Fusion Pharmaceuticals (FUSN) 10-Q 10 Nov 20 2020 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2020	Nov. 03, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Trading Symbol	FUSN
Entity Registrant Name	Fusion Pharmaceuticals Inc.
Entity Central Index Key	0001805890
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		41,725,797
Entity File Number	001-39344
Entity Address, Address Line One	270 Longwood Rd., S.
Entity Address, City or Town	Hamilton
Entity Address, State or Province	ON
Entity Address, Country	CA
Entity Address, Postal Zip Code	L8P 0A6
City Area Code	289
Local Phone Number	799-0891
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Document Quarterly Report	true
Document Transition Report	false
Entity Incorporation, State or Country Code	CA
Title of 12(b) Security	Common shares, no par value per share
Security Exchange Name	NASDAQ

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 248,793	$ 65,344
Restricted cash	280	280
Short-term investments	43,159
Prepaid expenses and other current assets	4,652	929
Total current assets	296,884	66,553
Property and equipment, net	1,981	1,272
Deferred tax assets	106	78
Restricted cash	1,465	1,497
Long-term investments	11,102
Other non-current assets	521
Total assets	312,059	69,400
Current liabilities:
Accounts payable	956	830
Accrued expenses	4,353	3,326
Income taxes payable		117
Total current liabilities	5,309	4,273
Deferred rent, net of current portion	3	28
Preferred share tranche right liability		5,741
Income taxes payable, net of current portion	293	293
Special voting shares redemption right liability (Notes 2 and 7)
Total liabilities	5,605	10,335
Commitments and contingencies (Note 12)
Convertible preferred shares, no par value; 0 shares and 132,207,290 shares authorized as of September 30, 2020 and December 31, 2019, respectively; 0 shares and 73,125,790 shares issued and outstanding as of September 30, 2020 and December 31, 2019, respectively; aggregate liquidation preference of $0 and $77,965 as of September 30, 2020 and December 31, 2019, respectively		71,592
Shareholders’ equity (deficit):
Common shares, no par value, unlimited shares authorized as of September 30, 2020 and December 31, 2019; 41,702,384 and 1,929,555 shares issued and outstanding as of September 30, 2020 and December 31, 2019, respectively
Additional paid-in capital	406,273	1,286
Accumulated other comprehensive income	(1)
Accumulated deficit	(99,818)	(34,774)
Total shareholders’ equity (deficit)	306,454	(33,488)
Total liabilities, non-controlling interest, convertible preferred shares and shareholders’ equity (deficit)	312,059	69,400
Fusion Pharmaceuticals (Ireland) Limited [Member]
Current liabilities:
Non-controlling interest in Fusion Pharmaceuticals (Ireland) Limited (Notes 2 and 7)		$ 20,961

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Preferred stock, no par value
Preferred stock, shares authorized	0	132,207,290
Preferred stock, shares issued	0	73,125,790
Preferred stock, shares outstanding	0	73,125,790
Aggregation liquidity preference	$ 0	$ 77,965
Common stock, no par value
Common stock, shares issued	41,702,384	1,929,555
Common stock, shares outstanding	41,702,384	1,929,555

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Operating expenses:
Research and development	$ 4,529	$ 2,238	$ 12,231	$ 7,216
General and administrative	5,790	1,922	14,105	4,864
Total operating expenses	10,319	4,160	26,336	12,080
Loss from operations	(10,319)	(4,160)	(26,336)	(12,080)
Other income (expense):
Change in fair value of preferred share tranche right liability		3,485	(32,722)	3,707
Change in fair value of preferred share warrant liability			(6,399)
Interest income (expense), net	80	247	249	395
Refundable investment tax credits	41	44	139	132
Other income (expense), net	20	(29)	148	76
Total other income (expense), net	141	3,747	(38,585)	4,310
Loss before (provision) benefit for income taxes	(10,178)	(413)	(64,921)	(7,770)
Income tax (provision) benefit	185	(181)	(27)	(213)
Net loss	(9,993)	(594)	(64,948)	(7,983)
Unrealized loss on investments	(1)		(1)
Comprehensive loss	(9,994)	(594)	(64,949)	(7,983)
Reconciliation of net loss to net loss attributable to common shareholders:
Net loss	(9,993)	(594)	(64,948)	(7,983)
Dividends paid to preferred shareholders in the form of warrants issued			(1,382)
Net loss attributable to common shareholders	$ (9,993)	$ (594)	$ (66,330)	$ (7,983)
Net loss per share attributable to common shareholders—basic and diluted	$ (0.24)	$ (0.31)	$ (4.30)	$ (4.18)
Weighted-average common shares outstanding—basic and diluted	41,682,797	1,929,555	15,422,375	1,910,695

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Non-Controlling Interest, Convertible Preferred Shares and Shareholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total	Class B Convertible Preferred Shares and Class B Preferred Exchangeable Shares [Member]	Non-Controlling Interest in Fusion Pharmaceuticals (Ireland) Limited [Member]	Non-Controlling Interest in Fusion Pharmaceuticals (Ireland) Limited [Member]Class B Preferred Exchangeable Shares and Class B Preferred Share Tranche Right [Member]	Class A and B Convertible Preferred Shares [Member]	Class A and B Convertible Preferred Shares [Member]Class B Convertible Preferred Shares and Class B Preferred Share Tranche Right [Member]	Common Shares [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]Class B Convertible Preferred Shares and Class B Preferred Exchangeable Shares [Member]	Accumulated Deficit [Member]	Accumulated Deficit [Member]Class B Convertible Preferred Shares and Class B Preferred Exchangeable Shares [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning Balance at Dec. 31, 2018	$ (17,917)		$ 15,168		$ 32,371			$ 668		$ (18,585)
Beginning Balance (in shares) at Dec. 31, 2018					42,918,661		1,872,975
Issuance of Class B convertible/exchangeable preferred shares and Class B preferred share tranche right, net of issuance costs				$ 6,711		$ 45,476
Issuance of Class B convertible/exchangeable preferred shares and Class B preferred share tranche right, net of issuance costs (in shares)						30,207,129
Initial fair value of Class B convertible preferred share tranche right liability			(918)		$ (6,255)
Share-based compensation expense	102							102
Net loss	(3,933)									(3,933)
Ending Balance at Mar. 31, 2019	(21,748)		20,961		$ 71,592			770		(22,518)
Ending Balance (in shares) at Mar. 31, 2019					73,125,790		1,872,975
Beginning Balance at Dec. 31, 2018	$ (17,917)		15,168		$ 32,371			668		(18,585)
Beginning Balance (in shares) at Dec. 31, 2018					42,918,661		1,872,975
Issuance of common shares upon exercise of common share warrants (in shares)	0
Net loss	$ (7,983)
Ending Balance at Sep. 30, 2019	(25,572)		20,961		$ 71,592			996		(26,568)
