G1 Therapeutics (GTHX) 10-Q 7 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 31, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Trading Symbol	GTHX
Entity Registrant Name	G1 THERAPEUTICS, INC.
Entity Central Index Key	1,560,241
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		37,185,032
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true

Condensed Balance Sheets

Condensed Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 390,524	$ 103,812
Prepaid expenses and other current assets	1,489	849
Total current assets	392,013	104,661
Property and equipment, net	1,006	510
Total assets	393,019	105,171
Current liabilities
Accounts payable	2,752	4,184
Accrued expenses	10,922	7,520
Total current liabilities	13,674	11,704
Other non-current liabilities	90	79
Total liabilities	13,764	11,783
Stockholders’ equity
Common stock, $0.0001 par value, 120,000,000 shares authorized as of September 30, 2018 and December 31, 2017, respectively; 37,165,969 and 28,420,511 shares issued as of September 30, 2018 and December 31, 2017, respectively; 37,139,303 and 28,393,845 shares outstanding as of September 30, 2018 and December 31, 2017, respectively	4	3
Treasury stock, 26,666 shares	(8)	(8)
Additional paid-in capital	569,574	222,511
Accumulated deficit	(190,315)	(129,118)
Total stockholders’ equity	379,255	93,388
Total liabilities and equity	$ 393,019	$ 105,171

Condensed Balance Sheets (Paren

Condensed Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	120,000,000	120,000,000
Common stock, shares, issued	37,165,969	28,420,511
Common stock, shares, outstanding	37,139,303	28,393,845
Treasury stock, shares	26,666	26,666

Condensed Statements of Operati

Condensed Statements of Operations (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating expenses
Research and development	$ 15,873	$ 14,054	$ 51,605	$ 38,806
General and administrative	4,949	1,875	11,595	4,881
Total operating expenses	20,822	15,929	63,200	43,687
Operating loss	(20,822)	(15,929)	(63,200)	(43,687)
Other income (expense)
Other income	904	328	2,003	588
Change in fair value in warrant liability and other liabilities				(41)
Total other income, net	904	328	2,003	547
Net loss	(19,918)	(15,601)	(61,197)	(43,140)
Accretion of redeemable convertible preferred stock				(4,757)
Net loss attributable to common stockholders	$ (19,918)	$ (15,601)	$ (61,197)	$ (47,897)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.59)	$ (0.55)	$ (1.91)	$ (3.24)
Weighted average common shares outstanding, basic and diluted	33,829,437	28,318,656	32,006,978	14,772,621

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows (unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities
Net loss	$ (61,197)	$ (43,140)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	115	59
Stock-based compensation	6,971	2,357
Gain/loss on disposal of property and equipment	7	6
Increase in fair value of warrant activity		41
Change in operating assets and liabilities
Prepaid expenses and other assets	(510)	(59)
Accounts payable	(1,648)	1,629
Accrued expenses	3,167	3,297
Deferred rent	16	37
Net cash used in operating activities	(53,079)	(35,773)
Cash flows from investing activities
Purchases of property and equipment	(598)	(270)
Net cash used in investing activities	(598)	(270)
Cash flows from financing activities
Proceeds from stock options and warrants exercised	1,414	13
Proceeds from public offerings, net of underwriting fees and commissions	339,589	108,503
Payment of public offering costs	(614)	(1,398)
Net cash provided by financing activities	340,389	107,118
Net change in cash and cash equivalents	286,712	71,075
Cash and cash equivalents
Beginning of period	103,812	47,305
End of period	390,524	118,380
Non-cash investing and financing activities
Accretion of redeemable convertible preferred stock		4,757
Purchases of equipment in accounts payable and accrued expenses	19
Conversion of preferred stock and preferred warrants to common stock and common warrants		112,337
Prepaid project costs in accounts payable	130
Deferred offering costs reclassified to additional paid-in capital		$ 1,398
Costs for public offering in accounts payable and accrued expenses	$ 297

