Principia Biopharma (PRNB) 10-K 19 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Mar. 12, 2019
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	PRNB
Entity Registrant Name	Principia Biopharma Inc.
Entity Central Index Key	0001510487
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Public Float	$ 443
Entity Common Stock, Shares Outstanding		23,865,451

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 34,489	$ 41,054
Short-term marketable securities	142,436	0
Restricted cash	82	100
Prepaid expenses and other current assets	3,765	1,778
Total current assets	180,772	42,932
Property and equipment, net	1,666	209
Long-term restricted cash	567	82
Long-term marketable securities	3,712	0
Other long-term assets	8,804	280
Total assets	195,521	43,503
Current liabilities:
Accounts payable	4,439	3,405
Deferred revenue	5,616	44,134
Accrued research and development liabilities	1,520	1,177
Accrued other liabilities	993	253
Accrued compensation	4,312	2,262
Total current liabilities	16,880	51,231
Deferred revenue, less current portion	0	5,619
Long-term deferred rent	8,781	8
Convertible preferred stock warrant liability	0	1,576
Total liabilities	25,661	58,434
Commitments and contingencies (Note 7)
Convertible preferred stock, $0.0001 par value, 20,000,000 and 113,680,282 shares authorized at December 31, 2018 and 2017, respectively; 0 and 12,285,434 shares issued and outstanding at December 31, 2018 and 2017, respectively (aggregate liquidation preference of $0 and $129,445 at December 31, 2018 and 2017)	0	128,531
Stockholders' equity (deficit)
Common stock, $0.0001 par value, 500,000,000 and 141,432,765 shares authorized at December 31, 2018 and 2017, respectively; 23,865,451 and 626,613 shares issued and outstanding at December 31, 2018 and 2017, respectively	2	1
Additional paid-in-capital	302,393	7,201
Accumulated other comprehensive loss	(128)	(90)
Accumulated deficit	(132,407)	(150,574)
Total stockholders’ equity (deficit)	169,860	(143,462)
Total liabilities, convertible preferred stock, and stockholders’ equity (deficit)	$ 195,521	$ 43,503

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Convertible preferred stock, par value	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	20,000,000	113,680,282
Convertible preferred stock, shares issued	0	12,285,434
Convertible preferred stock, shares outstanding	0	12,285,434
Convertible preferred stock, liquidation preference	$ 0	$ 129,445
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	500,000,000	141,432,765
Common stock, shares issued	23,865,451	626,613
Common stock, shares outstanding	23,865,451	626,613

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenue	$ 69,137	$ 5,247	$ 0
Operating expenses:
Research and development	40,533	25,390	22,290
General and administrative	11,462	6,443	4,763
Total operating expenses	51,995	31,833	27,053
Income (loss) from operations	17,142	(26,586)	(27,053)
Other income (expense), net	(653)	5,096	308
Interest income	1,678	16	5
Interest expense	0	(7,223)	(3,898)
Net income (loss)	18,167	(28,697)	(30,638)
Net income (loss) attributable to common stockholders	$ 4,917	$ (28,697)	$ (30,638)
Net income (loss) per share attributable to common stockholders
Basic	$ 0.65	$ (50.37)	$ (59.89)
Diluted	$ 0.57	$ (50.37)	$ (59.89)
Weighted-average shares used to calculate net income (loss) per share attributable to common stockholders
Basic	7,622,602	569,719	511,597
Diluted	8,627,473	569,719	511,597

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 18,167	$ (28,697)	$ (30,638)
Other comprehensive income (loss):
Net unrealized loss on available-for-sale securities	(38)	0	0
Foreign currency translation adjustment	0	52	(9)
Comprehensive income (loss)	$ 18,129	$ (28,645)	$ (30,647)

Consolidated Statements of Chan

Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Convertible Preferred Stock
Balance at Dec. 31, 2015						$ 104,926
Balance, Shares at Dec. 31, 2015						10,316,873
Balance at Dec. 31, 2015	$ (89,075)		$ 2,297	$ (133)	$ (91,239)
Balance, Shares at Dec. 31, 2015		484,496
Exercise of stock options and vesting of early exercise shares	143		143
Exercise of stock options and vesting of early exercise shares, Shares		46,501
Foreign currency translation adjustment	(9)			(9)
Stock-based compensation expense	1,053		1,053
Beneficial conversion features of the convertible loan	1,876		1,876
Net income (loss)	(30,638)				(30,638)
Balance at Dec. 31, 2016						$ 104,926
Balance, Shares at Dec. 31, 2016						10,316,873
Balance at Dec. 31, 2016	(116,650)		5,369	(142)	(121,877)
Balance, Shares at Dec. 31, 2016		530,997
Exercise of stock options and vesting of early exercise shares	318	$ 1	317
Exercise of stock options and vesting of early exercise shares, Shares		95,616
Issuance of preferred stock on conversion of notes payable						$ 23,605
Issuance of preferred stock on conversion of notes payable, Shares						1,968,561
Foreign currency translation adjustment	52			52
Stock-based compensation expense	1,027		1,027
Beneficial conversion features of the convertible loan	488		488
Net income (loss)	(28,697)				(28,697)
Balance at Dec. 31, 2017	$ 128,531					$ 128,531
Balance, Shares at Dec. 31, 2017	12,285,434					12,285,434
Balance at Dec. 31, 2017	$ (143,462)	$ 1	7,201	(90)	(150,574)
Balance, Shares at Dec. 31, 2017	626,613	626,613
Issuance of preferred stock from Series C offering						$ 49,800
Issuance of preferred stock from Series C offering, Shares						3,474,668
Conversion of preferred stock at initial public offering						$ (178,331)
Conversion of preferred stock at initial public offering, Shares						(15,760,102)
Conversion of preferred stock at initial public offering	$ 178,332	$ 1	178,331
Conversion of preferred stock at initial public offering, Shares		15,760,102
Conversion of warrants at initial public offering	2,153		2,153
Exercise of stock options and vesting of early exercise shares	$ 1,277		1,277
Exercise of stock options and vesting of early exercise shares, Shares	317,425	287,660
Issuance of stock from initial public offering	$ 110,574		110,574
Issuance of stock from initial public offering, Shares		7,187,500
Stock-based compensation expense	2,801		2,801
Issuance of common stock upon exercise of warrants	56		56
Issuance of common stock upon exercise of warrants, Shares		3,576
Unrealized loss on available-for-sale securities, net of taxes	(38)			(38)
Net income (loss)	18,167				18,167
Balance at Dec. 31, 2018	$ 0
Balance, Shares at Dec. 31, 2018	0
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

Consolidated Statements of Ch_2

Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - $ / shares	Dec. 31, 2018	Sep. 18, 2018	Dec. 31, 2017
Statement Of Stockholders Equity [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001	$ 0.0001

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities:
Net income (loss)	$ 18,167	$ (28,697)	$ (30,638)
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Change in fair value of convertible preferred stock warrant liability	577	526	(43)
Change in fair value of redemption features liability	0	(5,586)	241
Amortization of (discount) premium on marketable securities	(387)	0	0
Depreciation	252	178	232
Stock-based compensation	2,801	1,027	1,053
Deferred rent	745	0	0
Convertible loan issuance cost	0	0	(182)
Amortization of debt discount	0	5,495	3,458
Interest expense on convertible notes	0	1,669	442
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(2,187)	(843)	449
Deferred revenue	(44,137)	49,753	0
Accounts payable	1,034	1,280	696
Accrued liabilities	1,962	2,046	(552)
Net cash provided by (used in) operating activities	(21,173)	26,848	(24,844)
Investing activities:
Purchases of property and equipment	(1,067)	(90)	(12)
Maturities of marketable securities	13,500	0	0
Purchases of marketable securities	(159,299)	0	0
Net cash used in investing activities	(146,866)	(90)	(12)
Financing activities:
Net proceeds from initial public offering	110,574	0	0
Net proceeds from Series C preferred stock issuance	49,800	0	0
Proceeds from issuance of common stock	1,511	317	10
Proceeds from exercise of common stock warrants	56	0	0
Proceeds from issuance of convertible notes	0	8,060	13,433
Net cash provided by financing activities	161,941	8,377	13,443
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash	0	52	(9)
Net increase in cash, cash equivalents and restricted cash	(6,098)	35,187	(11,422)
Cash, cash equivalents and restricted cash at beginning of period	41,236	6,049	17,471
Cash, cash equivalents and restricted cash, at end of period	35,138	41,236	6,049
Supplemental disclosures of cash flow information
Tenant improvement allowance receivables	8,324	0	0
Purchases of property and equipment accrued but not yet paid	642	0	0
Conversion of convertible note and interest into convertible preferred stock	$ 0	$ 23,605	$ 0

Organization

Organization	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	1. Organization 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. Initial Public Offering On September 13, 2018, our Registration Statement on Form S-1 was declared effective by the U.S. Securities and Exchange Commission (“SEC”) for our initial public offering (“IPO”). In connection with the IPO, we issued and sold an aggregate of 7,187,500 shares of common stock, including 937,500 shares issued and sold pursuant to the underwriters’ full exercise of their over-allotment option to purchase additional shares, at an offering price to the public of $17.00 per share. Proceeds from the IPO, net of underwriting discounts and commissions, were $113.6 million In addition, all of our convertible preferred stock warrants were converted into warrants to purchase shares of common stock. In connection with our IPO, on September 18, 2018, our certificate of incorporation was amended and restated to provide for 500,000,000 authorized shares of common stock with a par value of $0.0001 per share and 20,000,000 authorized shares of preferred stock with a par value of $0.0001 per share .

