Kodiak Sciences (KOD) 10-Q 16 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Nov. 14, 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	KOD
Entity Registrant Name	Kodiak Sciences Inc.
Entity Central Index Key	1,468,748
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	true
Entity Common Stock, Shares Outstanding		36,829,857

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (unaudited) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 11,590	$ 1,395
Prepaid expenses and other current assets	703	200
Total current assets	12,293	1,595
Restricted cash	140	140
Property and equipment, net	1,198	1,509
Deferred offering costs	3,484
Total assets	17,115	3,244
Current liabilities:
Accounts payable	1,724	3,356
Accrued and other current liabilities	3,659	5,802
Total current liabilities	5,383	9,158
Convertible notes (includes $18,487 and $7,937 at September 30, 2018 and December 31, 2017 due to related parties)	41,476	9,921
Redeemable convertible preferred stock warrant liability (includes $4,000 and $1,840 at September 30, 2018 and December 31, 2017 attributable to warrants held by related parties)	5,000	2,300
Derivative instrument (includes $2,467 at September 30, 2018 attributable to related parties)	8,517
Other liabilities	552	586
Total liabilities	60,928	21,965
Commitments and contingencies (Note 7)
Redeemable convertible preferred stock, $0.0001 par value, 18,753,595 shares authorized at September 30, 2018 and December 31, 2017; 12,385,154 shares issued and outstanding at September 30, 2018 and December 31, 2017; liquidation value of $50,324 at September 30, 2018 and December 31, 2017	50,017	50,017
Stockholders’ deficit:
Common stock, $0.0001 par value, 28,500,000 shares authorized at September 30, 2018 and December 31, 2017; 8,011,734 shares issued and outstanding at September 30, 2018 and 7,936,434 shares issued and outstanding at December 31, 2017	1	1
Additional paid-in capital	2,273	584
Accumulated deficit	(96,104)	(69,323)
Total stockholders’ deficit	(93,830)	(68,738)
Total liabilities, redeemable convertible preferred stock and stockholders’ deficit	$ 17,115	$ 3,244

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) (unaudited) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Convertible notes related parties	$ 18,487	$ 7,937
Redeemable preferred stock warrant liability attributable to related parties	4,000	$ 1,840
Embedded derivative liability attributable to related parties	$ 2,467
Redeemable convertible preferred stock, par value	$ 0.0001	$ 0.0001
Redeemable convertible preferred stock, shares authorized	18,753,595	18,753,595
Redeemable convertible preferred stock, shares issued	12,385,154	12,385,154
Redeemable convertible preferred stock, share outstanding	12,385,154	12,385,154
Redeemable convertible preferred stock, liquidation value	$ 50,324	$ 50,324
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	28,500,000	28,500,000
Common stock, shares issued	8,011,734	7,936,434
Common stock, shares outstanding	8,011,734	7,936,434

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (unaudited) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating expenses
Research and development	$ 4,709	$ 3,148	$ 11,942	$ 13,246
General and administrative	1,671	831	5,075	2,517
Total operating expenses	6,380	3,979	17,017	15,763
Loss from operations	(6,380)	(3,979)	(17,017)	(15,763)
Interest expense (includes $1,076 and $311 attributable to related parties for the three months ended September 30, 2018 and 2017 and $2,944 and $311 attributable to related parties for the nine months ended September 30, 2018 and 2017)	(1,982)	(395)	(5,329)	(407)
Other income (expense), net (includes $1,129 and $176 other expense attributable to related parties for the three months ended September 30, 2018 and 2017 and $2,714 and $176 other expense attributable to related parties for the nine months ended September 30, 2018 and 2017)	(2,090)	(209)	(4,435)	(196)
Net loss and comprehensive loss	$ (10,452)	$ (4,583)	$ (26,781)	$ (16,366)
Net loss per share attributable to common stockholders, basic and diluted	$ (1.33)	$ (0.61)	$ (3.45)	$ (2.19)
Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted	7,851,560	7,549,711	7,764,888	7,479,523

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) (unaudited) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Interest expense attributable to related parties	$ 1,076	$ 311	$ 2,944	$ 311
Other income (expense) attributable to related parties	$ (1,129)	$ (176)	$ (2,714)	$ (176)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities
Net loss	$ (26,781)	$ (16,366)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	367	363
Non-cash interest expense and amortization of debt discount and issuance cost	5,295	392
Change in fair value of redeemable convertible preferred stock warrant liability	2,700	220
Change in fair value of derivative instrument	1,914	0
Stock-based compensation	1,549	207
Changes in assets and liabilities:
Prepaid expense and other current assets	(503)	423
Accounts payable	(1,754)	2,015
Accrued and other current liabilities	(3,511)	(45)
Other liabilities	33	33
Net cash used in operating activities	(20,691)	(12,758)
Cash flows from investing activities
Purchase of property and equipment	(56)	(159)
Net cash used in investing activities	(56)	(159)
Cash flows from financing activities
Proceeds from issuance of convertible notes (includes $9,560 and $8,000 from related parties for the nine months ended September 30, 2018 and 2017)	33,000	10,000
Deferred offering costs	(1,815)	0
Debt issuance cost	(140)	(181)
Principal payments of capital lease	(81)	(70)
Proceeds from issuance of common stock	49	3
Principal payments of tenant improvement allowance payable	(71)	(65)
Net cash provided by financing activities	30,942	9,687
Net increase (decrease) in cash, cash equivalents and restricted cash	10,195	(3,230)
Cash, cash equivalents and restricted cash, at beginning of period	1,535	9,762
Cash, cash equivalents and restricted cash, at end of period	11,730	6,532
Reconciliation of cash, cash equivalents and restricted cash to statement of financial position
Cash and cash equivalents	11,590	6,392
Restricted cash	140	140
Cash, cash equivalents and restricted cash, at end of period	11,730	6,532
Supplemental cash flow information:
Cash paid for interest	14	13
Supplemental disclosures of non-cash investing and financing information:
Acquisition of equipment through capital lease	0	73
Redeemable convertible preferred stock warrant issued in connection with convertible notes	0	1,040
Issuance of derivative instrument related to convertible notes payable	6,603	0
Unpaid deferred offering costs	1,577	0
Deferred offering costs paid in restricted stock awards	$ 91	$ 0

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows (Unaudited) (Parenthetical) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Statement Of Cash Flows [Abstract]
Proceeds from issuance of convertible notes	$ 9,560	$ 8,000

The Company

The Company	9 Months Ended
	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
The Company	1. The Company Kodiak Sciences Inc. (the “Company”) is a clinical stage biopharmaceutical company specializing in novel therapeutics to treat high-prevalence ophthalmic diseases. The Company devotes substantially all of its time and efforts to performing research and development, raising capital and recruiting personnel. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, protection of proprietary technology, dependence on key personnel, contract manufacturer and contract research organizations, compliance with government regulations and the need to obtain additional financing to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical studies, clinical trials and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance and reporting. The Company’s product candidates are in development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained or maintained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid technological change and substantial competition from other pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, consultants and other third parties. The Company has incurred significant losses and negative cash flows from operations since inception and had an accumulated deficit of $96.1 million as of September 30, 2018. The Company believes that its cash and cash equivalents as of September 30, 2018 along with the net proceeds from its initial public offering (“IPO”) will be sufficient for the Company to continue as a going concern for at least 12 months from the issuance date of these condensed consolidated financial statements for the period ended September 30, 2018. Initial Public Offering In October 2018, the Company sold and issued 9,000,000 shares of common stock at a price to the public of $10.00 per share for gross proceeds of $90.0 million. In November 2018, the Company sold and issued an additional 400,000 shares of common stock at $10.00 per share to the underwriters of the IPO following the partial exercise of their over-allotment option for gross proceeds of $4.0 million. The aggregate net proceeds to the Company from the IPO, inclusive of the partial over-allotment option exercise, were approximately $83.7 million after deducting underwriting discounts and commissions and other offering costs. Upon the closing of the IPO, all convertible preferred shares then outstanding automatically converted into 12,385,154 shares of common stock, 500,000 redeemable convertible preferred stock warrants automatically converted into common stock warrants and 100,000 of such warrants were exercised immediately following the closing of the IPO. The 2017 convertible notes converted into 2,637,292 shares of common stock and the 2018 convertible notes converted into 4,295,677 shares of common stock upon closing of the IPO. In connection with the IPO, the Company amended and restated its certificate of incorporation and bylaws. The condensed consolidated financial statements as of September 30, 2018, including share and per share amounts, do not give effect to the IPO, as it closed subsequent to September 30, 2018.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The Company’s unaudited interim condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (“U.S. GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2017 and notes thereto, included in the Company’s final prospectus for the IPO filed with the SEC pursuant to Rule 424(b)(4) on October 5, 2018 (“final prospectus”). The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited financial statements. In the opinion of the Company's management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary to present fairly the Company's financial position as of September 30, 2018, the results of its operations for the three and nine months ended September 30, 2018 and 2017 and cash flows for the nine months ended September 30, 2018 and 2017. Such adjustments are of a normal and recurring nature. The results for the three and nine months ended September 30, 2018 are not necessarily indicative of the results for the year ending December 31, 2018, or for any future period. The year-end condensed consolidated balance sheet data as of December 31, 2017 was derived from audited financial statements but does not include all disclosures required by U.S. GAAP. Use of Estimates The preparation of condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities as of the date of the condensed consolidated financial statements and expenses during the reporting period. Such estimates include the valuation of redeemable convertible preferred stock warrant liability, derivative instruments, deferred tax assets, useful lives of property and equipment, and stock-based compensation. Actual results could differ from those estimates. Summary There were no changes to the Company’s significant accounting policies, as described in the final prospectus, that have a material impact on these condensed consolidated financial statements. Recent a p From time to time, new accounting pronouncements are issued by the FASB, under its ASC or other standard setting bodies, and adopted by the Company as of the specified effective date, unless otherwise discussed below. Recently Adopted Accounting Pronouncements In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718), Scope of Modification Accounting In November 2016, the FASB issued ASU 2016-18, Restricted Cash In October 2016, the FASB issued ASU 2016-16, Income Taxes (Topic 740): Intra-Entity Transfer of Assets Other than Inventory In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments In January 2016, the FASB issued ASU 2016-01, Financial Instruments—Overall: Recognition and Measurement of Financial Assets and Financial Liabilities New Accounting Pronouncements Not Yet Adopted In August 2018, the FASB issued ASU 2018-13, Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurements In June 2018, the FASB issued ASU 2018-07, Compensation — Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260) Distinguishing Liabilities from Equity (Topic 480) Derivatives and Hedging (Topic 815) (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) Leases Leases (Topic 842), Codification Improvements Leases (Topic 842), Targeted Improvements

