Principia Biopharma (PRNB) 10-Q 6 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Nov. 01, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Trading Symbol	PRNB
Entity Registrant Name	Principia Biopharma Inc.
Entity Central Index Key	1,510,487
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		23,860,837

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 149,053	$ 41,054
Short-term investments	39,239	0
Restricted cash	82	100
Prepaid expenses and other current assets	2,506	1,778
Total current assets	190,880	42,932
Property and equipment, net	552	209
Long-term restricted cash	567	82
Other long-term assets	8,574	280
Total assets	200,573	43,503
Current liabilities:
Accounts payable	4,749	3,405
Deferred revenue	20,637	44,134
Accrued liabilities	4,205	1,430
Accrued compensation	1,953	2,262
Total current liabilities	31,544	51,231
Deferred revenue, less current portion	1,116	5,619
Long-term deferred rent	8,827	8
Convertible preferred stock warrant liability	0	1,576
Total liabilities	41,487	58,434
Commitments and contingencies (Note 7)
Convertible preferred stock, $0.0001 par value, 20,000,000 and 113,680,282 shares authorized at September 30, 2018 and December 31, 2017, respectively	0	128,531
Stockholders' equity
Common stock, $0.0001 par value, 500,000,000 and 141,432,765 shares authorized at September 30, 2018 and December 31, 2017, respectively; 23,860,837 and 626,613 shares issued and outstanding September 30, 2018 and December 31, 2017, respectively	2	1
Additional paid-in-capital	300,968	7,201
Accumulated other comprehensive loss	(100)	(90)
Accumulated deficit	(141,784)	(150,574)
Total stockholders’ equity (deficit)	159,086	(143,462)
Total liabilities, convertible preferred stock, and stockholders’ equity	$ 200,573	$ 43,503

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Sep. 30, 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
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	500,000,000	141,432,765
Common stock, shares issued	23,860,837	626,613
Common stock, shares outstanding	23,860,837	626,613

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Revenue	$ 18,564	$ 1,512	$ 43,000	$ 1,873
Operating expenses:
Research and development	9,200	6,061	26,855	17,847
General and administrative	2,887	1,329	7,265	4,429
Total operating expenses	12,087	7,390	34,120	22,276
Income (loss) from operations	6,477	(5,878)	8,880	(20,403)
Other income (expense), net	206	274	(90)	102
Interest expense	0	(1,521)	0	(5,566)
Net income (loss)	$ 6,683	$ (7,125)	$ 8,790	$ (25,867)
Net income (loss) per common share
Basic	$ 0	$ (12.26)	$ 0	$ (46.85)
Diluted	$ 0	$ (12.26)	$ 0	$ (46.85)
Weighted-average shares used to calculate net income (loss) per common share
Basic	5,087,792	581,355	2,149,583	552,119
Diluted	6,144,492	581,355	3,096,952	552,119

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 6,683	$ (7,125)	$ 8,790	$ (25,867)
Other comprehensive income (loss):
Net unrealized loss on available-for-sale securities	(10)	0	(10)	0
Foreign currency translation adjustment	0	0	0	52
Comprehensive income (loss)	$ 6,673	$ (7,125)	$ 8,780	$ (25,815)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating activities:
Net income (loss)	$ 8,790	$ (25,867)
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Change in fair value of convertible preferred stock warrant liability	577	240
Change in fair value of redemption features liability	0	(237)
Amortization of (discount) premium on short-term investments	(65)	0
Depreciation	176	138
Stock-based compensation	1,424	743
Deferred rent	468	0
Amortization of debt discount	0	4,298
Interest expense on convertible notes	0	1,223
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,165)	(261)
Deferred revenue	(28,000)	13,127
Accounts payable	1,344	(1,300)
Accrued liabilities	587	1,521
Net cash used by operating activities	(15,864)	(6,375)
Investing activities:
Purchases of property and equipment	(455)	(41)
Maturities of short-term investments	8,500	0
Purchases of short-term investments	(47,684)	0
Net cash used in investing activities	(39,639)	(41)
Financing activities:
Net proceeds from initial public offering	112,401	0
Net proceeds from Series-C preferred stock issuance	49,792	0
Proceeds from issuance of common stock	1,253	196
Proceeds from exercise of common stock warrants	56	0
Proceeds from debt obligations	0	8,060
Net cash provided by financing activities	163,502	8,256
Effect of foreign exchange rate changes on cash and cash equivalents	0	61
Net increase in cash and cash equivalents	107,999	1,901
Cash and cash equivalents at beginning of period	41,054	5,867
Cash and cash equivalents, at end of period	149,053	7,768
Supplemental disclosures of cash flow information
Tenant improvement allowance receivables	8,324	0
Stock issuance costs accrued but not yet paid	1,907	0
Purchases of property and equipment accrued but not yet paid	$ 64	$ 0

