Entasis Therapeutics (ETTX) 10-K 29 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Mar. 25, 2019	Jun. 30, 2018
Document And Entity Information Abstract
Entity Registrant Name	Entasis Therapeutics Holdings Inc.
Entity Central Index Key	0001724344
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Public Float			$ 0
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		13,127,128
Entity Shell Company	false
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 49,360	$ 55,101
Short-term investments	35,732
Grants receivable	1,706	722
Prepaid expenses and other current assets	1,994	497
Total current assets	88,792	56,320
Property and equipment, net	419	646
Deferred offering costs		1,765
Other assets	63	63
Total assets	89,274	58,794
Current liabilities:
Accounts payable	1,370	2,218
Accrued expenses and other current liabilities	4,846	7,615
Total current liabilities	6,216	9,833
Deferred rent	175	38
Total liabilities	6,391	9,871
Commitments (Note 14)
Temporary Equity
Redeemable Convertible Preferred Stock		104,713
Shareholders' equity (deficit):
Common stock, par value $0.001; 125,000,000 shares authorized and 13,124,842 shares issued and outstanding as of December 31, 2018; 250,000,000 shares authorized and 12,639 shares issued and outstanding as of December 31, 2017	13	3
Additional paid-in capital	172,988	1,377
Accumulated other comprehensive loss	(9)
Accumulated deficit	(90,109)	(57,170)
Total stockholders' equity (deficit)	82,883	(55,790)
Total liabilities, redeemable convertible preferred stock and stockholders' equity (deficit)	89,274	58,794
Series A Redeemable Convertible Preferred Stock
Temporary Equity
Redeemable Convertible Preferred Stock	0	23,866
Series B Redeemable Convertible Preferred Stock
Temporary Equity
Redeemable Convertible Preferred Stock	0	24,550
Series B‑1 Tranche A Redeemable Convertible Preferred Stock
Temporary Equity
Redeemable Convertible Preferred Stock	0	24,423
Series B‑1 Tranche B Redeemable Convertible Preferred Stock
Temporary Equity
Redeemable Convertible Preferred Stock	$ 0	$ 31,874

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Common stock par value (in dollar per share)	$ 0.001	$ 0.001
Common stock authorized (in shares)	125,000,000	250,000,000
Common stock issued (in shares)	13,124,842	12,639
Common stock outstanding (in shares)	13,124,842	12,639
Temporary Equity
Redeemable Convertible Preferred Shares issued (in shares)		154,940,578
Redeemable Convertible Preferred Shares outstanding (in shares)		154,940,578
Series A Redeemable Convertible Preferred Stock
Temporary Equity
Redeemable Convertible Preferred Shares par value (in dollar per share)	$ 0.001	$ 0.001
Redeemable Convertible Preferred Shares authorized (in shares)	0	33,499,900
Redeemable Convertible Preferred Shares issued (in shares)	0	33,499,900
Redeemable Convertible Preferred Shares outstanding (in shares)	0	33,499,900
Series B Redeemable Convertible Preferred Stock
Temporary Equity
Redeemable Convertible Preferred Shares par value (in dollar per share)	$ 0.001	$ 0.001
Redeemable Convertible Preferred Shares authorized (in shares)	0	25,000,000
Redeemable Convertible Preferred Shares issued (in shares)	0	25,000,000
Redeemable Convertible Preferred Shares outstanding (in shares)	0	25,000,000
Series B‑1 Tranche A Redeemable Convertible Preferred Stock
Temporary Equity
Redeemable Convertible Preferred Shares par value (in dollar per share)	$ 0.001	$ 0.001
Redeemable Convertible Preferred Shares authorized (in shares)	0	42,372,882
Redeemable Convertible Preferred Shares issued (in shares)	0	42,372,882
Redeemable Convertible Preferred Shares outstanding (in shares)	0	42,372,882
Series B‑1 Tranche B Redeemable Convertible Preferred Stock
Temporary Equity
Redeemable Convertible Preferred Shares par value (in dollar per share)	$ 0.001	$ 0.001
Redeemable Convertible Preferred Shares authorized (in shares)	0	54,067,796
Redeemable Convertible Preferred Shares issued (in shares)	0	54,067,796
Redeemable Convertible Preferred Shares outstanding (in shares)	0	54,067,796

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS
Revenue	$ 5,000
Operating expenses:
Research and development	33,046	$ 25,745
General and administrative	10,161	5,599
Total operating expenses	43,207	31,344
Loss from operations	(38,207)	(31,344)
Other income:
Grant income	5,337	1,396
Interest income	390	25
Total other income	5,727	1,421
Loss before income taxes	(32,480)	(29,923)
Provision for income taxes	472
Net loss	(32,952)	(29,923)
Dividends declared	(9,142)
Net Loss per Share
Net loss attributable to common stockholders—basic and diluted	$ (42,094)	$ (29,923)
Net loss per share attributable to common stockholders—basic and diluted (in dollar per share)	$ (12.31)	$ (13,795.76)
Weighted average common stock outstanding—basic and diluted (in shares)	3,419,720	2,169
Other comprehensive loss
Net loss	$ (32,952)	$ (29,923)
Net unrealized loss on investments held	(9)
Comprehensive loss	$ (32,961)	$ (29,923)

CONSOLIDATED STATEMENTS OF REDE

CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock	Series B‑1 Tranche A Redeemable Convertible Preferred Stock	Series B‑1 Tranche B Redeemable Convertible Preferred Stock	Common Stock	Additional Paid-in CapitalASU 2018-07	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Retained EarningsASU 2018-07	Retained Earnings	Total
Balance at Dec. 31, 2016	$ 23,866	$ 24,550
Balance (in shares) at Dec. 31, 2016	33,499,900	25,000,000
Increase (Decrease) in Temporary Equity
Issuance of series B redeemable convertible preference shares, net of issuance costs $450			$ 24,423	$ 31,874
Issuance of series B redeemable convertible preference shares, net of issuance costs $450 (in shares)			42,372,882	54,067,796
Balance at Dec. 31, 2017	$ 23,866	$ 24,550	$ 24,423	$ 31,874							$ 104,713
Balance (in shares) at Dec. 31, 2017	33,499,900	25,000,000	42,372,882	54,067,796							154,940,578
Balance at Dec. 31, 2016							$ 904			$ (27,247)	$ (26,343)
Balance (in shares) at Dec. 31, 2016					4
Increase (Decrease) in Temporary Equity
Stock-based compensation expense							420				420
Exercise of stock options					$ 3		53				56
Exercise of stock options (in shares)					12,635
Net loss										(29,923)	(29,923)
Balance at Dec. 31, 2017					$ 3		1,377			(57,170)	(55,790)
Balance (in shares) at Dec. 31, 2017					12,639
Balance at Dec. 31, 2018	$ 0	$ 0	$ 0	$ 0
Balance (in shares) at Dec. 31, 2018	0	0	0	0
Increase (Decrease) in Temporary Equity
Stock-based compensation expense							1,201				1,201
Reorganization adjustment					$ (3)		3
Conversion of preferred stock to common stock upon initial public offering	$ (23,866)	$ (24,550)	$ (24,423)	$ (31,874)	$ 8		104,705				104,713
Conversion of preferred stock into common stock upon initial public offering (in shares)	(33,499,900)	(25,000,000)	(42,372,882)	(54,067,796)	8,084,414
Issuance of common stock upon initial public offering, net of issuance costs of $9,378					$ 5		65,619				65,624
Issuance of common stock upon initial public offering, net of issuance costs of $9,378 (in shares)					5,000,000
Exercise of stock options							96				96
Exercise of stock options (in shares)					27,789
Net unrealized loss on investments held								$ (9)			(9)
Net loss										(32,952)	(32,952)
Balance at Dec. 31, 2018					$ 13		$ 172,988	$ (9)		$ (90,109)	$ 82,883
Balance (in shares) at Dec. 31, 2018					13,124,842
Increase (Decrease) in Temporary Equity
ASU 2018‑07 modified retrospective adjustment						$ (13)			$ 13

CONSOLIDATED STATEMENTS OF RE_2

CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY (DEFICIT)
Issuance costs	$ 9,378

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (32,952)	$ (29,923)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	164	194
Stock-based compensation expense	1,201	420
Loss on disposal of property and equipment	282
Amortization and accretion of investments	(21)
Changes in operating assets and liabilities:
Grants receivable	(984)	(722)
Prepaid expenses and other assets	(1,497)	(345)
Accounts payable	133	447
Due to related party		(620)
Accrued expenses and other current liabilities	(2,280)	3,386
Deferred rent	137	4
Net cash used in operating activities	(35,817)	(27,159)
Cash flows from investing activities:
Purchase of property and equipment	(370)	(286)
Purchase of short-term investments	(35,721)
Net cash used in investing activities	(36,091)	(286)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		56,531
Proceeds from exercise of stock options	96	56
Proceeds from initial public offering, net of issuance costs paid during the year	66,071	(297)
Net cash provided by financing activities	66,167	56,290
Net (decrease) increase in cash and cash equivalents	(5,741)	28,845
Cash and cash equivalents at beginning of the year	55,101	26,256
Cash and cash equivalents at end of the year	49,360	55,101
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable	37	190
Initial public offering costs included in accounts payable and accrued expenses	150
Deferred offering costs included in accounts payable and accrued expenses		$ 1,468
Conversion of preferred stock to common stock upon initial public offering	104,713
Supplemental disclosure of cash flow information:
Cash paid for taxes	$ 472

