Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2017	Feb. 16, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Trading Symbol	GTHX
Entity Registrant Name	G1 THERAPEUTICS, INC.
Entity Central Index Key	0001560241
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		28,444,863
Entity Public Float			$ 294,068,519

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets
Cash and cash equivalents	$ 103,812	$ 47,305
Prepaid expenses and other current assets	849	596
Total current assets	104,661	47,901
Property and equipment, net	510	311
Total assets	105,171	48,212
Current liabilities
Accounts payable	4,184	2,605
Accrued expenses	7,520	2,853
Warrant liability		167
Total current liabilities	11,704	5,625
Other non-current liabilities	79
Total liabilities	11,783	5,625
Commitments and contingencies
Stockholders’ equity (deficit)
Common stock, $0.0001 par value, 120,000,000 shares and 73,000,000 shares authorized as of December 31, 2017 and December 31, 2016, respectively; 28,420,511 and 1,504,947 shares issued as of December 31, 2017 and December 31, 2016, respectively; 28,393,845 and 1,478,281 shares outstanding as of December 31, 2017 and December 31, 2016, respectively	3
Treasury stock, 26,666 shares	(8)	(8)
Additional paid-in capital	222,511
Accumulated deficit	(129,118)	(64,985)
Total stockholders’ equity (deficit)	93,388	(64,993)
Total liabilities, mezzanine equity and equity	$ 105,171	48,212
Series C Redeemable Convertible Preferred Stock
Mezzanine equity
Redeemable convertible preferred stock		51,424
Series B Redeemable Convertible Preferred Stock
Mezzanine equity
Redeemable convertible preferred stock		40,355
Series A Redeemable Convertible Preferred Stock
Mezzanine equity
Redeemable convertible preferred stock		14,431
Series 1 Redeemable Convertible Preferred Stock
Mezzanine equity
Redeemable convertible preferred stock		$ 1,370

Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	120,000,000	73,000,000
Common stock, shares, issued	28,420,511	1,504,947
Common stock, shares, outstanding	28,393,845	1,478,281
Treasury stock, shares	26,666	26,666
Series C Redeemable Convertible Preferred Stock
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	0	17,000,000
Preferred stock, shares issued	0	5,609,398
Preferred stock, shares outstanding	0	5,609,398
Preferred stock, liquidation preference, value		$ 51,673
Series B Redeemable Convertible Preferred Stock
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	0	23,000,000
Preferred stock, shares issued	0	7,642,734
Preferred stock, shares outstanding	0	7,642,734
Preferred stock, liquidation preference, value		$ 35,722
Series A Redeemable Convertible Preferred Stock
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	0	14,996,692
Preferred stock, shares issued	0	4,998,895
Preferred stock, shares outstanding	0	4,998,895
Preferred stock, liquidation preference, value		$ 14,431
Series 1 Redeemable Convertible Preferred Stock
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	0	2,112,025
Preferred stock, shares issued	0	682,026
Preferred stock, shares outstanding	0	682,026
Preferred stock, liquidation preference, value		$ 931

Statements of Operations - USD ($) $ in Thousands	12 Months Ended
Statements of Operations - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Grant revenue			$ 522
Operating expenses
Research and development	$ 53,881	$ 25,161	12,730
General and administrative	7,087	5,230	3,216
Total operating expenses	60,968	30,391	15,946
Operating loss	(60,968)	(30,391)	(15,424)
Other income (expense)
Other income	888	182	18
Change in fair value in warrant liability and other liabilities	(41)	(82)	(85)
Change in fair value of Series B purchase option liability			(4,772)
Total other income (expense), net	847	100	(4,839)
Net loss	(60,121)	(30,291)	(20,263)
Accretion of redeemable convertible preferred stock	(4,757)	(4,405)	(1,427)
Net loss attributable to common stockholders	$ (64,878)	$ (34,696)	$ (21,690)
Net loss per share attributable to common stockholders, basic and diluted	$ (3.57)	$ (23.33)	$ (16.13)
Weighted average common shares outstanding, basic and diluted	18,197,970	1,486,986	1,344,584

Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series C Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock Series B Purchase Option	Series A Redeemable Convertible Preferred Stock	Series 1 Redeemable Convertible Preferred Stock	Common Stock	Treasury Stock	Additional Paid-in Capital	Accumulated Deficit
Balance at Dec. 31, 2014	$ (10,628)							$ (8)	$ 3	$ (10,623)
Temporary equity, balance (in shares) at Dec. 31, 2014					4,998,895	682,026
Temporary equity, balance at Dec. 31, 2014					$ 13,171	$ 931
Balance (in shares) at Dec. 31, 2014							1,273,831	(26,666)
Issuance of redeemable convertible preferred stock			$ 33,262
Issuance of redeemable convertible preferred stock (in shares)			7,642,734
Allocation of Series B proceeds to Series B purchase option liability				$ (1,935)
Temporary equity accretion of redeemable, convertible preferred stock			$ 797		$ 630
Accretion of redeemable, convertible preferred stock	(1,426)								(626)	(800)
Exercise of common stock options	53								53
Exercise of common stock options (in shares)							164,722
Stock-based compensation	389								389
Stock financing costs			$ (139)
Issuance of common shares for license agreement	181								181
Issuance of common shares for license agreement (in shares)							48,666
Exercise of Series B purchase option liability				$ 6,707
Net loss during year	(20,263)									(20,263)
Balance at Dec. 31, 2015	(31,694)							$ (8)		(31,686)
Temporary equity, balance (in shares) at Dec. 31, 2015			7,642,734		4,998,895	682,026
Temporary equity, balance at Dec. 31, 2015			$ 38,692		$ 13,801	$ 931
Balance (in shares) at Dec. 31, 2015							1,487,219	(26,666)
Issuance of redeemable convertible preferred stock		$ 50,000
Issuance of redeemable convertible preferred stock (in shares)		5,609,398
Temporary equity accretion of redeemable, convertible preferred stock		$ 1,673	$ 1,663		$ 630	$ 439
Accretion of redeemable, convertible preferred stock	(4,405)								(1,397)	(3,008)
Exercise of common stock options	6								6
Exercise of common stock options (in shares)							17,728
Stock-based compensation	1,391								1,391
Stock financing costs		$ (249)
Net loss during year	(30,291)									(30,291)
Balance at Dec. 31, 2016	(64,993)							$ (8)		(64,985)
Temporary equity, balance (in shares) at Dec. 31, 2016		5,609,398	7,642,734		4,998,895	682,026
Temporary equity, balance at Dec. 31, 2016		$ 51,424	$ 40,355		$ 14,431	$ 1,370
Balance (in shares) at Dec. 31, 2016							1,504,947	(26,666)
Temporary equity accretion of redeemable, convertible preferred stock		3,732	620		235	171
Accretion of redeemable, convertible preferred stock	(4,757)								(745)	(4,012)
Initial public offering	108,503						$ 1		108,502
Initial public offering (in shares)							7,781,564,000
Automatic conversion of preferred stock	112,337	$ (55,156)	$ (40,975)		$ (14,666)	$ (1,541)	$ 2		112,335
Automatic conversion of preferred stock, in shares		(5,609,398)	(7,642,734)		(4,998,895)	(682,026)	18,933,053
Automatic conversion of preferred warrants	208								208
Exercise of common stock options	$ 214								214
Exercise of common stock options (in shares)	173,035						160,579
Exercise of common stock warrants	$ 1								1
Exercise of common stock warrants, in shares							40,368
Stock-based compensation	3,394								3,394
IPO financing costs	(1,398)								(1,398)
Issuance of common shares for license agreement (in shares)			3,791,272
Net loss during year	(60,121)									(60,121)
Balance at Dec. 31, 2017	$ 93,388						$ 3	$ (8)	$ 222,511	$ (129,118)
Temporary equity, balance (in shares) at Dec. 31, 2017		0	0		0	0
Balance (in shares) at Dec. 31, 2017							28,420,511	(26,666)

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net loss	$ (60,121)	$ (30,291)	$ (20,263)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	89	67	42
Stock-based compensation	3,394	1,391	389
Purchase of license agreement			181
Gain/loss on disposal of property and equipment	6	18	2
Increase in fair value of warrant activity	41	82	85
Increase in fair value of Series B purchase option liability			4,773
Change in operating assets and liabilities
Prepaid expenses and other assets	(253)	216	(789)
Accounts payable and accrued expenses	6,325	3,376	1,735
Net cash used in operating activities	(50,519)	(25,141)	(13,845)
Cash flows from investing activities
Purchases of property and equipment	(294)	(250)	(87)
Net cash used in investing activities	(294)	(250)	(87)
Cash flows from financing activities
Proceeds from stock options and warrants exercised	215	7	53
Issuance costs for preferred share financings		(249)	(140)
Proceeds from initial public offering, net of underwriting fees and commissions	108,503
Payment of public offering costs	(1,398)
Net cash provided by financing activities	107,320	49,758	33,176
Net change in cash and cash equivalents	56,507	24,367	19,244
Cash and cash equivalents
Beginning of period	47,305	22,938	3,694
End of period	103,812	47,305	22,938
Non-cash investing and financing activities
Accretion of redeemable convertible preferred stock	4,757	4,405	1,427
Exercise of Series B purchase option			6,708
Common stock issued for patent rights			181
Conversion of preferred stock and preferred warrants to common stock and common warrants	$ 112,545
Series C Preferred Stock
Cash flows from financing activities
Proceeds from preferred stock		$ 50,000
Series B Preferred Stock
Cash flows from financing activities
Proceeds from preferred stock			$ 33,263

Description of Business	12 Months Ended
Description of Business	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Description of Business	1. Description of business G1 Therapeutics, Inc. (the “Company”) is a privately held clinical-stage biopharmaceutical company based in Research Triangle Park, North Carolina that focuses on the discovery and development of novel therapeutics for the treatment of cancer. The Company was incorporated on May 19, 2008 in the state of Delaware. The Company focuses on cyclin-dependent kinases (CDKs), a family of proteins that plays an important role in the growth and proliferation of all human cells. The Company has focused its CDK research on developing potent and selective inhibitors of the kinases CDK4 and CDK6, collectively known as CDK4/6. The Company is currently advancing two CDK4/6 inhibitor product candidates that each have broad applicability across multiple cancer indications. Trilaciclib, the Company’s most advanced clinical-stage candidate, is a potential first-in-class intravenous CDK4/6 inhibitor designed to preserve hematopoietic stem cells and enhance immune system function during chemotherapy. Based on compelling response rates and favorable tolerability shown in early-stage trials, trilaciclib is currently being evaluated in four randomized Phase 1b/2a trials: two in patients with small cell lung cancer, or SCLC, an additional Phase 2 trial in combination with Tecentriq in SCLC and a Phase 2 in patients with triple-negative breast cancer, or TNBC. G1T38, the Company’s second clinical-stage candidate, is a potential best-in-class oral CDK4/6 inhibitor, to be used in combination with other targeted therapies to treat multiple cancers. A Phase 1 trial of G1T38 in 75 healthy volunteers showed a favorable safety profile, and the Company initiated a Phase 1/2 trial in ER+, HER2- breast cancer in January 2017. The Company’s plans for G1T38 include future combinations in other cancers, such as non-small cell lung cancer, or NSCLC. As part of the Company’s strategy to develop wholly-owned proprietary combinations, the Company has exclusively in-licensed G1T48, a potential first/best-in-class oral selective estrogen receptor degrader, or SERD. The Company expects to initially develop G1T48 to be used in combination with G1T38 for the treatment of ER+, HER2- breast cancer. The Company plans to continue to leverage its proprietary assets and knowledge of CDK4/6 biology to explore additional combination treatments and to build a fully integrated oncology company. The Company’s financial statements have been prepared assuming that the Company will continue as a going concern, which contemplates the realization of assets and the settlement of liabilities and commitments in the normal course of business. As of December 31, 2017, the Company had an accumulated deficit of $129.1 million. The Company has reported a net loss in all fiscal periods since inception and expects to incur substantial losses in the future to conduct research and development and pre-commercialization activities. As of December 31, 2017, the Company had cash and cash equivalents of $103.8 million. The Company expects that its existing cash and cash equivalents will enable it to fund its operating expenses and capital expenditure requirements for greater than 12 months from the date of filing this Annual Report. The failure of the Company to obtain sufficient funds on acceptable terms when needed could have a material adverse effect on the Company’s results of operations and financial condition.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of significant accounting policies

Basis of presentation

The Company has prepared the accompanying financial statements in conformity with generally accepted accounting principles in the United States of America (“U.S. GAAP”).

