Kodiak Sciences (KOD) 10-K 27 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Mar. 06, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2018
Trading Symbol	KOD
Entity Registrant Name	Kodiak Sciences Inc.
Entity Central Index Key	0001468748
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Shell Company	false
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		36,909,857
Entity Public Float			$ 166

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 88,254	$ 1,395
Prepaid expenses and other current assets	2,195	200
Total current assets	90,449	1,595
Restricted cash	140	140
Property and equipment, net	1,097	1,509
Other assets	503
Total assets	92,189	3,244
Current liabilities:
Accounts payable	1,050	3,356
Accrued and other current liabilities	3,776	5,802
Total current liabilities	4,826	9,158
Convertible notes (includes $7,937 at December 31, 2017 due to related parties)		9,921
Redeemable convertible preferred stock warrant liability (includes $1,840 at December 31, 2017 attributable to warrants held by related parties)		2,300
Other liabilities	530	586
Total liabilities	5,356	21,965
Commitments and contingencies (Note 7)
Redeemable convertible preferred stock, $0.0001 par value, 10,000,000 and 18,753,595 shares authorized at December 31, 2018 and 2017; 0 and 12,385,154 shares issued and outstanding at December 31, 2018 and 2017; liquidation value of $0 and $50,324 at December 31, 2018 and 2017		50,017
Stockholders’ equity (deficit):
Common stock, $0.0001 par value, 490,000,000 and 28,500,000 shares authorized at December 31, 2018 and 2017; 36,829,857 and 7,936,434 shares issued and outstanding at December 31, 2018 and 2017, respectively	4	1
Additional paid-in capital	197,595	584
Accumulated deficit	(110,766)	(69,323)
Total stockholders’ equity (deficit)	86,833	(68,738)
Total liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)	$ 92,189	$ 3,244

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Convertible notes related parties		$ 7,937
Redeemable preferred stock warrant liability attributable to related parties		$ 1,840
Redeemable convertible preferred stock, par value	$ 0.0001	$ 0.0001
Redeemable convertible preferred stock, shares authorized	10,000,000	18,753,595
Redeemable convertible preferred stock, shares issued	0	12,385,154
Redeemable convertible preferred stock, share outstanding	0	12,385,154
Redeemable convertible preferred stock, liquidation value	$ 0	$ 50,324
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	490,000,000	28,500,000
Common stock, shares issued	36,829,857	7,936,434
Common stock, shares outstanding	36,829,857	7,936,434

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses
Research and development	$ 18,793	$ 22,022	$ 14,053
General and administrative	7,581	3,499	3,098
Total operating expenses	26,374	25,521	17,151
Loss from operations	(26,374)	(25,521)	(17,151)
Interest expense (includes $3,030 and $914 attributable to related parties for the years ended December 31, 2018 and 2017, respectively)	(5,519)	(1,185)	(6)
Other income (expense), net (includes $2,736 and $1,008 attributable to related parties for the years ended December 31, 2018 and 2017, respectively )	(4,071)	(1,230)	25
Loss on extinguishment of debt (includes $1,587 attributable to related parties for the year ended December 31, 2018)	(5,479)
Net loss and comprehensive loss	$ (41,443)	$ (27,936)	$ (17,132)
Net loss per share attributable to common stockholders, basic and diluted	$ (2.77)	$ (3.72)	$ (2.38)
Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted	14,976,515	7,515,336	7,211,360

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) (unaudited) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Interest expense attributable to related parties	$ 3,030	$ 914
Other income (expense) attributable to related parties	2,736	$ 1,008
Loss on extinguishment of debt attributable to related parties	$ 1,587

Consolidated Statements of Rede

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders Equity (Deficit) - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Deficit
Beginning Balance at Dec. 31, 2015	$ (24,236)		$ 1	$ 18	$ (24,255)
Redeemable convertible preferred stock, shares outstanding, beginning balance at Dec. 31, 2015		12,385,154
Redeemable convertible preferred stock, beginning balance at Dec. 31, 2015		$ 50,017
Beginning balance, shares at Dec. 31, 2015			7,828,908
Issuance of common stock upon exercise of stock options	5			5
Issuance of common stock upon exercise of stock options, shares			10,674
Vesting of early exercised stock options	12			12
Issuance of restricted stock awards, shares			142,938
Repurchase of early exercised stock options	(4)			(4)
Repurchase of early exercised stock options, shares			(51,689)
Stock-based compensation expense	272			272
Net loss	(17,132)				(17,132)
Ending Balance at Dec. 31, 2016	(41,083)		$ 1	303	(41,387)
Redeemable convertible preferred stock, shares outstanding, ending balance at Dec. 31, 2016		12,385,154
Redeemable convertible preferred stock, ending balance at Dec. 31, 2016		$ 50,017
Ending balance, shares at Dec. 31, 2016			7,930,831
Issuance of common stock upon exercise of stock options	3			3
Issuance of common stock upon exercise of stock options, shares			5,603
Vesting of early exercised stock options	3			3
Stock-based compensation expense	275			275
Net loss	(27,936)				(27,936)
Ending Balance at Dec. 31, 2017	$ (68,738)		$ 1	584	(69,323)
Redeemable convertible preferred stock, shares outstanding, ending balance at Dec. 31, 2017	12,385,154	12,385,154
Redeemable convertible preferred stock, ending balance at Dec. 31, 2017	$ 50,017	$ 50,017
Ending balance, shares at Dec. 31, 2017			7,936,434
Issuance of common stock upon exercise of stock options	49			49
Issuance of common stock upon exercise of stock options, shares			47,800
Issuance of restricted stock awards, shares			27,500
Conversion of redeemable convertible preferred stock into common stock	50,017		$ 1	50,016
Conversion of redeemable convertible preferred stock into common stock, shares		(12,385,154)
Conversion of redeemable convertible preferred stock into common stock		$ (50,017)
Conversion of redeemable convertible preferred stock into common stock, shares			12,385,154
Conversion of redeemable convertible preferred stock warrants into common stock warrants	5,000			5,000
Issuance of common stock upon exercise of common stock warrants , shares			100,000
Conversion of 2017 and 2018 convertible notes into common stock	55,733		$ 1	55,732
Conversion of 2017 and 2018 convertible notes into common stock, shares			6,932,969
Issuance of common stock upon initial public offering, net of issuance cost	83,459		$ 1	83,458
Issuance of common stock upon initial public offering, net of issuance cost, shares			9,400,000
Stock-based compensation expense	2,756			2,756
Net loss	(41,443)				(41,443)
Ending Balance at Dec. 31, 2018	$ 86,833		$ 4	$ 197,595	$ (110,766)
Redeemable convertible preferred stock, shares outstanding, ending balance at Dec. 31, 2018	0
Ending balance, shares at Dec. 31, 2018			36,829,857

Consolidated Statements of Re_2

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders Equity (Deficit) (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Statement Of Stockholders Equity [Abstract]
Issuance costs	$ 10,542

Consolidated Statements of Cash

Consolidated Statements of Cash Flows $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Cash flows from operating activities
Net loss	$ (41,443)	$ (27,936)	$ (17,132)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	490	549	257
Non-cash interest expense and amortization of debt discount and issuance cost	5,482	1,161	0
Change in fair value of redeemable convertible preferred stock warrant liability	2,700	1,260	0
Change in fair value of derivative instrument	1,988	0	0
Extinguishment of debt	5,479
Stock-based compensation	2,665	275	272
Changes in assets and liabilities:
Prepaid expense and other current assets	(1,995)	368	270
Other assets	0	8	35
Accounts payable	(2,323)	2,282	23
Accrued and other current liabilities	(2,105)	4,330	242
Other liabilities	31	48	(14)
Net cash used in operating activities	(29,031)	(17,655)	(16,047)
Cash flows from investing activities
Purchase of property and equipment	(78)	(209)	(771)
Deposits on property and equipment	(503)
Net cash used in investing activities	(581)	(209)	(771)
Cash flows from financing activities
Proceeds from issuance of common stock, net of offering costs	83,755
Payments for repurchase of early exercised stock options	0	0	(4)
Proceeds from issuance of convertible notes (includes $9,560 and $8,000 from related parties for the years ended December 31, 2018 and 2017, respectively)	33,000	10,000	0
Debt issuance cost	(140)	(181)	0
Principal payments of capital lease	(108)	(97)	(62)
Proceeds from issuance of common stock upon option exercise	49	3	5
Principal payments of tenant improvement allowance payable	(85)	(88)	(59)
Net cash provided by (used in) financing activities	116,471	9,637	(120)
Net increase (decrease) in cash, cash equivalents and restricted cash	86,859	(8,227)	(16,938)
Cash, cash equivalents and restricted cash, at beginning of year	1,535	9,762	26,700
Cash, cash equivalents and restricted cash, at end of year	88,394	1,535	9,762
Reconciliation of cash, cash equivalents and restricted cash to consolidated balance sheets
Cash and cash equivalents	88,254	1,395	9,622
Restricted cash	140	140	140
Cash, cash equivalents and restricted cash, at end of year	88,394	1,535	9,762
Supplemental cash flow information:
Cash paid for interest	19	24	6
Supplemental disclosures of non-cash investing and financing information:
Issuance of derivative instrument related to convertible notes payable	6,603	0	0
Unpaid offering costs	205
Offering costs paid in restricted stock awards	91
Redeemable convertible preferred stock warrant issued in connection with convertible notes	0	1,040	0
Acquisition of equipment through capital lease	0	73	246
Purchase of property and equipment under accounts payable and tenants improvement allowance payable	$ 0	$ 0	$ 731

Consolidated Statements of Ca_2

Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Statement Of Cash Flows [Abstract]
Proceeds from issuance of convertible notes	$ 9,560	$ 8,000

The Company

The Company	12 Months Ended
	Dec. 31, 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 was formed as a limited liability company on June 22, 2009 under the name Oligasis LLC. The Company changed its name and converted into a corporation which was incorporated in the state of Delaware on September 8, 2015. 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 initial public offering (“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 $83.5 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. Liquidity The Company has incurred significant losses and negative cash flows from operations since inception and had an accumulated deficit of $110.8 million as of December 31, 2018. The Company has historically financed its operations primarily through the sale of redeemable convertible preferred stock, convertible notes, warrants to purchase Series B redeemable convertible preferred stock and the sale of common stock in the IPO. To date, none of the Company’s product candidates have been approved for sale and therefore, the Company has not generated any revenue from product sales. Management expects operating losses to continue for the foreseeable future. The Company currently plans to raise additional funding as required based on the status of its clinical trials and projected cash flows, however t