Organization and Description of

Organization and Description of Business	12 Months Ended
	Dec. 31, 2018
Organization and Description of Business
Organization and Description of Business	ENTASIS THERAPEUTICS HOLDINGS INC. NOTES TO CONSOLIDATED FINANCIAL STATEMENTS 1. Organization and Description of Business Entasis Therapeutics Holdings Inc. (“Entasis” or the “Company”) is a clinical‑stage biopharmaceutical company focused on the discovery, development and commercialization of novel antibacterial products to treat serious infections caused by multidrug-resistant Gram‑negative bacteria. The Company has three subsidiaries: Entasis Therapeutics Limited; Entasis Therapeutics Inc.; and Entasis Therapeutics Security Corporation. The Company was initially formed as Entasis Therapeutics Limited (“Entasis Limited”) on March 6, 2015 in the United Kingdom (“U.K.”) as a wholly owned subsidiary of AstraZeneca AB (“AstraZeneca”). In connection with the spin‑out of Entasis Limited from AstraZeneca in May 2015, Entasis Limited issued 4 ordinary shares to AstraZeneca. Additionally, pursuant to a business transfer and subscription agreement with AstraZeneca (the “A Subscription Agreement”), Entasis Limited also issued 33,499,900 shares of A redeemable convertible preference shares (“A Preferred Stock”) to AstraZeneca in May 2015. In March 2016, Entasis Limited issued 25,000,000 shares of B redeemable convertible preference shares (“B Preferred Stock”) to third‑party investors, at which point AstraZeneca no longer held a controlling interest in Entasis Limited. On April 23, 2018, Entasis Limited completed a corporate reorganization (the “Reorganization”). As part of the Reorganization, Entasis Limited formed Entasis Therapeutics Holdings Inc., a Delaware corporation, in March 2018 with nominal assets and liabilities for the purpose of consummating the Reorganization. In connection with the Reorganization, the existing shareholders of Entasis Limited exchanged each of their classes of shares of Entasis Limited for the same number and classes of common stock and preferred stock of Entasis Therapeutics Holdings Inc. on a one‑to‑one basis. The newly issued stock of Entasis Therapeutics Holdings Inc. have substantially identical rights to the exchanged shares of Entasis Limited. As a result of the exchange, Entasis Therapeutics Holdings Inc. became the sole shareholder of Entasis Limited. Upon the completion of the Reorganization on April 23, 2018, the historical consolidated financial statements of Entasis Limited became the historical consolidated financial statements of Entasis Therapeutics Holdings Inc. On September 28, 2018, the Company completed an initial public offering of its common stock, in which the Company issued and sold 5,000,000 shares of common stock at a price to the public of $15.00 per share. The aggregate net proceeds to the Company from the initial public offering were approximately $65.6 million after deducting underwriting discounts and commissions and estimated offering expenses payable by the Company. Upon the completion of the Company’s initial public offering, all of the outstanding shares of redeemable convertible preferred stock of the Company, including accrued dividends, automatically converted into 8,084,414 shares of the Company’s common stock. Risks and Uncertainties As of December 31, 2018, the Company had $85.1 million in cash, cash equivalents and short-term investments and an accumulated deficit of $90.1 million. Since its inception through December 31, 2018, the Company has funded its operations primarily with proceeds from the sale of redeemable convertible preferred stock and the sale of its common stock. The Company has also either directly received funding or financial commitments from, or has had its program activities conducted and funded by, United States (“U.S.”) government agencies and non-profit entities. In the absence of positive cash flows from operations, the Company is highly dependent on its ability to find additional sources of funding in the form of debt or equity financing. The Company believes its existing cash, cash equivalents and short-term investments will enable it to fund its operating expenses and capital requirements into the fourth quarter of 2020. As an early-stage company, the Company is subject to a number of risks common to other life science companies, including, but not limited to, raising additional capital, development by its competitors of new technological innovations, risk of failure in preclinical and clinical studies, safety and efficacy of its product candidates in clinical trials, the risk of relying on external parties such as contract research organizations (“CROs”) and contract manufacturing organizations (“CMOs”), the regulatory approval process, market acceptance of the Company’s products once approved, lack of marketing and sales history, dependence on key personnel and protection of proprietary technology. The Company’s therapeutic programs are currently pre-commercial, spanning discovery through early development and will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization of any product candidates. These efforts require significant amounts of additional capital, adequate personnel, infrastructure, and extensive compliance-reporting capabilities. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate revenue from product sales. The Company may never achieve profitability, and unless and until it does, it will continue to need to raise additional capital or obtain financing from other sources, such as strategic collaborations or partnerships.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation and Consolidation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (“U.S. GAAP”). The consolidated financial statements include the accounts of Entasis Therapeutics Holdings Inc. and its wholly owned subsidiaries: Entasis Therapeutics Limited; Entasis Therapeutics Inc.; and Entasis Therapeutics Security Corporation. All intercompany accounts and transactions have been eliminated in consolidation. Use of Estimates The preparation of the Company’s consolidated financial statements in conformity with U.S. GAAP requires management to make estimates, judgments and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the recognition of revenue, the recognition of research and development expenses and the valuation of common stock used in the determination of stock-based compensation expense. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from the Company’s estimates. Fair Value Measurements The accounting standard for fair value measurements defines fair value, establishes a framework for measuring fair value in accordance with U.S. GAAP, and requires detailed disclosures about fair value measurements. Under this standard, fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. The valuation techniques are based on observable and unobservable inputs. Observable inputs reflect readily obtainable data from independent sources, while unobservable inputs reflect the Company’s assumptions. This standard classifies these inputs into the following hierarchy: Level 1 Inputs — Quoted prices in active markets for identical instruments; Level 2 Inputs —Quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; and model-derived valuations whose inputs are observable or whose significant value drivers are observable; Level 3 Inputs —Instruments with primarily unobservable value drivers. The Company evaluates transfers between levels at the end of each reporting period. There were no transfers of assets or liabilities between Level 1, Level 2 or Level 3 during the years ended December 31, 2018 and 2017. Cash and Cash Equivalents The Company considers all highly liquid investments purchased with original maturities of 90 days or less at acquisition to be cash equivalents. Cash and cash equivalents include cash held in banks, money market instruments, corporate and municipal notes, U.S. Treasury securities and federal agency securities. Cash equivalents are stated at fair value. The amount of cash equivalents included in cash and cash equivalents was approximately $38.6 million and zero at December 31, 2018 and 2017, respectively. Short-term Investments The Company classifies all short-term investments with an original maturity when purchased of greater than three months as available-for-sale. Available-for-sale securities are carried at fair value, with the unrealized gains and losses reported in other comprehensive loss. The amortized cost of debt securities in this category is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization is included in interest income. Realized gains and losses, and declines in value judged to be other than temporary on available-for-sale securities, are included in interest income. The cost of securities sold is based on the specific identification method. Interest and dividends on securities classified as available-for-sale are included in interest income. To determine whether an other-than-temporary impairment exists, the Company considers whether it has the ability and intent to hold the investment until a market price recovery, and whether evidence indicating the recoverability of the cost of the investment outweighs evidence to the contrary. Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and short-term investments. Periodically, the Company maintains deposits in accredited financial institutions in excess of federally insured limits. The Company maintains each of its cash balances with high‑quality, accredited, financial institutions and, accordingly, such funds are not exposed to significant credit risk. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third‑party manufacturers to supply drug substance products for research and development activities for its programs, including preclinical testing. These programs could be adversely affected by a significant interruption in the supply of such drug substance products. Deferred Offering Costs The Company capitalizes certain legal, professional accounting and other third‑party fees that are directly associated with in‑process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of proceeds generated as a result of the offering. Should a planned equity financing be abandoned, the deferred offering costs would be expensed immediately as a charge to operating expenses in the consolidated statement of operations. The Company recorded deferred offering costs of $1.8 million as of December 31, 2017. Property and Equipment Property and equipment is recorded at cost and depreciated over the estimated useful lives of the related assets using the straight‑line method. Upon disposal of an asset, the related cost and accumulated depreciation are removed from the asset accounts and any resulting gain or loss is included in the consolidated statement of operations. Repair and maintenance costs are expensed as incurred, whereas major improvements are capitalized as additions to property and equipment. The estimated useful lives of the Company’s respective assets are as follows: Estimated Useful Life Laboratory equipment 3 ‑ 5 years Computer software 3 years Computer equipment 3 years Furniture and fixtures 5 years Impairment of Long‑Lived Assets Long‑lived assets are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Whenever such events occur, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long‑lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows. Segment Information The Company manages its operations as a single operating segment for the purposes of assessing performance and making operating decisions. As of December 31, 2018 and 2017, all of the Company’s long‑lived assets were domiciled in the United States. Revenue Recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification, or ASC, Topic 606, Revenue from Contracts with Customers, or ASC 606. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance, and financial instruments. Under ASC 606, the Company recognizes revenue when a customer obtains control of promised goods or services, in an amount that reflects the consideration that the Company expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that the Company determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies a performance obligation. The Company only applies the five‑step model to contracts when it is probable that the Company will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assessed the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied as services are rendered. The Company enters into collaboration agreements for research, development, manufacturing and commercial services that are within the scope of ASC 606, under which the Company licenses certain rights to its product candidates to third parties. The terms of these arrangements typically include payment to the Company of one or more of the following: non‑refundable, upfront license fees; reimbursement of certain costs; customer option exercise fees; development, regulatory and commercial milestone payments; and royalties on net sales of licensed products. The amount of variable consideration is constrained until it is probable that the revenue is not at a significant risk of reversal in a future period. The contracts into which the Company enters generally do not include significant financing components. As part of the accounting for these arrangements, the Company may be required to use significant judgment to determine: (a) the performance obligations in the contract under step (ii) above, (b) the transaction price under step (iii) above and (c) the timing of revenue recognition, including the appropriate measure of progress in step (v) above. The Company uses judgment to determine whether milestones or other variable consideration, except for royalties, should be included in the transaction price, as described further below. The transaction price is allocated to each performance obligation on a relative stand‑alone selling price basis, for which the Company recognizes revenue as or when the performance obligations under the contract are satisfied. If a milestone or other variable consideration relates specifically to the Company’s efforts to satisfy a single performance obligation or to a specific outcome from satisfying the performance obligation, the Company generally allocates the milestone amount entirely to that performance obligation once it is probable that a significant revenue reversal would not occur. Amounts received prior to revenue recognition are recorded as deferred revenue. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date would be classified as current portion of deferred revenue in the consolidated balance sheet. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date would be classified as deferred revenue, net of current portion. Licenses of intellectual property In assessing whether a license is distinct from the other promises, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from a license for its intended purpose without the receipt of the remaining promise(s), whether the value of the license is dependent on the unsatisfied promise(s), whether there are other vendors that could provide the remaining promise(s), and whether it is separately identifiable from the remaining promise(s). For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Customer options If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent or include a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. The Company allocates the transaction price to material rights based on the relative standalone selling price, which is determined based on the identified discount and the probability that the customer will exercise. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised. Milestone payments At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant reversal of cumulative revenue would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant reversal of cumulative revenue would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch‑up basis, which would affect revenue and earnings in the period of adjustment. See Note 8 for a further discussion of accounting for revenue. Government Contracts and Grant Agreements Income from grants is recognized in the period during which the related specified expenses are incurred, provided that the conditions under which the grants or incentives were provided have been met. Grant funding that is received by the Company in advance of incurring specified expenses is recorded in the consolidated balance sheet as a liability. Grant income recognized upon incurring specified expenses in advance of receipt of grant funding is recorded in the consolidated balance sheet as a receivable. Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses include employee costs, such as salaries, equity‑based compensation and benefits, as well as consulting, contract research, third‑party license fees, depreciation, rent and other corporate or operational costs attributable to the Company’s research and development activities. These costs include allocated facility‑related expenses and external costs of outside vendors engaged to conduct both preclinical studies and clinical trials. Non‑refundable pre‑payments for goods or services that will be used or rendered for future research and development activities are deferred. Such amounts are recognized as expense as the goods or services are delivered or the related services are performed, or until it is no longer expected that the goods will be delivered or the services rendered. The Company has entered into various research and development contracts with research institutions and other companies. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs. Patent Costs The Company expenses patent costs as incurred and records such costs within general and administrative expenses. Stock‑Based Compensation The Company measures stock‑based awards granted based on the estimated fair value of the award on the date of the grant and recognizes compensation expense for those awarded to employees and directors over the requisite service period, which is generally the vesting period of the respective award, and for those awarded to nonemployees over the period during which services are rendered by nonemployees until completed. Forfeitures are accounted for as they occur. The Company has issued stock‑based awards with only service‑based vesting conditions and records the expense for these awards using the straight‑line method. The Company has not issued any stock‑based awards with performance‑based vesting conditions. The Company adopted ASU No. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”) in 2018 . Refer to Recently Adopted Accounting Pronouncements below for further information. The standard expands the scope of ASC 718 to include all share-based payment arrangements related to the acquisition of goods and services from both nonemployees and employees. Prior to the adoption of ASU 2018-07, for equity awards granted to nonemployees, the Company accounted for the related equity award compensation in accordance with the provisions of ASC 718 and ASC Topic 505, Equity , and recognized equity award compensation expense over the related service period of the nonemployee award. Equity awards issued to nonemployees were recorded at their fair values, using the then-current fair value of the incentive units, common stock and updated assumption inputs in the Black-Scholes option-pricing model, as applicable, and were periodically revalued as the equity instruments vested. After the adoption of ASU 2018-07, equity-classified share-based payment awards issued to nonemployees are measured at grant date fair value similarly to those of employees and are no longer revalued as the equity instruments vest. The new standard allows entities to use the expected term to measure nonemployee options or elect to use the contractual term as the expected term, on an award-by-award basis. The Company classifies stock‑based compensation expense in its consolidated statement of operations in the same manner in which the award recipients’ payroll costs are classified or in which the award recipients’ service payments are classified. The fair value of each stock option grant is estimated on the date of grant using the Black‑Scholes option pricing model. As there was no public market for the Company’s common stock prior to the initial public offering of its common stock, the estimated fair value of common stock was determined by the Company’s board of directors as of the date of each option grant, with input from management, considering third party valuations of its common stock as well as the Company’s board of directors’ assessment of additional objective and subjective factors that it believed were relevant and which may have changed from the date of the most recent third party valuation through the date of the grant. These third party valuations were performed in accordance with the guidance outlined in the American Institute of Certified Public Accountants’ Accounting and Valuation Guide, Valuation of Privately Held Company Equity Securities Issued as Compensation . The Company also lacked company‑specific historical and implied volatility information for its stock. Therefore, the Company estimates its expected stock price volatility based on the historical volatility of publicly traded peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method. The “simplified” method estimates the expected term of stock options as the mid‑point between the weighted average time to vesting and the contractual maturity. The risk‑free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. There is no expected dividend yield since the Company has never paid cash dividends on common stock and does not expect to pay any cash dividends in the foreseeable future. Income Taxes The Company accounts for income taxes using the asset and liability method which requires for the recognition of deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the carrying amounts and the tax bases of assets and liabilities. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax asset to a level which, more likely than not, will be realized. See Note 12 for further discussion of income taxes. Accounting for income taxes requires a two-step approach to recognize and measure uncertain tax positions. The first step is to evaluate the tax position for recognition by determining if, based on the technical merits, it is more likely than not that the position will be sustained upon audit, including resolution of related appeals or litigation processes, if any. The second step is to measure the tax benefit as the largest amount that is more than 50 percent likely of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties . Net Loss Per Share Basic and diluted net loss per share is determined by dividing net loss by the weighted‑average common stock outstanding during the period. For all periods presented, outstanding stock options, A Preferred Stock, B Preferred Stock, Series B-1 A redeemable convertible preferred stock (“B-1 A Preferred Stock”) and Series B‑1 B redeemable convertible preferred stock (“B‑1 B Stock”) have been excluded from the calculation because their effects would be anti‑dilutive. Therefore, the weighted‑average shares used to calculate both basic and diluted loss per share are the same. Recently Adopted Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standard Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which supersedes existing revenue recognition guidance under GAAP. The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those goods or services. The standard defines a five‑step process to achieve this principle, and will require companies to use more judgment and make more estimates than under the current guidance. ASU 2014‑09 also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, and is effective for public entities for annual periods beginning after December 15, 2017, including interim periods within those fiscal years. In March 2016, the FASB issued ASU No. 2016‑08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (“ASU 2016‑08”), which further clarifies the implementation guidance on principal versus agent considerations in ASU 2014‑09. In April 2016, the FASB issued ASU No. 2016‑10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing , clarifying the implementation guidance on identifying performance obligations and licensing. Specifically, the amendments in this update reduce the cost and complexity of identifying promised goods or services and improve the guidance for determining whether promises are separately identifiable. The amendments in this update also provide implementation guidance on determining whether an entity’s promise to grant a license provides a customer with either a right to use the entity’s intellectual property (which is satisfied at a point in time) or a right to access the entity’s intellectual property (which is satisfied over time). The Company adopted this guidance in connection with the execution of the license and collaboration agreement (the “Zai Agreement”) with Zai Lab (Shanghai) Co., Ltd. (“Zai Lab”) in April 2018. Prior to the Zai Agreement, the Company did not have any revenue from contracts with customers. In October 2016, the FASB issued ASU No. 2016‑16, Income Taxes (Topic 740): Intra‑Entity Transfer of Assets Other than Inventory (“ASU 2016‑16”), which requires the recognition of the income tax consequences of an intra‑entity transfer of an asset, other than inventory, when the transfer occurs. The standard is effective for annual periods beginning after December 15, 2017, including interim periods within those fiscal years. On January 1, 2018, the Company adopted this guidance, and the adoption did not have a material impact on the Company’s consolidated financial statements and related disclosures. In August 2016, the FASB issued ASU No. 2016‑15, Statement of Cash Flows (Topic 230), Classification of Certain Cash Receipts and Cash Payments (“ASU 2016‑15”). The amendments of ASU 2016‑15 were issued to address eight specific cash flow issues for which stakeholders have indicated to the FASB that a diversity in practice existed in how entities were presenting and classifying these items in the statement of cash flows. The issues addressed by ASU 2016‑15 include but are not limited to the classification of debt prepayment and debt extinguishment costs, payments made for contingent consideration for a business combination, proceeds from the settlement of insurance proceeds, distributions received from equity method investees and separately identifiable cash flows and the application of the predominance principle. The amendments of ASU 2016‑15 are effective for public entities for fiscal years beginning after December 15, 2017 and interim periods in those fiscal years. The adoption of ASU 2016‑15 is required to be applied retrospectively. On January 1, 2018, the Company adopted this guidance and the adoption did not have a material impact on the Company’s consolidated financial statements and related disclosures. In May 2017, the FASB issued ASU No. 2017‑09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017‑09”), which clarifies when to account for a change to the terms or conditions of a share‑based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. The standard is effective for annual periods beginning after December 15, 2017. On January 1, 2018, the Company adopted this guidance and the adoption did not have a material impact on the Company’s consolidated financial statements and related disclosures. In June 2018, the FASB issued ASU 2018‑07, which expands the scope of Topic 718 to include share‑based payment awards to nonemployees. The amendments in ASU 2018‑07 are effective for public business entities for fiscal years beginning after December 15, 2018, including interim periods within that fiscal year. For all other entities, the amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted, but no earlier than an entity’s adoption date of Topic 606. During the year ended December 31, 2018, the Company early adopted this guidance and the adoption did not have a material impact on the Company’s consolidated financial statements and related disclosures. Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02 – Leases (Topic 842) , which replaces the existing accounting guidance for leases. This standard requires entities that lease assets to recognize the assets and liabilities for the rights and obligations created by those leases on the balance sheet. The standard is effective for fiscal years and the interim periods within those fiscal years beginning after December 15, 2018. The guidance is required to be applied by the modified retrospective transition approach and early adoption is permitted. In July 2018, the FASB issued ASU 2018-11 Leases – Targeted Improvements, intended to ease the implementation of the new lease standard for financial statement preparers by, among other things, allowing for an additional transition method. In lieu of presenting transition requirements to comparative periods, as previously required, an entity may now elect to show a cumulative effect adjustment on the date of adoption without the requirement to recast prior period financial statements or disclosures presented in accordance with ASU 2016-02. The Company expects to adopt the new standard and elect to use the cumulative effect adjustment transition option effective January 1, 2019, which will be the initial date of application per ASU 2018-11. The Company expects to elect the available package of practical expedients which allows it to not reassess previous accounting conclusions around whether arrangements are or contain leases, the classification of its leases, and the treatment of initial direct costs. The Company also expects it will make an accounting policy election to keep leases with an initial term of 12 months or less off of the balance sheet. The Company is continuing to evaluate developments within the new lease guidance and is finalizing its evaluation of its existing population of contracts to ensure all contracts that meet the definition of a lease contract under the new standard are identified. The Company has assessed the impact that the adoption of this guidance will have on its financial statements and footnote disclosures. The standard will have a material impact on the consolidated balance sheet related to the recognition of right-of-use assets and lease liabilities for operating leases. The standard will not have a material impact on the consolidated statement of operations. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement, which eliminates, adds and modifies certain disclosure