Use of estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and the accompanying notes. Actual results could differ from those estimates. These estimates include the Company’s common stock valuation, warrant valuation and deferred tax asset valuation allowance.

Cash and cash equivalents

The Company considers all highly liquid investments purchased with an original maturity of three months or less at the date of purchase to be cash equivalents. Cash and cash equivalents at December 31, 2017 consist of amounts on deposit in banks, including checking accounts, money market accounts and certificates of deposit. Cash deposits are all in financial institutions in the United States.

Concentration of credit risk

Financial instruments that potentially subject the Company to credit risk consist of cash and cash equivalents. Deposits with financial institutions are insured, up to certain limits, by the Federal Deposit Insurance Corporation (“FDIC”). The Company’s cash deposits often exceed the FDIC insurance limit; however, all deposits are maintained with high credit quality institutions and the Company has not experienced any losses in such accounts. The financial condition of financial institutions is periodically reassessed, and the Company believes the risk of any loss is minimal. The Company believes the risk of any loss on cash due to credit risk is minimal.

Property and equipment

Property and equipment are stated at cost less accumulated depreciation. Depreciation is generally calculated using the straight-line method over the following estimated useful lives:

Computer equipment			5 years
Laboratory equipment			5 years
Furniture and fixtures			7 years
Leasehold improvements			7 years

Costs associated with maintenance and repairs are charged to expense as incurred. Property and equipment held under leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset.

Impairment of long-lived assets

The Company evaluates its long-lived assets for indicators of possible impairment by comparison of the carrying amounts to future net undiscounted cash flows expected to be generated by such assets when events or changes in circumstances indicate the carrying amount of an asset may not be recoverable. Should an impairment exist, the impairment loss would be measured based on the excess carrying value of the asset over the asset’s fair value based on discounted estimates of future cash flows. For the years ended December 31, 2017, 2016 and 2015, the Company’s management evaluated its long-lived assets and determined no impairment charge was needed.

Warrant liability

Warrants to purchase the Company’s redeemable convertible preferred stock have been classified as liabilities and are recorded at their estimated fair value. In each reporting period, any change in fair value of the warrants has been recorded as expense in the case of an increase in fair value and income in the case of a decrease in fair value.

Series B purchase option liability

Research and development

Research and development expenses consist of costs incurred to further the Company’s research and development activities and include salaries and related employee benefits, manufacturing of pharmaceutical active ingredients and drug products, costs associated with clinical trials, nonclinical activities, regulatory activities, research-related overhead expenses and fees paid to expert consultants, external service providers and contract research organizations which conduct certain research and development activities on behalf of the Company. Costs incurred in the research and development of products are charged to research and development expense as incurred.

Each reporting period, the Company estimates and accrues expenses, the largest of which is related to accrued research and development expenses. This process involves reviewing contracts and purchase orders, identifying services that have been performed on the Company’s behalf, and estimating the level of service performed and the associated cost incurred for the service when the Company has not yet been invoiced or otherwise notified of the actual costs.

Costs for preclinical studies and clinical trial activities are recognized based on an evaluation of vendors’ progress towards completion of specific tasks, using data such as patient enrollment, clinical site activations or information provided by vendors regarding their actual costs incurred. Payments for these activities are based on the terms of individual contracts and payment timing may differ significantly from the period in which the services were performed. The Company determines accrual estimates through reports from and discussions with applicable personnel and outside service providers as to the progress or state of completion of trials, or the services completed. The estimates of accrued expenses as of each balance sheet date are based on the facts and circumstances known at the time.

Revenue recognition

The Company received the majority of its revenues from grant programs authorized by Congress through the Small Business Innovation Research (SBIR) program and the Small Business Technology Transfer Act (STTR) of 1992. In addition, the Company was also awarded grant funds through other federal and state programs related to its research. Under the terms of the grants, the Company is entitled to receive reimbursement of its allowable direct expenses, allocated overhead and general and administrative expenses.

Revenue received under these grant programs is recognized as direct project costs are incurred, plus a portion of the Company’s indirect costs such as overhead and general and administrative expenses allocated to the project. The Company’s grant agreements are fixed fee arrangements.

In the event that the granting agency provides advance funding of a grant award, the Company records deferred revenues and then recognizes revenue as costs are incurred over the life of the grant.

To date, the Company has not generated any revenue from the commercial sale of its product candidates.

Fair value of financial instruments

The Company provides disclosure of financial assets and financial liabilities that are carried at fair value based on the price that would be received upon sale of an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Fair value measurements may be classified based on the amount of subjectivity associated with the inputs to fair valuation of these assets and liabilities using the following three levels:

Level 1		Inputs are unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.

Level 2		Inputs include quoted prices for similar assets and liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, inputs other than quoted prices that are observable for the asset or liability and inputs that are derived principally from or corroborated by observable market data by correlation or other means.

Level 3		Unobservable inputs that reflect the Company’s estimates of the assumptions that market participants would use in pricing the asset or liability. The Company develops these inputs based on the best information available, including its own data.

The carrying amounts of cash, cash equivalents, accounts payable and accrued liabilities approximate fair value because of their short-term nature.

At December 31, 2017 and 2016 these financials instruments and respective fair values have been classified as follows (in thousands):

	Quoted prices in active markets for identical assets (Level 1)		Significant other observable inputs (Level 2)		Significant other unobservable inputs (Level 3)		Balance at December 31, 2017
Assets
Money market funds	$	87,694	$	—	$	—	$	87,694
Certificates of Deposit		15,203		—		—		15,203
Total assets at fair value:	$	102,897	$	—	$	—	$	102,897
Liabilities:
Warrant Liability	$	—	$	—	$	—	$	—
Total liabilities at fair value:	$	—	$	—	$	—	$	—

	Quoted prices in active markets for identical assets (Level 1)		Significant other observable inputs (Level 2)		Significant other unobservable inputs (Level 3)		Balance at December 31, 2016
Assets
Money market funds	$	31,730	$	—	$	—	$	31,730
Certificates of Deposit		15,041		—		—		15,041
Total assets at fair value:	$	46,771	$	—	$	—	$	46,771
Liabilities:
Warrant Liability	$	—	$	—	$	167	$	167
Total liabilities at fair value:	$	—	$	—	$	167	$	167

The change in the fair value measurement using significant inputs (Level 3) is summarized below (in thousands):

Balance at December 31, 2015	$	85
Change in fair value in warrant liability		82
Balance at December 31, 2016	$	167
Change in fair value in warrant liability		41
Conversion of warrant to common stock warrant		(208	)
Balance at December 31, 2017	$	—

Patent costs

Costs associated with the submission of patent applications are expensed as incurred given the uncertainty of the future economic benefits of the patents. Patent-related legal expenses included in general and administrative costs were approximately $997, $1,034, and $691 for the years ended December 31, 2017, 2016 and 2015, respectively.

Income taxes

Income taxes are accounted for using the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to temporary differences between the financial statements carrying amounts of assets and liabilities and their respective tax bases, operating loss carryforwards, and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date.

In accordance with Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC) 740, Accounting for Income Taxes, the Company reflects in the financial statements the benefit of positions taken in a previously filed tax return or expected to be taken in a future tax return only when it is considered ‘more-likely-than-not’ that the position taken will be sustained by a taxing authority. As of December 31, 2017 and 2016, the Company had no unrecognized income tax benefits and correspondingly there is no impact on the Company’s effective income tax rate associated with these items. The Company’s policy for recording interest and penalties relating to uncertain income tax positions is to record them as a component of income tax expense in the accompanying statements of operations. As of December 31, 2017 and 2016, the Company had no such accruals.

Stock-based compensation

The primary type of stock-based payments utilized by the Company are stock options. The Company accounts for stock-based employee compensation arrangements by measuring the cost of employee services received in exchange for all equity awards granted based on the fair value of the award on the grant date. The fair value of each employee stock option is estimated on the date of grant using an options pricing model. The Company currently uses the Black-Scholes valuation model to estimate the fair value of its share-based payments. The model requires management to make a number of assumptions including expected volatility, expected life, risk-free interest rate and expected dividends.

The Company accounts for stock-based non-employee compensation arrangements by recording the expense of such services based on the fair value of the equity instrument as estimated using the Black-Scholes pricing model. The fair value of the equity instrument is charged to operating expense over the term of the service agreement.

Segment information

The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. All of the Company’s assets are held in the United States.

Comprehensive loss

Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. There was no difference between net loss and comprehensive loss for each of the periods presented in the accompanying financial statements.

Redeemable convertible preferred stock

The Company classifies its redeemable convertible preferred stock, for which the Company does not control the redemption, outside of permanent equity. The Company records redeemable convertible preferred stock at fair value upon issuance, net of any offering costs, and the carrying value is adjusted to the redemption value at the end of each reporting period. These adjustments are effected through charges against additional paid-in capital and accumulated deficit.

New accounting standards

In May 2017, the FASB issued ASU 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting. This guidance is intended to provide clarity and reduce diversity in practice as to when changes to the terms or conditions of share-based payments are accounted for as modifications. Under this new guidance, entities will apply modification accounting if the fair value, vesting conditions or classification of the award changes. This guidance will be effective for annual reporting periods beginning after December 15, 2017, including interim periods within those annual reporting periods, and early adoption is permitted. The guidance per ASU 2017-09 is to be adopted prospectively to an award modified on or after the adoption date. The Company does not anticipate a material impact to the Company’s financial statements as a result of the adoption of this guidance.