Business Description

Business Description	9 Months Ended
	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Business Description	1. Business Description G1 Therapeutics, Inc. (“G1” or the “Company”) is a clinical-stage biopharmaceutical company based in Research Triangle Park, North Carolina dedicated to the discovery, development and delivery of novel small molecule therapeutics that improve the lives of those affected by cancer. The Company was incorporated on May 19, 2008 in the state of Delaware. The Company’s product portfolio is built on a drug discovery platform that targets key cellular pathways with proprietary medicinal chemistry. The Company’s therapies are designed to enable more effective combination treatment strategies and improve outcomes for patients across multiple oncology indications. G1 has a deep expertise in cyclin-dependent kinases (CDKs), a family of proteins that plays an important role in the growth and proliferation of all human cells. The Company is developing potent and selective inhibitors of the kinases CDK4 and CDK6, collectively known as CDK4/6, which are a validated and promising class of targets for anti-cancer therapeutics. G1 is currently advancing two CDK4/6 inhibitor product candidates in clinical development, trilaciclib and lerociclib, each of which has broad therapeutic potential in many forms of cancer and may serve as the backbone of multiple combination regimens. These compounds were discovered and synthesized by G1; the Company has retained global development and commercial rights, and has created extensive patent portfolios, for both candidates. Trilaciclib, the Company’s most advanced clinical-stage candidate, is a potential first-in-class myelopreservation therapy designed to preserve hematopoietic stem and progenitor cell (HSPC) and immune system function during chemotherapy. Trilaciclib is a short-acting intravenous CDK4/6 inhibitor administered prior to chemotherapy. In March 2018, the Company announced positive trilaciclib myelopreservation results from a Phase 2 trial in first-line small cell lung cancer (SCLC) patients. Additional positive clinical data from that trial were presented at the European Society of Medical Oncology (ESMO) Congress in October 2018. Trilaciclib is currently being evaluated in three additional randomized trials: a Phase 2 trial in first line SCLC in combination with chemotherapy and the checkpoint inhibitor Tecentriq®, a Phase 2 trial in second/third line SCLC in combination with chemotherapy, and a Phase 2 trial in patients with metastatic triple-negative breast cancer (TNBC) in combination with chemotherapy. Lerociclib is a potential best-in-class oral CDK4/6 inhibitor, designed to be used in combination with other targeted therapies to treat multiple cancers. Preliminary Phase 1b data from a combination trial with Faslodex® in estrogen receptor-positive, HER2-negative (ER+, HER2-) breast cancer were presented in June 2018 at the American Society of Clinical Oncology (ASCO) Annual Meeting. These data showed promising safety, tolerability and anti-tumor activity when lerociclib was dosed continuously as a treatment for people with ER+, HER2- breast cancer. In April 2018, the Company announced the initiation of a Phase 1b/2 combination trial with the epidermal growth factor receptor (EGFR) inhibitor Tagrisso® in non-small cell lung cancer, or NSCLC. In addition to these CDK 4/6 inhibitors, the Company also has global development and commercialization rights for G1T48, a potential best-in-class oral selective estrogen receptor degrader, or SERD, which is targeted for the treatment of ER+ breast cancers. G1 initiated a Phase 1 clinical trial with G1T48 monotherapy for the treatment of ER+, HER2- breast cancer in the second quarter of 2018. Contingent on the findings of this trial, the Company plans to continue exploring G1T48 as monotherapy and initiate testing of a G1T48/lerociclib combination in breast cancer in 2019. G1 believes that it is the only emerging biopharmaceutical company with a wholly owned, proprietary all-oral SERD and CDK4/6 inhibitor combination regimen.

