diluted	25,792,101	1,412,928	23,896,788	1,508,584

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 7,115	$ 1,797	$ (6,572)	$ 2,107
Other comprehensive income (loss):
Net unrealized gain on available-for-sale securities	90	0	239	0
Comprehensive income (loss)	$ 7,205	$ 1,797	$ (6,333)	$ 2,107

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at Dec. 31, 2017	$ (143,462)		$ 1	$ 7,201	$ (90)	$ (150,574)
Balance, Shares at Dec. 31, 2017		12,285,434
Balance at Dec. 31, 2017		$ 128,531
Balance, Shares at Dec. 31, 2017			626,613
Stock-based compensation expense	366			366
Exercise of stock options and vesting of early exercise shares	126			126
Exercise of stock options and vesting of early exercise shares, Shares			32,169
Net income (loss)	310					310
Balance at Mar. 31, 2018	(142,660)		$ 1	7,693	(90)	(150,264)
Balance, Shares at Mar. 31, 2018		12,285,434
Balance at Mar. 31, 2018		$ 128,531
Balance, Shares at Mar. 31, 2018			658,782
Balance at Dec. 31, 2017	(143,462)		$ 1	7,201	(90)	(150,574)
Balance, Shares at Dec. 31, 2017		12,285,434
Balance at Dec. 31, 2017		$ 128,531
Balance, Shares at Dec. 31, 2017			626,613
Net income (loss)	2,107
Balance at Jun. 30, 2018	(140,306)		$ 1	8,250	(90)	(148,467)
Balance, Shares at Jun. 30, 2018		12,285,434
Balance at Jun. 30, 2018		$ 128,531
Balance, Shares at Jun. 30, 2018			718,008
Balance at Mar. 31, 2018	(142,660)		$ 1	7,693	(90)	(150,264)
Balance, Shares at Mar. 31, 2018		12,285,434
Balance at Mar. 31, 2018		$ 128,531
Balance, Shares at Mar. 31, 2018			658,782
Stock-based compensation expense	372			372
Exercise of stock options and vesting of early exercise shares	185			185
Exercise of stock options and vesting of early exercise shares, Shares			59,226
Net income (loss)	1,797					1,797
Balance at Jun. 30, 2018	(140,306)		$ 1	8,250	(90)	(148,467)
Balance, Shares at Jun. 30, 2018		12,285,434
Balance at Jun. 30, 2018		$ 128,531
Balance, Shares at Jun. 30, 2018			718,008
Balance at Dec. 31, 2018	$ 169,860		$ 2	302,393	(128)	(132,407)
Balance, Shares at Dec. 31, 2018	23,865,451		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
Cumulative-effect adjustment from adoption of ASC 606 accounting standard on revenue recognition | ASC 606	456					456
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		$ 2	302,393	(128)	(132,407)
Balance, Shares at Dec. 31, 2018	23,865,451		23,865,451
Exercise of stock options and vesting of early exercise shares, Shares	17,603
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		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		23,966,418

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders Equity (Deficit) (Unaudited) (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Statement Of Stockholders Equity [Abstract]
Common stock, par value	$ 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, 2019	Jun. 30, 2018
Operating activities:
Net income (loss)	$ (6,572)	$ 2,107
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Change in fair value of convertible preferred stock warrant liability	0	509
Amortization of discount on marketable securities	(962)	0
Depreciation	835	107
Stock-based compensation	5,681	738
Deferred rent	220	0
Changes in operating assets and liabilities:
Prepaid expenses and other assets	1,608	(9,573)
Deferred revenue	(5,160)	(14,436)
Accounts payable	(52)	(160)
Accrued liabilities	1,918	(1,013)
Net cash used in operating activities	(2,484)	(21,721)
Investing activities:
Purchases of property and equipment	(1,913)	(368)
Maturities of marketable securities	103,535	0
Purchases of marketable securities	(93,650)	(8,485)
Net cash provided by (used in) investing activities	7,972	(8,853)
Financing activities:
Proceeds from issuances of common stock upon exercise of options and participation in employee stock purchase plan	1,014	311
Deferred IPO costs	0	(385)
Net cash provided by (used in) financing activities	1,014	(74)
Net increase (decrease) in cash, cash equivalents and restricted cash	6,502	(30,648)
Cash, cash equivalents and restricted cash at beginning of period	35,138	41,236
Cash, cash equivalents and restricted cash, at end of period	41,640	10,588
Supplemental disclosures of cash flow information
Non cash tenant improvement allowance used for leasehold improvements	8,324	0
Purchases of property and equipment accrued but not yet paid	172	15
Deferred IPO costs accrued but not yet paid	$ 0	$ 886