Organization

Organization	9 Months Ended
	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	1. Description of Business We, Principia Biopharma Inc. (“Principia”), are a clinical-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	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Significant Accounting Policies	2. Significant Accounting Policies Basis of Presentation The accompanying interim condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) and include the accounts of Principia and our wholly-owned Australian subsidiary. All intercompany accounts, transactions and balances have been eliminated. The interim condensed consolidated balance sheet as of September 30, 2018, and the condensed consolidated statements of operations, comprehensive income (loss) and cash flows for the three and nine months ended September 30, 2018 and 2017 are unaudited. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the annual consolidated financial statements and reflect, in the opinion of management, all adjustments of a normal and recurring nature that are necessary for the fair presentation of our financial position as of September 30, 2018 and its results of operations and cash flows for the three and nine months ended September 30, 2018 and 2017. The financial data and the other financial information disclosed in the notes to the condensed consolidated financial statements related to the nine-month period are also unaudited. The condensed consolidated results of operations for any interim period are not necessarily indicative of the results to be expected for the full year or for any other future annual or interim period. The condensed consolidated balance sheet as of December 31, 2017 included herein was derived from the audited consolidated financial statements as of that date. These interim condensed consolidated financial statements should be read in conjunction with our audited consolidated financial statements set forth in our prospectus dated September 13, 2018 (“Prospectus”) that forms a part of our Registration Statements on Form S-1 (File No. 333-226922), as filed with the SEC pursuant to Rule 424 promulgated under the Securities Act of 1933, as amended. 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 condensed 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. Need for Additional Capital Since commencing operations in 2011, we have devoted substantially all of our resources to identifying and developing our drug candidates, including conducting preclinical studies and clinical trials, and providing general and administrative support for these operations. To date, we have financed our operations primarily through our IPO, private placements of our convertible preferred stock and convertible notes, and payments from license and research collaborations. From inception to date, we have received $299.5 million As of September 30, 2018 and December 31, 2017, we held cash, cash equivalents and short-term investments totaling $188.3 million and $41.1 million, respectively. We do not have any products for sale and have not generated any product revenue since our inception. To date, all of our revenue has been generated from the non-refundable payments received from our agreements with Sanofi and AbbVie. For the nine months ended September 30, 2018, our net income was $8.8 million. We have incurred significant operating losses since the commencement of our operations. As of September 30, 2018, we have an accumulated deficit of $141.8 million. We expect to continue to incur significant expenses and operating losses as we advance our clinical programs and expand our pipeline, seek regulatory approval and, if successful, proceed to commercial launch activities. Furthermore, we expect to incur additional costs associated with operating as a public company. We believe our cash, cash equivalents and short-term investments at September 30, 2018 are sufficient to fund our operations for at least the next 12 months from the date the unaudited financial statements are filed with the SEC. 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 short-term investments. 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 diversification of its investments in short-term investments and their maturities, which are designed to minimize risk and maintain liquidity. We are subject to a number of risks similar to other early-stage biopharmaceutical companies, including, but not limited to, the need to obtain adequate additional funding, possible failure of current or future preclinical studies or clinical trials, our reliance on third parties or partners to conduct our clinical trials, the need to obtain regulatory and marketing approvals for our drug candidates or to rely on partners to do so, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of our drug candidates, our right to develop and commercialize our drug candidates pursuant to the terms and conditions of the licenses granted to us, protection of proprietary technology, the ability to make 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 original maturities of 90 days or less at the date of purchase to be cash equivalents. Cash equivalents are stated at fair value. Short-term investments We invest in short-term investments, primarily money market funds, U.S. Treasury Securities and government agency securities. Investments with original maturities greater than 90 days that mature less than one year from the condensed consolidated balance sheet date are classified as short-term investments. Investments with a maturity date greater than one year at each balance sheet date are considered to be long-term investments. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on investments in short-term investments are included in other income (expense), net. The cost of securities sold is based on the specific-identification method. Interest earned on investments in short-term investments 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 September 30, 2018 and December 31, 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 condensed consolidated balance sheets. 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 as the related construction costs are incurred, up to the maximum aggregate allowances. Lease incentive obligations are classified as a component of deferred rent and are amortized on a straight-line basis over the lease term as a reduction of rent expense. 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 condensed 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 condensed 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 estimate 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 condensed 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 condensed consolidated statements of operations. 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 losses on foreign currency translation related to our wholly-owned Australian subsidiary and unrealized gains or losses related to our available-for-sale short-term investments are carried at estimated fair values on the condensed consolidated balance sheets. Net Income (Loss) per Share Basic net income (loss) per share attributable to common stockholders is calculated by dividing the net income (loss) attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period. Diluted net income (loss) per share includes the effect of potentially dilutive securities, which include outstanding warrants and stock options if the effect of their inclusion would be dilutive. In periods of net loss, diluted net loss per share is the same as basic net loss per share as the inclusion of potentially dilutive securities in the calculation would be anti-dilutive. We have issued securities other than common stock that participate in dividends (“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 condensed consolidated financial statements upon adoption. Under the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”), we meet the definition of an emerging growth company, and have elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act. Recently Adopted Accounting Standards Updates In March 2016, the FASB issued Accounting Standards Update (“ASU”) No. 2016-09, Compensation—Stock Compensation (Topic 718), which simplifies the accounting for employee share-based transactions. The amendments in this update cover such areas as the recognition of excess tax benefits and deficiencies, the classification of those excess tax benefits on the condensed consolidated statement of cash flows, an accounting policy election for forfeitures, the amount an employer can withhold to cover income taxes and still qualify for equity classification, and the classification of those taxes paid on the condensed consolidated statement of cash flows. For public entities, ASU 2016-09 was effective for fiscal years, and interim periods within those years, beginning after December 15, 2016. For entities other than public entities, the standard is effective for fiscal years beginning after December 15, 2017, and interim periods within fiscal years beginning after December 15, 2018. Early adoption is permitted. We early adopted ASU 2016-09 in 2017 and the adoption did not have a material impact on our condensed consolidated financial statements. 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) (“ASU 2014-09”), as subsequently amended, which outlines a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most recent current revenue recognition guidance, including industry-specific guidance. The core principle of the revenue model is that an entity recognizes revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. ASU 2014-09 defines a five-step process to achieve this core principle and, in doing so, it is possible more judgment and estimates may be required within the revenue recognition process than required under existing U.S. generally accepted accounting pronouncements. The guidance also specifies the accounting for certain incremental costs of obtaining a contract, and costs to fulfill a contract with a customer. 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 have elected the extended transition period provided by the JOBS Act and ASU 2014-09, as amended, becomes effective for us for fiscal years beginning after December 15, 2018, and interim periods beginning after December 15, 2019. Early adoption is permitted. We plan to adopt the accounting standard update in 2019 using the modified retrospective method and are currently in the early stages of analyzing our collaboration and license agreements to determine the differences in the accounting treatment under ASU 2014-09 compared to the current accounting standard. The consideration we are eligible to receive under these agreements includes upfront payments, research and development funding, milestone payments and royalties. The new revenue recognition standard differs from the current accounting standard in many respects, such as in the accounting for variable consideration and the measurement of progress toward completion of performance obligations. While we have not completed our assessment of the impact of adoption, adopting ASU 2014-09 may have a material effect on our condensed consolidated financial statements. In February 2016, the FASB issued ASU No. 2016-02 (Topic 842), Leases (“ASU 2016-02”). ASU 2016-02 requires an entity to recognize assets and liabilities arising from a lease for both financing and operating leases. The ASU will also require new qualitative and quantitative disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. We have elected the extended transition period provided by the JOBS Act and ASU 2016-02 is effective for us for fiscal years beginning after December 15, 2019, and interim periods beginning after December 15, 2020. Early adoption is permitted. The ASU is expected to impact our financial statements as we have certain operating lease agreements under which we are the lessee. We are currently evaluating the impact of the adoption of this ASU and anticipate the recognition of additional assets and corresponding liabilities on our balance sheet related to leases. In March 2018, the FASB issued ASU No. 2018-05 – Income Taxes (Topic 740): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118, or SAB 118, which provides guidance on accounting for the Tax Act, and provides that the measurement period should not extend beyond one year from the Tax Act enactment date for companies to complete the accounting. In accordance with this guidance, we considered our accounting for the impacts of the new tax law to be provisional. During the three and nine months ended September 30, 2018, no additional measurement-period adjustments were made by us related to the Tax Act. We continue to assess the impact of the recently enacted tax law and associated guidance on its business and consolidated financial statements and plans to complete its accounting for the income tax effects of the Tax Act within the one-year measurement period in 2018.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 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 September 30, 2018 and December 31, 2017 were as follows (in thousands): September 30, 2018 Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 11,529 $ — $ — $ 11,529 U.S. government agency securities — 69,248 — 69,248 U.S. treasury securities — 58,949 — 58,949 Short-term investments U.S. government agency securities — 11,372 — 11,372 U.S. treasury securities — 27,868 — 27,868 Total $ 11,529 $ 167,437 $ — $ 178,966 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 three and nine months ended September 30, 2018 or 2017.

