Evelo Biosciences (EVLO) 10-Q 5 Nov 19 2019 Q3 Quarterly report Financial data

Cover Page

Cover Page - shares	9 Months Ended
	Sep. 30, 2019	Oct. 31, 2019
Cover page. [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Entity Registrant Name	Evelo Biosciences, Inc.
Entity Central Index Key	0001694665
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Emerging Growth Company	true
Small Business Entity	true
Entity Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		32,085,829

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 97,061	$ 93,101
Short-term investments	0	54,818
Prepaid expenses and other current assets	4,170	3,703
Total current assets	101,231	151,622
Property and equipment, net	8,339	6,925
Other assets	1,570	1,320
Total assets	111,140	159,867
Current liabilities:
Accounts payable	1,358	1,519
Accrued expenses	7,919	4,965
Other current liabilities	398	2,751
Total current liabilities	9,675	9,235
Noncurrent liabilities:
Long-term debt, net of current portion	19,549	12,305
Deferred rent, net of current portion	1,146	1,071
Other noncurrent liabilities	224	307
Total liabilities	30,594	22,918
Commitments and contingencies
Stockholder’s equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized; no shares issued or outstanding at September 30, 2019 and December 31, 2018, respectively	0	0
Common stock, $0.001 par value; 200,000,000 shares authorized; 32,147,073 and 31,951,540 shares issued and 32,072,030 and 31,825,769 shares outstanding at September 30, 2019 and December 31, 2018, respectively	32	32
Additional paid-in capital	256,768	250,316
Accumulated other comprehensive loss	0	(18)
Accumulated deficit	(176,254)	(113,381)
Total stockholders’ equity	80,546	136,949
Total liabilities and stockholders’ equity	$ 111,140	$ 159,867

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, authorized (in shares)	10,000,000	10,000,000
Preferred stock, issued (in shares)	0	0
Preferred stock, outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	32,147,073	31,951,540
Common stock, shares outstanding (in shares)	32,072,030	31,825,769

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Operating expenses:
Research and development	$ 15,610	$ 11,227	$ 46,751	$ 28,542
General and administrative	5,886	5,230	16,936	13,568
Total operating expenses	21,496	16,457	63,687	42,110
Loss from operations	(21,496)	(16,457)	(63,687)	(42,110)
Other income (expense):
Interest income, net	81	600	1,032	1,013
Other expenses	(218)	0	(218)	(406)
Other income (expense), net	(137)	600	814	607
Net loss	(21,633)	(15,857)	(62,873)	(41,503)
Convertible preferred stock dividends	0	0	0	(3,937)
Net loss attributable to common stockholders	$ (21,633)	$ (15,857)	$ (62,873)	$ (45,440)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.67)	$ (0.50)	$ (1.96)	$ (2.45)
Weighted-average number of common shares outstanding, basic and diluted (in shares)	32,060,747	31,741,683	32,009,571	18,532,408
Comprehensive loss:
Net loss	$ (21,633)	$ (15,857)	$ (62,873)	$ (41,503)
Other comprehensive loss:
Unrealized gain (loss) on investments, net of tax of $0	(1)	(38)	18	(38)
Comprehensive loss	$ (21,634)	$ (15,895)	$ (62,855)	$ (41,541)

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Tax portion of unrealized loss on investments	$ 0	$ 0	$ 0	$ 0

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Convertible Preferred Stock, Preferred Stock and Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Total	Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	Convertible Preferred Stock	Convertible Preferred Stock, Series B and C
Beginning balance (in shares) at Dec. 31, 2017							65,833,096
Beginning balance at Dec. 31, 2017							$ 83,702
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series B and Series C Preferred Stock, net of issuance costs (in shares)								25,482,199
Issuance of Series B and Series C Preferred Stock, net of issuance costs								$ 82,076
Ending balance (in shares) at Mar. 31, 2018							91,315,295
Ending balance at Mar. 31, 2018							$ 165,778
Beginning balance (in shares) at Dec. 31, 2017		0	3,880,607
Beginning balance at Dec. 31, 2017	$ (54,723)	$ 0	$ 4	$ 1,708	$ 0	$ (56,435)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)			31,557
Vesting of restricted common stock	11			11
Exercise of stock options and warrants (in shares)			31,765
Exercise of stock options and warrants	35			35
Stock-based compensation expense	652			652
Net loss	(10,500)					(10,500)
Ending balance (in shares) at Mar. 31, 2018		0	3,943,929
Ending balance at Mar. 31, 2018	(64,525)	$ 0	$ 4	2,406	0	(66,935)
Beginning balance (in shares) at Dec. 31, 2017							65,833,096
Beginning balance at Dec. 31, 2017							$ 83,702
Ending balance (in shares) at Sep. 30, 2018							0
Ending balance at Sep. 30, 2018							$ 0
Beginning balance (in shares) at Dec. 31, 2017		0	3,880,607
Beginning balance at Dec. 31, 2017	(54,723)	$ 0	$ 4	1,708	0	(56,435)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized gain on investments	(38)
Net loss	(41,503)
Ending balance (in shares) at Sep. 30, 2018		0	31,757,707
Ending balance at Sep. 30, 2018	150,759	$ 0	$ 32	248,703	(38)	(97,938)
Beginning balance (in shares) at Mar. 31, 2018							91,315,295
Beginning balance at Mar. 31, 2018							$ 165,778
Increase (Decrease) in Temporary Equity [Roll Forward]
Conversion of preferred stock into common stock (in shares)			22,386,677				(91,315,295)
Conversion of preferred stock into common stock	165,778		$ 22	165,756			$ (165,778)
Ending balance (in shares) at Jun. 30, 2018							0
Ending balance at Jun. 30, 2018							$ 0
Beginning balance (in shares) at Mar. 31, 2018		0	3,943,929
Beginning balance at Mar. 31, 2018	(64,525)	$ 0	$ 4	2,406	0	(66,935)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from Initial Public Offering, net of underwriting costs and commissions (in shares)			5,312,500
Proceeds from Initial Public Offering, net of underwriting costs and commissions	75,798		$ 5	75,793
Reclassification of warrant liability	879			879
Vesting of restricted common stock (in shares)			31,572
Vesting of restricted common stock	11			11
Exercise of stock options and warrants (in shares)			51,408
Exercise of stock options and warrants	5		$ 1	4
Stock-based compensation expense	2,247			2,247
Net loss	(15,146)					(15,146)
Ending balance (in shares) at Jun. 30, 2018		0	31,726,086
Ending balance at Jun. 30, 2018	165,047	$ 0	$ 32	247,096	0	(82,081)
Ending balance (in shares) at Sep. 30, 2018							0
Ending balance at Sep. 30, 2018							$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)			25,816
Vesting of restricted common stock	7			7
Exercise of stock options and warrants (in shares)			5,805
Exercise of stock options and warrants	5			5
Stock-based compensation expense	1,595			1,595
Unrealized gain on investments	(38)				(38)
Net loss	(15,857)					(15,857)
Ending balance (in shares) at Sep. 30, 2018		0	31,757,707
Ending balance at Sep. 30, 2018	150,759	$ 0	$ 32	248,703	(38)	(97,938)
Beginning balance (in shares) at Dec. 31, 2018							0
Beginning balance at Dec. 31, 2018							$ 0
Ending balance (in shares) at Mar. 31, 2019							0
Ending balance at Mar. 31, 2019							$ 0
Beginning balance (in shares) at Dec. 31, 2018			31,825,769
Beginning balance at Dec. 31, 2018	136,949		$ 32	250,316	(18)	(113,381)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)			23,345
Vesting of restricted common stock	7			7
Exercise of stock options and warrants (in shares)			181,521
Exercise of stock options and warrants	257			257
Stock-based compensation expense	1,953			1,953
Unrealized gain on investments	16				16
Net loss	(20,299)					(20,299)
Ending balance (in shares) at Mar. 31, 2019			32,030,635
Ending balance at Mar. 31, 2019	118,883		$ 32	252,533	(2)	(133,680)
Beginning balance (in shares) at Dec. 31, 2018							0
Beginning balance at Dec. 31, 2018							$ 0
Ending balance (in shares) at Sep. 30, 2019							0
Ending balance at Sep. 30, 2019							$ 0
Beginning balance (in shares) at Dec. 31, 2018			31,825,769
Beginning balance at Dec. 31, 2018	$ 136,949		$ 32	250,316	(18)	(113,381)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options and warrants (in shares)	195,533
Unrealized gain on investments	$ 18
Net loss	(62,873)
Ending balance (in shares) at Sep. 30, 2019			32,072,030
Ending balance at Sep. 30, 2019	80,546		$ 32	256,768	0	(176,254)
Beginning balance (in shares) at Mar. 31, 2019							0
Beginning balance at Mar. 31, 2019							$ 0
Ending balance (in shares) at Jun. 30, 2019							0
Ending balance at Jun. 30, 2019							$ 0
Beginning balance (in shares) at Mar. 31, 2019			32,030,635
Beginning balance at Mar. 31, 2019	118,883		$ 32	252,533	(2)	(133,680)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)			13,692
Vesting of restricted common stock	7			7
Exercise of stock options and warrants (in shares)			1,379
Exercise of stock options and warrants	1			1
Stock-based compensation expense	2,135			2,135
Unrealized gain on investments	3				3
Net loss	(20,941)					(20,941)
Ending balance (in shares) at Jun. 30, 2019			32,045,706
Ending balance at Jun. 30, 2019	100,088		$ 32	254,676	1	(154,621)
Ending balance (in shares) at Sep. 30, 2019							0
Ending balance at Sep. 30, 2019							$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)			13,691
Vesting of restricted common stock	7			7
Exercise of stock options and warrants (in shares)			12,633
Exercise of stock options and warrants	23			23
Stock-based compensation expense	2,062			2,062
Unrealized gain on investments	(1)				(1)
Net loss	(21,633)					(21,633)
Ending balance (in shares) at Sep. 30, 2019			32,072,030
Ending balance at Sep. 30, 2019	$ 80,546		$ 32	$ 256,768	$ 0	$ (176,254)