Organization

Organization	6 Months Ended
	Jun. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	1. Description of Business We, Principia Biopharma Inc. (“Principia”), are a late-stage biopharmaceutical company focused on developing novel therapies for immunology and oncology. 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, 2019
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, 2018 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 19, 2019 (“2018 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. The majority of our cash and cash equivalents is maintained with one financial institution in the United States. Deposits with this financial institution have exceeded and will continue to exceed federally insured limits. We have not experienced any losses on our cash deposits. 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 early-stage biopharmaceutical companies, including, but not limited to, the need to obtain adequate additional funding, possible failure of current or future preclinical studies or clinical 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, 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. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on 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, 2019 and December 31, 2018, 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 lease agreements for our laboratory and office facilities. These leases are classified as operating leases. Rent expense is recognized on a straight-line basis over the term of the lease. The difference between rent expense recognized and rental payments is recorded as deferred rent in the condensed consolidated balance sheets. Lease incentives and allowance provided by our landlord for the construction of leasehold improvements are recorded as lease incentive obligations as the related construction costs are incurred, up to the maximum aggregate allowances. Lease incentive obligations are classified as a component of deferred rent and are amortized on a straight-line basis over the lease term as a reduction of rent expense. Revenue Recognition Effective January 1, 2019, we adopted Accounting Standards Codification, or ASC No. 2014-09, Revenue from Contracts with Customers, 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 . In determining the appropriate amount of revenue to be recognized as we fulfill our obligations under such licensing and collaboration agreements, we perform the five-step model under ASC 606. As part of the accounting for these arrangements, we must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. 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. The impact of the adoption of Topic 606 on the accompanying condensed consolidated balance sheet as of January 1, 2019 was as follows (in thousands): December 31, 2018 Adjustments Due to ASC 606 January 1, 2019 Liabilities Deferred revenue $ 5,616 $ (456 ) $ 5,160 Stockholders' Equity Accumulated deficit (132,407 ) 456 (131,951 ) The impact of the adoption of ASC 606 on our unaudited condensed consolidated statement of operations for the three and six months ended June 30, 2019 was as follows (in thousands): Three Months Ended June 30, 2019 As Reported Adjustments Balance without ASC 606 Adoption Revenue $ 30,000 $ — $ 30,000 Income (loss) from operations 6,049 — 6,049 Net income (loss) 7,115 — 7,115 Net income per share, basic 0.30 — 0.30 Net income per share, diluted 0.28 — 0.28 Six Months Ended June 30, 2019 As Reported Adjustments Balance without ASC 606 Adoption Revenue $ 35,160 $ 456 $ 35,616 Income (loss) from operations (8,821 ) 456 (8,365 ) Net income (loss) (6,572 ) 456 (6,116 ) Net loss per share, basic and diluted (0.28 ) 0.02 (0.26 ) During the three and six months ended June 30, 2019, we did not recognize any revenue from performance obligations satisfied in previous periods. Convertible Preferred Stock Warrants In connection with the issuance of certain convertible notes in 2016 and 2017 (the “Notes”), we issued warrants to purchase our capital stock. Freestanding warrants to purchase our convertible preferred stock were recorded as a liability on our condensed consolidated balance sheets because the underlying shares of convertible preferred stock are contingently redeemable, which, therefore, may obligate us to transfer assets to settle those warrants. The warrants are subject to revaluation at each balance sheet date, with changes in fair value recognized as a component of other income, net, on the condensed consolidated statements of operations. During the three and six months ended June 30, 2018, other expense, net, included $0.2 million and $0.5 million, respectively, related to the change in fair value of the preferred stock warrant liability. Upon the completion of our IPO in September 2018, all of our convertible preferred stock warrants were converted into warrants to purchase shares of common stock. We re-valued the convertible preferred stock warrants upon completion of our IPO and reclassified the estimated fair value of the warrants to additional paid in capital. 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 attributable to common stockholders is calculated by dividing the net income (loss) attributable to common stockholders 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. We have issued securities other than common stock that participate in dividends to the extent declared (“Participating Securities”), and therefore utilize the two-class method to calculate net income (loss) per share. These Participating Securities include Series A, Series B-1, Series B-2, Series B-3 and Series C redeemable convertible preferred stock. The two-class method requires a portion of net income (loss) to be allocated to the Participating Securities to determine net income (loss) attributable to common stockholders. Net income (loss) attributable to common stockholders is equal to the net income (loss) less dividends paid on preferred stock with any remaining earnings allocated in accordance with the bylaws between the outstanding common and redeemable convertible preferred stock as of the end of each period. 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. Recently Adopted Accounting Standards Updates In November 2016, the FASB issued accounting standard update (“ASU”) 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash We early adopted ASU 2016-18 during the fourth quarter of 2018 and the adoption did not have a material impact on our consolidated financial statements and related disclosures. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) Entities have the option of applying either a full retrospective approach to all periods presented, or a modified approach that reflects differences prior to the date of adoption as an adjustment to equity. We adopted ASC No. 2014-09 on January 1, 2019 using the modified retrospective method of transition applied to contracts that were not completed at January 1, 2019. Therefore, comparative information will not be adjusted and the impact of the transition is reflected as an adjustment to the opening accumulated deficit. A completed contract is a contract for which all, or substantially all, of the revenue was recognized in accordance with revenue guidance in effect before the date of initial application. The new revenue recognition standard differs from the previous accounting standard in many respects, such as in the accounting for variable consideration and the measurement of progress toward completion of performance obligations. Recently Issued Accounting Standards or Updates Not Yet Effective In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases (Topic 842) In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement . This ASU eliminates, modifies and adds disclosure requirements for fair value measurements. The amendments in this ASU are effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. We are currently evaluating the effects of this ASU on our consolidated financial statements and related disclosures. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. We are currently evaluating the effects of this ASU on our consolidated financial statements and related disclosures.

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2019
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, 2019 and December 31, 2018 were as follows (in thousands): June 30, 2019 Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 27,781 $ — $ — $ 27,781 Corporate commercial paper 8,980 — — 8,980 Short-term marketable securities Corporate commercial paper — 32,042 — 32,042 Corporate debt securities — 64,438 — 64,438 Government‑ sponsored enterprise securities — 32,012 — 32,012 Long-term marketable securities Government‑ sponsored enterprise securities — 8,972 — 8,972 Total $ 36,761 $ 137,464 $ — $ 174,225 December 31, 2018 Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 19,861 $ — $ — $ 19,861 Corporate commercial paper — 7,465 — 7,465 Corporate debt securities — 4,499 — 4,499 Short-term marketable securities Corporate commercial paper — 36,180 — 36,180 Corporate debt securities — 71,903 — 71,903 Government‑ sponsored enterprise securities — 9,906 — 9,906 U.S. Treasury securities — 24,447 — 24,447 Long-term marketable securities Corporate debt securities — 3,712 — 3,712 Total $ 19,861 $ 158,112 $ — $ 177,973 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, 2019 and 2018.

Cash Equivalents and Marketable

Cash Equivalents and Marketable Securities	6 Months Ended
	Jun. 30, 2019
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, 2019 and December 31, 2018 (in thousands): June 30, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents Money market funds $ 27,781 $ — $ — $ 27,781 Corporate commercial paper 8,980 — — 8,980 Total cash equivalents 36,761 — — 36,761 Short-term marketable securities Corporate commercial paper 31,988 54 — 32,042 Corporate debt securities 64,286 153 (1 ) 64,438 Government‑sponsored enterprise securities 32,006 6 — 32,012 Total short-term marketable securities 128,280 213 (1 ) 128,492 Long-term marketable securities Government‑sponsored enterprise securities 8,982 — (10 ) 8,972 Total long-term marketable securities 8,982 — (10 ) 8,972 Total $ 174,023 $ 213 $ (11 ) $ 174,225 December 31, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents Money market funds $ 19,861 $ — $ — $ 19,861 Corporate commercial paper 7,466 — (1 ) 7,465 Corporate debt securities 4,499 — — 4,499 Total cash equivalents 31,826 — (1 ) 31,825 Short-term marketable securities Corporate commercial paper 36,197 — (17 ) 36,180 Corporate debt securities 71,920 8 (25 ) 71,903 Government‑sponsored enterprise securities 9,911 — (5 ) 9,906 U.S. Treasury securities 24,451 — (4 ) 24,447 Total short-term marketable securities 142,479 8 (51 ) 142,436 Long-term marketable securities Corporate debt securities 3,706 6 — 3,712 Total long-term marketable securities 3,706 6 — 3,712 Total $ 178,011 $ 14 $ (52 ) $ 177,973 All our marketable securities are considered available-for-sale. There were no sales of available-for-sale marketable securities in any of the periods presented. The carrying value of marketable securities that were in unrealized loss positions totaled approximately $14.0 million as of June 30, 2019. We have determined that (i) we do not have the intent to sell any of these investments, and (ii) it is not more likely than not that we will be required to sell any of these investments before recovery of the entire amortized cost basis. We anticipate that we will recover the entire amortized cost basis of such securities and have determined there were no other-than-temporary impairments associated with any of these marketable securities before recovery of the entire amortized cost basis during the six months ended June 30, 2019. At June 30, 2019, the remaining contractual maturities of long-term marketable securities were less than two years.