Significant Accounting Policies

Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Significant Accounting Policies	2. Significant Accounting Policies Basis of Presentation The accompanying 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. Reverse Stock Split In August 2018, our board of directors approved an amendment to our amended and restated certificate of incorporation to effect a 1-for-9.0839 reverse split (the “Reverse Split”) of shares of our common and convertible preferred stock, which was effected on August 31, 2018. All share and per share information included in the accompanying consolidated financial statements has been adjusted to reflect the Reverse Split. Accordingly, all issued and outstanding common stock and preferred stock and related per share amounts contained in the financial statements have been retroactively adjusted to reflect the Reverse Split for all periods presented. 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 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 milestone, royalty or other payments due 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 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 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 December 31, 2018 and 2017, we had $0.6 million and $0.2 million, respectively, in combined restricted cash and long-term restricted cash that is used to secure financing through a company credit card and for a lease security deposit. This amount is separated from cash and cash equivalents on the consolidated balance sheets. Segments We have one operating segment. Our chief operating decision maker, our Chief Executive Officer, manages our operations on a consolidated basis in assessing performance and allocating resources. Property and Equipment Property and equipment consist of laboratory equipment, computer equipment, furniture and fixtures and leasehold improvements and are stated at cost, net of accumulated depreciation. Maintenance and repairs are charged to expense as incurred. Depreciation is recognized using the straight-line method over the estimated useful lives of the assets generally ranging from two years for computer equipment to five years for office furniture and fixtures. Leasehold improvements are amortized over the shorter of their estimated useful lives or the related lease term. 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. Deferred rent consists of the difference between cash payments and the recognition of rent expense for the buildings we occupy. Lease incentives and allowance provided by our landlord for the construction of leasehold improvements are recorded as lease incentive obligations, 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. Rent expense from operating leases is recognized on a straight-line basis over the lease term. The difference between rent expense recognized and rental payments is recorded as deferred rent in the consolidated balance sheets. Revenue Recognition We generate revenues from our collaboration agreements with Sanofi and with AbbVie. Revenue is recognized when the four revenue recognition criteria are met: (1) persuasive evidence of an arrangement exists; (2) delivery has occurred or services have been rendered; (3) the fee is fixed or determinable; and (4) collectability is reasonably assured. We account for multiple element arrangements, in which we may provide several deliverables, in accordance with Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) Subtopic 605-25, Multiple Element Arrangements. Revenue from non-refundable upfront fees that are not dependent on any future performance by us is recognized when such amounts are earned. If we have continuing obligations to perform under the arrangement, such fees are recognized over the estimated period of the continuing performance obligation. Where a portion of non-refundable upfront fees or other payments received is allocated to continuing performance obligations under the terms of the agreement, it is recorded as deferred revenue and recognized as revenue ratably over the term of our estimated performance period under the agreement. We determine the estimated performance periods and review them periodically based on the progress of the related program. The effect of any change made to an estimated performance period and, therefore revenue recognized, would occur on a prospective basis in the period that the change was made. We have received reimbursements for a portion of our research and development costs. Such reimbursements are recorded as a reduction to research and development expenses on our consolidated statements of operations. Our collaboration agreements may contain milestone payments that become due upon the achievement of certain milestones. We apply ASC Subtopic 605-28, Milestone Method. Under the milestone method, payments that are contingent upon achievement of a substantive milestone are recognized in the period in which the milestone is achieved. A milestone is defined as an event that can only be achieved based on our performance and there is substantive uncertainty about whether the event will be achieved at the inception of the arrangement. Events that are contingent only on the passage of time or only on counterparty performance are not considered milestones subject to this guidance. Further, for the milestone to be considered substantive, the consideration earned from the milestone must relate solely to prior performance, be reasonable relative to all of the deliverables, and the consideration must be commensurate with our performance to achieve the milestone or the enhancement of the value of the item delivered as a result of a specific outcome resulting from our performance to achieve the milestone. Non-substantive milestone or contingent payments are recognized as revenue over the estimated period of any remaining performance obligations or when earned as a result of counterparty performance in accordance with contractual terms and when such payments can be reasonably estimated and collectability of such payments is reasonably assured. Convertible Preferred Stock Warrants 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. Prior to the IPO, freestanding warrants to purchase our convertible preferred stock were recorded as a liability on the consolidated balance sheets. The warrants were subject to revaluation at each balance sheet date, with changes in fair value recognized as a component of other income, net, on the consolidated statements of operations. Freestanding warrants to purchase our convertible preferred stock were recorded as a liability on the consolidated balance sheets because the underlying shares of convertible preferred stock are contingently redeemable, which, therefore, may obligate us to transfer assets at some point in the future to settle these warrants. The warrants were subject to revaluation at each balance sheet date, with changes in fair value recognized as a component of other income, net, on the consolidated statements of operations. The fair value of the preferred stock warrants was determined using a hybrid approach that takes into account the probability of an IPO and non-IPO liquidity events. The non-IPO liquidity event scenario uses an option pricing model to allocate the total enterprise value to the various securities within our capital structure based on future expectations. The IPO scenario also uses an option pricing model to estimate the value of the warrant in the case of an IPO. See Note 8, Debt Obligation, to our consolidated financial statements included elsewhere in this report. In September 2018, upon our IPO, all convertible preferred stock warrants were converted into common stock warrants. Convertible Notes Redemption Features We recorded a derivative liability related to redemption features embedded within our convertible notes. The convertible notes issued in 2016 and 2017 included features that were determined to be an embedded derivative requiring bifurcation and separate accounting. This bifurcated redemption feature was initially recorded at fair value and was subject to revaluation at each balance sheet date. Changes in fair value were recognized as a component of other income, net, on the consolidated statements of operations. In December 2017, the convertible notes and interest payable were converted into Series B-3 preferred stock and the convertible notes redemption features liability was determined to have a fair value of zero. The fair value of our convertible notes redemption features was $3.6 million as of December 31, 2016, and $0 as of December 31, 2018 and 2017, respectively. See Note 8, Debt Obligation. We determined the fair value of the convertible notes redemption features liability based on a lattice model that utilizes projected outcomes and the probability of those outcomes. The inputs used to determine the estimated fair value of the derivative instrument include the probability of an underlying event triggering the exercise of the redemption features and its timing. The fair value measurement was based upon significant inputs not observable in the market at each valuation date. The inputs included our evaluation of the likelihood of various settlement scenarios for the convertible notes, including the probabilities of (a) subsequent preferred stock sales at various price ranges, (b) the completion of an IPO, (c) a change in control of the company, (d) conversion of the convertible notes upon maturity, and (e) repayment of the convertible notes upon maturity. The settlement scenarios that would have triggered payments under the redemption features included the conversion to a future round of financing at a discounted price and a change in control prior to the conversion or repayment of the convertible notes. The other scenarios would not have triggered payment under the redemption features. These assumptions were inherently subjective and involved significant management judgment. See Note 8, Debt Obligation, to our consolidated financial statements included elsewhere in this report. Research and Development Expenses Research and development expenses consist primarily of personnel costs, including salaries, benefits and stock-based compensation, costs related to clinical and preclinical studies, contract manufacturing, consulting fees, laboratory supplies, and allocated overhead and facility occupancy costs. Research and development costs are expensed as incurred. Non-refundable advance payments for goods or services that will be used or rendered for future research and development activities are capitalized and expensed as the goods are delivered or the related services are performed. Costs associated with co-development activities are included in research and development expenses, with any reimbursement of costs by our collaboration partners reflected as a reduction of research and development expenses. Stock-based Compensation We use the Black-Scholes valuation model to estimate the grant date fair value of stock option awards with time-based vesting terms. The determination of fair value for stock-based awards on the date of grant using an option-pricing model (OPM) requires management to make certain assumptions regarding a number of variables. Stock-based compensation expense is recognized based on the grant date fair value on a straight-line basis over the requisite service period and is reduced for forfeitures as they occur. Equity-based payments granted to non-employees are recorded at grant date fair value and are re-measured at the current fair value at the end of each reporting period until the underlying equity instruments vest. Non-employee stock-based compensation expense was immaterial for all periods presented. Income Taxes We account for income taxes under the asset and liability method. Deferred tax assets and liabilities are recognized for future tax consequences attributable to the differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and net operating loss and tax credit carryforwards. Valuation allowances are established when necessary to reduce deferred tax assets to the amount expected to be realized. Deferred tax assets and liabilities are measured at the balance sheet date using the enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in the period such tax rate changes are enacted. We record uncertain tax positions on the basis of a two-step process in which (1) it determines whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, we recognize the largest amount of tax benefit that is more than 50 percent likely to be realized upon ultimate settlement with the related tax authority. Our policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the underpayment of income taxes. 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 are carried at estimated fair values on the consolidated balance sheets and losses on foreign currency translation related to our wholly-owned Australian subsidiary. 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 (“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 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 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 has 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. Recently Issued Accounting Standards or Updates Not Yet Effective 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 will adopt ASU 2014-09 on January 1, 2019 using the modified retrospective method of transition applied to contracts that were not completed at January 1, 2019. 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. We are finalizing our assessment of the impact of adoption and expect to record a cumulative effect adjustment to our retained earnings during the first quarter of 2019 to reflect revenue being recognized based on a measurement of progress toward completion for the combined performance obligation, rather than on a straight-line basis, under the AbbVie agreement. We are also evaluating the impact the standard will have on our disclosures and are implementing changes to our current policies and practices, and internal controls over financial reporting to address the requirements of the standard. In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) In August 2018, the FASB issued ASU 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.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
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 December 31, 2018 and 2017 were as follows (in thousands): 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 December 31, 2017 Level 1 Level 2 Level 3 Total Assets Money market funds $ 30 $ — $ — $ 30 Liabilities Convertible preferred stock warrant liability $ — $ — $ 1,576 $ 1,576 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 years ended December 31, 2018 and 2017.

Cash Equivalents and Marketable

Cash Equivalents and Marketable Securities	12 Months Ended
	Dec. 31, 2018
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 December 31, 2018 (in thousands): 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 We had no marketable securities as of December 31, 2017. All of our marketable securities were considered available-for-sale at December 31, 2018. 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 $113.5 million as of December 31, 2018. We have determined that (i) we do not have the intent to sell any of these marketable securities, and (ii) it is not more likely than not that we will be required to sell any of these marketable securities 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 year ended December 31, 2018. At December 31, 2018, the remaining contractual maturities of long-term marketable securities were less than two years .

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	5. Balance Sheet Components Property and equipment as of December 31, 2018 and 2017 consisted of the following (in thousands): December 31, 2018 December 31, 2017 Laboratory equipment $ 2,500 $ 1,617 Computer equipment 508 281 Furniture and Fixtures 392 10 Leasehold improvements 420 203 3,820 2,111 Less accumulated depreciation and amortization (2,154 ) (1,902 ) Total $ 1,666 $ 209 Depreciation and leasehold improvement amortization expenses were $0.2 million for each of the years ended December 31, 2018 and 2017. Prepaid expenses and other current assets as of December 31, 2018 and December 31, 2017 consisted of the following (in thousands): December 31, 2018 December 31, 2017 Other accounts receivable $ 2,104 $ 378 Prepaid expenses 1,661 1,400 Total $ 3,765 $ 1,778 Other long-term assets as of December 31, 2018 and December 31, 2017 consisted of the following (in thousands): December 31, 2018 December 31, 2017 Tenant improvement allowance receivables $ 8,324 $ — Long-term deposits 480 280 Total $ 8,804 $ 280

License and Collaboration Agree

License and Collaboration Agreements	12 Months Ended
	Dec. 31, 2018
Research And Development [Abstract]
License and Collaboration Agreements	6. License and Collaboration Agreements Sanofi In November 2017, we entered into the Sanofi Agreement for an exclusive license to PRN2246/SAR442168 and backup molecules for development in MS and other CNS diseases. Under the Sanofi Agreement, we will complete Phase 1 activities, and Sanofi will take on all further development activities. We and Sanofi will each be responsible for certain early development costs, and Sanofi will be 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 that 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 payment to us of a field expansion payment as well as milestones and royalties within the expanded field. In December 2017, we received a $40.0 million upfront payment. Under the Sanofi Agreement, our deliverables are (i) grant a license of rights to PRN2246/SAR442168, (ii) transfer of technology (know-how) related to PRN2246/SAR442168, and (iii) performance of research and development services related to our responsibilities under the early development plan. We considered the provisions of the revenue recognition multiple-element arrangement guidance and concluded that the delivered license does not have standalone value at the inception of the arrangement due to our proprietary expertise with respect to the licensed compound and related ongoing developmental participation under the agreement, which is required for Sanofi to fully realize the value from the delivered license. Therefore, the $40.0 million received related to this combined unit of accounting was recognized as revenue ratably over the performance period, which has concluded as of December 31, 2018. 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 payment of $10.0 million 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. Under ASC Subtopic 605-28, Milestone Method, these milestones are not deemed substantive, and therefore, the $5.0 million payment we achieved in the third quarter of 2018 was recognized ratably over the remaining performance period of the collaboration through December 31, 2018. 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. For the years ended December 31, 2018 and 2017, we recognized $63.1 million and $1.9 million in revenue, respectively, related to the Sanofi Agreement. There was no deferred revenue related to the Sanofi Agreement at December 31, 2018. Deferred revenue related to the Sanofi Agreement was $38.1 million at December 31, 2017. Amounts for certain early development plan costs subject to reimbursement from Sanofi, recorded as a reduction of research and development expense, were approximately $2.4 million and $0.2 million for the years ended December 31, 2018 and 2017, respectively. Under the amended Sanofi Agreement, we may receive development, regulatory and commercial milestone payments up to an aggregate of $765.0 million, as well as royalties up to the mid-teens. As we have satisfied all deliverables under the Sanofi Agreement as of December 31, 2018, any future milestones achieved will be recognized as revenue, if and when earned. We have an option to fund a portion of Phase 3 development costs in return for, at Principia’s 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. Only the additional royalty option would be available if we are developing PRN1008 for major enumerated indications overseen by the FDA’s Division of Pulmonary, Allergy and Rheumatology Products or in certain changes of control involving Principia and 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. AbbVie In June 2017, we entered into a collaboration agreement with AbbVie to research and develop oral immunoproteasome inhibitors and received an upfront payment of $15.0 million. Under the AbbVie Agreement, our deliverables were (i) granting a license of rights to certain licensed compounds to develop and commercialize oral immunoproteasome inhibitors, (ii) transfer of technology (know-how) related to the oral immunoproteasome inhibitors program, and (iii) provide research and development services during the two-year research period, which can be extended for up to six months. We considered the provisions of the revenue recognition multiple-element arrangement guidance and concluded that the delivered license does not have standalone value at the inception of the arrangement due to our proprietary expertise with respect to the licensed compounds and related ongoing developmental participation under the AbbVie Agreement, which was 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 is recognized as revenue ratably over the estimated performance period, which was estimated as of December 31, 2018 to continue through the fourth quarter of 2019. In March 2019, we announced a mutual agreement with AbbVie to end our collaboration and to reacquire rights to the program. We and AbbVie have agreed to conclude the collaboration effective March 2019 and there are no further financial obligations between us, as of the date of the termination. For the years ended December 31, 2018 and 2017, we recognized $6.0 million and $3.4 million, respectively, in revenue related to the AbbVie Agreement. Deferred revenue related to the AbbVie Agreement was $5.6 million and $11.6 million at December 31, 2018 and 2017, respectively. University of California, San Francisco In November 2009, we entered into a license agreement, or the First Agreement, with the Regents of the University of California, or the Regents, which was amended in October 2010, February 2011, and amended and restated in May 2012. In September 2011, we entered into a separate license agreement, or the Second Agreement, with Regents which was amended and restated in December 2013. We refer to the First Agreement and the Second Agreement together as the UC Agreements. Under the UC Agreements, the Regents have granted to us exclusive, worldwide licenses, with the right to grant sublicenses, under the Regents’ patent rights in certain patent applications to make, use, sell, offer for sale, and import products and services and practice methods covered by such patent applications in all fields of use. We have paid the Regents license fees of $30,000 and $10,000 under the First Agreement and the Second Agreement, respectively, and we are required to pay the Regents annual license maintenance fees equal to $30,000 and $10,000 for the First Agreement and Second Agreement, respectively, prior to launching any licensed product. We may be obligated to make one-time regulatory and development milestone payments under the First Agreement or the Second Agreement, for future drug candidates. We are obligated to pay the Regents tiered royalty payments in the low single digits on net sales of the licensed products. The royalty is payable under the First Agreement or the Second Agreement, but not both. For licensed products under the First Agreement, we have to make a minimum annual royalty payment of $50,000 beginning in the calendar year after the first commercial sale of a licensed product occurs. The royalties are subject to standard reductions and are payable until the patents upon which such royalties are based expire or are held invalid. The patents issuing under the First Agreement will expire between 2024 and 2030 and under the Second Agreement will expire in 2033, subject to any potential patent term extensions. Additionally, we are obligated to pay the Regents payments on non-royalty licensing revenue we receive from our sub-licensees for products covered by UC patents. The UC Agreements require us to make IPO milestone payments under each of the First Agreement and Second Agreement upon the closing of an IPO. Pursuant to the UC agreements, we made IPO milestone payments totaling approximately $140,000 to the Regents in early October 2018. Under the UC Agreements, we are required to diligently proceed with the development, manufacture, regulatory approval, and sale of licensed products which include obligations to meet certain development-stage milestones within specified periods of time and to market the resulting licensed products in sufficient quantity to meet market demand. We have the right and option to extend the date by which we must meet any milestone in one year extensions by paying an extension fee for second and subsequent extension, provided we can demonstrate we made diligent efforts to meet the milestone.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies Leases Our corporate headquarters are located in South San Francisco, California, where we lease approximately 47,500 square feet of office, research and development, and laboratory space starting February 2019. 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 had been subject to several amendments to secure additional space, sublease certain office and laboratory space and/or extend the lease term. In 2015, we entered into agreements to sublease certain office and laboratory space to two different subtenants. Pursuant to the sublease agreements, we received sublease payments of $0.1 million for year ended December 31, 2017, which are recognized as an offset to rent expense. Both subleases were terminated during 2017. In April 2018, we signed a lease (the “Lease Agreement”) for approximately 47,500 square feet of office, research and development and laboratory space for a seven year period with an option to extend for another seven year period subject to certain conditions. We commenced occupancy in February 2019. 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 December 31, 2018. The Lease Agreement allows for a landlord provided tenant improvement allowance of up to $7.1 million to be applied to the cost of construction of tenant improvements to the new leased premises. The Lease Agreement also provides an option through July 1, 2019 for us to use an additional tenant improvement allowance of up to $1.2 million to apply toward the costs of tenant improvements. Any additional tenant improvement allowance used in connection with this option will be amortized and added to monthly rent payments commencing on the date the tenant improvements are completed. Any portion of the tenant improvement allowance not claimed or drawn by us prior to July 1, 2019 shall expire and shall no longer be available to us thereafter. 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 of December 31, 2018. Amounts that exceed the aggregate amount of allowances are not considered lease incentives and will not be recorded to deferred rent. 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 December 31, 2018 are as follows (in thousands): Year ended December 31, 2019 $ 2,354 2020 3,087 2021 3,187 2022 3,290 2023 3,397 2024 and beyond 7,434 Total $ 22,749 We recorded rent expense of $2.7 million for the year ended December 31, 2018 and rent expense, net of sublease payments, of $1.6 million and $1.0 million for the years ended December 31, 2017 and 2016, respectively. 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.

