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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2019	Jul. 31, 2019
Document Type	10-Q
Document Period End Date	Jun. 30, 2019
Entity Registrant Name	Kiniksa Pharmaceuticals, Ltd.
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Entity Central Index Key	0001730430
Amendment Flag	false
Common Shares
Entity Common Stock, Shares Outstanding		54,857,692
Class A common shares
Entity Common Stock, Shares Outstanding		19,165,265
Class B common shares
Entity Common Stock, Shares Outstanding		4,638,855
Class A1 common shares
Entity Common Stock, Shares Outstanding		14,995,954
Class B1 common shares
Entity Common Stock, Shares Outstanding		16,057,618

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 82,810	$ 71,976
Short-term investments	204,637	235,328
Prepaid expenses and other current assets	9,011	6,446
Total current assets	296,458	313,750
Property and equipment, net	6,359	6,356
Operating lease right-of-use assets	2,503
Restricted cash	210	210
Deferred offering costs		433
Deferred tax assets	2,607	1,216
Total assets	308,137	321,965
Current liabilities:
Accounts payable	9,338	10,918
Accrued expenses	19,179	16,418
Accrued milestones		15,000
Operating lease liabilities	1,496
Other current liabilities	165	218
Total current liabilities	30,178	42,554
Noncurrent liabilities:
Noncurrent operating lease liabilities	1,819
Other long-term liabilities	943	144
Total liabilities	32,940	42,698
Commitments and contingencies (Note 12)
Shareholders’ equity:
Additional paid-in capital	572,318	473,483
Accumulated other comprehensive income (loss)	101	(4)
Accumulated deficit	(297,237)	(194,225)
Total shareholders’ equity	275,197	279,267
Total liabilities and shareholders’ equity	308,137	321,965
Class A common shares
Shareholders’ equity:
Common stock value	6	4
Class B common shares
Shareholders’ equity:
Common stock value	1	1
Class A1 common shares
Shareholders’ equity:
Common stock value	4	4
Class B1 common shares
Shareholders’ equity:
Common stock value	$ 4	$ 4

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating expenses:
Research and development expense	$ 30,848	$ 17,200	$ 90,101	$ 29,831
General and administrative	8,441	4,327	16,835	8,036
Total operating expenses	39,289	21,527	106,936	37,867
Loss from operations	(39,289)	(21,527)	(106,936)	(37,867)
Interest income	1,724	1,066	3,533	1,371
Loss before benefit for income taxes	(37,565)	(20,461)	(103,403)	(36,496)
Benefit for income taxes	374	202	391	255
Net loss	$ (37,191)	$ (20,259)	$ (103,012)	$ (36,241)
Net loss per share attributable to common shareholders - basic and diluted	$ (0.68)	$ (1.11)	$ (1.94)	$ (3.45)
Weighted average common shares outstanding - basic and diluted	54,475,476	18,328,402	53,225,710	10,492,474
Comprehensive loss:
Net loss	$ (37,191)	$ (20,259)	$ (103,012)	$ (36,241)
Other comprehensive loss:
Unrealized gain on short-term investments	93	7	105
Total other comprehensive loss	93		105
Total comprehensive loss	$ (37,098)	$ (20,259)	$ (102,907)	$ (36,241)

CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED SHARES AND SHAREHOLDERS’ (DEFICIT) EQUITY - USD ($) $ in Thousands	Convertible Preferred Shares (Series A, B and C)	Common Shares Follow-On Offering	Common Shares Private Placement	Common Shares Initial Public Offering	Common Shares	Additional Paid-In Capital Follow-On Offering	Additional Paid-In Capital Private Placement	Additional Paid-In Capital Initial Public Offering	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Follow-On Offering	Private Placement	Initial Public Offering	Total
Balance at the beginning of the period at Dec. 31, 2017	$ 119,770
Balance at the beginning of the period (in shares) at Dec. 31, 2017	22,885,492
Changes in temporary equity
Issuance of Series C convertible preferred shares, net of issuance costs	$ 190,822
Issuance of Series C convertible preferred shares, net of issuance costs (in shares)	12,784,601
Balance at the end of the period at Mar. 31, 2018	$ 310,592
Balance at the end of the period (in shares) at Mar. 31, 2018	35,670,093
Balance at the beginning of the period at Dec. 31, 2017					$ 1				$ 1,289		$ (90,998)				$ (89,708)
Balance at the beginning of the period (in shares) at Dec. 31, 2017					4,288,329
Changes in equity
Exercise of options									17						17
Exercise of options (in shares)					4,574
Share-based compensation expense									558						558
Net loss											(15,982)				(15,982)
Balance at the end of the period at Mar. 31, 2018					$ 1				1,864		(106,980)				(105,115)
Balance at the end of the period (in shares) at Mar. 31, 2018					4,292,903
Balance at the beginning of the period at Dec. 31, 2017	$ 119,770
Balance at the beginning of the period (in shares) at Dec. 31, 2017	22,885,492
Balance at the beginning of the period at Dec. 31, 2017					$ 1				1,289		(90,998)				(89,708)
Balance at the beginning of the period (in shares) at Dec. 31, 2017					4,288,329
Changes in equity
Net loss															(36,241)
Balance at the end of the period at Jun. 30, 2018					$ 13				469,018	$ 7	(127,239)				341,799
Balance at the end of the period (in shares) at Jun. 30, 2018					49,447,198
Balance at the beginning of the period at Mar. 31, 2018	$ 310,592
Balance at the beginning of the period (in shares) at Mar. 31, 2018	35,670,093
Changes in temporary equity
Conversion of convertible preferred shares into common shares	$ (310,592)
Conversion of convertible preferred shares into common shares (in shares)	(35,670,093)
Balance at the beginning of the period at Mar. 31, 2018					$ 1				1,864		(106,980)				(105,115)
Balance at the beginning of the period (in shares) at Mar. 31, 2018					4,292,903
Changes in equity
Conversion of convertible preferred shares into common shares					$ 8				310,584						310,592
Conversion of convertible preferred shares into common shares (in shares)					35,670,093
Issuance of common shares upon completion of offering, net of underwriting discounts and commissions and offering costs				$ 4				$ 155,511						$ 155,515
Issuance of common shares upon completion of offering, net of underwriting discounts and commissions and offering costs (in shares)				9,484,202
Share-based compensation expense									1,059						1,059
Unrealized gain on short-term investments										7					7
Net loss											(20,259)				(20,259)
Balance at the end of the period at Jun. 30, 2018					$ 13				469,018	7	(127,239)				341,799
Balance at the end of the period (in shares) at Jun. 30, 2018					49,447,198
Balance at the beginning of the period at Dec. 31, 2018					$ 13				473,483	(4)	(194,225)				279,267
Balance at the beginning of the period (in shares) at Dec. 31, 2018					49,489,647
Changes in equity
Issuance of common shares upon completion of offering, net of underwriting discounts and commissions and offering costs		$ 2				$ 48,474	$ 34,511					$ 48,476	$ 34,511
Issuance of common shares upon completion of offering, net of underwriting discounts and commissions and offering costs (in shares)		2,816,110	2,000,000
Common shares issued or to be issued in connection with the acquisition of all issued and outstanding equity securities of Primatope Therapeutics, Inc.									7,000						7,000
Common shares issued or to be issued in connection with the acquisition of all issued and outstanding equity securities of Primatope Therapeutics, Inc. (in shares)					337,008
Exercise of options									181						181
Exercise of options (in shares)					50,070
Share-based compensation expense									2,893						2,893
Unrealized gain on short-term investments										12					12
Net loss											(65,821)				(65,821)
Balance at the end of the period at Mar. 31, 2019					$ 15				566,542	8	(260,046)				306,519
Balance at the end of the period (in shares) at Mar. 31, 2019					54,692,835
Balance at the beginning of the period at Dec. 31, 2018					$ 13				473,483	(4)	(194,225)				279,267
Balance at the beginning of the period (in shares) at Dec. 31, 2018					49,489,647
Changes in equity
Unrealized gain on short-term investments															105
Net loss															(103,012)
Balance at the end of the period at Jun. 30, 2019					$ 15				572,318	101	(297,237)				275,197
Balance at the end of the period (in shares) at Jun. 30, 2019					54,857,692
Balance at the beginning of the period at Mar. 31, 2019					$ 15				566,542	8	(260,046)				306,519
Balance at the beginning of the period (in shares) at Mar. 31, 2019					54,692,835
Changes in equity
Common shares issued or to be issued in connection with a milestone payment due to Primatope Therapeutics, Inc.									1,800						1,800
Common shares issued or to be issued in connection with a milestone payment due to Primatope Therapeutics, Inc. (in shares)					94,284
Exercise of options									512						512
Exercise of options (in shares)					70,573
Share-based compensation expense									3,464						3,464
Unrealized gain on short-term investments										93					93
Net loss											(37,191)				(37,191)
Balance at the end of the period at Jun. 30, 2019					$ 15				$ 572,318	$ 101	$ (297,237)				$ 275,197
Balance at the end of the period (in shares) at Jun. 30, 2019					54,857,692

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (103,012)	$ (36,241)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	979	20
Share-based compensation expense	6,357	1,617
Class A common shares issued or to be issued as consideration for Primatope, including milestone payments	8,800
Loss on disposal of property and equipment		66
Non-cash rent expense		208
Accretion of discounts on short-term investments	(2,055)
Deferred income taxes	(1,390)	(463)
Changes in operating assets and liabilities:
Prepaid expenses, right-of-use assets and other assets	(1,009)	(2,965)
Accounts payable	(1,663)	(747)
Accrued expenses and other liabilities	2,720	5,616
Accrued milestones	(15,000)
Operating lease liabilities	(602)
Other long-term liabilities	943
Net cash provided by (used in) operating activities	(104,932)	(32,889)
Cash flows from investing activities:
Purchases of property and equipment	(767)	(311)
Purchases of short-term investments	(273,488)	(73,676)
Proceeds from the maturities of short-term investments	306,340
Net cash used in investing activities	32,085	(73,987)
Cash flows from financing activities:
Payments of offering costs	(118)	(2,825)
Proceeds from exercise of options and employee share purchase plan	693	17
Net cash provided by financing activities	83,681	347,207
Net increase in cash and cash equivalents and restricted cash	10,834	240,331
Cash and cash equivalents and restricted cash at beginning of period	72,186	45,660
Cash and cash equivalents and restricted cash at end of period	83,020	285,991
Supplemental information:
Cash paid for income taxes	1,027	148
Supplemental disclosure of non-cash investing and financing activities:
Deferred offering costs included in accrued expenses and accounts payable		856
Property and equipment included in accrued expenses and accounts payable	253
Series C convertible preferred shares
Cash flows from financing activities:
Proceeds from issuance of convertible preferred shares, net of issuance costs		190,822
Follow-On Offering \| Class A common shares
Cash flows from financing activities:
Proceeds from issuance of common shares, net of underwriting commissions and discounts	48,595
Private Placement \| Class A1 common shares
Cash flows from financing activities:
Proceeds from issuance of common shares from private placement, net of underwriting commissions and discounts	$ 34,511
Initial Public Offering \| Class A common shares
Cash flows from financing activities:
Proceeds from issuance of common shares, net of underwriting commissions and discounts		$ 159,193