Balance Sheet Components

Balance Sheet Components	6 Months Ended
	Jun. 30, 2019
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, 2019 December 31, 2018 June 30, 2018 December 31, 2017 Cash and cash equivalents $ 41,073 $ 34,489 $ 9,839 $ 41,054 Restricted cash, current — 82 182 100 Restricted cash, non-current 567 567 567 82 Total $ 41,640 $ 35,138 $ 10,588 $ 41,236 Property and equipment as of June 30, 2019 and December 31, 2018 consisted of the following (in thousands): June 30, 2019 December 31, 2018 Laboratory equipment $ 1,973 $ 2,500 Computer equipment 289 508 Furniture and Fixtures 1,499 392 Leasehold improvements 8,324 420 12,085 3,820 Less accumulated depreciation and amortization (1,487 ) (2,154 ) Total $ 10,598 $ 1,666 Prepaid expenses and other current assets as of June 30, 2019 and December 31, 2018 consisted of the following (in thousands): June 30, 2019 December 31, 2018 Other accounts receivable $ 740 $ 2,104 Prepaid expenses 1,417 1,661 Total $ 2,157 $ 3,765

License and Collaboration Agree

License and Collaboration Agreements	6 Months Ended
	Jun. 30, 2019
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 central nervous system, retina and ophthalmic nerve. We have agreed not to develop other 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 Phase 2b clinical trial of PRN2246/SAR442168. 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 PRN1008 for major enumerated indications overseen by the Division of Pulmonary, Allergy and Rheumatology Products of the U.S. Food and Drug Administration’s (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 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. The transaction price was determined to be $65.0 million, which includes the $40.0 million upfront payment we received from Sanofi in December 2017 and an aggregate of $25.0 million milestone payments received in 2018. All potential future milestones and other payments were considered constrained at the inception of the Sanofi Agreement since the Company 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 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 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 completed. 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. We concluded upon analysis that there is no adjustment necessary in revenue recognized through December 31, 2018 under ASC 606 for the Sanofi Agreement. All deliverables under ASC 605 and all performance obligations under ASC 606 had been completed as of June 30, 2019. For the three and six months ended June 30, 2019, we recognize $30.0 million in revenue, respectively, related to the Sanofi Agreement. For the three and six months ended June 30, 2018, we recognized approximately $11.5 million and $21.4 million in revenue, respectively, related to the Sanofi Agreement. 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. We identified the following performance obligations under the AbbVie Agreement: (i) granting a license of rights to certain licensed compounds to develop and commercialize oral immunoproteasome inhibitors, (ii) transferring of technology (know-how) related to the oral immunoproteasome inhibitors program, and (iii) providing research and development services during the two-year research period, which can be extended for up to six months. We determined that each of these performance obligations individually is not distinct, rather each one represents a component of a single performance obligation of the contract. Prior to the adoption of ASC 606, we concluded under ASC 605 that the delivered license did not have standalone value at the inception of the arrangement due to our proprietary expertise with respect to the licensed compounds and related ongoing research participation under the AbbVie Agreement, which is required for AbbVie to fully realize the value from the licensed compound. Therefore, the $15.0 million received related to this combined unit of accounting was being recognized as revenue ratably over the estimated performance period, which was estimated to continue through the fourth quarter of 2019. On January 1, 2019, we adopted ASC 606 using the modified retrospective approach. Upon adoption, the fixed, non-refundable and non-creditable upfront payment of $15.0 million received in 2017 was determined to be the transaction price. Revenue is recognized based on a measurement of progress toward the completion of the performance obligation of providing research services for two years, which was subject to an extension for up to six months. In March 2019, we announced a mutual agreement with AbbVie to end our collaboration and to reacquire rights to the program. The measurement is calculated using an input-method based on research costs incurred by us during each reporting period compared to the total research costs projected to provide research services by us pursuant to the AbbVie Agreement. Such costs include external direct costs and internal direct labor consisting of the efforts of certain of our employees that dedicate their time providing research services pursuant to the AbbVie Agreement. 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. For the three and six months ended June 30, 2019, we recognized zero revenue and the remaining balance of the transaction price of $5.2 million in revenue, respectively, related to the AbbVie Agreement. For the three and six months ended June 30, 2018, we recognized approximately $1.5 million and $3.0 million in revenue, respectively, related to the AbbVie Agreement .