Property and Equipment. Net

Property and Equipment. Net	9 Months Ended
	Sep. 30, 2018
Property Plant And Equipment [Abstract]
Property and Equipment. Net	3. Property and Equipment, net Property and equipment, net consists of the following (in thousands): September 30, 2018 December 31, 2017 Leasehold improvement $ 1,260 $ 1,260 Laboratory equipment 1,225 1,174 Computer equipment 52 52 Computer software 178 173 Furniture and fixtures 225 225 Office equipment 79 79 Total property and equipment 3,019 2,963 Less: Accumulated depreciation (1,821 ) (1,454 ) Property and equipment, net $ 1,198 $ 1,509 All long-lived assets are maintained in the United States. Depreciation expense, including depreciation of assets under capital leases, was $0.1 million and $0.1 million for the three months ended September 30, 2018 and 2017, respectively, and $0.4 million and $0.4 million for the nine months ended September 30, 2018 and 2017, respectively.

Accrued Liabilities and Other C

Accrued Liabilities and Other Current Liabilities	9 Months Ended
	Sep. 30, 2018
Accounts Payable And Accrued Liabilities Current [Abstract]
Accrued Liabilities and Other Current Liabilities	4. Accrued Liabilities and Other Current Liabilities Accrued liabilities and other current liabilities consist of the following (in thousands): September 30, 2018 December 31, 2017 Accrued salaries and benefits $ 1,068 $ 1,129 Accrued legal fees 858 35 Accrued research and development 730 4,293 Accrued professional fees 704 19 Accrued other liabilities 299 326 Total accrued and other current liabilities $ 3,659 $ 5,802

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	5. Fair Value Measurements The Company applies fair value accounting for all financial assets and liabilities and non-financial assets and liabilities that are recognized or disclosed at fair value in the financial statements on a recurring basis. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, a three-tier fair value hierarchy has been established, which prioritizes the inputs used in measuring fair value as follows: Level 1 —Observable inputs, such as quoted prices in active markets for identical assets or liabilities at the measurement date. Level 2 —Observable inputs other than Level 1 prices such as quoted prices for similar assets or liabilities, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 —Unobservable inputs which reflect management’s best estimate of what market participants would use in pricing the asset or liability at the measurement date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model. The following tables present the Company’s fair value hierarchy for assets and liabilities measured at fair value on a recurring basis (in thousands): Fair Value Measurements at September 30, 2018 Quoted Price in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets: Money market funds $ 11,350 $ — $ — $ 11,350 Total $ 11,350 $ — $ — $ 11,350 Liabilities: Redeemable convertible preferred stock warrant liability $ — $ — $ 5,000 $ 5,000 2018 derivative instrument — — 8,517 8,517 Total $ — $ — $ 13,517 $ 13,517 Fair Value Measurements at December 31, 2017 Quoted Price in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets: Money market funds $ 1,217 $ — $ — $ 1,217 Total $ 1,217 $ — $ — $ 1,217 Liabilities: Redeemable convertible preferred stock warrant liability $ — $ — $ 2,300 $ 2,300 Total $ — $ — $ 2,300 $ 2,300 The following table sets forth a summary of the changes in the fair value of the Company’s Level 3 financial instruments (in thousands): Redeemable Convertible Preferred Stock Warrant Liability 2018 Derivative Instrument Liability Fair value as of December 31, 2017 $ 2,300 $ — Issuance of financial instruments — 6,603 Change in fair value included in other income (expense), net 2,700 1,914 Fair value as of September 30, 2018 $ 5,000 $ 8,517 The estimated fair value of the 2017 convertible notes (Level 3 instrument for disclosure purposes) was $26.4 million as of September 30, 2018 and $19.0 million as of December 31, 2017. The estimated fair value of the 2018 convertible notes (Level 3 instrument for disclosure purposes) was $43.0 million as of September 30, 2018. The fair value of the redeemable convertible preferred stock warrants, 2017 and 2018 convertible notes and derivative instruments as of September 30, 2018 was inferred from the $10.00 per share of common stock from the IPO completed in October 2018. Upon the closing of the IPO, the redeemable convertible preferred stock warrants automatically converted into common stock warrants and the 2017 and 2018 convertible notes were converted into common stock. The estimated fair value of the 2017 derivative instrument was immaterial as of September 30, 2018, due to the probability of occurrence of the underlying events being remote. The estimated fair value of the 2018 derivative instrument considered the probability and timing of occurrence of the IPO and assumed a discount rate of 40%. The Company used a hybrid method between the probability-weighted expected return method (“PWERM”) and the Black-Scholes option pricing model (“OPM”) to estimate the fair value of the warrants, 2017 and 2018 convertible notes and derivative instruments as of December 31, 2017. The PWERM is a scenario-based analysis that estimates value per share based on the probability-weighted present value of expected future investment returns, considering each of the possible outcomes available to the Company, as well as the economic and control rights of each share class. Under the OPM, shares are valued by creating a series of call options with exercise prices based on the liquidation preferences and conversion terms of each equity class. The values of the preferred and common stock are inferred by analyzing these options. The Company estimated the probability-weighted value across multiple scenarios, using the OPM to estimate the allocation of value within one of or more of those scenarios. In this valuation, the Company considered two possible outcomes under PWERM: (1) an IPO and (2) continued operations as a private company scenario, which is modeled using the OPM. For the IPO scenario, the fair value of the Company’s common stock is consistent with the methods outlined in the Practice Aid. Estimates of fair value using valuation the OPM are affected by assumptions regarding a number of complex variables, including expected term, expected volatility, expected dividend, and risk-free interest rate. The Company considered as inputs to the PWERM the probability of occurrence of an IPO, the enterprise value and the discount rate, which is a blended rate that reflects the risk associated with the business during the forecasted period. At December 31, 2017, the fair values recognized for the warrants and the derivative instruments, and for disclosure purposes of the fair value of convertible notes, assumed a discount rate of 57.5%, volatility of 75%, a risk-free rate of 1.97%, no dividends expected to be paid, and an expected term based on the timing for the IPO scenario and the private scenario.