Nature of the Business and Basis of Presentation	6 Months Ended
Nature of the Business and Basis of Presentation	Jun. 30, 2019
Nature of the Business and Basis of Presentation
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Kiniksa Pharmaceuticals, Ltd. (the “Company”) is a biopharmaceutical company focused on discovering, acquiring, developing and commercializing therapeutic medicines for patients suffering from debilitating diseases with significant unmet medical need. The Company was incorporated in July 2015 as a Bermuda exempted company. The Company has a pipeline of product candidates across various stages of development, focused on autoinflammatory and autoimmune conditions. The Company is subject to risks and uncertainties common to early‑stage companies in the biopharmaceutical industry. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s technology will be obtained, that any products developed will obtain necessary government regulatory approval or that any products, if approved, will be commercially viable. The Company does not currently generate revenue from sales of any products, and it may never be able to develop or commercialize a marketable product. The Company has not yet successfully completed any Phase 3 or other pivotal clinical trials, obtained any regulatory approvals, manufactured a commercial‑scale drug, or conducted sales and marketing activities. The Company operates in an environment of rapid technological innovation and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, consultants and service providers. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. Principles of Consolidation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of the Company and its wholly owned subsidiaries, Kiniksa Pharmaceuticals Corp. (“Kiniksa US”), Kiniksa Pharmaceuticals (UK), Ltd. (“Kiniksa UK”), Kiniksa Pharmaceuticals (Germany) GmbH (“Kiniksa Germany”), Kiniksa Pharmaceuticals (France) SARL (“Kiniksa France”) and Primatope Therapeutics, Inc. (“Primatope”), after elimination of all significant intercompany accounts and transactions. In assessing the consolidation requirement for variable interest entities (“VIEs”), the Company focuses on identifying whether it has both the power to direct the activities that most significantly impact the VIE’s economic performance and the obligation to absorb losses or the right to receive benefits from the VIE. In the event that the Company is the primary beneficiary of a VIE, the assets, liabilities, and results of operations of the VIE would be included in the Company’s consolidated financial statements. At December 31, 2018 and during the year then ended and at June 30, 2019 and during the three and six months then ended, the Company was not the primary beneficiary of a VIE. Use of Estimates The preparation of the Company’s consolidated financial statements in conformity with GAAP requires management to make estimates 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 expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual for research and development expenses and the valuation of common shares and share‑based awards. 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 those estimates. Reporting and Functional Currency The financial results of the Company's global activities are reported in U.S. dollars (“USD”) and its foreign subsidiaries generally utilize their respective local currency to be their functional currency. Transactions in other currencies are recorded in the functional currency at the rate of exchange prevailing when the transactions occur. Monetary assets and liabilities denominated in other currencies are re-measured into the functional currency at the rate of exchange in effect at the balance sheet date. Exchange rate gains and losses arising from re-measurement of foreign currency-denominated monetary assets and liabilities are included in income or losses in the period in which they occur. For the Company’s foreign subsidiaries where the local currency is the functional currency, assets and liabilities denominated in local currencies are translated into USD at end-of-period exchange rates and the resulting translation adjustments are reported as a component of accumulated other comprehensive loss within shareholders' equity. Unaudited Interim Consolidated Financial Information The accompanying unaudited consolidated financial statements have been prepared in accordance with GAAP for interim financial information. The accompanying unaudited consolidated financial statements do not include all of the information and footnotes required by GAAP for complete consolidated financial statements. The accompanying year-end consolidated balance sheet was derived from audited financial statements but does not include all disclosures required by GAAP. The unaudited interim consolidated financial statements have been prepared on the same basis as the audited annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of June 30, 2019 and the results of its operations for the three and six months ended June 30, 2019 and 2018 and its cash flows for the six months ended June 30, 2019 and 2018. The results for the three and six months ended June 30, 2019 are not necessarily indicative of results to be expected for the year ending December 31, 2019, any other interim periods or any future year or period. Reverse Stock Split On May 11, 2018, the Company effected a 1-for-2.73235 reverse share split of its authorized, designated, issued and outstanding common shares and preferred shares. Accordingly, all share and per share amounts for all periods presented in the accompanying consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse share split. Initial Public Offering On May 23, 2018, the Company’s registration statement on Form S-1 relating to its initial public offering of its Class A common shares (the “IPO”) was declared effective by the Securities and Exchange Commission (“SEC”). On May 29, 2018, the Company completed the IPO of 8,477,777 Class A common shares at a public offering price of $18.00 per share for gross proceeds of $152,600. In addition, on June 22, 2018, the Company completed the sale of 1,006,425 Class A common shares to the underwriters of the IPO following the exercise in part of their over-allotment option to purchase additional shares at a public offering price of $18.00 per share for gross proceeds of $18,116. The aggregate net proceeds to the Company from the IPO, inclusive of the over-allotment option exercise, was $155,536 after deducting underwriting discounts and commissions and other offering costs. Upon the closing of the IPO, all convertible preferred shares then outstanding automatically converted into 5,546,019 Class A common shares, 1,070,502 Class B common shares, 12,995,954 Class A1 common shares and 16,057,618 Class B1 common shares. In connection with the closing of the IPO, the Company amended and restated its bye-laws (the “Amended & Restated Bye-Laws”). Follow-on Offering and Private Placement On February 4, 2019, the Company completed a follow-on offering of 2,654,984 Class A common shares at a public offering price of $18.26 and a concurrent private placement of 2,000,000 Class A1 common shares at an offering price of $18.26 per share for aggregate gross proceeds of $85,000. In addition, on March 1, 2019, the Company completed the sale of 161,126 Class A common shares to the underwriters of the follow-on offering following the exercise in part of their over-allotment option to purchase additional shares at a public offering price of $18.26 per share for gross proceeds of $2,942. The aggregate net proceeds to the Company from the follow-on offering and concurrent private placement, inclusive of the over-allotment option exercise, was $82,985 after deducting underwriting discounts and commissions and other offering costs Liquidity In accordance with Accounting Standards Update (“ASU”) No. 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (Subtopic 205-40), the Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date the consolidated financial statements are issued. As of June 30, 2019, the Company had an accumulated deficit of $297,237. During the six months ended June 30, 2019, the Company incurred a net loss of $103,012 and used $104,932 of net cash in operating activities. The Company expects to continue to generate operating losses for the foreseeable future. As of June 30, 2019, the Company had cash, cash equivalents and short-term investments of $287,447. Based on its current operating plan, the Company expects that its cash, cash equivalents and short-term investments will be sufficient to fund its operating expenses and capital expenditure requirements for at least 12 months from the issuance date of these consolidated financial statements. The future viability of the Company beyond that point is dependent on its ability to raise additional capital to finance its operations. Although the Company has been successful in raising capital in the past, there is no assurance that it will be successful in obtaining such additional financing on terms acceptable to the Company, if at all. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations.

Summary of Significant Accounting Policies

6 Months Ended

Jun. 30, 2019

Summary of Significant Accounting Policies

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Cash and Cash Equivalents

The Company classifies deposits in banks, money market funds and cash invested temporarily in various instruments with maturities of three months or less at the time of purchase as cash and cash equivalents. At June 30, 2019 and December 31, 2018, cash and cash equivalents consisted principally of U.S. Treasury notes, amounts held in money market funds and cash on deposit at commercial banks.

Short-Term Investments

The Company generally invests its excess cash in money market funds and short-term investments in U.S. Treasury notes. Such investments included in short-term investments on the Company's consolidated balance sheets are considered available-for-sale debt securities and are reported at fair value with unrealized gains and losses included as a component of shareholders’ equity. Realized gains and losses, if any, on short-term investments are included in interest income.

The Company evaluates its short-term investments with unrealized losses for other-than-temporary impairment. When assessing short-term investments for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the consolidated statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented.

Concentrations of Credit Risk

Financial instruments that potentially subject the Company to concentrations of credit risk consist principally of cash, cash equivalents and short-term investments. At June 30, 2019 and December 31, 2018, substantially all of the Company’s cash, cash equivalents and short-term investments were held at two financial institutions. The Company generally maintains balances in various operating accounts at financial institutions that management believes to be of high credit quality, in amounts that may exceed federally insured limits. The Company has not experienced any losses related to its cash, cash equivalents and short-term investments and does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships.

Restricted Cash

In conjunction with the Company’s lease agreement entered into in March 2018 (see Note 5), the Company maintains a letter of credit for the benefit of the landlord. As of June 30, 2019 and December 31, 2018, the underlying cash balance of $210 securing this letter of credit, was classified as non‑current in its consolidated balance sheet.

Fair Value Measurements

Certain assets and liabilities of the Company are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:

Level 1—Quoted prices in active markets for identical assets or liabilities.

Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.

Level 3—Unobservable inputs that are supported by little or no market activity that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.

The Company’s restricted cash, which is held in a money market fund, is carried at fair value, determined based on Level 1 inputs in the fair value hierarchy described above (see Note 3). The Company’s cash equivalents and short-term investments, consisting of money market funds and U.S. Treasury notes, are carried at fair value, determined based on Level 1 and 2 inputs in the fair value hierarchy described above (see Note 3). The carrying values of the Company’s prepaid expenses and other current assets, accounts payable and accrued expenses approximate their fair values due to the short‑term nature of these assets and liabilities.

Leases

In February 2016, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which set forth the principles for recognition, measurement, presentation and disclosure of lease arrangements to enhance the transparency and comparability of financial reporting related to the arrangements. ASU 2016-02, including subsequently issued amendments, is collectively referred to as Accounting Standards Codification, Leases (Topic 842) (“ASC 842”). The Company adopted the new standard on January 1, 2019 using the modified retrospective transition approach as applied to leases existing as of the adoption date. The standard will be applied to all leases entered into after the initial adoption date.

At the inception of an arrangement, the Company determines whether the arrangement is or contains a “lease” as defined by ASC 842. A lease is an arrangement, or part of an arrangement, that conveys the right to control the use of identified property, plant, or equipment (an identified asset) for a period of time in exchange for consideration. The Company determines if the arrangement conveys the right to control the use of an identified asset for a period of time. It assesses throughout the period of use whether the Company has both of the following (1) the right to obtain substantially all of the economic benefits from use of the identified asset and (2) the right to direct the use of the identified asset. This determination is reassessed if the terms of the arrangement are changed. Leases are classified as operating or finance leases based on the terms of the lease agreement and certain characteristics of the identified asset. Right-of-use (“ROU”) assets and lease liabilities are recognized at lease commencement date based on the present value of the minimum future lease payments.

Most leases with a term greater than one year are recognized on the balance sheet as ROU assets with corresponding lease liabilities and, if applicable, long-term lease liabilities. The Company has elected not to recognize leases with a term of one year or less on its balance sheet. Operating leases, ROU assets and their corresponding lease liabilities are recorded based on the present value of lease payments over the expected remaining lease term. However, certain adjustments to the ROU assets may be required for items such as incentives received. The interest rate implicit in lease arrangements is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment.

In accordance with the guidance in ASU 2016-02, components of a lease should be split into three categories: lease components (e.g., land, building, etc.), non-lease components (e.g., common area maintenance, consumables, etc.), and non-components (e.g., property taxes, insurance, etc.); then the fixed and in-substance fixed contract consideration (including any related to non-components) must be allocated based on fair values to the lease components and non-lease components.

Although separation of lease and non-lease components is required, certain practical expedients are available. Companies may elect the practical expedient to not separate lease and non-lease components. In which case, the Company would account for each lease component and the related non-lease component together as a single component. The Company has elected to account for the lease and non-lease components of each of its operating leases as a single lease component and allocate all of the arrangement consideration to the lease component only. The lease component results in an operating right-of-use asset being recorded on the balance sheet and amortized on a straight-line basis as lease expense.

Research and Development Costs

Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred to discover, research and develop drug candidates, including personnel expenses, share‑based compensation expense, allocated facility‑related and depreciation expenses, third‑party license fees and external costs of outside vendors engaged to conduct preclinical and clinical development activities and clinical trials as well as to manufacture clinical trial materials. Non‑refundable prepayments for goods or services that will be used or rendered for future research and development activities are recorded as prepaid expenses. Such amounts are recognized as an expense as the goods 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.

Research Contract Costs and Accruals

The Company has entered into various research and development-related contracts with companies both inside and outside of the United States. The related costs 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 or clinical trials, 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 materially from the Company's estimates. The Company's historical accrual estimates have not been materially different from the actual costs.