Debt Obligation

Debt Obligation	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Debt Obligation	8. Debt Obligation Convertible Note Financing In August 2016, we entered into a Note and Warrant Purchase Agreement (the “Convertible Note Agreement”), as amended in September 2016 and March 2017, with certain of our existing investors, pursuant to which we issued convertible notes (the “Notes”) in the aggregate amount of $21.5 million. The Notes had an annual interest rate of 8.0% and a maturity date of December 31, 2017. At the time of issuance of the Notes in 2016 and 2017, we determined that a beneficial conversion feature existed based on the amount by which the fair value of the underlying shares of Series B-3 preferred stock exceeded the effective conversion rate of the Notes. An incremental debt discount of $0.5 million and $1.9 million was recorded related to this beneficial conversion feature in 2017 and 2016, respectively. Notes Redemption Features Liability The Notes also had redemption features that were determined to be an embedded derivative requiring bifurcation and separate accounting. The fair value of the embedded derivative upon issuance was determined to be $3.4 million and $1.9 million for the Notes issued in 2016 and 2017, respectively, and was recognized as a debt discount and as a derivative liability on the consolidated balance sheets. The embedded derivative associated with the Notes required re-measurement to fair value using Level 3 inputs while the Notes were outstanding at the end of each reporting period. In December 2017, the holders of the Notes exercised their option to convert the Notes and accrued interest into Series B-3 convertible preferred stock. In connection with the conversion, we issued 1,968,561 shares of Series B-3 convertible preferred stock at $11.9907 per share in a non-cash transaction and derecognized our convertible notes payable and related notes interest payable. Upon conversion, the fair value of the convertible notes redemption features liability was re-measured to zero. Prior to the conversion, we determined the fair value of the convertible notes redemption features liability based on a lattice model that utilizes projected outcomes and the probability of those outcomes. The inputs used to determine the estimated fair value of the derivative instrument included the probability of an underlying event triggering the exercise of the redemption features and its timing. The fair value measurement was based upon significant inputs not observable in the market at each valuation date. The inputs included our evaluation of the likelihood of various settlement scenarios for the convertible notes, including the probabilities of (a) subsequent preferred stock sales at various price ranges, (b) the completion of an IPO, (c) a change in control of the company, (d) conversion of the Notes upon maturity, and (e) repayment of the notes upon maturity. The settlement scenarios that would have triggered payments under the redemption features included the conversion to a future round of financing at a discounted price and a change in control prior to the conversion or repayment of the Notes. The other scenarios would not have triggered payment under the redemption features. Changes in fair value related to the notes redemption features liability were recorded as a component of other income, net. The following table summarizes the changes in fair value during the years ended December 31, 2016 and 2017 (in thousands): Estimated Fair Value of Convertible Notes Redemption Features Liability Balance at January 1, 2016 $ — Fair value of the redemption features upon issuance of convertible notes 3,407 Change in estimated fair value of redemption features liability included in other income, net 241 Balance at December 31, 2016 3,648 Fair value of the redemption features related to second tranche of convertible notes 1,938 Change in estimated fair value of redemption features liability included in other income, net (5,586 ) Balance at December 31, 2017 $ — In September 2018, upon our IPO, all shares of convertible preferred stock were converted into an equal number shares of common stock. Preferred Stock Warrant Liability In connection with the issuance of the Notes, we issued warrants to purchase our capital stock. Upon conversion of the Notes into Series B-3 preferred stock on December 29, 2017, we determined the total number of shares underlying such warrants to be 168,046 shares of our capital stock at an exercise price of $8.99 per share. The warrants are exercisable upon the issuance date and have a five-year term. The estimated fair value of the warrants upon issuance, of $1.1 million, was based on an option pricing model. We recorded the fair value of the warrants at issuance as a debt discount and as a convertible preferred stock warrant liability. The debt discount was accreted using the effective interest method as additional interest expense over the term of the Notes. The estimated fair value of the warrants is subject to re-measurement at each reporting period. As of December 31, 2017, the fair value of the outstanding warrants was $1.5 million. In September 2011, in connection with entering into a debt facility agreement, we issued warrants to purchase 4,954 shares of Series A convertible preferred stock at a price of $9.0839 per share. The warrants have a 10-year term can be exercised by paying cash or converted into the number of shares of convertible preferred stock equal in value to the intrinsic value of the warrants (“Net Exercises”). The estimated fair value of the warrants is subject to re-measurement at each balance sheet date. As of December 31, 2017, the fair value of outstanding Series A warrants was $27,000. The following table summarizes the changes in the estimated fair value of our convertible preferred stock warrant liability related to Series A and Series B-3 warrants for the years ended December 31, 2018, 2017 and 2016 (in thousands): Estimated Fair Value of Convertible Preferred Stock Warrant Liability Balance at January 1, 2016 $ 31 Warrants issued with convertible notes 1,062 Change in estimated fair value of convertible preferred stock warrant liability included in other income, net (43 ) Balance at December 31, 2016 1,050 Change in estimated fair value of convertible preferred stock warrant liability included in other income, net 526 Balance at December 31, 2017 1,576 Change in estimated fair value of convertible preferred stock warrant liability included in other income, net 634 Reclassification of warrant liability to additional paid-in-capital upon exercise of warrants (57 ) Reclassification of warrant liability to additional paid-in-capital upon initial public offering (2,153 ) Balance at December 31, 2018 $ — The fair value of the preferred stock warrants was determined using a hybrid approach that takes into account the probability of IPO and non-IPO liquidity events. The non-IPO liquidity event scenario uses an option pricing model to allocate the total enterprise value to the various securities within our capital structure based on future expectations. The IPO scenario also uses an option pricing model to estimate the value of the warrant in the case of an IPO. The two OPM models’ inputs reflect assumptions that market participants would use in pricing the instrument in a current period transaction. The fair values of our convertible preferred stock warrant liability related to Series A and Series B-3 warrants were approximately $1.6 million as of December 31, 2017. As of December 31, 2018, no preferred stock warrants were outstanding. Interest expense We had no interest expense during the year ended December 31, 2018. Interest expense of approximately $7.2 million and $3.9 million was recognized during the years ended December 31, 2017 and 2016, respectively. For the year ended December 31, 2017, interest expense included $5.5 million of accretion of debt discounts related to the beneficial conversion features, the convertible notes redemption features liability and the convertible preferred stock warranty liability, and $1.7 million of accrued interest expense on the Notes which was converted into preferred stock in December 2017. For the year ended December 31, 2016, interest expense included $3.5 million of accretion of debt discounts related to the beneficial conversion features, the convertible notes redemption features liability and the convertible preferred stock warranty liability and $0.4 million of accrued interest expense on the Notes.

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Convertible Preferred Stock	9. Convertible Preferred Stock We have issued two classes of stock, convertible preferred stock and common stock. Convertible preferred stock is carried at issuance price, net of issuance costs. In August 2018, we sold 3,474,668 shares of Series C convertible preferred stock at a price of $14.3898 per share for net proceeds of $49.8 million. Upon the completion of the IPO in September 2018, all the outstanding shares of the convertible preferred stock were converted into 15,760,102 shares of common stock, including shares of Series C convertible preferred stock that was issued in August 2018. In addition, all of our convertible preferred stock warrants were converted into warrants to purchase shares of common stock. As of December 31, 2018, we had no remaining shares of convertible preferred stock issued or outstanding. As of December 31, 2017, convertible preferred stock consisted of the following (in thousands, except share numbers): Shares Authorized Shares Issued and Outstanding Liquidation Amount Carrying Value Series A 40,094,998 4,408,893 $ 40,050 $ 39,630 Series B-1 22,957,067 2,527,221 25,253 25,028 Series B-2 19,130,887 2,106,014 25,252 25,027 Series B-3 31,497,330 3,243,306 38,890 38,846 Total 113,680,282 12,285,434 $ 129,445 $ 128,531 Series A Preferred Stock In 2011 and 2012, we sold a total of 4,408,893 shares of Series A convertible preferred stock, at a price per share of $9.0839, for gross proceeds of approximately $40.1 million. Series B Preferred Stock In April 2014, we completed the first tranche (“Series B-1”) of our Series B round of financing. Pursuant to the financing terms, we issued 2,527,221 shares of Series B-1 convertible preferred stock at $9.9923 per share for gross cash proceeds of $25.3 million. The Series B purchase agreement contemplated the sale in an additional closing, the second tranche (“Series B-2”), of up to 2,106,014 additional shares of our Series B preferred stock for aggregate gross proceeds of approximately $25.3 million. Series B-2 closed in November 2014 after we met the pre-defined milestones. Under the terms of the stock purchase agreement, we issued 2,106,014 shares of Series B-2 convertible preferred stock at $11.9907 per share for gross cash proceeds of approximately $25.3 million. In July 2015, we completed another financing with existing investors for Series B-3 convertible preferred stock. Pursuant to the financing terms, we issued 1,274,745 shares of Series B-3 convertible preferred stock at $11.9907 per share for gross cash proceeds of approximately $15.3 million. In December 2017, the holders of the outstanding convertible notes exercised their option to convert the Notes and accrued interest into Series B-3 convertible preferred stock. In connection with the conversion, we issued 1,968,561 shares of Series B-3 convertible preferred stock at $11.9907 per share in a non-cash transaction. Prior to their conversion upon closing of the IPO, the significant rights and obligations of our convertible preferred stock were as follows: Voting Rights —Each holder of convertible preferred stock is entitled to the number of votes equal to the number of shares of common stock into which such shares of convertible preferred stock are convertible. So long as 20% of the shares of convertible preferred stock remain outstanding, the holders of such shares of preferred stock are entitled to elect five members of our Board of Directors. Dividends— Holders of Series A, B-1, B-2, and B-3 convertible preferred stock are entitled to receive non-cumulative dividends, at the rate of 8.0%, 8.8%, 10.56%, and 10.56% per annum, respectively, for each share of Series A, B-1, B-2, and B-3 convertible preferred stock outstanding, when, as, and if declared by our Board of Directors. These dividends are payable in preference to common stock dividends. To date, we have not declared or paid any dividends. Liquidation— In the event of any liquidation, dissolution, or winding up of the company, the holders of Series A, B-1, B-2, and B-3 convertible preferred stock shall be entitled to receive payment from all assets legally available at a liquidation preference of $9.0839, $9.9923, $11.9907, and $11.9907 per share, respectively. In the event that the available funds and assets are insufficient for full payment to the holders of convertible preferred stock on a per-share basis as outlined above, our entire assets and funds legally available for distribution shall be distributed ratably among the holders of convertible preferred stock in proportion to the liquidation preference amount each such holder is otherwise entitled to receive. Upon completion of the distribution of assets as set forth above, all of the remaining assets, if any, shall be distributed ratably among the holders of preferred stock and common stock pro rata based on the number of shares of common stock held by each (assuming full conversion of all preferred stock to common stock at the then effective conversion rate). Conversion— Each share of convertible preferred stock is convertible at a 1:1 ratio at the option of the holder into fully paid and non-assessable shares of common stock. Conversion of each series of convertible preferred stock into common stock is automatic upon the earlier of (i) the closing of an IPO of our common stock, registered under the Securities Act of 1933, as amended, which results in aggregate proceeds equal to or exceeding $50.0 million to us, and the value of our shares immediately prior to the IPO is not less than $22.71 per share (as adjusted for any stock splits, stock dividends, combinations, subdivisions, recapitalizations or the like); or (ii) at any time upon the agreement of the holders of at least 60% of the then outstanding shares of preferred stock