Ending Balance (in shares) at Sep. 30, 2019					73,125,790		1,929,555
Beginning Balance at Mar. 31, 2019	(21,748)		20,961		$ 71,592			770		(22,518)
Beginning Balance (in shares) at Mar. 31, 2019					73,125,790		1,872,975
Issuance of common shares upon exercise of common share warrants (in shares)							56,580
Share-based compensation expense	88							88
Net loss	(3,456)									(3,456)
Ending Balance at Jun. 30, 2019	(25,116)		20,961		$ 71,592			858		(25,974)
Ending Balance (in shares) at Jun. 30, 2019					73,125,790		1,929,555
Share-based compensation expense	138							138
Net loss	(594)									(594)
Ending Balance at Sep. 30, 2019	(25,572)		20,961		$ 71,592			996		(26,568)
Ending Balance (in shares) at Sep. 30, 2019					73,125,790		1,929,555
Beginning Balance at Dec. 31, 2019	(33,488)		20,961		$ 71,592			1,286		(34,774)
Beginning Balance (in shares) at Dec. 31, 2019					73,125,790		1,929,555
Issuance of Class B convertible/exchangeable preferred shares and Class B preferred share tranche right, net of issuance costs						$ 9,907
Issuance of Class B convertible/exchangeable preferred shares and Class B preferred share tranche right, net of issuance costs (in shares)						6,598,917
Initial fair value of Class B convertible preferred share tranche right liability					$ (1,105)
Issuance of warrants to purchase Class B convertible preferred shares and Class B preferred exchangeable shares as a non-cash dividend to preferred shareholders		$ (1,382)							$ (1,286)		$ (96)
Share-based compensation expense	358							358
Net loss	(10,222)									(10,222)
Ending Balance at Mar. 31, 2020	(44,734)		20,961		$ 80,394			358		(45,092)
Ending Balance (in shares) at Mar. 31, 2020					79,724,707		1,929,555
Beginning Balance at Dec. 31, 2019	$ (33,488)		20,961		$ 71,592			1,286		(34,774)
Beginning Balance (in shares) at Dec. 31, 2019					73,125,790		1,929,555
Issuance of common shares upon exercise of common share warrants (in shares)	0
Unrealized loss on investments	$ (1)
Net loss	(64,948)
Ending Balance at Sep. 30, 2020	306,454							406,273		(99,818)		$ (1)
Ending Balance (in shares) at Sep. 30, 2020							41,702,384
Beginning Balance at Mar. 31, 2020	(44,734)		20,961		$ 80,394			358		(45,092)
Beginning Balance (in shares) at Mar. 31, 2020					79,724,707		1,929,555
Issuance of Class B convertible/exchangeable preferred shares and Class B preferred share tranche right, net of issuance costs				$ 6,722		$ 55,769
Issuance of Class B convertible/exchangeable preferred shares and Class B preferred share tranche right, net of issuance costs (in shares)						36,806,039
Reclassification of Class B convertible preferred share and preferred exchangeable			4,257		$ 35,311
Conversion of Class A and B preferred exchangeable shares into Class A and convertible preferred shares			$ (31,940)		$ 31,940
Conversion of Class A and B preferred exchangeable shares into Class A and B convertible preferred shares (in shares)					28,874,378
Conversion of Class A and B convertible preferred shares into common shares	203,414				$ (203,414)			203,414
Conversion of Class A and B convertible preferred shares into common shares (in shares)					(145,405,124)		27,234,489
Conversion of convertible preferred share warrants into common share warrants	7,781							7,781
Issuance of common shares upon closing of initial public offering, net of offering costs and underwriter fees	193,053							193,053
Issuance of common shares upon closing of initial public offering, net of offering costs and underwriter fees (in shares)							12,500,000
Share-based compensation expense	426							426
Net loss	(44,733)									(44,733)
Ending Balance at Jun. 30, 2020	315,207							405,032		(89,825)
Ending Balance (in shares) at Jun. 30, 2020							41,664,044
Issuance of common shares upon exercise of common share warrants (in shares)							38,340,000
Share-based compensation expense	1,241							1,241
Unrealized loss on investments	(1)											(1)
Net loss	(9,993)									(9,993)
Ending Balance at Sep. 30, 2020	$ 306,454							$ 406,273		$ (99,818)		$ (1)
Ending Balance (in shares) at Sep. 30, 2020							41,702,384

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Non-Controlling Interest, Convertible Preferred Shares and Shareholders' Equity (Deficit) (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2020	Mar. 31, 2019
Issuance of common shares upon closing of initial public offering, offering costs and underwriter fees	$ 19,447
Class B Convertible Preferred Shares and Class B Preferred Share Tranche Right [Member]
Issuance of preferred shares and tranche right, issuance costs	6	$ 299
Class B Preferred Exchangeable Shares and Class B Preferred Share Tranche Right [Member]
Issuance of preferred shares and tranche right, issuance costs	$ 2	$ 13

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities:
Net loss	$ (64,948)	$ (7,983)
Adjustments to reconcile net loss to net cash used in operating activities:
Share-based compensation expense	2,025	328
Depreciation and amortization expense	373	213
Non-cash rent expense	3	(2)
Change in fair value of preferred share tranche right liability	32,722	(3,707)
Change in fair value of preferred share warrant liability	6,399
Amortization of premiums (accretion of discounts) on investments, net	24
Deferred tax benefit	(28)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3,792)	16
Other non-current assets	(521)
Accounts payable	150	408
Accrued expenses	1,025	719
Income taxes payable	(117)	151
Net cash used in operating activities	(26,685)	(9,857)
Cash flows from investing activities:
Purchases of investments	(54,286)
Purchases of property and equipment	(1,063)	(340)
Net cash used in investing activities	(55,349)	(340)
Cash flows from financing activities:
Proceeds from issuance of Class B convertible preferred shares and Class B preferred share tranche right, net of issuance costs	65,676	45,476
Proceeds from issuance of Class B preferred exchangeable shares of Fusion Pharmaceuticals (Ireland) Limited and Class B preferred share tranche right, net of issuance costs	6,722	6,711
Proceeds from the issuance of common shares upon closing of initial public offering, net of underwriter fees	197,625
Payment of offering costs	(4,572)
Net cash provided by financing activities	265,451	52,187
Net increase in cash, cash equivalents and restricted cash	183,417	41,990
Cash, cash equivalents and restricted cash at beginning of period	67,121	29,080
Cash, cash equivalents and restricted cash at end of period	250,538	71,070
Supplemental disclosure of cash flow information:
Cash paid for income taxes	280	40
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses		4
Issuance of common shares upon net settlement of stock option exercise		$ 57