Convertible Notes

Convertible Notes	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Convertible Notes	6. Convertible Notes 2017 Convertible Notes In August 2017, the Company received $10.0 million in gross proceeds from the issuance of the 2017 convertible notes and warrants to purchase Series B redeemable convertible preferred stock (Note 11). Of this, $8.0 million aggregate principal amount of the 2017 convertible notes were issued to related parties. Interest on the unpaid principal balance of the 2017 convertible notes accrued and compounded monthly from October 1, 2017 at a rate of 2.5% per month and was payable at maturity. Unless converted or redeemed upon occurrence of certain events, the 2017 convertible notes were to mature on December 1, 2020. The 2017 convertible notes included embedded derivatives that were required to be bifurcated and accounted for separately as a single, compound derivative instrument (Note 12). The discount on 2017 convertible notes was amortizable over the contractual period of 3.31 years, using the effective interest rate method. The 2017 convertible notes had an annual effective interest rate of 38.18% per year. The 2017 convertible notes interest expense for the three months ended September 30, 2018 was $1.0 million, consisting of $1.0 million of contractual interest expense and less than $0.1 million of debt discount and issuance costs amortization. The 2017 convertible notes interest expense for the nine months ended September 30, 2018 was $2.8 million, consisting of $2.7 million of contractual interest expense and $0.1 million of debt discount and issuance costs amortization. The Company’s obligations with respect to the 2017 convertible notes were secured by all of its tangible and intangible assets. The 2017 convertible notes included covenants that restricted the Company’s ability to issue capital stock, repurchase or redeem capital stock, dispose of assets, incur debt, incur liens and make distributions to stockholders, including dividends. The 2017 convertible notes had customary events of default. After January 31, 2018, each holder of 2017 convertible notes may have at any time, at its option, elected to convert the principal amount and accrued interest of such convertible notes into shares of Series B redeemable convertible preferred stock at a price of $5.00 per share. In September 2018, the purchase agreement for the 2017 convertible notes was amended and effective immediately prior to the closing of the Company’s IPO. Following the amendment, the 2017 convertible notes were convertible into an equivalent number of shares of common stock in lieu of Series B redeemable convertible preferred stock and interest accrued from the initial public filing of a Registration Statement on Form S-1 on September 7, 2018 to immediately prior to the closing of the Company’s IPO would be waived. The Company issued 2,637,292 shares of common stock to the holders of the 2017 convertible notes at the closing of the Company’s IPO on October 9, 2018. 2018 Convertible Notes In February 2018, the Company received $33.0 million in gross proceeds from the issuance of 2018 convertible notes, of which the Company issued $31.2 million aggregate principal amount on February 2, 2018 (“first tranche”) and $1.8 million aggregate principal amount on February 23, 2018 (“second tranche”). Of this, $9.6 million were issued to related parties. Interest on the unpaid principal balance of the 2018 convertible notes accrued from the date of issuance and compounded monthly from February 28, 2018 at a rate of 6.0% per year and is payable at maturity. Unless converted, the 2018 convertible notes were to mature on the earlier of (1) December 1, 2020 and (2) the date of the consummation of a change of control. The 2018 convertible notes included embedded derivatives that are required to be bifurcated and accounted for separately as a single, compound derivative instrument (Note 12). The discount on 2018 convertible notes for the first and second tranche was amortizable over the contractual period of 2.83 years and 2.77 years, respectively, using the effective interest rate method. The 2018 convertible notes had an annual effective interest rate of 15.10% per year for the first tranche and 15.45% per year for the second tranche. The 2018 convertible notes interest expense for the three months ended September 30, 2018 was $1.0 million, consisting of $0.5 million of contractual interest expense and $0.5 million of debt discount and issuance costs amortization. The 2018 convertible notes interest expense for the nine months ended September 30, 2018 was $2.5 million, consisting of $1.3 million of contractual interest expense and $1.2 million of debt discount and issuance costs amortization. The Company’s obligations with respect to the 2018 convertible notes were unsecured and subordinated to its obligations with respect to the 2017 convertible notes. The 2018 convertible notes included covenants that restricted the Company’s ability to issue capital stock, repurchase or redeem capital stock, dispose of assets, incur debt, incur liens and make distributions to stockholders, including dividends. The 2018 convertible notes had customary events of default. The 2017 and 2018 convertible notes contained a clause in which failure to communicate to the lender any material adverse change or effect on the business, condition, operations, or ability to perform obligations under the terms of the 2017 and 2018 notes was considered an event of default. The 2018 convertible notes were automatically convertible into shares of the Company’s common stock at a price equal to (1) 80% of the initial price to public in a qualified initial public offering if such offering was completed prior to February 2, 2019 and (2) 75% of the initial price to public in a qualified initial public offering if such offering was completed on or after February 2, 2019. A qualified initial public offering for the purposes of the 2018 convertible notes was one in which the Company generated aggregate gross proceeds of at least $75.0 million or all of the 2017 convertible notes converted into shares of the Company’s common stock. The Company issued 4,295,677 shares of common stock to the holders of the 2018 convertible notes at the closing of the Company’s IPO on October 9, 2018.

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 In January 2013, the Company executed a non-cancellable lease agreement for office and laboratory space in Palo Alto, California. The lease began in October 2013 and would expire in October 2018. In March 2016, the Company executed a lease amendment agreement which was effective March 2016 and extended the lease term until October 2023. The Company recognizes rent expense on a straight-line basis over the lease period. Rent expense was $0.1 million and $0.1 million for the three months ended September 30, 2018 and 2017, respectively, and $0.4 million and $0.4 million for the nine months ended September 30, 2018 and 2017, respectively. The following table summarizes the Company’s future minimum commitments under non-cancelable contracts (in thousands): As of September 30, 2018: Operating Lease Remaining Fiscal 2018 $ 138 2019 564 2020 581 2021 598 2022 616 Thereafter 526 Total payments $ 3,023 Other Commitments; Contingencies The Company has entered into service agreements with Lonza AG and its affiliates (“Lonza”), pursuant to which Lonza agreed to perform activities in connection with the manufacturing process of certain compounds. Such agreements, and related amendments, state that planned activities that are included in the signed work orders are, in some cases, binding and, hence, obligate the Company to pay the full price of the work order upon satisfactory delivery of products and services. Per the terms of the agreements, the Company has the option to cancel signed orders at any time upon written notice, which may or may not be subject to payment of a cancellation fee. The level of cancellation fees is sometimes dependent on the timing of the written notice in relation to the commencement date of the work, with the maximum cancellation fee equal to the full price of the work order. As of September 30, 2018, the total amount of unconditional purchase obligations, including accrued amounts, under these agreements was $2.9 million. Purchases under this agreement for the nine months ended September 30, 2018 and 2017 were $2.1 million and $8.5 million, respectively. As of September 30, 2018, the Company had not incurred any cancellation fees for the work performed by Lonza. The Company is also party to a cancellable assignment and license agreement that would require the Company to make milestone payments of up to $33.2 million and royalty payments on net sales of products utilizing KSI-201 and related technology. Such milestones and royalties are dependent on future activity or product sales and are not estimable. Legal Proceedings From time to time, the Company may become involved in legal proceedings arising from the ordinary course of its business. Management is currently not aware of any matters that could have a material adverse effect on the Company’s financial position, results of operations or cash flows. The Company records a legal liability when it believes that it is both probable that a liability may be imputed, and the amount of the liability can be reasonably estimated. Significant judgment by the Company is required to determine both probability and the estimated amount. Indemnification To the extent permitted under Delaware law, the Company has agreed to indemnify its directors and officers for certain events or occurrences while the director or officer is, or was serving, at the Company’s request in such capacity. The indemnification period covers all pertinent events and occurrences during the director’s or officer’s service. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is not specified in the agreements; however, the Company has director and officer insurance coverage that reduces its exposure and enables the Company to recover a portion of any future amounts paid. The Company believes the estimated fair value of these indemnification agreements in excess of applicable insurance coverage is minimal.

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock	9 Months Ended
	Sep. 30, 2018
Temporary Equity Disclosure [Abstract]
Redeemable Convertible Preferred Stock	8. Redeemable Convertible Preferred Stock As of September 30, 2018 and December 31, 2017, the Company’s certificate of incorporation authorized the Company to issue up to 18,753,595 shares of redeemable convertible preferred stock. As of September 30, 2018 and December 31, 2017, redeemable convertible preferred stock consisted of the following (in thousands, except per share and share amounts): Redeemable Convertible Preferred Stock Liquidation Carrying Original Issuance Authorized Outstanding Value Value Price (1) Series A redeemable convertible preferred stock 6,253,595 5,593,154 $ 16,364 $ 16,283 $ 3.17 Series B redeemable convertible preferred stock 12,500,000 6,792,000 33,960 33,734 $ 5.00 Total 18,753,595 12,385,154 $ 50,324 $ 50,017 (1) In connection with the incorporation of the Company in the year ended December 31, 2015, all previously issued and outstanding Series A-1, Series B-1 and Series C-1 redeemable convertible preferred stock of Oligasis were converted into shares of the Company’s Series A redeemable convertible preferred stock at a par value of $0.0001 per share. The original issuance price of Series A redeemable convertible preferred stock is calculated based on the original issuance price of the Oligasis’ Series A-1, Series B-1 and Series C-1 redeemable convertible preferred stock of $1.34, $2.15, and $3.73, respectively, on a weighted-average basis. On October 9, 2018, upon the closing of the Company’s IPO, all outstanding redeemable convertible preferred stock automatically converted into shares of common stock. Prior to the closing of the Company’s IPO, the holders of redeemable convertible preferred stock had the following various rights and preferences: Liquidation Preference In the event of any liquidation event, the holders of the Series B redeemable convertible preferred stock are entitled to receive in any distribution of any of the assets of the Company in preference to the holders of the Series A redeemable convertible preferred stock or common stock, an amount equal to the original issue price, adjusted for any stock splits, stock dividends, recapitalizations, reclassifications, combinations or similar transactions (collectively, “anti-dilution adjustments”), plus all declared and unpaid dividends on such shares. After full payment to holders of the Series B redeemable convertible preferred stock, payment should be made to the holders of Series A redeemable convertible preferred stock, in preference to the holders of the common stock, an amount equal to the original issue price, adjusted for any anti-dilution adjustments, plus all declared and unpaid dividends on such shares. After the payment of the liquidation preference, all remaining assets available for distribution will be distributed ratably among the holders of the common stock. If available assets are insufficient to pay the full liquidation preference of a given series of redeemable convertible preferred stock, the assets available for distribution to holders of such preferred stock will be distributed among such holders on a pro rata basis. Notwithstanding the above, for purposes of determining the amount each holder of shares of redeemable convertible preferred stock is entitled to receive with respect to a liquidation event, each such holder of shares of a series of redeemable convertible preferred stock shall be deemed to have converted such holder’s shares of such series into shares of common stock immediately prior to the liquidation event if, as a result of an actual conversion, such holder would receive, in the aggregate, an amount greater than the amount that would be distributed to such holder if such holder did not convert such series of redeemable convertible preferred stock into shares of common stock. If any such holder shall be deemed to have converted shares of redeemable convertible preferred stock into common stock pursuant to this paragraph, then such holder shall not be entitled to receive any distribution that would otherwise be made to holders of redeemable convertible preferred stock that have not converted into shares of common stock. Conversion Shares of any series of redeemable convertible preferred stock can be converted, at the option of the stockholder, into such number of fully paid and non-assessable shares of common stock. The conversion price is determined by dividing the original issuance price applicable to each series of redeemable convertible preferred stock, adjusted for any anti-dilution adjustments, by the applicable conversion price for such series. As of September 30, 2018, the Company’s redeemable convertible preferred stock is convertible into the Company’s shares of common stock on a one-for-one basis. Shares of redeemable convertible preferred stock shall automatically be converted into shares of common stock at the then effective conversion rate for such share, upon earlier to occur of: (1) the date, or the occurrence of event, specified by the vote of or written consent of the holders of at least a majority of the redeemable convertible preferred stock voting together as a single class on an as-converted basis; and (2) immediately prior to the consummation of a firm commitment underwritten initial public offering pursuant to an effective registration statement filed under the Securities Act, covering the offer and sale of the Company’s common stock, provided that the per share price is at least $10.00 and gross proceeds to the Company are equal to or greater than $75.0 million. Dividends The redeemable convertible preferred stock dividends are not cumulative and are payable only when declared by the board of directors. No such dividends have been declared. Such dividends are in preference to any dividends to holders of common stock. Voting Rights Each holder of redeemable convertible preferred stock shall be entitled to the number of votes equal to the number of shares of common stock into which the shares of redeemable convertible preferred stock held by such holder could be converted as of the record date. Holders of redeemable convertible preferred stock and common stock generally vote as a single class. Redemption and Balance Sheet Classification The redeemable convertible preferred stock is recorded in mezzanine equity because while it is not mandatorily redeemable, it will become redeemable at the option of the stockholders upon the occurrence of certain deemed Liquidation Events that are considered not solely within the Company’s control.