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	10. Stock-Based Compensation Equity Incentive Plans In August 2018, our Board of Directors adopted the 2018 Equity Incentive Plan (the “2018 Plan”). The 2018 Plan provides for the grant of incentive stock options to employees and for the grant of non-statutory stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance stock awards, performance cash awards and other forms of stock awards to employees, directors and consultants. The 2018 Plan initially reserved 2,270,000 of new shares for issuance with automatic annual increases on January 1 of each year equal to 4% of the total number of shares of our capital stock outstanding on the last day of the preceding year, or a lesser number of shares determined by our Board of Directors. The exercise price of stock options is determined to be the market price of our common stock on the date immediately preceding the date of grant. Options granted under the 2018 Plan typically will vest over a four-year period but may be granted with different vesting terms. Awards that expire or terminate without being exercised are added back to the number of shares available for issuance under the 2018 Plan. Following our IPO and in connection with the effectiveness of the 2018 Plan, the 2008 Equity Incentive Plan (the “2008 Plan”) terminated and no further awards will be granted under that plan. All outstanding awards under the 2008 Plan will continue to be governed by their existing terms. The shares subject to stock options or other stock awards granted under the 2008 Plan and the shares that remained outstanding for issuance under the 2008 Plan were transferred into the 2018 Plan. As of December 31, 2018, there was an aggregate 5,194,378 shares of common stock authorized for issuance under the 2018 Plan. Prior to its termination, the 2008 Plan provided for the grant of incentive and non-statutory stock options to eligible employees, officers, directors, advisors and consultants. Options granted under the 2008 Plan generally vest over a four-year period and vested options are exercisable from the vesting date and expire 10 years from the date of grant. The 2008 Plan allowed for early exercise of certain options prior to vesting. Upon termination of employment, the unvested shares are subject to repurchase at the original exercise price. Stock Option Activity The following table summarizes our stock option activity under our stock plans and related information: Options Available for Grant Options Outstanding Weighted Average Exercise Price Weighted Average Remaining Contractual Life Aggregate Intrinsic Value (years) (in thousands) Balance at December 31, 2017 502,916 1,921,453 $ 4.54 7.64 $ 5,213 Options granted (1,418,610 ) 1,418,610 $ 14.84 Options forfeited 24,048 (24,048 ) $ 6.50 Options exercised — (317,425 ) $ 4.72 Options repurchased — — Shares added to plans 3,087,434 — Balance at December 31, 2018 2,195,788 2,998,590 $ 9.39 8.05 $ 53,994 Vested and expected to vest 2,998,590 $ 9.39 8.05 $ 53,994 Exercisable as of December 31, 2018 2,475,279 $ 6.85 7.69 $ 50,854 The weighted-average fair values of stock options granted during t he years 2018, 2017 and 2016 was $10.5, $4.3 and $3.5 per share, respectively At December 31, 2018 and 2017, 33,882 shares and 4,146 shares, respectively, were subject to repurchase by us at the original exercise price in the event that 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 $233,000 and $13,000 as of December 31, 2018 and 2017, respectively, and were included in accrued liabilities on the 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. Common shares are presented as outstanding on our consolidated balance sheets and consolidated statements of changes in convertible preferred stock and stockholders’ equity (deficit) as the shares vest and the underlying right of repurchase lapses. Valuation Assumptions We use the Black-Scholes valuation model to estimate fair value of stock options at the grant date. The Black-Scholes valuation model requires us to make certain estimates and assumptions, including assumptions related to the expected term, expected volatility, risk-free interest rate, and the expected dividend yield for our stock. The fair values for the stock options granted to employees and non-employee directors were estimated at the date of grant using the Black-Scholes valuation model with the following weighted-average assumptions: Twelve Months Ended December 31, 2018 2017 2016 Risk-free interest rate 2.5% - 3.1% 1.8% - 2.2% 1.3% - 1.6% Expected term (years) 5.0 - 6.1 5.8 - 6.1 5.8 - 6.0 Expected volatility 80.0% - 84.5% 81.0% - 85.3% 82.9% - 86.2% Dividend yield —% —% —% The valuation assumptions were determined as follows: • Risk-free interest rate—We base the risk-free interest rate on the interest rate payable on U.S. Treasury securities in effect at the time of grant for a period corresponding with the expected option term. • Expected term—The expected term of options represents the period of time options are expected to be outstanding. The expected term of the options granted is derived using the “simplified” method (that is, estimating the expected term as the mid-point between the vesting date and the end of the contractual term for each option). We use the simplified method because we do not have sufficient historical option exercise data to provide a reasonable basis upon which to estimate the expected term. • Expected volatility—The expected volatility used is based on historical volatilities of similar entities within our industry for periods corresponding with our expected term assumption. • Expected dividend yield—Historically, we have never paid dividends on our common stock and do not expect to pay a dividend in the foreseeable future. Therefore, we used an expected dividend yield of zero. Stock Options Granted to Employees Employee stock-based compensation is measured at the grant date based on the fair value of the award and is recognized as an expense, net of forfeitures, over the requisite service period, which is generally the vesting period of the respective award. Employee stock-based compensation expense includes expenses related to options granted to employees and non-employee directors. The following table summarizes stock-based compensation expense for options granted to employees and non-employee directors (in thousands): Twelve Months Ended December 31, 2018 2017 2016 Research and development $ 1,427 $ 462 $ 513 General and administrative 1,352 559 515 Total $ 2,779 $ 1,021 $ 1,028 As of December 31, 2018, there was approximately $15.6 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 December 31, 2018 is 8.1 years. Stock Options Granted to Outside Consultants For the year ended December 31, 2018, we granted to consultants a total of 1,101 stock options. We did not grant any stock options to consultants for the years ended December 31, 2017 and 2016. Stock-based compensation expense related to outside consultants was insignificant for each of the years ended December 31, 2018, 2017 and 2016. Employee Stock Purchase Plan In September 2018, we adopted the 2018 Employee Stock Purchase Plan (the “ESPP”), which initially reserved 250,000 shares of our common stock for employee purchases under terms and provisions established by the Board of Directors, with automatic annual increases on January 1 of each year equal to the lesser of (a) 1% of the total number of shares of our common stock outstanding on the last day of the preceding year, or (b) 500,000 shares of our common stock. Under the ESPP, employees may purchase common stock through payroll deductions at a price equal to 85% of the lower of the fair market value of common stock on the first trading day of each offering period or on the exercise date. The ESPP provides for consecutive 12-month offering periods. The offering periods may start on the first trading day on or after May 16 or November 16 of each year, except for the first offering period which commenced on September 13, 2018, the pricing date and the effective date of our registration statement. Contributions under the ESPP are limited to a maximum of 15% of an employee's eligible compensation. We recorded approximately $0.2 million of stock-based compensation expense related to our ESPP for the year ended December 31, 2018. No income tax benefit has been recognized related to stock-based compensation expense, and no tax benefits have been realized from exercised stock options.

Net Income (Loss) per Share

Net Income (Loss) per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Income (Loss) per Share	11. Net Income (Loss) per Share Net income is allocated between our common stock and other participating securities based on their participation rights. Our preferred stock (see Note 9) represents participating securities. 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): Year Ended December 31, 2018 2017 2016 Numerator Net income (loss) $ 18,167 $ (28,697 ) $ (30,638 ) Allocation of undistributed earnings to participating securities (13,250 ) — — Net income (loss) attributable to common stockholders 4,917 (28,697 ) (30,638 ) Denominator Weighted-average common shares outstanding, basic 7,622,602 569,719 511,597 Effect of dilutive shares: 1,004,871 — — Weighted-average shares outstanding used in per share calculation, diluted 8,627,473 569,719 511,597 Net income (loss) per share, basic $ 0.65 $ (50.37 ) $ (59.89 ) Net income (loss) per share, diluted $ 0.57 $ (50.37 ) $ (59.89 ) 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. Year Ended December 31, 2018 2017 2016 Series A-1 convertible preferred stock — 4,408,893 4,408,893 Series B-1 convertible preferred stock — 2,527,221 2,527,221 Series B-2 convertible preferred stock — 2,106,014 2,106,014 Series B-3 convertible preferred stock — 3,243,306 1,274,745 Employee stock purchase plan 27,751 — — Warrants to purchase capital stock 168,046 173,000 4,954 Common stock options issued and outstanding 1,993,720 1,921,453 1,430,187 Early exercised common stock subject to future vesting 33,882 4,146 28,854 Total 2,223,399 14,384,033 11,780,868

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	12. Income Taxes Income (loss) before provision for income taxes consisted of the following (in thousands): Year Ended December 31, 2018 2017 2016 United States $ 18,226 $ (28,589 ) $ (30,511 ) Foreign (59 ) (108 ) (127 ) Total $ 18,167 $ (28,697 ) $ (30,638 ) The income tax provision is summarized as follows: Year Ended December 31, 2018 2017 2016 Current Federal $ — $ — $ — State 800 800 800 International — — — Total current tax expense 800 800 800 Deferred Federal — — — State — — — International — — — Total deferred tax expense — — — Total tax expense $ 800 $ 800 $ 800 Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes, and operating losses and tax credit carryforwards. The tax effects of significant items comprising our net deferred tax assets are as follows (in thousands): December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 29,983 $ 36,237 Stock-based compensation 453 312 Deferred revenue 1,179 — Other accrued expenses 1,079 412 Fixed assets and intangibles — 121 Total deferred tax assets 32,694 37,082 Deferred tax liabilities: Fixed assets and intangibles (141 ) — Total deferred tax liabilities (141 ) — Valuation allowance (32,553 ) (37,082 ) Net deferred tax assets $ — $ — In December 2017, the Tax Cuts and Jobs Act (the “Tax Act”) was signed into law making significant changes to the Internal Revenue Code of 1986, as amended. Among other changes, the new tax law included a reduction in the federal corporate tax rate from 35% to 21% effective for tax years beginning after December 31, 2017. Changes in tax law are accounted for in the period of enactment. Under the Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”) issued by the SEC staff in December 2017, we are allowed to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. We calculated the best estimate of the impact of the Tax Act in accordance with our understanding of the Tax Act and guidance available at the time. As such, our consolidated financial statements as of December, 31, 2017 reflected the impact of the Tax Act, which primarily consisted of measuring our deferred tax assets and valuation allowance using the newly enacted U.S. corporate tax rate which resulted in In the fourth quarter of 2018, we completed the related analysis to determine the effect of the Tax Act. The valuation allowance decreased by $4.5 million and $8.0 million during the years ended December 31, 2018 and 2017, respectively, and increased by $9.5 million during the year ended December 31, 2016. Net operating losses and tax credit carryforwards as of December 31, 2018 are as follows (amounts in thousands): Amount Expiration Years Net operating losses, federal $ 104,246 2031 - 2037 Net operating losses, state 111,770 2031 - 2038 Tax credits, federal 5,407 2031 - 2038 Tax credits, state 3,551 No Expiration The effective tax rate of our provision for (benefit from) income taxes differs from the federal statutory income tax provision as follows: Year Ended December 31, 2018 2017 2016 Statutory rate 21.0 % 35.0 % 35.0 % State tax 0.0 % 0.0 % 0.0 % Stock option expense 1.6 % (0.6 %) (0.7 %) General business credit (14.7 %) 4.2 % 3.0 % Uncertain tax positions 14.7 % (4.2 %) (3.0 %) Change in valuation allowance (24.8 %) 30.1 % (28.7 %) Other permanent differences 2.2 % (3.3 %) (5.6 %) Tax reform—tax rate change 0.0 % (61.2 %) 0.0 % Total 0.0 % 0.0 % 0.0 % A reconciliation of our unrecognized tax benefits for the years ended December 31, 2018 and 2017 is as follows (in thousands): 2018 2017 Balance at beginning of year $ 6,119 $ 5,127 Addition for tax positions in prior years 736 105 Addition for tax positions in current year 2,103 887 Balance at end of year $ 8,958 $ 6,119 There is approximately $9.0 million of unrecognized tax benefits included in our consolidated balance sheets. Because of our valuation allowance in the United States, none of the $9.0 million of uncertain tax positions would, if recognized, affect the effective tax rate. We do not believe that our unrecognized tax benefits will significantly change within the next 12 months. It is our practice to recognize interest and penalties related to income tax matters in income tax expense. As of December 31, 2018, we had no accrued interest and penalties related to uncertain tax positions. We file federal, state and foreign income tax returns with varying statutes of limitations. A number of our tax returns remain subject to examination by the tax authorities. These include the U.S. federal and California returns for June 30, 2011 and later years.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2018
Selected Quarterly Financial Information [Abstract]
Selected Quarterly Financial Data (Unaudited)	13. Selected Quarterly Financial Data (Unaudited) The following tables present certain unaudited quarterly financial information for the years ended December 31, 2018 and 2017 (in thousands, except per share amounts): Quarter Ended December 31, 2018 September 30, 2018 June 30, 2018 March 31, 2018 Revenue $ 26,137 $ 18,564 $ 12,987 $ 11,449 Income from operations $ 8,262 $ 6,477 $ 1,871 $ 532 Net income $ 9,377 $ 6,683 $ 1,797 $ 310 Net income per share - basic $ 0.39 $ — $ — $ — Net income per share - diluted $ 0.37 $ — $ — $ — Quarter Ended December 31, 2017 September 30, 2017 June 30, 2017 March 31, 2017 Revenue $ 3,375 $ 1,512 $ 361 $ — Loss from operations $ (6,183 ) $ (5,878 ) $ (6,858 ) $ (7,667 ) Net loss $ (2,830 ) $ (7,125 ) $ (8,805 ) $ (9,937 ) Net loss per share - basic $ (4.55 ) $ (12.26 ) $ (16.27 ) $ (18.63 ) Net loss per share - diluted $ (4.55 ) $ (12.26 ) $ (16.27 ) $ (18.63 )

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Event	14. Subsequent Event In March 2019, we announced a mutual agreement with AbbVie to end our collaboration and to reacquire rights to the program. We and AbbVie have agreed to conclude the collaboration effective March 8, 2019 and there are no further financial obligations between us, as of the date of termination. We expect to recognize the remaining balance of the up-front payment as revenue upon termination of the agreement in March 2019, after considering any amounts recorded as an adjustment to opening retained earnings on January 1, 2019, upon adoption of ASU 2014-09.