Share‑Based Compensation

The Company measures all share-based awards granted to employees and directors based on their fair value on the date of the grant and recognizes compensation expense for those awards over the requisite service period, which is generally the vesting period of the respective award. Forfeitures are accounted for as they occur. The Company issues share-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 share-based awards with performance-based vesting conditions.

For share-based awards granted to consultants and non-employees, compensation expense is recognized over the vesting period of the awards, which is generally the period during which services are rendered by such consultants and non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is remeasured using the then-current fair value of the Company's Class A common shares and updated assumption inputs in the Black-Scholes option-pricing model.

The Company classifies share-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient's payroll costs are classified or in which the award recipient's service payments are classified.

The fair value of each restricted share award is estimated on the date of grant based on the fair value of the Company’s Class A common shares or Class B common shares on that same date. The fair value of each option grant is estimated using the Black-Scholes option-pricing model, which requires inputs based on certain subjective assumptions, including the expected share price volatility, the expected term of the award, the risk-free interest rate, and expected dividends (see Note 9). Prior to May 2018, the Company was a private company and, accordingly, lacks company-specific historical and implied volatility information for its shares. Therefore, it estimates its expected share 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 share price. The expected term of the Company’s options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of options granted to non-employees is equal to the contractual term of the option award. 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. Expected dividend yield is based on the fact that the Company has never paid cash dividends on common shares and does not expect to pay any cash dividends in the foreseeable future.

Comprehensive Loss

Comprehensive loss includes net loss as well as other changes in shareholders’ equity that result from transactions and economic events other than those with shareholders. For the three and six months ended June 30, 2019, the Company’s other comprehensive loss was primarily related to unrealized gain on short-term investments as well as cumulative translation adjustments. For the three and six months ended June 30, 2018, there was no difference between net loss and comprehensive loss.

Net Loss per Share

The Company follows the two‑class method when computing net loss per share as the Company has issued shares that meet the definition of participating securities. The two‑class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two‑class method requires income available to common shareholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed.

Basic net loss per share attributable to common shareholders is computed by dividing the net loss attributable to common shareholders by the weighted average number of common shares outstanding for the period. Diluted net loss attributable to common shareholders is computed by adjusting net loss attributable to common shareholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net loss per share attributable to common shareholders is computed by dividing the diluted net loss attributable to common shareholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares. For purpose of this calculation, outstanding options, unvested restricted common shares and convertible preferred shares are considered potential dilutive common shares.

Prior to the closing of the IPO, when the Company’s convertible preferred shares converted to common shares, the Company's convertible preferred shares contractually entitled the holders of such shares to participate in dividends but did not contractually require the holders of such shares to participate in losses of the Company. Accordingly, for periods in which the Company reported a net loss attributable to common shareholders, such losses were not allocated to convertible preferred shareholders. In periods in which the Company reports a net loss attributable to common shareholders, diluted net loss per share attributable to common shareholders is the same as basic net loss per share attributable to common shareholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. The Company reported a net loss attributable to common shareholders for the three and six months ended June 30, 2019 and 2018.

The Company identified an error in its calculation of weighted average shares for certain shares issued and outstanding during the three and six months ended June 30, 2018, which is not considered material to the previously issued financial statements. The previously issued financial statements were revised to reflect an adjustment to decrease weighted average common shares outstanding by 2,458,886 and 1,243,104 and increase net loss per share by $0.14 and $0.36 for the three and six months ended June 30, 2018, respectively.

Recently Adopted Accounting Pronouncements

In February 2016, the FASB issued ASU 2016-02, Leases. The standard, including subsequently issued amendments, collectively referred to as ASC 842, established principles of recognition, measurement, presentation and disclosure of lease arrangements applicable to lessees and lessors in order to enhance the transparency and compatibility of financial reporting related to the arrangements, including with respect to the amount, timing and uncertainty of cash flows arising from a lease. The Company adopted new accounting guidance regarding the accounting for leases as of January 1, 2019 using a modified retrospective transition approach that was applied to leases existing as of, or entered into prior to, January 1, 2019. See Note 2, Summary of Significant Accounting Policies, “Leases” for a discussion of the Company’s policy with respect to this standard and Note 5, “Leases” for a discussion of the Company’s adoption of this standard and its impact on its consolidated financial statements and related disclosures.

Upon the adoption of ASC 842, the Company recorded operating lease right-of-use assets of $3,682 and operating lease liabilities of $3,917 for its leases which were in effect and had commenced prior to January 1, 2019 and had original lease terms of more than 12 months.

Fair Value of Financial Assets and Liabilities

6 Months Ended

Jun. 30, 2019

Fair Value of Financial Assets and Liabilities

Fair Value of Financial Assets and Liabilities

3. Fair Value of Financial Assets and Liabilities

Short-term investments as of June 30, 2019 and December 31, 2018 consisted of U.S. Treasury notes which investments are each due within six months of such respective period. As of June 30, 2019 and December 31, 2018, the fair value of short-term investments was $204,637 and $235,328, respectively. As of June 30, 2019, the amortized cost was $204,536 and gross unrealized gain was $101. As of December 31, 2018, the amortized cost was $235,332 and gross unrealized loss was $4.

The following tables present information about the Company’s financial assets measured at fair value on a recurring basis and indicate the level of the fair value hierarchy used to determine such fair values:


	Fair Value Measurements
	as of June 30, 2019 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Restricted cash — money market funds	$	210	$	—	$	—	$	210
Cash equivalents — money market funds		5,460		—		—		5,460
Cash equivalents — U.S. Treasury notes		—		15,485		—		15,485
Short-term investments — U.S. Treasury notes		—		204,637		—		204,637
	$	5,670	$	220,122	$	—	$	225,792


	Fair Value Measurements
	as of December 31, 2018 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Restricted cash — money market funds	$	210	$	—	$	—	$	210
Cash equivalents — money market funds		29,721		—		—		29,721
Cash equivalents — U.S. Treasury notes		—		15,634		—		15,634
Short-term investments — U.S. Treasury notes		—		235,328		—		235,328
	$	29,931	$	250,962	$	—	$	280,893

During the periods ended June 30, 2019 and December 31, 2018 there were no transfers between Level 1, Level 2 and Level 3.

The money market funds were valued using quoted prices in active markets, which represent a Level 1 measurement in the fair value hierarchy. The Company's cash equivalents and short-term investments as of June 30, 2019 and December 31, 2018 consisted of U.S. Treasury notes, which are not traded on a daily basis and, therefore, represent a Level 2 measurement in the fair value hierarchy at each period end. All of the Company’s other assets and liabilities are recorded at fair value.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Convertible Preferred Shares	6 Months Ended
Convertible Preferred Shares	Jun. 30, 2019
Convertible Preferred Shares
Convertible Preferred Shares	7. Convertible Preferred Shares As of December 31, 2017, the Company’s bye-laws, as amended and restated, designated 22,885,492 authorized shares to be issued as convertible preferred shares with a par value of $0.000273235 per share, of which 17,128,120 shares were further designated as Series A convertible preferred shares (the “Series A preferred shares”) and 5,757,372 shares were further designated as Series B convertible preferred shares (the "Series B preferred shares"). In February 2018, the Company’s bye-laws were further amended and restated to, among other things, effect an increase in the number of authorized convertible preferred shares with a par value of $0.000273235 per share to 35,670,093 shares, of which 12,784,601 shares were further designated as Series C convertible preferred shares (the "Series C preferred shares"). The holders of convertible preferred shares had liquidation rights in the event of a deemed liquidation that, in certain situations, was not solely within the control of the Company. Therefore, the Series A, Series B and Series C convertible preferred shares (collectively, the “Preferred Shares”) were classified outside of shareholders' equity. In October 2015, the Company issued and sold 8,028,809 Series A preferred shares at a price of $4.6707 per share (the “Series A Original Issue Price”) for proceeds of $37,398, net of issuance costs of $102. In September 2016, the Company issued and sold an additional 9,099,311 Series A preferred shares at a price of $4.6707 per share for proceeds of $42,499, net of issuance costs of $1. In March 2017, the Company issued and sold 5,757,372 Series B preferred shares at a price of $6.9475 per share (the “Series B Original Issue Price”) for proceeds of $39,873, net of issuance costs of $127. In February 2018, the Company issued and sold 12,784,601 Series C preferred shares at a price of $15.6438 per share (the “Series C Original Issue Price”) for proceeds of $190,822, net of issuance costs of $9,178. In May 2018, upon the completion of the IPO (which qualified as a “Qualified IPO” under the Company’s bye-laws, as amended and restated), all of the outstanding Preferred Shares were converted into 5,546,019 Class A common shares, 1,070,502 Class B common shares, 12,995,954 Class A1 common shares and 16,057,618 Class B1 common shares in accordance with the Company’s bye-laws, as amended and restated. In connection with the completion of the IPO in May 2018, the Company adopted the Amended & Restated Bye-Laws to, among other things, authorize the issuance of undesignated preferred shares. As of June 30, 2019, no preferred shares were designated or issued. Prior to the conversion to common shares, the holders of the Preferred Shares had the following rights and preferences: Voting The holders of Preferred Shares were entitled to vote, together with the holders of common shares, on all matters submitted to shareholders for a vote. The holders of Series A preferred shares were entitled to the number of votes per Series A preferred share equal to the number of whole Class B common shares into which the Series A preferred shares were convertible at the time of such vote (which was ten votes for each Class B common share). The holders of Series B preferred shares were entitled to the number of votes per Series B preferred share equal to the number of whole Class A common shares into which the Series B preferred shares were convertible at the time of such vote (which was one vote for each Class A common share). The holders of Series C preferred shares were entitled to the number of votes per Series C preferred share equal to the number of whole Class A common shares into which the Series C preferred shares were convertible at the time of such vote (which was one vote for each Class A common share). Except as provided by law or by the other provisions of the Company’s bye-laws, holders of Preferred Shares voted together with the holders of common shares as a single class. The holders of Preferred Shares, voting together as a single class, were entitled to elect two directors of the Company. The holders of Preferred Shares, voting together with the holders of common shares as a single class, were entitled to elect the remaining directors of the Company, except for the one director that the holders of Class A common shares and Class B common shares, voting together as a single class were entitled to elect. Conversion Each Series A preferred share was convertible, at the option of the holder, at any time and from time to time, and without the payment of additional consideration by the holder thereof, in such manner as permitted by Bermuda law, into such number of fully paid and non-assessable Class B common shares determined by dividing the Series A Original Issue Price by the Series A Conversion Price (as defined below) in effect at the time of conversion. Each Series B preferred share was convertible, at the option of the holder, at any time and from time to time, and without the payment of additional consideration by the holder thereof, in such manner as permitted by Bermuda law, into such number of fully paid and non-assessable Class A common shares determined by dividing the Series B Original Issue Price by the Series B Conversion Price (as defined below) in effect at the time of conversion. Each Series C preferred share was convertible, at the option of the holder, at any time and from time to time, and without the payment of additional consideration by the holder thereof, in such manner as permitted by Bermuda law, into such number of fully paid and non-assessable Class A common shares determined by dividing the Series C Original Issue Price by the Series C Conversion Price (as defined below) in effect at the time of conversion. At the time of the IPO: the Series A Original Issue Price and Series A Conversion Price were equal to $4.6707; the Series B Original Issue Price and Series B Conversion Price were equal to $6.9475; and the Series C Original Issue Price and Series C Conversion Price were equal to $15.6438. Accordingly, each Series A preferred share was convertible into one Class B common share, each Series B preferred share was convertible into one Class A common share and each Series C preferred share was convertible into one Class A common share. Further, upon either (i) the closing of the sale of Class A common shares or Class B common shares to the public at a price of at least $15.6438 per share (subject to appropriate adjustment in the event of any share dividend, share split, combination or other similar recapitalization with respect to the applicable class of common shares) in an initial public offering resulting in at least $100,000 of gross proceeds to the Company (a “Qualified IPO”) or (ii) the date and time, or the occurrence of an event, specified by vote or written consent of the holders of a majority of the outstanding Preferred Shares, voting together as a single class on an as if converted to Class A common shares basis, all outstanding Series A preferred shares would automatically be converted, in such manner as permitted pursuant to Bermuda law, into Class B common shares at the then effective conversion rate, and all outstanding Series B and Series C preferred shares would automatically be converted, in such manner as permitted pursuant to Bermuda law, into Class A common shares at the then effective conversion rate. Notwithstanding the foregoing, in the event of a mandatory conversion of preferred shares as a result of a Qualified IPO, (a) holders of Series A preferred shares could have elected to receive Class B1 common shares in lieu of Class B common shares and (b) holders of Series B and Series C preferred shares could have elected to receive Class A1 common shares in lieu of Class A common shares. Dividends The holders of the Preferred Shares were entitled to receive noncumulative dividends when and if declared by the Company’s board of directors. The Company was not permitted to declare, pay or set aside any dividends on any other class or series of shares of the Company, other than dividends on common shares payable in common shares, unless the holders of the Preferred Share first received, or simultaneously received, a dividend on each outstanding Preferred Share equal to (i) in the case of a dividend on any class of common shares or any class or series was convertible into common shares, that dividend per Preferred Share as would have equaled the product of (a) the dividend payable on each share of such class or series determined, if applicable, as if all shares of such class or series had been converted into common shares and (b) the number of common shares issuable upon conversion of a share of the applicable series of Preferred Shares, or (ii) in the case of a dividend on any class or series that was not convertible into common shares, at a rate per Preferred Share determined by (a) dividing the amount of the dividend payable on each share of such class or series of shares by the original issue price of such class or series (subject to appropriate adjustment in the event of any bonus share, share dividend, share split, combination of or other similar recapitalization with respect to such class or series) and (b) multiplying such fraction by an amount equal to the applicable Series A, Series B or Series C Original Issue Price. No cash dividends were declared or paid on the Preferred Shares. Liquidation Preference In the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company or deemed liquidation event (as defined below), the holders of Preferred Shares then outstanding were entitled to be paid out of the assets of the Company available for distribution to its shareholders, on a pari passu basis, before any payment was made to the holders of common shares by reason of their ownership thereof, an amount per share equal to the greater of (i) one times the applicable Original Issue Price, plus any dividends declared but unpaid thereon, and (ii) such amount per share as would have been payable had all Preferred Shares been converted into common shares immediately prior to such liquidation, dissolution, winding up or deemed liquidation event. Thereafter, the remaining assets of the Company available for distribution to its shareholders would have been distributed among the holders of common shares, pro rata based on the number of shares held by each such holder. If upon any such liquidation, dissolution or winding up of the Company or deemed liquidation event, the assets of the Company available for distribution to its shareholders were insufficient to pay the holders of Preferred Shares the full amount to which they were entitled, the holders of Preferred Shares would have shared ratably in any distribution of the assets available for distribution in proportion to the respective amounts which would otherwise have been payable in respect of the shares held by such holders of Preferred Shares upon such distribution if all amounts payable on or with respect to such shares were paid in full. Unless a majority of the holders of the then outstanding Preferred Shares elected otherwise, a deemed liquidation event included a merger or consolidation (other than one in which shareholders of the Company owned a majority by voting power of the outstanding shares of the surviving or acquiring company or corporation) or a sale, lease, transfer, exclusive license or other disposition of all or substantially all of the assets of the Company. Redemption The Company’s bye-laws, as amended and restated, did not provide redemption rights to the holders of Preferred Shares.