Basis of Presentation and Summa

Basis of Presentation and Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation and Summary of Significant Accounting Policies	2. Basis of Presentation and Summary of Significant Accounting Policies Basis of Presentation The accompanying condensed financial statements of the Company have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”). In the opinion of management, the Company has made all necessary adjustments, which include normal recurring adjustments necessary for a fair statement of the Company’s financial position and results of operations for the interim periods presented. The information presented in the condensed financial statements and related notes as of September 30, 2018, and for the three and nine months ended September 30, 2018 and 2017, is unaudited. The results for the three months ended September 30, 2018 are not necessarily indicative of the results expected for the full fiscal year or any future period. These interim financial statements should be read in conjunction with the financial statements and notes set forth in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017 filed with the SEC on February 21, 2018. The December 31, 2017 condensed balance sheet included herein was derived from the audited financial statements as of that date, but does not include all disclosures, including notes, required by U.S. GAAP for complete financial statements. Reclassifications The Company has reclassified certain prior period amounts in its condensed statements of cash flows to conform to the current period presentation. The reclassification relates to separately presenting accounts payable, accrued expenses, and deferred rent, which were previously included in the change in the accounts payable and accrued expenses caption. For the nine months ended September 30, 2017, the Company reclassified $1.6 million into changes in accounts payable caption, $3.3 million into changes in the accrued expenses caption, and $37 thousand into the changes in deferred rent caption. 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. On an ongoing basis, the Company’s management evaluates its estimates which include, but are not limited to, estimates related to accrued expenses, accrued external clinical costs, stock-based compensation expense and deferred tax asset valuation allowance. The Company bases its estimates on historical experience and other market specific or other relevant assumptions it believes to be reasonable under the circumstances. Actual results could differ from those estimates. 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 Income taxes The Company did not record a federal or state income tax benefit for the nine months ended September 30, 2018 due to its conclusion that a full valuation allowance is required against the Company’s deferred tax assets. 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 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. Recent Accounting Pronouncements In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments In May 2014, the FASB and the International Accounting Standards Board jointly issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) The future impact of ASU 2014-09 will be dependent on the nature of the Company’s future revenue contracts and arrangements, if any. In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Codification Improvements to Topic 842, Leases Leases (Topic 842): Targeted Improvements In June 2018, the FASB issued ASU No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting In August 2018, the FASB issued ASU No. 2018-15, Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract .

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements 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 September 30, 2018 and December 31, 2017 these financial instruments and respective fair values have been classified as follows (in thousands): September 30, 2018 Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant other unobservable inputs (Level 3) Balance at September 30, 2018 (unaudited) (unaudited) (unaudited) (unaudited) Assets Money market funds $ 374,531 $ — $ — $ 374,531 Certificates of Deposit 15,413 — — 15,413 Total assets at fair value: $ 389,944 $ — $ — $ 389,944 December 31, 2017 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 During the three and nine months ended September 30, 2018 and the year ended December 31, 2017, there were no changes in valuation methodology.

Property and Equipment

Property and Equipment	9 Months Ended
	Sep. 30, 2018
Property Plant And Equipment [Abstract]
Property and Equipment	4. Property and equipment Property and equipment consists of the following (in thousands): September 30, 2018 December 31, 2017 (unaudited) Computer equipment $ 223 $ 112 Laboratory equipment 553 283 Furniture and fixtures 293 174 Leasehold improvements 190 121 Construction in progress 12 1 Accumulated depreciation (265 ) (181 ) Property and equipment, net $ 1,006 $ 510 Depreciation expense relating to property and equipment was $48 thousand and $115 thousand for the three and nine months ended September 30, 2018, respectively and $27 thousand and $59 thousand for the three and nine months ended September 30, 2017, respectively.

Patent License Agreement

Patent License Agreement	9 Months Ended
	Sep. 30, 2018
Patent License Agreement [Abstract]
Patent License Agreement	5. 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”), which was amended on March 24, 2017. Pursuant to the license agreement, as amended, 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 $0.5 million 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 royalty percentage on all net sales of products and a share of sublicensing revenues. In addition, the University is eligible to receive milestone payments of up to $2.6 million related to the initiation and execution of clinical trials, with payments made for the initial dosing for each phase of the clinical trials, as well as the first commercial sale of a product and the first commercial sale of a product in another country. To date, the Company has made milestone payments totaling $0.1 million, of which $75 thousand was incurred and accrued for during the current period with the enrollment and dosing of the first patient in the Phase 1 trial for G1T48. The Company will be responsible for any future patent prosecution costs that may arise. 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 such breach within forty-five (45) days after receipt of notice, (iii) makes a report to the University under the agreement that is determined 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.