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments. The FASB issued ASU 2016-09 to improve U.S. GAAP by providing guidance on the cash flow statement classification of eight specific areas where there is existing diversity in practice. The FASB expects that the guidance in this ASU will reduce the current and potential future diversity in practice in such areas. This ASU is effective for annual and interim periods beginning after December 15, 2017, with early adoption permitted. The Company is currently evaluating the impact of the adoption of this ASU on the Company’s financial statements.

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842). This guidance revises the accounting related to leases by requiring lessees to recognize a lease liability and a right-of-use asset for all leases. The new lease guidance also simplifies the accounting for sale and leaseback transactions. This ASU is effective for annual reporting periods beginning after December 15, 2018 and early adoption is permitted. The Company is currently evaluating the impact of the adoption of this ASU on the Company’s financial statements.

In May 2014, the FASB and the International Accounting Standards Board jointly issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes the revenue recognition requirements in ASC 605 and most industry-specific guidance. The new standard requires that an entity recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods and services. The update also requires additional disclosures about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgements and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. ASU 2014-09 is effective for public entities for annual and interim periods within those annual periods beginning after December 15, 2017. The Company is currently evaluating the method of adoption and the potential impact this standard may have on its financial position and results of operations. The future impact of ASU 2014-09 will be dependent on the nature of the Company’s future revenue contracts and arrangements, if any.

Property and Equipment

12 Months Ended

Dec. 31, 2017

Property Plant And Equipment [Abstract]

Property and Equipment

3. Property and equipment

Property and equipment consists of the following (in thousands):

	December 31, 2017			December 31, 2016

Computer equipment	$	112		$	67
Laboratory equipment		283			207
Furniture and fixtures		174			64
Leasehold improvements		121			80
Construction in progress		1			—
Accumulated depreciation		(181	)		(107	)
Property and equipment, net	$	510		$	311

Depreciation expenses relating to property and equipment were $89, $67, and $42 for the years ended December 31, 2017, 2016 and 2015, respectively.

Patent License Agreement	12 Months Ended
Patent License Agreement	Dec. 31, 2017
Patent License Agreement [Abstract]
Patent License Agreement	4. Patent license agreement On November 23, 2016, the Company entered into a license agreement with the Board of Trustees of the University of Illinois (the University) whereby the University licensed patent rights to the Company, with rights of sublicense, to make, have made, use, import, sell and offer for sale products covered by certain patent rights owned by the University. The rights licensed to the Company are exclusive, worldwide, non-transferable rights, for all fields of use. Under the terms of the agreement the Company paid a one-time only, non-refundable license issue fee in the amount of $500 which was charged to research and development expense in the fourth quarter of 2016. The Company is also obligated to pay annual maintenance fees to the University. All annual minimum payments are fully creditable against any royalty payments made by the Company. Under the terms of the agreement, the Company must pay the University a low single-digit royalty percentage on all net sales of products and a share of sublicensing revenues. The University is eligible to receive milestone payments of up to $2,625 related to the initiation and execution of clinical trials and first commercial sale of a product in multiple countries. The Company is also responsible for all future patent prosecution costs. The term of the license agreement will continue until the later of (i) the expiration of the last valid claim within the patent rights covering the product in such country, (ii) the expiration of market exclusivity in such country and (iii) the 10th anniversary of the first commercial sale in such country. The University may terminate the agreement in the event (i) the Company fails to pay any amount or make any report when required to be made and fails to cure such failure within thirty (30) days after receipt of notice from the University, (ii) is in breach of any provision of the agreement and fails to remedy within forty-five (45) days after receipt of notice, (iii) makes a report to the University under the agreement that is determine to be materially false, (iv) declares insolvency or bankruptcy or (v) takes an action that causes patent rights or technical information to be subject to lien or encumbrance and fails to remedy any such breach with in forty-five (45) days of receipt of notice from the University. The Company may terminate the agreement at any time on written notice to the University at least ninety (90) days prior to the termination date specified in the notice. Upon expiration or termination of the agreement, all rights revert to the University. On December 31, 2015 the Company entered into a non-exclusive, royalty-free license agreement for patent rights. As consideration for the patent rights, the Company issued 48,666 shares of common stock with a fair value of $181, and agreed to pay past and future patent prosecution costs related to the countries in which valid claims have or will have issued. The aggregate fair value of all consideration paid was expensed as a research and development cost in 2015.

Accrued Expenses

12 Months Ended

Dec. 31, 2017

Payables And Accruals [Abstract]

Accrued Expenses

5. Accrued expenses

Accrued expenses are comprised as follows (in thousands):

	December 31, 2017		December 31, 2016

Accrued external research and professional fees	$	1,402	$	295
Accrued external clinical study costs		4,788		1,897
Accrued compensation expense		1,328		617
Deferred rent, current portion		2		44
Accrued expenses	$	7,520	$	2,853

Lease Obligations

12 Months Ended

Dec. 31, 2017

Leases [Abstract]

Lease Obligations

6. Lease obligations

Operating lease commitments

Pursuant to a lease dated January 10, 2014, on April 1, 2014, the Company leased office and lab space under a lease agreement for $6 per month with a free rent period and escalating rent payments; the lease was set to expire on July 31, 2017.

On January 27, 2016, the Company signed an amendment to the Company’s existing lease to move to a larger office and lab space beginning in August 2016 for $16 per month with a discounted rent period and escalating rent payments; the lease was extended to December 31, 2022. The amendment also contained an option for a five year renewal and a right of first refusal to lease adjacent office space.

On March 27, 2017, the Company signed an amendment to the Company’s existing lease to lease additional, adjacent office space. Beginning August 2017, the combined space leased for $24 per month with a discounted rent period and escalating rent payments. The lease is set to expire on December 31, 2022. The Company has maintained an option for a five year renewal on the combined space.

Rent expense amounted to $252, $126 and $71 for the years ended December 31, 2017, 2016 and 2015, respectively.

The following is a schedule by years of minimum future rental payments on noncancelable operating leases as of December 31, 2017 (in thousands):

2018	$	294
2019		303
2020		312
2021		321
2022		331
2023		—
	$	1,561

Capitalization

12 Months Ended

Dec. 31, 2017

Capitalization Longterm Debt And Equity [Abstract]

Capitalization

7. Capitalization

Redeemable convertible preferred stock

The Company has determined that the Series C, Series B, Series A and Series 1 redeemable convertible preferred stock were redeemable, after a stated period of time, based on voting thresholds that vary by shareholder class, as outlined in the Company’s certificate of incorporation. The Company classified its redeemable convertible preferred stock outside of permanent equity and into mezzanine equity.

The Company recorded its redeemable convertible preferred stock at fair value upon issuance, net of any issuance costs or discounts, and the carrying value is increased by periodic accretion to its redemption value until the earliest possible date of redemption. These increases were recorded as charges against additional paid-in-capital until the additional paid-in-capital balance is reduced to zero. At that time, additional accretion adjustments were recorded as additions to accumulated deficit.

In May 2014, the Company issued 2,495,663 shares of its Series A redeemable convertible preferred stock for cash consideration of $2.52 per share. Total proceeds, including cancellation of indebtedness of $39, amounted to $6,289.

In February 2015, the Company’s Board of Directors and stockholders approved the Fourth Amended and Restated Certification of Incorporation, which increased the authorized number of shares of its redeemable convertible preferred stock to 39,908,717, of which 2,112,025 were designated as Series 1 redeemable convertible preferred stock, 14,996,692 as Series A redeemable convertible preferred stock and 22,800,000 as Series B redeemable convertible preferred stock. At the same time, the Company issued 3,794,024 shares of its Series B redeemable convertible preferred stock for cash consideration and cancellation of indebtedness at a price of $4.3521 per share. Total proceeds, including cancellation of indebtedness of $12, amounted to $16,512.

In December 2015, the Company approved a Certificate of Amendment to the Fourth Amended and Restated Certification of Incorporation which increased the authorized number of shares of its redeemable convertible preferred stock to 40,108,717, of which 2,112,025 were be designated as Series 1 redeemable convertible preferred stock, 14,996,692 as Series A redeemable convertible preferred stock and 23,000,000 as Series B redeemable convertible preferred stock. At the same time, the Company authorized 11,546,147 shares of its Series B redeemable convertible preferred stock and issued 3,848,710 shares of its Series B redeemable convertible preferred stock for cash consideration at a price of $4.3521 per share. Total additional proceeds amounted to $16,750.

In April 2016, the Company’s Board of Directors and stockholders approved the Fifth Amended and Restated Certification of Incorporation which increased the authorized number of shares of its redeemable convertible preferred stock to 57,108,717, of which 2,112,025 were be designated as Series 1 redeemable convertible preferred stock, 14,996,692 as Series A redeemable convertible preferred stock, 23,000,000 as Series B redeemable convertible preferred stock and 17,000,000 as Series C redeemable convertible preferred stock. In the second quarter of 2016, the Company authorized 16,828,217 shares of its Series C redeemable convertible preferred stock and issued 5,609,398 shares of its Series C redeemable convertible preferred stock for cash consideration at a price of $8.91 per share. Total additional proceeds amounted to $50,000.

Prior to the IPO, the holders of the Company’s convertible preferred stock had certain voting and dividend rights, as well as liquidation preferences and conversion privileges. All rights, preferences and privileges associated with the convertible preferred stock were terminated at the time of the Company’s IPO in conjunction with the conversion of all outstanding shares of convertible preferred stock into shares of common stock.

On May 22, 2017, the Company closed its IPO of 7,781,564 shares of the Company’s common stock at a public offering price of $15.00 per share, including 781,564 shares of common stock issued upon exercise by the underwriters of their option to purchase additional shares. The gross proceeds from the IPO were $116.7 million and net proceeds were $107.1 million, after deducting underwriting discounts and commissions and other offering expenses payable by the Company.

Upon closing of the IPO, all outstanding shares of the Company’s preferred stock were automatically converted into 18,933,053 shares of common stock. In connection with the IPO, the Board of Directors and the stockholders of the Company approved a one-for-three reverse stock split of the Company’s common stock. The reverse stock split became effective on May 11, 2017. All share and per share amounts in the financial statements have been retroactively adjusted for all periods presented to give effect to the reverse stock split, including reclassifying an amount equal to the reduction in par value to accumulated deficit.

Series B preferred stock tranche rights

Pursuant to the terms of the Series B Preferred Stock Purchase Agreement, the purchasers of Series B redeemable convertible preferred stock at the initial closing also committed to purchase an aggregate of 3,791,272 shares of Series B redeemable convertible preferred stock at $4.3521 per share (the “Second Tranche Shares”) at a second closing, subject to certain conditions, upon the achievement of one of the following milestones: (i) the Company’s enrollment of ten patients in an IV trilaciclib SCLC chemoprotection clinical trial with resulting data trending positively against historical controls or (ii) successful completion of IND-enabling studies for an oral trilaciclib antineoplastic program; provided, that either milestone is achieved prior to the earliest to occur of (i) a qualified initial public offering, (ii) a liquidation event or (iii) 12 months after the initial closing. Holders of at least sixty percent (60%) of the then-outstanding shares of Series B redeemable convertible preferred stock may elect to waive the foregoing conditions and to require the purchasers of Series B redeemable convertible preferred stock to purchase their pro rata portion of the Second Tranche Shares. Each purchaser also had the option, but not the obligation, to purchase all of its allocation of the Second Tranche Shares at any time prior to the earlier to occur of (i) the second closing, and (ii) 12 months after the initial closing.