Short-term Investments

Short-term Investments	12 Months Ended
	Dec. 31, 2018
Short-term Investment
Short-term Investments	3. Short-term Investments The following table summarizes the amortized cost and estimated fair value of the Company’s marketable securities, which are considered to be available-for-sale investments and were included in short-term investments on the consolidated balance sheets: Amortized Unrealized Unrealized Cost Gains Losses Fair Value (in thousands) Balance at December 31, 2018: U.S. government-sponsored enterprise securities $ 6,059 $ — $ — $ 6,059 U.S. Treasury securities 29,680 — (7) 29,673 Total $ 35,739 $ — $ (7) $ 35,732 Certain short-term debt securities with original maturities of less than 90 days are included in cash and cash equivalents on the consolidated balance sheets and are not included in the tables above. At December 31, 2018, all investments have contractual maturities within one year. The Company had no investments at December 31, 2017.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2018
Fair Value of Financial Instruments
Fair Value of Financial Instruments	4. Fair Value of Financial Instruments The following tables set forth the Company’s assets that were accounted for at fair value on a recurring basis as of December 31, 2018: December 31, 2018 Fair Value Measurement Using Level 1 Level 2 Level 3 Total (in thousands) Cash equivalents: Money market funds $ 18,609 $ — $ — $ 18,609 U.S. Treasury securities 19,964 — — 19,964 Short-term investments: U.S. government-sponsored enterprise securities — 6,059 — 6,059 U.S. Treasury securities 29,673 — — 29,673 Total $ 68,246 $ 6,059 $ — $ 74,305 The Company classifies its money market funds and U.S. Treasury securities as Level 1 assets under the fair value hierarchy as these assets have been valued using quoted market prices in active markets without any valuation adjustment. The Company classifies its corporate and municipal notes as Level 2 assets under the fair value hierarchy, as these assets have been valued using information obtained through a third-party pricing service at each balance sheet date, using observable market inputs that may include trade information, broker or dealer quotes, bids, offers, or a combination of these data sources. The carrying amounts of the Company’s cash equivalents, grants receivable, accounts payable and accrued expenses approximate their fair value due to the short-term nature of these amounts.

Property and Equipment, net

Property and Equipment, net	12 Months Ended
	Dec. 31, 2018
Property and Equipment, net
Property and Equipment, net	5. Property and Equipment, net Property and equipment, net consisted of the following (in thousands): December 31, 2018 2017 Laboratory equipment $ 929 $ 1,036 Computer software 71 71 Computer equipment 38 7 Furniture and fixtures 6 — Total 1,044 1,114 Less: accumulated depreciation (625) (468) Property and equipment, net $ 419 $ 646 Depreciation expense was $0.2 million for each of the years ended December 31, 2018 and 2017.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2018
Accrued Expenses and Other Current Liabilities.
Accrued Expenses and Other Current Liabilities	6. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following (in thousands): December 31, 2018 2017 Accrued compensation and benefits $ 1,526 $ 1,286 Accrued contract manufacturing 1,003 3,633 Accrued clinical costs 752 1,096 Accrued professional services 672 1,246 Accrued research costs 364 135 Other 529 219 Total accrued expenses and other current liabilities $ 4,846 $ 7,615

Funding Arrangements

Funding Arrangements	12 Months Ended
	Dec. 31, 2018
Funding Arrangements
Funding Arrangements	7. Funding Arrangements In December 2016, the Company entered into a funding arrangement with the U.S. Army Medical Research Acquisition Activity (the “USAMRAA grant”) that covers up to $1.1 million of specified research expenditures of the Company incurred from December 2016 through June 2019 (the “performance period”). The Company has until September 2022 to obtain the reimbursements from USAMRAA for the specified research expenditures incurred and paid by the Company during the performance period. The Company recognized grant income of $0.5 million and $0.5 million for the years ended December 31, 2018 and 2017, respectively. The Company received $0.9 million of funding under the grant for the year ended December 31, 2018 and no funding under the grant for the year ended December 31, 2017. The Company recorded a receivable to reflect unreimbursed, eligible costs incurred under the grant in the amount of $0.2 million and $0.5 million as of December 31, 2018 and December 31, 2017, respectively. In March 2017 and October 2017, the Company entered into funding arrangements with the Trustees of Boston University to utilize funds from the U.S. government through the Combating Antibiotic Resistant Bacteria Biopharmaceutical Accelerator (“CARB‑X”) program. These funding arrangements will cover up to $16.4 million of specified research expenditures of the Company from April 2017 through September 2021. The Company recognized grant income in connection with the CARB-X agreements of $4.8 million and $0.9 million for the years ended December 31, 2018 and 2017, respectively. The Company received $3.8 million and $0.7 million of funding under the grant for the year ended December 31, 2018 and 2017, respectively. The Company recorded a receivable to reflect unreimbursed, eligible costs incurred under the CARB-X agreements in the amount of $1.5 million and $0.2 million as of December 31, 2018 and December 31, 2017, respectively. Collaboration Agreement In July 2017, the Company entered into a collaboration agreement (the “Agreement”) with the Global Antibiotic Research and Development Partnership Drugs for Neglected Disease Initiative (“GARDP”) for the development, manufacture and commercialization of a product candidate containing zoliflodacin in certain countries. The Phase 3 clinical trial has not commenced and there have been no material transactions with respect to the agreement as of December 31, 2018. The Company also had zoliflodacin program activities conducted and funded by the U.S. government through its arrangements with the U.S. National Institute of Allergy and Infectious Diseases, or NIAID.

License and Collaboration Agree

License and Collaboration Agreement with Zai Lab	12 Months Ended
	Dec. 31, 2018
License and Collaboration Agreement with Zai Lab
License and Collaboration Agreement with Zai Lab	8. License and Collaboration Agreement with Zai Lab In April 2018, the Company entered into a license and collaboration agreement with Zai Lab, pursuant to which Zai Lab licensed exclusive rights to ETX2514 and ETX2514SUL in the Asia‑Pacific region (the “Zai Agreement”). Under the terms of the Zai Agreement, Zai Lab will fund most of the Company’s clinical trial costs in China for ETX2514SUL, including all costs in China for the Company’s planned Phase 3 clinical trial of ETX2514SUL, with the exception of patient drug supply. Zai Lab will conduct development activities, plan and obtain regulatory approval in a specified number of countries in the Asia‑Pacific region beyond China after regulatory approval of a licensed product in China. Zai Lab is also solely responsible for commercializing licensed products in the Asia‑Pacific region and will commercialize licensed products for which it has obtained regulatory approval. The Company is obligated to conduct specified development activities for the Asia‑Pacific region. The Company is also obligated to supply Zai Lab with the licensed products for clinical development, although Zai Lab may take over manufacturing responsibilities for its own commercialization activities within a specified time period following the effective date of the Zai Agreement. Both parties are prohibited from developing and commercializing products in the Asia‑Pacific region that would compete with the licensed products. In addition, under the Zai Agreement, either party may propose that Zai Lab pursue a combination of imipenem together with ETX2514SUL in the territory. If the parties decide to pursue an imipenem combination, Zai Lab would provide the Company with limited research and development support for the combination. The Company received an upfront, non‑refundable payment of $5.0 million, less applicable taxes of $0.8 million, and $0.3 million of research support funding, less applicable taxes, from Zai Lab in 2018. The Company is eligible to receive up to an aggregate of $98.3 million in additional research and development support payments and development, regulatory and sales milestone payments related to ETX2514SUL, imipenem and other combinations with the licensed products. In the event the China Food and Drug Administration requires a modification or supplement to the trial protocol, and the Company delays Zai Lab from providing the required information and subsequently from obtaining regulatory approval for the pivotal study of ETX2514SUL in China, then the sales‑based milestone payments that become due to the Company will be reduced by an agreed amount that increases with the length of the delay. Zai Lab will pay the Company a tiered royalty equal to a high‑single digit to low‑double digit percentage based on annual net sales of licensed products in the territory, subject to specified reductions for the market entry of competing products, loss of patent coverage of licensed products and for payments owed to third parties for additional rights necessary to commercialize licensed products in the territory. The Company evaluated the Zai Agreement under Topic 606 and identified two material promises: (1) an exclusive license to develop, manufacture and commercialize products containing ETX2514 or ETX2514SUL in the territory and (2) the initial technology transfer of licensed know‑how. The Company determined that the exclusive license and initial technology transfer were not distinct from one another, as the license has limited value without the transfer of the Company’s technology and Zai Lab would incur additional costs to recreate the Company’s know‑how. Therefore, the license and initial technology transfer were combined as a single performance obligation. The Company determined the $5.0 million non‑refundable upfront payment is the entire transaction price at the outset of the Zai Agreement. All other future potential milestone payments were excluded from the transaction price as they are fully constrained as the risk of significant reversal has not yet been resolved. The achievement of the future potential milestones is not within the Company’s control and is subject to certain research and development success, regulatory approvals or commercial success and therefore carry significant uncertainty. The Company will reevaluate the likelihood of achieving future milestones at the end of each reporting period. Future development milestone revenue from the arrangement will be recognized as revenue in the period when it is no longer probable that revenue attributable to the milestone will result in a significant reversal of cumulative revenue. The Company delivered the exclusive license and performed the initial technology transfer of licensed know-how prior to June 30, 2018. The Company recognized $5.0 million of revenue for the year ended December 31, 2018. Additionally, the Company recorded a provision for income taxes of $0.5 million for the year ended December 31, 2018, associated with China withholding taxes on the upfront payment under the Zai Agreement.