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Operating activities
Net loss	$ (62,873)	$ (41,503)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	6,150	4,494
Depreciation expense	1,264	1,575
Change in fair value of warrant and debt derivative liability	0	406
Net (accretion of discount)/amortization of premium on marketable securities	(164)	0
Non-cash interest expense	263	78
Gain on sale of fixed assets	(2)	0
Changes in assets and liabilities:
Prepaid expenses and other current assets	(633)	(3,180)
Accounts payable	90	1,375
Accrued expenses and other current liabilities	2,907	4,314
Other liabilities	12	618
Net cash used in operating activities	(52,986)	(31,823)
Investing activities
Purchase of investments	0	(84,653)
Proceeds from sales and maturities of investments	55,000	0
Purchases of property and equipment	(2,569)	(4,227)
Proceeds from the sale of fixed assets	2	171
Net cash provided by/(used in) investing activities	52,433	(88,709)
Financing activities
Proceeds from the issuance of common stock upon completion of initial public offering	0	85,000
Net proceeds from the issuance of convertible preferred stock	0	81,336
Net proceeds from the issuance of long-term debt	19,481	4,975
Repayment of long-term debt	(15,000)	0
Cash payment of initial public offering issuance costs	0	(9,171)
Settlement of derivative liability	0	(250)
Proceeds from the exercise of stock options, restricted common stock and warrants	282	104
Net cash provided by financing activities	4,763	161,994
Net increase in cash, cash equivalents and restricted cash	4,210	41,462
Cash, cash equivalents and restricted cash – beginning of period	94,351	39,746
Cash, cash equivalents and restricted cash – end of period	98,561	81,208
Supplemental disclosure of cash flow information
Cash paid for interest	728	535
Noncash investing and financing activities
Conversion of convertible preferred stock into common stock upon closing of initial public offering	0	165,778
Property and equipment additions in accounts payable and accrued expenses	290	176
Conversion of convertible preferred stock warrants into common stock warrants	0	819
Issuance of debt derivative liability in connection with long-term debt facility	0	150
Issuance of warrants in connection with long-term debt facility	$ 0	$ 89

Organization

Organization	9 Months Ended
	Sep. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization	Organization Evelo Biosciences, Inc. ("Evelo" or the "Company”) is a biotechnology company which was incorporated in Delaware on May 6, 2014. The Company is discovering and developing oral biologics designed to engage immune cells in the small intestine and drive therapeutic effects throughout the body. The Company is advancing these monoclonal microbials with the aim of treating a broad range of immune mediated diseases with an initial focus on inflammatory diseases and oncology. The Company is headquartered in Cambridge, Massachusetts. Since inception, the Company has devoted substantially all of its efforts to research and development and raising capital. The Company has not generated any revenue related to its primary business purpose to date . The Company is subject to a number of risks similar to those of other development stage companies, including dependence on key individuals, the need to develop commercially viable products, competition from other companies, many of whom are larger and better capitalized, and the need to obtain adequate additional financing to fund the development of its products. The Company has funded its operations from the issuance of convertible notes, convertible preferred stock, common stock and debt financing. At September 30, 2019 , the Company had cash and cash equivalents of $97.1 million and an accumulated deficit of $176.3 million . The Company has incurred operating losses since inception and expects such losses and negative operating cash flows to continue for the foreseeable future. The transition to profitability is dependent upon the successful development, approval, and commercialization of its products and product candidates and the achievement of a level of revenues adequate to support its cost structure. Based on the Company’s current operating plan, the Company believes that its cash and cash equivalents at September 30, 2019 , along with the capacity to borrow an additional $10.0 million under the 2019 Credit Facility (see note 6), will be sufficient to fund operations and capital expenditures for at least the twelve months following the filing of this Quarterly Report on Form 10-Q. Management’s belief with respect to its ability to fund operations is based on estimates that are subject to risks and uncertainties. If actual results are different from management’s estimates, the Company may need to seek additional funding sooner than would otherwise be expected. There can be no assurance that the Company will be able to obtain additional funding on acceptable terms, if at all.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Presentation The accompanying unaudited interim condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standard Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these financial statements should be read in conjunction with the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018 and notes thereto. The unaudited interim consolidated financial statements have been prepared on the same basis as the audited financial statements. In the opinion of the Company’s management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary to present fairly the Company’s financial position as of September 30, 2019 , the results of its operations and stockholders' equity for the three and nine months ended September 30, 2019 and 2018 and cash flows for the nine months ended September 30, 2019 and 2018 . Such adjustments are of a normal and recurring nature. The results for the three and nine months ended September 30, 2019 are not necessarily indicative of the results for the year ending December 31, 2019 , or for any future period. Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, the accrual of research and development expenses and the valuation of stock-based awards. The Company bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. Actual results could differ from those estimates. Principles of Consolidation The unaudited condensed consolidated financial statements include the accounts of the Company and its wholly owned, controlled subsidiaries. All intercompany transactions and balances have been eliminated in consolidation. Subsequent Event Considerations The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the consolidated financial statements to provide additional evidence for certain estimates or to identify matters that require additional disclosure. Subsequent events have been evaluated as required. The Company has evaluated all subsequent events and determined that there are no material recognized or unrecognized subsequent events requiring disclosure, other than those disclosed in this Quarterly Report on Form 10-Q. Emerging Growth Company Status Evelo is an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act of 2012 (the "JOBS Act"), and it may take advantage of reduced reporting requirements that are otherwise applicable to public companies. Evelo may take advantage of these exemptions until it is no longer an emerging growth company. Section 107 of the JOBS Act provides that an emerging growth company can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. Evelo has elected to use the extended transition period for complying with new or revised accounting standards, and, as a result of this election, its consolidated financial statements may not be comparable to companies that comply with public company effective dates. Evelo may take advantage of these exemptions up until the last day of the fiscal year following the fifth anniversary of its IPO or such earlier time that it is no longer an emerging growth company. Evelo would cease to be an emerging growth company if it has more than $1.07 billion in annual revenue, it has more than $700.0 million in market value of its stock held by non-affiliates (and has been a public company for at least 12 months and has filed one annual report on Form 10-K), or it has issued more than $1.0 billion of non-convertible debt securities over a three -year period. Comprehensive Loss Comprehensive loss includes net loss and certain changes in stockholders’ equity that are excluded from net loss. The Company's only element of other comprehensive income (loss) is unrealized gains and losses on available-for-sale investments. Comprehensive loss totaled $21.6 million and $62.9 million for the three and nine months ended September 30, 2019 , respectively, and $15.9 million and $41.5 million for the three and nine months ended September 30, 2018 , and was not significantly different than net loss in any period. For the nine months ended September 30, 2018 comprehensive loss was equal to net loss. Cash, Cash Equivalents, and Restricted Cash Cash equivalents are comprised of highly liquid investments that are readily convertible into cash with original maturities of three months or less. Cash and cash equivalents include cash held in banks and amounts held in money market funds and U.S. treasuries with original maturities of three months or less. Cash equivalents are stated at cost, which approximates market value. The Company’s restricted cash consists of restricted cash in connection with building leases for the Company’s office and laboratory premises and deposits held in relation to the company's credit card facility. Restricted cash totaled approximately $1.5 million and $1.3 million at September 30, 2019 and December 31, 2018 , respectively, and is classified within other assets on the accompanying condensed consolidated balance sheet. The following reconciles cash, cash equivalents and restricted cash as of September 30, 2019 and December 31, 2018 , as presented on the Company's statements of cash flows, to its related balance sheet accounts (in thousands): September 30, December 31, Cash and cash equivalents: Cash $ 1,304 $ 1,300 Money market funds 95,757 91,801 Total cash and cash equivalents 97,061 93,101 Restricted cash 1,500 1,250 Cash, cash equivalents and restricted cash $ 98,561 $ 94,351 Investments The Company accounts for and classifies its investments as either “available-for-sale,” “trading,” or “held-to-maturity,” in accordance with the accounting guidance related to the accounting and classification of certain investments in debt and equity securities. The determination of the appropriate classification is based primarily on management’s intent to sell the investment at the time of purchase. The Company did not have any investments at September 30, 2019 . As of December 31, 2018 all of the Company's investments were classified as available‑for‑sale securities. Available‑for‑sale securities are those securities which the Company views as available for use in current operations, if needed. The Company generally classifies its available‑for‑sale securities as short‑term investments, even though the stated maturity date may be one year or more beyond the current balance sheet date. Available‑for‑sale investments are stated at fair value with their unrealized gains and losses included in accumulated other comprehensive loss within stockholders’ (deficit) equity, until such gains and losses are realized in other income (expense) within the condensed consolidated statements of operations and comprehensive loss or until an unrealized loss is considered other‑than‑temporary. The Company recognizes other‑than‑temporary impairments of its investments in debt securities when there is a decline in fair value below the amortized cost basis and if (a) it has the intent to sell the security or (b) it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis. If either of these conditions is met, the Company recognizes the difference between the amortized cost of the security and its fair value at the impairment measurement date in the condensed consolidated statements of operations and comprehensive loss. If neither of these conditions is met, the Company must perform an additional analysis to evaluate whether the unrealized loss is associated with the creditworthiness of the issuer of the security rather than other factors, such as interest rates or market factors. If the Company determines from this analysis that it does not expect to receive cash flows sufficient to recover the entire amortized cost of the security, a credit loss exists, the impairment is considered other-than-temporary and is recognized in its condensed consolidated statements of operations and comprehensive loss. Fair Value of Financial Instruments ASC 820, Fair Value Measurement (“ASC 820”), establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability and are developed based on the best information available in the circumstances. ASC 820 identifies fair value as the exchange price, or exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, ASC 820 establishes a three-tier fair value hierarchy that distinguishes between the following: • Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities; • Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly; and • Level 3 inputs are unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. An entity may choose to measure many financial instruments and certain other items at fair value at specified election dates. Subsequent unrealized gains and losses on items for which the fair value option has been elected will be reported in earnings. The Company did not elect to measure any additional financial instruments or other items at fair value. Research and Development Costs Research and development costs are expensed in the period incurred. Research and development expenses consist of both internal and external costs such as payroll, consulting, and manufacturing costs associated with the development of the Company’s product candidates. Costs for certain development activities, such as clinical trials and manufacturing development activities, are recognized based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, and information provided to the Company by its vendors on their actual costs incurred or level of effort expended. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as prepaid or accrued research and development expenses. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred and capitalized. The capitalized amounts are expensed as the related goods are delivered or the services are performed. The Company has and may continue to acquire the rights to develop and commercialize new product candidates from third parties. The upfront payments to acquire license, product or rights, as well as any future milestone payments, are immediately recognized as research and development expense provided that there is no alternative future use of the rights in other research and development projects. Any milestone payments made for Intellectual Property after regulatory approval, or that have alternative future use, are capitalized and amortized. Stock-Based Compensation The Company records stock-based compensation for options granted to employees and directors based on the grant date fair value of awards issued. The expense is recorded over the requisite service period, which is the vesting period, on a straight-line basis. The Company uses the Black-Scholes option-pricing model to determine the fair value of stock options. The determination of the fair value of stock options on the date of grant using an option-pricing model is affected by the Company’s common stock price, as well as a number of other assumptions. The Company records forfeitures as they occur. The Company accounts for stock-based compensation arrangements with non-employees based upon the fair value of the consideration received or the equity instruments issued, whichever is more reliably measurable. The measurement date for non-employee awards is generally the date performance of services required from the non-employee is complete. Stock-based compensation costs for non-employee awards are recognized as services are provided, which is generally the vesting period, on a straight-line basis. The unvested portion of the stock options is subject to re-measurement over the vesting period and forfeitures are recorded as they occur. Segments The Company has one operating segment. The Company's chief operating decision maker, its Chief Executive Officer, manages the Company's operations on a consolidated basis for the purposes of allocating resources. Recently Adopted Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), and further updated through ASU 2016-12 (“ASU 2016-12”), which amends the existing accounting standards for revenue recognition. ASU 2014-09 is based on principles that govern the recognition of revenue at an amount to which an entity expects to be entitled when products are transferred to customers. This guidance is effective for annual reporting periods, and interim periods within those years, beginning after December 15, 2017, for public entities and no later than for annual reporting periods beginning after December 15, 2018, for non-public entities. The new revenue standard may be applied retrospectively to each prior period presented or retrospectively with the cumulative effect recognized as of the date of adoption. The Company adopted ASU 2014-09 on January 1, 2019 and has concluded that the adoption did not have a material impact on its consolidated financial statements as the Company is not yet generating revenues. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808)-Clarifying the Interaction between Topic 808 and Topic 606 ("ASU 2018-18"). The amendments in ASU 2018-18 make targeted improvements to U.S. GAAP for collaborative arrangements by clarifying that certain transactions between collaborative arrangement participants should be accounted for as revenue under Topic 606 when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in Topic 606 should be applied, including recognition, measurement, presentation, and disclosure requirements. In addition, unit-of-account guidance in Topic 808 was aligned with the guidance in Topic 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of Topic 606. ASU 2018-18 is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. Early adoption is permitted, including adoption in any interim period. The amendments in this Update should be applied retrospectively to the date of initial application of Topic 606. The Company adopted ASU 2018-18 on January 1, 2019 and has concluded the adoption did not have a material impact on its consolidated financial statements as the Company does not have any collaborative agreements under which any participant is considered a customer of the Company. Accounting Pronouncements Issued and Not Adopted as of September 30, 2019 In February 2016, the FASB issued ASU 2016-2, Leases (Topic 842) (“ASU 2016-2”), which supersedes the guidance in former ASC 840, Leases. The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to the historical guidance for operating leases. This guidance is effective for annual reporting periods, and interim periods within those years, beginning after December 15, 2018 for most public entities. Early adoption is permitted for all entities. Although its assessment is not complete, the Company currently expects the adoption of this guidance to result in the addition of material balances of leased assets and corresponding lease liabilities to its consolidated balance sheets, primarily relating to leases of office space. In June 2018, the FASB issued ASU No. 2018-07, Stock-based Compensation: Improvements to Nonemployee Share-based Payment Accounting (Topic 718) ("ASU 2018-07"), which amends the existing accounting standards for share-based payments to nonemployees. This ASU aligns much of the guidance on measuring and classifying nonemployee awards with that of awards to employees. Under the new guidance, the measurement of nonemployee equity awards is fixed on the grant date. This ASU becomes effective in the first quarter of fiscal year 2020 and early adoption is permitted but no earlier than the Company's adoption date of Topic 606. Entities will apply the ASU by recognizing a cumulative-effect adjustment to retained earnings as of the beginning of the annual period of adoption. The Company is currently evaluating the impact that ASU 2018-07 will have on its consolidated financial statements.