Cash Equivalents and Short-Term

Cash Equivalents and Short-Term Investments	9 Months Ended
	Sep. 30, 2018
Investments Debt And Equity Securities [Abstract]
Cash Equivalents and Short-Term Investments	4. Cash equivalents and Short-term investments Short-term investments consisted of the following as of September 30, 2018 (in thousands): September 30, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents Money market funds $ 11,529 $ — $ — $ 11,529 U.S. government agency securities 69,250 — (2 ) 69,248 U.S. treasury securities 58,945 4 — 58,949 Total cash equivalents $ 139,724 $ 4 $ (2 ) $ 139,726 Short-term investments U.S. government agency securities $ 11,376 $ — $ (5 ) $ 11,371 U.S. treasury securities 27,875 — (6 ) 27,869 Total short-term investments $ 39,251 $ — $ (11 ) $ 39,240 Total $ 178,975 $ 4 $ (13 ) $ 178,966 We had no short-term investments as of December 31, 2017. All of our available-for-sale short-term investments have effective maturities of less than one year. There were no sales of available-for-sale short-term investments in any of the periods presented. The carrying value of short-term investments that were in unrealized loss positions totaled $108.5 million as of September 30, 2018. We have determined that (i) we do not have the intent to sell any of these investments, and (ii) it is not more likely than not that we will be required to sell any of these investments before recovery of the entire amortized cost basis. We anticipate that it will recover the entire amortized cost basis of such securities and has determined that no other-than-temporary impairments associated with credit losses were required to be recognized during the three-month and nine-month periods ended September 30, 2018.

Balance Sheet Components

Balance Sheet Components	9 Months Ended
	Sep. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	5. Balance Sheet Components Property and equipment as of September 30, 2018 and December 31, 2017 consisted of the following (in thousands): September 30, 2018 December 31, 2017 Laboratory equipment $ 1,977 $ 1,617 Computer equipment 440 281 Furniture and Fixtures 10 10 Leasehold improvements 203 203 2,630 2,111 Less accumulated depreciation and amortization (2,078 ) (1,902 ) Total $ 552 $ 209 Prepaid expenses and other current assets as of September 30, 2018 and December 31, 2017 consisted of the following (in thousands): September 30, 2018 December 31, 2017 Other accounts receivable $ 1,596 $ 378 Prepaid expenses 910 1,400 Total $ 2,506 $ 1,778 Other long-term assets as of September 30, 2018 and December 31, 2017 consisted of the following (in thousands): September 30, 2018 December 31, 2017 Tenant improvement allowance receivables $ 8,324 $ — Long-term deposits 250 280 Total $ 8,574 $ 280

License and Collaboration Agree

License and Collaboration Agreements	9 Months Ended
	Sep. 30, 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 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 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 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, (ii) transfer of technology (know-how) related to PRN2246, 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 is recognized as revenue ratably over the estimated performance period, which is estimated to continue through the fourth quarter of 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 is recognized ratably over the remaining estimated performance period of the collaboration, which is estimated to continue through the fourth quarter of 2018. For the three and nine months ended September 30, 2018, we recognized approximately $17.1 million and $38.5 million in revenue, respectively, related to the Sanofi Agreement. Deferred revenue related to the Sanofi Agreement was $14.6 million and $38.1 million at September 30, 2018 and December 31, 2017, respectively. Amounts for certain early development plan costs subject to reimbursement from Sanofi, recorded as a reduction of research and development expense, were approximately $1.2 million and $1.9 million, respectively, for the three and nine months ended September 30, 2018. 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. None of the future milestones are considered substantive milestones and therefore any such amounts received will be recognized as revenue ratably over the remaining estimated performance period, if any. 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 the AbbVie Agreement related to the research and development of oral immunoproteasome inhibitors. Under the AbbVie Agreement, we and AbbVie will collaborate on the research and preclinical development of one or more oral immunoproteasome inhibitors with each company bearing its own costs. Following the expiration of the two-year research term (which can be extended by AbbVie for up to six months), AbbVie will become responsible, at its sole cost, for all further development and commercialization activities for each such compound. In June 2017, we received an upfront payment of $15.0 million. Under the AbbVie Agreement, we may receive additional development, regulatory and commercial milestones of up to an aggregate of $667.5 million, as well as tiered royalties in the high single digits. Royalties are subject to specified reductions and are payable, on a product-by-product and country-by-country basis, until the later of (i) the date that all patent rights that claim a composition of matter of such product expire in such country, (ii) the date of expiration of regulatory exclusivity for such product in such country, or (iii) the date that is ten years from the first commercial sale of such product in such country. Exclusivity obligations, which vary depending on the stage of development, apply to us and AbbVie with respect to research and development of certain other inhibitors of the immunoproteasome. None of the future milestones are considered substantive milestones and therefore any such amounts received will be recognized as revenue ratably over the remaining estimated performance period, if any. Under the AbbVie Agreement, our deliverables are (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 is required for AbbVie to fully realize the value from the licensed compound. Therefore, the $15.0 million received related to this combined unit of accounting is recognized as revenue ratably over the estimated performance period, which is estimated to continue through the fourth quarter of 2019. For the three and nine months ended September 30, 2018, we recognized approximately $1.5 million and $4.5 million, respectively, in revenue related to the AbbVie Agreement. Revenue related to the AbbVie Agreement was $1.5 million and $1.9 million during the three and nine months ended September 30, 2017, respectively. Deferred revenue related to the AbbVie Agreement was $7.1 million and $11.6 million at September 30, 2018 and December 31, 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	9 Months Ended
	Sep. 30, 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 30,000 square feet of office, research and development, and laboratory space under an operating lease. This facility houses all our personnel. The lease agreement has been subject to several amendments necessary to secure additional space, sublease certain office and laboratory space and/or extend the lease term. The term for the lease expires on January 31, 2019. 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 $41,000 and $120,000 for the three and nine months ended September 30, 2017, respectively, each of 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 with occupancy anticipated to commence on February 1, 2019 for a seven year period with an option to extend for another seven year period subject to certain conditions. Pursuant to the April 2018 Lease Agreement, we provided a letter of credit to the landlord for $567,000 which is recorded as long-term restricted cash at September 30, 2018. T he Lease Agreement allows for a landlord provided tenant improvement allowance of up to $7.1 million to be applied to the cost of construction of tenant improvements to the new leased premises. 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 expect to utilize the aggregate amount of allowances available to us and have recorded $8.3 million of tenant improvement costs as of September 30, 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 September 30, 2018 are as follows (in thousands): Year ended December 31, 2018 $ 316 2019 2,354 2020 3,087 2021 3,187 2022 3,290 2023 and beyond 10,831 Total $ 23,065 We recorded rent expense of $1.0 million and $1.9 million for the three and nine months ended September 30, 2018, respectively, and $0.4 million and $1.2 million, net of sublease payments, for the three and nine months ended September 30, 2017, 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	9 Months Ended
	Sep. 30, 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. 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 zero and $0.5 million was recorded related to this beneficial conversion feature during the three and nine months ended September 30, 2017, 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 condensed 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. In September 2018, upon our IPO, all shares of convertible preferred stocks were converted into an equal number of shares of common stocks. 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. In September 2018, upon our IPO, all the preferred stock warrants were converted into warrants to purchase shares of common stock. 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”). In September 2018, all Series A shares of warrants were net exercised and 3,576 shares of common stock were issued. 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 nine months ended September 30, 2018 (in thousands): Estimated Fair Value of Convertible Preferred Stock Warrant Liability 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 September 30, 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. Fair value 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 September 30, 2018, no preferred stock warrants were outstanding. Interest expense Interest expense of approximately $1.5 million and $5.6 million was recognized during the three and nine months ended September 30, 2017, respectively. They consist of accretion of debt discounts related to the beneficial conversion features, the convertible notes redemption features liability and the convertible preferred stock warrant liability, and accrued interest expense on the Notes which were converted into preferred stock in December 2017. We had no interest expense for the three and nine months ended September 30, 2018.