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2018
Redeemable Convertible Preferred Stock.
Redeemable Convertible Preferred Stock	9. Redeemable Convertible Preferred Stock On September 28, 2018, upon the closing of the Company’s IPO, all of the outstanding shares of Redeemable Convertible Preferred Stock (“Preferred Stock”) of the Company automatically converted into 8,084,414 shares of the Company’s common stock, which included 609,484 shares of the Company’s common stock as settlement of the accrued dividends through September 27, 2018. As of December 31, 2017, the Preferred Stock consisted of the following (in thousands, except share amounts): December 31, 2017 Liquidation Preferred Stock and Common stock Issued and Carrying Redemption Issuable Upon Outstanding Value Value Conversion A Preferred Stock 33,499,900 $ 23,866 $ 37,039 1,616,166 B Preferred Stock 25,000,000 24,550 26,759 1,206,096 B-1 Preferred Stock 96,440,678 56,297 57,330 4,652,668 154,940,578 $ 104,713 $ 121,128 7,474,930 The rights and privileges of the preferred stockholders were as follows: Voting Rights The Preferred Stock each had one vote for each share of common stock into which the share of Preferred Stock was convertible on a one-to-one basis. Preferred Stock and common stock voted together as a single class. In the event that the shares of A Preferred Stock would constitute greater than 50% of the common stock (on an as-converted basis), then the A Preferred Stock, as a class, would have votes equal to 49% of the common stock (on an as-converted basis) and the voting rights attaching to each of the A Preferred Stock would accordingly be reduced on a pro-rata basis. Distributions Preferred stockholders were entitled to receive, when and if declared by the board of directors, out of any funds legally available, dividends at the rate of 4% of the original issue price per share. On September 28, 2018, the Company completed its initial public offering and issued an aggregate of 609,484 shares of its common stock as settlement of the accrued dividends through September 27, 2018 due to the holders of Preferred Stock. No dividends accrue after September 27, 2018. Liquidation Preference Prior to the IPO, in the event of a Deemed Liquidation Event (as defined below), holders of the B-1 Preferred Stock then outstanding would have been entitled to be paid an amount equal to the greater of (a) $0.59 per share plus cumulative dividends, whether or not declared by the Company's board of directors, prior to any payment to the holders of the B Preferred Stock, A Preferred Stock and common shareholders or (b) the amount per share as would have been payable to the holders of the B-1 Preferred Stock had the conversion of the B-1 Preferred Stock into common stock taken place immediately prior to the date of the Deemed Liquidation Event (taking into account the conversion of all series of the Preferred Stock simultaneously). Next, the holders of the B Preferred Stock then outstanding would be entitled to be paid an amount equal to the greater of (a) $1.00 per share plus cumulative dividends, whether or not declared by the Company's board of directors, prior to any payment to the holders of the A Preferred Stock and common shareholders or (b) the amount per share as would have been payable to the holders of the B Preferred Stock had the conversion of the B Preferred Stock into common stock taken place immediately prior to the date of the Deemed Liquidation Event (taking into account the conversion of all series of the Preferred Stock simultaneously). Next, the holders of the A Preferred Stock then outstanding would be entitled to be paid an amount equal to the greater of (a) $1.00 per share plus cumulative dividends, whether or not declared by the Company's board of directors, prior to any payment to the common shareholders or (b) the amount per share as would have been payable to the holders of the A Preferred Stock had the conversion of the A Preferred Stock into common stock taken place immediately prior to the date of the Deemed Liquidation Event (taking into account the conversion of all series of Preferred Stock simultaneously). After payment to the holders of the Preferred Stock, any remaining assets of the Company available for distribution to its shareholders would have been distributed among the common shareholders pro rata based on the number of shares held by each such holder. A Deemed Liquidation Event was defined as (a) the appointment of a receiver or administrative receiver; (b) an administration order having been made; (c) the Company having stopped or suspended payment of its debts, becoming unable to pay its debts or otherwise becoming insolvent; (d) an unsatisfied judgement, order or award being outstanding against the Company; (e) the sale or transfer of the subsidiary to a third party; (f) the sale, transfer, exclusive license or other distribution of all or substantially all of the assets of the Company; (g) any consolidation or merger of the Company with or into any other corporation or other entity or person, or any other corporate reorganization, other than: any such consolidation, merger or reorganization in which the shares in issue immediately prior to such event continue to represent a majority of the voting power in the surviving entity immediately after such event; (h) any transaction or series of related transactions in which in excess of 50% of the voting power attaching to the shares in issue immediately prior to such transaction is transferred to a third party other than a direct or indirect wholly owned subsidiary of AstraZeneca; or (i) any other voluntary or involuntary dissolution, liquidation or winding up of the Company. Conversion The holders of the Preferred Stock had the following rights with respect to the conversion into common stock: · The Preferred Stock was convertible at the option of the holder, at any time into common stock on a one-for-20.728 basis. These rights terminated in the event of a change in control, Deemed Liquidation Event, or termination by the Company without cause. · All Preferred Stock would be automatically converted into common stock upon: (i) a public offering on the official list of the United Kingdom Listing Authority at a per share purchase price of at least two times the original purchase price of the B-1 Preferred Stock; (ii) a public offering pursuant to an effective registration statement under the Securities Act of 1933, as amended, resulting in at least $50.0 million of gross proceeds to the Company; or (iii) the election of 51% of the holders of outstanding Preferred Stock. Redemption The Preferred Stock was redeemable upon the occurrence of a Deemed Liquidation Event, which was not solely in control of the Company. Therefore, the Preferred Stock was classified as temporary equity at December 31, 2017.

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2018
Common Stock.
Common Stock	10. Common Stock As of December 31, 2018 and 2017, the Company’s certificate of incorporation authorized the Company to issue 125,000,000 and 250,000,000 shares, respectively, of $0.001 par value common stock. Each holder of common stock shall be entitled to one vote for each share of common stock held of record by such holder on all matters on which stockholders generally are entitled to vote. Common stockholders are entitled to receive dividends when and if declared by the board of directors, out of any funds legally available. As of December 31, 2018, no dividends have been declared or paid. In September 2018, the Company’s board of directors and stockholders approved, and the Company filed, an Amended and Restated Certificate of Incorporation effecting a 1-for-20.728 reverse stock split of its issued and outstanding common stock. All common share and per share data included in the consolidated financial statements reflect the reverse stock split. On September 28, 2018, the Company completed an initial public offering in which the Company issued and sold 5,000,000 shares of its common stock, at a public offering price of $15.00 per share, for gross proceeds of $75.0 million. The Company received approximately $65.6 million in net proceeds after deducting underwriting discounts and commissions and offering costs. In connection with this financing, all outstanding shares of Preferred Stock, including accrued dividends, converted into 8,084,414 shares of the Company’s common stock.

Stock-Based Compensation Expens

Stock-Based Compensation Expense	12 Months Ended
	Dec. 31, 2018
Stock-Based Compensation Expense
Stock-Based Compensation Expense	11. Stock‑Based Compensation Expense Stock Incentive Plan In connection with the Reorganization, Entasis Therapeutics Holdings Inc. assumed the Entasis Limited amended and restated stock incentive plan, and each outstanding share option to purchase ordinary shares of Entasis Limited was assumed by Entasis Therapeutics Holdings Inc. and converted into an option to purchase the same number of shares of common stock of Entasis Therapeutics Holdings Inc. at the same exercise price per share and on the same vesting schedule. Each new option has and is subject to the same terms and conditions as were in effect immediately prior to the assumption and conversion. No share options of Entasis Limited are outstanding following the assumption and conversion. In September 2018, the Company’s board of directors adopted and its stockholders approved the 2018 Equity Incentive Plan (the “2018 Plan”), which became effective on September 25, 2018, at which point no further grants will be made under the 2015 Stock Incentive Plan (the “2015 Plan”). Under the 2018 Plan, the Company may grant incentive stock options (“ISOs”), non-statutory stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. As of December 31, 2018, options to purchase an aggregate of 243,106 shares had been granted and 947,389 shares were available for future issuance under the 2018 Plan. The options issued under the 2018 Plan expire after 10 years. Initially, subject to adjustment as provided in the 2018 Plan, the aggregate number of shares of the Company’s common stock available for issuance under the 2018 Plan was 1,181,972. The number of shares of the Company’s common stock reserved for issuance under the 2018 Plan will automatically increase on January 1 of each year, for a period of 10 years, from January 1, 2019 continuing through January 1, 2028, by 4% of the total number of shares of the Company’s common stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares as may be determined by the Company’s board of directors. The maximum number of shares that may be issued pursuant to the exercise of ISOs under the 2018 Plan is 7,500,000. The maximum number of shares of the Company’s common stock subject to awards granted under the 2018 Plan or otherwise during a single calendar year to any nonemployee directors, taken together with any cash fees paid by the Company to such nonemployee director during the calendar year for serving on the Company’s board of directors, will not exceed $500,000 in total value, or, with respect to the calendar year in which a nonemployee director is first appointed or elected to the Company’s board of directors, $800,000. All options and awards granted under the 2015 Plan consisted of the Company’s common stock. As of September 25, 2018, no additional stock awards have been or will be granted under the 2015 plan. Although the 2015 Plan was terminated as to future awards in September 2018, it continues to govern the terms of options that remain outstanding under the 2015 Plan. Stock Option Activity Stock option activity under both plans for year ended December 31, 2018 is summarized as follows: Weighted- Weighted- Average Average Remaining Aggregate Number of Exercise Contractual Intrinsic Options Price Term (Years) Value (in thousands) Outstanding as of December 31, 2017 811,627 $ 3.73 9.09 $ 6,062 Granted 606,349 10.12 Exercised (27,789) 3.48 Cancelled or forfeited (14,457) 4.35 Outstanding as of December 31, 2018 1,375,730 $ 6.54 8.66 $ 426 Vested or expected to vest at December 31, 2018 1,375,730 $ 6.54 8.66 $ 426 Exercisable at December 31, 2018 492,995 $ 4.54 7.86 $ 152 The aggregate intrinsic value of options is calculated as the difference between the exercise price of the options and the fair value of the Company’s common stock for those options that had exercise prices lower than the fair value of the Company’s common stock. During the year ended December 31, 2018 and 2017 the weighted-average grant date fair value per granted option was $6.22 and $3.75, respectively. Employee Stock Purchase Plan In September 2018, the Company’s board of directors and its stockholders approved the 2018 Employee Stock Purchase Plan (the “ESPP”), which became effective as of September 25, 2018. The ESPP is intended to qualify as an “employee stock purchase plan” within the meaning of Section 423 of the U.S. Internal Revenue Code of 1986, as amended. The number of shares of common stock initially reserved for issuance under the ESPP was 140,000 shares. The ESPP provides for an annual increase on the first day of each year beginning in 2019 and ending in 2028, in each case subject to the approval of the board of directors, equal to the lesser of (i) 1% of the shares of common stock outstanding on the last day of the prior fiscal year or (ii) 250,000 shares; provided, that prior to the date of any such increase, the board of directors may determine that such increase will be less than the amount set forth in clauses (i) and (ii). As of December 31, 2018, no shares of common stock had been issued under the ESPP and 140,000 shares remained available for future issuance under the ESPP. No offering period under the ESPP has been set by the Company’s board of directors. AstraZeneca Shares Option and Incentive Plan Certain employees of the Company participated in the AstraZeneca Shares Option and Incentive Plan (the “AstraZeneca Plan”), whereby employees of the Company continue to vest in the restricted shares (“AstraZeneca RSUs”) of AstraZeneca ordinary shares issued by AstraZeneca to the employees prior to employment by the Company. AstraZeneca RSUs vested 100% after 36 months and were fully vested in March 2017 and therefore stock‑based compensation expense for the AstraZeneca RSUs was recognized in full by September 30, 2017. The Company recorded stock‑based compensation expense for AstraZeneca RSUs of $14,604 during the year ended December 31, 2017. Stock‑Based Compensation Stock‑based compensation expense was classified in the consolidated statement of operations as follows (in thousands): Year Ended December 31, 2018 2017 Research and development $ 479 $ 229 General and administrative 722 191 Total stock-based compensation expense $ 1,201 $ 420 As of December 31, 2018, total unrecognized stock‑based compensation expense related to unvested options was $4.5 million, which is expected to be recognized over the weighted average period of approximately 2.8 years. The total unrecognized stock-based compensation expense will be adjusted for actual forfeitures as they occur. The following weighted-average assumptions were used to calculate the fair value of each stock-based option award under the Black-Scholes option-pricing model: Year Ended December 31, 2018 2017 Expected stock price volatility 65.8 % 65.6 % Risk-free interest rate 3.0 % 2.2 % Expected annual dividend yield — — Expected life of options years Years