Issuance of warrants to purchase Class B preferred shares and Class B preferred exchangeable shares as a non-cash dividend to preferred shareholders	$ 1,382

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Fusion Pharmaceuticals Inc., together with its consolidated subsidiaries (“Fusion” or the “Company”), is a clinical-stage oncology company focused on developing next-generation radiopharmaceuticals as precision medicines. The Company was formed and subsequently incorporated as Fusion Pharmaceuticals Inc. in December 2014 under the Canada Business Corporations Act. The Company was founded to advance certain intellectual property relating to radiopharmaceuticals that had been developed by the Centre for Probe Development and Commercialization, a radiopharmaceutical research and good manufacturing practice production center. The Company is headquartered in Hamilton, Ontario, Canada. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, successful discovery and development of its product candidates, development by competitors of new technological innovations, dependence on key personnel, the ability to attract and retain qualified employees, protection of proprietary technology, compliance with governmental regulations, the impact of the COVID-19 pandemic, the ability to secure additional capital to fund operations and commercial success of its product candidates. Product candidates currently under development will require extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel, and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s drug development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. The accompanying condensed 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 subsidiary, Fusion Pharmaceuticals US Inc., and majority-owned subsidiary, Fusion Pharmaceuticals (Ireland) Limited. As a result of consolidating this majority-owned subsidiary, the Company reflected a non-controlling interest on the consolidated balance sheets; however, the Company does not recognize a non-controlling interest in the consolidated statements of operations and comprehensive loss as the majority-owned subsidiary has no operating activities and is an extension of the parent company (see Note 2). All intercompany accounts and transactions have been eliminated in consolidation. Closing of Class B Preferred Share Financing and Settlement of Class B Preferred Share Tranche Right Liability On May 15, 2020, the Company achieved the specified regulatory milestone associated with the Class B preferred share tranche right (see Note 7), which triggered the requirement of the Class B shareholders to participate in the Milestone Financing. Upon closing of the Milestone Financing on June 2, 2020, the Company issued and sold 36,806,039 Class B preferred shares at a price of $1.5154 per share and 4,437,189 Class B special voting shares at a price of $0.000001 per share and the Company’s Ireland subsidiary issued and sold 4,437,189 Class B preferred exchangeable shares at a price of $1.5154 per share, for aggregate gross proceeds of $62.5 million. The Class B preferred share tranche right liability (see Note 7) was settled in connection with the achievement of the regulatory milestone associated with the Class B preferred share tranche right. Specifically, the fair value of the Class B preferred share tranche right liability was remeasured for the last time as of the Milestone Financing closing date, resulting in the Company recognizing a loss in the condensed consolidated statement of operations and comprehensive loss for the nine months ended September 30, 2020 of $32.7 million for the change in the fair value of the tranche right liability between December 31, 2019 and June 2, 2020. Immediately thereafter, the balance of the Class B preferred share tranche right liability of $39.6 million was reclassified to Class B convertible preferred shares in an amount of $35.3 million and to non-controlling interest in Fusion Pharmaceuticals (Ireland) Limited in an amount of $4.3 million on the consolidated balance sheet. Reverse Share Split On June 19, 2020, the Company effected a one-for-5.339 reverse share split of its issued and outstanding common shares and a proportional adjustment to the existing conversion ratios for each class of the Company’s Preferred Shares (see Note 7) and Preferred Exchangeable Shares (see Note 7). Accordingly, all share and per share amounts for all periods presented in the accompanying condensed consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse share split and adjustment of the preferred share conversion ratios. Initial Public Offering On June 25, 2020, the Company completed an initial public offering (“IPO”) of its common shares and issued and sold 12,500,000 common shares at a public offering price of $17.00 per share, resulting in net proceeds of $193.1 million after deducting underwriting fees, and after deducting offering costs. Upon closing of the IPO, the Company’s outstanding Preferred Exchangeable Shares automatically converted into convertible preferred shares then the outstanding convertible preferred shares automatically converted into shares of common shares (see Note 7). Upon conversion of the convertible preferred shares, the Company reclassified the carrying value of the convertible preferred shares to common shares and additional paid-in capital. In addition, the warrants to purchase the Company’s Series B convertible preferred shares and warrants to purchase preferred exchangeable shares of Fusion Pharmaceuticals (Ireland) Limited were converted into warrants to purchase the Company’s common shares upon the closing of the IPO. As a result, the warrant liability was remeasured a final time on the closing date of the IPO and reclassified to shareholders’ equity (deficit) (see Note 3). In connection with the IPO on June 25, 2020, the Company filed an amended and restated articles of the corporation under laws governed by the Canada Business Corporations Act Basis of Presentation The accompanying condensed consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. Since inception, the Company has funded its operations primarily with proceeds from sales of its convertible preferred shares, including borrowings under a convertible promissory note, which converted into convertible preferred shares, proceeds from sales of its Ireland subsidiary’s preferred exchangeable shares, and most recently with the proceeds from the IPO completed in June 2020. 