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2018
Payables And Accruals [Abstract]
Accrued Expenses	6. Accrued expenses Accrued expenses are comprised as follows (in thousands): September 30, 2018 December 31, 2017 (unaudited) Accrued external research and professional fees $ 2,629 $ 1,402 Accrued external clinical study costs 6,788 4,788 Accrued compensation expense 1,498 1,328 Deferred rent, current portion 7 2 Accrued expenses $ 10,922 $ 7,520

Stockholders_ Equity

Stockholders’ Equity	9 Months Ended
	Sep. 30, 2018
Class Of Stock Disclosures [Abstract]
Stockholders’ Equity	7. Stockholders’ Equity Common Stock The Company is authorized to issue 120.0 million shares of common stock. Holders of common stock are entitled to one vote per share. Holders of common stock are entitled to receive dividends, as, if and when declared by the Company’s Board of Directors. On March 12, 2018, the Company closed an underwritten public offering of 3,910,000 shares of common stock at a public offering price of $29.50 per share, including 510,000 shares of common stock issued upon exercise by the underwriters of their option to purchase additional shares. The gross proceeds from the offering were $115.3 million and net proceeds were $107.9 million, after deducting underwriting discounts and commissions and other offering expenses payable by the Company. On June 15, 2018, the Company entered into a Sales Agreement for “at the market offerings” with Cowen and Company, LLC (“Cowen”), which allows G1 to issue and sell shares of common stock pursuant to a shelf registration statement for total gross sales proceeds of up to $125.0 million from time to time through Cowen, acting as our agent. Between June 18, 2018 and August 2, 2018, the Company sold 752,008 shares of common stock pursuant to this agreement resulting in $36.1 million in net proceeds, realizing $12.1 million in the second quarter and the remaining $24.0 million by August 2, 2018. On September 21, 2018, the Company closed on an underwritten public offering of 3,450,000 shares of its common stock at a public offering price of $60.00 per share, including 450,000 shares of common stock issued upon exercise by the underwriters of their option to purchase additional shares, pursuant to a shelf registration statement. The gross proceeds from the offering were $207.0 million and net proceeds were $194.9 million, after deducting underwriting discounts and commissions and other offering expenses payable by the Company. Preferred Stock The Company is authorized to issue 5.0 million shares of undesignated preferred stock in one or more series. As of September 30, 2018, no shares of preferred stock were issued or outstanding. Shares Reserved for Future Issuance The Company has reserved for future issuance the following number of shares of common stock: September 30, 2018 December 31, 2017 (unaudited) Common stock options outstanding 4,470,966 4,116,333 Options available for grant under Equity Incentive Plans 1,681,296 1,602,687 6,152,262 5,719,020

Stock-based Compensation

Stock-based Compensation	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based Compensation	8. Stock-based Compensation 2011 Equity Incentive Plan In March 2011, the Company adopted the 2011 Equity Incentive Plan (the “2011 Plan”). The 2011 Plan provided for the direct award or sale of the Company’s common stock and for the grant of stock options to employees, directors, officers, consultants and advisors of the Company. The 2011 Plan was subsequently amended in August 2012, October 2013, February 2015, December 2015, April 2016 and November 2016 to allow for the issuance of additional shares of common stock. 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. Effective January 1, 2018, and in accordance with the “evergreen” provision of the 2017 plan, an additional 1,066,692 shares were made available for issuance. 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 September 30, 2018, there were a total of 1,681,296 shares of common stock available for future issuance under the 2017 Plan. Stock Option Expense The Company recognizes compensation costs related to stock options granted to employees based on the estimated fair value of the awards on the date of grant, net of estimated forfeitures. The grant date fair value of the stock-based awards is generally recognized on a straight-line basis over the requisite service period, which is generally the vesting period of the respective awards. Share-based awards granted to non-employee directors as compensation for serving on the Company’s Board of Directors are accounted for in the same manner as employee share-based compensation awards. During the three and nine months ended September 30, 2018, the Company recorded employee share-based compensation expense of $2.7 million and $5.2 million, respectively. During the three and nine months ended September 30, 2017, the Company recorded employee share-based compensation expense of $0.5 million and $1.2 million, respectively. The Company recognizes compensation costs related to stock options granted to non-employees based on the estimated fair value of the awards on the date of grant in the same manner as employees; however, the fair value of the stock options granted to non-employees is re-measured each reporting period until the service is complete, and the resulting increase or decrease in value, if any, is recognized as expense or income, respectively, during the period the related services are rendered. During the three and nine months ended September 30, 2018, the Company recorded non-employee share-based compensation expense of $0.5 million and $1.8 million, respectively. During the three and nine months ended September 30, 2017, the Company recorded non-employee share-based compensation expense of $0.5 million and $1.2 million, respectively. The Company calculates the fair value of stock options using the Black-Scholes option pricing model. The Black-Scholes option-pricing model requires the use of subjective assumptions, including the expected volatility of the Company’s common stock, the assumed dividend yield, the expected term of the Company’s stock options and the fair value of the underlying common stock on the date of grant. Stock options— Black-Scholes inputs The fair value of stock options was estimated using the following weighted-average assumptions for the three and nine months ended September 30, 2018 and September 30, 2017: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (unaudited) (unaudited) Expected volatility 77.3 - 86.4% 74.6 - 75.1% 74.9 - 86.5% 74.2 - 79.3% Weighted-average risk free rate 2.7 - 2.9% 1.9 - 2.0% 2.3 - 2.9% 1.9 - 2.1% Dividend yield —% —% —% —% Expected term (in years) 6.06 5.94 6.03 5.98 The table below summarizes the stock-based compensation expense recognized in the Company’s statement of operations by classification (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (unaudited) (unaudited) Research and development $ 1,473 $ 792 $ 3,900 $ 1,796 General and administrative 1,803 248 3,071 561 Total stock-based compensation expense $ 3,276 $ 1,040 $ 6,971 $ 2,357 Stock Option Activity Stock option activity for the nine months ended September 30, 2018 is as follows: Weighted average Weighted average Remaining Aggregate Options exercise contractual intrinsic outstanding price life (Years) value (in thousands) Balance as of December 31, 2017 4,116,333 $ 4.41 7.8 $ 63,577 Cancelled (123,867 ) $ 4.89 Granted 1,111,950 37.68 Exercised (633,450 ) 2.23 Balance as of September 30, 2018 4,470,966 $ 12.98 7.7 $ 176,206 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 Exercisable at September 30, 2018 2,213,100 2.24 6.6 $ 110,770 Vested at September 30, 2018 and expected to vest 4,470,966 12.98 7.7 $ 176,206