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“US GAAP”). Principles of Consolidation The consolidated financial statements include the Company’s accounts and the accounts of Kodiak Sciences Financing Corporation, the Company’s direct wholly owned subsidiary, incorporated in the United States, and Kodiak Sciences GmbH, the Company’s indirect wholly owned subsidiary, incorporated in Switzerland. All intercompany accounts and transactions have been eliminated. The functional and reporting currency of the Company and its subsidiaries is the U.S. dollar. The aggregate foreign currency transaction loss included in determining net loss was $0.3 million, $0.1 million and less than $0.1 million for the years ended December 31, 2018, 2017 and 2016, respectively. Segments The Company operates and manages its business as one reportable and operating segment, which is the business of research and development of drugs for ophthalmic diseases. The Company’s Chief Executive Officer, who is the chief operating decision maker, reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance. Use of Estimates The preparation of 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 consolidated financial statements and expenses during the reporting period. Such estimates include, but are not limited to, the accrual for research and development expenses, the valuation of deferred tax assets, useful lives of property and equipment, Risk and Uncertainties The Company’s future results of operations involve a number of risks and uncertainties common to clinical-stage companies in the biotechnology industry. The Company’s product candidates are in development and the Company operates in an environment of rapid change in technology and substantial competition from other pharmaceutical and biotechnology companies. Products developed by the Company require approvals from the U.S. Food and Drug Administration (“FDA”) or other international regulatory agencies prior to commercial sales. There can be no assurance that any of the Company’s product candidates will receive the necessary approvals. If the Company is denied approval, approval is delayed or the Company is unable to maintain approvals, it could have a materially adverse impact on the Company. 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 expects to incur substantial operating losses for the next several years and will need to obtain additional financing in order to complete clinical trials and launch and commercialize any product candidates for which it receives regulatory approval. There can be no assurance that such financing will be available or will be on terms acceptable by the Company. Concentration of Credit Risk Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents. The Company’s cash and cash equivalents are held at two U.S. financial institutions. Such deposits may, at times, exceed federally insured limits. Cash and Cash Equivalents The Company considers all highly liquid investments with stated maturities of three months or less at the date of purchase to be cash equivalents. Cash and cash equivalents include money market funds. Restricted Cash As of December 31, 2018, and 2017, the Company had $0.1 million of long-term restricted cash deposited with a financial institution. The entire amount is held in a separate bank account to support a letter of credit agreement related to the Company’s headquarter facility lease which expires in 2023. Fair Value of Financial Instruments The carrying amounts of the Company’s financial instruments consisting of cash and cash equivalents, accounts payable and accrued liabilities and other current liabilities, approximate fair value due to their relatively short maturities. Property and Equipment, Net Property and equipment are stated at cost less accumulated depreciation for acquired assets. Depreciation is computed using the straight-line method over the estimated useful lives of assets, which is generally four years for laboratory equipment, three years for computer equipment and office equipment, five years for computer software and five to seven years for furniture and fixtures. Leasehold improvements are stated at cost and amortized over the shorter of the useful life of the assets or the length of the lease. Upon sale or retirement of assets, the costs and related accumulated depreciation are removed from the consolidated balance sheet and the resulting gain or loss is reflected in operations. Maintenance and repairs are charged to operations as incurred. Impairment of Long-Lived Assets The Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparison of the carrying amount to the future undiscounted net cash flows which the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the assets exceeds the projected discounted future net cash flows arising from the assets. There have been no such impairments of long-lived assets in the years ended December 31, 2018 and 2017. Redeemable Convertible Preferred Stock The Company recorded the redeemable convertible preferred stock at their respective fair values on the dates of issuance, net of issuance costs. The redeemable convertible preferred stock was recorded outside of permanent equity because in the event of certain deemed liquidation events considered not solely within the Company’s control, such as a merger, acquisition and sale of all or substantially all of the Company’s assets, the redeemable convertible preferred stock would become redeemable at the option of the holders. All outstanding shares of the redeemable convertible preferred stock converted into common stock upon effectiveness of the IPO. Redeemable Convertible Preferred Stock Warrants The Company issued redeemable convertible preferred stock warrants as additional consideration for the convertible notes issued in August 2017 (“preferred stock warrants”), which were classified as liabilities. The preferred stock warrants were recorded at fair value upon issuance and remeasured to fair value at each balance sheet date, with any changes in fair value recognized in the consolidated statements of operations and comprehensive loss. All outstanding preferred stock warrants converted into common stock warrants upon effectiveness of the IPO. The common stock warrants were reclassified as equity and the fair value on the date of reclassification was recorded to additional paid-in capital. Derivative Instruments The convertible senior secured promissory notes issued in August 2017 (“2017 convertible notes”) and convertible subordinated unsecured promissory notes issued in February 2018 (“2018 convertible notes”) contained embedded features that provided the lenders with multiple settlement alternatives. Certain of these settlement features provided the lenders a right to a fixed number of the Company’s shares upon conversion of the notes (“conversion option”). Other settlement features provided the lenders the right or the obligation to receive cash or a variable number of shares upon the completion of a capital raising transaction, change of control or default of the Company (“redemption features”). The conversion options of the 2017 convertible notes and 2018 convertible notes did not meet the requirements for separate accounting as an embedded derivative. However, the redemption features of the 2017 convertible notes and 2018 convertible notes met the requirements for separate accounting and are accounted for as a single, compound derivative instrument (“2017 derivative instrument” and “2018 derivative instrument”, respectively). The derivative instruments were recorded at fair value at inception and remeasured to fair value at each balance sheet date, with any changes in fair value recognized in the consolidated statements of operations and comprehensive loss (see Note 13). The fair value of the 2017 derivative instrument was immaterial upon the effectiveness of the IPO. The fair value of the 2018 derivative instrument upon the effectiveness of the IPO was recognized in the consolidated statements of operations and comprehensive loss and the derivative liability was extinguished. Research and Development Expenses Costs related to research, design and development of products are charged to research and development expense as incurred. Research and development costs include, but are not limited to, payroll and personnel expenses, including stock-based compensation, laboratory supplies, outside services and allocated overhead, including rent, equipment, depreciation and utilities. Accrued Research and Development The Company has entered into various agreements with contract research organizations (“CROs”) and contract manufacturing organizations (“CMOs”). The Company’s research and development accruals are estimated based on the level of services performed, progress of the studies, including the phase or completion of events, and contracted costs. The estimated costs of research and development provided, but not yet invoiced, are included in accrued and other current liabilities on the consolidated balance sheets. If the actual timing of the performance of services or the level of effort varies from the original estimates, the Company will adjust the accrual accordingly. Payments made to CROs or CMOs under these arrangements in advance of the performance of the related services are recorded as prepaid expenses and other current assets until the services are rendered. Stock-Based Compensation The Company accounts for stock-based compensation in accordance with the provisions of Accounting Standards Codification (“ASC”) 718, Compensation-Stock Compensation The Company accounts for equity instruments issued to non-employees using a fair value approach. 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. Fair Value of Common Stock Prior to the Company’s IPO, the fair value of the Company’s common stock was determined by the board of directors with assistance from management and external appraisers. Management’s approach to estimate the fair value of the Company’s common stock was consistent with the methods outlined in the American Institute of Certified Public Accountants’ Practice Aid, Valuation of Privately-Held-Company Equity Securities Issued as Compensation Income Taxes The Company accounts for income taxes under the asset and liability method, which requires, among other things, that deferred income taxes be provided for temporary differences between the tax basis of the Company’s assets and liabilities and their financial statement reported amounts. In addition, deferred tax assets are recorded for the future benefit of utilizing net operating losses (“NOLs”) and research and development credit carryforwards and are measured using the enacted tax rates and laws that will be in effect when such items are expected to reverse. A valuation allowance is provided against deferred tax assets unless it is more likely than not that they will be realized. The Company accounts for uncertain tax positions by assessing all material positions taken in any assessment or challenge by relevant taxing authorities. Assessing an uncertain tax position begins with the initial determination of the position’s sustainability and is measured at the largest amount of benefit that is greater than fifty percent likely of being realized upon ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits. Comprehensive Income (Loss) Comprehensive income (loss) is defined as a change in equity of a business enterprise during a period, resulting from transactions from non-owner sources. There have been no items qualifying as other comprehensive income (loss) and, therefore, for all periods presented, the Company’s comprehensive loss was the same as its reported net loss. Net Loss per Share Attributable to Common Stockholders Basic net loss per common share is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of common stock outstanding during the period, without consideration of potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common stock and potentially dilutive securities outstanding for the period. For purposes of this calculation, the redeemable convertible preferred stock, preferred stock warrants, convertible notes, common stock subject to repurchase, and stock options are considered to be potentially dilutive securities. Basic and diluted net loss attributable to common stockholders per share is presented in conformity with the two-class method required for participating securities as the redeemable convertible preferred stock is considered a participating security. The Company’s participating securities do not have a contractual obligation to share in the Company’s losses. As such, the net loss is attributed entirely to common stockholders. Since the Company has reported net loss for all periods presented, diluted net loss per share is the same as basic net loss per common share for those periods. 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 The Company plans to adopt ASU 2016-02 effective January 1, 2019 using the modified retrospective transition approach under ASU 2018-11. The Company is currently in the process of evaluating the impact that the adoption of the new lease standard will have on the consolidated financial statements. Upon adoption, the Company expects to recognize the right-of-use asset and related liability for the lease of its office and laboratory space in Palo Alto, California (Note 7).

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 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): December 31, 2018 December 31, 2017 Leasehold improvement $ 1,260 $ 1,260 Laboratory equipment 1,133 1,174 Furniture and fixtures 225 225 Computer software 85 173 Office equipment 79 79 Computer equipment — 52 Total property and equipment 2,782 2,963 Less: Accumulated depreciation (1,685 ) (1,454 ) Property and equipment, net $ 1,097 $ 1,509 All property and equipment are maintained in the United States. Depreciation expense, including depreciation of assets under capital leases, was $0.5 million, $0.5 million and $0.3 million for the years ended December 31, 2018, 2017 and 2016, respectively.

Accrued Liabilities and Other C

Accrued Liabilities and Other Current Liabilities	12 Months Ended
	Dec. 31, 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): December 31, 2018 December 31, 2017 Accrued salaries and benefits $ 2,061 $ 1,129 Accrued research and development 1,387 4,293 Accrued professional fees 117 19 Accrued legal fees 82 35 Accrued other liabilities 129 326 Total accrued and other current liabilities $ 3,776 $ 5,802

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 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 December 31, 2018 Level 1 Level 2 Level 3 Total Assets: Money market funds $ 87,957 $ — $ — $ 87,957 Total $ 87,957 $ — $ — $ 87,957 There was no liability measured at fair value on a recurring and non-recurring basis as of December 31, 2018. Fair Value Measurements at December 31, 2017 Level 1 Level 2 Level 3 Total Assets: Money market funds $ 1,217 $ — $ — $ 1,217 Total $ 1,217 $ — $ — $ 1,217 Liabilities: Preferred stock warrant liability $ — $ — $ 2,300 $ 2,300 Total $ — $ — $ 2,300 $ 2,300 There were no transfers of assets or liabilities between the fair value measurement levels during the years ended December 31, 2018 and 2017. The following table summarizes the changes in fair value of the Company’s Level 3 financial instruments (in thousands): Preferred Stock Warrant Liability 2018 Derivative Instrument Liability Fair value as of December 31, 2016 $ — $ — Issuance of financial instruments 1,040 — Change in fair value included in other income (expense), net 1,260 — 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 1,988 Conversion of preferred stock warrants into common stock warrants (5,000 ) — Extinguishment of 2018 derivative instrument liability — (8,591 ) Fair value as of December 31, 2018 $ — $ — In October 2018, the preferred stock warrants converted into common stock warrants and the 2018 derivative liability was extinguished in connection with the Company’s IPO. 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 preferred stock warrants, 2017 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. The Company considered 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, as inputs to the PWERM. At December 31, 2017, the fair values recognized for the preferred stock warrants, 2017 derivative instrument, and for disclosure purposes of the fair value of 2017 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 company scenario The estimated fair value of the 2017 convertible notes (Level 3 instrument for disclosure purposes) was $19.0 million as of December 31, 2017.