Share Based Compensation

6 Months Ended

Jun. 30, 2019

Share-Based Compensation

Share-Based Compensation

9. Share‑Based Compensation

In May 2018, the Company's board of directors and shareholders approved the 2018 Incentive Award Plan (the "2018 Plan"), which became effective on May 23, 2018. The 2018 plan provides for the grant of incentive options, nonqualified options, share appreciation rights, restricted shares, dividend equivalents, restricted share units and other share- or cash- based awards. Upon the effectiveness of the 2018 Plan, the Company ceased granting awards under its 2015 Equity Incentive Plan (as amended, the “2015 Plan” together with the 2018 Plan, the “Plans”).

In May 2018, the Company's board of directors and shareholders approved the 2018 Employee Share Purchase Plan (the "2018 ESPP"), which became effective on May 23, 2018. A total of 670,000 Class A common shares were initially reserved for issuance under the 2018 ESPP.

Options

Stock option activity under the Plans is summarized as follows:


		Weighted
	Number of	Average
	Shares	Fair Value
Outstanding as of December 31, 2018	5,960,939	$	6.98
Granted	1,808,688	$	12.35
Exercised	(91,640)	$	2.60
Forfeited	(302,731)	$	12.56
Outstanding as of June 30, 2019	7,375,256	$	8.12

Option Valuation

The assumptions that the Company used to determine the grant-date fair value of options granted to employees and directors under the Plans during the three and six months ended June 30, 2019 and 2018 were as follows, presented on a weighted-average basis:


	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2019		2018		2019		2018
Risk-free interest rate	2.16	%	2.87	%	2.48	%	2.72	%
Expected term (in years)	5.97		5.95		6.18		6.51
Expected volatility	78.88	%	74.83	%	78.35	%	75.22	%
Expected dividend yield	—	%	—	%	—	%	—	%

Share‑Based Compensation

Share‑based compensation expense was classified in the consolidated statements of operations and comprehensive loss as follows:


	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2019		2018		2019		2018
Research and development expenses	$	1,504	$	363	$	2,691	$	565
General and administrative expenses		1,960		696		3,666		1,052
	$	3,464	$	1,059	$	6,357	$	1,617

Restricted Shares

Under terms of the Class A and Class B restricted share agreements covering the Class A and Class B common shares, restricted common shares are subject to a vesting schedule. The restricted shares vest over a four‑year period during which time the Company has the right to repurchase up to all unvested shares at the amount paid if the relationship between the recipient and the Company ceases. Subject to the continued employment (or other engagement of the recipient by the Company as described in the restricted share agreements), all of the restricted common shares become fully vested within four years of the date of issuance.

The following table summarizes restricted share activity for the six months ended June 30, 2019:


	Class A			Class B
		Weighted			Weighted
		Average			Average
	Number of	Grant Date		Number of	Grant Date
	Shares	Fair Value		Shares	Fair Value
Unvested restricted shares outstanding as of December 31, 2018	133,812	$	0.000273235	743,407	$	0.000273235
Granted	—		—	—		—
Vested	(104,076)	$	0.000273235	(594,726)	$	0.000273235
Unvested restricted shares outstanding as of June 30, 2019	29,736	$	0.000273235	148,681	$	0.000273235