Convertible Preferred Stock

Convertible Preferred Stock	9 Months Ended
	Sep. 30, 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 September 30, 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

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 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. For stock options granted under the 2018 Plan, the option price shall be no less than 100% of the estimated fair value on 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 September 30, 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 the 2008 Plan (in thousands): Options Available for Grant Options Outstanding Weighted Average Exercise Price Balance at December 31, 2017 502,916 1,921,453 $ 4.54 Options granted (1,102,163 ) 1,102,163 $ 12.13 Options forfeited 17,697 (17,697 ) $ 5.70 Options exercised — (317,425 ) $ 4.72 Options repurchased — — Shares added to plans 3,087,434 — Balance at September 30, 2018 2,505,884 2,688,494 $ 7.63 Vested and expected to vest 2,688,494 $ 7.63 Exercisable as of September 30, 2018 2,473,063 $ 6.82 The weighted-average fair values of stock options granted during each of the nine months ended September 30, 2018 and 2017 were $12.17 and $3.19 per share, respectively. The total fair value of stock options vested was $0.4 million and $0.3 million during the three months ended September 30, 2018 and 2017, respectively, and $1.0 million and $0.7 million for the nine months ended September 30, 2018 and 2017, respectively. At September 30, 2018 and December 31, 2017, 38,496 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 $259,000 and $13,000 as of September 30, 2018 and December 31, 2017, respectively, and were included in accrued liabilities on the condensed consolidated balance sheets. Amounts from liabilities are reclassified into common stock and additional paid-in capital as the shares vest, which is generally over 48 months. Common shares are presented as outstanding on our condensed consolidated balance sheets and consolidated statements of changes in convertible preferred stock and stockholders’ 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: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Risk-free interest rate 2.73% - 2.88% 1.83% 2.73% - 2.88% 1.83% - 2.10% Expected term (years) 5.90 - 6.08 6.03 5.65 - 6.08 5.77 - 6.06 Expected volatility 83.31% - 83.69% 81.01% 83.05% - 84.45% 81.01% - 85.32% 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): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Research and development $ 307 $ 153 $ 647 $ 340 General and administrative 370 148 763 399 Total $ 677 $ 301 $ 1,410 $ 739 As of September 30, 2018, there was approximately $11.6 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 3.7 years. The weighted-average remaining contractual term of options outstanding at September 30, 2018, is 9.9 years. Stock Options Granted to Outside Consultants During the three and nine months ended September 30, 2018, we recorded approximately $10,000 of stock-based compensation expense related to outside consultants and $1,000 and $4,000, respectively, during the three and nine months ended September 30, 2017. 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 $32,000 of stock-based compensation expense related to our ESPP for the three and nine months ended September 30, 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	9 Months Ended
	Sep. 30, 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): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Numerator Net income (loss) $ 6,683 $ (7,125 ) $ 8,790 $ (25,867 ) Allocation of undistributed earnings to participating securities (6,683 ) — (8,790 ) — Net income (loss) attributable to common stockholders — (7,125 ) — (25,867 ) Denominator Weighted-average common shares outstanding, basic 5,087,792 581,355 2,149,583 552,119 Effect of dilutive shares: 1,056,700 — 947,369 — Weighted-average shares outstanding used in per share calculation, diluted 6,144,492 581,355 3,096,952 552,119 Net income (loss) per share, basic $ — $ (12.26 ) $ — $ (46.85 ) Net income (loss) per share, diluted $ — $ (12.26 ) $ — $ (46.85 ) The following outstanding shares of common stock equivalents were excluded in the computation of diluted net income (loss) per share attributable to common stockholders, because their effect would have been antidilutive for the periods presented. Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 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 — 1,274,745 — 1,274,745 Warrants to purchase capital stock 168,046 4,954 168,046 4,954 Common stock options issued and outstanding 1,631,794 1,440,214 1,741,125 1,440,214 Total 1,799,840 11,762,041 1,909,171 11,762,041

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	12. Income Taxes We did not record a provision for income taxes for the nine months ended September 30, 2018, because all of our taxable income will be fully offset by net operating losses generated in prior years. In addition, the deferred tax assets continue to be subject to a full valuation allowance.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2018
Subsequent Events [Abstract]
Subsequent Events	13. Subsequent Events In October 2018, we announced the achievement of $10.0 million in additional payments for successful development activities of PRN2246 related to the Sanofi collaboration.