Common Stock

Common Stock	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Common Stock	9. Common Stock As of September 30, 2018 and December 31, 2017, the Company’s certificate of incorporation authorized the Company to issue 28,500,000, shares of common stock at the par value of $0.0001 per share. The holder of each share of common stock is entitled to one vote per share. The number of authorized shares of common stock may be increased or decreased (but not below the number of shares thereof then outstanding or reserved for issuance) by the affirmative vote of the holders of a majority (assuming the conversion of all redeemable convertible preferred stock into shares of the Company’s common stock) of the capital stock of the Company entitled to vote and without a separate class vote of the common stock.

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 2015 Equity Incentive Plan In September 2015, the Company adopted the 2015 Equity Incentive Plan (the “2015 Plan”) under which 2,810,513 shares of common stock were reserved for issuance through grants of incentive stock options (“ISOs”), nonqualified stock options (“NSOs”) and restricted stock awards to employees, directors and consultants of the Company. The awards outstanding under the previously terminated 2009 Share Incentive Plan continued to be governed by their existing terms. Options under the Plans may be granted for periods of up to ten years and at prices based upon the estimated fair value of the shares on the date of grant as determined by the board of directors; provided, however, that (1) the exercise price of an option shall not be less than 100% of the estimated fair value of the shares on the date of grant, and (2) the exercise price of an ISO granted to a greater than 10% stockholder shall not be less than 110% of the estimated fair value of the shares on the date of grant, and (3) the term of an ISO granted to a greater than 10% stockholder should not exceed five years. Options granted generally vest over four years. Shares issued under the 2015 Plan may, but need not, be exercisable immediately, but subject to a right of repurchase by the Company of any unvested shares. Stock Options Stock option activity under the 2015 Plan is summarized as follows (in thousands, except share and per share data). Outstanding Awards Number of Shares Available for Grant Number of Shares Underlying Outstanding Options Weighted Average Exercise Price Weighted Average Remaining Contractual Term (in years) Aggregate Intrinsic Value Balance, December 31, 2017 605,557 1,204,414 $ 0.98 8.49 $ 41 Shares authorized 2,125,000 — RSAs granted (27,500 ) — Options granted (2,824,698 ) 2,824,698 $ 6.29 Options exercised — (47,800 ) $ 1.03 2 Options forfeited or canceled 185,000 (185,000 ) $ 5.19 Balance, September 30, 2018 63,359 3,796,312 $ 4.73 9.07 $ 21,117 Shares exercisable, September 30, 2018 1,632,095 $ 2.88 8.47 $ 12,094 Vested and expected to vest, September 30, 2018 3,796,312 $ 4.73 9.07 $ 21,117 During the nine months ended September 30, 2018 and 2017, the Company granted 2,429,698 and 107,500 stock options, respectively, to employees with a weighted-average grant date fair value of $3.46 and $0.62 per share, respectively. The total fair value of employee options vested during the nine months ended September 30, 2018 and 2017 was $0.6 million and $0.2 million, respectively. Fair Value of Options Granted The fair value of the shares of common stock underlying the stock options was determined by the board of directors as there has been no historical public market for the Company’s common stock prior to the IPO. The Company estimated the fair value of employee stock options using the Black-Scholes valuation model. The fair value of employee stock options was estimated using the following weighted-average assumptions: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Expected volatility 54 % N/A 55 % 63 % Risk-free interest rate 2.76 % N/A 2.71 % 1.89 % Dividend yield 0 % N/A 0 % 0 % Expected term 6.07 N/A 5.99 6.00 Non-Employee Stock-Based Compensation The Company granted 395,000 and 30,000 stock options to non-employees during the nine months ended September 30, 2018 and 2017, respectively. The fair value of stock options granted to non-employees is calculated at each grant date and remeasured at each reporting date using the Black-Scholes option pricing model. The fair value of non-employee stock options was estimated using the following weighted-average assumptions: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Expected volatility 58 % 63 % 56 % 63 % Risk-free interest rate 3.01 % 2.29 % 2.68 % 2.28 % Dividend yield 0 % 0 % 0 % 0 % Expected term 9.39 9.64 9.11 9.63 Restricted Stock Awards Restricted stock award (“RSAs”) activity is summarized as follows: Number of Shares Underlying Outstanding RSAs Weighted Average Grant Date Fair Value Unvested, December 31, 2017 291,633 $ 0.45 Granted 27,500 $ 5.38 Vested 210,959 $ 1.03 Unvested, September 30, 2018 108,174 $ 0.57 Stock-based compensation is classified in the condensed consolidated statements of operations and comprehensive loss as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Research and development $ 528 $ 43 $ 892 $ 129 General and administrative 272 26 600 78 Total stock-based compensation $ 800 $ 69 $ 1,492 $ 207 As of September 30, 2018, the unrecognized stock-based compensation of unvested employee options and unvested RSAs was $8.2 million and it is expected to be recognized over a weighted-average period of 1.59 years.

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock Warrants	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Redeemable Convertible Preferred Stock Warrants	11. Redeemable Convertible Preferred Stock Warrants On August 11, 2017 with the issuance of the 2017 convertible notes (Note 6), the Company issued warrants to purchase 500,000 shares of Series B redeemable convertible preferred stock at an exercise price of $0.01 per share, including warrants issued to related parties to purchase an aggregate of 400,000 shares of Series B redeemable convertible preferred stock. Upon the conversion of the Series B redeemable convertible preferred stock into shares of common stock, the outstanding warrants would convert into warrants to purchase 500,000 shares of common stock at an exercise price of $0.01 per share. The outstanding warrants terminate at the earlier of August 11, 2022 and a change of control unless exercised. These warrants have a net exercise provision under which their holders may, in lieu of payment of the exercise price in cash, surrender the warrant and receive a net number of shares based on the fair market value of the Company’s stock at the time of exercise of the warrants after deduction of the aggregate exercise price. These warrants contain provisions for adjustment of the exercise price and number of shares issuable upon the exercise of warrants in the event of certain stock dividends, stock splits, reorganizations, reclassifications and consolidations. The estimated fair value of the redeemable convertible preferred stock warrants on the date of issuance of $1.0 million was recorded as a debt discount. The redeemable convertible preferred stock warrant liability had a fair value of $5.0 million as of September 30, 2018 and $2.3 million as of December 31, 2017. The change in fair value was recorded in the condensed consolidated statements of operations and comprehensive loss. As of September 30, 2018, all redeemable convertible preferred stock warrants remained outstanding. Upon the closing of the IPO, 500,000 redeemable convertible preferred stock warrants automatically converted into common stock warrants and 100,000 of such warrants were exercised immediately following the closing of the IPO.

Derivative Instruments

Derivative Instruments	9 Months Ended
	Sep. 30, 2018
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Derivative Instruments	12. Derivative Instruments The redemption features of the 2017 convertible notes met the requirements for separate accounting and were accounted for as a single, compound derivative instrument. The 2017 derivative instrument is recorded at fair value, which was immaterial as of the issuance date, December 31, 2017 and September 30, 2018, due to the probability of occurrence of the underlying events being remote. The redemption features of the 2018 convertible notes met the requirements for separate accounting and are accounted for as a single, compound derivative instrument. The 2018 derivative instrument was recorded at fair value, which was $6.6 million as of the issuance date and $8.5 million as of September 30, 2018.