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying 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.
Reverse Stock Split	Reverse Stock Split In August 2018, our board of directors approved an amendment to our amended and restated certificate of incorporation to effect a 1-for-9.0839 reverse split (the “Reverse Split”) of shares of our common and convertible preferred stock, which was effected on August 31, 2018. All share and per share information included in the accompanying consolidated financial statements has been adjusted to reflect the Reverse Split. Accordingly, all issued and outstanding common stock and preferred stock and related per share amounts contained in the financial statements have been retroactively adjusted to reflect the Reverse Split for all periods presented.
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 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 milestone, royalty or other payments due 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 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 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 December 31, 2018 and 2017, we had $0.6 million and $0.2 million, respectively, in combined restricted cash and long-term restricted cash that is used to secure financing through a company credit card and for a lease security deposit. This amount is separated from cash and cash equivalents on the consolidated balance sheets.
Segments	Segments We have one operating segment. Our chief operating decision maker, our Chief Executive Officer, manages our operations on a consolidated basis in assessing performance and allocating resources.
Property and Equipment	Property and Equipment Property and equipment consist of laboratory equipment, computer equipment, furniture and fixtures and leasehold improvements and are stated at cost, net of accumulated depreciation. Maintenance and repairs are charged to expense as incurred. Depreciation is recognized using the straight-line method over the estimated useful lives of the assets generally ranging from two years for computer equipment to five years for office furniture and fixtures. Leasehold improvements are amortized over the shorter of their estimated useful lives or the related lease term.
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. Deferred rent consists of the difference between cash payments and the recognition of rent expense for the buildings we occupy. Lease incentives and allowance provided by our landlord for the construction of leasehold improvements are recorded as lease incentive obligations, 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. Rent expense from operating leases is recognized on a straight-line basis over the lease term. The difference between rent expense recognized and rental payments is recorded as deferred rent in the consolidated balance sheets.
Revenue Recognition	Revenue Recognition We generate revenues from our collaboration agreements with Sanofi and with AbbVie. Revenue is recognized when the four revenue recognition criteria are met: (1) persuasive evidence of an arrangement exists; (2) delivery has occurred or services have been rendered; (3) the fee is fixed or determinable; and (4) collectability is reasonably assured. We account for multiple element arrangements, in which we may provide several deliverables, in accordance with Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) Subtopic 605-25, Multiple Element Arrangements. Revenue from non-refundable upfront fees that are not dependent on any future performance by us is recognized when such amounts are earned. If we have continuing obligations to perform under the arrangement, such fees are recognized over the estimated period of the continuing performance obligation. Where a portion of non-refundable upfront fees or other payments received is allocated to continuing performance obligations under the terms of the agreement, it is recorded as deferred revenue and recognized as revenue ratably over the term of our estimated performance period under the agreement. We determine the estimated performance periods and review them periodically based on the progress of the related program. The effect of any change made to an estimated performance period and, therefore revenue recognized, would occur on a prospective basis in the period that the change was made. We have received reimbursements for a portion of our research and development costs. Such reimbursements are recorded as a reduction to research and development expenses on our consolidated statements of operations. Our collaboration agreements may contain milestone payments that become due upon the achievement of certain milestones. We apply ASC Subtopic 605-28, Milestone Method. Under the milestone method, payments that are contingent upon achievement of a substantive milestone are recognized in the period in which the milestone is achieved. A milestone is defined as an event that can only be achieved based on our performance and there is substantive uncertainty about whether the event will be achieved at the inception of the arrangement. Events that are contingent only on the passage of time or only on counterparty performance are not considered milestones subject to this guidance. Further, for the milestone to be considered substantive, the consideration earned from the milestone must relate solely to prior performance, be reasonable relative to all of the deliverables, and the consideration must be commensurate with our performance to achieve the milestone or the enhancement of the value of the item delivered as a result of a specific outcome resulting from our performance to achieve the milestone. Non-substantive milestone or contingent payments are recognized as revenue over the estimated period of any remaining performance obligations or when earned as a result of counterparty performance in accordance with contractual terms and when such payments can be reasonably estimated and collectability of such payments is reasonably assured.
Convertible Preferred Stock Warrants	Convertible Preferred Stock Warrants 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. Prior to the IPO, freestanding warrants to purchase our convertible preferred stock were recorded as a liability on the consolidated balance sheets. The warrants were subject to revaluation at each balance sheet date, with changes in fair value recognized as a component of other income, net, on the consolidated statements of operations. Freestanding warrants to purchase our convertible preferred stock were recorded as a liability on the consolidated balance sheets because the underlying shares of convertible preferred stock are contingently redeemable, which, therefore, may obligate us to transfer assets at some point in the future to settle these warrants. The warrants were subject to revaluation at each balance sheet date, with changes in fair value recognized as a component of other income, net, on the consolidated statements of operations. The fair value of the preferred stock warrants was determined using a hybrid approach that takes into account the probability of an IPO and non-IPO liquidity events. The non-IPO liquidity event scenario uses an option pricing model to allocate the total enterprise value to the various securities within our capital structure based on future expectations. The IPO scenario also uses an option pricing model to estimate the value of the warrant in the case of an IPO. See Note 8, Debt Obligation, to our consolidated financial statements included elsewhere in this report. In September 2018, upon our IPO, all convertible preferred stock warrants were converted into common stock warrants.
Convertible Notes Redemption Features	Convertible Notes Redemption Features We recorded a derivative liability related to redemption features embedded within our convertible notes. The convertible notes issued in 2016 and 2017 included features that were determined to be an embedded derivative requiring bifurcation and separate accounting. This bifurcated redemption feature was initially recorded at fair value and was subject to revaluation at each balance sheet date. Changes in fair value were recognized as a component of other income, net, on the consolidated statements of operations. In December 2017, the convertible notes and interest payable were converted into Series B-3 preferred stock and the convertible notes redemption features liability was determined to have a fair value of zero. The fair value of our convertible notes redemption features was $3.6 million as of December 31, 2016, and $0 as of December 31, 2018 and 2017, respectively. See Note 8, Debt Obligation. We determined the fair value of the convertible notes redemption features liability based on a lattice model that utilizes projected outcomes and the probability of those outcomes. The inputs used to determine the estimated fair value of the derivative instrument include the probability of an underlying event triggering the exercise of the redemption features and its timing. The fair value measurement was based upon significant inputs not observable in the market at each valuation date. The inputs included our evaluation of the likelihood of various settlement scenarios for the convertible notes, including the probabilities of (a) subsequent preferred stock sales at various price ranges, (b) the completion of an IPO, (c) a change in control of the company, (d) conversion of the convertible notes upon maturity, and (e) repayment of the convertible notes upon maturity. The settlement scenarios that would have triggered payments under the redemption features included the conversion to a future round of financing at a discounted price and a change in control prior to the conversion or repayment of the convertible notes. The other scenarios would not have triggered payment under the redemption features. These assumptions were inherently subjective and involved significant management judgment. See Note 8, Debt Obligation, to our consolidated financial statements included elsewhere in this report.
Research and Development Expenses	Research and Development Expenses Research and development expenses consist primarily of personnel costs, including salaries, benefits and stock-based compensation, costs related to clinical and preclinical studies, contract manufacturing, consulting fees, laboratory supplies, and allocated overhead and facility occupancy costs. Research and development costs are expensed as incurred. Non-refundable advance payments for goods or services that will be used or rendered for future research and development activities are capitalized and expensed as the goods are delivered or the related services are performed. Costs associated with co-development activities are included in research and development expenses, with any reimbursement of costs by our collaboration partners reflected as a reduction of research and development expenses.
Stock-based Compensation	Stock-based Compensation We use the Black-Scholes valuation model to estimate the grant date fair value of stock option awards with time-based vesting terms. The determination of fair value for stock-based awards on the date of grant using an option-pricing model (OPM) requires management to make certain assumptions regarding a number of variables. Stock-based compensation expense is recognized based on the grant date fair value on a straight-line basis over the requisite service period and is reduced for forfeitures as they occur. Equity-based payments granted to non-employees are recorded at grant date fair value and are re-measured at the current fair value at the end of each reporting period until the underlying equity instruments vest. Non-employee stock-based compensation expense was immaterial for all periods presented.
Income Taxes	Income Taxes We account for income taxes under the asset and liability method. Deferred tax assets and liabilities are recognized for future tax consequences attributable to the differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and net operating loss and tax credit carryforwards. Valuation allowances are established when necessary to reduce deferred tax assets to the amount expected to be realized. Deferred tax assets and liabilities are measured at the balance sheet date using the enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in the period such tax rate changes are enacted. We record uncertain tax positions on the basis of a two-step process in which (1) it determines whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, we recognize the largest amount of tax benefit that is more than 50 percent likely to be realized upon ultimate settlement with the related tax authority. Our policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the underpayment of income taxes.
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 are carried at estimated fair values on the consolidated balance sheets and losses on foreign currency translation related to our wholly-owned Australian subsidiary.
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 (“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 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 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 has 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. Recently Issued Accounting Standards or Updates Not Yet Effective 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 will adopt ASU 2014-09 on January 1, 2019 using the modified retrospective method of transition applied to contracts that were not completed at January 1, 2019. 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. We are finalizing our assessment of the impact of adoption and expect to record a cumulative effect adjustment to our retained earnings during the first quarter of 2019 to reflect revenue being recognized based on a measurement of progress toward completion for the combined performance obligation, rather than on a straight-line basis, under the AbbVie agreement. We are also evaluating the impact the standard will have on our disclosures and are implementing changes to our current policies and practices, and internal controls over financial reporting to address the requirements of the standard. In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) In August 2018, the FASB issued ASU 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.
Fair Value Measurements	Financial assets and liabilities are recorded at fair value. We determine fair value using the fair value hierarchy, which establishes three levels of inputs that may be used to measure fair value, as follows: Level 1 inputs include quoted prices in active markets for identical assets or liabilities. Level 2 inputs include observable inputs other than Level 1 inputs, such as quoted prices for similar assets or liabilities in active markets; quoted prices for identical or similar assets or liabilities in markets that are not active; and other inputs that are observable or can be corroborated by observable market data for substantially the full term of the asset or liability. Level 3 inputs include unobservable inputs that are supported by little or no market activity and are significant to the fair value of the underlying asset or liability. Such inputs reflect our best estimate of what market participants would use in pricing the asset or liability at the reporting date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model. Assets and liabilities measured at fair value are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. Our assessment of the significance of a particular input to the fair value measurement in its entirety requires our management to make judgments and consider factors specific to the asset or liability.

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
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 December 31, 2018 and 2017 were as follows (in thousands): 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 December 31, 2017 Level 1 Level 2 Level 3 Total Assets Money market funds $ 30 $ — $ — $ 30 Liabilities Convertible preferred stock warrant liability $ — $ — $ 1,576 $ 1,576

Cash Equivalents and Marketab_2

Cash Equivalents and Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Summary of Cash Equivalents and Marketable Securities	Cash equivalents and marketable securities consisted of the following as of December 31, 2018 (in thousands): 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)	12 Months Ended
	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
Summary of Property and Equipment	Property and equipment as of December 31, 2018 and 2017 consisted of the following (in thousands): December 31, 2018 December 31, 2017 Laboratory equipment $ 2,500 $ 1,617 Computer equipment 508 281 Furniture and Fixtures 392 10 Leasehold improvements 420 203 3,820 2,111 Less accumulated depreciation and amortization (2,154 ) (1,902 ) Total $ 1,666 $ 209
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets as of December 31, 2018 and December 31, 2017 consisted of the following (in thousands): December 31, 2018 December 31, 2017 Other accounts receivable $ 2,104 $ 378 Prepaid expenses 1,661 1,400 Total $ 3,765 $ 1,778
Summary of Other Long-Term Assets	Other long-term assets as of December 31, 2018 and December 31, 2017 consisted of the following (in thousands): December 31, 2018 December 31, 2017 Tenant improvement allowance receivables $ 8,324 $ — Long-term deposits 480 280 Total $ 8,804 $ 280

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Lease Payments For Operating Leases	Future minimum lease payments for operating leases at December 31, 2018 are as follows (in thousands): Year ended December 31, 2019 $ 2,354 2020 3,087 2021 3,187 2022 3,290 2023 3,397 2024 and beyond 7,434 Total $ 22,749

Debt Obligation (Tables)

Debt Obligation (Tables)	12 Months Ended
	Dec. 31, 2018
Convertible Notes Redemption Features Liability
Debt Instrument [Line Items]
Summary of Changes in Fair Value Related to Notes Redemption Features Liability and Warrant Liability Related to Series A and B-3 Warrants	Changes in fair value related to the notes redemption features liability were recorded as a component of other income, net. The following table summarizes the changes in fair value during the years ended December 31, 2016 and 2017 (in thousands): Estimated Fair Value of Convertible Notes Redemption Features Liability Balance at January 1, 2016 $ — Fair value of the redemption features upon issuance of convertible notes 3,407 Change in estimated fair value of redemption features liability included in other income, net 241 Balance at December 31, 2016 3,648 Fair value of the redemption features related to second tranche of convertible notes 1,938 Change in estimated fair value of redemption features liability included in other income, net (5,586 ) Balance at December 31, 2017 $ —
Convertible Preferred Stock
Debt Instrument [Line Items]
Summary of Changes in Fair Value Related to Notes Redemption Features Liability and Warrant Liability Related to Series A and B-3 Warrants	The following table summarizes the changes in the estimated fair value of our convertible preferred stock warrant liability related to Series A and Series B-3 warrants for the years ended December 31, 2018, 2017 and 2016 (in thousands): Estimated Fair Value of Convertible Preferred Stock Warrant Liability Balance at January 1, 2016 $ 31 Warrants issued with convertible notes 1,062 Change in estimated fair value of convertible preferred stock warrant liability included in other income, net (43 ) Balance at December 31, 2016 1,050 Change in estimated fair value of convertible preferred stock warrant liability included in other income, net 526 Balance at December 31, 2017 1,576 Change in estimated fair value of convertible preferred stock warrant liability included in other income, net 634 Reclassification of warrant liability to additional paid-in-capital upon exercise of warrants (57 ) Reclassification of warrant liability to additional paid-in-capital upon initial public offering (2,153 ) Balance at December 31, 2018 $ —