Net Loss Per Common Share

Net Loss Per Common Share	9 Months Ended
	Sep. 30, 2018
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 three months ended September 30, 2018 and 2017 and for the nine months ended September 30, 2018 and 2017 the following potentially dilutive securities have been excluded from the computations of diluted weighted-average shares outstanding because the effect would be anti-dilutive: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (unaudited) (unaudited) Stock options issued and outstanding 4,482,649 3,894,192 4,258,269 3,805,419 Stock warrants — — — 15,223 4,482,649 3,894,192 4,258,269 3,820,642 Amounts in the table above reflect the common stock equivalents of the noted instruments.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2018
Related Party Transactions [Abstract]
Related Party Transactions	10. Related party transactions The Company maintains a consulting agreement with the Chairman of the Board of Directors for scientific advisory services outside of his role on the Board of Directors that expires on June 30, 2020.

Basis of Presentation and Sum_2

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying condensed financial statements of the Company have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”). In the opinion of management, the Company has made all necessary adjustments, which include normal recurring adjustments necessary for a fair statement of the Company’s financial position and results of operations for the interim periods presented. The information presented in the condensed financial statements and related notes as of September 30, 2018, and for the three and nine months ended September 30, 2018 and 2017, is unaudited. The results for the three months ended September 30, 2018 are not necessarily indicative of the results expected for the full fiscal year or any future period. These interim financial statements should be read in conjunction with the financial statements and notes set forth in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017 filed with the SEC on February 21, 2018. The December 31, 2017 condensed balance sheet included herein was derived from the audited financial statements as of that date, but does not include all disclosures, including notes, required by U.S. GAAP for complete financial statements.
Reclassifications	Reclassifications The Company has reclassified certain prior period amounts in its condensed statements of cash flows to conform to the current period presentation. The reclassification relates to separately presenting accounts payable, accrued expenses, and deferred rent, which were previously included in the change in the accounts payable and accrued expenses caption. For the nine months ended September 30, 2017, the Company reclassified $1.6 million into changes in accounts payable caption, $3.3 million into changes in the accrued expenses caption, and $37 thousand into the changes in deferred rent caption.
Use of Estimates	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. On an ongoing basis, the Company’s management evaluates its estimates which include, but are not limited to, estimates related to accrued expenses, accrued external clinical costs, stock-based compensation expense and deferred tax asset valuation allowance. The Company bases its estimates on historical experience and other market specific or other relevant assumptions it believes to be reasonable under the circumstances. Actual results could differ from those estimates.
Research and Development	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
Income Taxes	Income taxes The Company did not record a federal or state income tax benefit for the nine months ended September 30, 2018 due to its conclusion that a full valuation allowance is required against the Company’s deferred tax assets. 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
Stock-Based Compensation	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.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments In May 2014, the FASB and the International Accounting Standards Board jointly issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) The future impact of ASU 2014-09 will be dependent on the nature of the Company’s future revenue contracts and arrangements, if any. In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Codification Improvements to Topic 842, Leases Leases (Topic 842): Targeted Improvements In June 2018, the FASB issued ASU No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting In August 2018, the FASB issued ASU No. 2018-15, Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract .

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Summary of Financial Instruments and Respective Fair Values	At September 30, 2018 and December 31, 2017 these financial instruments and respective fair values have been classified as follows (in thousands): September 30, 2018 Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant other unobservable inputs (Level 3) Balance at September 30, 2018 (unaudited) (unaudited) (unaudited) (unaudited) Assets Money market funds $ 374,531 $ — $ — $ 374,531 Certificates of Deposit 15,413 — — 15,413 Total assets at fair value: $ 389,944 $ — $ — $ 389,944 December 31, 2017 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

Property and Equipment (Tables)

Property and Equipment (Tables)	9 Months Ended
	Sep. 30, 2018
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consists of the following (in thousands): September 30, 2018 December 31, 2017 (unaudited) Computer equipment $ 223 $ 112 Laboratory equipment 553 283 Furniture and fixtures 293 174 Leasehold improvements 190 121 Construction in progress 12 1 Accumulated depreciation (265 ) (181 ) Property and equipment, net $ 1,006 $ 510

Accrued expenses (Tables)

Accrued expenses (Tables)	9 Months Ended
	Sep. 30, 2018
Payables And Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses are comprised as follows (in thousands): September 30, 2018 December 31, 2017 (unaudited) Accrued external research and professional fees $ 2,629 $ 1,402 Accrued external clinical study costs 6,788 4,788 Accrued compensation expense 1,498 1,328 Deferred rent, current portion 7 2 Accrued expenses $ 10,922 $ 7,520

Stockholders_ Equity (Tables)

Stockholders’ Equity (Tables)	9 Months Ended
	Sep. 30, 2018
Class Of Stock Disclosures [Abstract]
Summary of common stock shares reserved for future issuance	The Company has reserved for future issuance the following number of shares of common stock: September 30, 2018 December 31, 2017 (unaudited) Common stock options outstanding 4,470,966 4,116,333 Options available for grant under Equity Incentive Plans 1,681,296 1,602,687 6,152,262 5,719,020

Stock-based Compensation (Table

Stock-based Compensation (Tables)	9 Months Ended
	Sep. 30, 2018
Summary of Stock Option Activity	Stock option activity for the nine months ended September 30, 2018 is as follows: Weighted average Weighted average Remaining Aggregate Options exercise contractual intrinsic outstanding price life (Years) value (in thousands) Balance as of December 31, 2017 4,116,333 $ 4.41 7.8 $ 63,577 Cancelled (123,867 ) $ 4.89 Granted 1,111,950 37.68 Exercised (633,450 ) 2.23 Balance as of September 30, 2018 4,470,966 $ 12.98 7.7 $ 176,206 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 Exercisable at September 30, 2018 2,213,100 2.24 6.6 $ 110,770 Vested at September 30, 2018 and expected to vest 4,470,966 12.98 7.7 $ 176,206
Employee and Non-employee Stock Options
Summary of Fair Value of Stock Options Granted Using Black-Scholes Options Pricing Model	The fair value of stock options was estimated using the following weighted-average assumptions for the three and nine months ended September 30, 2018 and September 30, 2017: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (unaudited) (unaudited) Expected volatility 77.3 - 86.4% 74.6 - 75.1% 74.9 - 86.5% 74.2 - 79.3% Weighted-average risk free rate 2.7 - 2.9% 1.9 - 2.0% 2.3 - 2.9% 1.9 - 2.1% Dividend yield —% —% —% —% Expected term (in years) 6.06 5.94 6.03 5.98
Summary of Stock-Based Compensation Expense Recognized in Statement of Operations by Classification	The table below summarizes the stock-based compensation expense recognized in the Company’s statement of operations by classification (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (unaudited) (unaudited) Research and development $ 1,473 $ 792 $ 3,900 $ 1,796 General and administrative 1,803 248 3,071 561 Total stock-based compensation expense $ 3,276 $ 1,040 $ 6,971 $ 2,357