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying interim condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) and include the accounts of Principia and our wholly-owned Australian subsidiary. All intercompany accounts, transactions and balances have been eliminated. The interim condensed consolidated balance sheet as of September 30, 2018, and the condensed consolidated statements of operations, comprehensive income (loss) and cash flows for the three and nine months ended September 30, 2018 and 2017 are unaudited. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the annual consolidated financial statements and reflect, in the opinion of management, all adjustments of a normal and recurring nature that are necessary for the fair presentation of our financial position as of September 30, 2018 and its results of operations and cash flows for the three and nine months ended September 30, 2018 and 2017. The financial data and the other financial information disclosed in the notes to the condensed consolidated financial statements related to the nine-month period are also unaudited. The condensed consolidated results of operations for any interim period are not necessarily indicative of the results to be expected for the full year or for any other future annual or interim period. The condensed consolidated balance sheet as of December 31, 2017 included herein was derived from the audited consolidated financial statements as of that date. These interim condensed consolidated financial statements should be read in conjunction with our audited consolidated financial statements set forth in our prospectus dated September 13, 2018 (“Prospectus”) that forms a part of our Registration Statements on Form S-1 (File No. 333-226922), as filed with the SEC pursuant to Rule 424 promulgated under the Securities Act of 1933, as amended.
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 condensed 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.
Need for Additional Capital	Need for Additional Capital Since commencing operations in 2011, we have devoted substantially all of our resources to identifying and developing our drug candidates, including conducting preclinical studies and clinical trials, and providing general and administrative support for these operations. To date, we have financed our operations primarily through our IPO, private placements of our convertible preferred stock and convertible notes, and payments from license and research collaborations. From inception to date, we have received $299.5 million As of September 30, 2018 and December 31, 2017, we held cash, cash equivalents and short-term investments totaling $188.3 million and $41.1 million, respectively. We do not have any products for sale and have not generated any product revenue since our inception. To date, all of our revenue has been generated from the non-refundable payments received from our agreements with Sanofi and AbbVie. For the nine months ended September 30, 2018, our net income was $8.8 million. We have incurred significant operating losses since the commencement of our operations. As of September 30, 2018, we have an accumulated deficit of $141.8 million. We expect to continue to incur significant expenses and operating losses as we advance our clinical programs and expand our pipeline, seek regulatory approval and, if successful, proceed to commercial launch activities. Furthermore, we expect to incur additional costs associated with operating as a public company. We believe our cash, cash equivalents and short-term investments at September 30, 2018 are sufficient to fund our operations for at least the next 12 months from the date the unaudited financial statements are filed with the SEC.
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 short-term investments. 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 diversification of its investments in short-term investments and their maturities, which are designed to minimize risk and maintain liquidity. We are subject to a number of risks similar to other early-stage biopharmaceutical companies, including, but not limited to, the need to obtain adequate additional funding, possible failure of current or future preclinical studies or clinical trials, our reliance on third parties or partners to conduct our clinical trials, the need to obtain regulatory and marketing approvals for our drug candidates or to rely on partners to do so, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of our drug candidates, our right to develop and commercialize our drug candidates pursuant to the terms and conditions of the licenses granted to us, protection of proprietary technology, the ability to make 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 original maturities of 90 days or less at the date of purchase to be cash equivalents. Cash equivalents are stated at fair value.
Short-term Investments	Short-term investments We invest in short-term investments, primarily money market funds, U.S. Treasury Securities and government agency securities. Investments with original maturities greater than 90 days that mature less than one year from the condensed consolidated balance sheet date are classified as short-term investments. Investments with a maturity date greater than one year at each balance sheet date are considered to be long-term investments. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on investments in short-term investments are included in other income (expense), net. The cost of securities sold is based on the specific-identification method. Interest earned on investments in short-term investments 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 September 30, 2018 and December 31, 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 condensed consolidated balance sheets.
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 as the related construction costs are incurred, up to the maximum aggregate allowances. Lease incentive obligations are classified as a component of deferred rent and are amortized on a straight-line basis over the lease term as a reduction of rent expense. 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 condensed 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 condensed 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 estimate 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 condensed 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 condensed consolidated statements of operations.
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 losses on foreign currency translation related to our wholly-owned Australian subsidiary and unrealized gains or losses related to our available-for-sale short-term investments are carried at estimated fair values on the condensed consolidated balance sheets.
Net Income (Loss) per Share	Net Income (Loss) per Share Basic net income (loss) per share attributable to common stockholders is calculated by dividing the net income (loss) attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period. Diluted net income (loss) per share includes the effect of potentially dilutive securities, which include outstanding warrants and stock options if the effect of their inclusion would be dilutive. In periods of net loss, diluted net loss per share is the same as basic net loss per share as the inclusion of potentially dilutive securities in the calculation would be anti-dilutive. We have issued securities other than common stock that participate in dividends (“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 condensed consolidated financial statements upon adoption. Under the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”), we meet the definition of an emerging growth company, and have elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act. Recently Adopted Accounting Standards Updates In March 2016, the FASB issued Accounting Standards Update (“ASU”) No. 2016-09, Compensation—Stock Compensation (Topic 718), which simplifies the accounting for employee share-based transactions. The amendments in this update cover such areas as the recognition of excess tax benefits and deficiencies, the classification of those excess tax benefits on the condensed consolidated statement of cash flows, an accounting policy election for forfeitures, the amount an employer can withhold to cover income taxes and still qualify for equity classification, and the classification of those taxes paid on the condensed consolidated statement of cash flows. For public entities, ASU 2016-09 was effective for fiscal years, and interim periods within those years, beginning after December 15, 2016. For entities other than public entities, the standard is effective for fiscal years beginning after December 15, 2017, and interim periods within fiscal years beginning after December 15, 2018. Early adoption is permitted. We early adopted ASU 2016-09 in 2017 and the adoption did not have a material impact on our condensed consolidated financial statements. 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) (“ASU 2014-09”), as subsequently amended, which outlines a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most recent current revenue recognition guidance, including industry-specific guidance. The core principle of the revenue model is that an entity recognizes revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. ASU 2014-09 defines a five-step process to achieve this core principle and, in doing so, it is possible more judgment and estimates may be required within the revenue recognition process than required under existing U.S. generally accepted accounting pronouncements. The guidance also specifies the accounting for certain incremental costs of obtaining a contract, and costs to fulfill a contract with a customer. 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 have elected the extended transition period provided by the JOBS Act and ASU 2014-09, as amended, becomes effective for us for fiscal years beginning after December 15, 2018, and interim periods beginning after December 15, 2019. Early adoption is permitted. We plan to adopt the accounting standard update in 2019 using the modified retrospective method and are currently in the early stages of analyzing our collaboration and license agreements to determine the differences in the accounting treatment under ASU 2014-09 compared to the current accounting standard. The consideration we are eligible to receive under these agreements includes upfront payments, research and development funding, milestone payments and royalties. The new revenue recognition standard differs from the current accounting standard in many respects, such as in the accounting for variable consideration and the measurement of progress toward completion of performance obligations. While we have not completed our assessment of the impact of adoption, adopting ASU 2014-09 may have a material effect on our condensed consolidated financial statements. In February 2016, the FASB issued ASU No. 2016-02 (Topic 842), Leases (“ASU 2016-02”). ASU 2016-02 requires an entity to recognize assets and liabilities arising from a lease for both financing and operating leases. The ASU will also require new qualitative and quantitative disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. We have elected the extended transition period provided by the JOBS Act and ASU 2016-02 is effective for us for fiscal years beginning after December 15, 2019, and interim periods beginning after December 15, 2020. Early adoption is permitted. The ASU is expected to impact our financial statements as we have certain operating lease agreements under which we are the lessee. We are currently evaluating the impact of the adoption of this ASU and anticipate the recognition of additional assets and corresponding liabilities on our balance sheet related to leases. In March 2018, the FASB issued ASU No. 2018-05 – Income Taxes (Topic 740): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118, or SAB 118, which provides guidance on accounting for the Tax Act, and provides that the measurement period should not extend beyond one year from the Tax Act enactment date for companies to complete the accounting. In accordance with this guidance, we considered our accounting for the impacts of the new tax law to be provisional. During the three and nine months ended September 30, 2018, no additional measurement-period adjustments were made by us related to the Tax Act. We continue to assess the impact of the recently enacted tax law and associated guidance on its business and consolidated financial statements and plans to complete its accounting for the income tax effects of the Tax Act within the one-year measurement period in 2018.
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)	9 Months Ended
	Sep. 30, 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 September 30, 2018 and December 31, 2017 were as follows (in thousands): September 30, 2018 Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 11,529 $ — $ — $ 11,529 U.S. government agency securities — 69,248 — 69,248 U.S. treasury securities — 58,949 — 58,949 Short-term investments U.S. government agency securities — 11,372 — 11,372 U.S. treasury securities — 27,868 — 27,868 Total $ 11,529 $ 167,437 $ — $ 178,966 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 Short-Te_2

Cash Equivalents and Short-Term Investments (Tables)	9 Months Ended
	Sep. 30, 2018
Investments Debt And Equity Securities [Abstract]
Summary of Short-Term Investments	Short-term investments consisted of the following as of September 30, 2018 (in thousands): September 30, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents Money market funds $ 11,529 $ — $ — $ 11,529 U.S. government agency securities 69,250 — (2 ) 69,248 U.S. treasury securities 58,945 4 — 58,949 Total cash equivalents $ 139,724 $ 4 $ (2 ) $ 139,726 Short-term investments U.S. government agency securities $ 11,376 $ — $ (5 ) $ 11,371 U.S. treasury securities 27,875 — (6 ) 27,869 Total short-term investments $ 39,251 $ — $ (11 ) $ 39,240 Total $ 178,975 $ 4 $ (13 ) $ 178,966