The option to purchase shares of Series B redeemable convertible preferred stock in the second tranche has been accounted for as a free-standing instrument and classified as a liability. On February 4, 2015, upon purchase of the first tranche of Series B redeemable convertible preferred stock, the option to purchase additional shares was recorded at its fair value, with the remaining cash proceeds received on that date allocated to Series B redeemable convertible preferred stock. As the value of the option to purchase shares in the second tranche increased over time, a change in the fair value of the liability was recorded as “Change in fair value of Series B purchase option liability” in the accompanying statement of operations. This free-standing instrument was exercised on December 10, 2015 when the holders of at least sixty percent (60%) of the then-outstanding shares of Series B redeemable convertible preferred stock elected to waive the conditions set forth above and required the purchasers of Series B redeemable convertible preferred stock to purchase their pro rata portion of the second tranche shares resulting in an outstanding liability of $0 on December 31, 2016. The Company relied on an independent third-party valuation in estimating the fair value of its Series B purchase option liability. The valuations used significant assumptions including the estimated volatility range of 68%-71%, risk free interest rate range of 0.47%-0.94%, estimated fair value of the redeemable convertible preferred stock and a 2-year estimated life of the purchase option. These assumptions were used in the option pricing method and the probability weighted expected return method, a blend of which were considered in establishing fair value.

Preferred stock

Upon completion of the IPO, all outstanding preferred stock was automatically converted into 18,933,053 shares of common stock. The Company is also authorized to issue 5.0 million shares of undesignated preferred stock in one or more series. As December 31, 2017, no shares of preferred stock were issued or outstanding.

Common stock

The Company’s common stock has a par value of $0.0001 per share and consists of 120,000,000 and 73,000,000 authorized shares as of December 31, 2017 and 2016, respectively. Holders of common stock are entitled to one vote per share and are entitled to receive dividends, as if and when declared by the Company’s Board of Directors.

The Company has reserved authorized shares of common stock for future issuance at December 31, 2017 and December 31, 2016 as follows:

	December 31, 2017		December 31, 2016

Conversion of Series C Preferred Stock on a fully-diluted basis		—		5,609,398
Conversion of Series B Preferred Stock on a fully-diluted basis		—		7,642,734
Conversion of Series A Preferred Stock on a fully-diluted basis		—		4,998,895
Conversion of Series 1 Preferred Stock on a fully-diluted basis		—		682,026
Common stock warrants issued with promissory notes		—		16,666
Other common stock warrants		—		3,466
Series 1 Preferred Stock warrants issued with promissory notes		—		21,978
Common stock options outstanding		4,116,333		3,690,058
Options available for grant under Equity Incentive Plans		1,602,687		176,919
		5,719,020		22,842,140

Stock option plan

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock option plan

8. Stock option plan

2011 Equity Incentive Plan

In March 2011, the Company adopted the 2011 Equity Incentive Plan (the “Plan”). As amended, 4,400,640 shares of common stock were reserved for issuance under the 2011 Plan. Eligible plan participants included employees, directors, officers, consultants and advisors of the Company. The 2011 plan permitted the granting of incentive stock options, nonqualified stock options and other stock- based awards. In connection with the adoption of the 2017 Plan (as defined below), the 2011 Plan was terminated and no further awards will be made under the 2011 Plan.

2017 Equity Incentive Plan

In May 2017, the Company adopted the 2017 Equity Incentive Plan (the “2017 Plan”). The 2017 Plan provided for the direct award or sale of the Company’s common stock and for the grant of up to 1,932,000 stock options to employees, directors, officers, consultants and advisors of the Company. The 2017 Plan provides for the grant of incentive stock options, non-statutory stock options or restricted stock.

Under both the 2011 Plan and the 2017 Plan, options to purchase the Company’s common stock may be granted at a price no less than the fair market value of a share of common stock on the date of grant. The fair value shall be the closing sales price for a share as quoted on any established securities exchange for such grant date or the last preceding date for which such quotation exists. Vesting terms of options issued are determined by the board of directors or compensation committee of the board. The Company’s stock options vest based on terms in the stock option agreements. Stock options have a maximum term of ten years.

As of December 31, 2017, there were a total of 1,602,687 shares of common stock available for future issuance under the 2017 Plan.

Stock-based Compensation

During the years ended December 31, 2017, 2016 and 2015, the Company recorded employee share-based compensation expense of $1,772, $907, and $180, respectively. The Company recorded non-employee share-based compensation expense of $1,622, $484, and $209 during the years ended December 31, 2017, 2016 and 2015, respectively. Total share-based compensation expense included in the statements of operations is as follows:

	Year Ended December 31,
	2017		2016		2015
	in thousands
Research and development	$	2,531	$	911	$	221
General and administrative		863		480		168
Total stock-based compensation expense	$	3,394	$	1,391	$	389

The fair value of each option grant is estimated on the grant date using the Black-Scholes option-pricing model, using the following weighted average assumptions:

	Year Ended December 31,
	2017		2016		2015

Expected volatility	74.2 - 79.3%		74.8 - 83.9%		66.8 - 69.3%
Weighted-average risk-free rate	1.9 - 2.2%		1.2 - 2.1%		1.5 - 1.7%
Dividend yield	—%		—%		—%
Expected term (in years)	6.01			6.40	6.56
Weighted-average grant-date fair value per share	$	10.71	$	3.24	$	1.59

The expected term of stock options represents the weighted-average period the stock options are expected to remain outstanding and is based on the option vesting term, contractual terms and industry peers as the Company did not have sufficient historical information to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior.

The expected stock price volatility assumptions for the Company’s stock options were determined by examining the historical volatilities for industry peers.

The risk-free interest rate assumption at the date of grant is based on the U.S. Treasury instruments whose term was consistent with the expected term of the Company’s stock options.

The expected dividend assumption is based on the Company’s history and expectation of dividend payouts.

Stock option activity during 2017 is as follows:

						Weighted average
				Weighted		Remaining
				average		contractual		Aggregate
	Options			exercise		for		intrinsic
	outstanding			price		life (Years)		value
								(in thousands)
Balance as of December 31, 2016		3,690,058		$	2.13		8.4	$	17,463
Cancelled		(89,687	)	$	3.81
Granted		688,997			15.96
Exercised		(173,035	)		2.24
Balance as of December 31, 2017		4,116,333		$	4.41		7.8	$	63,577
Exercisable at December 31, 2017		2,225,970			1.64		7.2	$	40,523
Vested at December 31, 2017 and expected to vest		4,116,333			4.41		7.8	$	63,577

As of December 31, 2017, there was $10,404 of total unrecognized share-based compensation costs, net of estimated forfeitures, which is expected to be recognized over a weighted-average period of 2.93 years.

In 2016, in connection with preparation of the 2015 financials, the Company reassessed the determination of the fair value of the common shares underlying stock options granted throughout 2015. As a result, the Company determined that the fair value of the common shares was $0.75, $2.40 and $2.73 per share at February 27, 2015, July 15, 2015 and September 7, 2015, respectively, which was higher than the fair value as initially determined by the Board of Directors on the dates of grant. The use of this higher share price increased both recognized and unrecognized share-based compensation expense and also impacted the valuation of the non-employee share-based compensation expense which is marked to market at each reporting date.

In 2017, in connection with the Company’s initial public offering and after discussions with their underwriters, the Company reassessed the determination of the fair value of the common shares underlying the 1,099,320 stock options granted throughout 2016, and the 119,997 options granted up through the IPO date of May 16, 2017, and determined that no adjustment was necessary.

Since the IPO, the board of directors has determined the fair value of each common share underlying share-based awards based on the closing price of the common shares as reported by Nasdaq on the date of grant.

Net Loss Per Common Share

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Net Loss Per Common Share

9. Net loss per common share

Basic net loss per common share is computed using the weighted average number of common shares outstanding during the period including nominal issuances of common stock warrants. Diluted net loss per common share is computed using the sum of the weighted average number of common shares outstanding during the period and, if dilutive, the weighted average number of potential shares of common stock, including the assumed exercise of stock options, stock warrants and unvested restricted common stock. For the years ended December 31, 2017, 2016 and 2015, the following potentially dilutive securities have been excluded from the computations of diluted weighted-average shares outstanding because the effect would be anti-dilutive:

	Year Ended December 31,
	2017		2016		2015

Stock options issued and outstanding		3,838,358		3,224,682		1,867,325
Stock warrants		11,385		25,444		25,444
		3,849,743		3,250,126		1,892,769

Amounts in the table above reflect the common stock equivalents of the noted instruments.

Income Taxes

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Income Taxes

10. Income taxes

In accordance with FASB ASC 740, Accounting for Income Taxes, the Company reflects in the financial statements the benefit of positions taken in a previously filed tax return or expected to be taken in a future tax return only when it is considered ‘more-likely-than-not’ that the position taken will be sustained by a taxing authority. As of December 31, 2017 and 2016, the Company had no unrecognized income tax benefits and correspondingly there is no impact on the Company’s effective income tax rate associated with these items. The Company’s policy for recording interest and penalties relating to uncertain income tax positions is to record them as a component of income tax expense in the accompanying statements of operations. As of December 31, 2017 and 2016, the Company had no such accruals.