Net Loss per Share Attributable

Net Loss per Share Attributable to Common Stockholders	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Net Loss per Share Attributable to Common Stockholders	13. Net Loss per Share Attributable to Common Stockholders The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders which excludes shares which are legally outstanding, but subject to repurchase by the Company (in thousands, except share and per share data): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Numerator: Net loss attributable to common stockholders $ (10,452 ) $ (4,583 ) $ (26,781 ) $ (16,366 ) Denominator: Weighted-average shares outstanding 7,980,457 7,933,130 7,954,583 7,931,606 Less: weighted-average unvested restricted shares and shares subject to repurchase (128,897 ) (383,419 ) (189,695 ) (452,083 ) Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted 7,851,560 7,549,711 7,764,888 7,479,523 Net loss per share attributable to common stockholders, basic and diluted $ (1.33 ) $ (0.61 ) $ (3.45 ) $ (2.19 ) The following potentially dilutive securities, presented on an as-converted to common stock basis, were excluded from the computation of diluted net loss per share attributable to common stockholders for the period presented because including them would have been antidilutive: As of September 30, 2018 2017 Redeemable convertible preferred stock 12,385,154 12,385,154 2017 convertible notes (1) 2,689,744 2,000,000 2018 convertible notes (2) 4,289,955 — Series B redeemable convertible preferred stock warrants 500,000 500,000 Options to purchase common stock 3,796,312 1,229,212 Unvested restricted stock awards 108,174 356,062 Unvested early exercised common stock options — 4,043 Total 23,769,339 16,474,471 (1) Calculated as $10.0 million in principal and $3.5 million in accrued but unpaid interest as of September 30, 2018 and $10.0 million in principal and no accrued but unpaid interest as of September 30, 2017, convertible at $5.00 per share of common stock. (2) Calculated as $33.0 million in principal and $1.3 million in accrued but unpaid interest as of September 30, 2018, convertible at 80% of the $10.00 per share of common stock from the Company’s IPO.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2018
Related Party Transactions [Abstract]
Related Party Transactions	14. Related Party Transactions Baker Bros. Advisors LP, which holds more than 5% of the Company’s stock, purchased $6.6 million aggregate principal amount of 2018 convertible notes and $3.0 million aggregate principal amount of 2017 convertible notes and warrants to purchase an aggregate of 150,000 shares of the Company’s Series B redeemable convertible preferred stock (see Note 6 and 11). The Dustin Moskovitz Trust DTD 12/27/05, which holds more than 5% of the Company’s stock, purchased $3.0 million aggregate principal amount of 2018 convertible notes and $5.0 million aggregate principal amount of 2017 convertible notes and warrants to purchase an aggregate of 250,000 shares of the Company’s Series B redeemable convertible preferred stock (see Note 6 and 11).

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2018
Subsequent Events [Abstract]
Subsequent Events	15. Subsequent Events The Company evaluated subsequent events that occurred after the balance sheet date up to the date on which the condensed consolidated financial statements were available for issuance. 2018 Equity Incentive Plan The Company’s board of directors approved the 2018 Equity Incentive Plan (“2018 Plan”) in August 2018, which became effective on the business day prior to the effectiveness of the registration statement relating to the IPO. The 2018 Plan permits the grant of incentive stock options, nonstatutory stock options, restricted stock, restricted stock units, stock appreciation rights, performance units and performance shares. A total of 4,300,000 shares of common stock were initially reserved for issuance under the 2018 Plan. The 2018 Plan was approved by the Company stockholders after its adoption by the board of directors. 2018 Employee Share Purchase Plan The Company’s board of directors approved the 2018 Employee Share Purchase Plan (“ESPP”) in August 2018, which became effective on the business day prior to the effectiveness of the registration statement relating to the IPO. A total of 460,000 shares of common stock were initially reserved for issuance under the ESPP. The ESPP was approved by the Company stockholders after its adoption by the board of directors. Corporate Housing In November 2018, the Company entered into an agreement with our CEO to lease his personal property, located 0.5 miles from Kodiak’s Palo Alto facility, to the Company at fair market value to provide flexible corporate housing for relocating employees. Total expected lease payments under the agreement are $0.1 million.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The Company’s unaudited interim condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (“U.S. GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2017 and notes thereto, included in the Company’s final prospectus for the IPO filed with the SEC pursuant to Rule 424(b)(4) on October 5, 2018 (“final prospectus”). The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited financial statements. In the opinion of the Company's management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary to present fairly the Company's financial position as of September 30, 2018, the results of its operations for the three and nine months ended September 30, 2018 and 2017 and cash flows for the nine months ended September 30, 2018 and 2017. Such adjustments are of a normal and recurring nature. The results for the three and nine months ended September 30, 2018 are not necessarily indicative of the results for the year ending December 31, 2018, or for any future period. The year-end condensed consolidated balance sheet data as of December 31, 2017 was derived from audited financial statements but does not include all disclosures required by U.S. GAAP.
Use of Estimates	Use of Estimates The preparation of condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities as of the date of the condensed consolidated financial statements and expenses during the reporting period. Such estimates include the valuation of redeemable convertible preferred stock warrant liability, derivative instruments, deferred tax assets, useful lives of property and equipment, and stock-based compensation. Actual results could differ from those estimates.
Recent Accounting Pronouncements	Recent a p From time to time, new accounting pronouncements are issued by the FASB, under its ASC or other standard setting bodies, and adopted by the Company as of the specified effective date, unless otherwise discussed below. Recently Adopted Accounting Pronouncements In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718), Scope of Modification Accounting In November 2016, the FASB issued ASU 2016-18, Restricted Cash In October 2016, the FASB issued ASU 2016-16, Income Taxes (Topic 740): Intra-Entity Transfer of Assets Other than Inventory In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments In January 2016, the FASB issued ASU 2016-01, Financial Instruments—Overall: Recognition and Measurement of Financial Assets and Financial Liabilities New Accounting Pronouncements Not Yet Adopted In August 2018, the FASB issued ASU 2018-13, Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurements In June 2018, the FASB issued ASU 2018-07, Compensation — Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260) Distinguishing Liabilities from Equity (Topic 480) Derivatives and Hedging (Topic 815) (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) Leases Leases (Topic 842), Codification Improvements Leases (Topic 842), Targeted Improvements

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	9 Months Ended
	Sep. 30, 2018
Property Plant And Equipment [Abstract]
Summary of Property and Equipment, Net	Property and equipment, net consists of the following (in thousands): September 30, 2018 December 31, 2017 Leasehold improvement $ 1,260 $ 1,260 Laboratory equipment 1,225 1,174 Computer equipment 52 52 Computer software 178 173 Furniture and fixtures 225 225 Office equipment 79 79 Total property and equipment 3,019 2,963 Less: Accumulated depreciation (1,821 ) (1,454 ) Property and equipment, net $ 1,198 $ 1,509

Accrued Liabilities and Other_2

Accrued Liabilities and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2018
Accounts Payable And Accrued Liabilities Current [Abstract]
Schedule of Accrued Liabilities and Other Current Liabilities	Accrued liabilities and other current liabilities consist of the following (in thousands): September 30, 2018 December 31, 2017 Accrued salaries and benefits $ 1,068 $ 1,129 Accrued legal fees 858 35 Accrued research and development 730 4,293 Accrued professional fees 704 19 Accrued other liabilities 299 326 Total accrued and other current liabilities $ 3,659 $ 5,802

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Summary of Fair Value Hierarchy of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables present the Company’s fair value hierarchy for assets and liabilities measured at fair value on a recurring basis (in thousands): Fair Value Measurements at September 30, 2018 Quoted Price in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets: Money market funds $ 11,350 $ — $ — $ 11,350 Total $ 11,350 $ — $ — $ 11,350 Liabilities: Redeemable convertible preferred stock warrant liability $ — $ — $ 5,000 $ 5,000 2018 derivative instrument — — 8,517 8,517 Total $ — $ — $ 13,517 $ 13,517 Fair Value Measurements at December 31, 2017 Quoted Price in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets: Money market funds $ 1,217 $ — $ — $ 1,217 Total $ 1,217 $ — $ — $ 1,217 Liabilities: Redeemable convertible preferred stock warrant liability $ — $ — $ 2,300 $ 2,300 Total $ — $ — $ 2,300 $ 2,300
Summary of Changes in Fair Value of Level 3 Financial Instruments	The following table sets forth a summary of the changes in the fair value of the Company’s Level 3 financial instruments (in thousands): Redeemable Convertible Preferred Stock Warrant Liability 2018 Derivative Instrument Liability Fair value as of December 31, 2017 $ 2,300 $ — Issuance of financial instruments — 6,603 Change in fair value included in other income (expense), net 2,700 1,914 Fair value as of September 30, 2018 $ 5,000 $ 8,517

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Commitments Under Non-Cancelable Contracts	The following table summarizes the Company’s future minimum commitments under non-cancelable contracts (in thousands): As of September 30, 2018: Operating Lease Remaining Fiscal 2018 $ 138 2019 564 2020 581 2021 598 2022 616 Thereafter 526 Total payments $ 3,023

Redeemable Convertible Prefer_3

Redeemable Convertible Preferred Stock (Tables)	9 Months Ended
	Sep. 30, 2018
Temporary Equity Disclosure [Abstract]
Schedule of Redeemable Convertible Preferred Stock	As of September 30, 2018 and December 31, 2017, redeemable convertible preferred stock consisted of the following (in thousands, except per share and share amounts): Redeemable Convertible Preferred Stock Liquidation Carrying Original Issuance Authorized Outstanding Value Value Price (1) Series A redeemable convertible preferred stock 6,253,595 5,593,154 $ 16,364 $ 16,283 $ 3.17 Series B redeemable convertible preferred stock 12,500,000 6,792,000 33,960 33,734 $ 5.00 Total 18,753,595 12,385,154 $ 50,324 $ 50,017 (1) In connection with the incorporation of the Company in the year ended December 31, 2015, all previously issued and outstanding Series A-1, Series B-1 and Series C-1 redeemable convertible preferred stock of Oligasis were converted into shares of the Company’s Series A redeemable convertible preferred stock at a par value of $0.0001 per share. The original issuance price of Series A redeemable convertible preferred stock is calculated based on the original issuance price of the Oligasis’ Series A-1, Series B-1 and Series C-1 redeemable convertible preferred stock of $1.34, $2.15, and $3.73, respectively, on a weighted-average basis.