Balance Sheet Components (Table

Balance Sheet Components (Tables)	9 Months Ended
	Sep. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Summary of Property and Equipment	Property and equipment as of September 30, 2018 and December 31, 2017 consisted of the following (in thousands): September 30, 2018 December 31, 2017 Laboratory equipment $ 1,977 $ 1,617 Computer equipment 440 281 Furniture and Fixtures 10 10 Leasehold improvements 203 203 2,630 2,111 Less accumulated depreciation and amortization (2,078 ) (1,902 ) Total $ 552 $ 209
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets as of September 30, 2018 and December 31, 2017 consisted of the following (in thousands): September 30, 2018 December 31, 2017 Other accounts receivable $ 1,596 $ 378 Prepaid expenses 910 1,400 Total $ 2,506 $ 1,778
Summary of Other Long-Term Assets	Other long-term assets as of September 30, 2018 and December 31, 2017 consisted of the following (in thousands): September 30, 2018 December 31, 2017 Tenant improvement allowance receivables $ 8,324 $ — Long-term deposits 250 280 Total $ 8,574 $ 280

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Lease Payments For Operating Leases	Future minimum lease payments for operating leases at September 30, 2018 are as follows (in thousands): Year ended December 31, 2018 $ 316 2019 2,354 2020 3,087 2021 3,187 2022 3,290 2023 and beyond 10,831 Total $ 23,065

Debt Obligation (Tables)

Debt Obligation (Tables)	9 Months Ended
	Sep. 30, 2018
Convertible Preferred Stock
Class Of Warrant Or Right [Line Items]
Summary of Changes in Estimated Fair Value of 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 nine months ended September 30, 2018 (in thousands): Estimated Fair Value of Convertible Preferred Stock Warrant Liability 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 September 30, 2018 $ —

Convertible Preferred Stock (Ta

Convertible Preferred Stock (Tables)	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Summary of Convertible Preferred Stock	As of September 30, 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

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Activity	The following table summarizes our stock option activity under the 2008 Plan (in thousands): Options Available for Grant Options Outstanding Weighted Average Exercise Price Balance at December 31, 2017 502,916 1,921,453 $ 4.54 Options granted (1,102,163 ) 1,102,163 $ 12.13 Options forfeited 17,697 (17,697 ) $ 5.70 Options exercised — (317,425 ) $ 4.72 Options repurchased — — Shares added to plans 3,087,434 — Balance at September 30, 2018 2,505,884 2,688,494 $ 7.63 Vested and expected to vest 2,688,494 $ 7.63 Exercisable as of September 30, 2018 2,473,063 $ 6.82
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: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Risk-free interest rate 2.73% - 2.88% 1.83% 2.73% - 2.88% 1.83% - 2.10% Expected term (years) 5.90 - 6.08 6.03 5.65 - 6.08 5.77 - 6.06 Expected volatility 83.31% - 83.69% 81.01% 83.05% - 84.45% 81.01% - 85.32% 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): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Research and development $ 307 $ 153 $ 647 $ 340 General and administrative 370 148 763 399 Total $ 677 $ 301 $ 1,410 $ 739

Net Income (Loss) per Share (Ta

Net Income (Loss) per Share (Tables)	9 Months Ended
	Sep. 30, 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): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Numerator Net income (loss) $ 6,683 $ (7,125 ) $ 8,790 $ (25,867 ) Allocation of undistributed earnings to participating securities (6,683 ) — (8,790 ) — Net income (loss) attributable to common stockholders — (7,125 ) — (25,867 ) Denominator Weighted-average common shares outstanding, basic 5,087,792 581,355 2,149,583 552,119 Effect of dilutive shares: 1,056,700 — 947,369 — Weighted-average shares outstanding used in per share calculation, diluted 6,144,492 581,355 3,096,952 552,119 Net income (loss) per share, basic $ — $ (12.26 ) $ — $ (46.85 ) Net income (loss) per share, diluted $ — $ (12.26 ) $ — $ (46.85 )
Summary of Common Stock Equivalents Excluded Used in Computation of Diluted Net Income (loss) Per Share	The following outstanding shares of common stock equivalents were excluded in the computation of diluted net income (loss) per share attributable to common stockholders, because their effect would have been antidilutive for the periods presented. Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 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 — 1,274,745 — 1,274,745 Warrants to purchase capital stock 168,046 4,954 168,046 4,954 Common stock options issued and outstanding 1,631,794 1,440,214 1,741,125 1,440,214 Total 1,799,840 11,762,041 1,909,171 11,762,041

Organization - Additional Infor

Organization - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	Sep. 13, 2018	Jul. 23, 2018	Sep. 30, 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 | IPO
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Issuance of common stock	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 common stock	937,500

Significant Accounting Polici_3

Significant Accounting Policies - Additional Information (Details) $ in Thousands	1 Months Ended				3 Months Ended		9 Months Ended		120 Months Ended
	Aug. 31, 2018USD ($)	Jul. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Jun. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)
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 and short-term investments			$ 41,100		$ 188,300		$ 188,300		$ 188,300
Net income (loss)					6,683	$ (7,125)	8,790	$ (25,867)
Accumulated deficit			150,574		141,784		141,784		141,784
Restricted cash			200		$ 600		$ 600		600
Sanofi Agreement
Summary Of Significant Accounting Policies [Line Items]
Non-refundable upfront payment received	$ 5,000	$ 10,000	$ 40,000
AbbVie Agreement
Summary Of Significant Accounting Policies [Line Items]
Non-refundable upfront payment received				$ 15,000
Amended Sanofi Agreement
Summary Of Significant Accounting Policies [Line Items]
Non-refundable upfront payment received	$ 5,000	$ 10,000
Convertible Preferred Stock
Summary Of Significant Accounting Policies [Line Items]
Aggregae proceeds through financings									$ 299,500