Investments

Investments	9 Months Ended
	Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Investments	Investments As of September 30, 2019 the Company did not hold any short-term investments. As of December 31, 2018 , the Company had short-term investments, consisting entirely of U.S. treasury securities, of $54.8 million , respectively. The following tables summarize the Company's investments held at December 31, 2018 , which are all classified as available-for-sale (in thousands): Description Amortized cost Unrealized Gain Unrealized Loss Fair Value December 31, 2018: U.S. treasury securities $ 54,836 $ — $ (18 ) $ 54,818 Total $ 54,836 $ — $ (18 ) $ 54,818 The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At September 30, 2019 the Company had no accumulated other comprehensive loss. There were no material realized gains or losses recognized on the sale or maturity of available-for-sale securities during the three and nine months ended September 30, 2019 or 2018, and as a result, the Company did no t reclassify any amounts out of accumulated other comprehensive loss for the same periods. As of December 31, 2018 , the aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months was $54.8 million , and none of these securities had remaining maturities of greater than three years. The Company has the intent and ability to hold such securities until recovery. The Company determined that there has been no material change in the credit risk of the above investments. As a result, the Company determined it did not hold any investments with any other-than-temporary impairment as of December 31, 2018 .

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The following tables present information about the Company’s financial assets and liabilities that have been measured at fair value as of September 30, 2019 and December 31, 2018 (in thousands): Description September 30, Active Markets (Level 1) Observable Inputs (Level 2) Unobservable Inputs (Level 3) Assets: Money market funds included within cash and cash equivalents $ 95,757 $ 95,757 $ — $ — Total $ 95,757 $ 95,757 $ — $ — Description December 31, Active Markets (Level 1) Observable Inputs (Level 2) Unobservable Inputs (Level 3) Assets: Money market funds included within cash and cash equivalents $ 91,801 $ 91,801 $ — $ — U.S. treasury securities included within short-term investments 54,818 — 54,818 — Total $ 146,619 $ 91,801 $ 54,818 $ — As of September 30, 2019 and December 31, 2018 , the Company's cash equivalents and short-term investments have been initially valued at the transaction price and subsequently valued utilizing a third party pricing service. The Company validates the prices provided by its third-party pricing service by understanding the models used and obtaining market values from other pricing sources.

Property and Equipment, Net

Property and Equipment, Net	9 Months Ended
	Sep. 30, 2019
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	Property and Equipment, Net Property and equipment consists of the following (in thousands): September 30, December 31, Property and equipment: Lab equipment $ 7,300 $ 5,393 Leasehold improvements 1,944 1,824 Furniture and fixtures 701 525 Computers and software 200 115 Office equipment 9 9 Construction-in-process 1,394 1,011 Property and equipment 11,548 8,877 Less: accumulated depreciation (3,209 ) (1,952 ) Property and equipment, net $ 8,339 $ 6,925 The Company recognized $0.5 million and $1.3 million of depreciation expense for the three and nine months ended September 30, 2019 , respectively, and $0.3 million and $1.6 million for the three and nine months ended September 30, 2018 , respectively.

Loan and Security Agreements

Loan and Security Agreements	9 Months Ended
	Sep. 30, 2019
Debt Disclosure [Abstract]
Loan and Security Agreements	Loan and Security Agreements 2016 Credit Facility In 2016, the Company entered into a credit facility (the “2016 Credit Facility”) with a bank that allowed the Company to borrow up to $15.0 million . Borrowings under the 2016 Credit Facility were secured by a lien on all Company assets, excluding intellectual property. Prior to 2018, a total of $10.0 million was drawn under the 2016 Credit Facility, and in February 2018, the Company drew the remaining $5.0 million available. This resulted in an increase to the interest rate to the higher of (i) prime plus 0.25% or (ii) 4.50% per annum and extended the interest only payment period was extended through to August 15, 2019. Upon the expiration of the interest only period, amounts borrowed will be repaid over 24 equal monthly payments of principal plus interest accrued through August 15, 2021 . The Company had the ability to prepay the outstanding balance of the 2016 Credit Facility at its option with a prepayment fee of 2% of principal amount if prepayment was made before August 15, 2018 or 0.5% if the prepayment was made between August 15, 2018 and August 15, 2019 . In conjunction with the February 2018 drawdown, the Company issued a warrant to purchase up to 34,722 shares of the Company’s Series B preferred stock at an exercise price of $1.80 per share. As part of the February 2018 drawdown, the loan and security agreement was amended to include the payment of a $0.3 million success fee to the financial institution in the event of a liquidation event, including an initial public offering. The success fee represented an embedded derivative which the Company bifurcated from the debt arrangement and carried at fair value. In May 2018, the Company completed its initial public offering (the "IPO") and paid the success fee of $0.3 million . In addition, the warrant issued in February 2018 was exercised in May 2018. The 2016 Credit Facility contained negative covenants restricting the Company’s activities, including limitations on cash deposits, dispositions, mergers or acquisitions, incurring indebtedness or liens, paying dividends or making investments and certain other business transactions. There were no financial covenants associated with the agreement. 2019 Credit Facility On July 19, 2019, the Company entered into a loan and security agreement (as amended, the "2019 Credit Facility") with K2 HealthVentures LLC ("K2HV") pursuant to which the K2HV agreed to make term loans in an aggregate principal amount of up to $45.0 million available to the Company in three tranches. The initial tranche of $20.0 million was funded upon closing on July 19, 2019. The second tranche of $10.0 million is available to be funded at the Company's election between December 1, 2019 and June 1, 2020, subject to certain customary conditions. The third tranche of $15.0 million is available to be funded at the Company's election on or before January 15, 2021, subject to certain customary conditions and the achievement of certain clinical development milestones. Borrowings under the Loan Agreement are collateralized by substantially all of the Company's personal property, excluding intellectual property, and the Company pledged its equity interests in its subsidiaries, subject to certain limitations with respect to its foreign subsidiaries. Interest on the outstanding loan balance will accrue at a variable annual rate equal to the greater of (i) 8.65% and (ii) the prime rate plus 3.15% . The Company is required to make interest-only payments on the loans on a monthly basis through February 28, 2022. If the Company elects to draw the third tranche, the interest-only period is extended through August 31, 2022. Subsequent to the interest only periods, the Company is required to make equal monthly payments of principal plus interest until the loans mature on August 1, 2024. Upon final payment or prepayment of the loans, the Company must pay a final payment equal to 4.3% of the loans borrowed, which is being accrued to interest expense over the term of the loan using the effective-interest method. The Company incurred fees associated with establishing the facility of $0.4 million . The Company has an option to prepay the loans in whole, subject to a prepayment fee of 2% of the amount prepaid or, if the prepayment occurs after the 18 -month anniversary of the funding date of the loans, 1% of the amount prepaid. The Loan Agreement contains customary representations, warranties and covenants and also includes customary events of default, including payment defaults, breaches of covenants, change of control and occurrence of a material adverse effect. The Company has determined that the risk of subjective acceleration under the material adverse events clause was remote and therefore has classified the long-term portion of the outstanding principal in non-current liabilities. Upon the occurrence and continuation of an event of default, a default interest rate of an additional 5% per annum may be applied to the outstanding loan balances, and the administrative agent, collateral agent, and lenders may declare all outstanding obligations immediately due and payable and exercise all of their rights and remedies as set forth in the Loan Agreement and under applicable law. The Company used the proceeds from the initial $20.0 million tranche to prepay the full $15.0 million loan balance outstanding under the 2016 Credit Facility on July 19, 2019. The Company has the following minimum aggregate future loan payments at September 30, 2019 (in thousands): Twelve month period ending September 30, Amount 2020 $ 1,759 2021 1,754 2022 6,318 2023 9,022 2024 8,515 Total minimum payments $ 27,368 Less amounts representing interest and discount (7,819 ) Long-term debt $ 19,549 Interest expense related to the Company's 2016 Credit Facility was approximately $0.5 million for the nine months ended September 30, 2019 and $0.2 million and $0.6 million for the three and nine months ended September 30, 2018 , respectively. Interest expense related to the Company's 2019 Credit Facility was approximately $0.4 million for both the three and nine months ended September 30, 2019 .