Convertible Preferred Stock (Ta

Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Summary of Convertible Preferred Stock	As of December 31, 2017, convertible preferred stock consisted of the following (in thousands, except share numbers): Shares Authorized Shares Issued and Outstanding Liquidation Amount Carrying Value Series A 40,094,998 4,408,893 $ 40,050 $ 39,630 Series B-1 22,957,067 2,527,221 25,253 25,028 Series B-2 19,130,887 2,106,014 25,252 25,027 Series B-3 31,497,330 3,243,306 38,890 38,846 Total 113,680,282 12,285,434 $ 129,445 $ 128,531

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
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 Available for Grant Options Outstanding Weighted Average Exercise Price Weighted Average Remaining Contractual Life Aggregate Intrinsic Value (years) (in thousands) Balance at December 31, 2017 502,916 1,921,453 $ 4.54 7.64 $ 5,213 Options granted (1,418,610 ) 1,418,610 $ 14.84 Options forfeited 24,048 (24,048 ) $ 6.50 Options exercised — (317,425 ) $ 4.72 Options repurchased — — Shares added to plans 3,087,434 — Balance at December 31, 2018 2,195,788 2,998,590 $ 9.39 8.05 $ 53,994 Vested and expected to vest 2,998,590 $ 9.39 8.05 $ 53,994 Exercisable as of December 31, 2018 2,475,279 $ 6.85 7.69 $ 50,854
Summary of Fair Values for Stock Options Granted at Estimated Date of Grant	The fair values for the stock options granted to employees and non-employee directors were estimated at the date of grant using the Black-Scholes valuation model with the following weighted-average assumptions: Twelve Months Ended December 31, 2018 2017 2016 Risk-free interest rate 2.5% - 3.1% 1.8% - 2.2% 1.3% - 1.6% Expected term (years) 5.0 - 6.1 5.8 - 6.1 5.8 - 6.0 Expected volatility 80.0% - 84.5% 81.0% - 85.3% 82.9% - 86.2% Dividend yield —% —% —%
Summary of Stock-based Compensation Expense	The following table summarizes stock-based compensation expense for options granted to employees and non-employee directors (in thousands): Twelve Months Ended December 31, 2018 2017 2016 Research and development $ 1,427 $ 462 $ 513 General and administrative 1,352 559 515 Total $ 2,779 $ 1,021 $ 1,028

Net Income (Loss) per Share (Ta

Net Income (Loss) per Share (Tables)	12 Months Ended
	Dec. 31, 2018
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): Year Ended December 31, 2018 2017 2016 Numerator Net income (loss) $ 18,167 $ (28,697 ) $ (30,638 ) Allocation of undistributed earnings to participating securities (13,250 ) — — Net income (loss) attributable to common stockholders 4,917 (28,697 ) (30,638 ) Denominator Weighted-average common shares outstanding, basic 7,622,602 569,719 511,597 Effect of dilutive shares: 1,004,871 — — Weighted-average shares outstanding used in per share calculation, diluted 8,627,473 569,719 511,597 Net income (loss) per share, basic $ 0.65 $ (50.37 ) $ (59.89 ) Net income (loss) per share, diluted $ 0.57 $ (50.37 ) $ (59.89 )
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. Year Ended December 31, 2018 2017 2016 Series A-1 convertible preferred stock — 4,408,893 4,408,893 Series B-1 convertible preferred stock — 2,527,221 2,527,221 Series B-2 convertible preferred stock — 2,106,014 2,106,014 Series B-3 convertible preferred stock — 3,243,306 1,274,745 Employee stock purchase plan 27,751 — — Warrants to purchase capital stock 168,046 173,000 4,954 Common stock options issued and outstanding 1,993,720 1,921,453 1,430,187 Early exercised common stock subject to future vesting 33,882 4,146 28,854 Total 2,223,399 14,384,033 11,780,868

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Summary of Income (Loss) Before Provision for Income Taxes	Income (loss) before provision for income taxes consisted of the following (in thousands): Year Ended December 31, 2018 2017 2016 United States $ 18,226 $ (28,589 ) $ (30,511 ) Foreign (59 ) (108 ) (127 ) Total $ 18,167 $ (28,697 ) $ (30,638 )
Components of Income Tax Provision	The income tax provision is summarized as follows: Year Ended December 31, 2018 2017 2016 Current Federal $ — $ — $ — State 800 800 800 International — — — Total current tax expense 800 800 800 Deferred Federal — — — State — — — International — — — Total deferred tax expense — — — Total tax expense $ 800 $ 800 $ 800
Schedule of Components of Net Deferred Tax Assets	The tax effects of significant items comprising our net deferred tax assets are as follows (in thousands): December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 29,983 $ 36,237 Stock-based compensation 453 312 Deferred revenue 1,179 — Other accrued expenses 1,079 412 Fixed assets and intangibles — 121 Total deferred tax assets 32,694 37,082 Deferred tax liabilities: Fixed assets and intangibles (141 ) — Total deferred tax liabilities (141 ) — Valuation allowance (32,553 ) (37,082 ) Net deferred tax assets $ — $ —
Summary of Net Operating Losses and Tax Credit Carryforwards	Net operating losses and tax credit carryforwards as of December 31, 2018 are as follows (amounts in thousands): Amount Expiration Years Net operating losses, federal $ 104,246 2031 - 2037 Net operating losses, state 111,770 2031 - 2038 Tax credits, federal 5,407 2031 - 2038 Tax credits, state 3,551 No Expiration
Summary of Effective Tax Rate Provision For (Benefit From) Income Taxes Differs From The Federal Statutory Income Tax Provision	The effective tax rate of our provision for (benefit from) income taxes differs from the federal statutory income tax provision as follows: Year Ended December 31, 2018 2017 2016 Statutory rate 21.0 % 35.0 % 35.0 % State tax 0.0 % 0.0 % 0.0 % Stock option expense 1.6 % (0.6 %) (0.7 %) General business credit (14.7 %) 4.2 % 3.0 % Uncertain tax positions 14.7 % (4.2 %) (3.0 %) Change in valuation allowance (24.8 %) 30.1 % (28.7 %) Other permanent differences 2.2 % (3.3 %) (5.6 %) Tax reform—tax rate change 0.0 % (61.2 %) 0.0 % Total 0.0 % 0.0 % 0.0 %
Schedule of Reconciliation of Unrecognized Tax Benefits	A reconciliation of our unrecognized tax benefits for the years ended December 31, 2018 and 2017 is as follows (in thousands): 2018 2017 Balance at beginning of year $ 6,119 $ 5,127 Addition for tax positions in prior years 736 105 Addition for tax positions in current year 2,103 887 Balance at end of year $ 8,958 $ 6,119

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Selected Quarterly Financial Information [Abstract]
Schedule of Unaudited Quarterly Financial Information	The following tables present certain unaudited quarterly financial information for the years ended December 31, 2018 and 2017 (in thousands, except per share amounts): Quarter Ended December 31, 2018 September 30, 2018 June 30, 2018 March 31, 2018 Revenue $ 26,137 $ 18,564 $ 12,987 $ 11,449 Income from operations $ 8,262 $ 6,477 $ 1,871 $ 532 Net income $ 9,377 $ 6,683 $ 1,797 $ 310 Net income per share - basic $ 0.39 $ — $ — $ — Net income per share - diluted $ 0.37 $ — $ — $ — Quarter Ended December 31, 2017 September 30, 2017 June 30, 2017 March 31, 2017 Revenue $ 3,375 $ 1,512 $ 361 $ — Loss from operations $ (6,183 ) $ (5,878 ) $ (6,858 ) $ (7,667 ) Net loss $ (2,830 ) $ (7,125 ) $ (8,805 ) $ (9,937 ) Net loss per share - basic $ (4.55 ) $ (12.26 ) $ (16.27 ) $ (18.63 ) Net loss per share - diluted $ (4.55 ) $ (12.26 ) $ (16.27 ) $ (18.63 )

Organization - Additional Infor

Organization - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	Sep. 13, 2018	Dec. 31, 2018	Sep. 18, 2018	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Date of incorporation		Oct. 6, 2008
Common stock, shares authorized		500,000,000	500,000,000	141,432,765
Common stock, par value		$ 0.0001	$ 0.0001	$ 0.0001
Preferred stock shares authorized			20,000,000
Preferred stock par value			$ 0.0001
Common Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Issuance of stock, shares		7,187,500
Number of common stock issued on conversion of convertible preferred stock		15,760,102
Common Stock | IPO
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Issuance of stock, shares	7,187,500
Offering price per share	$ 17
Proceeds from IPO, net of underwriting discounts and commissions	$ 113.6
Number of common stock issued on conversion of convertible preferred stock	15,760,102
Common Stock | IPO | Underwriters
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Issuance of stock, shares	937,500

Significant Accounting Polici_3

Significant Accounting Policies - Additional Information (Details)	1 Months Ended	12 Months Ended
	Aug. 31, 2018	Dec. 31, 2018USD ($)Segment	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Summary Of Significant Accounting Policies [Line Items]
Reverse stock split, description		1-for-9.0839
Reverse stock split, conversion ratio	0.110084875
Cash, cash equivalents, marketable securities		$ 180,600,000	$ 41,100,000
Restricted cash		$ 600,000	200,000
Number of operating segment | Segment		1
Unrecognized tax benefits, penalties and interest expense		$ 0
Series B-3 Convertible Preferred Stock
Summary Of Significant Accounting Policies [Line Items]
Fair value of convertible notes redemption features liability		$ 0	$ 0	$ 3,600,000
Computer Equipment
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of assets		2 years
Office Furniture and Fixtures
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of assets		5 years

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	Dec. 31, 2018	Dec. 31, 2017
Assets
Assets	$ 177,973
Convertible Preferred Stock Warrant Liability
Liabilities
Liabilities		$ 1,576
Money Market Funds
Assets
Assets		30
Level 1
Assets
Assets	19,861
Level 1 | Convertible Preferred Stock Warrant Liability
Liabilities
Liabilities		0
Level 1 | Money Market Funds
Assets
Assets		30
Level 2
Assets
Assets	158,112
Level 2 | Convertible Preferred Stock Warrant Liability
Liabilities
Liabilities		0
Level 2 | Money Market Funds
Assets
Assets		0
Level 3
Assets
Assets	0
Level 3 | Convertible Preferred Stock Warrant Liability
Liabilities
Liabilities		$ 1,576
Cash Equivalents | Money Market Funds
Assets
Assets	19,861
Cash Equivalents | Corporate Commercial Paper
Assets
Assets	7,465
Cash Equivalents | Corporate Debt Securities
Assets
Assets	4,499
Cash Equivalents | Level 1 | Money Market Funds
Assets
Assets	19,861
Cash Equivalents | Level 1 | Corporate Commercial Paper
Assets
Assets	0
Cash Equivalents | Level 1 | Corporate Debt Securities
Assets
Assets	0
Cash Equivalents | Level 2 | Money Market Funds
Assets
Assets	0
Cash Equivalents | Level 2 | Corporate Commercial Paper
Assets
Assets	7,465
Cash Equivalents | Level 2 | Corporate Debt Securities
Assets
Assets	4,499
Cash Equivalents | Level 3 | Money Market Funds
Assets
Assets	0
Cash Equivalents | Level 3 | Corporate Commercial Paper
Assets
Assets	0
Cash Equivalents | Level 3 | Corporate Debt Securities
Assets
Assets	0
Short-term marketable securities | Corporate Commercial Paper
Assets
Assets	36,180
Short-term marketable securities | Corporate Debt Securities
Assets
Assets	71,903
Short-term marketable securities | Government‑ sponsored Enterprise Securities
Assets
Assets	9,906
Short-term marketable securities | U.S. treasury securities
Assets
Assets	24,447
Short-term marketable securities | Level 1 | Corporate Commercial Paper
Assets
Assets	0
Short-term marketable securities | Level 1 | Corporate Debt Securities
Assets
Assets	0
Short-term marketable securities | Level 1 | Government‑ sponsored Enterprise Securities
Assets
Assets	0
Short-term marketable securities | Level 1 | U.S. treasury securities
Assets
Assets	0
Short-term marketable securities | Level 2 | Corporate Commercial Paper
Assets
Assets	36,180
Short-term marketable securities | Level 2 | Corporate Debt Securities
Assets
Assets	71,903
Short-term marketable securities | Level 2 | Government‑ sponsored Enterprise Securities
Assets
Assets	9,906
Short-term marketable securities | Level 2 | U.S. treasury securities
Assets
Assets	24,447
Short-term marketable securities | Level 3 | Corporate Commercial Paper
Assets
Assets	0
Short-term marketable securities | Level 3 | Corporate Debt Securities
Assets
Assets	0
Short-term marketable securities | Level 3 | Government‑ sponsored Enterprise Securities
Assets
Assets	0
Short-term marketable securities | Level 3 | U.S. treasury securities
Assets
Assets	0
Long-term marketable securities | Corporate Debt Securities
Assets
Assets	3,712
Long-term marketable securities | Level 1 | Corporate Debt Securities
Assets
Assets	0
Long-term marketable securities | Level 2 | Corporate Debt Securities
Assets
Assets	3,712
Long-term marketable securities | Level 3 | Corporate Debt Securities
Assets
Assets	$ 0