Convertible Notes

Convertible Notes	12 Months Ended
	Dec. 31, 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 12). 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 13). The discount on 2017 convertible notes was amortized 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 year ended December 31, 2018 was $2.8 million, consisting of $2.7 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 year ended December 31, 2017 was $1.1 million, consisting of $0.8 million of contractual interest expense and $0.3 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. The Company recorded the carrying value of the debt including principal and accrued interest of $13.5 million, net of the unamortized debt discount of $0.7 million, into its capital accounts on the consolidated balance sheets on the date of conversion upon the closing of the IPO. 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 13). The discount on 2018 convertible notes for the first and second tranche was amortized 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 year ended December 31, 2018 was $2.6 million, consisting of $1.4 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 would automatically convert 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 2018 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. The Company accounted for the conversion of the 2018 convertible notes into shares of common stock as a debt extinguishment and recorded $5.5 million loss on such extinguishment in its consolidated statements of operations for the year ended December 31, 2018. The Company recorded the carrying value of the debt including principal and accrued interest of $34.4 million, net of the unamortized debt discount of $5.5 million, into its capital accounts on the consolidated balance sheet on the date of conversion upon the closing of the IPO.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies Leases 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 $0.6 million and $0.6 million and $0.5 million for the years ended December 31, 2018, 2017 and 2016, respectively. The following table summarizes the Company’s future minimum commitments under non-cancelable contracts (in thousands): As of December 31, Operating Lease 2019 $ 564 2020 581 2021 598 2022 616 2023 526 Total payments $ 2,885 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. Other Commitments and 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 may be 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 December 31, 2018 and 2017, the total amount of unconditional purchase obligations, including accrued amounts, under these agreements was $3.2 million and 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. Tenant Improvement Allowance Payable In May 2013, the Company entered into a tenant improvement allowance agreement with its landlord. The agreement allowed the Company to draw down $0.3 million for tenant improvements related to the office lease over the period from the execution of the agreement to October 2018. The interest rate is 8% per year over the lease period. This tenant improvement allowance was repaid in October 2018. In March 2016, the Company entered into a lease amendment, under which the Company is allowed to draw down an additional allowance of $0.4 million for tenant improvements related to the office lease over the period from the execution of the agreement to October 2023. The interest rate is 8% per year over 10 years. Principal and interest are payable on the first day of every month. As of December 31, 2018 and 2017, the current portion of the tenant improvement allowance payable in accrued and other current liabilities was less than $0.1 and $0.1 million, respectively. As of December 31, 2018 and 2017, the non-current portion of the tenant improvement allowance payable in other liabilities was $0.3 million and $0.4 million, respectively. 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.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	8. Income Taxes The Company has not recorded any income tax expense. The Company has a net operating loss and has provided a valuation allowance against net deferred tax assets due to uncertainties regarding the Company’s ability to realize these assets. The components of loss before income taxes were as follows (in thousands): Year Ended December 31, 2018 Year Ended December 31, 2017 Year Ended December 31, 2016 United States $ (17,273 ) $ (3,240 ) $ (14,632 ) Foreign (24,170 ) (24,696 ) (2,500 ) Total loss before income taxes $ (41,443 ) $ (27,936 ) $ (17,132 ) The tax effects of temporary differences that give rise to significant components of the deferred tax assets are as follows (in thousands): December 31, 2018 December 31, 2017 December 31, 2016 Deferred tax assets: Net operating loss carryforwards $ 11,044 $ 11,270 $ 7,022 Intangible assets 7,588 618 783 Research and development tax credits 1,559 788 235 Accruals 700 393 172 Stock-based compensation 394 — — Property and equipment 109 97 88 Other — 26 65 Total gross deferred tax asset 21,394 13,192 8,365 Valuation allowance (21,394 ) (13,192 ) (8,365 ) Net deferred tax assets $ — $ — $ — The Company has recorded a full valuation allowance against its net deferred tax assets due to the uncertainty as to whether such assets will be realized. The net change in the total valuation allowance for the years ended December 31, 2018, 2017 and 2016 was an increase of approximately $8.2 million, $4.8 million and $6.2 million, respectively. On December 22, 2017, H.R. 1 (“Tax Act”) was enacted and included broad tax reforms. The Tax Act reduced the U.S. corporate tax rate from 35% to 21% effective January 1, 2018. The rate change resulted in a $2.5 million reduction in the Company’s deferred tax assets as of December 31, 2017 with an offsetting change in the valuation allowance. The Tax Act also imposed a deemed repatriation of foreign earnings of subsidiaries; Kodiak Sciences GmbH is considered an E&P deficit corporation for the purposes of this provision and thus no income inclusion was required. The Company has elected to treat taxes on Global Intangible Low Tax Income (“GILTI”) as period costs starting in 2018. The SEC released SAB 118 on December 22, 2017 to provide guidance in the application of U.S. GAAP in situations when a registrant does not have the necessary information available, prepared or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the tax reform. The Company had no adjustments related to changes in interpretation of tax reform since December 31, 2017. NOLs and tax credit carry-forwards are subject to review and possible adjustment by the Internal Revenue Service (“IRS”) and may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant shareholders over a three-year period in excess of 50% as defined under Sections 382 and 383 in the Internal Revenue Code, which could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the Company’s value immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. The Company has completed a Section 382 study through December 31, 2018 which concluded no such ownership change had occurred through December 31, 2018. As of December 31, 2018, the Company has $21.9 million of federal and $71.3 million of state net operating loss available to offset future taxable income. The federal net operating loss carryforwards begin to expire in 2035 and the state net operating loss carryforwards begin to expire in 2035, if not utilized. As of December 31, 2018, the Company also had federal and state research and development credit carryforwards of $1.2 million and $0.7 million, respectively. The federal research and development credit carryforwards expire beginning 2035. The California tax credit can be carried forward indefinitely. A reconciliation of the Company’s effective tax rate to the statutory U.S. federal rate is as follows: December 31, 2018 December 31, 2017 December 31, 2016 Federal statutory income tax rate 21.0 % 34.0 % 34.0 % State taxes (tax effected) 5.6 7.1 5.1 Foreign tax rate differential (3.8 ) (17.1 ) (2.8 ) Research tax credit 1.3 0.8 0.4 Stock-based compensation (0.6 ) (0.3 ) (0.2 ) Other (0.1 ) (0.3 ) (0.3 ) Remeasurement of deferred tax due to tax law change — (8.8 ) — Fair value adjustments (2.4 ) — — Extinguishment of convertible note (2.7 ) — — Change in valuation allowance (18.3 ) (15.4 ) (36.2 ) Provision for income taxes 0.0 % 0.0 % 0.0 % The Company recognizes benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. It is the Company’s policy to include penalties and interest expense related to income taxes as a component of other expense, net as necessary. The beginning and ending unrecognized tax benefits amounts are as follows (in thousands): December 31, 2018 December 31, 2017 December 31, 2016 Unrecognized tax benefits at beginning of period $ 357 $ 235 $ 128 Increases related to prior year tax positions — 102 — Increases related to current year tax positions 41 20 107 Unrecognized tax benefits at end of period $ 398 $ 357 $ 235 The Company files income tax returns in the United States and Switzerland. The Company is not currently under examination by income tax authorities in federal, state or other jurisdictions. All tax returns remain open for examination by the federal and state authorities for three and four years, respectively, from the date of utilization of any net operating loss or credits.

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2018
Temporary Equity Disclosure [Abstract]
Redeemable Convertible Preferred Stock	9. Redeemable Convertible Preferred Stock As of December 31, 2018 and 2017, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue up to 10,000,000 and 18,753,595 shares of preferred stock at the par value of $0.0001 per share. Upon the closing of the Company’s IPO on October 9, 2018, all outstanding redeemable convertible preferred stock converted into shares of common stock. As of December 31, 2018, there are no holders of the Company’s preferred stock. As of 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 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 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 were 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 would 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 would be distributed ratably among the holders of the common stock. If available assets were 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 would 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 was entitled to receive with respect to a liquidation event, each such holder of shares of a series of redeemable convertible preferred stock would 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 would be deemed to have converted shares of redeemable convertible preferred stock into common stock pursuant to this paragraph, then such holder would not be entitled to receive any distribution that would otherwise be made to holders of redeemable convertible preferred stock that had not converted into shares of common stock. Conversion Shares of any series of redeemable convertible preferred stock could be converted, at the option of the stockholder, into such number of fully paid and non-assessable shares of common stock. The conversion price was 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. The Company’s redeemable convertible preferred stock was convertible into the Company’s shares of common stock on a one-for-one basis. Shares of redeemable convertible preferred stock would 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 was at least $10.00 and gross proceeds to the Company were equal to or greater than $75.0 million. Dividends The redeemable convertible preferred stock dividends were not cumulative and were payable only when declared by the board of directors. No such dividends were declared. Such dividends were in preference to any dividends to holders of common stock. Voting Rights Each holder of redeemable convertible preferred stock would 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 was recorded in mezzanine equity because while it was not mandatorily redeemable, it would become redeemable at the option of the stockholders upon the occurrence of certain deemed liquidation events that were considered not solely within the Company’s control.

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Common Stock	10. Common Stock As of December 31, 2018 and 2017, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue 490,000,000 and 28,500,000 shares of common stock at the par value of $0.0001 per share. Each share of common stock is entitled to one vote. The board of directors may declare and pay dividends to holders of common stock. The Company has never declared or paid any dividends on common stock. The Company had reserved common stock for future issuances as follows: December 31, 2018 December 31, 2017 Exercise of options outstanding 5,135,267 1,204,414 Exercise of common stock warrants outstanding 400,000 — Issuance of common stock under the 2018 Equity Incentive Plan 3,024,404 — Issuance of common stock under the 2018 Employee Share Purchase Plan 460,000 — Issuance of common stock under the 2015 Equity Incentive Plan — 605,557 Conversion of redeemable convertible preferred stock — 12,385,154 Conversion of convertible notes outstanding — 2,153,781 Exercise of preferred stock warrants outstanding — 500,000 Total 9,019,671 16,848,906