The components of income tax expense (benefit) attributable to continuing operations are as follows:

	Year ended December 31,
	2017		2016		2015
Current Expense:
Federal	$	—	$	—	$	—
State		—		—		—
		—		—		—
Deferred Expense:
Federal		—		—		—
State		—		—		—
	$	—	$	—	$	—

The differences between the company’s income tax expense attributable to continuing operation and the expense computed at the 34% U.S. statutory income tax rate were as follows (in thousands):

	Year ended December 31,
	2017			2016			2015
Federal income tax expense at statutory rate:	$	(20,441	)	$	(10,299	)	$	(6,890	)
Increase (reduction) in income tax resulting from:
State Income Taxes		(1,623	)		(397	)		(335	)
Increase in Valuation Allowance		8,977			10,936			6,109
Increase in fair value of Series B purchase option liability		—			—			1,623
Equity Financing Expenses		39			371			—
Stock Compensation		152			200			48
Research and Development Credit		(1,882	)		(803	)		(509	)
Effect on Tax Cuts & Job Acts Rate Reduction		14,770			—			—
Other		8			(8	)		(46	)
	$	—		$	—		$	—

The tax effects of temporary differences and operating loss carryforwards that gave rise to significant portions of the deferred tax assets and deferred tax liabilities were as follows at December 31, 2017 and 2016 (in thousands):

	Year ended December 31,
	2017			2016
Deferred tax assets
Accrued expenses	$	1,412		$	751
Deferred rent		20			16
Stock compensation		360			386
Charitable contributions		2			1
Capitalized patents and licenses		1,225			1,474
R&D credits		3,333			1,451
Net operating loss carryforwards		23,556			16,844
Deferred tax assets		29,908			20,923
Deferred tax liabilities
Property, plant and equipment, primarily due to differences in depreciation		(21	)		(13	)
Deferred tax liabilities		(21	)		(13	)
Valuation allowance		(29,887	)		(20,910	)
Net deferred tax assets	$	—		$	—

At December 31, 2017 and December 31, 2016, the Company evaluated all significant available positive and negative evidence, including the existence of losses in recent years and management’s forecast of future taxable income, and, as a result, determined it was more likely than not that federal and state deferred tax assets, including benefits related to net operating loss carryforwards, would not be realized. The valuation allowance was increased from $20,910 at December 31, 2016 to $29,887 at December 31, 2017. The increase in valuation allowance was due primarily to the increase in net operating loss carryforwards and income tax credits offset by the impact of the Tax Cuts and Job Act rate reduction.

The table below summarizes changes in the deferred tax valuation allowance (in thousands):

	2017		2016		2015
Balance at beginning of year	$	20,910	$	9,974	$	3,865
Charges to costs and expenses		8,977		10,936		6,109
Write-offs		—		—		—
Balance at end of year		29,887		20,910		9,974

At December 31, 2017, the Company has federal net operating loss carryforwards of approximately $100,795, which are available to offset future taxable income. The federal net operating loss carryforwards begin to expire in 2028. In addition, the Company has state net operating loss carryforwards totaling approximately $100,793, which are available to offset future state taxable income. State net operating losses begin to expire in 2023. Because the Company has incurred cumulative net operating losses since inception, all tax years remain open to examination by U.S. federal and state income tax authorities.

In accordance with FASB ASC 740, Accounting for Income Taxes, the Company reflects in the financial statements the benefit of positions taken in a previously filed tax return or expected to be taken in a future tax return only when it is considered ‘more-likely-than-not’ that the position taken will be sustained by a taxing authority. As of December 31, 2017 and 2016, the Company had no unrecognized income tax benefits and correspondingly there is no impact on the Company’s effective income tax rate associated with these items. The Company’s policy for recording interest and penalties relating to uncertain income tax positions is to record them as a component of income tax expense in the accompanying statements of income. As of December 31, 2017 and 2016, the Company had no such accruals.

On December 22, 2017, the Tax Cuts and Jobs Act (“Tax Act”) was signed into law. The Tax Act lowered the Federal corporate tax rate from 34% to 21% and made numerous other tax law changes. The Company has measured deferred tax assets at the enacted tax rate expected to apply when these temporary differences are expected to be realized or settled, resulting in a provisional charge of $14.8 million for the revaluation of the Company’s deferred tax assets. U.S. GAAP requires companies to recognize the effect of tax law changes in the period of enactment. Reasonable estimates were made based on the Company’s analysis of the Tax Act. These provisional amounts may be adjusted during 2018 when additional information is obtained. Additional information that may affect our provisional amounts would include further clarification and guidance on how the Internal Revenue Service will implement the Tax Act, including guidance with respect to guidance on how state taxing authorities will implement tax reform and the related effect on our state income tax returns, completion of our 2017 tax return filings, and the potential for additional guidance from the Financial Accounting Standards Board related to the Tax Act. Under the Tax Act, NOLs arising after December 31, 2017 may be carried forward indefinitely. However, for NOLs arising after December 31, 2017, NOL carryforwards will be limited to 80% of taxable income. The Company’s NOLs generated in 2017 and in prior years will not be subject to the limitations under the Tax Act.

Potential 382 Limitation

The Company’s ability to utilize its net operating loss (NOL) and research and development (R&D) credit carryforwards may be substantially limited due to ownership changes that may have occurred or that could occur in the future, as required by Section 382 of the Internal Revenue Code of 1986, as amended (the Code), as well as similar state provisions. These ownership changes may limit the amount of NOL and R&D credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. In general, an “ownership change,” as defined by Section 382 of the Code, results from a transaction or series of transactions over a three-year period resulting in an ownership change of more than 50 percent of the outstanding stock of a company by certain stockholders or public groups.

The Company has not completed a study to assess whether one or more ownership changes have occurred since the Company became a loss corporation under the definition of Section 382. If the Company has experienced an ownership change, utilization of the NOL or R&D credit carryforwards would be subject to an annual limitation, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term, tax-exempt rate, and then could be subject to additional adjustments, as required. Any such limitation may result in the expiration of a portion of the NOL or R&D credit carryforwards before utilization. Until a study is completed and any limitation known, no amounts are being considered as an uncertain tax position or disclosed as an unrecognized tax benefit under ASC-740. Any carryforwards that expire prior to utilization as a result of such limitations will be removed from deferred tax assets with a corresponding reduction of the valuation allowance. Due to the existence of the valuation allowance, it is not expected that any possible limitation will have an impact on the results of operations of the Company.

Related Party Transactions	12 Months Ended
Related Party Transactions	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	11. Related party transactions Two co-founders and shareholders of the Company had consulting agreements with the Company for their continued development work through June 30, 2017. The Company renewed its consulting agreement with one of these founders through June 30, 2019. Combined consulting fees paid to both founders were approximately $64, $108, and $104 for the years ended December 31, 2017, 2016 and 2015, respectively. The Company paid approximately $11, $14, $20 to the Chairman of the Board of Directors for scientific advisory services outside of his role on the board of directors during the years ended December 31, 2017, 2016 and 2015, respectively.

Quarterly Results of Operations (Unaudited)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Quarterly Results of Operations (Unaudited)

12. Quarterly Results of Operations (Unaudited)

The following table contains quarterly financial information for 2017 and 2016. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.

	Three Months Ended
	(unaudited)
	(in thousands, except share and per share amounts)
	March 31,			June 30,			September 30,			December 31,
	2017			2017			2017			2017
Total operating expenses	$	12,378		$	15,379		$	15,929		$	17,282
Operating loss		(12,378	)		(15,379	)		(15,929	)		(17,282	)
Total other income (expense), net		33			185			328			301
Net loss	$	(12,345	)	$	(15,194	)	$	(15,601	)	$	(16,981	)
Accretion of redeemable convertible preferred stock		(4,468	)		(289	)		—			—
Net loss attributable to common stockholders	$	(16,813	)	$	(15,483	)	$	(15,601	)	$	(16,981	)
Net loss per share attributable to common stockholders, basic and diluted	$	(11.24	)	$	(1.09	)	$	(0.55	)	$	(0.60	)
Weighted average common shares outstanding, basic and diluted		1,496,336			14,208,115			28,318,656			28,362,323


	March 31,			June 30,			September 30,			December 31,
	2016			2016			2016			2016
Total operating expenses	$	5,946		$	7,429		$	6,613		$	10,403
Operating loss		(5,946	)		(7,429	)		(6,613	)		(10,403	)
Total other income (expense), net		(8	)		50			56			2
Net loss and comprehensive loss	$	(5,954	)	$	(7,379	)	$	(6,557	)	$	(10,401	)
Accretion of redeemable convertible preferred stock		(1,009	)		(986	)		(1,205	)		(1,205	)
Net loss attributable to common stockholders	$	(6,963	)	$	(8,365	)	$	(7,762	)	$	(11,606	)
Net loss per share attributable to common stockholders, basic and diluted	$	(4.71	)	$	(5.63	)	$	(5.21	)	$	(7.77	)
Weighted average common shares outstanding, basic and diluted		1,447,219			1,486,303			1,490,552			1,493,753

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Basis of Presentation

Basis of presentation

The Company has prepared the accompanying financial statements in conformity with generally accepted accounting principles in the United States of America (“U.S. GAAP”).

Use of Estimates

Use of estimates

Cash and Cash Equivalents

Cash and cash equivalents

Concentration of Credit Risk

Concentration of credit risk

Property and Equipment

Property and equipment

Property and equipment are stated at cost less accumulated depreciation. Depreciation is generally calculated using the straight-line method over the following estimated useful lives:

Computer equipment			5 years
Laboratory equipment			5 years
Furniture and fixtures			7 years
Leasehold improvements			7 years

Impairment of Long-Lived Assets

Impairment of long-lived assets

Warrant Liability

Warrant liability

Series B Purchase Option Liability

Series B purchase option liability

Research and Development

Research and development

Revenue Recognition

Revenue recognition

In the event that the granting agency provides advance funding of a grant award, the Company records deferred revenues and then recognizes revenue as costs are incurred over the life of the grant.

To date, the Company has not generated any revenue from the commercial sale of its product candidates.

Fair Value of Financial Instruments

Fair value of financial instruments

Level 1		Inputs are unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.

Level 2		Inputs include quoted prices for similar assets and liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, inputs other than quoted prices that are observable for the asset or liability and inputs that are derived principally from or corroborated by observable market data by correlation or other means.

Level 3		Unobservable inputs that reflect the Company’s estimates of the assumptions that market participants would use in pricing the asset or liability. The Company develops these inputs based on the best information available, including its own data.

The carrying amounts of cash, cash equivalents, accounts payable and accrued liabilities approximate fair value because of their short-term nature.

At December 31, 2017 and 2016 these financials instruments and respective fair values have been classified as follows (in thousands):

	Quoted prices in active markets for identical assets (Level 1)		Significant other observable inputs (Level 2)		Significant other unobservable inputs (Level 3)		Balance at December 31, 2017
Assets
Money market funds	$	87,694	$	—	$	—	$	87,694
Certificates of Deposit		15,203		—		—		15,203
Total assets at fair value:	$	102,897	$	—	$	—	$	102,897
Liabilities:
Warrant Liability	$	—	$	—	$	—	$	—
Total liabilities at fair value:	$	—	$	—	$	—	$	—

	Quoted prices in active markets for identical assets (Level 1)		Significant other observable inputs (Level 2)		Significant other unobservable inputs (Level 3)		Balance at December 31, 2016
Assets
Money market funds	$	31,730	$	—	$	—	$	31,730
Certificates of Deposit		15,041		—		—		15,041
Total assets at fair value:	$	46,771	$	—	$	—	$	46,771
Liabilities:
Warrant Liability	$	—	$	—	$	167	$	167
Total liabilities at fair value:	$	—	$	—	$	167	$	167

The change in the fair value measurement using significant inputs (Level 3) is summarized below (in thousands):

Balance at December 31, 2015	$	85
Change in fair value in warrant liability		82
Balance at December 31, 2016	$	167
Change in fair value in warrant liability		41
Conversion of warrant to common stock warrant		(208	)
Balance at December 31, 2017	$	—

Patent Costs

Patent costs

Income Taxes

Income taxes

Stock-Based Compensation

Stock-based compensation

Segment Information

Segment information

The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. All of the Company’s assets are held in the United States.