Stock-based Compensation (Table

Stock-based Compensation (Tables)	9 Months Ended
	Sep. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Stock Options Activity Under 2015 Plan	Stock option activity under the 2015 Plan is summarized as follows (in thousands, except share and per share data). Outstanding Awards Number of Shares Available for Grant Number of Shares Underlying Outstanding Options Weighted Average Exercise Price Weighted Average Remaining Contractual Term (in years) Aggregate Intrinsic Value Balance, December 31, 2017 605,557 1,204,414 $ 0.98 8.49 $ 41 Shares authorized 2,125,000 — RSAs granted (27,500 ) — Options granted (2,824,698 ) 2,824,698 $ 6.29 Options exercised — (47,800 ) $ 1.03 2 Options forfeited or canceled 185,000 (185,000 ) $ 5.19 Balance, September 30, 2018 63,359 3,796,312 $ 4.73 9.07 $ 21,117 Shares exercisable, September 30, 2018 1,632,095 $ 2.88 8.47 $ 12,094 Vested and expected to vest, September 30, 2018 3,796,312 $ 4.73 9.07 $ 21,117
Summary of Restricted Stock Award Activity	Restricted stock award (“RSAs”) activity is summarized as follows: Number of Shares Underlying Outstanding RSAs Weighted Average Grant Date Fair Value Unvested, December 31, 2017 291,633 $ 0.45 Granted 27,500 $ 5.38 Vested 210,959 $ 1.03 Unvested, September 30, 2018 108,174 $ 0.57
Summary of Stock-based Compensation Classified in Condensed Consolidated Statements of Operations and Comprehensive Loss	Stock-based compensation is classified in the condensed consolidated statements of operations and comprehensive loss as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Research and development $ 528 $ 43 $ 892 $ 129 General and administrative 272 26 600 78 Total stock-based compensation $ 800 $ 69 $ 1,492 $ 207
Employee Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair Value of Stock Options Estimated Using Weighted-average Assumptions	The fair value of employee stock options was estimated using the following weighted-average assumptions: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Expected volatility 54 % N/A 55 % 63 % Risk-free interest rate 2.76 % N/A 2.71 % 1.89 % Dividend yield 0 % N/A 0 % 0 % Expected term 6.07 N/A 5.99 6.00
Non-employee Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair Value of Stock Options Estimated Using Weighted-average Assumptions	The fair value of stock options granted to non-employees is calculated at each grant date and remeasured at each reporting date using the Black-Scholes option pricing model. The fair value of non-employee stock options was estimated using the following weighted-average assumptions: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Expected volatility 58 % 63 % 56 % 63 % Risk-free interest rate 3.01 % 2.29 % 2.68 % 2.28 % Dividend yield 0 % 0 % 0 % 0 % Expected term 9.39 9.64 9.11 9.63

Net Loss per Share Attributab_2

Net Loss per Share Attributable to Common Stockholders (Tables)	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss per Share Attributable to Common Stockholders	The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders which excludes shares which are legally outstanding, but subject to repurchase by the Company (in thousands, except share and per share data): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Numerator: Net loss attributable to common stockholders $ (10,452 ) $ (4,583 ) $ (26,781 ) $ (16,366 ) Denominator: Weighted-average shares outstanding 7,980,457 7,933,130 7,954,583 7,931,606 Less: weighted-average unvested restricted shares and shares subject to repurchase (128,897 ) (383,419 ) (189,695 ) (452,083 ) Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted 7,851,560 7,549,711 7,764,888 7,479,523 Net loss per share attributable to common stockholders, basic and diluted $ (1.33 ) $ (0.61 ) $ (3.45 ) $ (2.19 )
Summary of Anti-dilutive Securities Excluded from Computation of Diluted Net Loss per Share	The following potentially dilutive securities, presented on an as-converted to common stock basis, were excluded from the computation of diluted net loss per share attributable to common stockholders for the period presented because including them would have been antidilutive: As of September 30, 2018 2017 Redeemable convertible preferred stock 12,385,154 12,385,154 2017 convertible notes (1) 2,689,744 2,000,000 2018 convertible notes (2) 4,289,955 — Series B redeemable convertible preferred stock warrants 500,000 500,000 Options to purchase common stock 3,796,312 1,229,212 Unvested restricted stock awards 108,174 356,062 Unvested early exercised common stock options — 4,043 Total 23,769,339 16,474,471 (1) Calculated as $10.0 million in principal and $3.5 million in accrued but unpaid interest as of September 30, 2018 and $10.0 million in principal and no accrued but unpaid interest as of September 30, 2017, convertible at $5.00 per share of common stock. (2) Calculated as $33.0 million in principal and $1.3 million in accrued but unpaid interest as of September 30, 2018, convertible at 80% of the $10.00 per share of common stock from the Company’s IPO.

The Company - Additional Inform

The Company - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Nov. 16, 2018	Oct. 09, 2018	Oct. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Accumulated deficit				$ (96,104)	$ (69,323)
Subsequent Event
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net proceeds from initial public offering			$ 83,700
Initial Public Offering | Common Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Share issued price, per share				$ 10
Initial Public Offering | Common Stock | Subsequent Event
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Stock issued during period			9,000,000
Share issued price, per share			$ 10
Gross proceeds from initial public offering			$ 90,000
Conversion of convertible preferred stock, common stock issued		12,385,154
Initial Public Offering | Common Stock | Subsequent Event | 2017 Convertible Notes
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Conversion of convertible securities, common stock issued		2,637,292
Initial Public Offering | Common Stock | Subsequent Event | 2018 Convertible Notes
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Conversion of convertible securities, common stock issued		4,295,677
Initial Public Offering | Common Stock | Subsequent Event | Redeemable Convertible Preferred Stock | Warrants Exercised into Convertible Preferred Shares then converted
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Warrants exercised into common stock shares		100,000
Initial Public Offering | Common Stock | Subsequent Event | Redeemable Convertible Preferred Stock | Warrant
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Preferred stock warrants converted into common stock warrants		500,000
Over-allotment Option | Common Stock | Subsequent Event
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Stock issued during period	400,000
Share issued price, per share	$ 10
Gross proceeds from initial public offering	$ 4,000

Property and Equipment, Net - S

Property and Equipment, Net - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property Plant and Equipment [Line Items]
Total property and equipment	$ 3,019	$ 2,963
Less: Accumulated depreciation	(1,821)	(1,454)
Property and equipment, net	1,198	1,509
Leasehold Improvement
Property Plant and Equipment [Line Items]
Total property and equipment	1,260	1,260
Laboratory Equipment
Property Plant and Equipment [Line Items]
Total property and equipment	1,225	1,174
Computer Equipment
Property Plant and Equipment [Line Items]
Total property and equipment	52	52
Computer Software
Property Plant and Equipment [Line Items]
Total property and equipment	178	173
Furniture and Fixtures
Property Plant and Equipment [Line Items]
Total property and equipment	225	225
Office Equipment
Property Plant and Equipment [Line Items]
Total property and equipment	$ 79	$ 79

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Property Plant And Equipment [Abstract]
Depreciation	$ 100	$ 100	$ 367	$ 363

Accrued Liabilities and Other_3

Accrued Liabilities and Other Current Liabilities - Schedule of Accrued Liabilities and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts Payable And Accrued Liabilities Current [Abstract]
Accrued salaries and benefits	$ 1,068	$ 1,129
Accrued legal fees	858	35
Accrued research and development	730	4,293
Accrued professional fees	704	19
Accrued other liabilities	299	326
Total accrued and other current liabilities	$ 3,659	$ 5,802