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2019
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 period with an option to extend for another seven year period subject to certain conditions. Pursuant to the April 2018 Lease Agreement, we provided a letter of credit to the landlord for $0.6 million which is recorded as long-term restricted cash at June 30, 2019. T he 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. Reimbursable construction costs incurred will be recorded as a leasehold improvement with a corresponding lease incentive obligation, which is classified as a component of deferred rent. Amounts that will be reimbursed under the tenant improvement allowance will be recorded as deferred rent and amortized as a reduction to rent expense over the lease term. We utilized the aggregate amount of allowances available to us and have recorded $8.3 million of tenant improvement costs as leasehold improvements as of June 30, 2019. We recognize rent expense on a straight-line basis over the non-cancellable lease term and record the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. 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 recorded tenant improvement allowances as deferred rent and the associated expenditures as leasehold improvements. Leasehold improvements are being amortized 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 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. Future minimum lease payments for operating leases at June 30, 2019 are as follows (in thousands): Year ended December 31, 2019 (remaining 6 months) $ 1,502 2020 3,093 2021 3,193 2022 3,296 2023 3,403 2024 and beyond 7,447 Total $ 21,934 We recorded rent expense of $0.7 million and $1.4 million for the three and six months ended June 30, 2019, respectively, and $0.5 million and $0.9 million for the three and six months ended June 30, 2018, respectively. Our previous lease, which expired on January 31, 2019, was for 30,000 square feet of office, research and development, and laboratory space in South San Francisco. It was subject to several amendments to secure additional space, sublease certain office and laboratory space and/or extend the lease term. 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 indemnification issues for any periods presented. We indemnify each of our officers and directors for certain events or occurrences, subject to certain limitations, while the officer or director 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 and officers. The term of the indemnification period lasts as long as an officer or a director may be subject to any proceeding arising out of acts or omissions of such officer or director 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. 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, 2019
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, 2018 2,998,590 $ 9.39 Options granted 1,247,125 $ 34.38 Options forfeited (103,754 ) $ 15.09 Options exercised (17,603 ) $ 6.64 Options repurchased — Shares added to plans — Balance at June 30, 2019 4,124,358 $ 16.81 Vested and expected to vest 4,124,358 $ 16.81 Exercisable as of June 30, 2019 2,428,297 $ 6.96 At June 30, 2019 and December 31, 2018, 29,460 shares and 33,882 shares, respectively, were subject to repurchase by us at the original exercise price in the event the optionee’s employment is terminated either voluntarily or involuntarily. We classify cash received from the exercise of unvested options as a short term liability. Liabilities related to the early exercise of stock options were approximately $0.2 million as of June 30, 2019 and December 31, 2018, respectively, and were included in accrued liabilities on the condensed consolidated balance sheets. Amounts from liabilities are reclassified into common stock and additional paid-in capital as the shares vest, which is generally over 48 months. 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, 2019 2018 2019 2018 Research and development $ 1,776 $ 179 $ 2,926 $ 341 General and administrative 1,700 188 2,755 392 Total $ 3,476 $ 367 $ 5,681 $ 733 As of June 30, 2019, there was approximately $40.5 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 3.3 years. The weighted-average remaining contractual term of options outstanding at June 30, 2019, was 8.0 years. Our stock-based compensation expense for the three and six months ended June 30, 2019 includes approximately $0.7 million in expense related to modifications of stock options for members of our Board of Directors.

Net Income (Loss) per Share

Net Income (Loss) per Share	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Net Income (Loss) per Share	9. Net Income (Loss) per Share Net income is allocated between our common stock and other participating securities based on their participation rights. Additionally, during the periods in which we have net income, the diluted net income per share has been computed using the weighted average number of shares of common stock outstanding and other dilutive securities. For the periods presented with a net loss herein, the computation of basic earnings per share using the two-class method is not applicable as the participating securities do not have contractual rights and obligations to share in our losses. 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, 2019 2018 2019 2018 Numerator Net income (loss) $ 7,115 $ 1,797 (6,572 ) $ 2,107 Allocation of undistributed earnings to participating securities — (1,797 ) — (2,107 ) Net income (loss) attributable to common stockholders 7,115 — (6,572 ) — Denominator Weighted-average common shares outstanding, basic 23,927,172 681,616 23,896,788 656,129 Effect of dilutive shares: 1,864,929 731,312 — 852,455 Weighted-average shares outstanding used in per share calculation, diluted 25,792,101 1,412,928 23,896,788 1,508,584 Net income (loss) per share, basic $ 0.30 $ — $ (0.28 ) $ — Net income (loss) per share, diluted $ 0.28 $ — $ (0.28 ) $ — The following outstanding shares of common stock equivalents were excluded in the computation of diluted net income (loss) per share attributable to common stockholders, because their effect would have been antidilutive for the periods presented: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Employee stock purchase plan 11,326 — 43,745 — Warrants to purchase capital stock 48,406 173,000 168,046 173,000 Common stock options issued and outstanding 2,390,056 1,323,262 4,124,358 1,202,118 Early exercised common stock subject to future vesting 29,460 21,835 29,460 21,835 Total 2,479,248 1,518,097 4,365,609 1,396,953

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2019
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, 2019 and 2018 and for the six months ended June 30, 2018, 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, 2019
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, 2018 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 19, 2019 (“2018 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. The majority of our cash and cash equivalents is maintained with one financial institution in the United States. Deposits with this financial institution have exceeded and will continue to exceed federally insured limits. We have not experienced any losses on our cash deposits. 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 early-stage biopharmaceutical companies, including, but not limited to, the need to obtain adequate additional funding, possible failure of current or future preclinical studies or clinical 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, 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. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on 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, 2019 and December 31, 2018, 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 lease agreements for our laboratory and office facilities. These leases are classified as operating leases. Rent expense is recognized on a straight-line basis over the term of the lease. The difference between rent expense recognized and rental payments is recorded as deferred rent in the condensed consolidated balance sheets. Lease incentives and allowance provided by our landlord for the construction of leasehold improvements are recorded as lease incentive obligations as the related construction costs are incurred, up to the maximum aggregate allowances. Lease incentive obligations are classified as a component of deferred rent and are amortized on a straight-line basis over the lease term as a reduction of rent expense.
Revenue Recognition	Revenue Recognition Effective January 1, 2019, we adopted Accounting Standards Codification, or ASC No. 2014-09, Revenue from Contracts with Customers, 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 . In determining the appropriate amount of revenue to be recognized as we fulfill our obligations under such licensing and collaboration agreements, we perform the five-step model under ASC 606. As part of the accounting for these arrangements, we must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. 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. The impact of the adoption of Topic 606 on the accompanying condensed consolidated balance sheet as of January 1, 2019 was as follows (in thousands): December 31, 2018 Adjustments Due to ASC 606 January 1, 2019 Liabilities Deferred revenue $ 5,616 $ (456 ) $ 5,160 Stockholders' Equity Accumulated deficit (132,407 ) 456 (131,951 ) The impact of the adoption of ASC 606 on our unaudited condensed consolidated statement of operations for the three and six months ended June 30, 2019 was as follows (in thousands): Three Months Ended June 30, 2019 As Reported Adjustments Balance without ASC 606 Adoption Revenue $ 30,000 $ — $ 30,000 Income (loss) from operations 6,049 — 6,049 Net income (loss) 7,115 — 7,115 Net income per share, basic 0.30 — 0.30 Net income per share, diluted 0.28 — 0.28 Six Months Ended June 30, 2019 As Reported Adjustments Balance without ASC 606 Adoption Revenue $ 35,160 $ 456 $ 35,616 Income (loss) from operations (8,821 ) 456 (8,365 ) Net income (loss) (6,572 ) 456 (6,116 ) Net loss per share, basic and diluted (0.28 ) 0.02 (0.26 ) During the three and six months ended June 30, 2019, we did not recognize any revenue from performance obligations satisfied in previous periods.
Convertible Preferred Stock Warrants	Convertible Preferred Stock Warrants In connection with the issuance of certain convertible notes in 2016 and 2017 (the “Notes”), we issued warrants to purchase our capital stock. Freestanding warrants to purchase our convertible preferred stock were recorded as a liability on our condensed consolidated balance sheets because the underlying shares of convertible preferred stock are contingently redeemable, which, therefore, may obligate us to transfer assets to settle those warrants. The warrants are subject to revaluation at each balance sheet date, with changes in fair value recognized as a component of other income, net, on the condensed consolidated statements of operations. During the three and six months ended June 30, 2018, other expense, net, included $0.2 million and $0.5 million, respectively, related to the change in fair value of the preferred stock warrant liability. Upon the completion of our IPO in September 2018, all of our convertible preferred stock warrants were converted into warrants to purchase shares of common stock. We re-valued the convertible preferred stock warrants upon completion of our IPO and reclassified the estimated fair value of the warrants to additional paid in capital.
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 attributable to common stockholders is calculated by dividing the net income (loss) attributable to common stockholders 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. We have issued securities other than common stock that participate in dividends to the extent declared (“Participating Securities”), and therefore utilize the two-class method to calculate net income (loss) per share. These Participating Securities include Series A, Series B-1, Series B-2, Series B-3 and Series C redeemable convertible preferred stock. The two-class method requires a portion of net income (loss) to be allocated to the Participating Securities to determine net income (loss) attributable to common stockholders. Net income (loss) attributable to common stockholders is equal to the net income (loss) less dividends paid on preferred stock with any remaining earnings allocated in accordance with the bylaws between the outstanding common and redeemable convertible preferred stock as of the end of each period.
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. Recently Adopted Accounting Standards Updates In November 2016, the FASB issued accounting standard update (“ASU”) 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash We early adopted ASU 2016-18 during the fourth quarter of 2018 and the adoption did not have a material impact on our consolidated financial statements and related disclosures. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) Entities have the option of applying either a full retrospective approach to all periods presented, or a modified approach that reflects differences prior to the date of adoption as an adjustment to equity. We adopted ASC No. 2014-09 on January 1, 2019 using the modified retrospective method of transition applied to contracts that were not completed at January 1, 2019. Therefore, comparative information will not be adjusted and the impact of the transition is reflected as an adjustment to the opening accumulated deficit. A completed contract is a contract for which all, or substantially all, of the revenue was recognized in accordance with revenue guidance in effect before the date of initial application. The new revenue recognition standard differs from the previous accounting standard in many respects, such as in the accounting for variable consideration and the measurement of progress toward completion of performance obligations. Recently Issued Accounting Standards or Updates Not Yet Effective In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases (Topic 842) In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement . This ASU eliminates, modifies and adds disclosure requirements for fair value measurements. The amendments in this ASU are effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. We are currently evaluating the effects of this ASU on our consolidated financial statements and related disclosures. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. We are currently evaluating the effects of this ASU on our 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.