Comprehensive Loss

Comprehensive loss

Redeemable Convertible Preferred Stock

Redeemable convertible preferred stock

New Accounting Standards

New accounting standards

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Schedule of Property and Equipment Estimated Useful Lives

Depreciation is generally calculated using the straight-line method over the following estimated useful lives:

Computer equipment			5 years
Laboratory equipment			5 years
Furniture and fixtures			7 years
Leasehold improvements			7 years

Summary of Financials Instruments and Respective Fair Values

At December 31, 2017 and 2016 these financials instruments and respective fair values have been classified as follows (in thousands):

	Quoted prices in active markets for identical assets (Level 1)		Significant other observable inputs (Level 2)		Significant other unobservable inputs (Level 3)		Balance at December 31, 2017
Assets
Money market funds	$	87,694	$	—	$	—	$	87,694
Certificates of Deposit		15,203		—		—		15,203
Total assets at fair value:	$	102,897	$	—	$	—	$	102,897
Liabilities:
Warrant Liability	$	—	$	—	$	—	$	—
Total liabilities at fair value:	$	—	$	—	$	—	$	—

	Quoted prices in active markets for identical assets (Level 1)		Significant other observable inputs (Level 2)		Significant other unobservable inputs (Level 3)		Balance at December 31, 2016
Assets
Money market funds	$	31,730	$	—	$	—	$	31,730
Certificates of Deposit		15,041		—		—		15,041
Total assets at fair value:	$	46,771	$	—	$	—	$	46,771
Liabilities:
Warrant Liability	$	—	$	—	$	167	$	167
Total liabilities at fair value:	$	—	$	—	$	167	$	167

Summary of Change in Fair Value Measurement Using Significant Inputs (Level 3)

The change in the fair value measurement using significant inputs (Level 3) is summarized below (in thousands):

Balance at December 31, 2015	$	85
Change in fair value in warrant liability		82
Balance at December 31, 2016	$	167
Change in fair value in warrant liability		41
Conversion of warrant to common stock warrant		(208	)
Balance at December 31, 2017	$	—

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2017

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment consists of the following (in thousands):

	December 31, 2017			December 31, 2016

Computer equipment	$	112		$	67
Laboratory equipment		283			207
Furniture and fixtures		174			64
Leasehold improvements		121			80
Construction in progress		1			—
Accumulated depreciation		(181	)		(107	)
Property and equipment, net	$	510		$	311

Accrued expenses (Tables)

12 Months Ended

Dec. 31, 2017

Payables And Accruals [Abstract]

Schedule of Accrued Expenses

Accrued expenses are comprised as follows (in thousands):

	December 31, 2017		December 31, 2016

Accrued external research and professional fees	$	1,402	$	295
Accrued external clinical study costs		4,788		1,897
Accrued compensation expense		1,328		617
Deferred rent, current portion		2		44
Accrued expenses	$	7,520	$	2,853

Lease Obligations (Tables)

12 Months Ended

Dec. 31, 2017

Leases [Abstract]

Schedule of Minimum Future Rental Payments on Noncancelable Operating Leases

The following is a schedule by years of minimum future rental payments on noncancelable operating leases as of December 31, 2017 (in thousands):

2018	$	294
2019		303
2020		312
2021		321
2022		331
2023		—
	$	1,561

Capitalization (Tables)

12 Months Ended

Dec. 31, 2017

Capitalization Longterm Debt And Equity [Abstract]

Summary of common stock shares reserved for future issuance

The Company has reserved authorized shares of common stock for future issuance at December 31, 2017 and December 31, 2016 as follows:

	December 31, 2017		December 31, 2016

Conversion of Series C Preferred Stock on a fully-diluted basis		—		5,609,398
Conversion of Series B Preferred Stock on a fully-diluted basis		—		7,642,734
Conversion of Series A Preferred Stock on a fully-diluted basis		—		4,998,895
Conversion of Series 1 Preferred Stock on a fully-diluted basis		—		682,026
Common stock warrants issued with promissory notes		—		16,666
Other common stock warrants		—		3,466
Series 1 Preferred Stock warrants issued with promissory notes		—		21,978
Common stock options outstanding		4,116,333		3,690,058
Options available for grant under Equity Incentive Plans		1,602,687		176,919
		5,719,020		22,842,140

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section S99
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=65877616&loc=d3e177068-122764

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(d))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
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-Paragraph 1
-Subparagraph (SX 210.3-04)
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Reference 6: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21553-112644

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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Reference 9: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-SubTopic 10
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-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=65888546&loc=d3e21300-112643

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(27))
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Reference 12: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21538-112644

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21488-112644

+ Details

Stock option plan (Tables)

12 Months Ended

Dec. 31, 2017

Summary of Stock Option Activity

Stock option activity during 2017 is as follows:

						Weighted average
				Weighted		Remaining
				average		contractual		Aggregate
	Options			exercise		for		intrinsic
	outstanding			price		life (Years)		value
								(in thousands)
Balance as of December 31, 2016		3,690,058		$	2.13		8.4	$	17,463
Cancelled		(89,687	)	$	3.81
Granted		688,997			15.96
Exercised		(173,035	)		2.24
Balance as of December 31, 2017		4,116,333		$	4.41		7.8	$	63,577
Exercisable at December 31, 2017		2,225,970			1.64		7.2	$	40,523
Vested at December 31, 2017 and expected to vest		4,116,333			4.41		7.8	$	63,577

Employee and Non-employee Stock Options

Summary of Share-Based Compensation Expense Included in the Statement of Operations

Total share-based compensation expense included in the statements of operations is as follows:

	Year Ended December 31,
	2017		2016		2015
	in thousands
Research and development	$	2,531	$	911	$	221
General and administrative		863		480		168
Total stock-based compensation expense	$	3,394	$	1,391	$	389

Summary of Fair Value of Stock Options Granted Using Black-Scholes Options Pricing Model

The fair value of each option grant is estimated on the grant date using the Black-Scholes option-pricing model, using the following weighted average assumptions:

	Year Ended December 31,
	2017		2016		2015

Expected volatility	74.2 - 79.3%		74.8 - 83.9%		66.8 - 69.3%
Weighted-average risk-free rate	1.9 - 2.2%		1.2 - 2.1%		1.5 - 1.7%
Dividend yield	—%		—%		—%
Expected term (in years)	6.01			6.40	6.56
Weighted-average grant-date fair value per share	$	10.71	$	3.24	$	1.59

Net Loss Per Common Share (Tables)

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Schedule of Potentially Dilutive Securities Excluded from Computations of Diluted Weighted-average Shares Outstanding

For the years ended December 31, 2017, 2016 and 2015, the following potentially dilutive securities have been excluded from the computations of diluted weighted-average shares outstanding because the effect would be anti-dilutive:

	Year Ended December 31,
	2017		2016		2015

Stock options issued and outstanding		3,838,358		3,224,682		1,867,325
Stock warrants		11,385		25,444		25,444
		3,849,743		3,250,126		1,892,769

Income taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Components of Income Tax Expense (Benefit) Attributable to Continuing Operations

The components of income tax expense (benefit) attributable to continuing operations are as follows:

	Year ended December 31,
	2017		2016		2015
Current Expense:
Federal	$	—	$	—	$	—
State		—		—		—
		—		—		—
Deferred Expense:
Federal		—		—		—
State		—		—		—
	$	—	$	—	$	—

Summary of Differences Between the Income Tax Expense Attributable to Continuing Operation and the Expense Computed at U.S. Statutory Income Tax Rate

The differences between the company’s income tax expense attributable to continuing operation and the expense computed at the 34% U.S. statutory income tax rate were as follows (in thousands):

	Year ended December 31,
	2017			2016			2015
Federal income tax expense at statutory rate:	$	(20,441	)	$	(10,299	)	$	(6,890	)
Increase (reduction) in income tax resulting from:
State Income Taxes		(1,623	)		(397	)		(335	)
Increase in Valuation Allowance		8,977			10,936			6,109
Increase in fair value of Series B purchase option liability		—			—			1,623
Equity Financing Expenses		39			371			—
Stock Compensation		152			200			48
Research and Development Credit		(1,882	)		(803	)		(509	)
Effect on Tax Cuts & Job Acts Rate Reduction		14,770			—			—
Other		8			(8	)		(46	)
	$	—		$	—		$	—

Components of Tax Effects of Temporary Differences and Operating Loss Carryforwards Including Significant Portions of Deferred Tax Assets and Deferred Tax Liabilities

	Year ended December 31,
	2017			2016
Deferred tax assets
Accrued expenses	$	1,412		$	751
Deferred rent		20			16
Stock compensation		360			386
Charitable contributions		2			1
Capitalized patents and licenses		1,225			1,474
R&D credits		3,333			1,451
Net operating loss carryforwards		23,556			16,844
Deferred tax assets		29,908			20,923
Deferred tax liabilities
Property, plant and equipment, primarily due to differences in depreciation		(21	)		(13	)
Deferred tax liabilities		(21	)		(13	)
Valuation allowance		(29,887	)		(20,910	)
Net deferred tax assets	$	—		$	—

Summary of Changes in the Deferred Tax Valuation Allowance

The table below summarizes changes in the deferred tax valuation allowance (in thousands):

	2017		2016		2015
Balance at beginning of year	$	20,910	$	9,974	$	3,865
Charges to costs and expenses		8,977		10,936		6,109
Write-offs		—		—		—
Balance at end of year		29,887		20,910		9,974

Quarterly Results of Operations (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Quarterly Results of Operations

	Three Months Ended
	(unaudited)
	(in thousands, except share and per share amounts)
	March 31,			June 30,			September 30,			December 31,
	2017			2017			2017			2017
Total operating expenses	$	12,378		$	15,379		$	15,929		$	17,282
Operating loss		(12,378	)		(15,379	)		(15,929	)		(17,282	)
Total other income (expense), net		33			185			328			301
Net loss	$	(12,345	)	$	(15,194	)	$	(15,601	)	$	(16,981	)
Accretion of redeemable convertible preferred stock		(4,468	)		(289	)		—			—
Net loss attributable to common stockholders	$	(16,813	)	$	(15,483	)	$	(15,601	)	$	(16,981	)
Net loss per share attributable to common stockholders, basic and diluted	$	(11.24	)	$	(1.09	)	$	(0.55	)	$	(0.60	)
Weighted average common shares outstanding, basic and diluted		1,496,336			14,208,115			28,318,656			28,362,323