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Fair Value Hierarchy of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring Basis - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Total Assets	$ 11,350	$ 1,217
Liabilities:
Total Liabilities	13,517	2,300
Redeemable Convertible Preferred Stock Warrant Liability
Liabilities:
Total Liabilities	5,000	2,300
2018 Derivative Instrument
Liabilities:
Total Liabilities	8,517
Money Market Funds
Assets:
Total Assets	11,350	1,217
Quoted Price in Active Markets for Identical Assets (Level 1)
Assets:
Total Assets	11,350	1,217
Liabilities:
Total Liabilities	0	0
Quoted Price in Active Markets for Identical Assets (Level 1) | Redeemable Convertible Preferred Stock Warrant Liability
Liabilities:
Total Liabilities	0	0
Quoted Price in Active Markets for Identical Assets (Level 1) | 2018 Derivative Instrument
Liabilities:
Total Liabilities	0
Quoted Price in Active Markets for Identical Assets (Level 1) | Money Market Funds
Assets:
Total Assets	11,350	1,217
Significant Other Observable Inputs (Level 2)
Assets:
Total Assets	0	0
Liabilities:
Total Liabilities	0	0
Significant Other Observable Inputs (Level 2) | Redeemable Convertible Preferred Stock Warrant Liability
Liabilities:
Total Liabilities	0	0
Significant Other Observable Inputs (Level 2) | 2018 Derivative Instrument
Liabilities:
Total Liabilities	0
Significant Other Observable Inputs (Level 2) | Money Market Funds
Assets:
Total Assets	0	0
Significant Unobservable Inputs (Level 3)
Assets:
Total Assets	0	0
Liabilities:
Total Liabilities	13,517	2,300
Significant Unobservable Inputs (Level 3) | Redeemable Convertible Preferred Stock Warrant Liability
Liabilities:
Total Liabilities	5,000	2,300
Significant Unobservable Inputs (Level 3) | 2018 Derivative Instrument
Liabilities:
Total Liabilities	8,517
Significant Unobservable Inputs (Level 3) | Money Market Funds
Assets:
Total Assets	$ 0	$ 0

Fair Value Measurements - Sum_2

Fair Value Measurements - Summary of Changes in Fair Value of Level 3 Financial Instruments (Details) - Significant Unobservable Inputs (Level 3) $ in Thousands	9 Months Ended
	Sep. 30, 2018USD ($)
Redeemable Convertible Preferred Stock Warrant Liability
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value as of December 31, 2017	$ 2,300
Issuance of financial instruments	0
Change in fair value included in other income (expense), net	2,700
Fair value as of September 30, 2018	5,000
2018 Derivative Instrument Liability
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value as of December 31, 2017	0
Issuance of financial instruments	6,603
Change in fair value included in other income (expense), net	1,914
Fair value as of September 30, 2018	$ 8,517

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) $ / shares in Units, $ in Millions	Sep. 30, 2018USD ($)$ / shares	Dec. 31, 2017USD ($)
Initial Public Offering | Common Stock
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Issued price per share | $ / shares	$ 10
Initial Public Offering | Discount Rate
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value measurement input rate		0.575
Initial Public Offering | Discount Rate | 2018 Derivative Instrument
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value measurement input rate	0.40
Initial Public Offering | Measurement Input, Volatility
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value measurement input rate		0.75
Initial Public Offering | Measurement Input, Risk-free Rate
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value measurement input rate		0.0197
Initial Public Offering | Measurement Input, Expected Dividend Payment
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value measurement input rate		0
Significant Unobservable Inputs (Level 3) | 2017 Convertible Notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Estimated fair value of financial liabilities	$ 26.4	$ 19
Significant Unobservable Inputs (Level 3) | 2018 Convertible Notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Estimated fair value of financial liabilities	$ 43

Convertible Notes - Additional

Convertible Notes - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Oct. 09, 2018	Feb. 28, 2018	Feb. 23, 2018	Feb. 02, 2018	Feb. 28, 2018	Aug. 31, 2017	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Gross proceeds from issuance of convertible notes								$ 33,000	$ 10,000
Aggregate principal amount of convertible notes								9,560	8,000
Non-cash interest expense and amortization of debt discount and issuance cost								5,295	$ 392
2017 Convertible Notes and Series B Redeemable Convertible Preferred Stock
Debt Instrument [Line Items]
Gross proceeds from issuance of convertible notes						$ 10,000
2017 Convertible Notes
Debt Instrument [Line Items]
Aggregate principal amount of convertible notes						$ 8,000
Interest on unpaid principal balance of convertible notes						2.50%
Convertible notes mature date						Dec. 1, 2020
Discount on convertible notes amortized over the contractual period						3 years 3 months 21 days
Convertible notes, annual effective interest rate						38.18%
Convertible notes interest expense							$ 1,000	2,800
Contractual interest expense							1,000	2,700
Non-cash interest expense and amortization of debt discount and issuance cost								100
2017 Convertible Notes | Series B Redeemable Convertible Preferred Stock
Debt Instrument [Line Items]
Conversion price per share						$ 5
2017 Convertible Notes | Initial Public Offering | Subsequent Event | Common Stock
Debt Instrument [Line Items]
Common stock, shares issued	2,637,292
2018 Convertible Notes
Debt Instrument [Line Items]
Gross proceeds from issuance of convertible notes			$ 1,800	$ 31,200	$ 33,000
Aggregate principal amount of convertible notes		$ 9,600
Interest on unpaid principal balance of convertible notes		6.00%			6.00%
Convertible notes interest expense							1,000	2,500
Contractual interest expense							500	1,300
Non-cash interest expense and amortization of debt discount and issuance cost							$ 500	$ 1,200
2018 Convertible Notes | February 2, 2018, First Tranche
Debt Instrument [Line Items]
Discount on convertible notes amortized over the contractual period					2 years 9 months 29 days
Convertible notes, annual effective interest rate		15.10%			15.10%
2018 Convertible Notes | February 23, 2018 Second Tranche
Debt Instrument [Line Items]
Discount on convertible notes amortized over the contractual period					2 years 9 months 7 days
Convertible notes, annual effective interest rate		15.45%			15.45%
2018 Convertible Notes | Initial Public Offering | Subsequent Event | Common Stock
Debt Instrument [Line Items]
Common stock, shares issued	4,295,677

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Commitments And Contingencies Disclosure [Line Items]
Rent expense	$ 100,000	$ 100,000	$ 400,000	$ 400,000
Cancellable Assignment and License Agreement | Maximum
Commitments And Contingencies Disclosure [Line Items]
Milestone payments			33,200,000
Lonza | Service Agreements
Commitments And Contingencies Disclosure [Line Items]
Unconditional purchase obligations, including accrued amounts	$ 2,900,000		2,900,000
Unrecorded unconditional purchase obligation, purchases			$ 2,100,000	$ 8,500,000

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Future Minimum Commitments Under Non-Cancelable Contracts (Details) $ in Thousands	Sep. 30, 2018USD ($)
Operating Lease
Remaining Fiscal 2018	$ 138
2,019	564
2,020	581
2,021	598
2,022	616
Thereafter	526
Total payments	$ 3,023

Redeemable Convertible Prefer_4

Redeemable Convertible Preferred Stock - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized	18,753,595	18,753,595
Minimum | Common Stock
Temporary Equity [Line Items]
Share price	$ 10
Minimum | Common Stock | Initial Public Offering
Temporary Equity [Line Items]
Gross proceeds from Issuance of Common Stock	$ 75,000,000
Redeemable Convertible Preferred Stock | Common Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, terms of conversion	one-for-one
Redeemable Convertible Preferred Stock | Maximum
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized	18,753,595	18,753,595

Redeemable Convertible Prefer_5

Redeemable Convertible Preferred Stock - Schedule of Redeemable Convertible Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2015
Temporary Equity [Line Items]
Redeemable Convertible Preferred Stock, Authorized	18,753,595	18,753,595
Redeemable Convertible Preferred Stock, Outstanding	12,385,154	12,385,154
Liquidation Value	$ 50,324	$ 50,324
Carrying Value	$ 50,017	$ 50,017
Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Stock, Authorized	6,253,595	6,253,595
Redeemable Convertible Preferred Stock, Outstanding	5,593,154	5,593,154
Liquidation Value	$ 16,364	$ 16,364
Carrying Value	$ 16,283	$ 16,283
Original Issuance Price			$ 3.17
Series B Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Stock, Authorized	12,500,000	12,500,000
Redeemable Convertible Preferred Stock, Outstanding	6,792,000	6,792,000
Liquidation Value	$ 33,960	$ 33,960
Carrying Value	$ 33,734	$ 33,734
Original Issuance Price			$ 5

Redeemable Convertible Prefer_6

Redeemable Convertible Preferred Stock - Schedule of Redeemable Convertible Preferred Stock (Parenthetical) (Details) - $ / shares	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2015
Temporary Equity [Line Items]
Redeemable convertible preferred stock, par value	$ 0.0001	$ 0.0001
Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, par value			$ 0.0001
Original Issuance Price			3.17
Series A-1, Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Original Issuance Price			1.34
Series B-1, Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Original Issuance Price			2.15
Series C-1, Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Original Issuance Price			$ 3.73

Common Stock - Additional Infor

Common Stock - Additional Information (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2018Vote$ / sharesshares	Dec. 31, 2017Vote$ / sharesshares
Equity [Abstract]
Common stock, shares authorized | shares	28,500,000	28,500,000
Common stock, par value | $ / shares	$ 0.0001	$ 0.0001
Common stock, number of vote per share | Vote	1	1

Stock-based Compensation - Addi

Stock-based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation of unvested stock options and unvested RSAs		$ 8.2
Unrecognized stock-based compensation of unvested stock awards, weighted-average period expected for recognition		1 year 7 months 2 days
Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Awards vesting period	4 years
Options | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options, periods granted	10 years
Options | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options exercise price, percentage of estimated fair value of shares on grant date	100.00%
ISO Granted to a Greater than 10% Stockholder | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options, periods granted	5 years
ISO Granted to a Greater than 10% Stockholder | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options exercise price, percentage of estimated fair value of shares on grant date	110.00%
Employee Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options, granted		2,429,698	107,500
Stock options, weighted-average grant date fair value		$ 3.46	$ 0.62
Total fair value of employee options vested		$ 0.6	$ 0.2
Non-employee Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options, granted		395,000	30,000
2015 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance	2,810,513