License and Acquisition Agreements	6 Months Ended
License and Acquisition Agreements	Jun. 30, 2019
License and Acquisition Agreements
License and Acquisition Agreements	10. License and Acquisition Agreements Biogen Asset Purchase Agreement In September 2016, the Company entered into an asset purchase agreement (the "Biogen Agreement") with Biogen MA Inc. ("Biogen") to acquire all of Biogen's right, title and interest in and to certain assets used in or relating to KPL-716 and other antibodies covered by certain patent rights, including patents and other intellectual property rights, clinical data, know-how, and clinical drug supply. In addition, Biogen granted to the Company a non-exclusive, sublicensable, worldwide license to certain background patent rights related to the KPL-716 program. The Company is obligated to use commercially reasonable efforts to develop and commercialize such acquired products. In exchange for these rights, the Company made an upfront payment to Biogen of $11,500 and a technology transfer payment of $500. The Company accounted for the acquisition of technology as an asset acquisition because it did not meet the definition of a business. The Company recorded the upfront payment and technology transfer payment as research and development expense in the consolidated statement of operations and comprehensive loss because the acquired technology represented in‑process research and development and had no alternative future use. Under the Biogen Agreement, the Company is obligated to make milestone payments to Biogen of up to $179,000 upon the achievement of specified clinical and regulatory milestones in multiple indications in various territories, including milestone payments of $4,000 and $10,000 paid during the year ended December 31, 2017 and the six months ended June 30, 2019, respectively, each payment was associated with the achievement of a specified clinical milestone event. Additionally, the Company could be obligated to make up to an aggregate of up to $150,000 of payments upon the achievement of specified annual net sales milestones and to pay tiered royalties on escalating tiers of annual net sales of licensed products starting in the high single-digit percentages and ending below the teens. The Company also agreed to pay certain obligations under third‑party contracts retained by Biogen that relate to the KPL‑716 program. Under these retained contracts, the Company paid a one‑time upfront sublicense fee of $150 and is obligated to pay insignificant annual maintenance fees as well as clinical and regulatory milestone payments of up to an aggregate of $1,575. The Biogen Agreement will terminate upon the expiration of all payment obligations with respect to the last product in all countries in the territory. The Company has the right to terminate the agreement with 90 days’ prior written notice. Both parties may terminate by mutual written consent or in the event of material breach of the agreement by the other party that remains uncured for 90 days (or 30 days for payment‑related breaches). During the three and six months ended June 30, 2019, the Company recorded research and development expense of $164 and $10,166, respectively, primarily related to milestone payment and other payments associated with the achievement of a specified clinical milestone event due under the Biogen Agreement. During the three and six months ended June 30, 2018, the Company recorded research and development expense of $19 and $30 in connection with annual maintenance fees for the retained contracts due under the Biogen Agreement. Novo Nordisk License Agreement In August 2017, the Company entered into a license agreement (the “Novo Nordisk Agreement”) with Novo Nordisk A/S (“Novo Nordisk”), pursuant to which the Company has been granted an exclusive, sublicensable, worldwide license under certain intellectual property rights controlled by Novo Nordisk to make, use, develop and commercialize KPL‑045 for all indications. The Company is obligated to use commercially reasonable efforts to develop and commercialize such licensed products. In consideration for the license, the Company made an upfront payment of $1,500 to Novo Nordisk. The Company accounted for the acquisition of technology as an asset acquisition because it did not meet the definition of a business. The Company recorded the upfront payment as research and development expense in the consolidated statement of operations and comprehensive loss because the acquired technology represented in‑process research and development and had no alternative future use. Under the Novo Nordisk Agreement, the Company was also required to make a payment of $150 upon completion of the technology transfer by Novo Nordisk. The technology was transferred during the six months ended June 30, 2018 and, as a result, this payment was made and is recorded in the Company's consolidated statement of operations for the six months ended June 30, 2018. In addition, the Company is obligated to make milestone payments upon the achievement of specified clinical, regulatory and initial sales milestones and upon the achievement of annual net sales thresholds, including a payment of $1,000 upon the earlier to occur of a specified regulatory milestone and January 2020, unless the Novo Nordisk Agreement is earlier terminated by either party. As of June 30, 2019 and December 31, 2018, the Company determined that the payment related to the milestone was not probable and, therefore, no amount was recorded in the Company's consolidated statement of operations and comprehensive loss during the three and six months ended June 30, 2019 and 2018. The Company has also agreed to pay royalties on annual net sales of products licensed under the agreement. Under the Novo Nordisk Agreement, the Company is solely responsible for all development, regulatory and commercial activities and costs. The Company is also responsible for costs related to filing, prosecuting and maintaining the licensed patent rights. The Novo Nordisk Agreement will terminate upon expiration of the last‑to‑expire royalty term for any licensed product in the territories, as defined in the agreement. Either party may terminate the agreement upon the other party’s insolvency or bankruptcy or for uncured material breach of the agreement by the other party. Novo Nordisk has the right to terminate the agreement if the Company challenges any of the licensed patent rights. The Company may also terminate the agreement for any reason upon prior written notice to Novo Nordisk. The Company did not incur any research and development expense directly related to milestone payments due under the Novo Nordisk Agreement during the three and six months ended June 30, 2019 and the three months ended June 30, 2018. During the six months ended June 30, 2018 the Company recorded research and development expense of $150, in connection with the completion of technology transfer payment due under the Novo Nordisk Agreement. Primatope Stock Purchase Option Agreement In September 2017, the Company entered into a stock purchase option agreement (the “Primatope Agreement”) with Primatope Therapeutics, Inc. (“Primatope”), pursuant to which the Company was granted a license to certain intellectual property rights owned or controlled by Primatope to research, develop, and manufacture the preclinical antibody, KPL‑404. The agreement provided the Company with an exclusive call option to purchase 100% of the equity securities of Primatope. Upon execution of the agreement, the Company made $500 in upfront payments for the initial option period through April 2018 (the “Initial Option Period”). The Primatope Agreement allowed for up to three extensions of the Initial Option Period through January 2019 (including the initial option period, the “Option Period”) for total extension payments of up to $800. Through December 31, 2018, the Company made payments totaling $800 to extend the Option Period to January 15, 2019. During the Option Period, the Company could conduct research and preclinical work to assess the viability of the asset. The Company determined that the call option represented a variable interest in Primatope and that Primatope is a VIE. However, as the Company had no ability to control the board of directors or direct the ongoing activities of Primatope during the Option Period, the Company did not have power over the activities that most significantly impact Primatope’s economic performance and was not the primary beneficiary of Primatope. As a result, the Company did not consolidate the assets, liabilities, and results of operations of Primatope. In January 2019, the Company exercised the call option and in March 2019, the Company acquired all of the issued and outstanding equity securities of Primatope (the “Primatope Acquisition”) in exchange for $18,000 comprised of upfront consideration of $10,000 at closing and milestone payments of $5,000,which had been achieved as of the closing date, and a milestone of $3,000, which was achieved during the six months ended June 30, 2019, each paid in a combination of cash and Class A common shares (inclusive of escrow and holdback amounts) in accordance with the Primatope Agreement. At the closing of the Primatope Acquisition, Primatope became a wholly owned subsidiary of the Company and the acquisition was accounted for as an asset acquisition as it did not meet the definition of a business. The Company recorded the upfront payment and milestone payments as research and development expense in the consolidated statement of operations and comprehensive loss because the acquired technology represented in‑process research and development and had no alternative future use. During the three months ended June 30, 2019, the Company did not incur any research and development expense directly in connection with milestone or other payments related to the Primatope Acquisition or the Primatope Agreement. During the six months ended June 30, 2019, the Company recorded research and development expense of $18,000 related to the Primatope Acquisition. During the three and six months ended June 30, 2018, the Company recorded research and development expense of $250, related to the extension of the option period under the Primatope Agreement. Beth Israel Deaconess Medical Center License Agreement As a result of the Primatope Acquisition, the Company acquired the rights to an exclusive license to certain intellectual property rights controlled by Beth Israel Deaconess Medical Center, Inc. (“BIDMC”) to make, use, develop and commercialize KPL-404 (the “BIDMC Agreement”). Under the BIDMC Agreement, the Company is solely responsible for all development, regulatory and commercial activities and costs. The Company is also responsible for costs related to filing, prosecuting and maintaining the licensed patent rights. Under the BIDMC Agreement, the Company is obligated to pay an insignificant annual maintenance fee as well as clinical and regulatory milestone payments of up to an aggregate of $1,200 to BIDMC. The Company is also obligated to pay a low single-digit royalty on annual net sales of products licensed under the agreement. The Company did not incur any research and development expense in connection with the BIDMC Agreement during the three and six months ended June 30, 2019. Regeneron License Agreement In September 2017, the Company entered into a license agreement (the “Regeneron Agreement”) with Regeneron Pharmaceuticals, Inc. (“Regeneron”), pursuant to which the Company has been granted an exclusive, sublicensable license under certain intellectual property rights controlled by Regeneron to develop and commercialize rilonacept in certain fields and territories. The Company is obligated to use commercially reasonable efforts to develop and commercialize such licensed products. In exchange for these rights, the Company made an upfront payment of $5,000. The Company accounted for the acquisition of technology as an asset acquisition because it did not meet the definition of a business. The Company recorded the upfront payment as research and development expense in the consolidated statement of operations and comprehensive loss because the acquired technology represented in‑process research and development and had no alternative future use. Under the Regeneron Agreement, the Company is also obligated to make payments to Regeneron of up to an aggregate of $27,500 upon the achievement of specified regulatory milestones. Upon commercialization of the licensed products, the parties will share profits equally, after deducting certain commercialization expenses subject to specified limits. Under the Regeneron Agreement, the Company is solely responsible for all development and commercialization activities and costs in its territories. The Company is also responsible for costs related to the filing, prosecution and maintenance of certain licensed patent rights. The parties also entered into a clinical supply agreement under which Regeneron agreed to manufacture the developed product during the clinical phase. During the three and six months ended June 30, 2019, the Company recorded research and development expense of $3,127 and $3,642, respectively, and during the three and six months ended June 30, 2018, the Company recorded research and development expense of $1,201 and $1,577, respectively, related to the purchase of drug materials under this agreement. As of June 30, 2019 and December 31, 2018, the Company has non-cancelable purchase commitments under the clinical supply agreement (see Note 12). The Regeneron Agreement will expire when the Company is no longer developing or commercializing any licensed product under the Regeneron Agreement. Either party may terminate the agreement upon the other party’s insolvency or bankruptcy or for material breach of the agreement by the other party that remains uncured for 90 days (or 30 days for payment‑related breaches). Regeneron has the right to terminate the agreement if the Company suspends its development or commercialization activities for a consecutive 12-month period or does not grant a sublicense to a third‑party to perform such activities, or if the Company challenges any of the licensed patent rights. The Company may terminate the agreement at any time that is 18 months after the effective date of the agreement with 180 days’ written notice or with one years’ written notice if the Company terminates the agreement following U.S. marketing approval of a rilonacept product developed by the Company. The Company may also terminate the agreement with three month’s written notice. During the three and six months ended June 30, 2019 and June 30, 2018, the Company recorded research and development expense of $3,127, $3,642, $1,201 and $1,577, respectively, related to the purchase of drug materials under the clinical supply agreement with Regeneron. During the three and six months ended June 30, 2019 and 2018, the Company did not record research and development expense in connection with milestone payments due under the Regeneron Agreement. MedImmune License Agreement In December 2017, the Company entered into a license agreement (the “MedImmune Agreement”) with MedImmune, Limited (“MedImmune”), pursuant to which MedImmune granted the Company an exclusive, sublicensable, worldwide license to certain intellectual property rights to make, use, develop and commercialize mavrilimumab. Under the MedImmune Agreement, the Company also acquired reference rights to relevant manufacturing and regulatory documents and MedImmune’s existing supply of mavrilimumab drug substance and product. The Company is obligated to use commercially reasonable efforts to develop and commercialize the licensed products. In exchange for these rights, the Company made an upfront payment of $8,000. The Company accounted for the acquisition of technology as an asset acquisition because it did not meet the definition of a business. The Company recorded the upfront payment as research and development expense in the consolidated statement of operations and comprehensive loss because the acquired technology represented in‑process research and development and had no alternative future use. In addition, the Company is obligated to make clinical, regulatory and initial sales milestone payments of up to $72,500 in aggregate for the first two indications, including, a $5,000 pass‑through payment due upon the achievement of a specified clinical milestone event which was met in the fourth quarter of 2018. Also included is a milestone payment of $10,000 due upon the earlier to occur of a specified regulatory milestone and December 31, 2018, unless the MedImmune Agreement is earlier terminated by either party. As of December 31, 2018 and 2017, the Company determined that the payment related to this milestone was probable and, therefore, recognized research and development expense and an accrued milestone of $10,000 during the year ended December 31, 2017. During the six months ended June 30, 2019, the Company made both the $5,000 and $10,000 milestone payments in accordance with the MedImmune Agreement. In addition, the Company is obligated to make clinical and regulatory milestone payments of up to $15,000 in the aggregate for each subsequent indication. The Company is obligated to make milestone payments to MedImmune of up to $85,000 upon the achievement of annual net sales thresholds up to, but excluding, $1,000,000 in annual net sales as well as additional milestone payments aggregating up to $1,100,000 upon the achievement of additional specified annual net sales thresholds starting at $1,000,000 and higher. The Company has also agreed to pay tiered royalties on escalating tiers of annual net sales of licensed products starting in the low double‑digit percentages and ending at twenty percent. Royalty rates are subject to reductions upon certain events. The Company is solely responsible for all development, manufacturing, and commercial activities and costs of the licensed products, including clinical studies or other tests necessary to support the use of a licensed product. The Company is also responsible for costs related to the filing, prosecution and maintenance of the licensed patent rights. The MedImmune Agreement will expire upon the expiration of the royalty term in the last country for the last indication, as defined in the agreement. Either party may terminate the agreement upon the other party’s insolvency or bankruptcy or for material breach of the agreement by the other party that remains uncured for 90 days. MedImmune has the right to terminate the agreement if the Company challenges any of the licensed patent rights. The Company may terminate the agreement at any time upon 90 days’ prior written notice. During the three and six months ended June 30, 2019 and 2018, the Company did not record research and development expense in connection with milestone payments due under the MedImmune Agreement.

Net Loss per Share

6 Months Ended

Jun. 30, 2019

Net Loss per Share

Net Loss per Share

11. Net Loss per Share

Net Loss per Share Attributable to Common Shareholders

The rights, including the liquidation and dividend rights, of the holders of Class A, Class B, Class A1 and B1 common shares are identical, except with respect to voting rights, transferability and conversion (see Note 8). As the liquidation and dividend rights are identical, losses are allocated on a proportionate basis and the resulting net loss per share attributed to common shareholders will, therefore, be the same for both Class A and Class B common shares on an individual or combined basis.

Basic and diluted net loss per share attributable to common shareholders was calculated as follows:


	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2019		2018		2019		2018
Numerator:
Net loss attributable to common shareholders	$	(37,191)	$	(20,259)	$	(103,012)	$	(36,241)
Denominator:
Weighted average common shares outstanding—basic and diluted		54,475,476		18,328,402		53,225,710		10,492,474
Net loss per share attributable to common shareholders— basic and diluted	$	(0.68)	$	(1.11)	$	(1.94)	$	(3.45)

The Company’s unvested restricted common shares have been excluded from the computation of basic net loss per share attributable to common shareholders.

The Company's potentially dilutive securities, which include options and unvested restricted shares have been excluded from the computation of diluted net loss per share attributable to common shareholders as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common shareholders is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common shareholders for the periods indicated because including them would have had an anti-dilutive effect:


	As of June 30,
	2019	2018
Options to purchase common shares	7,375,256	4,805,037
Unvested restricted shares	178,417	1,412,472
	7,553,673	6,217,509

Summary of Significant Accounting Policies (Policies)

6 Months Ended

Jun. 30, 2019

Summary of Significant Accounting Policies

Principles of Consolidation

The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of the Company and its wholly owned subsidiaries, Kiniksa Pharmaceuticals Corp. (“Kiniksa US”), Kiniksa Pharmaceuticals (UK), Ltd. (“Kiniksa UK”), Kiniksa Pharmaceuticals (Germany) GmbH (“Kiniksa Germany”), Kiniksa Pharmaceuticals (France) SARL (“Kiniksa France”) and Primatope Therapeutics, Inc. (“Primatope”), after elimination of all significant intercompany accounts and transactions.