	March 31,			June 30,			September 30,			December 31,
	2016			2016			2016			2016
Total operating expenses	$	5,946		$	7,429		$	6,613		$	10,403
Operating loss		(5,946	)		(7,429	)		(6,613	)		(10,403	)
Total other income (expense), net		(8	)		50			56			2
Net loss and comprehensive loss	$	(5,954	)	$	(7,379	)	$	(6,557	)	$	(10,401	)
Accretion of redeemable convertible preferred stock		(1,009	)		(986	)		(1,205	)		(1,205	)
Net loss attributable to common stockholders	$	(6,963	)	$	(8,365	)	$	(7,762	)	$	(11,606	)
Net loss per share attributable to common stockholders, basic and diluted	$	(4.71	)	$	(5.63	)	$	(5.21	)	$	(7.77	)
Weighted average common shares outstanding, basic and diluted		1,447,219			1,486,303			1,490,552			1,493,753

Description of Business - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Description Of Business [Line Items]
Accumulated deficit	$ 129,118	$ 64,985
Cash and cash equivalents	$ 103,812	$ 47,305	$ 22,938	$ 3,694
Minimum
Description Of Business [Line Items]
Expected period to fund operations by existing cash resources	12 months

Summary of Significant Accounting Policies - Schedule of Property and Equipment Estimated Useful Lives (Details)	12 Months Ended
	Dec. 31, 2017
Computer Equipment
Property Plant And Equipment [Line Items]
Estimated useful lives	5 years
Laboratory Equipment
Property Plant And Equipment [Line Items]
Estimated useful lives	5 years
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Estimated useful lives	7 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Estimated useful lives	7 years

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items]
Impairment charge of long-lived assets	$ 0	$ 0	$ 0
Unrecognized income tax benefits	0	0
Accrued income taxes	0	0
Patent \| General and Administrative Costs
Summary Of Significant Accounting Policies [Line Items]
Legal expenses	997,000	1,034,000	$ 691,000
Series B Purchase Option
Summary Of Significant Accounting Policies [Line Items]
Outstanding liability	$ 0	$ 0
Free-standing instrument exercised date	Dec. 10, 2015
Series B Purchase Option \| Second Tranche
Summary Of Significant Accounting Policies [Line Items]
Free-standing instrument exercised date	Dec. 10, 2015

Summary of Significant Accounting Policies - Summary of Financials Instruments and Respective Fair Values (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets
Assets at fair value	$ 102,897	$ 46,771
Liabilities:
Liabilities at fair value		167
Money Market Funds
Assets
Assets at fair value	87,694	31,730
Certificates of Deposit
Assets
Assets at fair value	15,203	15,041
Warrant Liability
Liabilities:
Liabilities at fair value		167
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Assets at fair value	102,897	46,771
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Money Market Funds
Assets
Assets at fair value	87,694	31,730
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Certificates of Deposit
Assets
Assets at fair value	$ 15,203	15,041
Significant Other Unobservable Inputs (Level 3)
Liabilities:
Liabilities at fair value		167
Significant Other Unobservable Inputs (Level 3) \| Warrant Liability
Liabilities:
Liabilities at fair value		$ 167

Summary of Significant Accounting Policies - Summary of Change in Fair Value Measurement Using Significant Inputs (Level 3) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 167	$ 85
Change in fair value	41
Ending balance	0	167
Warrant
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Change in fair value		$ 82
Common Stock
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Conversion of warrant	$ (208)

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Accumulated depreciation	$ (181)	$ (107)
Property and equipment, net	510	311
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	112	67
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	283	207
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	174	64
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	121	$ 80
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 1

Patent License Agreement - Additional Information (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2015
Patent License Agreement [Line Items]
License agreement date	Dec. 31, 2015
Common stock, shares issued	48,666
Common stock, fair value of stock issued	$ 181			$ 181
University
Patent License Agreement [Line Items]
License agreement date			Nov. 23, 2016
Non-refundable license issue fee		$ 500
Term of license agreement description			The term of the license agreement will continue until the later of (i) the expiration of the last valid claim within the patent rights covering the product in such country, (ii) the expiration of market exclusivity in such country and (iii) the 10th anniversary of the first commercial sale in such country. The University may terminate the agreement in the event (i) the Company fails to pay any amount or make any report when required to be made and fails to cure such failure within thirty (30) days after receipt of notice from the University, (ii) is in breach of any provision of the agreement and fails to remedy within forty-five (45) days after receipt of notice, (iii) makes a report to the University under the agreement that is determine to be materially false, (iv) declares insolvency or bankruptcy or (v) takes an action that causes patent rights or technical information to be subject to lien or encumbrance and fails to remedy any such breach with in forty-five (45) days of receipt of notice from the University. The Company may terminate the agreement at any time on written notice to the University at least ninety (90) days prior to the termination date specified in the notice. Upon expiration or termination of the agreement, all rights revert to the University.
License agreement termination notice period			90 days
University \| Maximum [Member]
Patent License Agreement [Line Items]
Milestone payments			$ 2,625

Accrued Expenses - Schedule of Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued external research and professional fees	$ 1,402	$ 295
Accrued external clinical study costs	4,788	1,897
Accrued compensation expense	1,328	617
Deferred rent, current portion	2	44
Accrued expenses	$ 7,520	$ 2,853

Lease Obligations - Additional Information (Details) - USD ($) $ in Thousands					1 Months Ended		12 Months Ended
	Mar. 27, 2017	Jan. 27, 2016	Apr. 01, 2014	Jan. 10, 2014	Aug. 31, 2017	Aug. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Leases [Abstract]
Rent per month			$ 6		$ 24	$ 16
Lease expiration date	Dec. 31, 2022	Dec. 31, 2022		Jul. 31, 2017
Renewal term	5 years	5 years
Rent expense							$ 252	$ 126	$ 71

Lease Obligations - Schedule of Minimum Future Rental Payments on Noncancelable Operating Leases (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Leases [Abstract]
2018	$ 294
2019	303
2020	312
2021	321
2022	331
Total minimum future rentals	$ 1,561

Capitalization - Additional Information (Details)			1 Months Ended		12 Months Ended
Capitalization - Additional Information (Details)	May 22, 2017 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Feb. 28, 2015 USD ($) $ / shares shares	May 31, 2014 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) Patient $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Jun. 30, 2016 USD ($) $ / shares shares	Apr. 30, 2016 shares
Schedule Of Capitalization [Line Items]
Cancellation of Indebtedness \| $				$ 39,000
Number of aggregate shares committed by purchasers		48,666
Proceeds from initial public offering, net of underwriting discounts and commissions \| $					$ 108,503,000
Reverse stock split description					In connection with the IPO, the Board of Directors and the stockholders of the Company approved a one-for-three reverse stock split of the Company’s common stock.
Reverse stock split ratio	0.33
Reverse stock split effective date	May 11, 2017
Undesignated preferred stock, shares authorized to issue					5,000,000
Preferred stock, shares issued					0
Preferred stock, shares outstanding					0
Common stock, par value \| $ / shares					$ 0.0001	$ 0.0001
Common stock, shares authorized					120,000,000	73,000,000
Common Stock Voting Rights					Holders of common stock are entitled to one vote per share
Series B Purchase Option
Schedule Of Capitalization [Line Items]
Free-standing instrument exercised date					Dec. 10, 2015
Outstanding liability \| $						$ 0
Estimated volatility range, minimum					68.00%
Estimated volatility range, maximum					71.00%
Risk free interest rate, minimum					0.47%
Risk free interest rate, maximum					0.94%
Estimated life of purchase option					2 years
IPO
Schedule Of Capitalization [Line Items]
Number of aggregate shares committed by purchasers	7,781,564
Common stock, price per share \| $ / shares	$ 15.00
Gross proceeds from issuance of common stock \| $	$ 116,700,000
Proceeds from initial public offering, net of underwriting discounts and commissions \| $	$ 107,100,000
Underwriters Option to Purchase
Schedule Of Capitalization [Line Items]
Number of aggregate shares committed by purchasers	781,564
Fourth Amended and Restated Certification of Incorporation
Schedule Of Capitalization [Line Items]
Cancellation of Indebtedness \| $			$ 12,000
Certificate of Amendment
Schedule Of Capitalization [Line Items]
Additional proceeds amount \| $		$ 16,750,000
Fifth Amended and Restated Certification of Incorporation
Schedule Of Capitalization [Line Items]
Preferred stock, shares authorized								57,108,717
Additional proceeds amount \| $							$ 50,000,000
Series A Redeemable Convertible Preferred Stock
Schedule Of Capitalization [Line Items]
Preferred stock, shares issued				2,495,663	0	4,998,895
Preferred stock, par value \| $ / shares				$ 2.52	$ 0.0001	$ 0.0001
Proceeds from shares issued including cancellation of indebtedness, total \| $				$ 6,289,000
Preferred stock, shares authorized					0	14,996,692
Series A Redeemable Convertible Preferred Stock \| Fourth Amended and Restated Certification of Incorporation
Schedule Of Capitalization [Line Items]
Preferred stock, shares authorized			14,996,692
Series A Redeemable Convertible Preferred Stock \| Certificate of Amendment
Schedule Of Capitalization [Line Items]
Preferred stock, shares authorized		14,996,692
Series A Redeemable Convertible Preferred Stock \| Fifth Amended and Restated Certification of Incorporation
Schedule Of Capitalization [Line Items]
Preferred stock, shares authorized								14,996,692
Redeemable Convertible Preferred Stock \| Fourth Amended and Restated Certification of Incorporation
Schedule Of Capitalization [Line Items]
Preferred stock, shares authorized			39,908,717
Redeemable Convertible Preferred Stock \| Certificate of Amendment
Schedule Of Capitalization [Line Items]
Preferred stock, shares authorized		40,108,717
Series 1 Redeemable Convertible Preferred Stock
Schedule Of Capitalization [Line Items]
Preferred stock, shares issued					0	682,026
Preferred stock, par value \| $ / shares					$ 0.0001	$ 0.0001
Preferred stock, shares authorized					0	2,112,025
Series 1 Redeemable Convertible Preferred Stock \| Fourth Amended and Restated Certification of Incorporation
Schedule Of Capitalization [Line Items]
Preferred stock, shares authorized			2,112,025
Series 1 Redeemable Convertible Preferred Stock \| Certificate of Amendment
Schedule Of Capitalization [Line Items]
Preferred stock, shares authorized		2,112,025
Series 1 Redeemable Convertible Preferred Stock \| Fifth Amended and Restated Certification of Incorporation
Schedule Of Capitalization [Line Items]
Preferred stock, shares authorized								2,112,025
Series B Redeemable Convertible Preferred Stock
Schedule Of Capitalization [Line Items]
Preferred stock, shares issued					0	7,642,734
Preferred stock, par value \| $ / shares					$ 0.0001	$ 0.0001
Preferred stock, shares authorized					0	23,000,000
Number of aggregate shares committed by purchasers					3,791,272
Price per share \| $ / shares					$ 4.3521
Number of patients for clinical trial \| Patient					10
Percentage of redeemable convertible preferred stock outstanding					60.00%
Description for milestone to be achieved					Each purchaser also had the option, but not the obligation, to purchase all of its allocation of the Second Tranche Shares at any time prior to the earlier to occur of (i) the second closing, and (ii) 12 months after the initial closing.
Series B Redeemable Convertible Preferred Stock \| Fourth Amended and Restated Certification of Incorporation
Schedule Of Capitalization [Line Items]
Preferred stock, shares issued			3,794,024
Proceeds from shares issued including cancellation of indebtedness, total \| $			$ 16,512,000
Preferred stock, shares authorized			22,800,000
Preferred stock, price per shares \| $ / shares			$ 4.3521
Series B Redeemable Convertible Preferred Stock \| Certificate of Amendment
Schedule Of Capitalization [Line Items]
Preferred stock, shares issued		3,848,710
Preferred stock, shares authorized		23,000,000
Preferred stock, price per shares \| $ / shares		$ 4.3521
Additional preferred stock, shares authorized		11,546,147
Series B Redeemable Convertible Preferred Stock \| Fifth Amended and Restated Certification of Incorporation
Schedule Of Capitalization [Line Items]
Preferred stock, shares authorized								23,000,000
Series C Redeemable Convertible Preferred Stock
Schedule Of Capitalization [Line Items]
Preferred stock, shares issued					0	5,609,398
Preferred stock, par value \| $ / shares					$ 0.0001	$ 0.0001
Preferred stock, shares authorized					0	17,000,000
Series C Redeemable Convertible Preferred Stock \| Fifth Amended and Restated Certification of Incorporation
Schedule Of Capitalization [Line Items]
Preferred stock, shares issued							5,609,398
Preferred stock, shares authorized							16,828,217	17,000,000
Preferred stock, price per shares \| $ / shares							$ 8.91
Convertible Preferred Stock \| IPO
Schedule Of Capitalization [Line Items]
Conversion of outstanding preferred stock to common stock	18,933,053