Significant Accounting Polici_3

Significant Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Summary of Impact of the Adoption of Topic 606 on Condensed Consolidated Balance Sheet	The impact of the adoption of Topic 606 on the accompanying condensed consolidated balance sheet as of January 1, 2019 was as follows (in thousands): December 31, 2018 Adjustments Due to ASC 606 January 1, 2019 Liabilities Deferred revenue $ 5,616 $ (456 ) $ 5,160 Stockholders' Equity Accumulated deficit (132,407 ) 456 (131,951 )
Summary of Impact of the Adoption of ASC 606 on Unaudited Condensed Consolidated Statement of Operations	The impact of the adoption of ASC 606 on our unaudited condensed consolidated statement of operations for the three and six months ended June 30, 2019 was as follows (in thousands): Three Months Ended June 30, 2019 As Reported Adjustments Balance without ASC 606 Adoption Revenue $ 30,000 $ — $ 30,000 Income (loss) from operations 6,049 — 6,049 Net income (loss) 7,115 — 7,115 Net income per share, basic 0.30 — 0.30 Net income per share, diluted 0.28 — 0.28 Six Months Ended June 30, 2019 As Reported Adjustments Balance without ASC 606 Adoption Revenue $ 35,160 $ 456 $ 35,616 Income (loss) from operations (8,821 ) 456 (8,365 ) Net income (loss) (6,572 ) 456 (6,116 ) Net loss per share, basic and diluted (0.28 ) 0.02 (0.26 )

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2019
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, 2019 and December 31, 2018 were as follows (in thousands): June 30, 2019 Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 27,781 $ — $ — $ 27,781 Corporate commercial paper 8,980 — — 8,980 Short-term marketable securities Corporate commercial paper — 32,042 — 32,042 Corporate debt securities — 64,438 — 64,438 Government‑ sponsored enterprise securities — 32,012 — 32,012 Long-term marketable securities Government‑ sponsored enterprise securities — 8,972 — 8,972 Total $ 36,761 $ 137,464 $ — $ 174,225 December 31, 2018 Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 19,861 $ — $ — $ 19,861 Corporate commercial paper — 7,465 — 7,465 Corporate debt securities — 4,499 — 4,499 Short-term marketable securities Corporate commercial paper — 36,180 — 36,180 Corporate debt securities — 71,903 — 71,903 Government‑ sponsored enterprise securities — 9,906 — 9,906 U.S. Treasury securities — 24,447 — 24,447 Long-term marketable securities Corporate debt securities — 3,712 — 3,712 Total $ 19,861 $ 158,112 $ — $ 177,973

Cash Equivalents and Marketab_2

Cash Equivalents and Marketable Securities (Tables)	6 Months Ended
	Jun. 30, 2019
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, 2019 and December 31, 2018 (in thousands): June 30, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents Money market funds $ 27,781 $ — $ — $ 27,781 Corporate commercial paper 8,980 — — 8,980 Total cash equivalents 36,761 — — 36,761 Short-term marketable securities Corporate commercial paper 31,988 54 — 32,042 Corporate debt securities 64,286 153 (1 ) 64,438 Government‑sponsored enterprise securities 32,006 6 — 32,012 Total short-term marketable securities 128,280 213 (1 ) 128,492 Long-term marketable securities Government‑sponsored enterprise securities 8,982 — (10 ) 8,972 Total long-term marketable securities 8,982 — (10 ) 8,972 Total $ 174,023 $ 213 $ (11 ) $ 174,225 December 31, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents Money market funds $ 19,861 $ — $ — $ 19,861 Corporate commercial paper 7,466 — (1 ) 7,465 Corporate debt securities 4,499 — — 4,499 Total cash equivalents 31,826 — (1 ) 31,825 Short-term marketable securities Corporate commercial paper 36,197 — (17 ) 36,180 Corporate debt securities 71,920 8 (25 ) 71,903 Government‑sponsored enterprise securities 9,911 — (5 ) 9,906 U.S. Treasury securities 24,451 — (4 ) 24,447 Total short-term marketable securities 142,479 8 (51 ) 142,436 Long-term marketable securities Corporate debt securities 3,706 6 — 3,712 Total long-term marketable securities 3,706 6 — 3,712 Total $ 178,011 $ 14 $ (52 ) $ 177,973