Net Loss Per Common Share (Tabl

Net Loss Per Common Share (Tables)	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Schedule of Potentially Dilutive Securities Excluded from Computations of Diluted Weighted-average Shares Outstanding	For the three months ended September 30, 2018 and 2017 and for the nine months ended September 30, 2018 and 2017 the following potentially dilutive securities have been excluded from the computations of diluted weighted-average shares outstanding because the effect would be anti-dilutive: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (unaudited) (unaudited) Stock options issued and outstanding 4,482,649 3,894,192 4,258,269 3,805,419 Stock warrants — — — 15,223 4,482,649 3,894,192 4,258,269 3,820,642

Business Description - Addition

Business Description - Additional Information (Details)	9 Months Ended
	Sep. 30, 2018Product
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Number of clinical-stage product candidates from pipeline	3

Basis of Presentation and Sum_3

Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accounting Policies [Abstract]
Accounts payable	$ (1,648,000)	$ 1,629,000
Accrued expenses	3,167,000	3,297,000
Deferred rent	16,000	$ 37,000
Federal, state income tax benefit	0
Unrecognized income tax benefits	0		$ 0
Accrued income taxes	$ 0		$ 0

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Instruments and Respective Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets
Assets at fair value	$ 389,944	$ 102,897
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Assets at fair value	389,944	102,897
Money Market Funds
Assets
Assets at fair value	374,531	87,694
Money Market Funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Assets at fair value	374,531	87,694
Certificates of Deposit
Assets
Assets at fair value	15,413	15,203
Certificates of Deposit | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Assets at fair value	$ 15,413	$ 15,203

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Changes in valuation methodology	$ 0	$ 0	$ 0

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Accumulated depreciation	$ (265)	$ (181)
Property and equipment, net	1,006	510
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	223	112
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	553	283
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	293	174
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	190	121
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 12	$ 1

Property and Equipment - Additi

Property and Equipment - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Property Plant And Equipment [Abstract]
Depreciation expenses relating to property and equipment	$ 48	$ 27	$ 115	$ 59

Patent License Agreement - Addi

Patent License Agreement - Additional Information (Details) - University - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	22 Months Ended
	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2018
Patent License Agreement [Line Items]
License agreement date		Nov. 23, 2016
License agreement amendment date		Mar. 24, 2017
Milestone payments		$ 75	$ 100
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 such breach within forty-five (45) days after receipt of notice, (iii) makes a report to the University under the agreement that is determined 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
Maximum
Patent License Agreement [Line Items]
Milestone payments		$ 2,600
License
Patent License Agreement [Line Items]
Non-refundable license issue fee	$ 500

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued external research and professional fees	$ 2,629	$ 1,402
Accrued external clinical study costs	6,788	4,788
Accrued compensation expense	1,498	1,328
Deferred rent, current portion	7	2
Accrued expenses	$ 10,922	$ 7,520

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 21, 2018	Jun. 15, 2018	Mar. 12, 2018	Aug. 02, 2018	Aug. 02, 2018	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Class Of Stock [Line Items]
Common stock, shares authorized							120,000,000		120,000,000
Common stock, voting rights							Holders of common stock are entitled to one vote per share.
Proceeds from public offering, net of underwriting fees and commissions							$ 339,589	$ 108,503
Undesignated preferred stock, shares authorized to issue							5,000,000
Preferred stock, shares issued							0
Preferred stock, shares outstanding							0
Sales Agreement | Cowen and Company, LLC
Class Of Stock [Line Items]
Maximum gross proceeds of common stock allowed from issuance and sell		$ 125,000
Net proceeds from issuance of common stock				$ 24,000	$ 36,100
Sales Agreement | Cowen and Company, LLC | Common Stock
Class Of Stock [Line Items]
Common stock, shares issued					752,008
Sales Agreement | Cowen and Company, LLC | Common Stock | Weighted Average
Class Of Stock [Line Items]
Net proceeds from issuance of common stock						$ 12,100
Underwritten Public Offering
Class Of Stock [Line Items]
Common stock, shares issued	3,450,000		3,910,000
Common stock, price per share	$ 60		$ 29.50
Gross proceeds from issuance of common stock	$ 207,000		$ 115,300
Proceeds from public offering, net of underwriting fees and commissions	$ 194,900		$ 107,900
Underwriters Option to Purchase
Class Of Stock [Line Items]
Common stock, shares issued	450,000		510,000