Capitalization - Summary of common stock shares reserved for future issuance (Details) - shares	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Capitalization [Line Items]
Common stock, shares reserved for future issuance	5,719,020	22,842,140
Conversion of Series C Preferred Stock On A Fully-Diluted Basis
Schedule Of Capitalization [Line Items]
Common stock, shares reserved for future issuance		5,609,398
Conversion of Series B Preferred Stock On A Fully-Diluted Basis
Schedule Of Capitalization [Line Items]
Common stock, shares reserved for future issuance		7,642,734
Conversion of Series A Preferred Stock On A Fully-Diluted Basis
Schedule Of Capitalization [Line Items]
Common stock, shares reserved for future issuance		4,998,895
Conversion of Series 1 Preferred Stock On A Fully-Diluted Basis
Schedule Of Capitalization [Line Items]
Common stock, shares reserved for future issuance		682,026
Common Stock Warrants Issued With Promissory Notes
Schedule Of Capitalization [Line Items]
Common stock, shares reserved for future issuance		16,666
Other Common Stock Warrants
Schedule Of Capitalization [Line Items]
Common stock, shares reserved for future issuance		3,466
Series 1 Preferred Stock Warrants Issued With Promissory Notes
Schedule Of Capitalization [Line Items]
Common stock, shares reserved for future issuance		21,978
Common Stock Options Outstanding
Schedule Of Capitalization [Line Items]
Common stock, shares reserved for future issuance	4,116,333	3,690,058
Options Available For Grant Under Equity Incentive Plan
Schedule Of Capitalization [Line Items]
Common stock, shares reserved for future issuance	1,602,687	176,919

Stock option plan - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	May 16, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 31, 2017	Sep. 07, 2015	Jul. 15, 2015	Feb. 27, 2015	Mar. 31, 2011
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation		$ 3,394	$ 1,391	$ 389
Granted		688,997
Employee Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation		$ 1,772	$ 907	180
Unrecognized stock-based compensation costs		$ 10,404
Weighted-average recognition period		2 years 11 months 4 days
Share Price						$ 2.73	$ 2.40	$ 0.75
Granted	119,997		1,099,320
Non-employee Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation		$ 1,622	$ 484	$ 209
2011 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares approved for grant under equity incentive plan									4,400,640
Number of shares available for grant under equity incentive plan		0
2017 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares approved for grant under equity incentive plan					1,932,000
Number of shares available for grant under equity incentive plan		1,602,687
Stock options, maximum term		10 years

Stock option plan - Summary of Share-Based Compensation Expense Included in the Consolidated Statement of Operations (Details) - Employee and Non-employee Stock Options - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 3,394	$ 1,391	$ 389
Research and development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	2,531	911	221
General and administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 863	$ 480	$ 168

Stock option plan - Fair Value of Stock Options Granted Using Black-Scholes Options Pricing Model (Details) - Employee and Non-employee Stock Options - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	74.20%	74.80%	66.80%
Expected volatility, maximum	79.30%	83.90%	69.30%
Weighted-average risk free rate, minimum	1.90%	1.20%	1.50%
Weighted-average risk free rate, maximum	2.20%	2.10%	1.70%
Expected term (in years)	6 years 3 days	6 years 4 months 24 days	6 years 6 months 21 days
Weighted-average grant-date fair value per share	$ 10.71	$ 3.24	$ 1.59

Stock option plan - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Options outstanding
Beginning balance	3,690,058
Cancelled	(89,687)
Granted	688,997
Exercised	(173,035)
Ending balance	4,116,333	3,690,058
Exercisable	2,225,970
Vested and expected to vest	4,116,333
Weighted average exercise price
Beginning balance	$ 2.13
Cancelled	3.81
Granted	15.96
Exercised	2.24
Ending balance	4.41	$ 2.13
Exercisable	1.64
Vested and expected to vest	$ 4.41
Weighted average, Remaining contractual for life (Years)
Balance	7 years 9 months 18 days	8 years 4 months 24 days
Exercisable	7 years 2 months 12 days
Vested and expected to vest	7 years 9 months 18 days
Weighted average, Aggregate intrinsic value
Balance	$ 63,577	$ 17,463
Exercisable	40,523
Vested and expected to vest	$ 63,577

Net Loss Per Common Share (Details) - shares	12 Months Ended
Net Loss Per Common Share (Details) - shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computations of diluted weighted-average shares outstanding	3,849,743	3,250,126	1,892,769
Employee and Non-employee Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computations of diluted weighted-average shares outstanding	3,838,358	3,224,682	1,867,325
Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computations of diluted weighted-average shares outstanding	11,385	25,444	25,444

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Unrecognized income tax benefits		$ 0	$ 0
Unrecognized income tax benefits impact on effective income tax rate		0	0
Accrued income taxes		$ 0	$ 0
U.S. statutory income tax rate		34.00%	34.00%	34.00%
Valuation allowance		$ 29,887,000	$ 20,910,000
Effect on provisional charge for revaluation of deferred tax assets		14,770,000
Scenario, Forecast
Income Taxes [Line Items]
U.S. statutory income tax rate	21.00%
Percentage of limitations on NOL carryforwards to taxable income	80.00%
Federal
Income Taxes [Line Items]
Net operating loss carryforwards		$ 100,795,000
Net operating loss carryforwards expiration year		2028
State
Income Taxes [Line Items]
Net operating loss carryforwards		$ 100,793,000
Net operating loss carryforwards expiration year		2023

Income Taxes - Summary of Differences Between the Income Tax Expense Attributable to Continuing Operation and the Expense Computed at U.S. Statutory Income Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal income tax expense at statutory rate:	$ (20,441)	$ (10,299)	$ (6,890)
Increase (reduction) in income tax resulting from:
State Income Taxes	(1,623)	(397)	(335)
Increase in Valuation Allowance	8,977	10,936	6,109
Increase in fair value of Series B purchase option liability			1,623
Equity Financing Expenses	39	371
Stock Compensation	152	200	48
Research and Development Credit	(1,882)	(803)	(509)
Effect on Tax Cuts & Job Acts Rate Reduction	14,770
Other	$ 8	$ (8)	$ (46)

Income Taxes - Components of Tax Effects of Temporary Differences and Operating Loss Carryforwards Including Significant Portions of Deferred Tax Assets and Deferred Tax Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets
Accrued expenses	$ 1,412	$ 751
Deferred rent	20	16
Stock compensation	360	386
Charitable contributions	2	1
Capitalized patents and licenses	1,225	1,474
R&D credits	3,333	1,451
Net operating loss carryforwards	23,556	16,844
Deferred tax assets	29,908	20,923
Deferred tax liabilities
Property, plant and equipment, primarily due to differences in depreciation	(21)	(13)
Deferred tax liabilities	(21)	(13)
Valuation allowance	$ (29,887)	$ (20,910)

Income Taxes - Summary of Changes in the Deferred Tax Valuation Allowance (Details) - Valuation Allowance of Deferred Tax Assets - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation Allowance [Line Items]
Balance at beginning of year	$ 20,910	$ 9,974	$ 3,865
Charges to costs and expenses	8,977	10,936	6,109
Balance at end of year	$ 29,887	$ 20,910	$ 9,974

Related Party Transactions - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Founders
Related Party Transaction [Line Items]
Consulting agreements completion date	Jun. 30, 2017
Consulting agreements renewal date	Jun. 30, 2019
Consulting fees	$ 64	$ 108	$ 104
Consulting Services \| Chairman of the Board of Directors
Related Party Transaction [Line Items]
Consulting fees	$ 11	$ 14	$ 20

Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total operating expenses	$ 17,282	$ 15,929	$ 15,379	$ 12,378	$ 10,403	$ 6,613	$ 7,429	$ 5,946	$ 60,968	$ 30,391	$ 15,946
Operating loss	(17,282)	(15,929)	(15,379)	(12,378)	(10,403)	(6,613)	(7,429)	(5,946)	(60,968)	(30,391)	(15,424)
Total other income (expense), net	301	328	185	33	2	56	50	(8)	847	100	(4,839)
Net loss	(16,981)	(15,601)	(15,194)	(12,345)	(10,401)	(6,557)	(7,379)	(5,954)	(60,121)	(30,291)	(20,263)
Accretion of redeemable convertible preferred stock			(289)	(4,468)	(1,205)	(1,205)	(986)	(1,009)	(4,757)	(4,405)	(1,427)
Net loss attributable to common stockholders	$ (16,981)	$ (15,601)	$ (15,483)	$ (16,813)	$ (11,606)	$ (7,762)	$ (8,365)	$ (6,963)	$ (64,878)	$ (34,696)	$ (21,690)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.60)	$ (0.55)	$ (1.09)	$ (11.24)	$ (7.77)	$ (5.21)	$ (5.63)	$ (4.71)	$ (3.57)	$ (23.33)	$ (16.13)
Weighted average common shares outstanding, basic and diluted	28,362,323	28,318,656	14,208,115	1,496,336	1,493,753	1,490,552	1,486,303	1,447,219	18,197,970	1,486,986	1,344,584

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	13. Subsequent events In January 2018, the Company signed an amendment to its current lease to secure additional 6,395 square feet of office space in its existing building. The Company expects the space to be available in July 2018.

Subsequent Events - Additional Information (Details)	Jan. 31, 2018 ft²
Subsequent Event
Subsequent Event [Line Items]
Additional office space lease to secure	6,395