Stock-based Compensation - Summ

Stock-based Compensation - Summary of Stock Options Activity Under 2015 Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
2015 Equity Incentive Plan
Number of Shares Available for Grant
Beginning balance	605,557
Shares authorized	2,125,000
Options granted	(2,824,698)
Options forfeited or canceled	185,000
Ending balance	63,359	605,557
Number of Shares Underlying Outstanding Options
Beginning balance	1,204,414
Options granted	2,824,698
Options exercised	(47,800)
Options forfeited or canceled	(185,000)
Ending balance	3,796,312	1,204,414
Shares exercisable, September 30, 2018	1,632,095
Vested and expected to vest, September 30, 2018	3,796,312
Weighted Average Exercise Price
Beginning balance	$ 0.98
Options granted	6.29
Options exercised	1.03
Options forfeited or canceled	5.19
Ending balance	4.73	$ 0.98
Shares exercisable, September 30, 2018	2.88
Vested and expected to vest, September 30, 2018	$ 4.73
Weighted Average Remaining Contractual Term (in years)
Weighted Average Remaining Contractual Term (in years)	9 years 25 days	8 years 5 months 26 days
Shares exercisable, September 30, 2018	8 years 5 months 19 days
Vested and expected to vest, September 30, 2018	9 years 25 days
Aggregate Intrinsic Value
Beginning balance	$ 41
Options exercised	2
Ending balance	21,117	$ 41
Shares exercisable, September 30, 2018	12,094
Vested and expected to vest, September 30, 2018	$ 21,117
RSA
Number of Shares Available for Grant
RSAs granted	(27,500)

Stock-based Compensation - Fair

Stock-based Compensation - Fair Value of Employee Stock Options Estimated Using Weighted-average Assumptions (Details) - Employee 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]
Expected volatility	54.00%	0.00%	55.00%	63.00%
Risk-free interest rate	2.76%	0.00%	2.71%	1.89%
Dividend yield	0.00%	0.00%	0.00%	0.00%
Expected term	6 years 25 days	0 years	5 years 11 months 26 days	6 years

Stock-based Compensation - Fa_2

Stock-based Compensation - Fair Value of Non-employee Stock Options Estimated Using Weighted-average Assumptions (Details) - Non-employee 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]
Expected volatility	58.00%	63.00%	56.00%	63.00%
Risk-free interest rate	3.01%	2.29%	2.68%	2.28%
Dividend yield	0.00%	0.00%	0.00%	0.00%
Expected term	9 years 4 months 20 days	9 years 7 months 20 days	9 years 1 month 9 days	9 years 7 months 17 days

Stock-based Compensation - Su_2

Stock-based Compensation - Summary of Restricted Stock Award Activity (Details) - RSA	9 Months Ended
	Sep. 30, 2018$ / sharesshares
Number of Shares Underlying Outstanding RSAs
Unvested, beginning balance | shares	291,633
Granted | shares	27,500
Vested | shares	210,959
Unvested, ending balance | shares	108,174
Weighted Average Grant Date Fair Value
Unvested, beginning balance | $ / shares	$ 0.45
Granted | $ / shares	5.38
Vested | $ / shares	1.03
Unvested, ending balance | $ / shares	$ 0.57

Stock-based Compensation - Su_3

Stock-based Compensation - Summary of Stock-based Compensation Classified in Condensed Consolidated Statements of Operations and Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 800	$ 69	$ 1,492	$ 207
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	528	43	892	129
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 272	$ 26	$ 600	$ 78

Redeemable Convertible Prefer_7

Redeemable Convertible Preferred Stock Warrants - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Aug. 11, 2017	Oct. 09, 2018	Sep. 30, 2018	Dec. 31, 2017
Class Of Warrant Or Right [Line Items]
Redeemable convertible preferred stock warrant liability (includes $4,000 and $1,840 at September 30, 2018 and December 31, 2017 attributable to warrants held by related parties)	$ 1,000		$ 5,000	$ 2,300
Common Stock | Redeemable Convertible Preferred Stock Warrant Liability | Subsequent Event | Initial Public Offering
Class Of Warrant Or Right [Line Items]
Warrants exercised		100,000
Common Stock | Redeemable Convertible Preferred Stock Warrant Liability | Warrant | Subsequent Event | Initial Public Offering
Class Of Warrant Or Right [Line Items]
Warrants converted		500,000
Series B Redeemable Convertible Preferred Stock | Common Stock
Class Of Warrant Or Right [Line Items]
Exercise price per share	$ 0.01
Warrants issued	500,000
2017 Convertible Notes | Series B Redeemable Convertible Preferred Stock
Class Of Warrant Or Right [Line Items]
Warrants issued to purchase stock	500,000
Exercise price per share	$ 0.01
2017 Convertible Notes | Series B Redeemable Convertible Preferred Stock | Related Parties
Class Of Warrant Or Right [Line Items]
Warrants issued	400,000

Derivative Instruments - Additi

Derivative Instruments - Additional Information (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Feb. 28, 2018
Embedded Derivative [Line Items]
Derivative instrument, fair value	$ 8,517
2018 Convertible Notes
Embedded Derivative [Line Items]
Derivative instrument, fair value	$ 8,500	$ 6,600

Net Loss per Share Attributab_3

Net Loss per Share Attributable to Common Stockholders - Computation of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (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 loss attributable to common stockholders	$ (10,452)	$ (4,583)	$ (26,781)	$ (16,366)
Denominator:
Weighted-average shares outstanding	7,980,457	7,933,130	7,954,583	7,931,606
Less: weighted-average unvested restricted shares and shares subject to repurchase	(128,897)	(383,419)	(189,695)	(452,083)
Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted	7,851,560	7,549,711	7,764,888	7,479,523
Net loss per share attributable to common stockholders, basic and diluted	$ (1.33)	$ (0.61)	$ (3.45)	$ (2.19)

Net Loss per Share Attributab_4

Net Loss per Share Attributable to Common Stockholders - Summary of Anti-dilutive Securities Excluded from Computation of Diluted Net Loss per Share (Details) - shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	23,769,339	16,474,471
2017 Convertible Notes
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	2,689,744	2,000,000
2018 Convertible Notes
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	4,289,955	0
Series B Redeemable Convertible Preferred Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	500,000	500,000
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	3,796,312	1,229,212
Unvested Restricted Stock Awards
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	108,174	356,062
Unvested Early Exercised Common Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	0	4,043
Redeemable Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	12,385,154	12,385,154

Net Loss per Share Attributab_5

Net Loss per Share Attributable to Common Stockholders - Summary of Anti-dilutive Securities Excluded from Computation of Diluted Net Loss per Share (Parenthetical) (Details) - USD ($)	Sep. 30, 2018	Sep. 30, 2017
Initial Public Offering | Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Issued price per share	$ 10
2017 Convertible Notes
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Convertible notes, principal amount	$ 10,000,000	$ 10,000,000
Convertible notes, accrued but unpaid interest	$ 3,500,000	$ 0
Convertible notes, conversion price per share of common stock	$ 5	$ 5
2018 Convertible Notes | Initial Public Offering
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Convertible notes, principal amount	$ 33,000,000
Convertible notes, accrued but unpaid interest	$ 1,300,000
Percentage of initial public offering per share price	80.00%
2018 Convertible Notes | Initial Public Offering | Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Issued price per share	$ 10

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)shares
Baker Bros. Advisors LP | Series B Redeemable Convertible Preferred Stock
Related Party Transaction [Line Items]
Number of warrants issued to related parties | shares	150,000
Baker Bros. Advisors LP | 2018 Convertible Notes
Related Party Transaction [Line Items]
Aggregate principal amount of convertible notes	$ 6.6
Baker Bros. Advisors LP | 2017 Convertible Notes
Related Party Transaction [Line Items]
Aggregate principal amount of convertible notes	$ 3
Baker Bros. Advisors LP | Minimum
Related Party Transaction [Line Items]
Ownership percentage threshold	5.00%
Dustin Moskovitz Trust DTD 12/27/05 | Series B Redeemable Convertible Preferred Stock
Related Party Transaction [Line Items]
Number of warrants issued to related parties | shares	250,000
Dustin Moskovitz Trust DTD 12/27/05 | 2018 Convertible Notes
Related Party Transaction [Line Items]
Aggregate principal amount of convertible notes	$ 3
Dustin Moskovitz Trust DTD 12/27/05 | 2017 Convertible Notes
Related Party Transaction [Line Items]
Aggregate principal amount of convertible notes	$ 5
Dustin Moskovitz Trust DTD 12/27/05 | Minimum
Related Party Transaction [Line Items]
Ownership percentage threshold	5.00%

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - USD ($) $ in Millions	Nov. 16, 2018	Sep. 30, 2018
Subsequent Event | Corporate Housing
Subsequent Event [Line Items]
Expected lease payments	$ 0.1
2018 Equity Incentive Plan
Subsequent Event [Line Items]
Common stock reserved for issuance		4,300,000
2018 Employee Share Purchase Plan
Subsequent Event [Line Items]
Common stock reserved for issuance		460,000