Balance Sheet Components (Table

Balance Sheet Components (Tables)	6 Months Ended
	Jun. 30, 2019
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, 2019 December 31, 2018 June 30, 2018 December 31, 2017 Cash and cash equivalents $ 41,073 $ 34,489 $ 9,839 $ 41,054 Restricted cash, current — 82 182 100 Restricted cash, non-current 567 567 567 82 Total $ 41,640 $ 35,138 $ 10,588 $ 41,236
Summary of Property and Equipment	Property and equipment as of June 30, 2019 and December 31, 2018 consisted of the following (in thousands): June 30, 2019 December 31, 2018 Laboratory equipment $ 1,973 $ 2,500 Computer equipment 289 508 Furniture and Fixtures 1,499 392 Leasehold improvements 8,324 420 12,085 3,820 Less accumulated depreciation and amortization (1,487 ) (2,154 ) Total $ 10,598 $ 1,666
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets as of June 30, 2019 and December 31, 2018 consisted of the following (in thousands): June 30, 2019 December 31, 2018 Other accounts receivable $ 740 $ 2,104 Prepaid expenses 1,417 1,661 Total $ 2,157 $ 3,765

Commitments and Contingencies (

Commitments and Contingencies (Tables)	6 Months Ended
	Jun. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Lease Payments For Operating Leases	Future minimum lease payments for operating leases at June 30, 2019 are as follows (in thousands): Year ended December 31, 2019 (remaining 6 months) $ 1,502 2020 3,093 2021 3,193 2022 3,296 2023 3,403 2024 and beyond 7,447 Total $ 21,934

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	6 Months Ended
	Jun. 30, 2019
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, 2018 2,998,590 $ 9.39 Options granted 1,247,125 $ 34.38 Options forfeited (103,754 ) $ 15.09 Options exercised (17,603 ) $ 6.64 Options repurchased — Shares added to plans — Balance at June 30, 2019 4,124,358 $ 16.81 Vested and expected to vest 4,124,358 $ 16.81 Exercisable as of June 30, 2019 2,428,297 $ 6.96
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, 2019 2018 2019 2018 Research and development $ 1,776 $ 179 $ 2,926 $ 341 General and administrative 1,700 188 2,755 392 Total $ 3,476 $ 367 $ 5,681 $ 733

Net Income (Loss) per Share (Ta

Net Income (Loss) per Share (Tables)	6 Months Ended
	Jun. 30, 2019
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, 2019 2018 2019 2018 Numerator Net income (loss) $ 7,115 $ 1,797 (6,572 ) $ 2,107 Allocation of undistributed earnings to participating securities — (1,797 ) — (2,107 ) Net income (loss) attributable to common stockholders 7,115 — (6,572 ) — Denominator Weighted-average common shares outstanding, basic 23,927,172 681,616 23,896,788 656,129 Effect of dilutive shares: 1,864,929 731,312 — 852,455 Weighted-average shares outstanding used in per share calculation, diluted 25,792,101 1,412,928 23,896,788 1,508,584 Net income (loss) per share, basic $ 0.30 $ — $ (0.28 ) $ — Net income (loss) per share, diluted $ 0.28 $ — $ (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 attributable to common stockholders, because their effect would have been antidilutive for the periods presented: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Employee stock purchase plan 11,326 — 43,745 — Warrants to purchase capital stock 48,406 173,000 168,046 173,000 Common stock options issued and outstanding 2,390,056 1,323,262 4,124,358 1,202,118 Early exercised common stock subject to future vesting 29,460 21,835 29,460 21,835 Total 2,479,248 1,518,097 4,365,609 1,396,953

Organization - Additional Infor

Organization - Additional Information (Details)	6 Months Ended
	Jun. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Date of incorporation	Oct. 6, 2008

Significant Accounting Polici_4

Significant Accounting Policies - Additional Information (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)Segment	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)
Summary Of Significant Accounting Policies [Line Items]
Cash cash equivalents and marketable securities	$ 178,500,000		$ 178,500,000		$ 180,600,000
Restricted cash	600,000		$ 600,000		$ 600,000
Number of operating segment | Segment			1
Revenue recognized from performance obligations satisfied in previous periods	$ 0		$ 0
Change in fair value of convertible preferred stock warrant liability		$ 200,000	0	$ 509,000
ASC 606 | Adjustments
Summary Of Significant Accounting Policies [Line Items]
Cumulative adjustment to decrease accumulated deficit and deferred revenue			$ 400,000

Significant Accounting Polici_5

Significant Accounting Policies - Summary of Impact of the Adoption of Topic 606 on Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Liabilities
Deferred revenue	$ 0		$ 5,616
Stockholders' equity (deficit)
Accumulated deficit	(138,523)		$ (132,407)
ASC 606
Liabilities
Deferred revenue		$ 5,160
Stockholders' equity (deficit)
Accumulated deficit		$ (131,951)
Adjustments | ASC 606
Liabilities
Deferred revenue	(456)
Stockholders' equity (deficit)
Accumulated deficit	$ 456

Significant Accounting Polici_6

Significant Accounting Policies - Summary of Impact of the Adoption of ASC 606 on Unaudited Condensed Consolidated Statement of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 30,000		$ 12,987		$ 35,160	$ 24,436
Income (loss) from operations	6,049		1,871		(8,821)	2,403
Net income (loss)	$ 7,115	$ (13,687)	$ 1,797	$ 310	$ (6,572)	$ 2,107
Net income per share, basic	$ 0.30		$ 0		$ (0.28)	$ 0
Net income per share, diluted	$ 0.28		$ 0		(0.28)	$ 0
Net loss per share, basic and diluted					$ (0.28)
Adjustments | ASC 606
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenue					$ 456
Income (loss) from operations					456
Net income (loss)					$ 456
Net loss per share, basic and diluted					$ 0.02
Balance without ASC 606 Adoption | ASC 606
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 30,000				$ 35,616
Income (loss) from operations	6,049				(8,365)
Net income (loss)	$ 7,115				$ (6,116)
Net income per share, basic	$ 0.30
Net income per share, diluted	$ 0.28
Net loss per share, basic and diluted					$ (0.26)