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Taxes
Income Taxes	12. Income Taxes During the years ended December 31, 2018 and 2017, the Company recorded no income tax benefits for the net operating losses incurred due to its uncertainty of realizing a benefit from those items. Net loss before the provision for income taxes for the years ended December 31, 2018 and 2017, consisted of the following (in thousands): Year Ended December 31, 2018 2017 United Kingdom $ 28,123 $ 21,806 United States 4,357 8,117 $ 32,480 $ 29,923 As discussed in Note 1, on April 23, 2018, Entasis Limited completed a Reorganization resulting in Entasis Therapeutics Holdings Inc. becoming the sole shareholder of Entasis Limited. Upon the completion of the Reorganization, the historical consolidated financial statements of Entasis Limited became the historical consolidated financial statements of Entasis Therapeutics Holdings Inc. As a result of this Reorganization, the reconciliation below of statutory income tax rates to the Company’s effective income tax rates is based on U.S. statutory rates in 2018 and U.K. statutory rates in 2017: Year Ended December 31, 2018 2017 Income tax benefit computed at statutory tax rate 21.0 % 19.0 % State taxes, net of federal benefit 4.6 1.3 Foreign rate differential (3.4) 4.1 Research and development tax credits 1.4 3.0 Permanent difference 11.5 (1.3) Valuation allowances (35.1) (17.7) Rate changes — (8.4) Foreign withholding tax (1.4) — Effective income tax rate (1.4) % (0.0) % The Company recorded a tax provision of $0.5 million in 2018 associated with China withholding taxes on the upfront payment under the Zai Agreement. Net deferred tax assets as of December 31, 2018 and 2017 consisted of the following (in thousands): December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 20,051 $ 8,967 Tax credit carryforwards 2,321 1,763 Depreciation and amortization — 7 Accrued expenses and other 1,097 987 Total deferred tax assets 23,469 11,724 Depreciation and amortization (10) — Total deferred tax liabilities (10) — Valuation allowance (23,459) (11,724) Net deferred tax assets $ — $ — Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2018 and 2017 related primarily to the increases in NOLs and research and development tax credit carryforwards and were as follows (in thousands): Year Ended December 31, 2018 2017 Valuation allowance at beginning of year $ (11,724) $ (6,415) Increases recorded to income tax provision (11,735) (5,309) Valuation allowance at end of year $ (23,459) $ (11,724) On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act (“TCJA”) tax reform legislation. The legislation reduced the U.S. corporate tax rate from the current rate of 35% down to 21% effective for tax years beginning after December 31, 2017. As a result of the TCJA, at December 31, 2017 the Company revalued deferred tax assets and liabilities at the new rate. This revaluation resulted in a reduction in deferred tax assets of $1.7 million with a corresponding reduction in the valuation allowance. There was no impact to the Company’s statement of operations as a result of a reduction in tax rate. Also as a result of passage of the TCJA, net operating losses generated in years ending after December 31, 2018 will be carried forward indefinitely and can no longer be carried back, and net operating losses generated in years beginning after December 31, 2017, can only reduce taxable income by 80% when utilized in a future period. The exact ramifications of the legislation are subject to interpretation and could have a material impact on the Company’s financial position and/or results of operations. The TCJA is complex and far-reaching, and its effect, whether adverse or favorable, may not become evident for some period of time. As of December 31, 2018, the Company had U.S. federal and state net operating loss carryforwards (“NOLs”) of $33.8 million and $34.5 million, respectively, which begin to expire in 2035. Included in the $33.8 million of federal net operating losses are losses of $21.2 million that will carry forward indefinitely as a result of the Tax Cuts and Jobs Act. As of December 31, 2018, the Company had federal and state research and development tax credits carryforwards of $1.8 million and $0.7 million, which begin to expire in 2035 and 2026, respectively. Utilization of the U.S. federal NOLs and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986, as amended, due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain shareholders or public groups in the shares of a corporation by more than 50% over a three‑year period. The Company has not conducted a study to assess whether a change of control has occurred or whether there have been multiple changes of control since inception, due to the significant complexity and cost associated with such a study. If the Company has experienced a change of control, as defined by Section 382, at any time since inception, utilization of the NOLs or research and development tax credit carryforwards would be subject to an annual limitation under Section 382, which is determined by first multiplying the value of the Company’s shares at the time of the ownership change by the applicable long‑term tax‑exempt rate, and then could be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the NOLs or research and development tax credit carryforwards before utilization. As of December 31, 2018, the Company had NOLs in the United Kingdom of $63.3 million to offset future taxable income. The NOLs in the United Kingdom can be carried forward indefinitely. The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of its deferred tax assets. Accordingly, a full valuation allowance of $23.5 million has been established against the deferred tax assets as of December 31, 2018. The Company has not recorded an amount for unrecognized tax benefits or related interest and penalties accrued as of December 31, 2018. The Company files income tax returns in the United States, Massachusetts and the United Kingdom. The U.S. federal and state returns are generally subject to tax examinations for the tax years ended December 31, 2015 to the present. The statute of limitations for assessment by the United Kingdom is open for the tax years since 2015. There are currently no pending tax examinations. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service and state tax authorities to the extent utilized in a future or prior period. The Company’s policy is to record interest and penalties related to income taxes as part of its income tax provision.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2018
Net Loss per Share
Net Loss per Share	13. Net Loss per Share Basic and diluted net loss per share of the Company was calculated as follows (in thousands, except share and per share amounts): Year Ended December 31, 2018 2017 Numerator: Net loss $ (32,952) $ (29,923) Less: Dividends declared (9,142) — Net loss attributable to common stockholders—basic and diluted $ (42,094) $ (29,923) Denominator: Weighted average common stock outstanding—basic and diluted 3,419,720 2,169 Net loss per share attributable to common stockholders—basic and diluted $ (12.31) $ (13,795.76) The Company excluded the following potential shares of common stock, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share for the periods indicated because including them would have had an anti‑dilutive effect. Therefore, the weighted average number of shares of common stock outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. As of December 31, 2018 2017 Options to purchase shares of common stock 1,375,730 811,627 Redeemable convertible preferred stock (as converted to common stock) — 7,474,930 1,375,730 8,286,557

Commitments

Commitments	12 Months Ended
	Dec. 31, 2018
Commitments
Commitments	14. Commitments Lease Commitments In May 2015, the Company entered into an operating lease agreement for its office and laboratory space with AstraZeneca, which extended through May 2020. In February 2018, the Company amended its operating lease to extend the term through December 2022 and expand the premises to include an additional 7,257 square feet. The facility lease requires the Company to pay certain operating costs. During the year ended December 31, 2018 and 2017, the Company recorded rental expense of $0.6 million and $0.4 million, respectively. Future minimum lease payments for the combined spaces as of December 31, 2018 were as follows (in thousands): Year Ending December 31, 2019 $ 597 2020 656 2021 710 2022 730 $ 2,693 A Subscription Agreement In connection with the A Subscription Agreement, the Company agreed to pay AstraZeneca a one‑time milestone payment of $5.0 million within three months of achieving a specified cumulative net sales milestone for ETX2514. This milestone payment will be automatically waived should the Company’s common stock trade on Nasdaq at or above a specified price at the time it achieves such specified cumulative net sales milestone for ETX2514. The Company is also obligated to pay AstraZeneca a one‑time milestone payment of $10.0 million within two years of achieving the first commercial sale of zoliflodacin. At the Company’s election, either milestone payment may be paid in cash, common stock, or a combination of cash and common stock. Additionally, the Company is obligated to pay AstraZeneca tiered, single‑digit, per‑country royalties on the annual worldwide net sales of ETX2514 and zoliflodacin.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2018
Related Party Transactions
Related Party Transactions	15. Related Party Transactions The Company was formed in May 2015 as a wholly owned subsidiary of AstraZeneca, which maintained a controlling interest in the Company until the B Preferred Stock were issued in March 2016. Prior to the closing of the initial public offering on September 28, 2018, AstraZeneca was the sole A Preferred Stockholder. Upon the closing of the initial public offering, all shares of preferred stock converted into shares of common stock. Subscription Receivable Due from AstraZeneca In connection with the issuance and sale of A Preferred Stock, AstraZeneca agreed to provide cash management services for the net proceeds due to the Company under the A Preferred Stock financing for as long as the Company remained a majority-controlled subsidiary. As a result, the full amount of the funds due to the Company were held by AstraZeneca, as property of the Company. This arrangement ceased upon the closing the B Preferred Stock financing in March 2016. During 2016, $17.6 million of the funds were transferred to the Company, with the remaining $0.2 million received in 2017. Lease Commitments The Company has an operating lease agreement for its office and laboratory space with AstraZeneca. See Note 14.