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	11. Stock-Based Compensation 2018 Equity Incentive Plan In August 2018, the Company adopted the 2018 Equity Incentive Plan (“2018 Plan”), which became effective on the business day prior to the effectiveness of the registration statement relating to the IPO. The 2018 Plan initially reserved 4,300,000 shares of common stock for the issuance of incentive stock options ("ISOs"), nonstatutory stock options, restricted stock, restricted stock units (“RSUs”), stock appreciation rights, performance units and performance shares to employees, directors and consultants of the Company. The number of shares available for issuance will increase annually on the first day of each fiscal year beginning in 2019 equal to the least of (1) 4,300,000 shares, and (2) 4% of outstanding shares of common stock as of the last day of the immediately preceding year, and (3) such other amount as determined by the board of directors. The exercise price of options must be equal to at least the fair market value of the common stock on the grant date. For ISOs, the term may not exceed The 2015 Equity Incentive Plan was terminated in connection with the adoption of the 2018 Plan and the 63,359 shares that were then unissued and available for future award under the 2015 Equity Incentive Plan became available under the 2018 Plan. The awards outstanding under the 2015 Equity Incentive Plan continue to be governed by their existing terms. As of December 31, 2018, there were 3,024,404 shares available for grant under the 2018 Plan. 2015 Equity Incentive Plan In September 2015, the Company adopted the 2015 Equity Incentive Plan (“2015 Plan”) under which 2,810,513 shares of common stock were reserved for issuance through grants of incentive stock options, nonqualified stock options and restricted stock awards (“RSAs”) to employees, directors and consultants of the Company. During 2018, the board of directors approved an increase of 2,125,000 shares to the common stock reserved under the 2015 Plan. The awards outstanding under the previously terminated 2009 Share Incentive Plan continue to be governed by their existing terms. Stock Options Stock option activity under the 2018 Plan and 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, January 1, 2016 1,814,560 102,937 $ 0.23 7.92 $ 86 Options granted (1,209,595 ) 1,209,595 $ 1.04 9.53 Options exercised — (10,674 ) $ 0.45 6.76 7 Options forfeited or canceled 38,101 (38,101 ) $ 0.97 9.38 Repurchase of early exercised options 51,689 — $ 0.12 RSAs granted(1) (142,938 ) — Balance, December 31, 2016 551,817 1,263,757 $ 0.98 9.42 $ 100 Options granted (137,500 ) 137,500 $ 1.06 Options exercised — (5,603 ) $ 0.50 3 Options forfeited or canceled 191,240 (191,240 ) $ 1.04 Balance, December 31, 2017 605,557 1,204,414 $ 0.98 8.49 $ 41 Shares authorized 6,425,000 — RSAs granted (27,500 ) — RSUs granted (60,000 ) 60,000 $ 10.00 Options granted (4,103,653 ) 4,103,653 $ 7.45 Options exercised — (47,800 ) $ 1.03 290 Options forfeited or canceled 185,000 (185,000 ) $ 5.19 Balance, December 31, 2018 3,024,404 5,135,267 $ 5.99 9.07 $ 10,681 Shares exercisable, December 31, 2017 863,791 $ 0.97 8.38 $ 30 Vested and expected to vest, December 31, 2017 1,204,414 $ 0.98 8.49 $ 41 Shares exercisable, December 31, 2018 1,766,385 $ 3.11 8.28 $ 7,182 Vested and expected to vest, December 31, 2018 5,135,267 $ 5.99 9.07 $ 10,681 (1) Reflects an adjustment to shares available for grant related to the issuance of restricted stock awards under the 2015 Plan. During the years ended December 31, 2018, 2017 and 2016, the Company granted 3,888,653, 107,500 and 1,205,940 stock options, respectively, to employees with a weighted-average grant date fair value of $4.36, $0.61 and $0.60 per share, respectively. Shares Subject to Repurchase The Company has a right of repurchase with respect to unvested shares issued upon early exercise of options at an amount equal to the lower of (1) the exercise price of each restricted share being repurchased and (2) the fair market value of such restricted share at the time the Company’s right of repurchase is exercised. The Company’s right to repurchase these shares lapses as those shares vest over the requisite service period. Shares purchased by employees pursuant to the early exercise of stock options are not deemed, for accounting purposes, to be issued until those shares vest according to their respective vesting schedules. Cash received for early exercised stock options is recorded as accrued liabilities and other current liabilities on the consolidated balance sheet and is reclassified to common stock and additional paid-in capital as such shares vest. At December 31, 2018 and 2017, 0 and 2,887 shares, respectively, remained subject to the Company’s right of repurchase as a result of the early exercised stock options. Fair Value of Options Granted Prior to the Company’s IPO, the fair value of the shares of common stock underlying the stock options was determined by the board of directors with assistance from management and external appraisers as there has been no historical public market for the Company’s common stock. Subsequent to the Company’s IPO, the fair value of the Company’s common stock is determined based on its closing market price. 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: Year Ended December 31, 2018 Year Ended December 31, 2017 Year Ended December 31, 2016 Expected volatility 59 % 63 % 64 % Risk-free interest rate 2.82 % 1.89 % 1.22 % Dividend yield 0 % 0 % 0 % Expected term 6.06 6.00 5.84 Expected Term . The expected term is calculated using the simplified method, which is available where there is insufficient historical data about exercise patterns and post-vesting employment termination behavior. The simplified method is based on the vesting period and the contractual term for each grant, or for each vesting-tranche for awards with graded vesting. The mid-point between the vesting date and the maximum contractual expiration date is used as the expected term under this method. For awards with multiple vesting-tranches, the times from grant until the mid-points for each of the tranches may be averaged to provide an overall expected term. Expected Volatility . The Company used an average historical stock price volatility of a peer group of publicly traded companies to be representative of its expected future stock price volatility, as the Company does not have sufficient trading history for its common stock. For purposes of identifying these peer companies, the Company considered the industry, stage of development, size and financial leverage of potential comparable companies. For each grant, the Company measured historical volatility over a period equivalent to the expected term. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available. Risk-Free Interest Rate . The risk-free interest rate is based on the implied yield currently available on U.S. Treasury zero-coupon issues with a remaining term equivalent to the expected term of a stock award. Expected Dividend Rate . The Company has not paid and does not anticipate paying any dividends in the near future. Accordingly, the Company has estimated the dividend yield to be zero. The total value of employee options vested during the years ended December 31, 2018, 2017 and 2016 was $1.2 million, $0.2 million and $0.1 million, respectively. Stock-based compensation expense recognized during the years ended December 31, 2018, 2017 and 2016 for options granted to employees was $2.0 million, $0.2 million and $0.2 million, respectively. Non-Employee Stock-Based Compensation The Company granted 215,000, 30,000 and 3,655 stock options to non-employees during the years ended December 31, 2018, 2017 and 2016, 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: Year Ended December 31, 2018 Year Ended December 31, 2017 Year Ended December 31, 2016 Expected volatility 68 % 63 % 65 % Risk-free interest rate 2.71 % 2.31 % 1.86 % Dividend yield 0 % 0 % 0 % Expected term 9.30 9.35 9.09 Stock-based compensation expense recognized during the years ended December 31, 2018, 2017 and 2016 for options granted to non-employees was $0.4 million, less than $0.1 million and less than $0.1 million, respectively. 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, 2016 549,351 $ 0.41 Vested (257,718 ) $ 1.00 Unvested, December 31, 2017 291,633 $ 0.45 Granted 27,500 $ 5.38 Vested (268,683 ) $ 0.89 Unvested, December 31, 2018 50,450 $ 0.79 Under the terms of the restricted stock agreements, 1/48th of the award vests monthly over four years, which is the requisite service period. Recipients of restricted stock awards generally have voting and dividend rights with respect to such shares upon grant without regard to vesting. Shares of restricted stock that do not vest are subject to forfeiture. The Company recognizes stock-based compensation expense for RSAs on a straight-line basis over the requisite service period for the entire award. The total value of RSAs vested during the years ended December 31, 2018, 2017 and 2016 was $0.2 million, $0.1 million and $0.1 million, respectively. During the years ended December 31, 2018, 2017 and 2016, stock-based compensation expense recognized for RSAs was $0.2 million, $0.1 million and $0.1 million, respectively. The Company granted 60,000 RSUs under the 2018 Plan and recognized stock-based compensation expense of less than $0.1 million for the year ended December 31, 2018. 2018 Employee Share Purchase Plan In August 2018, the Company adopted the 2018 Employee Share Purchase Plan (“ESPP”), 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 offering period and purchase period will be determined by the board of directors. As of December 31, 2018, no offerings have been authorized to date. Stock-Based Compensation Expense Stock-based compensation is classified in the consolidated statements of operations and comprehensive loss as follows (in thousands): Year Ended December 31, 2018 Year Ended December 31, 2017 Year Ended December 31, 2016 Research and development $ 1,535 $ 171 $ 88 General and administrative 1,073 104 184 Total stock-based compensation $ 2,608 $ 275 $ 272 As of December 31, 2018, the unrecognized stock-based compensation of unvested employee options and unvested RSAs and RSUs was $15.7 million and expected to be recognized over a weighted-average period of 1.69 years. As of December 31, 2017, the unrecognized stock-based compensation of unvested employee options and unvested RSAs was $0.5 million and expected to be recognized over a weighted-average period of 0.95 years.

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock Warrants	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Redeemable Convertible Preferred Stock Warrants	12. 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 preferred stock warrants would convert into common stock warrants at an exercise price of $0.01 per share. The outstanding preferred stock warrants terminated at the earlier of August 11, 2022 and a change of control unless exercised. These warrants had 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 contained 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 preferred stock warrants on the date of issuance of $1.0 million was recorded as a debt discount. The preferred stock warrant liability had a fair value of $5.0 million immediately prior to conversion into common stock warrants and $2.3 million as of December 31, 2017. The change in fair value was recorded in the consolidated statements of operations and comprehensive loss. Upon the closing of the IPO, 500,000 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 common stock warrants were reclassified as equity and the fair value on the date of reclassification was recorded to additional paid-in capital. As of December 31, 2018, 400,000 common stock warrants remained outstanding.

Derivative Instruments

Derivative Instruments	12 Months Ended
	Dec. 31, 2018
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Derivative Instruments	13. 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 immediately prior to closing of the IPO 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 $8.6 million immediately prior to closing of the IPO.

Net Loss per Share Attributable

Net Loss per Share Attributable to Common Stockholders	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Loss per Share Attributable to Common Stockholders	14. 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): Year Ended December 31, 2018 Year Ended December 31, 2017 Year Ended December 31, 2016 Numerator: Net loss attributable to common stockholders $ (41,443 ) $ (27,936 ) $ (17,132 ) Denominator: Weighted-average shares outstanding 15,136,197 7,932,717 7,850,183 Less: weighted-average unvested restricted shares and shares subject to repurchase (159,682 ) (417,381 ) (638,823 ) Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted 14,976,515 7,515,336 7,211,360 Net loss per share attributable to common stockholders, basic and diluted $ (2.77 ) $ (3.72 ) $ (2.38 ) 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: Year Ended December 31, 2018 Year Ended December 31, 2017 Year Ended December 31, 2016 Exercise of options outstanding 5,135,267 1,204,414 1,263,757 Unvested restricted stock awards 50,450 291,633 549,351 Unvested early exercised common stock options — 2,887 19,336 Conversion of redeemable convertible preferred stock — 12,385,154 12,385,154 Conversion of convertible notes outstanding — 2,153,781 — Exercise of preferred stock warrants outstanding — 500,000 — Total 5,185,717 16,537,869 14,217,598

401(k) Plan

401(k) Plan	12 Months Ended
	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
401(k) plan	15. 401(k) Plan In 2011, the Company adopted a 401(k) retirement and savings plan covering all employees. The 401(k) plan allows employees to make pre- and post-tax contributions up to the maximum allowable amount set by the Internal Revenue Service. The Company does not make matching contributions to the 401(k) plan on behalf of participants.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party Transactions	16. 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 12). 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 12).