10.0 64.9 0.6 8.0 September 30, 2020 99.8 Impact of the COVID-19 Pandemic The COVID-19 pandemic, which began in December 2019 and has spread worldwide, has caused many governments to implement measures to slow the spread of the outbreak through quarantines, travel restrictions, heightened border security and other measures. The impact of this pandemic has been, and will likely continue to be, extensive in many aspects of society, which has resulted, and will likely continue to result, in significant disruptions to the global economy as well as businesses and capital markets around the world. The future progression of the pandemic and its effects on the Company’s business and operations are uncertain. In response to public health directives and orders and to help minimize the risk of the virus to employees, the Company has taken precautionary measures, including implementing work-from-home policies for certain employees. The impact of the virus, including work-from-home policies, may negatively impact productivity, disrupt the Company’s business, and delay its preclinical research and clinical trial activities and its development program timelines, the magnitude of which will depend, in part, on the length and severity of the restrictions and other limitations on the Company’s ability to conduct its business in the ordinary course. Specifically, the Company may not be able to enroll additional patient cohorts on its planned timeline due to disruptions at its clinical trial sites and is unable to predict how the COVID-19 pandemic may affect its ability to successfully progress its clinical programs in the future. Other impacts to the Company’s business may include temporary closures of its suppliers and disruptions or restrictions on its employees’ ability to travel. Any prolonged material disruption to the Company’s employees or suppliers could adversely impact the Company’s preclinical research and clinical trial activities, financial condition and results of operations, including its ability to obtain financing. The Company is monitoring the potential impact of the COVID-19 pandemic on its business and condensed consolidated financial statements. To date, the Company has not experienced material business disruptions or incurred impairment losses in the carrying values of its assets as a result of the pandemic and it is not aware of any specific related event or circumstance that would require it to revise its estimates reflected in these condensed consolidated financial statements.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates The preparation of the Company’s condensed 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 condensed consolidated financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, the accrual of research and development expenses and the valuations of common shares, stock options, preferred share tranche rights and preferred share warrants. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates when there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results may differ from those estimates or assumptions. Unaudited Interim Financial Information The accompanying condensed consolidated balance sheet as of September 30, 2020, the condensed consolidated statement of operations and comprehensive loss, and the condensed consolidated statement of non-controlling interest, convertible preferred shares and shareholders’ equity (deficit) for the three and nine months ended September 30, 2020 and 2019, and the condensed consolidated statement of cash flows for the nine months ended September 30, 2020 and 2019 are unaudited. The unaudited interim condensed 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 September 30, 2020 and the results of its operations for three and nine months ended September 30, 2020 and 2019 and its cash flows for the nine months ended September 30, 2020 and 2019. The financial data and other information disclosed in these notes related to the three and nine months ended September 30, 2020 and 2019 are also unaudited. The results for the three and nine months ended September 30, 2020 are not necessarily indicative of results to be expected for the year ending December 31, 2020, any other interim periods, or any future year or period. The accompanying balance sheet as of December 31, 2019 has been derived from the Company’s audited financial statements for the year ended December 31, 2019. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These unaudited interim condensed consolidated financial statements should be read in conjunction with the audited annual consolidated financial statements and notes thereto as of December 31, 2019 and for each of the three years in the period ended December 31, 2019 included in the Company’s Registration Statement on Form S-1, as amended, on file with the SEC. Foreign Currency and Currency Translation The reporting currency of the Company is the U.S. dollar. The functional currency of the Company’s operating company in Canada, operating company in the U.S. and non-operating company in Ireland is also the U.S. dollar. As a result, the Company records no cumulative translation adjustments related to translation of unrealized foreign exchange gains or losses. For the remeasurement of local currencies to the U.S. dollar functional currency of the Canadian and Irish entities, assets and liabilities are translated into U.S. dollars at the exchange rate in effect on the balance sheet date, and income items and expenses are translated into U.S. dollars at the average exchange rate in effect during the period. Resulting transaction gains (losses) are included in other income (expense), net in the consolidated statements of operations and comprehensive loss, as incurred. Adjustments that arise from exchange rate changes on transactions denominated in a currency other than the local currency are included in other income (expense), net in the consolidated statements of operations and comprehensive loss, as incurred. During the three and nine months ended September 30, 2020, the Company recorded less than ($0.1) million of foreign currency losses in the consolidated statements of operations and comprehensive loss for both periods. During the three and nine months ended September 30, 2019, the Company recorded less than ($0.1) million and $0.1 million of foreign currency gains (losses) in the consolidated statements of operations and comprehensive loss for both periods. Cash, Cash Equivalents and Restricted Cash Cash and cash equivalents consist of standard checking accounts, money market accounts, and all highly liquid investments with an original maturity of three months or less at the date of purchase. As of September 30, 2020 and December 31, 2019, the Company was required to maintain a separate cash balance of $0.3 million to collateralize corporate credit cards with a bank, which was classified as restricted cash (current) on its condensed consolidated balance sheets. In connection