In assessing the consolidation requirement for variable interest entities (“VIEs”), the Company focuses on identifying whether it has both the power to direct the activities that most significantly impact the VIE’s economic performance and the obligation to absorb losses or the right to receive benefits from the VIE. In the event that the Company is the primary beneficiary of a VIE, the assets, liabilities, and results of operations of the VIE would be included in the Company’s consolidated financial statements. At December 31, 2018 and during the year then ended and at June 30, 2019 and during the three and six months then ended, the Company was not the primary beneficiary of a VIE.

Use of Estimates

The preparation of the Company’s consolidated financial statements in conformity with GAAP requires management to make estimates 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 expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual for research and development expenses and the valuation of common shares and share‑based awards. 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 those estimates.

Reporting and Functional Currency

The financial results of the Company's global activities are reported in U.S. dollars (“USD”) and its foreign subsidiaries generally utilize their respective local currency to be their functional currency.

Transactions in other currencies are recorded in the functional currency at the rate of exchange prevailing when the transactions occur. Monetary assets and liabilities denominated in other currencies are re-measured into the functional currency at the rate of exchange in effect at the balance sheet date. Exchange rate gains and losses arising from re-measurement of foreign currency-denominated monetary assets and liabilities are included in income or losses in the period in which they occur.

For the Company’s foreign subsidiaries where the local currency is the functional currency, assets and liabilities denominated in local currencies are translated into USD at end-of-period exchange rates and the resulting translation adjustments are reported as a component of accumulated other comprehensive loss within shareholders' equity.

Unaudited Interim Consolidated Financial Information

The accompanying unaudited consolidated financial statements have been prepared in accordance with GAAP for interim financial information. The accompanying unaudited consolidated financial statements do not include all of the information and footnotes required by GAAP for complete consolidated financial statements. The accompanying year-end consolidated balance sheet was derived from audited financial statements but does not include all disclosures required by GAAP. The unaudited interim consolidated financial statements have been prepared on the same basis as the audited annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of June 30, 2019 and the results of its operations for the three and six months ended June 30, 2019 and 2018 and its cash flows for the six months ended June 30, 2019 and 2018. The results for the three and six months ended June 30, 2019 are not necessarily indicative of results to be expected for the year ending December 31, 2019, any other interim periods or any future year or period.

Cash and Cash Equivalents

Short-term Investments

Short-Term Investments

Concentrations of Credit Risk

Restricted Cash

Fair Value Measurements

Level 1—Quoted prices in active markets for identical assets or liabilities.

Leases

Research and Development Costs

Research Contract Costs and Accruals

Share-Based Compensation

Share‑Based Compensation

Comprehensive Loss

Net Loss per Share

Recently Adopted Accounting Pronouncements

Fair Value of Financial Assets and Liabilities (Tables)

6 Months Ended

Jun. 30, 2019

Fair Value of Financial Assets and Liabilities

Schedule of financial assets and liabilities measured at fair value on a recurring basis


	Fair Value Measurements
	as of June 30, 2019 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Restricted cash — money market funds	$	210	$	—	$	—	$	210
Cash equivalents — money market funds		5,460		—		—		5,460
Cash equivalents — U.S. Treasury notes		—		15,485		—		15,485
Short-term investments — U.S. Treasury notes		—		204,637		—		204,637
	$	5,670	$	220,122	$	—	$	225,792


	Fair Value Measurements
	as of December 31, 2018 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Restricted cash — money market funds	$	210	$	—	$	—	$	210
Cash equivalents — money market funds		29,721		—		—		29,721
Cash equivalents — U.S. Treasury notes		—		15,634		—		15,634
Short-term investments — U.S. Treasury notes		—		235,328		—		235,328
	$	29,931	$	250,962	$	—	$	280,893

Share Based Compensation (Tables)

6 Months Ended

Jun. 30, 2019

Schedule of stock option activity under the Plans


		Weighted
	Number of	Average
	Shares	Fair Value
Outstanding as of December 31, 2018	5,960,939	$	6.98
Granted	1,808,688	$	12.35
Exercised	(91,640)	$	2.60
Forfeited	(302,731)	$	12.56
Outstanding as of June 30, 2019	7,375,256	$	8.12

Schedule of restricted share activity


	Class A			Class B
		Weighted			Weighted
		Average			Average
	Number of	Grant Date		Number of	Grant Date
	Shares	Fair Value		Shares	Fair Value
Unvested restricted shares outstanding as of December 31, 2018	133,812	$	0.000273235	743,407	$	0.000273235
Granted	—		—	—		—
Vested	(104,076)	$	0.000273235	(594,726)	$	0.000273235
Unvested restricted shares outstanding as of June 30, 2019	29,736	$	0.000273235	148,681	$	0.000273235

Schedule of share-based compensation expense was classified in the consolidated statements of operations and comprehensive loss


	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2019		2018		2019		2018
Research and development expenses	$	1,504	$	363	$	2,691	$	565
General and administrative expenses		1,960		696		3,666		1,052
	$	3,464	$	1,059	$	6,357	$	1,617

Employees and directors

Schedule of stock option valuation assumptions presented on weighted average basis


	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2019		2018		2019		2018
Risk-free interest rate	2.16	%	2.87	%	2.48	%	2.72	%
Expected term (in years)	5.97		5.95		6.18		6.51
Expected volatility	78.88	%	74.83	%	78.35	%	75.22	%
Expected dividend yield	—	%	—	%	—	%	—	%

Nature of the Business and Basis of Presentation (Details) $ / shares in Units, $ in Thousands						3 Months Ended				6 Months Ended
	Mar. 01, 2019 USD ($) $ / shares shares	Feb. 04, 2019 USD ($) $ / shares shares	Jun. 22, 2018 USD ($) $ / shares shares	May 29, 2018 USD ($) $ / shares shares	May 11, 2018	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	May 31, 2018 shares
Nature of the Business and Basis of Presentation
Reverse stock split					2.73235
Proceeds from IPO net of underwriting and offering costs										$ 155,536
Aggregate proceeds from offerings net of underwriter discounts and commissions and other offering costs										82,985
Liquidity
Accumulated deficit						$ 297,237				297,237		$ 194,225
Net losses						37,191	$ 65,821	$ 20,259	$ 15,982	103,012	$ 36,241
Cash used in operations										104,932	32,889
Cash, cash equivalents and short-term investments						$ 287,447				287,447
Class A common shares
Nature of the Business and Basis of Presentation
Number of shares issued on conversion of convertible preferred shares \| shares													5,546,019
Class B common shares
Nature of the Business and Basis of Presentation
Number of shares issued on conversion of convertible preferred shares \| shares													1,070,502
Class A1 common shares
Nature of the Business and Basis of Presentation
Number of shares issued on conversion of convertible preferred shares \| shares													12,995,954
Class B1 common shares
Nature of the Business and Basis of Presentation
Number of shares issued on conversion of convertible preferred shares \| shares													16,057,618
Common Class A and Class A1 common shares
Nature of the Business and Basis of Presentation
Gross proceeds from offering		$ 85,000
Initial Public Offering \| Class A common shares
Nature of the Business and Basis of Presentation
Issuance of stock (in shares) \| shares				8,477,777
Share price (in dollars per share) \| $ / shares				$ 18.00
Gross proceeds from IPO				$ 152,600
Gross proceeds from offering											$ 159,193
Over-allotment
Nature of the Business and Basis of Presentation
Gross proceeds from offering	$ 2,942
Over-allotment \| Class A common shares
Nature of the Business and Basis of Presentation
Issuance of stock (in shares) \| shares	161,126		1,006,425
Share price (in dollars per share) \| $ / shares	$ 18.26	$ 18.26	$ 18.00
Gross proceeds from IPO			$ 18,116
Gross proceeds from offering	$ 2,942
Follow-On Public Offering and Private Placement
Nature of the Business and Basis of Presentation
Gross proceeds from offering		$ 85,000
Follow-On Offering \| Class A common shares
Nature of the Business and Basis of Presentation
Issuance of stock (in shares) \| shares		2,654,984
Share price (in dollars per share) \| $ / shares		$ 18.26
Gross proceeds from offering										$ 48,595
Private Placement \| Class A1 common shares
Nature of the Business and Basis of Presentation
Issuance of stock (in shares) \| shares		2,000,000
Share price (in dollars per share) \| $ / shares		$ 18.26

Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets Transfers
Fair value of assets transferred from Level 1 to Level 2	$ 0	$ 0
Fair value of assets transferred from Level 2 to Level 1	0	0
Fair value of assets transferred from Level 1 to Level 3	0	0
Fair value of assets transferred from Level 2 to Level 3	0	0
Fair value of assets transferred from Level 3 to Level 1	0	0
Fair value of assets transferred from Level 3 to Level 2	0	0
U.S. Treasury Notes
Assets
Amortized cost of short-term investments	204,536	235,332
Unrealized gain (loss) on short-term investments	101	(4)
Fair Value \| U.S. Treasury Notes
Assets
Short-term investments	204,637	235,328
Fair Value \| Recurring basis
Assets
Assets	225,792	280,893
Fair Value \| Recurring basis \| Money Market Funds
Assets
Restricted cash	210	210
Cash equivalents	5,460	29,721
Fair Value \| Recurring basis \| U.S. Treasury Notes
Assets
Cash equivalents	15,485	15,634
Short-term investments	204,637	235,328
Fair Value \| Recurring basis \| Level 1
Assets
Assets	5,670	29,931
Fair Value \| Recurring basis \| Level 1 \| Money Market Funds
Assets
Restricted cash	210	210
Cash equivalents	5,460	29,721
Fair Value \| Recurring basis \| Level 2
Assets
Assets	220,122	250,962
Fair Value \| Recurring basis \| Level 2 \| U.S. Treasury Notes
Assets
Cash equivalents	15,485	15,634
Short-term investments	$ 204,637	$ 235,328

Leases - Lease Cost and Maturities of Operating Lease Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Lease cost
Operating lease cost	$ 361	$ 762
Variable lease cost	$ 87	$ 109
Weighted average lease term	1 year 11 months 23 days	1 year 11 months 23 days
Discount rate	7.16%	7.16%
Maturities of operating lease liabilities
2019	$ 767	$ 767
2020	1,821	1,821
2021	972	972
Total future minimum lease payments	3,560	3,560
Less imputed interest	(245)	(245)
Present value of lease liabilities	$ 3,315	$ 3,315	$ 3,917
Future minimum lease payments
2019				$ 1,394
2020				1,821
2021				972
Total future minimum lease payments				$ 4,187