Benefit Plans

Benefit Plans	12 Months Ended
	Dec. 31, 2018
Benefits Plans
Benefit Plans	16. Benefit Plans The Company has a tax‑qualified employee savings and retirement 401(k) plan, covering all qualified employees. Participants may elect a salary deferral up to the statutorily prescribed annual limit for tax‑deferred contributions. The Company made matching contributions of $0.2 million and $0.1 million for each of the years ended December 31, 2018 and 2017, respectively.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Summary of Significant Accounting Policies
Basis of Presentation and Consolidation	Basis of Presentation and Consolidation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (“U.S. GAAP”). The consolidated financial statements include the accounts of Entasis Therapeutics Holdings Inc. and its wholly owned subsidiaries: Entasis Therapeutics Limited; Entasis Therapeutics Inc.; and Entasis Therapeutics Security Corporation. All intercompany accounts and transactions have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of the Company’s consolidated financial statements in conformity with U.S. GAAP requires management to make estimates, judgments and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the recognition of revenue, the recognition of research and development expenses and the valuation of common stock used in the determination of stock-based compensation expense. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from the Company’s estimates.
Fair Value Measurements	Fair Value Measurements The accounting standard for fair value measurements defines fair value, establishes a framework for measuring fair value in accordance with U.S. GAAP, and requires detailed disclosures about fair value measurements. Under this standard, fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. The valuation techniques are based on observable and unobservable inputs. Observable inputs reflect readily obtainable data from independent sources, while unobservable inputs reflect the Company’s assumptions. This standard classifies these inputs into the following hierarchy: Level 1 Inputs — Quoted prices in active markets for identical instruments; Level 2 Inputs —Quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; and model-derived valuations whose inputs are observable or whose significant value drivers are observable; Level 3 Inputs —Instruments with primarily unobservable value drivers. The Company evaluates transfers between levels at the end of each reporting period. There were no transfers of assets or liabilities between Level 1, Level 2 or Level 3 during the years ended December 31, 2018 and 2017.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments purchased with original maturities of 90 days or less at acquisition to be cash equivalents. Cash and cash equivalents include cash held in banks, money market instruments, corporate and municipal notes, U.S. Treasury securities and federal agency securities. Cash equivalents are stated at fair value. The amount of cash equivalents included in cash and cash equivalents was approximately $38.6 million and zero at December 31, 2018 and 2017, respectively.
Short-term Investments	Short-term Investments The Company classifies all short-term investments with an original maturity when purchased of greater than three months as available-for-sale. Available-for-sale securities are carried at fair value, with the unrealized gains and losses reported in other comprehensive loss. The amortized cost of debt securities in this category is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization is included in interest income. Realized gains and losses, and declines in value judged to be other than temporary on available-for-sale securities, are included in interest income. The cost of securities sold is based on the specific identification method. Interest and dividends on securities classified as available-for-sale are included in interest income. To determine whether an other-than-temporary impairment exists, the Company considers whether it has the ability and intent to hold the investment until a market price recovery, and whether evidence indicating the recoverability of the cost of the investment outweighs evidence to the contrary.
Concentrations of Credit Risk and of Significant Suppliers	Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and short-term investments. Periodically, the Company maintains deposits in accredited financial institutions in excess of federally insured limits. The Company maintains each of its cash balances with high‑quality, accredited, financial institutions and, accordingly, such funds are not exposed to significant credit risk. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third‑party manufacturers to supply drug substance products for research and development activities for its programs, including preclinical testing. These programs could be adversely affected by a significant interruption in the supply of such drug substance products.
Deferred Offering Costs	Deferred Offering Costs The Company capitalizes certain legal, professional accounting and other third‑party fees that are directly associated with in‑process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of proceeds generated as a result of the offering. Should a planned equity financing be abandoned, the deferred offering costs would be expensed immediately as a charge to operating expenses in the consolidated statement of operations. The Company recorded deferred offering costs of $1.8 million as of December 31, 2017.
Property and Equipment	Property and Equipment Property and equipment is recorded at cost and depreciated over the estimated useful lives of the related assets using the straight‑line method. Upon disposal of an asset, the related cost and accumulated depreciation are removed from the asset accounts and any resulting gain or loss is included in the consolidated statement of operations. Repair and maintenance costs are expensed as incurred, whereas major improvements are capitalized as additions to property and equipment. The estimated useful lives of the Company’s respective assets are as follows: Estimated Useful Life Laboratory equipment 3 ‑ 5 years Computer software 3 years Computer equipment 3 years Furniture and fixtures 5 years
Impairment of Long-Lived Assets	Impairment of Long‑Lived Assets Long‑lived assets are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Whenever such events occur, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long‑lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows.
Segment Information	Segment Information The Company manages its operations as a single operating segment for the purposes of assessing performance and making operating decisions. As of December 31, 2018 and 2017, all of the Company’s long‑lived assets were domiciled in the United States.
Revenue Recognition	Revenue Recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification, or ASC, Topic 606, Revenue from Contracts with Customers, or ASC 606. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance, and financial instruments. Under ASC 606, the Company recognizes revenue when a customer obtains control of promised goods or services, in an amount that reflects the consideration that the Company expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that the Company determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies a performance obligation. The Company only applies the five‑step model to contracts when it is probable that the Company will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assessed the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied as services are rendered. The Company enters into collaboration agreements for research, development, manufacturing and commercial services that are within the scope of ASC 606, under which the Company licenses certain rights to its product candidates to third parties. The terms of these arrangements typically include payment to the Company of one or more of the following: non‑refundable, upfront license fees; reimbursement of certain costs; customer option exercise fees; development, regulatory and commercial milestone payments; and royalties on net sales of licensed products. The amount of variable consideration is constrained until it is probable that the revenue is not at a significant risk of reversal in a future period. The contracts into which the Company enters generally do not include significant financing components. As part of the accounting for these arrangements, the Company may be required to use significant judgment to determine: (a) the performance obligations in the contract under step (ii) above, (b) the transaction price under step (iii) above and (c) the timing of revenue recognition, including the appropriate measure of progress in step (v) above. The Company uses judgment to determine whether milestones or other variable consideration, except for royalties, should be included in the transaction price, as described further below. The transaction price is allocated to each performance obligation on a relative stand‑alone selling price basis, for which the Company recognizes revenue as or when the performance obligations under the contract are satisfied. If a milestone or other variable consideration relates specifically to the Company’s efforts to satisfy a single performance obligation or to a specific outcome from satisfying the performance obligation, the Company generally allocates the milestone amount entirely to that performance obligation once it is probable that a significant revenue reversal would not occur. Amounts received prior to revenue recognition are recorded as deferred revenue. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date would be classified as current portion of deferred revenue in the consolidated balance sheet. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date would be classified as deferred revenue, net of current portion. Licenses of intellectual property In assessing whether a license is distinct from the other promises, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from a license for its intended purpose without the receipt of the remaining promise(s), whether the value of the license is dependent on the unsatisfied promise(s), whether there are other vendors that could provide the remaining promise(s), and whether it is separately identifiable from the remaining promise(s). For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Customer options If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent or include a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. The Company allocates the transaction price to material rights based on the relative standalone selling price, which is determined based on the identified discount and the probability that the customer will exercise. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised. Milestone payments At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant reversal of cumulative revenue would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant reversal of cumulative revenue would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch‑up basis, which would affect revenue and earnings in the period of adjustment. See Note 8 for a further discussion of accounting for revenue.
Government Contracts and Grant Agreements	Government Contracts and Grant Agreements Income from grants is recognized in the period during which the related specified expenses are incurred, provided that the conditions under which the grants or incentives were provided have been met. Grant funding that is received by the Company in advance of incurring specified expenses is recorded in the consolidated balance sheet as a liability. Grant income recognized upon incurring specified expenses in advance of receipt of grant funding is recorded in the consolidated balance sheet as a receivable.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses include employee costs, such as salaries, equity‑based compensation and benefits, as well as consulting, contract research, third‑party license fees, depreciation, rent and other corporate or operational costs attributable to the Company’s research and development activities. These costs include allocated facility‑related expenses and external costs of outside vendors engaged to conduct both preclinical studies and clinical trials. Non‑refundable pre‑payments for goods or services that will be used or rendered for future research and development activities are deferred. Such amounts are recognized as expense as the goods or services are delivered or the related services are performed, or until it is no longer expected that the goods will be delivered or the services rendered. The Company has entered into various research and development contracts with research institutions and other companies. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.
Patent Costs	Patent Costs The Company expenses patent costs as incurred and records such costs within general and administrative expenses.
Stock-Based Compensation	Stock‑Based Compensation The Company measures stock‑based awards granted based on the estimated fair value of the award on the date of the grant and recognizes compensation expense for those awarded to employees and directors over the requisite service period, which is generally the vesting period of the respective award, and for those awarded to nonemployees over the period during which services are rendered by nonemployees until completed. Forfeitures are accounted for as they occur. The Company has issued stock‑based awards with only service‑based vesting conditions and records the expense for these awards using the straight‑line method. The Company has not issued any stock‑based awards with performance‑based vesting conditions. The Company adopted ASU No. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”) in 2018 . Refer to Recently Adopted Accounting Pronouncements below for further information. The standard expands the scope of ASC 718 to include all share-based payment arrangements related to the acquisition of goods and services from both nonemployees and employees. Prior to the adoption of ASU 2018-07, for equity awards granted to nonemployees, the Company accounted for the related equity award compensation in accordance with the provisions of ASC 718 and ASC Topic 505, Equity , and recognized equity award compensation expense over the related service period of the nonemployee award. Equity awards issued to nonemployees were recorded at their fair values, using the then-current fair value of the incentive units, common stock and updated assumption inputs in the Black-Scholes option-pricing model, as applicable, and were periodically revalued as the equity instruments vested. After the adoption of ASU 2018-07, equity-classified share-based payment awards issued to nonemployees are measured at grant date fair value similarly to those of employees and are no longer revalued as the equity instruments vest. The new standard allows entities to use the expected term to measure nonemployee options or elect to use the contractual term as the expected term, on an award-by-award basis. The Company classifies stock‑based compensation expense in its consolidated statement of operations in the same manner in which the award recipients’ payroll costs are classified or in which the award recipients’ service payments are classified. The fair value of each stock option grant is estimated on the date of grant using the Black‑Scholes option pricing model. As there was no public market for the Company’s common stock prior to the initial public offering of its common stock, the estimated fair value of common stock was determined by the Company’s board of directors as of the date of each option grant, with input from management, considering third party valuations of its common stock as well as the Company’s board of directors’ assessment of additional objective and subjective factors that it believed were relevant and which may have changed from the date of the most recent third party valuation through the date of the grant. These third party valuations were performed in accordance with the guidance outlined in the American Institute of Certified Public Accountants’ Accounting and Valuation Guide, Valuation of Privately Held Company Equity Securities Issued as Compensation . The Company also lacked company‑specific historical and implied volatility information for its stock. Therefore, the Company estimates its expected stock price volatility based on the historical volatility of publicly traded peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method. The “simplified” method estimates the expected term of stock options as the mid‑point between the weighted average time to vesting and the contractual maturity. The risk‑free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. There is no expected dividend yield since the Company has never paid cash dividends on common stock and does not expect to pay any cash dividends in the foreseeable future.
Income Taxes	Income Taxes The Company accounts for income taxes using the asset and liability method which requires for the recognition of deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the carrying amounts and the tax bases of assets and liabilities. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax asset to a level which, more likely than not, will be realized. See Note 12 for further discussion of income taxes. Accounting for income taxes requires a two-step approach to recognize and measure uncertain tax positions. The first step is to evaluate the tax position for recognition by determining if, based on the technical merits, it is more likely than not that the position will be sustained upon audit, including resolution of related appeals or litigation processes, if any. The second step is to measure the tax benefit as the largest amount that is more than 50 percent likely of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties .
Net Loss Per Share	Net Loss Per Share Basic and diluted net loss per share is determined by dividing net loss by the weighted‑average common stock outstanding during the period. For all periods presented, outstanding stock options, A Preferred Stock, B Preferred Stock, Series B-1 A redeemable convertible preferred stock (“B-1 A Preferred Stock”) and Series B‑1 B redeemable convertible preferred stock (“B‑1 B Stock”) have been excluded from the calculation because their effects would be anti‑dilutive. Therefore, the weighted‑average shares used to calculate both basic and diluted loss per share are the same.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standard Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which supersedes existing revenue recognition guidance under GAAP. The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those goods or services. The standard defines a five‑step process to achieve this principle, and will require companies to use more judgment and make more estimates than under the current guidance. ASU 2014‑09 also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, and is effective for public entities for annual periods beginning after December 15, 2017, including interim periods within those fiscal years. In March 2016, the FASB issued ASU No. 2016‑08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (“ASU 2016‑08”), which further clarifies the implementation guidance on principal versus agent considerations in ASU 2014‑09. In April 2016, the FASB issued ASU No. 2016‑10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing , clarifying the implementation guidance on identifying performance obligations and licensing. Specifically, the amendments in this update reduce the cost and complexity of identifying promised goods or services and improve the guidance for determining whether promises are separately identifiable. The amendments in this update also provide implementation guidance on determining whether an entity’s promise to grant a license provides a customer with either a right to use the entity’s intellectual property (which is satisfied at a point in time) or a right to access the entity’s intellectual property (which is satisfied over time). The Company adopted this guidance in connection with the execution of the license and collaboration agreement (the “Zai Agreement”) with Zai Lab (Shanghai) Co., Ltd. (“Zai Lab”) in April 2018. Prior to the Zai Agreement, the Company did not have any revenue from contracts with customers. In October 2016, the FASB issued ASU No. 2016‑16, Income Taxes (Topic 740): Intra‑Entity Transfer of Assets Other than Inventory (“ASU 2016‑16”), which requires the recognition of the income tax consequences of an intra‑entity transfer of an asset, other than inventory, when the transfer occurs. The standard is effective for annual periods beginning after December 15, 2017, including interim periods within those fiscal years. On January 1, 2018, the Company adopted this guidance, and the adoption did not have a material impact on the Company’s consolidated financial statements and related disclosures. In August 2016, the FASB issued ASU No. 2016‑15, Statement of Cash Flows (Topic 230), Classification of Certain Cash Receipts and Cash Payments (“ASU 2016‑15”). The amendments of ASU 2016‑15 were issued to address eight specific cash flow issues for which stakeholders have indicated to the FASB that a diversity in practice existed in how entities were presenting and classifying these items in the statement of cash flows. The issues addressed by ASU 2016‑15 include but are not limited to the classification of debt prepayment and debt extinguishment costs, payments made for contingent consideration for a business combination, proceeds from the settlement of insurance proceeds, distributions received from equity method investees and separately identifiable cash flows and the application of the predominance principle. The amendments of ASU 2016‑15 are effective for public entities for fiscal years beginning after December 15, 2017 and interim periods in those fiscal years. The adoption of ASU 2016‑15 is required to be applied retrospectively. On January 1, 2018, the Company adopted this guidance and the adoption did not have a material impact on the Company’s consolidated financial statements and related disclosures. In May 2017, the FASB issued ASU No. 2017‑09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017‑09”), which clarifies when to account for a change to the terms or conditions of a share‑based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. The standard is effective for annual periods beginning after December 15, 2017. On January 1, 2018, the Company adopted this guidance and the adoption did not have a material impact on the Company’s consolidated financial statements and related disclosures. In June 2018, the FASB issued ASU 2018‑07, which expands the scope of Topic 718 to include share‑based payment awards to nonemployees. The amendments in ASU 2018‑07 are effective for public business entities for fiscal years beginning after December 15, 2018, including interim periods within that fiscal year. For all other entities, the amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted, but no earlier than an entity’s adoption date of Topic 606. During the year ended December 31, 2018, the Company early adopted this guidance and the adoption did not have a material impact on the Company’s consolidated financial statements and related disclosures.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02 – Leases (Topic 842) , which replaces the existing accounting guidance for leases. This standard requires entities that lease assets to recognize the assets and liabilities for the rights and obligations created by those leases on the balance sheet. The standard is effective for fiscal years and the interim periods within those fiscal years beginning after December 15, 2018. The guidance is required to be applied by the modified retrospective transition approach and early adoption is permitted. In July 2018, the FASB issued ASU 2018-11 Leases – Targeted Improvements, intended to ease the implementation of the new lease standard for financial statement preparers by, among other things, allowing for an additional transition method. In lieu of presenting transition requirements to comparative periods, as previously required, an entity may now elect to show a cumulative effect adjustment on the date of adoption without the requirement to recast prior period financial statements or disclosures presented in accordance with ASU 2016-02. The Company expects to adopt the new standard and elect to use the cumulative effect adjustment transition option effective January 1, 2019, which will be the initial date of application per ASU 2018-11. The Company expects to elect the available package of practical expedients which allows it to not reassess previous accounting conclusions around whether arrangements are or contain leases, the classification of its leases, and the treatment of initial direct costs. The Company also expects it will make an accounting policy election to keep leases with an initial term of 12 months or less off of the balance sheet. The Company is continuing to evaluate developments within the new lease guidance and is finalizing its evaluation of its existing population of contracts to ensure all contracts that meet the definition of a lease contract under the new standard are identified. The Company has assessed the impact that the adoption of this guidance will have on its financial statements and footnote disclosures. The standard will have a material impact on the consolidated balance sheet related to the recognition of right-of-use assets and lease liabilities for operating leases. The standard will not have a material impact on the consolidated statement of operations. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement, which eliminates, adds and modifies certain disclosure requirements for fair value measurements. The guidance is effective for all entities for fiscal years beginning after December 15, 2019 and for interim periods within those fiscal years, but entities are permitted to early adopt either the entire standard or only the provisions that eliminate or modify the requirements. The Company does not expect the adoption of the new guidance to have a material effect on its consolidated financial statements. In November 2018, the FASB issued ASU No. 2018-18 — Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606. This update clarifies the interaction between Topic 808, Collaborative Arrangements, and Topic 606, Revenue from Contracts with Customers. The standard is effective for fiscal years and the interim periods within those fiscal years beginning after December 15, 2019. The guidance is required to be applied retrospectively to the date of initial application of Topic 606. An entity should recognize the cumulative effect of initially applying the amendments as an adjustment to the opening balance of retained earnings of the later of the earliest annual period presented and the annual period that includes the date of the entity’s initial application of Topic 606. The Company is currently assessing the impact that adoption of this guidance will have on its consolidated financial statements.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Summary of Significant Accounting Policies
Schedule of estimated useful lives of property and equipment	Estimated Useful Life Laboratory equipment 3 ‑ 5 years Computer software 3 years Computer equipment 3 years Furniture and fixtures 5 years

Short-term Investments (Tables)

Short-term Investments (Tables)	12 Months Ended
	Dec. 31, 2018
Short-term Investment
Summary of amortized cost and estimated fair value of Company’s marketable securities	Amortized Unrealized Unrealized Cost Gains Losses Fair Value (in thousands) Balance at December 31, 2018: U.S. government-sponsored enterprise securities $ 6,059 $ — $ — $ 6,059 U.S. Treasury securities 29,680 — (7) 29,673 Total $ 35,739 $ — $ (7) $ 35,732