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	Sep. 30, 2018	Dec. 31, 2017
Assets
Assets	$ 178,966
Convertible Preferred Stock Warrant Liability
Liabilities
Liabilities		$ 1,576
Money Market Funds
Assets
Assets		30
Cash Equivalents | Money Market Funds
Assets
Assets	11,529
Cash Equivalents | U.S. Government Agency Securities
Assets
Assets	69,248
Cash Equivalents | U.S. Treasury Securities
Assets
Assets	58,949
Short-term Investments | U.S. Government Agency Securities
Assets
Assets	11,372
Short-term Investments | U.S. Treasury Securities
Assets
Assets	27,868
Level 1
Assets
Assets	11,529
Level 1 | Convertible Preferred Stock Warrant Liability
Liabilities
Liabilities		0
Level 1 | Money Market Funds
Assets
Assets		30
Level 1 | Cash Equivalents | Money Market Funds
Assets
Assets	11,529
Level 1 | Cash Equivalents | U.S. Government Agency Securities
Assets
Assets	0
Level 1 | Cash Equivalents | U.S. Treasury Securities
Assets
Assets	0
Level 1 | Short-term Investments | U.S. Government Agency Securities
Assets
Assets	0
Level 1 | Short-term Investments | U.S. Treasury Securities
Assets
Assets	0
Level 2
Assets
Assets	167,437
Level 2 | Convertible Preferred Stock Warrant Liability
Liabilities
Liabilities		0
Level 2 | Money Market Funds
Assets
Assets		0
Level 2 | Cash Equivalents | Money Market Funds
Assets
Assets	0
Level 2 | Cash Equivalents | U.S. Government Agency Securities
Assets
Assets	69,248
Level 2 | Cash Equivalents | U.S. Treasury Securities
Assets
Assets	58,949
Level 2 | Short-term Investments | U.S. Government Agency Securities
Assets
Assets	11,372
Level 2 | Short-term Investments | U.S. Treasury Securities
Assets
Assets	27,868
Level 3
Assets
Assets	0
Level 3 | Convertible Preferred Stock Warrant Liability
Liabilities
Liabilities		$ 1,576
Level 3 | Cash Equivalents | Money Market Funds
Assets
Assets	0
Level 3 | Cash Equivalents | U.S. Government Agency Securities
Assets
Assets	0
Level 3 | Cash Equivalents | U.S. Treasury Securities
Assets
Assets	0
Level 3 | Short-term Investments | U.S. Government Agency Securities
Assets
Assets	0
Level 3 | Short-term Investments | U.S. Treasury Securities
Assets
Assets	$ 0

Cash Equivalents and Short-Te_3

Cash Equivalents and Short-Term Investments - Summary of Short-Term Investments (Details) $ in Thousands	Sep. 30, 2018USD ($)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	$ 178,975
Unrealized Gains, Total	4
Unrealized Losses, Total	(13)
Fair Value, Total	178,966
Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	139,724
Unrealized Gains, Total	4
Unrealized Losses, Total	(2)
Fair Value, Total	139,726
Cash Equivalents | U.S. Government Agency Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	69,250
Unrealized Gains, Total	0
Unrealized Losses, Total	(2)
Fair Value, Total	69,248
Cash Equivalents | U.S. Treasury Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	58,945
Unrealized Gains, Total	4
Unrealized Losses, Total	0
Fair Value, Total	58,949
Cash Equivalents | Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	11,529
Unrealized Gains, Total	0
Unrealized Losses, Total	0
Fair Value, Total	11,529
Short-term Investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	39,251
Unrealized Gains, Total	0
Unrealized Losses, Total	(11)
Fair Value, Total	39,240
Short-term Investments | U.S. Government Agency Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	11,376
Unrealized Gains, Total	0
Unrealized Losses, Total	(5)
Fair Value, Total	11,371
Short-term Investments | U.S. Treasury Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	27,875
Unrealized Gains, Total	0
Unrealized Losses, Total	(6)
Fair Value, Total	$ 27,869

Cash Equivalents and Short-Te_4

Cash Equivalents and Short-Term Investments - Additional Information (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Short-term investments			$ 0
Short-term investments in unrealized loss position	$ 108,500,000	$ 108,500,000
Other than temporary impairments	$ 0	$ 0

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 2,630	$ 2,111
Less accumulated depreciation and amortization	(2,078)	(1,902)
Total	552	209
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,977	1,617
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	440	281
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	10	10
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 203	$ 203

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Prepaid Expense And Other Assets Current [Abstract]
Other accounts receivable	$ 1,596	$ 378
Prepaid expenses	910	1,400
Total	$ 2,506	$ 1,778

Balance Sheet Components - Su_3

Balance Sheet Components - Summary of Other Long-Term Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Other Assets Noncurrent Disclosure [Abstract]
Tenant improvement allowance receivables	$ 8,324	$ 0
Long-term deposits	250	280
Total	$ 8,574	$ 280

License and Collaboration Agr_2

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

1 Months Ended

3 Months Ended

9 Months Ended

Aug. 31, 2018

Jul. 31, 2018

Dec. 31, 2017

Nov. 30, 2017

Jun. 30, 2017

Feb. 28, 2011

Nov. 30, 2009

Sep. 30, 2018

Sep. 30, 2017

Sep. 30, 2018

Sep. 30, 2017

Oct. 01, 2018

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenues

$ 18,564,000

$ 1,512,000

$ 43,000,000

$ 1,873,000

University Of California San Francisco | IPO | Subsequent Event

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

IPO milestone payment

$ 140,000

Sanofi Agreement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Payment received

$ 5,000,000

$ 10,000,000

$ 40,000,000

Revenue recognized

17,100,000

38,500,000

Deferred revenue

38,100,000

14,600,000

14,600,000

Reduction of research and development expense

(1,200,000)

(1,900,000)

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

AbbVie Agreement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Payment received

$ 15,000,000

Royalty expiration period

10 years

Expiration of research term

2 years

Research and development period

2 years

Revenues

1,500,000

$ 1,500,000

4,500,000

$ 1,900,000

Deferred revenue

$ 11,600,000

$ 7,100,000

$ 7,100,000

AbbVie Agreement | Maximum

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Additional development, regulatory and commercial milestones payments

$ 667,500,000

Extension of research and development period

6 months

AbbVie Agreement | Up-front Payment Arrangement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Payment received

$ 15,000,000

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

2,024

Latest agreement expire year

2,030

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

2,033

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Apr. 30, 2018USD ($)ft²	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)ft²	Sep. 30, 2017USD ($)	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			$ 41,000		$ 120,000
Sublease agreement termination year				2,017
Description of subleases to terminate				Both subleases were terminated during 2017.
Rent expense net of sublease payment		$ 1,000,000	$ 400,000	$ 1,900,000	$ 1,200,000
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		7 years
Option to extend period		7 years		7 years
Letter of credit		$ 567,000		$ 567,000
Tenant improvement cost		8,300,000		$ 8,300,000
Lease agreement commencement date				Aug. 1, 2018
April 2018 Lease Agreement | Leasehold Improvements
Commitments And Contingencies [Line Items]
Tenant improvement allowance claim date				Jul. 1, 2019
April 2018 Lease Agreement | Maximum
Commitments And Contingencies [Line Items]
Tenant improvement allowance		$ 7,100,000		$ 7,100,000
Additional tenant improvement allowance	$ 1,200,000

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Future Minimum Lease Payments For Operating Leases (Details) $ in Thousands	Sep. 30, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,018	$ 316
2,019	2,354
2,020	3,087
2,021	3,187
2,022	3,290
2023 and beyond	10,831
Total	$ 23,065