Convertible Preferred Shares (Details) $ / shares in Units, $ in Thousands	1 Months Ended					6 Months Ended		12 Months Ended
	May 31, 2018 $ / shares shares	Feb. 28, 2018 USD ($) $ / shares shares	Mar. 31, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares shares	Oct. 31, 2015 USD ($) $ / shares shares	Jun. 30, 2019 USD ($) director Vote shares	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) director $ / shares shares	Dec. 31, 2016 USD ($)
Convertible Preferred Shares
Preferred shares designated						0
Preferred stock, shares issued (in shares)						0
Payment of offering expenses \| $						$ 118	$ 2,825
Cash dividend declared \| $						$ 0		$ 0	$ 0	$ 0
Convertible Preferred Shares (Series A, B and C)
Convertible Preferred Shares
Preferred shares designated		35,670,093				0			22,885,492
Preferred stock, shares issued (in shares)						0
Convertible preferred shares, par value (in dollars per share) \| $ / shares		$ 0.000273235							$ 0.000273235
Number of directors entitled to elect \| director						2			2
Series A convertible preferred shares
Convertible Preferred Shares
Preferred shares designated									17,128,120
Preferred stock, shares issued (in shares)				9,099,311	8,028,809
Share price per share \| $ / shares				$ 4.6707	$ 4.6707
Proceeds from issuance of shares \| $				$ 42,499	$ 37,398
Payment of offering expenses \| $				$ 1	$ 102
Number of votes for each Class B common share \| Vote						10
Original issue price (per share) \| $ / shares	$ 4.6707
Convertible ratio to Class B common share	1
Series B convertible preferred shares
Convertible Preferred Shares
Preferred shares designated									5,757,372
Preferred stock, shares issued (in shares)			5,757,372
Share price per share \| $ / shares			$ 6.9475
Proceeds from issuance of shares \| $			$ 39,873
Payment of offering expenses \| $			$ 127
Number of votes for each Class A common share \| Vote						1
Original issue price (per share) \| $ / shares	$ 6.9475
Convertible ratio to Class A common share	1
Series C convertible preferred shares
Convertible Preferred Shares
Preferred shares designated		12,784,601
Preferred stock, shares issued (in shares)		12,784,601
Share price per share \| $ / shares		$ 15.6438
Proceeds from issuance of shares \| $		$ 190,822
Payment of offering expenses \| $		$ 9,178
Original issue price (per share) \| $ / shares	$ 15.6438
Convertible ratio to Class A common share	1
Common Class A and Class B
Convertible Preferred Shares
Number of directors entitled to elect \| director						1			1
Class A common shares
Convertible Preferred Shares
Number of shares issued on conversion of convertible preferred shares	5,546,019
Class B common shares
Convertible Preferred Shares
Number of shares issued on conversion of convertible preferred shares	1,070,502
Class A1 common shares
Convertible Preferred Shares
Number of shares issued on conversion of convertible preferred shares	12,995,954
Class B1 common shares
Convertible Preferred Shares
Number of shares issued on conversion of convertible preferred shares	16,057,618
Class C common shares
Convertible Preferred Shares
Number of votes for each Class A common share \| Vote						1
Common Class A or Class B \| Minimum
Convertible Preferred Shares
Original issue price (per share) \| $ / shares	$ 15.6438
Gross proceeds from IPO \| $						$ 100,000

Common Shares (Details) $ / shares in Units, $ in Thousands					1 Months Ended	6 Months Ended
Common Shares (Details) $ / shares in Units, $ in Thousands	Mar. 01, 2019 USD ($) $ / shares shares	Feb. 04, 2019 USD ($) $ / shares shares	Jun. 22, 2018 USD ($) $ / shares shares	May 29, 2018 USD ($) $ / shares shares	Mar. 01, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($) director Vote $ / shares shares	Jun. 30, 2018 USD ($)	Dec. 31, 2018 $ / shares	May 31, 2018 $ / shares shares	Feb. 28, 2018 shares	Dec. 31, 2017 director $ / shares shares
Nature of the Business and Basis of Presentation
Common share authorized (in shares)									200,000,000	44,746,463	43,918,239
Common stock, par value (in dollars per share) \| $ / shares									$ 0.000273235		$ 0.000273235
Common shares or Preferred Shares not designated											11,956,456
Proceeds from IPO net of underwriting and offering costs \| $						$ 155,536
Authorized capital									54,647,000
Preferred shares designated						0
Preferred stock, shares issued (in shares)						0
Common stock, cash dividends declared \| $						$ 0
Common stock, cash dividends paid \| $						0
Preferred stock, cash dividends declared \| $						0
Preferred stock, cash dividends paid \| $						$ 0
Common Class A and Class B
Nature of the Business and Basis of Presentation
Number of directors entitled to elect \| director						1					1
Class A common shares
Nature of the Business and Basis of Presentation
Common share authorized (in shares)										5,507,938	5,507,938
Common stock, par value (in dollars per share) \| $ / shares						$ 0.000273235		$ 0.000273235
Number of shares issued on conversion of convertible preferred shares									5,546,019
Number of votes (per share) \| Vote						1
Class B common shares
Nature of the Business and Basis of Presentation
Common share authorized (in shares)										3,568,353	3,568,353
Common stock, par value (in dollars per share) \| $ / shares						$ 0.000273235		0.000273235
Number of shares issued on conversion of convertible preferred shares									1,070,502
Number of votes (per share) \| Vote						10
Convertible ratio to Class A common share						1
Convertible ratio to Class B1 common share						1
Common Class A and Class A1 common shares
Nature of the Business and Basis of Presentation
Gross proceeds from offering \| $		$ 85,000
Class A1 common shares
Nature of the Business and Basis of Presentation
Common share authorized (in shares)											0
Common stock, par value (in dollars per share) \| $ / shares						$ 0.000273235		0.000273235
Number of shares issued on conversion of convertible preferred shares									12,995,954
Number of votes (per share) \| Vote						0
Convertible ratio to Class A common share						1
Class B1 common shares
Nature of the Business and Basis of Presentation
Common share authorized (in shares)											0
Common stock, par value (in dollars per share) \| $ / shares						$ 0.000273235		$ 0.000273235
Number of shares issued on conversion of convertible preferred shares									16,057,618
Number of votes (per share) \| Vote						0
Convertible ratio to Class A common share						1
Convertible ratio to Class B common share						1
Convertible Preferred Shares (Series A, B and C)
Nature of the Business and Basis of Presentation
Preferred shares designated						0				35,670,093	22,885,492
Preferred stock, shares issued (in shares)						0
Number of directors entitled to elect \| director						2					2
Initial Public Offering \| Class A common shares
Nature of the Business and Basis of Presentation
Issuance of stock (in shares)				8,477,777
Share price (in dollars per share) \| $ / shares				$ 18.00
Gross proceeds from IPO \| $				$ 152,600
Gross proceeds from offering \| $							$ 159,193
Over-allotment
Nature of the Business and Basis of Presentation
Gross proceeds from offering \| $	$ 2,942
Over-allotment \| Class A common shares
Nature of the Business and Basis of Presentation
Issuance of stock (in shares)	161,126		1,006,425
Share price (in dollars per share) \| $ / shares	$ 18.26	$ 18.26	$ 18.00		$ 18.26
Gross proceeds from IPO \| $			$ 18,116
Gross proceeds from offering \| $	$ 2,942
Follow-On Public Offering and Private Placement
Nature of the Business and Basis of Presentation
Gross proceeds from offering \| $		$ 85,000
Follow-On Public Offering and Private Placement \| Common Class A and Class A1 common shares
Nature of the Business and Basis of Presentation
Proceeds from offering net of underwriting and offering costs \| $					$ 82,985
Follow-On Offering \| Class A common shares
Nature of the Business and Basis of Presentation
Issuance of stock (in shares)		2,654,984
Share price (in dollars per share) \| $ / shares		$ 18.26
Gross proceeds from offering \| $						$ 48,595
Private Placement \| Class A1 common shares
Nature of the Business and Basis of Presentation
Issuance of stock (in shares)		2,000,000
Share price (in dollars per share) \| $ / shares		$ 18.26

License and Acquisition Agreements - MedImmune License Agreement (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Dec. 31, 2017	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
License and Acquisition Agreements
Accrued milestone payments						$ 15,000
Research and development expense		$ 30,848	$ 17,200	$ 90,101	$ 29,831
MedImmune Agreement
License and Acquisition Agreements
Upfront payment for exchange of rights	$ 8,000
Accrued milestone payments	10,000					$ 5,000
Payment of accrued milestones				10,000
Pass-through payment paid upon clinical milestone achievement				5,000
Annual net sales excluded from calculation of specified annual net sales thresholds	$ 1,000,000
Maximum percentage of royalty payable on annual net sales of licensed products	20.00%
Notice period to terminate the agreement by both parties for material breaches	90 days
Notice period to terminate the agreement	90 days
Research and development expense	$ 10,000	$ 0	$ 0	$ 0	$ 0
Minimum \| MedImmune Agreement
License and Acquisition Agreements
Additional specified annual net sales threshold for additional milestone payment.	1,000,000
Maximum \| MedImmune Agreement
License and Acquisition Agreements
Milestone payment to be paid upon specified milestone achievements for first two indications	72,500
Milestone payment to be paid upon clinical and regulatory milestone achievement	15,000
Milestone payment to be paid upon specified annual sales milestone achievements	85,000
Additional milestone payments payable after achievement of additional annual sales	$ 1,100,000

Common Shares	6 Months Ended
Common Shares	Jun. 30, 2019
Common Shares
Common Shares	8. Common Shares As of December 31, 2017, the Company’s bye-laws, as amended and restated, authorized the Company to issue 43,918,239 total shares with a par value of $0.000273235, of which 5,507,938 and 3,568,353 shares were designated as Class A and Class B common shares, respectively. In February 2018, the Company’s bye-laws were further amended and restated to, among other things, effect an increase in the number of authorized common shares to 44,746,463 shares, of which 5,507,938 shares were designated as Class A common shares and 3,568,353 shares were designated as Class B common shares. The remaining 11,956,456 shares that were not designated as common shares or Preferred Shares as of December 31, 2017 could have been designated to any class at any time in the future by the Company’s board of directors. No Class A1 common shares or Class B1 common shares were designated as of December 31, 2017. On May 23, 2018, the Company’s registration statement on Form S-1 relating to the IPO was declared effective by the SEC. On May 29, 2018, the Company completed the IPO of 8,477,777 Class A common shares at a public offering price of $18.00 per share for gross proceeds of $152,600. In addition, on June 22, 2018, the Company completed the sale of 1,006,425 Class A common shares to the underwriters of the IPO following the exercise in part of their over-allotment option to purchase additional shares at a public offering price of $18.00 per share for gross proceeds of $18,116. The aggregate net proceeds to the Company from the IPO, inclusive of the over-allotment option exercise, was $155,536 after deducting underwriting discounts and commissions and other offering costs. In May 2018, upon completion of the IPO (which qualified as a “Qualified IPO” under the Company’s bye-laws, as amended and restated), all outstanding Preferred Shares were converted into 5,546,019 Class A common shares, 1,070,502 Class B common shares, 12,995,954 Class A1 common shares and 16,057,618 Class B1 common shares in accordance with the Company’s bye-laws, as amended and restated. In connection with the completion of the IPO in May 2018, the Company increased the authorized capital of the Company to $54,647 consisting of 200,000,000 shares of $0.000273235 par value per share and, among other things, amended the description of different classes of shares under the Amended & Restated Bye-Laws. On February 4, 2019, the Company completed a follow-on offering of 2,654,984 Class A common shares at a public offering price of $18.26 and a concurrent private placement of 2,000,000 Class A1 common shares at an offering price of $18.26 per share for aggregate gross proceeds of $85,000. In addition, on March 1, 2019, the Company completed the sale of 161,126 Class A common shares to the underwriters of the follow-on offering following the exercise in part of their over-allotment option to purchase additional shares at a public offering price of $18.26 per share for gross proceeds of $2,942. The aggregate net proceeds to the Company from the follow-on offering and concurrent private placement, inclusive of the over-allotment option exercise, was $82,985 after deducting underwriting discounts and commissions and other offering costs. The rights of the holders of the Company’s Class A common shares, Class B common shares, Class A1 common shares and Class B1 common shares are identical, except with respect to voting and conversion, as described below. As of December 31, 2017, the voting, dividend and liquidation rights of the holders of the Company’s common shares were subject to and qualified by the rights, powers and preferences of the holders of the Preferred Shares as set forth above. In May 2018, following the conversion of the Preferred Shares into common shares, the voting, dividend and liquidation rights of the holders of the Company’s common shares were then subject to and qualified by the rights, powers and preferences of the holders of the preferred shares, if any. As of June 30, 2019, no preferred shares were designated or issued. Voting Each Class A common share entitles the holder to one vote on all matters submitted to the shareholders for a vote. Each Class B common share entitles the holder to ten votes on all matters submitted to the shareholders for a vote. Holders of Class A1 common shares and Class B1 common shares have no voting rights. As of December 31, 2017, the holders of Class A and Class B common shares, voting together as a single class, were entitled to elect one director of the Company. The holders of Class A and Class B common shares, voting together as a single class with the holders of the Preferred Shares, were entitled to elect the remaining directors of the Company, except for the two directors of the Company that the holders of the Preferred Shares, voting together as a single class, were entitled to elect. In May 2018, following the conversion of the Preferred Shares into common shares, the holders of Class A and Class B common shares, voting together as a single class, are entitled to elect the directors of the Company. Dividends Common shareholders are entitled to receive dividends, as may be declared by the Company’s board of directors. As of December 31, 2017, any such dividends would have been subject to the preferential dividend rights of the holders of the Preferred Shares. In May 2018, following the conversion of the Preferred Shares into common shares, any such dividends would be subject to the rights, powers and preferences of any preferred shares that may be issued by the Company. As of June 30, 2019, no preferred shares were designated or issued. Through June 30, 2019, no cash dividends have been declared or paid. Conversion Each Class B common share shall automatically convert into one Class A common share upon certain transfers of such shares by the holder thereof (subject to certain exceptions). Each Class B common share is convertible, at the holder’s election into one Class A common share or one Class B1 common share. Each Class A1 common share is convertible into one Class A common share at the holder’s election. Each Class B1 common share shall automatically convert into one Class A common share upon certain transfers of such shares by the holder thereof (subject to certain exceptions). Each Class B1 common share is convertible into one Class A common share or one Class B common share at the holder’s election. There are no conversion rights associated with the Class A common shares.