In-License Agreements

In-License Agreements	9 Months Ended
	Sep. 30, 2019
Research and Development [Abstract]
In-License Agreements	In-License Agreements Mayo Foundation for Medical Education and Research On June 10, 2016, the Company entered into a Research and License Agreement, (the “2016 Mayo License Agreement”) with Mayo Foundation for Medical Education and Research (“Mayo”). Under the Mayo License Agreement, Mayo was entitled to certain participation rights in connection with the issuance and sale of Series B Preferred Stock. The 2016 Mayo License Agreement allowed Mayo to purchase shares at the same price paid as other investors and is considered to be a fair value contract. In 2017, Mayo purchased 1,666,667 shares of Series B Preferred Stock at $1.80 per share. Also pursuant to the 2016 Mayo License Agreement, Mayo received 490 shares of common stock upon the completion of certain project milestones as well as warrants to purchase common stock (the “Mayo Warrants”) exercisable for 18 shares and 116 shares of common stock upon the completion of certain additional project milestones. The Mayo Warrants were fully vested and expensed in 2016. On April 9, 2018, Mayo Foundation exercised its warrant and was issued 134 shares of common stock. On August 6, 2017, the Company and Mayo entered into a license agreement (“2017 Mayo License Agreement”). Under the 2017 Mayo License Agreement, Mayo granted the Company (i) an exclusive, worldwide, sublicensable license under Mayo’s rights to certain intellectual property and microbial strains (ii) a non-exclusive, worldwide, sublicensable license to certain related know-how, in each case, to develop and commercialize certain microbial strains and licensed products incorporating any such strains. As consideration, the Company paid a nonrefundable upfront fee of $0.2 million and annual license maintenance fees. Nonrefundable upfront fees were expensed in full to research and development expense in 2017. Annual maintenance fees will be expensed as incurred over the term of the agreement. The Company may owe Mayo milestone payments upon the achievement of certain development, regulatory, and commercial milestones, up to a maximum of $56.0 million in the aggregate, as well as royalties on net sales of licensed products in low single-digit percentages. As of September 30, 2019 , the Company has incurred milestone payments to date totaling approximately $0.2 million under the agreement of which no amounts are currently due. University of Chicago On March 10, 2016, the Company and the University of Chicago entered into a patent license agreement (“2016 University of Chicago Agreement”). Under the 2016 University of Chicago Agreement, the University of Chicago granted the Company (i) an exclusive, royalty-bearing and sublicensable license under the Licensed Patents and (ii) a non-exclusive, royalty-bearing, sublicensable license to access the technical information to diligently develop and commercialize Licensed Products. As consideration, the Company paid a nonrefundable upfront fee of less than $0.5 million and will pay annual license maintenance fees. Nonrefundable upfront fees were expensed in full to research and development expense in 2016. Annual maintenance fees will be expensed as incurred over the term of the agreement. The Company may owe the University of Chicago milestone payments, totaling an aggregate of approximately $60.9 million upon the achievement of certain development, regulatory, and commercial milestones, as well as royalties on net sales of licensed products ranging from low to high single-digit percentages. As of September 30, 2019 , the Company has incurred milestone payments to date totaling approximately $0.4 million . In addition, the Company also agreed to pay the University of Chicago a share of sublicense revenue. The University of Chicago maintains control of patent prosecution, defense and maintenance on their patent rights. The Company has the first right, but not obligation, to control any post-grant proceedings and to take action in the prosecution or prevention of any infringement by a third party to patent rights. The Company is responsible for reimbursing the University of Chicago, or directly paying, for patent prosecution, defense and/or maintenance costs incurred. This includes any costs associated with defense activities related to one of the patents underlying the 2016 University of Chicago Agreement and to which in April 2019 the United States Patent and Trademark Office instituted a post-grant review based on a petition from a third party. For the nine months ended September 30, 2019 , the Company has incurred expenses of $1.1 million related to the post grant review.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Lease Obligations In January 2018, the Company entered into an operating sublease arrangement to lease approximately 40,765 square feet for its office and research development space at 620 Memorial Drive, Cambridge, MA 02139 from February 2018 to September 2025. The Company maintains an additional separate operating lease for office and laboratory space that is scheduled to expire in 2020 . The leases require security deposits, which the Company has primarily met with letters of credit from a financial institution that are secured with cash on deposit. In June 2018, the Company entered into a sublease arrangement with a third party to lease space subject to an operating lease that is scheduled to expire in 2020 . The future minimum rental payments to be received under this agreement total $0.3 million and are equivalent to the minimum payments due from the Company to the landlord. The Company recorded rent expense of $0.7 million and $2.0 million for the three and nine months ended September 30, 2019 , respectively. Rent expense for the three and nine months ended September 30, 2019 is net of sublease rental income of $0.1 million and $0.4 million , respectively. Rent expense totaled $0.8 million and $1.5 million for the three and nine months ended September 30, 2018 , which is net of sublease rental income of $0.1 million in both periods. The minimum aggregate future lease commitments, exclusive of any offsetting sublease rental payments, at September 30, 2019 , are as follows (in thousands): Twelve month period ending September 30, Amount 2020 $ 3,187 2021 2,951 2022 3,040 2023 3,131 2024 3,225 Thereafter 3,321 $ 18,855 Collaboration Agreement with Sacco S.r.l. In July 2019, the Company entered into a Collaboration Agreement with Sacco S.r.l. ("Sacco"), an affiliate of one of the Company’s existing contract manufacturing organizations, pursuant to which and subject to certain exceptions for pre-existing products for pre-existing customers, Sacco will manufacture and supply single strain, non-genetically modified microbes intended for oral delivery or oral use in pharmaceutical products exclusively for the Company for a period of five years . Sacco may terminate the agreement if the provision of manufacturing services has been, or is scheduled to be, inactive for a period of six consecutive months. The Company has agreed to pay Sacco an aggregate of €3.0 million , €0.6 million annually, during the exclusivity period. The Company recognized €0.6 million in expense associated with this agreement during the three months ended September 30, 2019 . Equipment Funding Arrangement In July 2019, the Company entered into an arrangement with one of its external manufacturing partners providing the Company with priority access to future manufacturing services which will be rendered using certain dedicated equipment. In return for such access, the Company committed to provide funding for the purchase of the dedicated equipment in an aggregate amount of £0.8 million , £0.4 million of which to be paid upfront and, subject to the manufacturer's installation and qualification of the equipment, an additional two installments of £0.2 million will be paid in January and April 2020. Biose Industrie On February 15, 2018, the Company entered into an Exclusivity and Commitment Agreement with Biose Industrie (“Biose”), a French corporation, in which Biose has agreed to exclusively manufacture certain microbial biotherapeutic products for the Company and reserve agreed upon manufacturing resources to conduct manufacturing runs for such products. Under the terms of this agreement, the Company agreed to annual fees in the mid-six digits in consideration of both exclusivity for the manufacture of monoclonal microbials and for a set minimum number of manufacturing runs per year. Exclusivity fees paid and any minimum commitments are expensed as incurred. The Company has agreed to pay an exclusivity fee of $0.3 million per year. Litigation and Other Proceedings The Company may periodically become subject to legal proceedings and claims arising in connection with on-going business activities, including claims or disputes related to patents that have been issued or that are pending in the field of research on which the Company is focused. The Company is not a party to any material litigation and does not have contingency reserves established for any litigation liabilities. In April 2019, the United States Patent and Trademark Office ("USPTO"), granted a third party petition to initiate a post-grant review of a patent issued to the University of Chicago, to which the Company has an exclusive license from the University of Chicago. Although the Company believes that the subject patent is valid, there is a possibility that the USPTO could invalidate the patent or require the University of Chicago to narrow the claims contained in the patent. Under the terms of the license agreement, the Company is responsible for reimbursing the University of Chicago for patent defense costs.