with the Company’s lease agreement entered into in October 2019 (see Note 12), the Company maintains a letter of credit of $1.5 million for the benefit of the landlord. As of September 30, 2020 and December 31, 2019, the underlying cash balance collateralizing this letter of credit was classified as restricted cash (non-current) on its condensed consolidated balance sheets based on the release date of the restrictions of this cash. As of September 30, 2020 and 2019, the cash, cash equivalents and restricted cash of $250.5 million and $71.1 million, respectively, presented in the condensed consolidated statements of cash flows included cash and cash equivalents of $248.8 million and $69.3 million, respectively, and restricted cash of $1.7 million and $1.8 million, respectively. Investments The Company determines the appropriate classification of its investments in debt securities at the time of purchase and re-evaluates such determination at each balance sheet date. The Company classifies its investments as current or non-current based on each instrument’s underlying maturity date. Investments with original maturities of greater than three months and less than twelve months are classified as current and are included in short-term investments in the condensed consolidated balance sheets. Investments with original maturities greater than one year from the balance sheet date are classified as non-current and are included in long-term investments in the condensed consolidated balance sheets. The Company’s investments are classified as available-for-sale, are reported at fair value and consist of U.S. government agency securities, corporate bonds, and commercial paper. Unrealized gains and losses included in other comprehensive income (loss) as a component of shareholders’ equity (deficit) until realized. Amortization and accretion of premiums and discounts are recorded in interest income (expense). Realized gains and losses on debt securities are included in other income (expense), net. If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is other than temporary and, if so, marks the investment to market on the Company’s condensed consolidated statements of operations and comprehensive loss. Deferred Offering Costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of an equity financing, these costs are recorded as a reduction of the proceeds from the offering, either as a reduction to the carrying value of the preferred exchangeable shares or convertible preferred shares or in shareholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the offering. Should an in-process equity financing be abandoned, the deferred offering costs would be expensed immediately as a charge to operating expenses in the consolidated statements of operations and comprehensive loss. The Company did not record any deferred offering costs as of September 30, 2020 or December 31, 2019. Offering costs of $4.6 million incurred during 2020 have been recorded in shareholders’ equity (deficit) as a reduction of the gross proceeds generated from the Company’s initial public offering of common shares. Business Combinations In determining whether an acquisition should be accounted for as a business combination or asset acquisition, the Company first determines whether substantially all of the fair value of the gross assets acquired is concentrated in a single identifiable asset or a group of similar identifiable assets. If this is the case, the single identifiable asset or the group of similar assets is not deemed to be a business, and is instead deemed to be an asset. If this is not the case, the Company then further evaluates whether the single identifiable asset or group of similar identifiable assets and activities includes, at a minimum, an input and a substantive process that together significantly contribute to the ability to create outputs. If so, the Company concludes that the single identifiable asset or group of similar identifiable assets and activities is a business. The Company accounts for business combinations using the acquisition method of accounting. Application of this method of accounting requires that (i) identifiable assets acquired (including identifiable intangible assets) and liabilities assumed generally be measured and recognized at fair value as of the acquisition date and (ii) the excess of the purchase price over the net fair value of identifiable assets acquired and liabilities assumed be recognized as goodwill, which is not amortized for accounting purposes but is subject to testing for impairment at least annually. Acquired in-process research and development (“IPR&D”) is recognized at fair value and initially characterized as an indefinite-lived intangible asset, irrespective of whether the acquired IPR&D has an alternative future use. Transaction costs related to business combinations are expensed as incurred. Determining the fair value of assets acquired and liabilities assumed in a business combination requires management to use significant judgment and estimates, especially with respect to intangible assets. During the measurement period, which extends no later than one year from the acquisition date, the Company may record certain adjustments to the carrying value of the assets acquired and liabilities assumed with the corresponding offset to goodwill. After the measurement period, all adjustments are recorded in the consolidated statements of operations as operating expenses or income. To date, the Company has not recorded any asset acquisitions as a business combination. Asset Acquisitions The Company measures and recognizes asset acquisitions that are not deemed to be business combinations based on the cost to acquire the assets, which includes transaction costs. Goodwill is not recognized in asset acquisitions. In an asset acquisition, the cost allocated to acquire IPR&D with no alternative future use is charged to expense at the acquisition date. Contingent consideration in asset acquisitions payable in the form of cash is recognized when payment becomes probable and reasonably estimable, unless the contingent consideration meets the definition of a derivative, in which case the amount becomes part of the asset acquisition cost when acquired. Contingent consideration payable in the form of a fixed number of the Company’s own shares is measured at fair value as of the acquisition date and recognized when the issuance of the shares becomes probable. Upon recognition of the contingent consideration payment, the amount is included in the cost of the acquired asset or group of assets, or, if related to IPR&D with no alternative future use, charged to expense. 