Debt Obligation - Additional in

Debt Obligation - Additional information (Details)	Dec. 29, 2017$ / sharesshares	Sep. 30, 2018USD ($)shares	Dec. 31, 2017USD ($)$ / sharesshares	Aug. 31, 2016USD ($)	Sep. 30, 2011$ / sharesshares	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)Model	Sep. 30, 2017USD ($)	Apr. 30, 2017USD ($)	Dec. 31, 2016USD ($)
Debt Instrument [Line Items]
Incremental debt discount							$ 0		$ 500,000
Fair value of embedded derivative upon issuance										$ 1,900,000	$ 3,400,000
Number of option pricing models | Model								2
Convertible preferred stock warrant liability outstanding		$ 0	$ 1,576,000			$ 0		$ 0
Convertible Notes
Debt Instrument [Line Items]
Interest expense						0	$ 1,500,000	0	$ 5,600,000
Series A Warrants
Debt Instrument [Line Items]
Warrants term					10 years
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
Remeasurement of fair value of convertible notes redemption features liability			$ 0
Warrants issued during period to purchase capital stock | shares	168,046
Exercise price per share | $ / shares	$ 8.99
Warrants term	5 years
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		$ 0	$ 1,576,000			$ 0		$ 0
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 Estimated Fair Value of Warrant Liability Related to Series A and B-3 Warrants (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Class Of Warrant Or Right [Line Items]
Balance at December 31, 2017	$ 1,576,000
Change in estimated fair value of convertible preferred stock warrant liability included in other income, net	(577,000)	$ (240,000)
Balance at September 30, 2018	0
Convertible Preferred Stock
Class Of Warrant Or Right [Line Items]
Balance at December 31, 2017	1,576,000
Change in estimated fair value of convertible preferred stock warrant liability included in other income, net	634,000
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)
Balance at September 30, 2018	$ 0

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	9 Months Ended
	Aug. 31, 2018	Sep. 30, 2018	Sep. 30, 2017
Temporary Equity [Line Items]
Net proceeds from sale of convertible preferred stock		$ 49,792	$ 0
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]
Convertible preferred stock, Shares Issued	3,474,668
Shares price per share	$ 14.3898
Net proceeds from sale of convertible preferred stock	$ 49,800
Number of common stock issued on conversion of convertible preferred stock	15,760,102

Convertible Preferred Stock - S

Convertible Preferred Stock - Summary of Convertible Preferred Stock (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized	20,000,000	113,680,282
Convertible preferred stock, Carrying Value	$ 0	$ 128,531
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
Convertible preferred stock, Carrying Value		$ 39,630
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
Convertible preferred stock, Carrying Value		$ 25,028
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
Convertible preferred stock, Carrying Value		$ 25,027
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
Convertible preferred stock, Carrying Value		$ 38,846
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
Convertible preferred stock, Liquidation Amount		$ 129,445
Convertible preferred stock, Carrying Value		$ 128,531

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2018	Aug. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period					48 months
Weighted average fair value stock options granted					$ 12.17	$ 3.19
Fair value of stock options vested			$ 400,000	$ 300,000	$ 1,000,000	$ 700,000
Shares repurchase in the event of employment termination	38,496		38,496		38,496		4,146
Unamortized compensation cost related to stock awards	$ 11,600,000		$ 11,600,000		$ 11,600,000
Expected weighted average period					3 years 8 months 12 days
Weighted-average remaining contractual term options outstanding					9 years 10 months 24 days
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		250,000		250,000
Automatic annual increase of capital stock outstanding	1.00%
Exercise price as a percentage of estimated fair value of shares on the date of grant	85.00%
Stock-based compensation expense			$ 32,000		$ 32,000
Additional shares of common stock	500,000
Offering period	12 months
Consultant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			10,000	$ 1,000	10,000	$ 4,000
Early Exercised Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Early exercised stock options in accrued liabilities	$ 259,000		$ 259,000		$ 259,000		$ 13,000
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		5,194,378		5,194,378
2018 Equity Incentive Plans | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price as a percentage of estimated fair value of shares on the date of grant		100.00%
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)	9 Months Ended
	Sep. 30, 2018$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted Average Remaining Contractual Life (years)	9 years 10 months 24 days
2008 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options Available for Grant, Beginning Balance	502,916
Options Available for Grant, Options granted	(1,102,163)
Options Available for Grant, Options forfeited	17,697
Options Available for Grant, Options exercised	0
Options Available for Grant, Options repurchased	0
Options Available for Grant, Options repurchased, Shares added to plans	3,087,434
Options Available for Grant, Ending Balance	2,505,884
Options Outstanding, Beginning Balance	1,921,453
Options Outstanding, Options granted	1,102,163
Options Outstanding, Options forfeited	(17,697)
Options Outstanding, Options exercised	(317,425)
Options Outstanding, Options repurchased	0
Options Outstanding, Shares added to plans	0
Options Outstanding, Ending Balance	2,688,494
Options Outstanding, Vested and expected to vest	2,688,494
Options Outstanding, Exercisable as of September 30, 2018	2,473,063
Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 4.54
Weighted Average Exercise Price, Options granted | $ / shares	12.13
Weighted Average Exercise Price, Options forfeited | $ / shares	5.70
Weighted Average Exercise Price, Options exercised | $ / shares	4.72
Weighted Average Exercise Price, Ending Balance | $ / shares	7.63
Weighted Average Exercise Price, Vested and expected to vest | $ / shares	7.63
Weighted Average Exercise Price, Exercisable as of September 30, 2018 | $ / shares	$ 6.82

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	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate		1.83%
Risk-free interest rate, minimum	2.73%		2.73%	1.83%
Risk-free interest rate, maximum	2.88%		2.88%	2.10%
Expected term (years)		6 years 10 days
Expected volatility, minimum	83.31%		83.05%	81.01%
Expected volatility, maximum	83.69%		84.45%	85.32%
Expected volatility		81.01%
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	5 years 10 months 24 days		5 years 7 months 24 days	5 years 9 months 7 days
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	6 years 29 days		6 years 29 days	6 years 21 days

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Stock-based Compensation Expense (Details) - Employees and Nonemployee Directors Stock Options - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 677	$ 301	$ 1,410	$ 739
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	307	153	647	340
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 370	$ 148	$ 763	$ 399

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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator
Net income (loss)	$ 6,683	$ (7,125)	$ 8,790	$ (25,867)
Allocation of undistributed earnings to participating securities	(6,683)	0	(8,790)	0
Net income (loss) attributable to common stockholders	$ 0	$ (7,125)	$ 0	$ (25,867)
Denominator
Weighted-average common shares outstanding, basic	5,087,792	581,355	2,149,583	552,119
Effect of dilutive shares:	1,056,700	0	947,369	0
Weighted-average shares outstanding used in per share calculation, diluted	6,144,492	581,355	3,096,952	552,119
Net income (loss) per share, basic	$ 0	$ (12.26)	$ 0	$ (46.85)
Net income (loss) per share, diluted	$ 0	$ (12.26)	$ 0	$ (46.85)

Net Income (Loss) per Share -_2

Net Income (Loss) per Share - Summary of Common Stock Equivalents Excluded Used in Computation of Diluted Net Income (loss) Per Share (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	1,799,840	11,762,041	1,909,171	11,762,041
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	0	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	0	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	0	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	1,274,745	0	1,274,745
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	4,954	168,046	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	1,631,794	1,440,214	1,741,125	1,440,214

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details)	9 Months Ended
	Sep. 30, 2018USD ($)
Income Tax Disclosure [Abstract]
Provision for income taxes	$ 0

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - Amended Sanofi Agreement - USD ($) $ in Millions	1 Months Ended
	Oct. 31, 2018	Aug. 31, 2018	Jul. 31, 2018
Subsequent Event [Line Items]
Additional payment received for successful development activities related to collaboration		$ 5	$ 10
Subsequent Event
Subsequent Event [Line Items]
Additional payment received for successful development activities related to collaboration	$ 10