Stockholders_ Equity and Conver

Stockholders’ Equity and Convertible Preferred Stock	9 Months Ended
	Sep. 30, 2019
Temporary Equity Disclosure [Abstract]
Stockholders’ Equity and Convertible Preferred Stock	Stockholders’ Equity and Convertible Preferred Stock Common Stock On April 27, 2018, the Company filed an amendment to its certificate of incorporation with the Secretary of State of the State of Delaware, to effect a 1-for-4.079 reverse stock split of the Company’s common stock. All share and per share data shown in the accompanying condensed consolidated financial statements and related notes have been retroactively revised to reflect the reverse stock split. On May 11, 2018, the Company completed an IPO of 5,312,500 shares of its common stock for aggregate gross proceeds of $85.0 million . The Company received approximately $75.8 million in net proceeds after deducting underwriting discounts and commissions and other estimated offering expenses payable by the Company. Upon closing of the IPO, all of the outstanding shares of convertible preferred stock automatically converted into 22,386,677 shares of common stock at the applicable conversion ratio then in effect On May 11, 2018, the Company filed a restated certificate of incorporation with the Secretary of the State of Delaware, which became effective in connection with the closing of the IPO. Pursuant to the restated certificate of incorporation, the Company is authorized to issue 200,000,000 shares of common stock and 10,000,000 shares of preferred stock. On June 3, 2019, the Company filed a Registration Statement on Form S-3 (File No. 333-231911) (the “Shelf”) with the SEC in relation to the registration of common stock, preferred stock, debt securities, warrants and/or units of any combination thereof in the aggregate amount of up to $200.0 million for a period up to three years from the date of the filing. The Company also simultaneously entered into a sales agreement with Cowen and Company, LLC, as sales agent, providing for the offering, issuance and sale by the Company of up to an aggregate $50.0 million of its common stock from time to time in “at-the-market” offerings under the Shelf. As of September 30, 2019 , no securities have been issued pursuant to the sales agreement. Convertible Preferred Stock Upon closing of the IPO in May 2018, all 91,315,295 outstanding shares of the Series A, Series A-1, Series A-2, Series A-3, Series B and Series C Preferred Stock (collectively, the "Preferred Stock") automatically converted into 22,386,677 shares of the Company’s common stock at the applicable conversion ratio of 1-for-4.079. Prior to conversion, all shares of Preferred Stock accrued a cumulative dividend of 8% per annum. Dividends for the applicable periods are included in net loss attributable to common shareholders on the condensed consolidated statement of operations through the conversion date. All accrued dividends earned on Preferred Stock were forfeited as of the conversion. In February and March 2018, the Company issued a total of 25,232,199 shares of Series C Preferred Stock at purchase price of $3.23 for gross proceeds of $81.5 million under the same terms as the Series B Preferred Stock.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	Stock-Based Compensation 2018 Incentive Award Plan The Company’s board of directors adopted on April 18, 2018, and the Company’s stockholders approved, the 2018 Incentive Award Plan (the “2018 Plan”), which became effective May 8, 2018 and under which the Company may grant cash and equity-based incentive awards to the Company’s employees, officers, directors, consultants and advisors. Following the effectiveness of the 2018 Plan, the Company ceased making grants under the 2015 Stock Incentive Plan (as amended the "2015 Plan"). The 2018 Plan allows the Company to grant awards for up to 1,344,692 shares of common stock plus that number of shares of common stock subject to awards outstanding under the 2015 Plan, that expire, lapse or become terminated or are exchanged for cash, surrendered, repurchased, canceled without having been fully exercised or forfeited. Each year starting with 2019 and ending in and including 2028, the, the number of shares available for grants of awards under the 2018 Plan will be increased automatically on January 1 by a number of shares of common stock equal to the lesser of 4% of the shares of common stock outstanding on the final day of the preceding calendar year or the number of shares determined by the Company’s board of directors. Accordingly, effective January 1, 2019, the number of shares authorized for issuance under the 2018 Plan was increased by 1,273,031 shares. The 2015 Plan continues to govern the terms and conditions of the outstanding awards granted under it. The exercise price of stock options granted under the 2018 Plan is not less than the fair market value of a share of the Company’s common stock on the grant date. Other terms of awards, including vesting requirements, are determined by the board of directors and are subject to the provisions of the 2018 Plan. Stock options granted to employees generally vest over a four -year period but may be granted with different vesting terms. Certain options provide for accelerated vesting in the event of a change in control. Awards granted to non-employee consultants generally vest monthly over a period of one to four years . Stock options granted under the 2018 Plan expire no more than 10 years from the date of grant. As of September 30, 2019 , equity-based incentive awards for the purchase of up to 2,135,946 shares of the Company’s common stock have been issued under the 2018 Plan, of which 99,196 have been canceled and none have been exercised. As of September 30, 2019 , 947,381 shares of common stock are available for future grant under the 2018 Plan, which includes 366,408 shares subject to awards that were originally granted, and have, since the effective date of the 2018 Plan, been canceled or repurchased under the 2015 Plan. 2015 Stock Incentive Plan Prior to the approval of the 2018 Plan, the Company granted equity awards under the 2015 Plan, which originally provided for grant of incentive stock options, non-qualified stock options, restricted stock awards ("RSAs"), and other stock-based awards to the Company’s employees, officers, directors, consultants and advisors. The terms of stock awards agreements, including vesting requirements, are determined by the board of directors and are subject to the provisions of the 2015 Plan. Stock options granted to employees generally vest over a four -year period but may be granted with different vesting terms. A limited number of awards contain performance-based vesting criteria and for such awards that are deemed probable of vesting, the Company records expense in the period in which such determination is made through any estimated remaining vesting period. Certain options provide for accelerated vesting in the event of a change in control. Awards granted to non-employee consultants generally vest monthly over a period of one to four years. Stock options issued under the 2015 Plan expire no more than 10 years from the date of grant. As of the adoption of the 2018 Plan, the Company ceased making awards under the 2015 Plan. Under the 2015 Plan, the Company was authorized to grant equity awards up to an aggregate of 5,417,044 shares of common stock. As of September 30, 2019 , an aggregate of 5,758,518 options and other equity awards had been granted under the 2015 Plan, of which 1,100,083 have been exercised and 802,417 have been canceled and 18,468 have been repurchased as of September 30, 2019 . A total of 113,006 shares previously reserved under the 2015 Plan that had not been exercised or were otherwise subject to outstanding exercise awards were no longer authorized as of May 8, 2018. Stock-Based Compensation Expense Stock-based compensation expense included in the Company’s condensed consolidated statements of operations and comprehensive loss is as follows (in thousands): Three Months Ended Nine Months Ended 2019 2018 2019 2018 General and administrative $ 1,082 $ 831 $ 3,306 $ 2,727 Research and development 980 764 2,844 1,767 Total stock-based compensation expense $ 2,062 $ 1,595 $ 6,150 $ 4,494 Stock Options A summary of the Company’s stock option activity and related information is as follows: Shares Weighted- Average Exercise Price Options outstanding at December 31, 2018 4,917,811 $ 5.64 Granted 1,431,970 11.35 Exercised (195,533 ) 1.44 Canceled (261,467 ) 8.38 Options outstanding at September 30, 2019 5,892,781 $ 7.05 Exercisable as of September 30, 2019 2,230,234 $ 4.00 The weighted-average fair value of options granted during the nine months ended September 30, 2019 and 2018 was $7.66 and $8.18 , respectively. On January 30, 2018, the Company issued 250,000 shares of Series B Preferred Stock to a non-employee consultant as part of the consideration for services performed and completed in 2017. The Company has recognized $0.7 million as general and administrative expense in the condensed consolidated statement of operations and comprehensive loss, of which $0.1 million was recorded in the nine months ended September 30, 2018 . As of September 30, 2019 , total unrecognized stock-based compensation expense relating to unvested stock options was $21.6 million . This amount is subject to change as the unvested portion of the stock options granted to non-employees is subject to re-measurement over the vesting period. This amount is expected to be recognized over a weighted average period of 2.71 years. 2018 Employee Stock Purchase Plan The Company's board of directors adopted on April 18, 2018, and the Company’s stockholders approved, the 2018 Employee Stock Purchase Plan (the “ESPP”), which became effective on May 8, 2018. A total of 336,356 shares of common stock were initially reserved for issuance under the ESPP. In addition, the number of shares of common stock that may be issued under the ESPP will automatically increase on the first day of each calendar year, beginning in 2020 and ending in and including 2028, by an amount equal to the lesser of (i) 1% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (ii) an amount determined by the Company’s board of directors. The plan administrators have proposed, and the Company's board of directors has authorized, an initial offering period under the ESPP commencing on February 1, 2020.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Deferred tax assets and deferred tax liabilities are recognized based on temporary differences between the financial reporting and tax basis of assets and liabilities using statutory rates. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. There were no significant income tax provisions or benefits for the three or nine months ended September 30, 2019 . Due to losses incurred since inception and the uncertainty surrounding the realization of the favorable tax attributes in future tax returns, the Company has recorded a full valuation allowance against the Company’s otherwise recognizable net deferred tax assets.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Net Loss Per Share	Net Loss Per Share Basic and diluted net loss per common share is determined by dividing net loss attributable to common stockholders by the weighted-average common shares outstanding during the period. The Company has computed diluted net loss per common share after giving consideration to all potentially dilutive common shares, including options to purchase common stock, restricted common stock, convertible preferred stock and warrants to purchase convertible preferred stock, outstanding during the period determined using the if-converted and treasury stock methods, except where the effect of including such securities would be antidilutive. Because the Company has reported net losses since inception, these potential common shares have been anti-dilutive and therefore basic and diluted net loss per share have been equivalent. The following table presents securities that have been excluded from the computations of diluted weighted-average shares outstanding as they would be anti-dilutive: Nine Months Ended 2019 2018 Unvested common stock from early exercise of options 75,043 168,947 Stock options to purchase common stock 5,892,781 4,912,281 Total 5,967,824 5,081,228

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2019
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions The Company receives clinical advisory services from Weatherden Ltd. (“Weatherden”) under agreements that were entered into during 2017 and 2018. Duncan McHale, the Company’s Chief Medical Officer, is a part owner of Weatherden. During the nine months ended September 30, 2019 and 2018 , the Company paid Weatherden $0.7 million and $0.5 million , respectively. As of September 30, 2019 and 2018 , the amounts due to Weatherden under the supply of service agreement were $0.1 million in both periods. In June 2018, the Company entered into a subleasing arrangement with Ring Therapeutics (formerly VL46), an affiliate of one of its stockholders, Flagship Venture Funds. Under the terms of the sublease, the Company will invoice Ring Therapeutics for an aggregate $0.9 million in rent payments which are due during the period from July 1, 2018 through May 31, 2020 plus any related taxes and lease operating costs. For the nine months ended September 30, 2019 , $0.5 million related to this sublease agreement, inclusive of taxes and operating expenses, has been recorded as an offset to operating expense within the consolidated statements of operations and comprehensive loss. The Company entered into a consulting agreement with David Epstein (the "Consulting Agreement"), the Company's Chairman of the Board, effective September 16, 2019 pursuant to which Mr. Epstein will provide strategic advisory and other consulting services to the Company. The Consulting Agreement has an one year term and may be earlier terminated by either Mr. Epstein or the Company upon 30 days ’ notice, or 24 hours ’ notice by the non-breaching party in the event of a breach. In accordance with the terms of the Consulting Agreement, Mr. Epstein was granted an option to purchase 75,000 shares of the Company’s common stock, which award vests in 36 equal monthly installments subject to his continued provision of consulting services to the Company pursuant to the Consulting Agreement on the applicable vesting date. Under the Consulting Agreement, Mr. Epstein also is entitled to receive (i) an annual equity award on each anniversary of the effective date of the Consulting Agreement in the form of an option to purchase shares of the Company’s common stock having an aggregate grant date fair market value equal to approximately $0.2 million , as determined by the Board in its discretion based on customary option pricing methodologies, which award vests in full on the first anniversary of the grant date, subject to his continued provision of consulting services to the Company pursuant to the Consulting Agreement on the applicable vesting date, and (ii) an aggregate annual cash consulting fee of $0.3 million for his consulting services. All of the foregoing options, to the extent then outstanding, will be subject to accelerated vesting upon the occurrence of a change in control of the Company.