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 and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. Prior to the settlement of the Company’s preferred share tranche right liability and prior to the conversion of the Company’s preferred share warrant liability, these instruments were carried at fair value, determined according to Level 3 inputs in the fair value hierarchy described above (see Note 3). The Company’s cash equivalents and investments are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s amounts due for refundable investment tax credits and Canadian harmonized sales tax, accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. Preferred Share Tranche Right Liability The subscription agreements for the Company’s Class B convertible preferred shares (see Note 7) and its Ireland subsidiary’s Class B preferred exchangeable shares (see Note 7) provides investors the right, or obligates investors, to participate in subsequent offerings of Class B convertible preferred shares or Class B preferred exchangeable shares together with Class B special voting shares in the event that specified development or regulatory milestones are achieved (the “Class B preferred share tranche right liability”). The Company classifies these preferred share tranche rights as a liability on its consolidated balance sheets as each preferred share tranche right is a freestanding financial instrument that may require the Company to transfer assets upon the achievement of specified milestone events. Each preferred share tranche right liability was initially recorded at fair value upon the date of issuance of each preferred share tranche right and is subsequently remeasured to fair value at each reporting date. Changes in the fair value of the preferred share tranche right liability are recognized as a component of other income (expense) in the consolidated statement of operations and comprehensive loss. Changes in the fair value of the preferred share tranche right liability will continue to be recognized until the respective preferred share tranche right is settled upon achievement of the specified milestones or expires. On May 15, 2020, the Company achieved the specified regulatory milestone associated with the Class B preferred share tranche right (see Note 7), which triggered the requirement of the Class B shareholders to participate in the Milestone Financing. Upon closing of the Milestone Financing on June 2, 2020, the Company issued and sold 36,806,039 Class B preferred shares at a price of $1.5154 per share and 4,437,189 Class B special voting shares at a price of $0.000001 per share and the Company’s Ireland subsidiary issued and sold 4,437,189 Class B preferred exchangeable shares at a price of $1.5154 per share, for aggregate gross proceeds of $62.5 million. The Class B preferred share tranche right liability (see Note 7) was settled in connection with the achievement of the regulatory milestone associated with the Class B preferred share tranche right. Specifically, the fair value of the Class B preferred share tranche right liability was remeasured for the last time as of the Milestone Financing closing date, resulting in the Company recognizing a loss in the consolidated statement of operations and comprehensive loss for the nine months ended September 30, 2020 of $32.7 million for the change in the fair value of the tranche right liability between December 31, 2019 and June 2, 2020. Immediately thereafter, the balance of the Class B preferred share tranche right liability of $39.6 million was reclassified to Class B convertible preferred shares in an amount of $35.3 million and to non-controlling interest in Fusion Pharmaceuticals (Ireland) Limited in an amount of $4.3 million on the consolidated balance sheet. Preferred Share Warrant Liability The Company classifies warrants to purchase its convertible preferred shares and warrants to purchase preferred exchangeable shares of Fusion Pharmaceuticals (Ireland) Limited as a liability on its consolidated balance sheets as these warrants are freestanding financial instruments that may require the Company to transfer assets upon exercise (see Note 7). The preferred share warrant liability, which consists of warrants to purchase Class B convertible preferred shares of the Company and warrants to purchase Class B preferred exchangeable shares of Fusion Pharmaceuticals (Ireland) Limited, was initially recorded at fair value upon the date of issuance of each warrant and is subsequently remeasured to fair value at each reporting date. Changes in the fair value of the preferred share warrant liability are recognized as a component of other income (expense) in the consolidated statement of operations and comprehensive loss. Changes in the fair value of the preferred share warrant liability will continue to be recognized until each respective warrant is exercised, expires or qualifies for equity classification. Upon the closing of the IPO, the warrants to purchase its convertible preferred shares and warrants to purchase preferred exchangeable shares of Fusion Pharmaceuticals (Ireland) Limited were converted into warrants to purchase shares of the Company’s common shares. As a result, the warrant liability was remeasured a final time on the closing date of the IPO and reclassified to shareholders’ equity (deficit) as the warrants qualify for equity classification. Research, Development and Manufacturing Contract Costs and Accruals The Company has entered into various research, development and manufacturing contracts with research institutions and other companies. These agreements are generally cancelable, and related costs are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research, development and manufacturing costs. When billing terms under these contracts do not coincide with the timing of when the work is performed, the Company is required to make estimates of outstanding obligations to those third parties as of period end. Any accrual estimates are based on a number of factors, including the Company’s knowledge of the progress towards completion of the research, development and manufacturing activities, invoicing to date under the contracts, communication from the research institutions and other companies of any actual costs incurred during the period that have not yet been invoiced and the costs included in the contracts. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical accrual estimates made by the Company have not been materially different from the actual costs. Net Loss per Share The Company follows the two-class method when computing net income (loss) per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net income (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 income (loss) per share attributable to common shareholders is computed by dividing the net income (loss) attributable to common shareholders by the weighted-average number of common shares outstanding for the period. Diluted net income (loss) attributable to common shareholders is computed by adjusting net income (loss) attributable to common shareholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common shareholders is computed by dividing the diluted net income (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 stock options, warrants and convertible preferred shares are considered potential dilutive common shares. The Company’s convertible preferred shares contractually entitle the holders of such shares to participate in dividends but do not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common shareholders, such losses are not allocated to such participating securities. In periods in which the Company reported 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 nine months ended September 30, 2020 and 2019. Recently Adopted Accounting Pronouncements In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement Recently Issued Accounting Pronouncements The Company qualifies as “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012 and has elected to “opt in” to the extended transition related to complying with new or revised accounting standards, which means that when a standard is issued or revised and it has different application dates for public and nonpublic companies, the Company will adopt the new or revised standard at the time nonpublic companies adopt the new or revised standard and will do so until such time that the Company either (i) irrevocably elects to “opt out” of such extended transition period or (ii) no longer qualifies as an emerging growth company. The Company may choose to early adopt any new or revised accounting standards whenever such early adoption is permitted for private companies. In February 2016, the FASB issued ASU No. 2016-02 , Leases (Topic 842) ASU No. 2018-11, Leases (Topic 842) In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments Codification Improvements to Topic 326, Financial Instruments—Credit Losses Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The following tables present information about the Company’s financial assets and liabilities that are measured at fair value on a recurring basis and indicates the level of the fair value hierarchy used to determine such fair values (in thousands): Fair Value Measurements as of September 30, 2020 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 28,670 $ — $ — $ 28,670 Commercial paper — 2,999 — 2,999 Corporate bonds — 1,862 — 1,862 U.S. Government agencies — 20,998 — 20,998 Investments: Commercial paper — 6,994 — 6,994 Corporate bonds — 905 — 905 U.S. Government agencies — 46,362 — 46,362 $ 28,670 $ 80,120 $ — $ 108,790 Fair Value Measurements as of December 31, 2019 Using: Level 1 Level 2 Level 3 Total Liabilities: Preferred share tranche right liability $ — $ — $ 5,741 $ 5,741 $ — $ — $ 5,741 $ 5,741 During the year ended December 31, 2019 and the nine months ended September 30, 2020, there were no transfers between Level 1, Level 2 and Level 3. Valuation of Preferred Share Tranche Right Liability The preferred share tranche right liability in the table below is composed of the fair value of rights to purchase Class B convertible preferred shares and Class B preferred exchangeable shares with Class B special voting shares (see Note 7). The fair value of the preferred share tranche right liability was determined based on significant inputs not observable in the market, which represent a Level 3 measurement within the fair value hierarchy. The fair value of the preferred share tranche right liability was determined using the forward contract pricing model, which considered as inputs the probability and timing of achieving the specified milestones as of each valuation date, the estimated fair value of the preferred shares as of each valuation date, and the risk-free interest rate. The most significant assumption in the forward contract pricing model impacting the fair value of the preferred share tranche right liability is the fair value of the Company’s Class B convertible preferred shares as of each measurement date. The Company determines the fair value per share of the underlying Class B convertible preferred shares by taking into consideration the most recent sales of its convertible preferred shares, results obtained from third-party valuations and additional factors the Company deems relevant. As of June 2, 2020, the date of final measurement as described below, and December 31, 2019, the fair value of each Class B convertible preferred share was $2.47 per share and $1.45 per share, respectively. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve for time periods approximately equal to the remaining estimated time period of achievement of the specified milestones underlying the preferred share tranche rights. The preferred share tranche right liability was initially recorded at fair value upon the date of issuance of each preferred share tranche right and is subsequently remeasured to fair value at each reporting date. Changes in the fair value of the preferred share tranche right liability are recognized as a component of other income (expense) in the consolidated statements of operations and comprehensive loss. Changes in the fair value of the preferred share tranche right liability will continue to be recognized until the respective preferred share tranche right is settled upon achievement of the specified milestones or expires. On May 15, 2020, the Company achieved the specified regulatory milestone associated with the Class B preferred share tranche right (see Note 7), which triggered the requirement of the Class B shareholders to participate in the Milestone Financing. Upon closing of the Milestone Financing on June 2, 2020, the Company issued and sold 36,806,039 Class B preferred shares at a price of $1.5154 per share and 4,437,189 Class B special voting shares at a price of $0.000001 per share and the Company’s Ireland subsidiary issued and sold 4,437,189 Class B preferred exchangeable shares at a price of $1.5154 per share, for aggregate gross proceeds of $62.5 million. The Class B preferred share tranche right liability (see Note 7) was settled in connection with the achievement of the regulatory milestone associated with the Class B preferred share tranche right. Specifically, the fair value of the Class B preferred share tranche right liability was remeasured for the last time as of the Milestone Financing closing date, resulting in the Company recognizing a loss in the consolidated statement of operations and