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (Unaudited)	17. Selected Quarterly Financial Data (unaudited) The following table provides the selected quarterly financial information for the years 2018 and 2017 (in thousands, except per share data): Three Months Ended March 31, 2018 June 30, 2018 September 30, 2018 December 31, 2018 Loss from operations $ (5,547 ) $ (5,090 ) $ (6,380 ) $ (9,357 ) Net loss and comprehensive loss $ (8,920 ) $ (7,409 ) $ (10,452 ) $ (14,662 ) Net loss per share attributable to common stockholders, basic and diluted $ (1.16 ) $ (0.96 ) $ (1.33 ) $ (0.40 ) Three Months Ended March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 Loss from operations $ (5,043 ) $ (6,742 ) $ (3,979 ) $ (9,757 ) Net loss and comprehensive loss $ (5,041 ) $ (6,742 ) $ (4,583 ) $ (11,570 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.68 ) $ (0.90 ) $ (0.61 ) $ (1.52 )

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“US GAAP”).
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the Company’s accounts and the accounts of Kodiak Sciences Financing Corporation, the Company’s direct wholly owned subsidiary, incorporated in the United States, and Kodiak Sciences GmbH, the Company’s indirect wholly owned subsidiary, incorporated in Switzerland. All intercompany accounts and transactions have been eliminated. The functional and reporting currency of the Company and its subsidiaries is the U.S. dollar. The aggregate foreign currency transaction loss included in determining net loss was $0.3 million, $0.1 million and less than $0.1 million for the years ended December 31, 2018, 2017 and 2016, respectively.
Segments	Segments The Company operates and manages its business as one reportable and operating segment, which is the business of research and development of drugs for ophthalmic diseases. The Company’s Chief Executive Officer, who is the chief operating decision maker, reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance.
Use of Estimates	Use of Estimates The preparation of 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 consolidated financial statements and expenses during the reporting period. Such estimates include, but are not limited to, the accrual for research and development expenses, the valuation of deferred tax assets, useful lives of property and equipment,
Risk and Uncertainties	Risk and Uncertainties The Company’s future results of operations involve a number of risks and uncertainties common to clinical-stage companies in the biotechnology industry. The Company’s product candidates are in development and the Company operates in an environment of rapid change in technology and substantial competition from other pharmaceutical and biotechnology companies. Products developed by the Company require approvals from the U.S. Food and Drug Administration (“FDA”) or other international regulatory agencies prior to commercial sales. There can be no assurance that any of the Company’s product candidates will receive the necessary approvals. If the Company is denied approval, approval is delayed or the Company is unable to maintain approvals, it could have a materially adverse impact on the Company. 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 expects to incur substantial operating losses for the next several years and will need to obtain additional financing in order to complete clinical trials and launch and commercialize any product candidates for which it receives regulatory approval. There can be no assurance that such financing will be available or will be on terms acceptable by the Company.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents. The Company’s cash and cash equivalents are held at two U.S. financial institutions. Such deposits may, at times, exceed federally insured limits.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with stated maturities of three months or less at the date of purchase to be cash equivalents. Cash and cash equivalents include money market funds.
Restricted Cash	Restricted Cash As of December 31, 2018, and 2017, the Company had $0.1 million of long-term restricted cash deposited with a financial institution. The entire amount is held in a separate bank account to support a letter of credit agreement related to the Company’s headquarter facility lease which expires in 2023.
Fair Value Measurements	Fair Value of Financial Instruments The carrying amounts of the Company’s financial instruments consisting of cash and cash equivalents, accounts payable and accrued liabilities and other current liabilities, approximate fair value due to their relatively short maturities.
Property and Equipment, Net	Property and Equipment, Net Property and equipment are stated at cost less accumulated depreciation for acquired assets. Depreciation is computed using the straight-line method over the estimated useful lives of assets, which is generally four years for laboratory equipment, three years for computer equipment and office equipment, five years for computer software and five to seven years for furniture and fixtures. Leasehold improvements are stated at cost and amortized over the shorter of the useful life of the assets or the length of the lease. Upon sale or retirement of assets, the costs and related accumulated depreciation are removed from the consolidated balance sheet and the resulting gain or loss is reflected in operations. Maintenance and repairs are charged to operations as incurred.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparison of the carrying amount to the future undiscounted net cash flows which the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the assets exceeds the projected discounted future net cash flows arising from the assets. There have been no such impairments of long-lived assets in the years ended December 31, 2018 and 2017.
Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock The Company recorded the redeemable convertible preferred stock at their respective fair values on the dates of issuance, net of issuance costs. The redeemable convertible preferred stock was recorded outside of permanent equity because in the event of certain deemed liquidation events considered not solely within the Company’s control, such as a merger, acquisition and sale of all or substantially all of the Company’s assets, the redeemable convertible preferred stock would become redeemable at the option of the holders. All outstanding shares of the redeemable convertible preferred stock converted into common stock upon effectiveness of the IPO.
Redeemable Convertible Preferred Stock Warrants	Redeemable Convertible Preferred Stock Warrants The Company issued redeemable convertible preferred stock warrants as additional consideration for the convertible notes issued in August 2017 (“preferred stock warrants”), which were classified as liabilities. The preferred stock warrants were recorded at fair value upon issuance and remeasured to fair value at each balance sheet date, with any changes in fair value recognized in the consolidated statements of operations and comprehensive loss. All outstanding preferred stock warrants converted into common stock warrants upon effectiveness of the IPO. The common stock warrants were reclassified as equity and the fair value on the date of reclassification was recorded to additional paid-in capital.
Derivative Instruments	Derivative Instruments The convertible senior secured promissory notes issued in August 2017 (“2017 convertible notes”) and convertible subordinated unsecured promissory notes issued in February 2018 (“2018 convertible notes”) contained embedded features that provided the lenders with multiple settlement alternatives. Certain of these settlement features provided the lenders a right to a fixed number of the Company’s shares upon conversion of the notes (“conversion option”). Other settlement features provided the lenders the right or the obligation to receive cash or a variable number of shares upon the completion of a capital raising transaction, change of control or default of the Company (“redemption features”). The conversion options of the 2017 convertible notes and 2018 convertible notes did not meet the requirements for separate accounting as an embedded derivative. However, the redemption features of the 2017 convertible notes and 2018 convertible notes met the requirements for separate accounting and are accounted for as a single, compound derivative instrument (“2017 derivative instrument” and “2018 derivative instrument”, respectively). The derivative instruments were recorded at fair value at inception and remeasured to fair value at each balance sheet date, with any changes in fair value recognized in the consolidated statements of operations and comprehensive loss (see Note 13). The fair value of the 2017 derivative instrument was immaterial upon the effectiveness of the IPO. The fair value of the 2018 derivative instrument upon the effectiveness of the IPO was recognized in the consolidated statements of operations and comprehensive loss and the derivative liability was extinguished.
Research and Development Expenses	Research and Development Expenses Costs related to research, design and development of products are charged to research and development expense as incurred. Research and development costs include, but are not limited to, payroll and personnel expenses, including stock-based compensation, laboratory supplies, outside services and allocated overhead, including rent, equipment, depreciation and utilities.
Accrued Research And Development	Accrued Research and Development The Company has entered into various agreements with contract research organizations (“CROs”) and contract manufacturing organizations (“CMOs”). The Company’s research and development accruals are estimated based on the level of services performed, progress of the studies, including the phase or completion of events, and contracted costs. The estimated costs of research and development provided, but not yet invoiced, are included in accrued and other current liabilities on the consolidated balance sheets. If the actual timing of the performance of services or the level of effort varies from the original estimates, the Company will adjust the accrual accordingly. Payments made to CROs or CMOs under these arrangements in advance of the performance of the related services are recorded as prepaid expenses and other current assets until the services are rendered.
Stock-Based Compensation	Stock-Based Compensation The Company accounts for stock-based compensation in accordance with the provisions of Accounting Standards Codification (“ASC”) 718, Compensation-Stock Compensation The Company accounts for equity instruments issued to non-employees using a fair value approach. 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.
Fair Value of Common Stock	Fair Value of Common Stock Prior to the Company’s IPO, the fair value of the Company’s common stock was determined by the board of directors with assistance from management and external appraisers. Management’s approach to estimate the fair value of the Company’s common stock was consistent with the methods outlined in the American Institute of Certified Public Accountants’ Practice Aid, Valuation of Privately-Held-Company Equity Securities Issued as Compensation
Income Taxes	Income Taxes The Company accounts for income taxes under the asset and liability method, which requires, among other things, that deferred income taxes be provided for temporary differences between the tax basis of the Company’s assets and liabilities and their financial statement reported amounts. In addition, deferred tax assets are recorded for the future benefit of utilizing net operating losses (“NOLs”) and research and development credit carryforwards and are measured using the enacted tax rates and laws that will be in effect when such items are expected to reverse. A valuation allowance is provided against deferred tax assets unless it is more likely than not that they will be realized. The Company accounts for uncertain tax positions by assessing all material positions taken in any assessment or challenge by relevant taxing authorities. Assessing an uncertain tax position begins with the initial determination of the position’s sustainability and is measured at the largest amount of benefit that is greater than fifty percent likely of being realized upon ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) is defined as a change in equity of a business enterprise during a period, resulting from transactions from non-owner sources. There have been no items qualifying as other comprehensive income (loss) and, therefore, for all periods presented, the Company’s comprehensive loss was the same as its reported net loss.
Net Loss per Share Attributable to Common Stockholders	Net Loss per Share Attributable to Common Stockholders Basic net loss per common share is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of common stock outstanding during the period, without consideration of potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common stock and potentially dilutive securities outstanding for the period. For purposes of this calculation, the redeemable convertible preferred stock, preferred stock warrants, convertible notes, common stock subject to repurchase, and stock options are considered to be potentially dilutive securities. Basic and diluted net loss attributable to common stockholders per share is presented in conformity with the two-class method required for participating securities as the redeemable convertible preferred stock is considered a participating security. The Company’s participating securities do not have a contractual obligation to share in the Company’s losses. As such, the net loss is attributed entirely to common stockholders. Since the Company has reported net loss for all periods presented, diluted net loss per share is the same as basic net loss per common share for those periods.
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 The Company plans to adopt ASU 2016-02 effective January 1, 2019 using the modified retrospective transition approach under ASU 2018-11. The Company is currently in the process of evaluating the impact that the adoption of the new lease standard will have on the consolidated financial statements. Upon adoption, the Company expects to recognize the right-of-use asset and related liability for the lease of its office and laboratory space in Palo Alto, California (Note 7).

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Summary of Property and Equipment, net	Property and equipment, net consists of the following (in thousands): December 31, 2018 December 31, 2017 Leasehold improvement $ 1,260 $ 1,260 Laboratory equipment 1,133 1,174 Furniture and fixtures 225 225 Computer software 85 173 Office equipment 79 79 Computer equipment — 52 Total property and equipment 2,782 2,963 Less: Accumulated depreciation (1,685 ) (1,454 ) Property and equipment, net $ 1,097 $ 1,509

Accrued Liabilities and Other_2

Accrued Liabilities and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 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): December 31, 2018 December 31, 2017 Accrued salaries and benefits $ 2,061 $ 1,129 Accrued research and development 1,387 4,293 Accrued professional fees 117 19 Accrued legal fees 82 35 Accrued other liabilities 129 326 Total accrued and other current liabilities $ 3,776 $ 5,802

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 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 December 31, 2018 Level 1 Level 2 Level 3 Total Assets: Money market funds $ 87,957 $ — $ — $ 87,957 Total $ 87,957 $ — $ — $ 87,957 There was no liability measured at fair value on a recurring and non-recurring basis as of December 31, 2018. Fair Value Measurements at December 31, 2017 Level 1 Level 2 Level 3 Total Assets: Money market funds $ 1,217 $ — $ — $ 1,217 Total $ 1,217 $ — $ — $ 1,217 Liabilities: Preferred stock warrant liability $ — $ — $ 2,300 $ 2,300 Total $ — $ — $ 2,300 $ 2,300
Summarizes the Changes in Fair Value of Level 3 Financial Instruments	The following table summarizes the changes in fair value of the Company’s Level 3 financial instruments (in thousands): Preferred Stock Warrant Liability 2018 Derivative Instrument Liability Fair value as of December 31, 2016 $ — $ — Issuance of financial instruments 1,040 — Change in fair value included in other income (expense), net 1,260 — 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 1,988 Conversion of preferred stock warrants into common stock warrants (5,000 ) — Extinguishment of 2018 derivative instrument liability — (8,591 ) Fair value as of December 31, 2018 $ — $ —

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 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 December 31, Operating Lease 2019 $ 564 2020 581 2021 598 2022 616 2023 526 Total payments $ 2,885