Subsequent Event

Subsequent Event	9 Months Ended
	Sep. 30, 2019
Subsequent Events [Abstract]
Subsequent Event	Subsequent Event [placeholder if any]

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited interim condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standard Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these financial statements should be read in conjunction with the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018 and notes thereto. The unaudited interim consolidated financial statements have been prepared on the same basis as the audited financial statements. In the opinion of the Company’s management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary to present fairly the Company’s financial position as of September 30, 2019 , the results of its operations and stockholders' equity for the three and nine months ended September 30, 2019 and 2018 and cash flows for the nine months ended September 30, 2019 and 2018 . Such adjustments are of a normal and recurring nature. The results for the three and nine months ended September 30, 2019 are not necessarily indicative of the results for the year ending December 31, 2019 , or for any future period.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, the accrual of research and development expenses and the valuation of stock-based awards. The Company bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. Actual results could differ from those estimates.
Principles of Consolidation	Principles of Consolidation The unaudited condensed consolidated financial statements include the accounts of the Company and its wholly owned, controlled subsidiaries. All intercompany transactions and balances have been eliminated in consolidation.
Subsequent Event Considerations	Subsequent Event Considerations The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the consolidated financial statements to provide additional evidence for certain estimates or to identify matters that require additional disclosure. Subsequent events have been evaluated as required. The Company has evaluated all subsequent events and determined that there are no material recognized or unrecognized subsequent events requiring disclosure, other than those disclosed in this Quarterly Report on Form 10-Q.
Emerging Growth Company Status	Emerging Growth Company Status Evelo is an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act of 2012 (the "JOBS Act"), and it may take advantage of reduced reporting requirements that are otherwise applicable to public companies. Evelo may take advantage of these exemptions until it is no longer an emerging growth company. Section 107 of the JOBS Act provides that an emerging growth company can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. Evelo has elected to use the extended transition period for complying with new or revised accounting standards, and, as a result of this election, its consolidated financial statements may not be comparable to companies that comply with public company effective dates. Evelo may take advantage of these exemptions up until the last day of the fiscal year following the fifth anniversary of its IPO or such earlier time that it is no longer an emerging growth company. Evelo would cease to be an emerging growth company if it has more than $1.07 billion in annual revenue, it has more than $700.0 million in market value of its stock held by non-affiliates (and has been a public company for at least 12 months and has filed one annual report on Form 10-K), or it has issued more than $1.0 billion of non-convertible debt securities over a three -year period.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss and certain changes in stockholders’ equity that are excluded from net loss.
Cash, Cash Equivalents and Restricted Cash	Cash, Cash Equivalents, and Restricted Cash Cash equivalents are comprised of highly liquid investments that are readily convertible into cash with original maturities of three months or less. Cash and cash equivalents include cash held in banks and amounts held in money market funds and U.S. treasuries with original maturities of three months or less. Cash equivalents are stated at cost, which approximates market value. The Company’s restricted cash consists of restricted cash in connection with building leases for the Company’s office and laboratory premises and deposits held in relation to the company's credit card facility.
Investments	Investments The Company accounts for and classifies its investments as either “available-for-sale,” “trading,” or “held-to-maturity,” in accordance with the accounting guidance related to the accounting and classification of certain investments in debt and equity securities. The determination of the appropriate classification is based primarily on management’s intent to sell the investment at the time of purchase. The Company did not have any investments at September 30, 2019 . As of December 31, 2018 all of the Company's investments were classified as available‑for‑sale securities. Available‑for‑sale securities are those securities which the Company views as available for use in current operations, if needed. The Company generally classifies its available‑for‑sale securities as short‑term investments, even though the stated maturity date may be one year or more beyond the current balance sheet date. Available‑for‑sale investments are stated at fair value with their unrealized gains and losses included in accumulated other comprehensive loss within stockholders’ (deficit) equity, until such gains and losses are realized in other income (expense) within the condensed consolidated statements of operations and comprehensive loss or until an unrealized loss is considered other‑than‑temporary. The Company recognizes other‑than‑temporary impairments of its investments in debt securities when there is a decline in fair value below the amortized cost basis and if (a) it has the intent to sell the security or (b) it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis. If either of these conditions is met, the Company recognizes the difference between the amortized cost of the security and its fair value at the impairment measurement date in the condensed consolidated statements of operations and comprehensive loss. If neither of these conditions is met, the Company must perform an additional analysis to evaluate whether the unrealized loss is associated with the creditworthiness of the issuer of the security rather than other factors, such as interest rates or market factors. If the Company determines from this analysis that it does not expect to receive cash flows sufficient to recover the entire amortized cost of the security, a credit loss exists, the impairment is considered other-than-temporary and is recognized in its condensed consolidated statements of operations and comprehensive loss.
Fair Value of Financial Instruments	Fair Value of Financial Instruments ASC 820, Fair Value Measurement (“ASC 820”), establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability and are developed based on the best information available in the circumstances. ASC 820 identifies fair value as the exchange price, or exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, ASC 820 establishes a three-tier fair value hierarchy that distinguishes between the following: • Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities; • Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly; and • Level 3 inputs are unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. An entity may choose to measure many financial instruments and certain other items at fair value at specified election dates. Subsequent unrealized gains and losses on items for which the fair value option has been elected will be reported in earnings. The Company did not elect to measure any additional financial instruments or other items at fair value.
Research and Development Costs	Research and Development Costs Research and development costs are expensed in the period incurred. Research and development expenses consist of both internal and external costs such as payroll, consulting, and manufacturing costs associated with the development of the Company’s product candidates. Costs for certain development activities, such as clinical trials and manufacturing development activities, are recognized based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, and information provided to the Company by its vendors on their actual costs incurred or level of effort expended. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as prepaid or accrued research and development expenses. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred and capitalized. The capitalized amounts are expensed as the related goods are delivered or the services are performed. The Company has and may continue to acquire the rights to develop and commercialize new product candidates from third parties. The upfront payments to acquire license, product or rights, as well as any future milestone payments, are immediately recognized as research and development expense provided that there is no alternative future use of the rights in other research and development projects. Any milestone payments made for Intellectual Property after regulatory approval, or that have alternative future use, are capitalized and amortized.
Stock-Based Compensation	Stock-Based Compensation The Company records stock-based compensation for options granted to employees and directors based on the grant date fair value of awards issued. The expense is recorded over the requisite service period, which is the vesting period, on a straight-line basis. The Company uses the Black-Scholes option-pricing model to determine the fair value of stock options. The determination of the fair value of stock options on the date of grant using an option-pricing model is affected by the Company’s common stock price, as well as a number of other assumptions. The Company records forfeitures as they occur. The Company accounts for stock-based compensation arrangements with non-employees based upon the fair value of the consideration received or the equity instruments issued, whichever is more reliably measurable. The measurement date for non-employee awards is generally the date performance of services required from the non-employee is complete. Stock-based compensation costs for non-employee awards are recognized as services are provided, which is generally the vesting period, on a straight-line basis. The unvested portion of the stock options is subject to re-measurement over the vesting period and forfeitures are recorded as they occur.
Segments	Segments The Company has one operating segment. The Company's chief operating decision maker, its Chief Executive Officer, manages the Company's operations on a consolidated basis for the purposes of allocating resources.
Recently Adopted and Accounting Pronouncements Issued and Not Yet Adopted	Recently Adopted Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), and further updated through ASU 2016-12 (“ASU 2016-12”), which amends the existing accounting standards for revenue recognition. ASU 2014-09 is based on principles that govern the recognition of revenue at an amount to which an entity expects to be entitled when products are transferred to customers. This guidance is effective for annual reporting periods, and interim periods within those years, beginning after December 15, 2017, for public entities and no later than for annual reporting periods beginning after December 15, 2018, for non-public entities. The new revenue standard may be applied retrospectively to each prior period presented or retrospectively with the cumulative effect recognized as of the date of adoption. The Company adopted ASU 2014-09 on January 1, 2019 and has concluded that the adoption did not have a material impact on its consolidated financial statements as the Company is not yet generating revenues. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808)-Clarifying the Interaction between Topic 808 and Topic 606 ("ASU 2018-18"). The amendments in ASU 2018-18 make targeted improvements to U.S. GAAP for collaborative arrangements by clarifying that certain transactions between collaborative arrangement participants should be accounted for as revenue under Topic 606 when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in Topic 606 should be applied, including recognition, measurement, presentation, and disclosure requirements. In addition, unit-of-account guidance in Topic 808 was aligned with the guidance in Topic 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of Topic 606. ASU 2018-18 is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. Early adoption is permitted, including adoption in any interim period. The amendments in this Update should be applied retrospectively to the date of initial application of Topic 606. The Company adopted ASU 2018-18 on January 1, 2019 and has concluded the adoption did not have a material impact on its consolidated financial statements as the Company does not have any collaborative agreements under which any participant is considered a customer of the Company. Accounting Pronouncements Issued and Not Adopted as of September 30, 2019 In February 2016, the FASB issued ASU 2016-2, Leases (Topic 842) (“ASU 2016-2”), which supersedes the guidance in former ASC 840, Leases. The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to the historical guidance for operating leases. This guidance is effective for annual reporting periods, and interim periods within those years, beginning after December 15, 2018 for most public entities. Early adoption is permitted for all entities. Although its assessment is not complete, the Company currently expects the adoption of this guidance to result in the addition of material balances of leased assets and corresponding lease liabilities to its consolidated balance sheets, primarily relating to leases of office space. In June 2018, the FASB issued ASU No. 2018-07, Stock-based Compensation: Improvements to Nonemployee Share-based Payment Accounting (Topic 718) ("ASU 2018-07"), which amends the existing accounting standards for share-based payments to nonemployees. This ASU aligns much of the guidance on measuring and classifying nonemployee awards with that of awards to employees. Under the new guidance, the measurement of nonemployee equity awards is fixed on the grant date. This ASU becomes effective in the first quarter of fiscal year 2020 and early adoption is permitted but no earlier than the Company's adoption date of Topic 606. Entities will apply the ASU by recognizing a cumulative-effect adjustment to retained earnings as of the beginning of the annual period of adoption. The Company is currently evaluating the impact that ASU 2018-07 will have on its consolidated financial statements.
Income Taxes	Income Taxes Deferred tax assets and deferred tax liabilities are recognized based on temporary differences between the financial reporting and tax basis of assets and liabilities using statutory rates. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following reconciles cash, cash equivalents and restricted cash as of September 30, 2019 and December 31, 2018 , as presented on the Company's statements of cash flows, to its related balance sheet accounts (in thousands): September 30, December 31, Cash and cash equivalents: Cash $ 1,304 $ 1,300 Money market funds 95,757 91,801 Total cash and cash equivalents 97,061 93,101 Restricted cash 1,500 1,250 Cash, cash equivalents and restricted cash $ 98,561 $ 94,351