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Common Stock Shares Reserved for Future Issuance (Details) - shares	Sep. 30, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Common stock, shares reserved for future issuance	6,152,262	5,719,020
Common Stock Options Outstanding
Class Of Stock [Line Items]
Common stock, shares reserved for future issuance	4,470,966	4,116,333
Options Available For Grant Under Equity Incentive Plan
Class Of Stock [Line Items]
Common stock, shares reserved for future issuance	1,681,296	1,602,687

Stock-based Compensation - Addi

Stock-based Compensation - Additional Information (Details) - USD ($) $ in Thousands	Jan. 01, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	May 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation				$ 6,971	$ 2,357
Employee Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options, maximum term				10 years
Stock-based compensation		$ 2,700	$ 500	$ 5,200	1,200
Non-employee Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation		$ 500	$ 500	$ 1,800	$ 1,200
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 additional shares approved for grant under equity incentive plan	1,066,692
Number of shares available for grant under equity incentive plan		1,681,296		1,681,296

Stock-based Compensation - Fair

Stock-based Compensation - Fair Value of Stock Options Granted Using Black-Scholes Options Pricing Model (Details) - Employee and Non-employee Stock Options	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	77.30%	74.60%	74.90%	74.20%
Expected volatility, maximum	86.40%	75.10%	86.50%	79.30%
Weighted-average risk free rate, minimum	2.70%	1.90%	2.30%	1.90%
Weighted-average risk free rate, maximum	2.90%	2.00%	2.90%	2.10%
Expected term (in years)	6 years 21 days	5 years 11 months 8 days	6 years 10 days	5 years 11 months 23 days

Stock-based Compensation - Summ

Stock-based Compensation - Summary of Stock-Based Compensation Expense Recognized in Statement of Operations by Classification (Details) - Employee and Non-employee Stock Options - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 3,276	$ 1,040	$ 6,971	$ 2,357
Research and development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	1,473	792	3,900	1,796
General and administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 1,803	$ 248	$ 3,071	$ 561

Stock-based Compensation - Su_2

Stock-based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Options outstanding
Beginning balance	4,116,333
Cancelled	(123,867)
Granted	1,111,950
Exercised	(633,450)
Ending balance	4,470,966	4,116,333
Exercisable	2,213,100	2,225,970
Vested and expected to vest	4,470,966	4,116,333
Weighted average exercise price
Beginning balance	$ 4.41
Cancelled	4.89
Granted	37.68
Exercised	2.23
Ending balance	12.98	$ 4.41
Exercisable	2.24	1.64
Vested and expected to vest	$ 12.98	$ 4.41
Weighted average, Remaining contractual for life (Years)
Balance	7 years 8 months 12 days	7 years 9 months 18 days
Exercisable	6 years 7 months 6 days	7 years 2 months 12 days
Vested and expected to vest	7 years 8 months 12 days	7 years 9 months 18 days
Weighted average, Aggregate intrinsic value
Balance	$ 176,206	$ 63,577
Exercisable	110,770	40,523
Vested and expected to vest	$ 176,206	$ 63,577

Net Loss Per Common Share (Deta

Net Loss Per Common Share (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computations of diluted weighted-average shares outstanding	4,482,649	3,894,192	4,258,269	3,820,642
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	4,482,649	3,894,192	4,258,269	3,805,419
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				15,223

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details)	9 Months Ended
	Sep. 30, 2018
Scientific Advisory | Chairman of the Board of Directors
Related Party Transaction [Line Items]
Consulting agreements expiration date	Jun. 30, 2020