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Components of Loss before Income Taxes	The components of loss before income taxes were as follows (in thousands): Year Ended December 31, 2018 Year Ended December 31, 2017 Year Ended December 31, 2016 United States $ (17,273 ) $ (3,240 ) $ (14,632 ) Foreign (24,170 ) (24,696 ) (2,500 ) Total loss before income taxes $ (41,443 ) $ (27,936 ) $ (17,132 )
Components of Deferred Tax Assets	The tax effects of temporary differences that give rise to significant components of the deferred tax assets are as follows (in thousands): December 31, 2018 December 31, 2017 December 31, 2016 Deferred tax assets: Net operating loss carryforwards $ 11,044 $ 11,270 $ 7,022 Intangible assets 7,588 618 783 Research and development tax credits 1,559 788 235 Accruals 700 393 172 Stock-based compensation 394 — — Property and equipment 109 97 88 Other — 26 65 Total gross deferred tax asset 21,394 13,192 8,365 Valuation allowance (21,394 ) (13,192 ) (8,365 ) Net deferred tax assets $ — $ — $ —
Schedule of Reconciliation of Effective Tax Rate to Statutory U.S. Federal Rate	A reconciliation of the Company’s effective tax rate to the statutory U.S. federal rate is as follows: December 31, 2018 December 31, 2017 December 31, 2016 Federal statutory income tax rate 21.0 % 34.0 % 34.0 % State taxes (tax effected) 5.6 7.1 5.1 Foreign tax rate differential (3.8 ) (17.1 ) (2.8 ) Research tax credit 1.3 0.8 0.4 Stock-based compensation (0.6 ) (0.3 ) (0.2 ) Other (0.1 ) (0.3 ) (0.3 ) Remeasurement of deferred tax due to tax law change — (8.8 ) — Fair value adjustments (2.4 ) — — Extinguishment of convertible note (2.7 ) — — Change in valuation allowance (18.3 ) (15.4 ) (36.2 ) Provision for income taxes 0.0 % 0.0 % 0.0 %
Summary of Unrecognized Tax Benefits Amounts	The beginning and ending unrecognized tax benefits amounts are as follows (in thousands): December 31, 2018 December 31, 2017 December 31, 2016 Unrecognized tax benefits at beginning of period $ 357 $ 235 $ 128 Increases related to prior year tax positions — 102 — Increases related to current year tax positions 41 20 107 Unrecognized tax benefits at end of period $ 398 $ 357 $ 235

Redeemable Convertible Prefer_3

Redeemable Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2018
Temporary Equity Disclosure [Abstract]
Schedule of Redeemable Convertible Preferred Stock	As of 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 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

Common Stock (Tables)

Common Stock (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Schedule of Common Stock Reserved for Future Issuances	The Company had reserved common stock for future issuances as follows: December 31, 2018 December 31, 2017 Exercise of options outstanding 5,135,267 1,204,414 Exercise of common stock warrants outstanding 400,000 — Issuance of common stock under the 2018 Equity Incentive Plan 3,024,404 — Issuance of common stock under the 2018 Employee Share Purchase Plan 460,000 — Issuance of common stock under the 2015 Equity Incentive Plan — 605,557 Conversion of redeemable convertible preferred stock — 12,385,154 Conversion of convertible notes outstanding — 2,153,781 Exercise of preferred stock warrants outstanding — 500,000 Total 9,019,671 16,848,906

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Stock Options Activity Under 2018 Plan and 2015 Plan	Stock option activity under the 2018 Plan and 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, January 1, 2016 1,814,560 102,937 $ 0.23 7.92 $ 86 Options granted (1,209,595 ) 1,209,595 $ 1.04 9.53 Options exercised — (10,674 ) $ 0.45 6.76 7 Options forfeited or canceled 38,101 (38,101 ) $ 0.97 9.38 Repurchase of early exercised options 51,689 — $ 0.12 RSAs granted(1) (142,938 ) — Balance, December 31, 2016 551,817 1,263,757 $ 0.98 9.42 $ 100 Options granted (137,500 ) 137,500 $ 1.06 Options exercised — (5,603 ) $ 0.50 3 Options forfeited or canceled 191,240 (191,240 ) $ 1.04 Balance, December 31, 2017 605,557 1,204,414 $ 0.98 8.49 $ 41 Shares authorized 6,425,000 — RSAs granted (27,500 ) — RSUs granted (60,000 ) 60,000 $ 10.00 Options granted (4,103,653 ) 4,103,653 $ 7.45 Options exercised — (47,800 ) $ 1.03 290 Options forfeited or canceled 185,000 (185,000 ) $ 5.19 Balance, December 31, 2018 3,024,404 5,135,267 $ 5.99 9.07 $ 10,681 Shares exercisable, December 31, 2017 863,791 $ 0.97 8.38 $ 30 Vested and expected to vest, December 31, 2017 1,204,414 $ 0.98 8.49 $ 41 Shares exercisable, December 31, 2018 1,766,385 $ 3.11 8.28 $ 7,182 Vested and expected to vest, December 31, 2018 5,135,267 $ 5.99 9.07 $ 10,681 (1) Reflects an adjustment to shares available for grant related to the issuance of restricted stock awards under the 2015 Plan.
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, 2016 549,351 $ 0.41 Vested (257,718 ) $ 1.00 Unvested, December 31, 2017 291,633 $ 0.45 Granted 27,500 $ 5.38 Vested (268,683 ) $ 0.89 Unvested, December 31, 2018 50,450 $ 0.79
Summary of Stock-based Compensation Classified in Condensed Consolidated Statements of Operations and Comprehensive Loss	Stock-based compensation is classified in the consolidated statements of operations and comprehensive loss as follows (in thousands): Year Ended December 31, 2018 Year Ended December 31, 2017 Year Ended December 31, 2016 Research and development $ 1,535 $ 171 $ 88 General and administrative 1,073 104 184 Total stock-based compensation $ 2,608 $ 275 $ 272
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 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: Year Ended December 31, 2018 Year Ended December 31, 2017 Year Ended December 31, 2016 Expected volatility 59 % 63 % 64 % Risk-free interest rate 2.82 % 1.89 % 1.22 % Dividend yield 0 % 0 % 0 % Expected term 6.06 6.00 5.84
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: Year Ended December 31, 2018 Year Ended December 31, 2017 Year Ended December 31, 2016 Expected volatility 68 % 63 % 65 % Risk-free interest rate 2.71 % 2.31 % 1.86 % Dividend yield 0 % 0 % 0 % Expected term 9.30 9.35 9.09

Net Loss per Share Attributab_2

Net Loss per Share Attributable to Common Stockholders (Tables)	12 Months Ended
	Dec. 31, 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): Year Ended December 31, 2018 Year Ended December 31, 2017 Year Ended December 31, 2016 Numerator: Net loss attributable to common stockholders $ (41,443 ) $ (27,936 ) $ (17,132 ) Denominator: Weighted-average shares outstanding 15,136,197 7,932,717 7,850,183 Less: weighted-average unvested restricted shares and shares subject to repurchase (159,682 ) (417,381 ) (638,823 ) Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted 14,976,515 7,515,336 7,211,360 Net loss per share attributable to common stockholders, basic and diluted $ (2.77 ) $ (3.72 ) $ (2.38 )
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: Year Ended December 31, 2018 Year Ended December 31, 2017 Year Ended December 31, 2016 Exercise of options outstanding 5,135,267 1,204,414 1,263,757 Unvested restricted stock awards 50,450 291,633 549,351 Unvested early exercised common stock options — 2,887 19,336 Conversion of redeemable convertible preferred stock — 12,385,154 12,385,154 Conversion of convertible notes outstanding — 2,153,781 — Exercise of preferred stock warrants outstanding — 500,000 — Total 5,185,717 16,537,869 14,217,598

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	The following table provides the selected quarterly financial information for the years 2018 and 2017 (in thousands, except per share data): Three Months Ended March 31, 2018 June 30, 2018 September 30, 2018 December 31, 2018 Loss from operations $ (5,547 ) $ (5,090 ) $ (6,380 ) $ (9,357 ) Net loss and comprehensive loss $ (8,920 ) $ (7,409 ) $ (10,452 ) $ (14,662 ) Net loss per share attributable to common stockholders, basic and diluted $ (1.16 ) $ (0.96 ) $ (1.33 ) $ (0.40 ) Three Months Ended March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 Loss from operations $ (5,043 ) $ (6,742 ) $ (3,979 ) $ (9,757 ) Net loss and comprehensive loss $ (5,041 ) $ (6,742 ) $ (4,583 ) $ (11,570 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.68 ) $ (0.90 ) $ (0.61 ) $ (1.52 )

The Company - Additional Inform

The Company - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Nov. 16, 2018	Oct. 09, 2018	Oct. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net proceeds from initial public offering			$ 83,500
Accumulated deficit				$ (110,766)	$ (69,323)
Redeemable Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Conversion of redeemable convertible preferred stock into common stock, shares				(12,385,154)
Common Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Stock issued during period				9,400,000
Conversion of 2017 and 2018 convertible notes into common stock, shares				6,932,969
Initial Public Offering | Common Stock
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 redeemable convertible preferred stock into common stock, shares		12,385,154
Initial Public Offering | Common Stock | 2017 Convertible Notes
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Conversion of 2017 and 2018 convertible notes into common stock, shares		2,637,292
Initial Public Offering | Common Stock | 2018 Convertible Notes
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Conversion of 2017 and 2018 convertible notes into common stock, shares		4,295,677
Initial Public Offering | Common Stock | 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 | Redeemable Convertible Preferred Stock | Warrant
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Conversion of preferred stock warrants into common stock warrants, shares		500,000
Over-Allotment Option | Common Stock
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
Oligasis LLC
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Limited liability company, formation date				Jun. 22, 2009
Date of incorporation				Sep. 8, 2015

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018USD ($)SegmentFinancialInstitution	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Summary Of Significant Accounting Policies [Line Items]
Foreign currency transaction loss	$ (300,000)	$ (100,000)
Number of reportable segments | Segment	1
Number of operating segments | Segment	1
Number of financial institution cash and cash equivalents held | FinancialInstitution	2
Long-term restricted cash	$ 140,000	140,000	$ 140,000
Lease expiration year	2023
Impairment of long-lived assets	$ 0	0
Unrecognized tax benefits, interest and panalities charged	$ 0
2017 Convertible Notes
Summary Of Significant Accounting Policies [Line Items]
Convertible notes, issuance date	Aug. 31, 2017
2018 Convertible Notes
Summary Of Significant Accounting Policies [Line Items]
Convertible notes, issuance date	Feb. 28, 2018
Laboratory Equipment
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives of assets	4 years
Computer Equipment and Office Equipment
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives of assets	3 years
Computer Software
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives of assets	5 years
Deposited with Financial Institution
Summary Of Significant Accounting Policies [Line Items]
Long-term restricted cash	$ 100,000	$ 100,000
Maximum
Summary Of Significant Accounting Policies [Line Items]
Foreign currency transaction loss			$ (100,000)
Maximum | Furniture and Fixtures
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives of assets	7 years
Minimum | Furniture and Fixtures
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives of assets	5 years

Property and Equipment, Net - S

Property and Equipment, Net - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant and Equipment [Line Items]
Total property and equipment	$ 2,782	$ 2,963
Less: Accumulated depreciation	(1,685)	(1,454)
Property and equipment, net	1,097	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	1,133	1,174
Computer Equipment
Property Plant and Equipment [Line Items]
Total property and equipment		52
Computer Software
Property Plant and Equipment [Line Items]
Total property and equipment	85	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	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Depreciation	$ 490	$ 549	$ 257