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Summary of Short-Term Investments Classified as Available-For-Sale	The following tables summarize the Company's investments held at December 31, 2018 , which are all classified as available-for-sale (in thousands): Description Amortized cost Unrealized Gain Unrealized Loss Fair Value December 31, 2018: U.S. treasury securities $ 54,836 $ — $ (18 ) $ 54,818 Total $ 54,836 $ — $ (18 ) $ 54,818

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Summary of Financial Assets and Liabilities Measured at Fair Value	The following tables present information about the Company’s financial assets and liabilities that have been measured at fair value as of September 30, 2019 and December 31, 2018 (in thousands): Description September 30, Active Markets (Level 1) Observable Inputs (Level 2) Unobservable Inputs (Level 3) Assets: Money market funds included within cash and cash equivalents $ 95,757 $ 95,757 $ — $ — Total $ 95,757 $ 95,757 $ — $ — Description December 31, Active Markets (Level 1) Observable Inputs (Level 2) Unobservable Inputs (Level 3) Assets: Money market funds included within cash and cash equivalents $ 91,801 $ 91,801 $ — $ — U.S. treasury securities included within short-term investments 54,818 — 54,818 — Total $ 146,619 $ 91,801 $ 54,818 $ —

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	9 Months Ended
	Sep. 30, 2019
Property, Plant and Equipment [Abstract]
Summary of Property and Equipment	Property and equipment consists of the following (in thousands): September 30, December 31, Property and equipment: Lab equipment $ 7,300 $ 5,393 Leasehold improvements 1,944 1,824 Furniture and fixtures 701 525 Computers and software 200 115 Office equipment 9 9 Construction-in-process 1,394 1,011 Property and equipment 11,548 8,877 Less: accumulated depreciation (3,209 ) (1,952 ) Property and equipment, net $ 8,339 $ 6,925

Loan and Security Agreements (T

Loan and Security Agreements (Tables)	9 Months Ended
	Sep. 30, 2019
Debt Disclosure [Abstract]
Schedule of Minimum Aggregate Future Loan Payments	The Company has the following minimum aggregate future loan payments at September 30, 2019 (in thousands): Twelve month period ending September 30, Amount 2020 $ 1,759 2021 1,754 2022 6,318 2023 9,022 2024 8,515 Total minimum payments $ 27,368 Less amounts representing interest and discount (7,819 ) Long-term debt $ 19,549

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Schedule of Minimum Aggregate Future Lease Commitments	The minimum aggregate future lease commitments, exclusive of any offsetting sublease rental payments, at September 30, 2019 , are as follows (in thousands): Twelve month period ending September 30, Amount 2020 $ 3,187 2021 2,951 2022 3,040 2023 3,131 2024 3,225 Thereafter 3,321 $ 18,855

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Stock-based Compensation Expense	Stock-based compensation expense included in the Company’s condensed consolidated statements of operations and comprehensive loss is as follows (in thousands): Three Months Ended Nine Months Ended 2019 2018 2019 2018 General and administrative $ 1,082 $ 831 $ 3,306 $ 2,727 Research and development 980 764 2,844 1,767 Total stock-based compensation expense $ 2,062 $ 1,595 $ 6,150 $ 4,494
Summary of Stock Option Activity	A summary of the Company’s stock option activity and related information is as follows: Shares Weighted- Average Exercise Price Options outstanding at December 31, 2018 4,917,811 $ 5.64 Granted 1,431,970 11.35 Exercised (195,533 ) 1.44 Canceled (261,467 ) 8.38 Options outstanding at September 30, 2019 5,892,781 $ 7.05 Exercisable as of September 30, 2019 2,230,234 $ 4.00

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Weighted-Average Shares Outstanding	The following table presents securities that have been excluded from the computations of diluted weighted-average shares outstanding as they would be anti-dilutive: Nine Months Ended 2019 2018 Unvested common stock from early exercise of options 75,043 168,947 Stock options to purchase common stock 5,892,781 4,912,281 Total 5,967,824 5,081,228

Organization - Additional Infor

Organization - Additional Information (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash, cash equivalents and short-term investments	$ 97,100,000
Accumulated deficit	176,254,000	$ 113,381,000
2019 Credit Facility | Line of Credit
Debt Instrument [Line Items]
Additional borrowing capacity	$ 10,000,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)segment	Sep. 30, 2018USD ($)	Dec. 31, 2018USD ($)
Accounting Policies [Abstract]
Comprehensive loss	$ 21,634	$ 15,895	$ 62,855	$ 41,541
Restricted cash	$ 1,500		$ 1,500		$ 1,250
Number of operating segments | segment			1

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Cash and cash equivalents:
Cash	$ 1,304	$ 1,300
Money market funds	95,757	91,801
Total cash and cash equivalents	97,061	93,101
Restricted cash	1,500	1,250
Cash, cash equivalents and restricted cash	$ 98,561	$ 94,351	$ 81,208	$ 39,746

Investments - Narrative (Detail

Investments - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Short term investments					$ 54,818,000
Realized gain (loss) on sale or maturity of available-for-sale securities	$ 0	$ 0	$ 0	$ 0
Reclassification out of other comprehensive loss	$ 0	$ 0	$ 0	$ 0
Value of securities in unrealized loss position for less than twelve months					54,800,000
Maturity period			3 years
U.S. treasury securities
Debt and Equity Securities, FV-NI [Line Items]
Short term investments					$ 54,818,000

Investments - Summary of Short-

Investments - Summary of Short-Term Investments Classified as Available-For-Sale (Details) $ in Thousands	Dec. 31, 2018USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 54,836
Unrealized Gain	0
Unrealized Loss	(18)
Fair Value	54,818
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	54,836
Unrealized Gain	0
Unrealized Loss	(18)
Fair Value	$ 54,818

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets:
Total	$ 95,757	$ 146,619
Money market funds included within cash and cash equivalents
Assets:
Cash and cash equivalents, fair value disclosure	95,757	91,801
U.S. treasury securities included within short-term investments
Assets:
Investments, fair value disclosure		54,818
Active Markets (Level 1)
Assets:
Total	95,757	91,801
Active Markets (Level 1) | Money market funds included within cash and cash equivalents
Assets:
Cash and cash equivalents, fair value disclosure	95,757	91,801
Active Markets (Level 1) | U.S. treasury securities included within short-term investments
Assets:
Investments, fair value disclosure		0
Observable Inputs (Level 2)
Assets:
Total	0	54,818
Observable Inputs (Level 2) | Money market funds included within cash and cash equivalents
Assets:
Cash and cash equivalents, fair value disclosure	0	0
Observable Inputs (Level 2) | U.S. treasury securities included within short-term investments
Assets:
Investments, fair value disclosure		54,818
Unobservable Inputs (Level 3)
Assets:
Total	0	0
Unobservable Inputs (Level 3) | Money market funds included within cash and cash equivalents
Assets:
Cash and cash equivalents, fair value disclosure	$ 0	0
Unobservable Inputs (Level 3) | U.S. treasury securities included within short-term investments
Assets:
Investments, fair value disclosure		$ 0

Property and Equipment, Net - S

Property and Equipment, Net - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Property and equipment:
Property and equipment	$ 11,548	$ 8,877
Less: accumulated depreciation	(3,209)	(1,952)
Property and equipment, net	8,339	6,925
Lab equipment
Property and equipment:
Property and equipment	7,300	5,393
Leasehold improvements
Property and equipment:
Property and equipment	1,944	1,824
Furniture and fixtures
Property and equipment:
Property and equipment	701	525
Computers and software
Property and equipment:
Property and equipment	200	115
Office equipment
Property and equipment:
Property and equipment	9	9
Construction-in-process
Property and equipment:
Property and equipment	$ 1,394	$ 1,011

Property and Equipment, Net - N

Property and Equipment, Net - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Property, Plant and Equipment [Abstract]
Depreciation	$ 500	$ 300	$ 1,264	$ 1,575

Loan and Security Agreements -

Loan and Security Agreements - Narrative (Details)	Jul. 19, 2019USD ($)tranche	Feb. 28, 2018USD ($)$ / sharesshares	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)payment	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)	May 31, 2018USD ($)
Line of Credit Facility [Line Items]
Proceeds from Issuance of Long-term Debt					$ 19,481,000	$ 4,975,000
2016 Credit Facility
Line of Credit Facility [Line Items]
Aggregate principal amount			$ 15,000,000		$ 15,000,000
Credit facility, additional amount borrowed		$ 5,000,000					$ 10,000,000
Number of monthly payments | payment					24
Interest expense				$ 200,000	$ 500,000	$ 600,000
2019 Credit Facility
Line of Credit Facility [Line Items]
Interest expense			400,000		400,000
Line of Credit
Line of Credit Facility [Line Items]
Success fee		$ 300,000						$ 300,000
Line of Credit | 2016 Credit Facility
Line of Credit Facility [Line Items]
Aggregate principal amount			$ 15,000,000		$ 15,000,000
Loan and security agreement, interest rate		4.50%
Prepayment fee percentage		2.00%
Prepayment fee percentage after specified period		0.50%
Line of Credit | 2019 Credit Facility
Line of Credit Facility [Line Items]
Aggregate principal amount	$ 45,000,000
Loan and security agreement, interest rate	8.65%
Prepayment fee percentage	2.00%
Prepayment fee percentage after specified period	1.00%
Number of tranches | tranche	3
Final payment under agreement as a percentage of loans borrowed	4.30%
Fees incurred to establish facility	$ 400,000
Anniversary of funding date for determining prepayment fee percentage	18 months
Debt default interest rate per annum	5.00%
Line of Credit | Security And Loan Agreement, Tranche One | 2019 Credit Facility
Line of Credit Facility [Line Items]
Proceeds from Issuance of Long-term Debt	$ 20,000,000
Line of Credit | Security And Loan Agreement, Tranche Two | 2019 Credit Facility
Line of Credit Facility [Line Items]
Proceeds from Issuance of Long-term Debt	10,000,000
Line of Credit | Security And Loan Agreement, Tranche Three | 2019 Credit Facility
Line of Credit Facility [Line Items]
Proceeds from Issuance of Long-term Debt	$ 15,000,000
Line of Credit | Prime Plus | 2016 Credit Facility
Line of Credit Facility [Line Items]
Loan and security agreement, basis spread on interest rate		0.25%
Line of Credit | Prime Plus | 2019 Credit Facility
Line of Credit Facility [Line Items]
Loan and security agreement, basis spread on interest rate	3.15%
Series B Preferred Stock
Line of Credit Facility [Line Items]
Number of shares called by warrants (in shares) | shares		34,722
Warrants exercise price per share (in dollars per share) | $ / shares		$ 1.80