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, 2019	Dec. 31, 2018
Assets
Assets	$ 174,225	$ 177,973
Cash Equivalents | Money Market Funds
Assets
Assets	27,781	19,861
Cash Equivalents | Corporate Commercial Paper
Assets
Assets	8,980	7,465
Cash Equivalents | Corporate Debt Securities
Assets
Assets		4,499
Short-term marketable securities | Corporate Commercial Paper
Assets
Assets	32,042	36,180
Short-term marketable securities | Corporate Debt Securities
Assets
Assets	64,438	71,903
Short-term marketable securities | Government- sponsored Enterprise Securities
Assets
Assets	32,012	9,906
Short-term marketable securities | U.S. treasury securities
Assets
Assets		24,447
Long-term marketable securities | Corporate Debt Securities
Assets
Assets		3,712
Long-term marketable securities | Government- sponsored Enterprise Securities
Assets
Assets	8,972
Level 1
Assets
Assets	36,761	19,861
Level 1 | Cash Equivalents | Money Market Funds
Assets
Assets	27,781	19,861
Level 1 | Cash Equivalents | Corporate Commercial Paper
Assets
Assets	8,980	0
Level 1 | Cash Equivalents | Corporate Debt Securities
Assets
Assets		0
Level 1 | Short-term marketable securities | Corporate Commercial Paper
Assets
Assets	0	0
Level 1 | Short-term marketable securities | Corporate Debt Securities
Assets
Assets	0	0
Level 1 | Short-term marketable securities | Government- sponsored Enterprise Securities
Assets
Assets	0	0
Level 1 | Short-term marketable securities | U.S. treasury securities
Assets
Assets		0
Level 1 | Long-term marketable securities | Corporate Debt Securities
Assets
Assets		0
Level 1 | Long-term marketable securities | Government- sponsored Enterprise Securities
Assets
Assets	0
Level 2
Assets
Assets	137,464	158,112
Level 2 | Cash Equivalents | Money Market Funds
Assets
Assets	0	0
Level 2 | Cash Equivalents | Corporate Commercial Paper
Assets
Assets	0	7,465
Level 2 | Cash Equivalents | Corporate Debt Securities
Assets
Assets		4,499
Level 2 | Short-term marketable securities | Corporate Commercial Paper
Assets
Assets	32,042	36,180
Level 2 | Short-term marketable securities | Corporate Debt Securities
Assets
Assets	64,438	71,903
Level 2 | Short-term marketable securities | Government- sponsored Enterprise Securities
Assets
Assets	32,012	9,906
Level 2 | Short-term marketable securities | U.S. treasury securities
Assets
Assets		24,447
Level 2 | Long-term marketable securities | Corporate Debt Securities
Assets
Assets		3,712
Level 2 | Long-term marketable securities | Government- sponsored Enterprise Securities
Assets
Assets	8,972
Level 3
Assets
Assets	0	0
Level 3 | Cash Equivalents | Money Market Funds
Assets
Assets	0	0
Level 3 | Cash Equivalents | Corporate Commercial Paper
Assets
Assets	0	0
Level 3 | Cash Equivalents | Corporate Debt Securities
Assets
Assets		0
Level 3 | Short-term marketable securities | Corporate Commercial Paper
Assets
Assets	0	0
Level 3 | Short-term marketable securities | Corporate Debt Securities
Assets
Assets	0	0
Level 3 | Short-term marketable securities | Government- sponsored Enterprise Securities
Assets
Assets	0	0
Level 3 | Short-term marketable securities | U.S. treasury securities
Assets
Assets		0
Level 3 | Long-term marketable securities | Corporate Debt Securities
Assets
Assets		$ 0
Level 3 | Long-term marketable securities | Government- sponsored Enterprise 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, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	$ 174,023	$ 178,011
Unrealized Gains, Total	213	14
Unrealized Losses, Total	(11)	(52)
Fair Value, Total	174,225	177,973
Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	36,761	31,826
Unrealized Gains, Total	0	0
Unrealized Losses, Total	0	(1)
Fair Value, Total	36,761	31,825
Cash Equivalents | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total		4,499
Unrealized Gains, Total		0
Unrealized Losses, Total		0
Fair Value, Total		4,499
Cash Equivalents | Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	27,781	19,861
Unrealized Gains, Total	0	0
Unrealized Losses, Total	0	0
Fair Value, Total	27,781	19,861
Cash Equivalents | Corporate Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	8,980	7,466
Unrealized Gains, Total	0	0
Unrealized Losses, Total	0	(1)
Fair Value, Total	8,980	7,465
Short-term marketable securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	128,280	142,479
Unrealized Gains, Total	213	8
Unrealized Losses, Total	(1)	(51)
Fair Value, Total	128,492	142,436
Short-term marketable securities | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	64,286	71,920
Unrealized Gains, Total	153	8
Unrealized Losses, Total	(1)	(25)
Fair Value, Total	64,438	71,903
Short-term marketable securities | Corporate Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	31,988	36,197
Unrealized Gains, Total	54	0
Unrealized Losses, Total	0	(17)
Fair Value, Total	32,042	36,180
Short-term marketable securities | Government- sponsored Enterprise Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	32,006	9,911
Unrealized Gains, Total	6	0
Unrealized Losses, Total	0	(5)
Fair Value, Total	32,012	9,906
Short-term marketable securities | U.S. treasury securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total		24,451
Unrealized Gains, Total		0
Unrealized Losses, Total		(4)
Fair Value, Total		24,447
Long-term marketable securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	8,982	3,706
Unrealized Gains, Total	0	6
Unrealized Losses, Total	(10)	0
Fair Value, Total	8,972	3,712
Long-term marketable securities | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total		3,706
Unrealized Gains, Total		6
Unrealized Losses, Total		0
Fair Value, Total		$ 3,712
Long-term marketable securities | Government- sponsored Enterprise Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	8,982
Unrealized Gains, Total	0
Unrealized Losses, Total	(10)
Fair Value, Total	$ 8,972

Cash Equivalents and Marketab_4

Cash Equivalents and Marketable Securities - Additional Information (Details)	6 Months Ended
	Jun. 30, 2019USD ($)
Schedule Of Available For Sale Securities [Line Items]
Marketable securities	$ 0
Short-term marketable securities in unrealized loss position	14,000,000
Other than temporary impairments	$ 0
Maximum
Schedule Of Available For Sale Securities [Line Items]
Remaining contractual maturities of long-term marketable securities	2 years

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 41,073	$ 34,489	$ 9,839	$ 41,054
Restricted cash, current	0	82	182	100
Restricted cash, non-current	567	567	567	82
Total	$ 41,640	$ 35,138	$ 10,588	$ 41,236