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value of Financial Instruments
Schedule of assets that were accounted for at fair value on recurring basis	December 31, 2018 Fair Value Measurement Using Level 1 Level 2 Level 3 Total (in thousands) Cash equivalents: Money market funds $ 18,609 $ — $ — $ 18,609 U.S. Treasury securities 19,964 — — 19,964 Short-term investments: U.S. government-sponsored enterprise securities — 6,059 — 6,059 U.S. Treasury securities 29,673 — — 29,673 Total $ 68,246 $ 6,059 $ — $ 74,305

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2018
Property and Equipment, net
Schedule of property and equipment, net	Property and equipment, net consisted of the following (in thousands): December 31, 2018 2017 Laboratory equipment $ 929 $ 1,036 Computer software 71 71 Computer equipment 38 7 Furniture and fixtures 6 — Total 1,044 1,114 Less: accumulated depreciation (625) (468) Property and equipment, net $ 419 $ 646

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Accrued Expenses and Other Current Liabilities.
Schedule of accrued expenses and other current liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): December 31, 2018 2017 Accrued compensation and benefits $ 1,526 $ 1,286 Accrued contract manufacturing 1,003 3,633 Accrued clinical costs 752 1,096 Accrued professional services 672 1,246 Accrued research costs 364 135 Other 529 219 Total accrued expenses and other current liabilities $ 4,846 $ 7,615

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2018
Redeemable Convertible Preferred Stock.
Schedule of rights and privileges of preferred stockholders	As of December 31, 2017, the Preferred Stock consisted of the following (in thousands, except share amounts): December 31, 2017 Liquidation Preferred Stock and Common stock Issued and Carrying Redemption Issuable Upon Outstanding Value Value Conversion A Preferred Stock 33,499,900 $ 23,866 $ 37,039 1,616,166 B Preferred Stock 25,000,000 24,550 26,759 1,206,096 B-1 Preferred Stock 96,440,678 56,297 57,330 4,652,668 154,940,578 $ 104,713 $ 121,128 7,474,930

Stock-Based Compensation Expe_2

Stock-Based Compensation Expense (Tables)	12 Months Ended
	Dec. 31, 2018
Stock-Based Compensation Expense
Summary of Stock option activity under the 2015 and 2018 Plan	Weighted- Weighted- Average Average Remaining Aggregate Number of Exercise Contractual Intrinsic Options Price Term (Years) Value (in thousands) Outstanding as of December 31, 2017 811,627 $ 3.73 9.09 $ 6,062 Granted 606,349 10.12 Exercised (27,789) 3.48 Cancelled or forfeited (14,457) 4.35 Outstanding as of December 31, 2018 1,375,730 $ 6.54 8.66 $ 426 Vested or expected to vest at December 31, 2018 1,375,730 $ 6.54 8.66 $ 426 Exercisable at December 31, 2018 492,995 $ 4.54 7.86 $ 152
Schedule of stock-based compensation expense	Stock‑based compensation expense was classified in the consolidated statement of operations as follows (in thousands): Year Ended December 31, 2018 2017 Research and development $ 479 $ 229 General and administrative 722 191 Total stock-based compensation expense $ 1,201 $ 420
Schedule of Black-Scholes option-pricing model	Year Ended December 31, 2018 2017 Expected stock price volatility 65.8 % 65.6 % Risk-free interest rate 3.0 % 2.2 % Expected annual dividend yield — — Expected life of options years Years

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Taxes
Schedule of net loss before provision for income taxes	Net loss before the provision for income taxes for the years ended December 31, 2018 and 2017, consisted of the following (in thousands): Year Ended December 31, 2018 2017 United Kingdom $ 28,123 $ 21,806 United States 4,357 8,117 $ 32,480 $ 29,923
Schedule of reconciliation of the federal statutory income tax rate to the Company’s effective income tax rate	Year Ended December 31, 2018 2017 Income tax benefit computed at statutory tax rate 21.0 % 19.0 % State taxes, net of federal benefit 4.6 1.3 Foreign rate differential (3.4) 4.1 Research and development tax credits 1.4 3.0 Permanent difference 11.5 (1.3) Valuation allowances (35.1) (17.7) Rate changes — (8.4) Foreign withholding tax (1.4) — Effective income tax rate (1.4) % (0.0) %
Schedule of net deferred tax assets	Net deferred tax assets as of December 31, 2018 and 2017 consisted of the following (in thousands): December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 20,051 $ 8,967 Tax credit carryforwards 2,321 1,763 Depreciation and amortization — 7 Accrued expenses and other 1,097 987 Total deferred tax assets 23,469 11,724 Depreciation and amortization (10) — Total deferred tax liabilities (10) — Valuation allowance (23,459) (11,724) Net deferred tax assets $ — $ —
Schedule of changes in valuation allowance for deferred tax assets	Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2018 and 2017 related primarily to the increases in NOLs and research and development tax credit carryforwards and were as follows (in thousands): Year Ended December 31, 2018 2017 Valuation allowance at beginning of year $ (11,724) $ (6,415) Increases recorded to income tax provision (11,735) (5,309) Valuation allowance at end of year $ (23,459) $ (11,724)

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Net Loss per Share
Schedule of computation of basic and diluted net loss per share	Basic and diluted net loss per share of the Company was calculated as follows (in thousands, except share and per share amounts): Year Ended December 31, 2018 2017 Numerator: Net loss $ (32,952) $ (29,923) Less: Dividends declared (9,142) — Net loss attributable to common stockholders—basic and diluted $ (42,094) $ (29,923) Denominator: Weighted average common stock outstanding—basic and diluted 3,419,720 2,169 Net loss per share attributable to common stockholders—basic and diluted $ (12.31) $ (13,795.76)
Schedule of anti-dilutive securities	As of December 31, 2018 2017 Options to purchase shares of common stock 1,375,730 811,627 Redeemable convertible preferred stock (as converted to common stock) — 7,474,930 1,375,730 8,286,557

Commitments (Tables)

Commitments (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments
Schedule of future minimum lease payments for the combined spaces	Future minimum lease payments for the combined spaces as of December 31, 2018 were as follows (in thousands): Year Ending December 31, 2019 $ 597 2020 656 2021 710 2022 730 $ 2,693

Organization and Description _2

Organization and Description of Business (Details) $ / shares in Units, $ in Thousands	Sep. 28, 2018USD ($)$ / sharesshares	Apr. 23, 2018	Mar. 31, 2016shares	May 31, 2015shares	Dec. 31, 2018USD ($)subsidiaryshares	Dec. 31, 2017USD ($)
Number of subsidiaries | subsidiary					3
Shares issued (in shares)				4
Shares exchange ratio (as a percent)		100
Proceeds from initial public offering, net | $					$ 66,071	$ (297)
Preferred stock including accrued dividends, converted into common stock	8,084,414
Risks and Uncertainties
Cash, cash equivalents and short-term investments | $					85,100
Accumulated deficit | $					$ (90,109)	$ (57,170)
Initial Public Offering
Shares issued (in shares)	5,000,000
Share price (in dollars per share) | $ / shares	$ 15
Proceeds from initial public offering, net | $	$ 65,600
Preferred stock including accrued dividends, converted into common stock	8,084,414
Series A Redeemable Convertible Preferred Stock
Shares issued (in shares)				33,499,900
Preferred stock including accrued dividends, converted into common stock					(33,499,900)
Series B Redeemable Convertible Preferred Stock
Shares issued (in shares)			25,000,000
Preferred stock including accrued dividends, converted into common stock					(25,000,000)

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2018USD ($)segment	Dec. 31, 2017USD ($)segment
Fair Value Measurements
Transfers between level 1 and level 2	$ 0	$ 0
Transfers between level 2 and level 1	0	0
Asset transfers into level 3	0	0
Asset transfers out of level 3	0	0
Cash and Cash Equivalents
Cash equivalents	$ 38,600,000	0
Deferred Offering Costs
Deferred Offering Costs		$ 1,765,000
Segment Information
Number of operating segments | segment	1	1
Laboratory equipment | Minimum
Property and Equipment
Estimated useful lives (in years)	3 years
Laboratory equipment | Maximum
Property and Equipment
Estimated useful lives (in years)	5 years
Computer software
Property and Equipment
Estimated useful lives (in years)	3 years
Computer equipment
Property and Equipment
Estimated useful lives (in years)	3 years
Furniture and fixtures
Property and Equipment
Estimated useful lives (in years)	5 years

Short-term Investments (Details

Short-term Investments (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Table summarizes the amortized cost and estimated fair value of the Company’s marketable securities
Amortized Cost	$ 35,739,000
Unrealized Losses	(7,000)
Fair Value	35,732,000	$ 0
U.S. government-sponsored enterprise securities
Table summarizes the amortized cost and estimated fair value of the Company’s marketable securities
Amortized Cost	6,059,000
Fair Value	6,059,000
U.S. Treasury securities
Table summarizes the amortized cost and estimated fair value of the Company’s marketable securities
Amortized Cost	29,680,000
Unrealized Losses	(7,000)
Fair Value	$ 29,673,000

Fair Value of Financial Instr_3

Fair Value of Financial Instruments (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Company’s assets that were accounted for at fair value on a recurring basis
Short-term investments	$ 35,732,000	$ 0
Recurring
Company’s assets that were accounted for at fair value on a recurring basis
Total	74,305,000
Recurring | Level 1
Company’s assets that were accounted for at fair value on a recurring basis
Total	68,246,000
Recurring | Level 2
Company’s assets that were accounted for at fair value on a recurring basis
Total	6,059,000
Recurring | Money market funds
Company’s assets that were accounted for at fair value on a recurring basis
Cash equivalents	18,609,000
Recurring | Money market funds | Level 1
Company’s assets that were accounted for at fair value on a recurring basis
Cash equivalents	18,609,000
Recurring | U.S. Treasury securities
Company’s assets that were accounted for at fair value on a recurring basis
Cash equivalents	19,964,000
Short-term investments	29,673,000
Recurring | U.S. Treasury securities | Level 1
Company’s assets that were accounted for at fair value on a recurring basis
Cash equivalents	19,964,000
Short-term investments	29,673,000
Recurring | U.S. government-sponsored enterprise securities
Company’s assets that were accounted for at fair value on a recurring basis
Short-term investments	6,059,000
Recurring | U.S. government-sponsored enterprise securities | Level 2
Company’s assets that were accounted for at fair value on a recurring basis
Short-term investments	$ 6,059,000

Property and Equipment, net (De

Property and Equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property and equipment, net
Total	$ 1,044	$ 1,114
Less: accumulated depreciation	(625)	(468)
Property and equipment, net	419	646
Depreciation expense	164	194
Laboratory equipment
Property and equipment, net
Total	929	1,036
Computer software
Property and equipment, net
Total	71	71
Computer equipment
Property and equipment, net
Total	38	$ 7
Furniture and fixtures
Property and equipment, net
Total	$ 6

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Expenses and Other Current Liabilities.
Accrued compensation and benefits	$ 1,526	$ 1,286
Accrued contract manufacturing	1,003	3,633
Accrued clinical costs	752	1,096
Accrued professional services	672	1,246
Accrued research costs	364	135
Other	529	219
Total accrued expenses and other current liabilities	$ 4,846	$ 7,615

Funding Arrangements (Details)

Funding Arrangements (Details) - USD ($) $ in Millions	1 Months Ended	8 Months Ended	12 Months Ended
	Dec. 31, 2016	Oct. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
USAMRAA grant
Funding Arrangements
Maximum reimbursable research expenditures	$ 1.1
Grant income recognized			$ 0.5	$ 0.5
Funds received during the period			0.9	0
Grants receivable			0.2	0.5
CARB-X program
Funding Arrangements
Maximum reimbursable research expenditures		$ 16.4
Grant income recognized			4.8	0.9
Funds received during the period			3.8	0.7
Grants receivable			$ 1.5	$ 0.2

License and Collaboration Agr_2

License and Collaboration Agreement with Zai Lab (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Apr. 30, 2018	Dec. 31, 2018
License and Collaboration Agreement with Zai Lab
Revenue		$ 5,000
Provision for income taxes		472
License and collaboration agreement with Zai Lab
License and Collaboration Agreement with Zai Lab
Upfront, non-refundable payment received, net of tax	$ 5,000
Upfront, non-refundable payment received, tax expense	800
Upfront, non-refundable research support funding, net of tax	300
Eligible amount receivable in research and development support payments and development, regulatory and sales milestone payments	$ 98,300
Revenue		5,000
Provision for income taxes		$ 500

Redeemable Convertible Prefer_3

Redeemable Convertible Preferred Stock - General (Details) - USD ($) $ in Thousands	Sep. 28, 2018	Sep. 27, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Redeemable Convertible Preferred Stock
Preferred stock including accrued dividends, converted into common stock	8,084,414
Stock issued as settlement of accrued dividends		609,484
Rights and privileges of preferred stockholders:
Preferred Stock Issued				154,940,578
Preferred Stock Outstanding				154,940,578
Carrying value				$ 104,713
Liquidation value				121,128
Redemption value				$ 121,128
Common Stock Issuable Upon Conversion				7,474,930
Series A Redeemable Convertible Preferred Stock
Redeemable Convertible Preferred Stock
Preferred stock including accrued dividends, converted into common stock			(33,499,900)
Rights and privileges of preferred stockholders:
Preferred Stock Issued			0	33,499,900
Preferred Stock Outstanding			0	33,499,900	33,499,900
Carrying value			$ 0	$ 23,866	$ 23,866
Liquidation value				37,039
Redemption value				$ 37,039
Common Stock Issuable Upon Conversion				1,616,166
Series B Redeemable Convertible Preferred Stock
Redeemable Convertible Preferred Stock
Preferred stock including accrued dividends, converted into common stock			(25,000,000)
Rights and privileges of preferred stockholders:
Preferred Stock Issued			0	25,000,000
Preferred Stock Outstanding			0	25,000,000	25,000,000
Carrying value			$ 0	$ 24,550	$ 24,550
Liquidation value				26,759
Redemption value				$ 26,759
Common Stock Issuable Upon Conversion				1,206,096
B-1 Preferred Stock
Rights and privileges of preferred stockholders:
Preferred Stock Issued				96,440,678
Preferred Stock Outstanding				96,440,678
Carrying value				$ 56,297
Liquidation value				57,330
Redemption value				$ 57,330
Common Stock Issuable Upon Conversion				4,652,668