Accrued Liabilities and Other_3

Accrued Liabilities and Other Current Liabilities - Schedule of Accrued Liabilities and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounts Payable And Accrued Liabilities Current [Abstract]
Accrued salaries and benefits	$ 2,061	$ 1,129
Accrued research and development	1,387	4,293
Accrued professional fees	117	19
Accrued legal fees	82	35
Accrued other liabilities	129	326
Total accrued and other current liabilities	$ 3,776	$ 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 - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Total Assets	$ 87,957	$ 1,217
Liabilities:
Total Liabilities		2,300
Preferred Stock Warrant Liability
Liabilities:
Total Liabilities		2,300
Money Market Funds
Assets:
Total Assets	87,957	1,217
Level 1
Assets:
Total Assets	87,957	1,217
Liabilities:
Total Liabilities		0
Level 1 | Preferred Stock Warrant Liability
Liabilities:
Total Liabilities		0
Level 1 | Money Market Funds
Assets:
Total Assets	87,957	1,217
Level 2
Assets:
Total Assets	0	0
Liabilities:
Total Liabilities		0
Level 2 | Preferred Stock Warrant Liability
Liabilities:
Total Liabilities		0
Level 2 | Money Market Funds
Assets:
Total Assets	0	0
Level 3
Assets:
Total Assets	0	0
Liabilities:
Total Liabilities		2,300
Level 3 | Preferred Stock Warrant Liability
Liabilities:
Total Liabilities		2,300
Level 3 | Money Market Funds
Assets:
Total Assets	$ 0	$ 0

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Transfers of assets or liabilities between the fair value measurement levels	$ 0	$ 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		$ 19,000,000
Measurement Input, Discount Rate | Initial Public Offering
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value measurement input rate		0.575
Measurement Input, Volatility | Initial Public Offering
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value measurement input rate		0.75
Measurement Input, Risk-free Rate | Initial Public Offering
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value measurement input rate		0.0197
Measurement Input, Expected Dividend Payment | Initial Public Offering
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value measurement input rate		0
Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liability measured at fair value	0
Nonrecurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liability measured at fair value	$ 0

Fair Value Measurements - Sum_2

Fair Value Measurements - Summarizes the Changes in Fair Value of Level 3 Financial Instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Conversion of preferred stock warrants into common stock warrants	$ 5,000
Preferred Stock Warrant Liability | Significant Unobservable Inputs (Level 3)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value, beginning balance	2,300	$ 0
Issuance of financial instruments	0	1,040
Change in fair value included in other income (expense), net	2,700	1,260
Conversion of preferred stock warrants into common stock warrants	(5,000)
Extinguishment of 2018 derivative instrument liability	0
Fair value, ending balance	0	2,300
2018 Derivative Instrument Liability | Significant Unobservable Inputs (Level 3)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value, beginning balance	0	0
Issuance of financial instruments	6,603	0
Change in fair value included in other income (expense), net	1,988	0
Conversion of preferred stock warrants into common stock warrants	0
Extinguishment of 2018 derivative instrument liability	(8,591)
Fair value, ending balance	$ 0	$ 0

Convertible Notes - Additional

Convertible Notes - Additional Information (Details) - USD ($)	Oct. 09, 2018	Feb. 28, 2018	Feb. 23, 2018	Feb. 02, 2018	Feb. 28, 2018	Aug. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Aug. 11, 2017
Debt Instrument [Line Items]
Gross proceeds from issuance of convertible notes							$ 33,000,000	$ 10,000,000	$ 0
Aggregate principal amount of convertible notes							9,560,000	8,000,000
Non-cash interest expense and amortization of debt discount and issuance cost							5,482,000	1,161,000	$ 0
Net unamortized debt discount										$ 1,000,000
Loss on extinguishment of debt							$ (5,479,000)
Common Stock
Debt Instrument [Line Items]
Common stock, shares issued							12,385,154
2017 Convertible Notes and Series B Redeemable Convertible Preferred Stock
Debt Instrument [Line Items]
Gross proceeds from issuance of convertible notes						$ 10,000,000
2017 Convertible Notes
Debt Instrument [Line Items]
Aggregate principal amount of convertible notes						$ 8,000,000
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							$ 2,800,000	1,100,000
Contractual interest expense							2,700,000	800,000
Non-cash interest expense and amortization of debt discount and issuance cost								300,000
Carrying value of the debt including principal and accrued interest								13,500,000
Net unamortized debt discount								$ 700,000
2017 Convertible Notes | Series B Redeemable Convertible Preferred Stock
Debt Instrument [Line Items]
Conversion price per share						$ 5
2017 Convertible Notes | Initial Public Offering | Common Stock
Debt Instrument [Line Items]
Common stock, shares issued	2,637,292
2017 Convertible Notes | Maximum
Debt Instrument [Line Items]
Non-cash interest expense and amortization of debt discount and issuance cost							100,000
2018 Convertible Notes
Debt Instrument [Line Items]
Gross proceeds from issuance of convertible notes			$ 1,800,000	$ 31,200,000	$ 33,000,000
Aggregate principal amount of convertible notes		$ 9,600,000
Interest on unpaid principal balance of convertible notes		6.00%			6.00%
Convertible notes interest expense							2,600,000
Contractual interest expense							1,400,000
Non-cash interest expense and amortization of debt discount and issuance cost							1,200,000
Carrying value of the debt including principal and accrued interest							34,400,000
Net unamortized debt discount							5,500,000
Loss on extinguishment of debt							5,500,000
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 | Common Stock
Debt Instrument [Line Items]
Common stock, shares issued	4,295,677
2018 Convertible Notes | Minimum | Initial Public Offering | Common Stock
Debt Instrument [Line Items]
Gross proceeds from Issuance of Common Stock							$ 75,000,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	1 Months Ended			12 Months Ended
	Nov. 30, 2018USD ($)mi	Mar. 31, 2016USD ($)	May 31, 2013USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Commitments And Contingencies Disclosure [Line Items]
Rent expense				$ 600,000	$ 600,000	$ 500,000
Number of miles | mi	0.5
Expected lease payments	$ 100,000
Current portion of tenant improvement allowance payable				3,776,000	5,802,000
Other liabilities, non-current portion of tenant Improvement allowance payable				530,000	586,000
Tenant Improvement Allowance Payable
Commitments And Contingencies Disclosure [Line Items]
Other liabilities, non-current portion of tenant Improvement allowance payable				300,000	400,000
Leasehold Improvements | Palo Alto, California
Commitments And Contingencies Disclosure [Line Items]
Improvements allowance payable to be drawn		$ 400,000
Interest rate		8.00%
Lease term		Oct. 31, 2023
Debt instrument term		10 years
Tenant Improvement Allowance Agreement | Leasehold Improvements | Palo Alto, California
Commitments And Contingencies Disclosure [Line Items]
Improvements allowance payable to be drawn			$ 300,000
Interest rate			8.00%
Lease term			Oct. 31, 2018
Maximum | Tenant Improvement Allowance Payable
Commitments And Contingencies Disclosure [Line Items]
Current portion of tenant improvement allowance payable				100,000	100,000
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				3,200,000	10,900,000
Unrecorded unconditional purchase obligation, purchases				2,800,000	$ 13,900,000	$ 6,100,000
Unrecorded unconditional purchase obligation, cancellation fees				$ 0

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Future Minimum Commitments Under Non-Cancelable Contracts (Details) $ in Thousands	Dec. 31, 2018USD ($)
Operating Lease
2019	$ 564
2020	581
2021	598
2022	616
2023	526
Total payments	$ 2,885

Income Taxes - Components of Lo

Income Taxes - Components of Loss before Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
United States	$ (17,273)	$ (3,240)	$ (14,632)
Foreign	(24,170)	(24,696)	(2,500)
Total loss before income taxes	$ (41,443)	$ (27,936)	$ (17,132)

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 11,044	$ 11,270	$ 7,022
Intangible assets	7,588	618	783
Research and development tax credits	1,559	788	235
Accruals	700	393	172
Stock-based compensation	394	0	0
Property and equipment	109	97	88
Other	0	26	65
Total gross deferred tax asset	21,394	13,192	8,365
Valuation allowance	(21,394)	(13,192)	(8,365)
Net deferred tax assets	$ 0	$ 0	$ 0

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Increase in valuation allowance	$ 8,200	$ 4,800	$ 6,200
U.S. corporate tax rate	21.00%	34.00%	34.00%
Tax Acts, reduction in deferred tax assets		$ 2,500
Tax Adjustments, settlements, and unusual provisions		0
Research and development credit carryforwards	$ 1,559	$ 788	$ 235
Federal
Income Taxes [Line Items]
Net operating loss	$ 21,900
Net operating loss carryforwards expiration start year	2035
Research and development credit carryforwards	$ 1,200
Tax credit carryforwards expiration start year	2035
Number of periods open for examination	3 years
State
Income Taxes [Line Items]
Net operating loss	$ 71,300
Net operating loss carryforwards expiration start year	2035
Research and development credit carryforwards	$ 700
Number of periods open for examination	4 years
California
Income Taxes [Line Items]
Tax credit carryforward, expiration year	0

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Effective Tax Rate to Statutory U.S. Federal Rate (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	34.00%	34.00%
State taxes (tax effected)	5.60%	7.10%	5.10%
Foreign tax rate differential	(3.80%)	(17.10%)	(2.80%)
Research tax credit	1.30%	0.80%	0.40%
Stock-based compensation	(0.60%)	(0.30%)	(0.20%)
Other	(0.10%)	(0.30%)	(0.30%)
Remeasurement of deferred tax due to tax law change		(8.80%)
Fair value adjustments	(2.40%)
Extinguishment of convertible note	(2.70%)
Change in valuation allowance	(18.30%)	(15.40%)	(36.20%)
Provision for income taxes	0.00%	0.00%	0.00%

Income Taxes - Summary of Unrec

Income Taxes - Summary of Unrecognized Tax Benefits Amounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Unrecognized tax benefits at beginning of period	$ 357	$ 235	$ 128
Increases related to prior year tax positions	0	102	0
Increases related to current year tax positions	41	20	107
Unrecognized tax benefits at end of period	$ 398	$ 357	$ 235

Redeemable Convertible Prefer_4

Redeemable Convertible Preferred Stock - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized	10,000,000	18,753,595
Redeemable convertible preferred stock, par value	$ 0.0001	$ 0.0001
Minimum | Common Stock | Initial Public Offering
Temporary Equity [Line Items]
Share price	$ 10
Gross proceeds from Issuance of Common Stock	$ 75,000,000
Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, par value	$ 0.0001	$ 0.0001
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	10,000,000	18,753,595

Redeemable Convertible Prefer_5

Redeemable Convertible Preferred Stock - Schedule of Redeemable Convertible Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Redeemable Convertible Preferred Stock, Authorized	10,000,000	18,753,595
Redeemable Convertible Preferred Stock, Outstanding	0	12,385,154
Liquidation Value	$ 0	$ 50,324
Carrying Value		$ 50,017
Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Stock, Authorized		6,253,595
Redeemable Convertible Preferred Stock, Outstanding		5,593,154
Liquidation Value		$ 16,364
Carrying Value		$ 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
Redeemable Convertible Preferred Stock, Outstanding		6,792,000
Liquidation Value		$ 33,960
Carrying Value		$ 33,734
Original Issuance Price		$ 5

Common Stock - Additional Infor

Common Stock - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018Vote$ / sharesshares	Dec. 31, 2017Vote$ / sharesshares
Equity [Abstract]
Common stock, shares authorized | shares	490,000,000	28,500,000
Common stock, par value | $ / shares	$ 0.0001	$ 0.0001
Common stock, number of vote per share | Vote	1	1