Cash Equivalents and Marketab_3

Cash Equivalents and Marketable Securities - Summary of Cash Equivalents and Marketable Securities (Details) $ in Thousands	Dec. 31, 2018USD ($)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	$ 178,011
Unrealized Gains, Total	14
Unrealized Losses, Total	(52)
Fair Value, Total	177,973
Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	31,826
Unrealized Losses, Total	(1)
Fair Value, Total	31,825
Cash Equivalents | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	4,499
Fair Value, Total	4,499
Cash Equivalents | Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	19,861
Fair Value, Total	19,861
Cash Equivalents | Corporate Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	7,466
Unrealized Losses, Total	(1)
Fair Value, Total	7,465
Short-term marketable securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	142,479
Unrealized Gains, Total	8
Unrealized Losses, Total	(51)
Fair Value, Total	142,436
Short-term marketable securities | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	71,920
Unrealized Gains, Total	8
Unrealized Losses, Total	(25)
Fair Value, Total	71,903
Short-term marketable securities | Corporate Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	36,197
Unrealized Losses, Total	(17)
Fair Value, Total	36,180
Short-term marketable securities | Government‑ sponsored Enterprise Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	9,911
Unrealized Losses, Total	(5)
Fair Value, Total	9,906
Short-term marketable securities | U.S. treasury securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	24,451
Unrealized Losses, Total	(4)
Fair Value, Total	24,447
Long-term marketable securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	3,706
Unrealized Gains, Total	6
Fair Value, Total	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
Fair Value, Total	$ 3,712

Cash Equivalents and Marketab_4

Cash Equivalents and Marketable Securities - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Marketable securities		$ 0
Short-term marketable securities in unrealized loss position	$ 113,500,000
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 Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 3,820	$ 2,111
Less accumulated depreciation and amortization	(2,154)	(1,902)
Total	1,666	209
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,500	1,617
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	508	281
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	392	10
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 420	$ 203

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Depreciation and leasehold improvement amortization expense	$ 0.2	$ 0.2

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Prepaid Expense And Other Assets Current [Abstract]
Other accounts receivable	$ 2,104	$ 378
Prepaid expenses	1,661	1,400
Total	$ 3,765	$ 1,778

Balance Sheet Components - Su_3

Balance Sheet Components - Summary of Other Long-Term Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Other Assets Noncurrent Disclosure [Abstract]
Tenant improvement allowance receivables	$ 8,324	$ 0
Long-term deposits	480	280
Total	$ 8,804	$ 280

License and Collaboration Agr_2

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

1 Months Ended

3 Months Ended

12 Months Ended

Nov. 30, 2018

Aug. 31, 2018

Jul. 31, 2018

Dec. 31, 2017

Nov. 30, 2017

Jun. 30, 2017

Feb. 28, 2011

Nov. 30, 2009

Dec. 31, 2018

Sep. 30, 2018

Jun. 30, 2018

Mar. 31, 2018

Dec. 31, 2017

Sep. 30, 2017

Jun. 30, 2017

Dec. 31, 2018

Dec. 31, 2017

Dec. 31, 2016

Oct. 01, 2018

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenues

$ 26,137,000

$ 18,564,000

$ 12,987,000

$ 11,449,000

$ 3,375,000

$ 1,512,000

$ 361,000

$ 69,137,000

$ 5,247,000

$ 0

University Of California San Francisco | IPO

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

IPO milestone payment

$ 140,000

Abb Vie Agreement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Deferred revenue

$ 11,600,000

5,600,000

11,600,000

5,600,000

11,600,000

Research and development period

2 years

Revenues

6,000,000

3,400,000

Abb Vie Agreement | Maximum

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Extension of research and development period

6 months

Abb Vie Agreement | Up-front Payment Arrangement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Payment received

$ 15,000,000

Sanofi Agreement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Payment received

$ 10,000,000

$ 5,000,000

$ 10,000,000

Revenue recognized

63,100,000

1,900,000

Deferred revenue

38,100,000

$ 0

$ 38,100,000

0

38,100,000

Reduction of research and development expense

$ 2,400,000

$ 200,000

Royalty expiration period

10 years

Sanofi Agreement | Maximum

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

License agreement milestone payment receive regulatory milestones and commercial milestones

$ 765,000,000

Sanofi Agreement | Up-front Payment Arrangement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Payment received

$ 40,000,000

First Agreement | University Of California San Francisco

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Licensing fees

$ 30,000

Annual licenses maintenance fee

30,000

Royalty payment

$ 50,000

Earliest agreement expire year

2024

Latest agreement expire year

2030

Second Agreement | University Of California San Francisco

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Licensing fees

$ 10,000

Annual licenses maintenance fee

$ 10,000

Agreement expire year

2033

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	1 Months Ended	12 Months Ended
	Apr. 30, 2018USD ($)ft²	Dec. 31, 2018USD ($)ft²	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015Tenant
Commitments And Contingencies [Line Items]
Area under lease | ft²		30,000
Lease expiration date		Jan. 31, 2019
Number of subtenants | Tenant					2
Sublease payments received			$ 100,000
Sublease agreement termination year		2017
Description of subleases to terminate		Both subleases were terminated during 2017.
Rent expense net of sublease payment		$ 2,700,000	$ 1,600,000	$ 1,000,000
Accruals for expenses related to indemnification issues		$ 0
April 2018 Lease Agreement
Commitments And Contingencies [Line Items]
Area under lease | ft²	47,500
Anticipated commencement date		Feb. 1, 2019
Term of lease agreement		7 years
Option to extend period		7 years
Letter of credit		$ 600,000
Tenant improvement cost		$ 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
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	Dec. 31, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 2,354
2020	3,087
2021	3,187
2022	3,290
2023	3,397
2024 and beyond	7,434
Total	$ 22,749

Debt Obligation - Additional in

Debt Obligation - Additional information (Details)	Dec. 29, 2017USD ($)$ / sharesshares	Sep. 30, 2018shares	Dec. 31, 2017USD ($)$ / sharesshares	Aug. 31, 2016USD ($)	Sep. 30, 2011$ / sharesshares	Dec. 31, 2018USD ($)Modelshares	Dec. 31, 2017USD ($)$ / shares	Dec. 31, 2016USD ($)	Apr. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Dec. 31, 2015USD ($)
Debt Instrument [Line Items]
Incremental debt discount							$ 500,000	$ 1,900,000
Debt instrument annual interest rate				8.00%
Debt instrument, maturity date				Dec. 31, 2017
Fair value of embedded derivative upon issuance									$ 1,900,000	$ 3,400,000
Convertible preferred stock warrant liability outstanding			$ 1,576,000			$ 0	1,576,000
Number of option pricing models | Model						2
Convertible Notes
Debt Instrument [Line Items]
Interest expense						$ 0	7,200,000	3,900,000
Common Stock
Debt Instrument [Line Items]
Shares issued upon conversion of convertible securities | shares						15,760,102
Series A Warrants
Debt Instrument [Line Items]
Warrants term					10 years
Convertible preferred stock warrant liability outstanding			$ 27,000				$ 27,000
Series A Warrants | Common Stock
Debt Instrument [Line Items]
Shares issued upon conversion of convertible securities | shares		3,576
Series B-3 Convertible Preferred Stock
Debt Instrument [Line Items]
Shares issued upon conversion of convertible securities | shares			1,968,561
Offering price per share | $ / shares			$ 11.9907				$ 11.9907
Remeasurement of fair value of convertible notes redemption features liability			$ 0			$ 0	$ 0	3,600,000
Warrants issued during period to purchase capital stock | shares	168,046
Exercise price per share | $ / shares	$ 8.99
Warrants term	5 years
Convertible preferred stock warrant liability outstanding	$ 1,100,000		1,500,000				1,500,000
Series A Convertible Preferred Stock | Series A Warrants
Debt Instrument [Line Items]
Exercise price per share | $ / shares					$ 9.0839
Warrants to purchase convertible shares | shares					4,954
Convertible Preferred Stock
Debt Instrument [Line Items]
Convertible preferred stock warrant liability outstanding			$ 1,576,000			$ 0	1,576,000	1,050,000			$ 31,000
Convertible Preferred Stock | Convertible Notes
Debt Instrument [Line Items]
Interest expense							1,700,000	400,000
Convertible Notes Redemption Features Liability | Convertible Notes
Debt Instrument [Line Items]
Interest expense							$ 5,500,000	$ 3,500,000
Note and Warrant Purchase Agreement
Debt Instrument [Line Items]
Aggregate amount of convertible notes issued				$ 21,500,000

Debt Obligation - Summary of Ch

Debt Obligation - Summary of Changes in Fair Value Related to Notes Redemption Features Liability (Details) - Convertible Notes Redemption Features Liability - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 3,648	$ 0
Fair value of the redemption features upon issuance of convertible notes	1,938	3,407
Change in estimated fair value of warrant liability included in other income, net	(5,586)	241
Ending Balance	$ 0	$ 3,648

Debt Obligation - Summary of _2

Debt Obligation - Summary of Changes in Estimated Fair Value of Warrant Liability Related to Series A and B-3 Warrants (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class Of Warrant Or Right [Line Items]
Beginning Balance	$ 1,576,000
Change in fair value of convertible preferred stock warrant liability	577,000	$ 526,000	$ (43,000)
Ending Balance	0	1,576,000
Convertible Preferred Stock
Class Of Warrant Or Right [Line Items]
Beginning Balance	1,576,000	1,050,000	31,000
Warrants issued with convertible notes			1,062,000
Change in fair value of convertible preferred stock warrant liability	634,000	526,000	(43,000)
Reclassification of Warrant Liability to Additional Paid-in-Capital Upon Exercise of Warrants	(57,000)
Reclassification of Warrant Liability to Additional Paid-in-Capital Upon Initial Public Offering	(2,153,000)
Ending Balance	$ 0	$ 1,576,000	$ 1,050,000

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Details) $ / shares in Units, $ in Thousands	Sep. 13, 2018shares	Aug. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017$ / sharesshares	Jul. 31, 2015USD ($)$ / sharesshares	Nov. 30, 2014USD ($)$ / sharesshares	Apr. 30, 2014USD ($)$ / sharesshares	Dec. 31, 2018USD ($)BoardofDirector$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)shares	Dec. 31, 2012USD ($)$ / sharesshares	Dec. 31, 2015shares
Temporary Equity [Line Items]
Proceeds from sale of convertible preferred stock | $							$ 49,800	$ 0	$ 0
Convertible preferred stock, Shares Outstanding			12,285,434				0	12,285,434
Convertible preferred stock, Shares Issued			12,285,434				0	12,285,434
Convertible preferred stock, conversion term							Each share of convertible preferred stock is convertible at a 1:1 ratio at the option of the holder into fully paid and non-assessable shares of common stock. Conversion of each series of convertible preferred stock into common stock is automatic upon the earlier of (i) the closing of an IPO of our common stock, registered under the Securities Act of 1933, as amended, which results in aggregate proceeds equal to or exceeding $50.0 million to us, and the value of our shares immediately prior to the IPO is not less than $22.71 per share (as adjusted for any stock splits, stock dividends, combinations, subdivisions, recapitalizations or the like); or (ii) at any time upon the agreement of the holders of at least 60% of the then outstanding shares of preferred stock
Common Stock
Temporary Equity [Line Items]
Issuance of stock, shares							7,187,500
Number of convertible preferred stock converted into common stock							15,760,102
Conversion ratio, convertible preferred stock to common stock							100.00%
Minimum percentage of convertible preferred shareholders required to request conversion							60.00%
Minimum | Common Stock
Temporary Equity [Line Items]
Proceeds from IPO | $							$ 50,000
IPO | Common Stock
Temporary Equity [Line Items]
Issuance of stock, shares	7,187,500
Number of convertible preferred stock converted into common stock	15,760,102
IPO | Minimum | Common Stock
Temporary Equity [Line Items]
Share price | $ / shares							$ 22.71
Series C Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, Shares Outstanding							0
Convertible preferred stock, Shares Issued							0
Series C Convertible Preferred Stock | IPO
Temporary Equity [Line Items]
Issuance of stock, shares		3,474,668
Shares price per share | $ / shares		$ 14.3898
Proceeds from sale of convertible preferred stock | $		$ 49,800
Number of convertible preferred stock converted into common stock	15,760,102
Series A
Temporary Equity [Line Items]
Issuance of stock, shares										4,408,893
Shares price per share | $ / shares										$ 9.0839
Proceeds from sale of convertible preferred stock | $										$ 40,100
Convertible preferred stock, Shares Outstanding			4,408,893					4,408,893
Convertible preferred stock, Shares Issued			4,408,893					4,408,893
Percentage of non-cumulative dividends							8.00%
Convertible preferred stock, liquidation preference per share | $ / shares							$ 9.0839
Series B-1
Temporary Equity [Line Items]
Issuance of stock, shares						2,527,221
Shares price per share | $ / shares						$ 9.9923
Proceeds from sale of convertible preferred stock | $						$ 25,300
Convertible preferred stock, Shares Outstanding			2,527,221					2,527,221
Convertible preferred stock, Shares Issued			2,527,221					2,527,221
Percentage of non-cumulative dividends							8.80%
Convertible preferred stock, liquidation preference per share | $ / shares							$ 9.9923
Series B-2
Temporary Equity [Line Items]
Issuance of stock, shares					2,106,014
Shares price per share | $ / shares					$ 11.9907
Proceeds from sale of convertible preferred stock | $					$ 25,300
Convertible preferred stock, Shares Outstanding			2,106,014					2,106,014
Convertible preferred stock, Shares Issued			2,106,014					2,106,014
Percentage of non-cumulative dividends							10.56%
Convertible preferred stock, liquidation preference per share | $ / shares							$ 11.9907
Series B-2 | Maximum
Temporary Equity [Line Items]
Issuance of stock, shares					2,106,014
Series B-3 Convertible Preferred Stock
Temporary Equity [Line Items]
Issuance of stock, shares			1,968,561	1,274,745
Shares price per share | $ / shares			$ 11.9907	$ 11.9907				$ 11.9907
Proceeds from sale of convertible preferred stock | $				$ 15,300
Number of convertible preferred stock converted into common stock			1,968,561
Convertible preferred stock, Shares Outstanding			3,243,306					3,243,306
Convertible preferred stock, Shares Issued			3,243,306					3,243,306
Percentage of non-cumulative dividends							10.56%
Convertible preferred stock, liquidation preference per share | $ / shares							$ 11.9907
Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, Shares Outstanding			12,285,434					12,285,434	10,316,873		10,316,873
Convertible preferred stock, Shares Issued			12,285,434					12,285,434
Convertible preferred stock voting rights, description							Each holder of convertible preferred stock is entitled to the number of votes equal to the number of shares of common stock into which such shares of convertible preferred stock are convertible. So long as 20% of the shares of convertible preferred stock remain outstanding, the holders of such shares of preferred stock are entitled to elect five members of our Board of Directors
Percentage of converitble preferred stock shares remain outstanding							20.00%
Number of directors are entitled to elect by preferred stock holders | BoardofDirector							5