Redeemable Convertible Prefer_4

Redeemable Convertible Preferred Stock - Voting rights (Details)	12 Months Ended
	Dec. 31, 2018Voteshares
Redeemable Convertible Preferred Stock
Number of shares of common stock | shares	1
Series A Redeemable Convertible Preferred Stock
Redeemable Convertible Preferred Stock
Number of Votes Per Share	1
Votes, as percentage of common stock	49.00%
Series A Redeemable Convertible Preferred Stock | Maximum
Redeemable Convertible Preferred Stock
Percentage of common stock	50.00%
Series B Redeemable Convertible Preferred Stock
Redeemable Convertible Preferred Stock
Number of Votes Per Share	1
B-1 Preferred Stock
Redeemable Convertible Preferred Stock
Number of Votes Per Share	1

Redeemable Convertible Prefer_5

Redeemable Convertible Preferred Stock - Distributions (Details) - USD ($)	Sep. 27, 2018	Dec. 31, 2018
Redeemable Convertible Preferred Stock.
Dividend rate		4.00%
Stock issued as settlement of accrued dividends	609,484
Dividend declared or paid		$ 0

Redeemable Convertible Prefer_6

Redeemable Convertible Preferred Stock - Liquidation preference (Details)	12 Months Ended
	Dec. 31, 2018$ / shares
B-1 Preferred Stock
Redeemable Convertible Preferred Stock
Liquidation preference, per share	$ 0.59
Series B Redeemable Convertible Preferred Stock
Redeemable Convertible Preferred Stock
Liquidation preference, per share	1
Series A Redeemable Convertible Preferred Stock
Redeemable Convertible Preferred Stock
Liquidation preference, per share	$ 1
Minimum
Redeemable Convertible Preferred Stock
Percentage of voting power in case of deemed liquidation event	50.00%

Redeemable Convertible Prefer_7

Redeemable Convertible Preferred Stock - Conversion (Details) $ in Millions	1 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018USD ($)item
Redeemable Convertible Preferred Stock
Conversion ratio	0.048243921	0.048243921
Threshold gross proceeds for automatic conversion into common stock | $		$ 50
Threshold percentage of outstanding preferred stock elected for automatic conversion into common stock		51.00%
B-1 Preferred Stock
Redeemable Convertible Preferred Stock
Threshold number of times of original purchase price for automatic conversion into common stock | item		2

Common Stock (Details)

Common Stock (Details)	Sep. 28, 2018USD ($)$ / sharesshares	Sep. 30, 2018	May 31, 2015shares	Dec. 31, 2018USD ($)Vote$ / sharesshares	Dec. 31, 2017$ / sharesshares
Common Stock
Authorized shares				125,000,000	250,000,000
Par value of common stock | $ / shares				$ 0.001	$ 0.001
Dividend declared or paid | $				$ 0
Conversion ratio		0.048243921		0.048243921
Shares issued (in shares)			4
Net proceeds after deducting underwriting discounts and commissions and offering costs | $				$ 65,624,000
Preferred stock including accrued dividends, converted into common stock	8,084,414
Initial Public Offering
Common Stock
Shares issued (in shares)	5,000,000
Price per share | $ / shares	$ 15
Gross proceeds | $	$ 75,000,000
Net proceeds after deducting underwriting discounts and commissions and offering costs | $	$ 65,600,000
Preferred stock including accrued dividends, converted into common stock	8,084,414
Common Stock
Common Stock
Number of votes per share | Vote				1
Shares issued (in shares)				5,000,000
Net proceeds after deducting underwriting discounts and commissions and offering costs | $				$ 5,000
Preferred stock including accrued dividends, converted into common stock				8,084,414

Stock-Based Compensation Expe_3

Stock-Based Compensation Expense - Stock Incentive Plan (Details) - USD ($)	Sep. 25, 2018	Sep. 30, 2018	Dec. 31, 2018
2015 Stock Incentive Plan
Stock-Based Compensation
Number of options granted (in shares)	0
2018 Stock Incentive Plan | Non-employee directors
Stock-Based Compensation
Maximum cash fee paid		$ 500,000
2018 Stock Incentive Plan | Board of directors
Stock-Based Compensation
Maximum cash fee paid		$ 800,000
2018 Stock Incentive Plan | Stock Options
Stock-Based Compensation
Number of options granted (in shares)			243,106
Number of shares available for future issuance (in shares)			947,389
Number of shares available for grant (in shares)		1,181,972
Period of options		10 years	10 years
Percentage of annual increase to the total number of common stock outstanding (as a percent)		4.00%
2018 Stock Incentive Plan | Incentive stock options
Stock-Based Compensation
Maximum number of shares issued (in shares)		7,500,000

Stock-Based Compensation Expe_4

Stock-Based Compensation Expense - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 25, 2018	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Employee Stock Purchase Plan
Aggregate Intrinsic Value
Number of shares available for future issuance (in shares)		140,000	140,000
Percentage of annual increase to the total number of common stock outstanding (as a percent)		1.00%
Annual increase in the number of shares to the shares of common stock (in shares)		250,000
Maximum number of shares issued (in shares)			0
2015 Stock Incentive Plan
Number of Options
Granted (in shares)	0
2015 and 2018 Stock Incentive Plan
Number of Options
Outstanding, as of beginning of period (in shares)			811,627
Granted (in shares)			606,349
Exercised (in shares)			(27,789)
Cancelled or forfeited (in shares)			(14,457)
Outstanding, as of end of period (in shares)			1,375,730	811,627
Vested or expected to vest (in shares)			1,375,730
Exerciseable (in shares)			492,995
Weighted Average Exercise Price
Outstanding, as of beginning of period (in dollar per share)			$ 3.73
Granted (in dollar per share)			10.12
Exercised (in dollar per share)			3.48
Cancelled or forfeited (in dollar per share)			4.35
Outstanding, as of end of period (in dollar per share)			6.54	$ 3.73
Vested or expected to vest (in dollar per share)			6.54
Exercisable (in dollar per share)			$ 4.54
Weighted Average Remaining Contractual Term (Years)
Options outstanding			8 years 7 months 28 days	9 years 1 month 2 days
Vested or expected to vest			8 years 7 months 28 days
Exercisable			7 years 10 months 10 days
Aggregate Intrinsic Value
Options outstanding			$ 426	$ 6,062
Vested or expected to vest			426
Exercisable			$ 152
Weighted average grant date fair value per share of options granted (in dollar per share)			$ 6.22	$ 3.75

Stock-Based Compensation Expe_5

Stock-Based Compensation Expense - AstraZeneca Shares Option and Incentive Plan (Details) - AstraZeneca Plan - RSUs - USD ($)	1 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2017
Stock-Based Compensation
Vesting percentage (as a percent)	100.00%
Vesting period	36 months
RSU compensation expense		$ 14,604

Stock-Based Compensation Expe_6

Stock-Based Compensation Expense - Stock-Based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Stock-Based Compensation
Total stock-based compensation expense	$ 1,201	$ 420
Unrecognized compensation cost	$ 4,500
Period for recognition	2 years 9 months 18 days
Research and development.
Stock-Based Compensation
Total stock-based compensation expense	$ 479	229
General and administrative
Stock-Based Compensation
Total stock-based compensation expense	$ 722	$ 191

Stock-Based Compensation Expe_7

Stock-Based Compensation Expense - Black-Scholes option pricing model (Details) - Stock Options	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Stock-Based Compensation
Expected stock price volatility (as a percent)	65.80%	65.60%
Risk-free interest rate (as a percent)	3.00%	2.20%
Expected life of options	6 years 1 month 6 days	6 years 1 month 6 days

Income Taxes - Net Loss Before

Income Taxes - Net Loss Before Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Taxes
United Kingdom	$ 28,123	$ 21,806
United States	4,357	8,117
Net loss before the provision for income taxes	$ 32,480	$ 29,923

Income Taxes - Reconciliation (

Income Taxes - Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of the federal statutory income tax rate to the Company’s effective income tax rate
Income tax benefit computed at statutory tax rate (as a percent)	21.00%	35.00%
State taxes, net of federal benefit (as a percent)	4.60%	1.30%
Foreign rate differential (as a percent)	(3.40%)	4.10%
Research and development tax credits (as a percent)	1.40%	3.00%
Permanent difference (as a percent)	11.50%	(1.30%)
Valuation allowances (as a percent)	(35.10%)	(17.70%)
Rate changes (as a percent)		(8.40%)
Foreign withholding tax (as a percent)	(1.40%)
Effective income tax rate (as a percent)	(1.40%)	0.00%
Provision for income taxes	$ 472
U.K.
Reconciliation of the federal statutory income tax rate to the Company’s effective income tax rate
Income tax benefit computed at statutory tax rate (as a percent)		19.00%
CHINA
Reconciliation of the federal statutory income tax rate to the Company’s effective income tax rate
Provision for income taxes	$ 500

Income Taxes - Net Deferred Tax

Income Taxes - Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 20,051	$ 8,967
Tax credit carryforwards	2,321	1,763
Depreciation and amortization		7
Accrued expenses and other	1,097	987
Total deferred tax assets	23,469	11,724
Depreciation and amortization	(10)
Total deferred tax liabilities	(10)
Valuation allowance	$ (23,459)	$ (11,724)	$ (6,415)

Income Taxes - Valuation Allowa

Income Taxes - Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Taxes
Valuation allowance at beginning of year	$ (11,724)	$ (6,415)
Increases recorded to income tax provision	(11,735)	(5,309)
Valuation allowance at end of year	$ (23,459)	$ (11,724)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
TCJA, NOL and Tax Credit
Income tax benefit	$ 472
U.S. corporate tax rate (as a percent)	21.00%	35.00%
Reduction in deferred tax assets	$ 1,700
Maximum percent of net operating losses utilized in future periods (as a percent)	80.00%
Federal indefinite net operating loss carry forwards from JOBS act	$ 21,200
Valuation allowance	23,459	$ 11,724	$ 6,415
U.K.
TCJA, NOL and Tax Credit
U.S. corporate tax rate (as a percent)		19.00%
Net operating loss carryforwards	63,300
U.S. federal
TCJA, NOL and Tax Credit
Net operating loss carryforwards	33,800
U.S. federal | Research and development
TCJA, NOL and Tax Credit
Tax credits carryforwards	1,800
State
TCJA, NOL and Tax Credit
Net operating loss carryforwards	34,500
State | Research and development
TCJA, NOL and Tax Credit
Tax credits carryforwards	$ 700

Net Loss per Share - Calculatio

Net Loss per Share - Calculation (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss	$ (32,952)	$ (29,923)
Less: Dividends declared	(9,142)
Net loss attributable to common stockholders—basic and diluted	$ (42,094)	$ (29,923)
Denominator:
Weighted average common stock outstanding—basic and diluted (in shares)	3,419,720	2,169
Net loss per share attributable to common stockholders—basic and diluted (in dollar per share)	$ (12.31)	$ (13,795.76)

Net Loss per Share - Antidiluti

Net Loss per Share - Antidilutive Securities Excluded from Computation (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Net Loss per Share
Antidilutive securities excluded from computation of earnings per share (in shares)	1,375,730	8,286,557
Stock Options
Net Loss per Share
Antidilutive securities excluded from computation of earnings per share (in shares)	1,375,730	811,627
Redeemable convertible preferred stock
Net Loss per Share
Antidilutive securities excluded from computation of earnings per share (in shares)		7,474,930

Commitments - Lease Commitments

Commitments - Lease Commitments (Details) - AstraZeneca $ in Millions	1 Months Ended	12 Months Ended
	Feb. 28, 2018ft²	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Commitments and Contingencies
Additional space | ft²	7,257
Rental expense | $		$ 0.6	$ 0.4

Commitments - Future minimum le

Commitments - Future minimum lease payments (Details) $ in Thousands	Dec. 31, 2018USD ($)
Future minimum lease payments for the combined spaces
2019	$ 597
2020	656
2021	710
2022	730
Total	$ 2,693

Commitments - A Subscription Ag

Commitments - A Subscription Agreement (Details) - AstraZeneca $ in Millions	1 Months Ended
	Apr. 30, 2018USD ($)
Commitments and Contingencies
Subscription agreement one-time milestone payment on net sales	$ 5
Term of the milestone payment on net sales	3 months
Subscription agreement one-time milestone payment on first commercial sale	$ 10
Term of the milestone payment on first commercial sale	2 years

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
AstraZeneca
Related Party Transactions
Funds received	$ 0.2	$ 17.6

Benefit Plans (Details)

Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Benefits Plans
Company's matching contributions	$ 0.2	$ 0.1