Common Stock - Schedule of Comm

Common Stock - Schedule of Common Stock Reserved for Future Issuances (Details) - shares	Dec. 31, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Common stock reserved for future issuances	9,019,671	16,848,906
Employee Stock Options
Class Of Stock [Line Items]
Common stock reserved for future issuances	5,135,267	1,204,414
2018 Equity Incentive Plan
Class Of Stock [Line Items]
Common stock reserved for future issuances	3,024,404	0
2018 Employee Share Purchase Plan
Class Of Stock [Line Items]
Common stock reserved for future issuances	460,000	0
2015 Equity Incentive Plan
Class Of Stock [Line Items]
Common stock reserved for future issuances	0	605,557
Warrant
Class Of Stock [Line Items]
Common stock reserved for future issuances	400,000	0
Preferred Stock Warrants Outstanding
Class Of Stock [Line Items]
Common stock reserved for future issuances	0	500,000
Convertible Notes
Class Of Stock [Line Items]
Common stock reserved for future issuances	0	2,153,781
Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Common stock reserved for future issuances	0	12,385,154

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance		9,019,671	16,848,906
Right of shares repurchase as early exercised stock options.		0	2,887
Stock-based compensation expense recognized		$ 2,608,000	$ 275,000	$ 272,000
Employee Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance		5,135,267	1,204,414
Stock options, granted		3,888,653	107,500	1,205,940
Stock options, weighted-average grant date fair value		$ 4.36	$ 0.61	$ 0.60
Dividends paid		$ 0
Expected dividend payment		$ 0
Dividend yield		0.00%	0.00%	0.00%
Total value of employee options vested		$ 1,200,000	$ 200,000	$ 100,000
Stock-based compensation expense recognized		2,000,000	200,000	$ 200,000
Unrecognized stock-based compensation of unvested stock options and unvested RSAs and RSUs		$ 15,700,000	$ 500,000
Unrecognized stock-based compensation of unvested stock awards, weighted-average period expected for recognition		1 year 8 months 8 days	11 months 12 days
Non-employee Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options, granted		215,000	30,000	3,655
Dividend yield		0.00%	0.00%	0.00%
Options Granted to Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense recognized		$ 400,000	$ 100,000	$ 100,000
Restricted Stock Awards (RSAs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Awards vesting period		4 years
Stock-based compensation expense recognized		$ 200,000	100,000	100,000
Stock-based compensation of total value vested		$ 200,000	$ 100,000	$ 100,000
Shares granted		27,500		142,938
Restricted Stock Awards (RSAs) | Share Based Compensation Award Monthly Vesting
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award monthly vesting percentage		2.083%
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares granted		60,000
2018 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance		63,359
Percentage of outstanding stock		4.00%
Awards vesting period		4 years
Number of shares available for grant		3,024,404
2018 Equity Incentive Plan | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares increases annually		4,300,000
2018 Equity Incentive Plan | Incentive Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance		4,300,000
2018 Equity Incentive Plan | Incentive Stock Options | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options, periods granted		10 years
2018 Equity Incentive Plan | 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
Options exercise price, percentage of estimated fair value of shares on grant date		110.00%
2018 Equity Incentive Plan | Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares granted		60,000
2018 Equity Incentive Plan | Restricted Stock Units (RSUs) | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense recognized		$ 100,000
2015 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance	2,125,000
Common stock, shares reserved for issuance	2,810,513
2018 Employee Share Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance		460,000
Common stock, shares authorized		0

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Options Activity Under 2018 Plan and 2015 Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
2018 Plan And 2015 Equity Incentive Plan
Number of Shares Available for Grant
Beginning balance	605,557	551,817	1,814,560
Shares authorized	6,425,000
Options granted	(4,103,653)	(137,500)	(1,209,595)
Options forfeited or canceled	185,000	191,240	38,101
Repurchase of early exercised options			51,689
Ending balance	3,024,404	605,557	551,817	1,814,560
Number of Shares Underlying Outstanding Options
Beginning balance	1,204,414	1,263,757	102,937
Options granted	4,103,653	137,500	1,209,595
Options exercised	(47,800)	(5,603)	(10,674)
Options forfeited or canceled	(185,000)	(191,240)	(38,101)
Ending balance	5,135,267	1,204,414	1,263,757	102,937
Shares exercisable	1,766,385	863,791
Vested and expected to vest	5,135,267	1,204,414
Weighted Average Exercise Price
Beginning balance	$ 0.98	$ 0.98	$ 0.23
Options granted	7.45	1.06	1.04
Options exercised	1.03	0.50	0.45
Options forfeited or canceled	5.19	1.04	0.97
Repurchase of early exercised options			0.12
Ending balance	5.99	0.98	$ 0.98	$ 0.23
Shares exercisable	3.11	0.97
Vested and expected to vest	$ 5.99	$ 0.98
Weighted Average Remaining Contractual Term (in years)
Weighted Average Remaining Contractual Term (in years)	9 years 25 days	8 years 5 months 26 days	9 years 5 months 1 day	7 years 11 months 1 day
Option granted, Weighted Average Remaining Contractual Term (in years)			9 years 6 months 10 days
Option exercised, Weighted Average Remaining Contractual Term (in years)			6 years 9 months 3 days
Option forfeited or canceled, Weighted Average Remaining Contractual Term (in years)			9 years 4 months 17 days
Shares exercisable	8 years 3 months 10 days	8 years 4 months 17 days
Vested and expected to vest	9 years 25 days	8 years 5 months 26 days
Aggregate Intrinsic Value
Beginning balance	$ 41	$ 100	$ 86
Options exercised	290	3	7
Ending balance	10,681	41	$ 100	$ 86
Shares exercisable	7,182	30
Vested and expected to vest	$ 10,681	$ 41
Restricted Stock Awards (RSAs)
Number of Shares Available for Grant
Shares granted	(27,500)		(142,938)
Restricted Stock Units (RSUs)
Number of Shares Available for Grant
Shares granted	(60,000)
Number of Shares Underlying Outstanding Options
RSUs granted	60,000
Weighted Average Exercise Price
RSUs granted	$ 10

Stock-Based Compensation - Fair

Stock-Based Compensation - Fair Value of Employee Stock Options Estimated Using Weighted-average Assumptions (Details) - Employee Stock Options	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	59.00%	63.00%	64.00%
Risk-free interest rate	2.82%	1.89%	1.22%
Dividend yield	0.00%	0.00%	0.00%
Expected term	6 years 21 days	6 years	5 years 10 months 2 days

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	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	68.00%	63.00%	65.00%
Risk-free interest rate	2.71%	2.31%	1.86%
Dividend yield	0.00%	0.00%	0.00%
Expected term	9 years 3 months 18 days	9 years 4 months 6 days	9 years 1 month 2 days

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Restricted Stock Award Activity (Details) - Restricted Stock Awards (RSAs) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Shares Underlying Outstanding RSAs
Unvested, beginning balance	291,633	549,351
Granted	27,500		142,938
Vested	(268,683)	(257,718)
Unvested, ending balance	50,450	291,633	549,351
Weighted Average Grant Date Fair Value
Unvested, December 31, 2016	$ 0.45	$ 0.41
Granted	5.38
Vested	0.89	1
Unvested, December 31, 2017	$ 0.79	$ 0.45	$ 0.41

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	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 2,608	$ 275	$ 272
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	1,535	171	88
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 1,073	$ 104	$ 184

Redeemable Convertible Prefer_6

Redeemable Convertible Preferred Stock Warrants - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Oct. 09, 2018	Aug. 11, 2017	Dec. 31, 2018	Dec. 31, 2017
Class Of Warrant Or Right [Line Items]
Fair value of warrants liability		$ 1,000
Fair value of warrants liability				$ 2,300
Conversion of preferred stock warrants into common stock warrants			$ 5,000
Common Stock | Redeemable Convertible Preferred Stock Warrant Liability | Initial Public Offering
Class Of Warrant Or Right [Line Items]
Warrants exercised	100,000
Common Stock | Redeemable Convertible Preferred Stock Warrant Liability | Warrant | Initial Public Offering
Class Of Warrant Or Right [Line Items]
Conversion of preferred stock warrants into common stock warrants, shares	500,000
Common stock warrants remained outstanding			400,000
Series B Redeemable Convertible Preferred Stock | Common Stock
Class Of Warrant Or Right [Line Items]
Exercise price per price		$ 0.01
2017 Convertible Notes
Class Of Warrant Or Right [Line Items]
Fair value of warrants liability				$ 700
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 price		$ 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 Millions	Oct. 08, 2018	Feb. 28, 2018
2018 Convertible Notes
Embedded Derivative [Line Items]
Derivative instrument, fair value	$ 8.6	$ 6.6

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								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss attributable to common stockholders									$ (41,443)	$ (27,936)	$ (17,132)
Denominator:
Weighted-average shares outstanding									15,136,197	7,932,717	7,850,183
Less: weighted-average unvested restricted shares and shares subject to repurchase									(159,682)	(417,381)	(638,823)
Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted									14,976,515	7,515,336	7,211,360
Net loss per share attributable to common stockholders, basic and diluted	$ (0.40)	$ (1.33)	$ (0.96)	$ (1.16)	$ (1.52)	$ (0.61)	$ (0.90)	$ (0.68)	$ (2.77)	$ (3.72)	$ (2.38)

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	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	5,185,717	16,537,869	14,217,598
Employee 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	5,135,267	1,204,414	1,263,757
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	50,450	291,633	549,351
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	2,887	19,336
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	0	2,153,781	0
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	0	500,000	0
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	0	12,385,154	12,385,154

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - USD ($) $ in Thousands	Feb. 28, 2018	Aug. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Aggregate principal amount of convertible notes			$ 9,560	$ 8,000
2018 Convertible Notes
Related Party Transaction [Line Items]
Aggregate principal amount of convertible notes	$ 9,600
2017 Convertible Notes
Related Party Transaction [Line Items]
Aggregate principal amount of convertible notes		$ 8,000
Baker Bros. Advisors LP | Series B Redeemable Convertible Preferred Stock
Related Party Transaction [Line Items]
Number of warrants issued to related parties			150,000
Baker Bros. Advisors LP | 2018 Convertible Notes
Related Party Transaction [Line Items]
Aggregate principal amount of convertible notes			$ 6,600
Baker Bros. Advisors LP | 2017 Convertible Notes
Related Party Transaction [Line Items]
Aggregate principal amount of convertible notes			$ 3,000
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			250,000
Dustin Moskovitz Trust DTD 12/27/05 | 2018 Convertible Notes
Related Party Transaction [Line Items]
Aggregate principal amount of convertible notes			$ 3,000
Dustin Moskovitz Trust DTD 12/27/05 | 2017 Convertible Notes
Related Party Transaction [Line Items]
Aggregate principal amount of convertible notes			$ 5,000
Dustin Moskovitz Trust DTD 12/27/05 | Minimum
Related Party Transaction [Line Items]
Ownership percentage threshold			5.00%

Selected Quarterly Financial _3

Selected Quarterly Financial Data - Schedule of Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Loss from operations	$ (9,357)	$ (6,380)	$ (5,090)	$ (5,547)	$ (9,757)	$ (3,979)	$ (6,742)	$ (5,043)	$ (26,374)	$ (25,521)	$ (17,151)
Net loss and comprehensive loss	$ (14,662)	$ (10,452)	$ (7,409)	$ (8,920)	$ (11,570)	$ (4,583)	$ (6,742)	$ (5,041)	$ (41,443)	$ (27,936)	$ (17,132)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.40)	$ (1.33)	$ (0.96)	$ (1.16)	$ (1.52)	$ (0.61)	$ (0.90)	$ (0.68)	$ (2.77)	$ (3.72)	$ (2.38)