Loan and Security Agreements _2

Loan and Security Agreements - Schedule of Minimum Aggregate Future Loan Payments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Long-term debt	$ 19,549	$ 12,305
Line of Credit
Line of Credit Facility [Line Items]
2020	1,759
2021	1,754
2022	6,318
2023	9,022
2024	8,515
Total minimum payments	27,368
Less amounts representing interest and discount	(7,819)
Long-term debt	$ 19,549

In-License Agreements - Narrati

In-License Agreements - Narrative (Details) - USD ($)	Aug. 06, 2017	Mar. 10, 2016	Sep. 30, 2019	Dec. 31, 2018	Apr. 09, 2018	Dec. 31, 2017	Jun. 10, 2016
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Purchase of preferred stock (in shares)			0	0
Preferred stock par value per share (in dollars per share)			$ 0.001	$ 0.001
Common stock shares issued (in shares)			32,147,073	31,951,540
2016 Mayo License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Purchase of preferred stock (in shares)						1,666,667
Preferred stock par value per share (in dollars per share)						$ 1.8
Common stock shares issued (in shares)							490
Mayo Warrants
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Common stock shares issued (in shares)					134
Warrants to purchase of common stock exercisable (in shares)							18
Number of shares of common stock upon completion (in shares)							116
2017 Mayo License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Non-refundable upfront fee	$ 200,000
Milestone payments upon achievement of certain development, regulatory, and commercial events			$ 200,000
Amount due under license agreement			0
2017 Mayo License Agreement | Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments upon achievement of certain development, regulatory, and commercial events	$ 56,000,000
2016 University of Chicago Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments upon achievement of certain development, regulatory, and commercial events		$ 60,900,000
Milestone payments for development and commercialization of licensed products			400,000
Expenses incurred related to the post grant review			$ 1,100,000
2016 University of Chicago Agreement | Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Non-refundable upfront fee		$ 500,000

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) € in Millions, £ in Millions, $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Jul. 31, 2019GBP (£)installment	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Jul. 31, 2019EUR (€)	Jul. 31, 2019GBP (£)	Feb. 15, 2018USD ($)	Jan. 31, 2018ft²
Commitment And Contingencies [Line Items]
Area of leased office and research development space (in square feet) | ft²									40,765
Future minimum rental payments to be received		$ 0.3		$ 0.3
Rent expense		0.7	$ 0.8	2	$ 1.5
Sublease rent income		$ 0.1	$ 0.1	$ 0.4	$ 0.2
Equipment purchase obligation | £							£ 0.8
Upfront payments to acquire equipment | £	£ 0.4
Payments to acquire equipment, number of installments | installment	2
Payments due to acquire equipment | £							£ 0.2
Exclusivity and Commitment Agreement | Biose Industrie
Commitment And Contingencies [Line Items]
Exclusivity fee payable								$ 0.3
Sacco | Collaborative Arrangement
Commitment And Contingencies [Line Items]
Term of collaboration arrangement	5 years
Period of inactive manufacturing services causing termination under collaborative arrangement	6 months
Aggregate amount due under collaborative arrangement | €						€ 3
Annual amount due under collaborative arrangement | €						€ 0.6

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Minimum Aggregate Future Lease Commitments (Details) $ in Thousands	Sep. 30, 2019USD ($)
Commitments and Contingencies Disclosure [Abstract]
2020	$ 3,187
2021	2,951
2022	3,040
2023	3,131
2024	3,225
Thereafter	3,321
Aggregate future lease commitments	$ 18,855

Stockholders_ Equity and Conv_2

Stockholders’ Equity and Convertible Preferred Stock - Narrative (Details)	Jun. 03, 2019USD ($)	May 11, 2018USD ($)shares	Apr. 27, 2018	Mar. 31, 2018USD ($)$ / sharesshares	Jun. 30, 2018shares	Sep. 30, 2019shares	Jun. 30, 2019shares	Mar. 31, 2019shares	Dec. 31, 2018shares	Sep. 30, 2018shares	Dec. 31, 2017shares
Temporary Equity [Line Items]
Common stock, shares authorized (in shares)		200,000,000				200,000,000			200,000,000
Preferred stock, authorized (in shares)		10,000,000				10,000,000			10,000,000
Preferred stock dividend percentage		8.00%
Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares outstanding (in shares)		91,315,295		91,315,295	0	0	0	0	0	0	65,833,096
Series C Preferred Stock
Temporary Equity [Line Items]
Preferred stock, issued (in shares)				25,232,199
Purchase price per share (in dollars per share) | $ / shares				$ 3.23
Gross proceeds | $				$ 81,500,000
Registration Statement
Temporary Equity [Line Items]
Registration of equity instruments, aggregate authorized amount | $	$ 200,000,000
Sale of stock, term	3 years
Sales Agreement, Cowen And Company, LLC
Temporary Equity [Line Items]
Registration of equity instruments, aggregate authorized amount | $	$ 50,000,000
Common Stock
Temporary Equity [Line Items]
Reverse stock split ratio			0.2451
Number of shares issued in transaction (in shares)					5,312,500
Common Stock | IPO
Temporary Equity [Line Items]
Number of shares issued in transaction (in shares)		5,312,500
Aggregate gross proceeds from transaction | $		$ 85,000,000
Net proceeds from transaction | $		$ 75,800,000
Conversion of shares, shares converted (in shares)		22,386,677

Stock-Based Compensation - Narr

Stock-Based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	Jan. 01, 2019	May 08, 2018	Apr. 18, 2018	Jan. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares canceled (in shares)					261,467
Number of shares exercised (in shares)					195,533
Weighted average fair value of options granted (in dollars per share)					$ 7.66	$ 8.18
Employee and Non-Employee Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock based compensation expense, stock options					$ 21.6
Compensation cost not yet recognized, period for recognition					2 years 8 months 14 days
Series B Preferred Stock | Non Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued in exchange for service (in shares)				250,000
Series B Preferred Stock | Non Employees | General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of underlying shares				$ 0.7		$ 0.1
2018 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized for issuance (in shares)			336,356
Number of shares canceled (in shares)					99,196
Number of shares exercised (in shares)					0
Number of common stock available for future grant (in shares)					947,381
Percentage of outstanding shares			1.00%
2018 Stock Incentive Plan | Stock options to purchase common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years
2018 Stock Incentive Plan | Minimum | Non-Employee Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					1 year
2018 Stock Incentive Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares originally authorized for issuance (in shares)		1,344,692
Common stock outstanding percentage to be declared as an increase in grants					4.00%
Number of additional shares authorized for issuance (in shares)	1,273,031
Options granted, maximum expiration period					10 years
Number of shares issued for purchase (in shares)					2,135,946
2018 Stock Incentive Plan | Maximum | Non-Employee Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years
2015 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares originally authorized for issuance (in shares)		0
Number of shares canceled (in shares)					366,408
Number of common stock available for future grant (in shares)					5,417,044
Number of shares, options and other equity awards granted (in shares)					5,758,518
Number of shares, options and other equity awards exercised (in shares)					1,100,083
Number of shares, options and other equity awards canceled (in shares)					802,417
Number of shares repurchased (in shares)					18,468
Number of shares no longer authorized (in shares)		113,006
2015 Stock Incentive Plan | Stock options to purchase common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years
2015 Stock Incentive Plan | Minimum | Non-Employee Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					1 year
2015 Stock Incentive Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted, maximum expiration period					10 years
2015 Stock Incentive Plan | Maximum | Non-Employee Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 2,062	$ 1,595	$ 6,150	$ 4,494
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	1,082	831	3,306	2,727
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 980	$ 764	$ 2,844	$ 1,767

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Stock Option Activity (Details) - $ / shares	9 Months Ended
	Sep. 30, 2019
Shares
Options outstanding, beginning balance (in shares)	4,917,811
Granted (in shares)	1,431,970
Exercised (in shares)	(195,533)
Canceled (in shares)	(261,467)
Options outstanding, ending balance (in shares)	5,892,781
Exercisable as of period end (in shares)	2,230,234
Weighted- Average Exercise Price
Options outstanding, beginning balance (in dollars per share)	$ 5.64
Granted (in dollars per share)	11.35
Exercised (in dollars per share)	1.44
Canceled (in dollars per share)	8.38
Options outstanding, ending balance (in dollars per share)	7.05
Exercisable as of period end (in dollars per share)	$ 4

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income tax provision (benefit)	$ 0	$ 0	$ 0	$ 0

Net Loss Per Share - Summary of

Net Loss Per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Weighted-Average Shares Outstanding (Details) - shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total number of securities excluded from computation of diluted weighted-average shares outstanding (in shares)	5,967,824	5,081,228
Unvested common stock from early exercise of options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total number of securities excluded from computation of diluted weighted-average shares outstanding (in shares)	75,043	168,947
Stock options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total number of securities excluded from computation of diluted weighted-average shares outstanding (in shares)	5,892,781	4,912,281

Related Party Transactions - Na

Related Party Transactions - Narrative (Details) $ in Millions	Sep. 16, 2019USD ($)installmentshares	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)shares	Sep. 30, 2018USD ($)
Related Party Transaction [Line Items]
Rent due from sublease agreement		$ 0.3		$ 0.3
Offset to rent expense from sublease agreement		0.1	$ 0.1	$ 0.4	$ 0.2
Number of stock options granted (in shares) | shares				1,431,970
Weatherden Ltd
Related Party Transaction [Line Items]
Payment to related party				$ 0.7	0.5
Amounts due to related party		0.1	$ 0.1	0.1	$ 0.1
VL46
Related Party Transaction [Line Items]
Rent due from sublease agreement		$ 0.9		0.9
Offset to rent expense from sublease agreement				$ 0.5
Consulting Agreement | Mr. Epstein
Related Party Transaction [Line Items]
Consulting arrangement term	1 year
Termination notice period of consulting arrangement	30 days
Termination notice period by non-breaching party in event of a breach	24 hours
Number of stock options granted (in shares) | shares	75,000
Number of equal monthly installments | installment	36
Aggregate grant date fair value	$ 0.2
Aggregate annual cash consulting fee	$ 0.3