Commitments and Contingencies	6 Months Ended
Commitments and Contingencies	Jun. 30, 2019
Commitments and Contingencies
Commitments and Contingencies	12. Commitments and Contingencies License Agreements The Company has entered into license agreements with various parties under which it is obligated to make contingent and non‑contingent payments (see Note 10). Manufacturing Commitments The Company entered into agreements with several contract manufacturing organizations to provide preclinical and clinical trial materials. As of June 30, 2019 and December 31, 2018, the Company had committed to minimum payments under these agreements totaling $7,021 and $12,012, respectively. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it did not accrue any liabilities related to such obligations in its consolidated financial statements as of June 30, 2019 and December 31, 2018. Legal Proceedings The Company is not a party to any litigation and does not have contingency reserves established for any litigation liabilities.

Property and Equipment, Net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Property and Equipment, Net
Total property and equipment	$ 7,657		$ 7,657		$ 6,675
Less: Accumulated depreciation	(1,298)		(1,298)		(319)
total property and equipment, net	6,359		6,359		6,356
Depreciation expense	508	$ 12	979	$ 20
Furniture, fixtures, and vehicles
Property and Equipment, Net
Total property and equipment	91		91		91
Computer hardware and software
Property and Equipment, Net
Total property and equipment	269		269		249
Leasehold improvements
Property and Equipment, Net
Total property and equipment	3,225		3,225		2,676
Lab Equipment
Property and Equipment, Net
Total property and equipment	3,939		3,939		3,107
Construction in progress
Property and Equipment, Net
Total property and equipment	$ 133		$ 133		$ 552

Leases - Operating Leases (Details) $ in Thousands						6 Months Ended	12 Months Ended
Leases - Operating Leases (Details) $ in Thousands	Dec. 01, 2019 USD ($)	Feb. 01, 2019 USD ($)	Jan. 01, 2019 USD ($)	Dec. 21, 2018 USD ($) ft²	Mar. 31, 2017 USD ($)	Jun. 30, 2019	Dec. 31, 2018 USD ($)	Nov. 07, 2018 ft²	Jun. 26, 2018 ft²	Mar. 31, 2016 ft²
Maximum
Operating leases
Remaining lease terms						3 years
Wellesley Hills office
Operating leases
Area of leased space after expansion \| ft²										10,800
Monthly lease payments, inclusive of base rent and ancillary charges \| $					$ 27
Lexington office
Operating leases
Area of leased space after expansion \| ft²								55,924	27,244
Monthly base rent payment \| $	$ 138	$ 110	$ 101				$ 73
San Diego office
Operating leases
Area of leased space after expansion \| ft²				4,400
Monthly base rent payment \| $				$ 13

Accrued Expenses (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accrued Expenses
Accrued employee compensation and benefits	$ 3,983	$ 5,678
Accrued research and development expenses	13,709	9,656
Accrued legal and professional fees	1,128	994
Other	359	90
Accrued expenses	$ 19,179	$ 16,418

CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED SHARES AND SHAREHOLDERS’ (DEFICIT) EQUITY (Parenthetical) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Series C convertible preferred shares
Issuance costs	$ 9,178

Summary of Significant Accounting Policies - Concentrations of Credit Risk (Details) - item	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Cash and cash equivalents \| Credit concentration
Concentration risk
Number of financial institutions holding cash, cash equivalents and short-term investments	2	2

Summary of Significant Accounting Policies - Recently Adopted Accounting Pronouncements (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019
Operating lease assets and liabilities
Operating lease right-of-use assets	$ 2,503	$ 3,682
Operating lease liabilities	$ 3,315	$ 3,917

Share Based Compensation (Details)	May 31, 2018 shares
2018 Employee Share Purchase Plan \| Class A common shares
Share-based compensation
Total number of common shares authorized to issue	670,000

Share Based Compensation - Options (Details) - Class A common shares - Stock options	6 Months Ended
	Jun. 30, 2019 $ / shares shares
Number of Shares
Outstanding, beginning of the period \| shares	5,960,939
Granted \| shares	1,808,688
Exercised \| shares	(91,640)
Forfeited \| shares	(302,731)
Outstanding, end of the period \| shares	7,375,256
Weighted Average Exercise Price
Outstanding, beginning of the period \| $ / shares	$ 6.98
Granted \| $ / shares	12.35
Exercised \| $ / shares	2.60
Forfeited \| $ / shares	12.56
Outstanding, end of the period \| $ / shares	$ 8.12

Share Based Compensation - Restricted Shares (Details) - Unvested restricted shares	6 Months Ended
	Jun. 30, 2019 $ / shares shares
Share-based compensation
Vesting period	4 years
Class A common shares
Number of shares
Unvested restricted shares outstanding, Beginning of the period \| shares	133,812
Unvested restricted shares vested \| shares	(104,076)
Unvested restricted shares outstanding, End of the period \| shares	29,736
Weighted Average Fair Value at Issuance
Unvested restricted shares outstanding, Beginning of the period \| $ / shares	$ 0.000273235
Unvested restricted shares vested \| $ / shares	0.000273235
Unvested restricted shares outstanding, End of the period \| $ / shares	$ 0.000273235
Class B common shares
Number of shares
Unvested restricted shares outstanding, Beginning of the period \| shares	743,407
Unvested restricted shares vested \| shares	(594,726)
Unvested restricted shares outstanding, End of the period \| shares	148,681
Weighted Average Fair Value at Issuance
Unvested restricted shares outstanding, Beginning of the period \| $ / shares	$ 0.000273235
Unvested restricted shares vested \| $ / shares	0.000273235
Unvested restricted shares outstanding, End of the period \| $ / shares	$ 0.000273235

License and Acquisition Agreements - Biogen Asset Purchase Agreement (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Sep. 30, 2016	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2018
License and Acquisition Agreements
Accrued milestones							$ 15,000
Research and development expense		$ 30,848	$ 17,200	$ 90,101	$ 29,831
Biogen Agreement
License and Acquisition Agreements
Upfront payment for exchange of rights	$ 11,500
Technology transfer payment	500
Milestone payment upon achievement of clinical milestone event				10,000		$ 4,000
One-time payment of upfront sublicense fee on retained contracts	150
Maximum aggregate obligation to pay insignificant annual maintenance fees as well as clinical and regulatory milestone payments	$ 1,575
Notice period to terminate the agreement	90 days
Uncured period which causes termination of the agreement, due to insolvency or bankruptcy or for material breach of the agreement by the other party that remains uncured	90 days
Notice period to terminate the agreement under payment-related breaches	30 days
Research and development expense		$ 164	$ 19	$ 10,166	$ 30
Maximum \| Biogen Agreement
License and Acquisition Agreements
Milestone payment to be paid upon clinical and regulatory milestone achievement	$ 179,000
Milestone payment to be paid upon net sales milestone achievement	$ 150,000

License and Acquisition Agreements - Novo Nordisk License Agreement (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Aug. 31, 2017	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
License and Acquisition Agreements
Research and development expense		$ 30,848	$ 17,200	$ 90,101	$ 29,831
Novo Nordisk Agreement
License and Acquisition Agreements
Upfront payment for exchange of rights	$ 1,500
Consideration payable upon completion of technology transfer			150		150
Milestone payment payable after achievement of specified milestones	$ 1,000
Milestone payments due		0	$ 0	0	0
Research and development expense		$ 0		$ 0	$ 150

License and Acquisition Agreements - Primatope Stock Purchase Option Agreement (Details) $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
	Jan. 31, 2019 USD ($)	Sep. 30, 2017 USD ($) item	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
License and Acquisition Agreements
Accrued milestones							$ 15,000
Research and development expense			$ 30,848	$ 17,200	$ 90,101	$ 29,831
Primatope Agreement
License and Acquisition Agreements
Percentage of capital stock available to purchase in call option		100.00%
Upfront payment for initial option period		$ 500
Payment for extension of option period							$ 800
Research and development expense			0	$ 250		$ 250
Primatope Acquisition
License and Acquisition Agreements
Purchase price	$ 18,000
Upfront consideration payable on exercise of call option to acquire all outstanding equity	10,000
Milestone payment payable after achievement of specified milestones	$ 5,000		3,000		3,000
Research and development expense			$ 0		$ 18,000
Maximum \| Primatope Agreement
License and Acquisition Agreements
Number of extensions allowed for initial option period \| item		3
Total aggregate extension payments		$ 800

License and Acquisition Agreements - Regeneron License Agreement (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
License and Acquisition Agreements
Research and development expense		$ 30,848	$ 17,200	$ 90,101	$ 29,831
Regeneron Agreement
License and Acquisition Agreements
Upfront payment for exchange of rights	$ 5,000
Uncured period which causes termination of the agreement, due to insolvency or bankruptcy or for material breach of the agreement by the other party that remains uncured	90 days
Research and development expense		0	0	0	0
Notice period to terminate the agreement under payment-related breaches	30 days
Consecutive suspension of activities period for termination of agreement	12 months
Period after which the agreement can be terminated	18 months
Notice period to terminate the agreement	180 days
Notice period to terminate the agreement after US marketing approval of the developed product	1 year
Notice period to terminate the agreement if products are having safety concerns	3 months
Regeneron Clinical Supply Agreement
License and Acquisition Agreements
Research and development expense		$ 3,127	$ 1,201	$ 3,642	$ 1,577
Maximum \| Regeneron Agreement
License and Acquisition Agreements
Milestone payment to be paid upon regulatory milestone achievement	$ 27,500

Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net loss attributable to common shareholders	$ (37,191)	$ (65,821)	$ (20,259)	$ (15,982)	$ (103,012)	$ (36,241)
Denominator:
Weighted average common shares outstanding — basic and diluted	54,475,476		18,328,402		53,225,710	10,492,474
Net loss per share attributable to common shareholders — basic and diluted	$ (0.68)		$ (1.11)		$ (1.94)	$ (3.45)

Net Loss per Share - Anti-dilutive securities (Details) - shares	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Anti-dilutive securities excluded from computation of earnings per share
Total, anti-dilutive securities excluded from computation of earnings per share	7,553,673	6,217,509
Stock options
Anti-dilutive securities excluded from computation of earnings per share
Total, anti-dilutive securities excluded from computation of earnings per share	7,375,256	4,805,037
Unvested restricted shares
Anti-dilutive securities excluded from computation of earnings per share
Total, anti-dilutive securities excluded from computation of earnings per share	178,417	1,412,472

Commitments and Contingencies (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Manufacturing commitments
Non-cancelable purchase commitments	$ 7,021	$ 12,012