Convertible Preferred Stock - S

Convertible Preferred Stock - Summary of Convertible Preferred Stock (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized	20,000,000	113,680,282
Convertible preferred stock, Shares Issued	0	12,285,434
Convertible preferred stock, Shares Outstanding	0	12,285,434
Convertible preferred stock, Liquidation Amount	$ 0	$ 129,445
Convertible preferred stock, Carrying Value	$ 0	$ 128,531,000
Series A
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized		40,094,998
Convertible preferred stock, Shares Issued		4,408,893
Convertible preferred stock, Shares Outstanding		4,408,893
Convertible preferred stock, Liquidation Amount		$ 40,050,000
Convertible preferred stock, Carrying Value		$ 39,630,000
Series B-1
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized		22,957,067
Convertible preferred stock, Shares Issued		2,527,221
Convertible preferred stock, Shares Outstanding		2,527,221
Convertible preferred stock, Liquidation Amount		$ 25,253,000
Convertible preferred stock, Carrying Value		$ 25,028,000
Series B-2
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized		19,130,887
Convertible preferred stock, Shares Issued		2,106,014
Convertible preferred stock, Shares Outstanding		2,106,014
Convertible preferred stock, Liquidation Amount		$ 25,252,000
Convertible preferred stock, Carrying Value		$ 25,027,000
Series B-3
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized		31,497,330
Convertible preferred stock, Shares Issued		3,243,306
Convertible preferred stock, Shares Outstanding		3,243,306
Convertible preferred stock, Liquidation Amount		$ 38,890,000
Convertible preferred stock, Carrying Value		$ 38,846,000
Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized		113,680,282
Convertible preferred stock, Shares Issued		12,285,434
Convertible preferred stock, Shares Outstanding		12,285,434	10,316,873	10,316,873
Convertible preferred stock, Liquidation Amount		$ 129,445,000
Convertible preferred stock, Carrying Value		$ 128,531,000	$ 104,926,000	$ 104,926,000

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	Sep. 30, 2018	Aug. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period			48 months
Weighted average fair value stock options granted			$ 10.5	$ 4.3	$ 3.5
Fair value of stock options vested			$ 2,600,000	$ 1,100,000	$ 1,000,000
Shares repurchase in the event of employment termination			33,882	4,146
Weighted-average remaining contractual term options outstanding			8 years 18 days	7 years 7 months 20 days
Stock options granted			1,418,610
Income tax benefit recognized from stock-based compensation expense			$ 0
Tax benefits realized from exercised stock options			0
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance	250,000
Automatic annual increase of capital stock outstanding	1.00%
Additional shares of common stock	500,000
Percentage of purchase price of common stock to fair value	85.00%
Offering period	12 months
Stock-based compensation expense			$ 200,000
Consultant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted			1,101	0	0
Early Exercised Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Early exercised stock options in accrued liabilities			$ 233,000	$ 13,000
Employees and Nonemployee Directors Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unamortized compensation cost related to stock awards			$ 15,600,000
Expected weighted average period			3 years 3 months 18 days
Weighted-average remaining contractual term options outstanding			8 years 1 month 6 days
Stock-based compensation expense			$ 2,779,000	$ 1,021,000	$ 1,028,000
Maximum | Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of employee's eligible compensation contribution	15.00%
2018 Equity Incentive Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance		2,270,000
Automatic annual increase of capital stock outstanding		4.00%
Options vesting period		4 years
Number of shares of common stock authorized for issuance			5,194,378
2008 Equity Incentive Plan | Employees and Nonemployees Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period			4 years
Options expiration period			10 years

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Options Available for Grant, Beginning Balance	502,916
Options Available for Grant, Options granted	(1,418,610)
Options Available for Grant, Options forfeited	24,048
Options Available for Grant, Options exercised	0
Options Available for Grant, Options repurchased	0
Options Available for Grant, Shares added to plans	3,087,434
Options Available for Grant, Ending Balance	2,195,788	502,916
Options Outstanding, Beginning Balance	1,921,453
Options Outstanding, Options granted	1,418,610
Options Outstanding, Options forfeited	(24,048)
Options Outstanding, Options exercised	(317,425)
Options Outstanding, Options repurchased	0
Options Outstanding, Shares added to plans	0
Options Outstanding, Ending Balance	2,998,590	1,921,453
Options Outstanding, Vested and expected to vest	2,998,590
Options Outstanding, Exercisable as of December 31, 2018	2,475,279
Weighted Average Exercise Price, Beginning Balance	$ 4.54
Weighted Average Exercise Price, Options granted	14.84
Weighted Average Exercise Price, Options forfeited	6.50
Weighted Average Exercise Price, Options exercised	4.72
Weighted Average Exercise Price, Ending Balance	9.39	$ 4.54
Weighted Average Exercise Price, Vested and expected to vest	9.39
Weighted Average Exercise Price, Exercisable as of December 31, 2018	$ 6.85
Weighted Average Remaining Contractual Life	8 years 18 days	7 years 7 months 20 days
Weighted Average Remaining Contractual Life, Vested and expected to vest	8 years 18 days
Weighted Average Remaining Contractual Life, Exercisable as of December 31, 2018	7 years 8 months 8 days
Aggregate Intrinsic Value	$ 53,994	$ 5,213
Aggregate Intrinsic Value, Vested and expected to vest	53,994
Aggregate Intrinsic Value, Exercisable as of December 31, 2018	$ 50,854

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Fair Values for Stock Options Granted at Estimated Date of Grant (Details) - Employees and Nonemployee Directors Stock Options	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	2.50%	1.80%	1.30%
Risk-free interest rate, maximum	3.10%	2.20%	1.60%
Expected volatility, minimum	80.00%	81.00%	82.90%
Expected volatility, maximum	84.50%	85.30%	86.20%
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	5 years	5 years 9 months 18 days	5 years 9 months 18 days
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	6 years 1 month 6 days	6 years 1 month 6 days	6 years

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Stock-based Compensation Expense (Details) - Employees and Nonemployee Directors Stock Options - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 2,779	$ 1,021	$ 1,028
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	1,427	462	513
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 1,352	$ 559	$ 515

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								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator
Net income (loss)	$ 9,377	$ 6,683	$ 1,797	$ 310	$ (2,830)	$ (7,125)	$ (8,805)	$ (9,937)	$ 18,167	$ (28,697)	$ (30,638)
Allocation of undistributed earnings to participating securities									(13,250)	0	0
Net income (loss) attributable to common stockholders									$ 4,917	$ (28,697)	$ (30,638)
Weighted-average shares used to calculate net income (loss) per share attributable to common stockholders
Weighted-average common shares outstanding, basic									7,622,602	569,719	511,597
Effect of dilutive shares:									1,004,871	0	0
Weighted-average shares outstanding used in per share calculation, diluted									8,627,473	569,719	511,597
Net income (loss) per share, basic	$ 0.39				$ (4.55)	$ (12.26)	$ (16.27)	$ (18.63)	$ 0.65	$ (50.37)	$ (59.89)
Net income (loss) per share, diluted	$ 0.37				$ (4.55)	$ (12.26)	$ (16.27)	$ (18.63)	$ 0.57	$ (50.37)	$ (59.89)

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	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	2,223,399	14,384,033	11,780,868
Series A-1 Convertible Preferred 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	0	4,408,893	4,408,893
Series B-1 Convertible Preferred 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	0	2,527,221	2,527,221
Series B-2 Convertible Preferred 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	0	2,106,014	2,106,014
Series B-3 Convertible Preferred 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	0	3,243,306	1,274,745
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	27,751	0	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	168,046	173,000	4,954
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	1,993,720	1,921,453	1,430,187
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	33,882	4,146	28,854

Income Taxes - Summary of Incom

Income Taxes - Summary of Income (Loss) Before Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
United States	$ 18,226	$ (28,589)	$ (30,511)
Foreign	(59)	(108)	(127)
Total	$ 18,167	$ (28,697)	$ (30,638)

Income Taxes - Components of In

Income Taxes - Components of Income Tax Provision (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current
Federal	$ 0	$ 0	$ 0
State	800	800	800
International	0	0	0
Total current tax expense	800	800	800
Deferred
Federal	0	0	0
State	0	0	0
International	0	0	0
Total deferred tax expense	0	0	0
Total tax expense	$ 800	$ 800	$ 800

Income Taxes - Summary of Net D

Income Taxes - Summary of Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 29,983	$ 36,237
Stock-based compensation	453	312
Deferred revenue	1,179	0
Other accrued expenses	1,079	412
Fixed assets and intangibles	0	121
Total deferred tax assets	32,694	37,082
Deferred tax liabilities:
Fixed assets and intangibles	(141)	0
Total deferred tax liabilities	(141)	0
Valuation allowance	(32,553)	(37,082)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal corporate tax rate	21.00%	35.00%	35.00%
Reduction of deferred tax asset change in valuation allowance		$ 17,400,000
Increase (decrease) in valuation allowance	$ (4,500,000)	(8,000,000)	$ 9,500,000
Unrecognized tax benefits	8,958,000	$ 6,119,000	$ 5,127,000
Unrecognized tax benefits that would impact effective tax rate	0
Accrued interest and penalties	$ 0

Income Taxes - Summary of Net O

Income Taxes - Summary of Net Operating Losses and Tax Credit Carryforwards (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Tax Credit Carryforward [Line Items]
Net operating losses, federal, amount	$ 104,246
Net operating losses, state, amount	111,770
Federal
Tax Credit Carryforward [Line Items]
Tax credits	$ 5,407
Net operating losses, Expiration years	2031
Net operating losses, Expiration years	2037
Tax credits, Expiration years	2031
Tax credits, Expiration years	2038
State
Tax Credit Carryforward [Line Items]
Tax credits	$ 3,551
Net operating losses, Expiration years	2031
Net operating losses, Expiration years	2038
Tax credits, Expiration years	No Expiration

Income Taxes - Summary of Effec

Income Taxes - Summary of Effective Tax Rate Provision For (Benefit From) Income Taxes Differs From The Federal Statutory Income Tax Provision (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]
Statutory rate	21.00%	35.00%	35.00%
State tax	0.00%	0.00%	0.00%
Stock option expense	1.60%	(0.60%)	(0.70%)
General business credit	(14.70%)	4.20%	3.00%
Uncertain tax positions	14.70%	(4.20%)	(3.00%)
Change in valuation allowance	(24.80%)	30.10%	(28.70%)
Other permanent differences	2.20%	(3.30%)	(5.60%)
Tax reform—tax rate change	0.00%	(61.20%)	0.00%
Total	0.00%	0.00%	0.00%

Income Taxes - Summary of Recon

Income Taxes - Summary of Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 6,119	$ 5,127
Addition for tax positions in prior years	736	105
Addition for tax positions in current year	2,103	887
Balance at end of year	$ 8,958	$ 6,119

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Unaudited) - Schedule of Unaudited Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Information [Abstract]
Revenue	$ 26,137	$ 18,564	$ 12,987	$ 11,449	$ 3,375	$ 1,512	$ 361		$ 69,137	$ 5,247	$ 0
Income from operations	8,262	6,477	1,871	532	(6,183)	(5,878)	(6,858)	$ (7,667)	17,142	(26,586)	(27,053)
Net income (loss)	$ 9,377	$ 6,683	$ 1,797	$ 310	$ (2,830)	$ (7,125)	$ (8,805)	$ (9,937)	$ 18,167	$ (28,697)	$ (30,638)
Net income (loss) per share, basic	$ 0.39				$ (4.55)	$ (12.26)	$ (16.27)	$ (18.63)	$ 0.65	$ (50.37)	$ (59.89)
Net income (loss) per share, diluted	$ 0.37				$ (4.55)	$ (12.26)	$ (16.27)	$ (18.63)	$ 0.57	$ (50.37)	$ (59.89)