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 12,085	$ 3,820
Less accumulated depreciation and amortization	(1,487)	(2,154)
Total	10,598	1,666
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,973	2,500
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	289	508
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,499	392
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 8,324	$ 420

Balance Sheet Components - Su_3

Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Prepaid Expense And Other Assets Current [Abstract]
Other accounts receivable	$ 740	$ 2,104
Prepaid expenses	1,417	1,661
Total	$ 2,157	$ 3,765

License and Collaboration Agr_2

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

1 Months Ended

3 Months Ended

6 Months Ended

12 Months Ended

Nov. 30, 2018

Aug. 31, 2018

Jul. 31, 2018

Dec. 31, 2017

Nov. 30, 2017

Jun. 30, 2017

Jun. 30, 2019

Jun. 30, 2018

Jun. 30, 2019

Jun. 30, 2018

Dec. 31, 2018

Dec. 31, 2017

May 01, 2019

Jan. 01, 2019

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Accumulated deficit

$ 138,523,000

$ 138,523,000

$ 132,407,000

Deferred revenue

0

0

5,616,000

ASC 606

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Accumulated deficit

$ 131,951,000

Deferred revenue

5,160,000

ASC 606 | Adjustments

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Accumulated deficit

(456,000)

(456,000)

Deferred revenue

(456,000)

(456,000)

Sanofi Agreement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Payment received

$ 10,000,000

$ 5,000,000

$ 10,000,000

$ 40,000,000

Milestone payments

30,000,000

30,000,000

25,000,000

Royalty expiration period

10 years

Upfront payment recognized

40,000,000

Transaction price determined

65,000,000

Revenue recognized

30,000,000

$ 11,500,000

30,000,000

$ 21,400,000

Sanofi Agreement | Clinical Development Milestones

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Milestone payments

$ 30,000,000

Revenue recognized

30,000,000

Increase in transaction price

30,000,000

Sanofi Agreement | Maximum

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

License agreement milestone payment receive regulatory milestones and commercial milestones

$ 765,000,000

Abb Vie Agreement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Payment received

$ 15,000,000

$ 15,000,000

Revenue recognized

$ 0

$ 1,500,000

$ 5,200,000

$ 3,000,000

Research and development period

2 years

Abb Vie Agreement | ASC 606

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenue recognized

9,800,000

Abb Vie Agreement | ASC 606 | ASC 605

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenue recognized

$ 9,400,000

Abb Vie Agreement | ASC 606 | Adjustments

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Accumulated deficit

400,000

Deferred revenue

$ 400,000

Abb Vie 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)	Jan. 31, 2019ft²	Apr. 30, 2018USD ($)ft²	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)
Commitments And Contingencies [Line Items]
Area under lease | ft²	30,000
Rent expense			$ 700,000	$ 500,000	$ 1,400,000	$ 900,000
Lease expiration date					Jan. 31, 2019
Accruals for expenses related to indemnification issues					$ 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			$ 600,000		$ 600,000
Tenant improvement cost			8,300,000		$ 8,300,000
Lease agreement commencement date					Aug. 1, 2018
April 2018 Lease Agreement | Leasehold Improvements
Commitments And Contingencies [Line Items]
Tenant improvement allowance claim date					Jul. 1, 2019
April 2018 Lease Agreement | Maximum
Commitments And Contingencies [Line Items]
Tenant improvement allowance			$ 7,100,000		$ 7,100,000
Additional tenant improvement allowance		$ 1,200,000

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Future Minimum Lease Payments For Operating Leases (Details) $ in Thousands	Jun. 30, 2019USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019 (remaining 6 months)	$ 1,502
2020	3,093
2021	3,193
2022	3,296
2023	3,403
2024 and beyond	7,447
Total	$ 21,934

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details) - $ / shares	6 Months Ended
	Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Options Outstanding, Beginning Balance	2,998,590
Options Outstanding, Options granted	1,247,125
Options Outstanding, Options forfeited	(103,754)
Options Outstanding, Options exercised	(17,603)
Options Outstanding, Options repurchased	0
Options Outstanding, Shares added to plans	0
Options Outstanding, Ending Balance	4,124,358
Options Outstanding, Vested and expected to vest	4,124,358
Options Outstanding, Exercisable as of June 30, 2019	2,428,297
Weighted Average Exercise Price, Beginning Balance	$ 9.39
Weighted Average Exercise Price, Options granted	34.38
Weighted Average Exercise Price, Options forfeited	15.09
Weighted Average Exercise Price, Options exercised	6.64
Weighted Average Exercise Price, Ending Balance	16.81
Weighted Average Exercise Price, Vested and expected to vest	16.81
Weighted Average Exercise Price, Exercisable as of June 30, 2019	$ 6.96

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares repurchase in the event of employment termination	29,460	29,460	33,882
Options vesting period		48 months
Unamortized compensation cost related to stock awards	$ 40.5	$ 40.5
Expected weighted average period		3 years 3 months 18 days
Weighted-average remaining contractual term options outstanding		8 years
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	0.7	$ 0.7
Early Exercised Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Early exercised stock options in accrued liabilities	$ 0.2	$ 0.2	$ 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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 3,476	$ 367	$ 5,681	$ 733
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	1,776	179	2,926	341
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 1,700	$ 188	$ 2,755	$ 392

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, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator
Net income (loss)	$ 7,115	$ (13,687)	$ 1,797	$ 310	$ (6,572)	$ 2,107
Allocation of undistributed earnings to participating securities	0		(1,797)		0	(2,107)
Net income (loss) attributable to common stockholders	$ 7,115		$ 0		$ (6,572)	$ 0
Denominator
Weighted-average common shares outstanding, basic	23,927,172		681,616		23,896,788	656,129
Effect of dilutive shares:	1,864,929		731,312		0	852,455
Weighted-average shares outstanding used in per share calculation, diluted	25,792,101		1,412,928		23,896,788	1,508,584
Net income (loss) per share, basic	$ 0.30		$ 0		$ (0.28)	$ 0
Net income per share, diluted	$ 0.28		$ 0		$ (0.28)	$ 0

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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	2,479,248	1,518,097	4,365,609	1,396,953
Employee Stock Purchase Plan
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	11,326	0	43,745	0
Warrants to Purchase Capital Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	48,406	173,000	168,046	173,000
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 income (loss) per share	2,390,056	1,323,262	4,124,358	1,202,118
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 income (loss) per share	29,460	21,835	29,460	21,835

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Provision for income taxes	$ 0	$ 0	$ 0