comprehensive loss for the nine months ended September 30, 2020 of $32.7 million for the change in the fair value of the tranche right liability between December 31, 2019 and June 2, 2020. Immediately thereafter, the balance of the Class B preferred share tranche right liability of $39.6 million was reclassified to Class B convertible preferred shares in an amount of $35.3 million and to non-controlling interest in Fusion Pharmaceuticals (Ireland) Limited in an amount of $4.3 million on the consolidated balance sheet. Valuation of Preferred Share Warrant Liability The preferred share warrant liability in the table below is composed of the fair value of warrants to purchase Class B convertible preferred shares of the Company and warrants to purchase Class B preferred exchangeable shares of Fusion Pharmaceuticals (Ireland) Limited that were issued in January 2020 in connection with the Company’s Class B preferred share financing (see Note 7). The fair value of the preferred share warrant liability was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of the preferred share warrant liability was determined using a hybrid method, which is a probability-weighted expected return method, or PWERM, where the equity value in one or more of the scenarios is calculated using an option-pricing model, or OPM. The PWERM is a scenario-based methodology that estimates the fair value of the Company’s different classes of equity based upon an analysis of future values for the Company, assuming various outcomes. Under both models, assumptions and estimates are used to value the preferred share warrants. The Company assesses these assumptions and estimates on a quarterly basis as additional information impacting the assumptions is obtained. The quantitative elements associated with the Company’s Level 3 inputs impacting the fair value measurement of the preferred share warrant liability include the fair value per share of the underlying Class B convertible preferred shares, the timing, form and overall value of the expected exits for the shareholders, the risk-free interest rate, the expected dividend yield and the expected volatility of the Company’s shares. The most significant assumption impacting the fair value of the preferred share warrant liability is the fair value of the Company’s Class B convertible preferred shares as of each measurement date. The Company determines the fair value per share of the underlying Class B convertible preferred shares by taking into consideration the most recent sales of its preferred shares, results obtained from third-party valuations and additional factors it deems relevant. In January 2020, upon issuance of the preferred share warrants, the fair value of each Class B convertible preferred share was $1.45 per share. As of June 2, 2020, the date of final measurement of the preferred share tranche right liability as described below, the fair value of each Class B convertible preferred share was $2.47 per share. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve for time periods approximately equal to the remaining contractual term of the warrants. The Company estimated a 0% dividend yield based on the expected dividend yield and the fact that the Company has never paid or declared cash dividends. The preferred share warrant liability was initially recorded at fair value upon the date the warrants were issued and exercisable and is subsequently remeasured to fair value at each reporting date. Changes in the fair value of the preferred share warrant liability are recognized as a component of other income (expense) in the consolidated statements of operations and comprehensive loss. Changes in the fair value of the preferred share warrant liability will continue to be recognized until each respective warrant is exercised, expires or qualifies for equity classification. Upon the closing of the IPO, the warrants to purchase its convertible preferred shares and warrants to purchase preferred exchangeable shares of Fusion Pharmaceuticals (Ireland) Limited were converted into warrants to purchase shares of the Company’s common shares. As a result, the warrant liability was remeasured a final time on the closing date of the IPO and reclassified to shareholders’ equity (deficit) as the warrants qualify for equity classification. The following table provides a roll-forward of the aggregate fair value of the Company’s preferred share tranche right liability and preferred share warrant liability, for which fair value is determined using Level 3 inputs (in thousands): Preferred Share Tranche Right Liability Preferred Share Warrant Liability Balance as of December 31, 2019 $ 5,741 $ — Initial fair value of Class B preferred share tranche right liability 1,105 — Initial fair value of Class B preferred share warrant liability — 1,382 Change in fair value of Class B preferred share tranche right liability 32,722 — Change in fair value of Class B preferred share warrant liability — 6,399 Reclassification of Class B preferred share tranche right liability upon settlement (39,568 ) — Conversion of Class B preferred shares warrants into common share warrants — (7,781 ) Balance as of September 30, 2020 $ — $ —

Investments

Investments	9 Months Ended
	Sep. 30, 2020
Investments Debt And Equity Securities [Abstract]
Investments	4. Investments Investments consisted of the following (in thousands): September 30, 2020 Amortized Cost Fair Value Due within one year or less $ 43,160 $ 43,159 Due after one year through three years 11,102 11,102 $ 54,262 $ 54,261 As of September 30, 2020, the amortized cost and estimated fair value of investments, by contractual maturity, was as follows (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Current Non-Current Commercial paper $ 6,994 $ — $ — $ 6,994 $ 6,994 $ — Corporate bonds 905 — — 905 905 — U.S. Government agencies 46,363 1 (2 ) 46,362 35,260 11,102 $ 54,262 $ 1 $ (2 ) $ 54,261 $ 43,159 $ 11,102 The Company did not hold any investments as of December 31, 2019.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	9 Months Ended
	Sep. 30, 2020
Prepaid Expense And Other Assets Current [Abstract]
Prepaid Expenses and Other Current Assets	5 . Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following (in thousands): September 30, 2020 December 31, 2019 Prepaid clinical trial expenses $ — $ 94 Prepaid insurance 3,136 11 Prepaid software subscriptions 173 129 Income tax receivable 109 — Interest receivable 229 62 Canadian harmonized sales tax receivable 238 252 Refundable investment tax credits 315 176 Other 452 205 $ 4,652 $ 929

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2020
Payables And Accruals [Abstract]
Accrued Expenses	6 . Accrued Expenses Accrued expenses consisted of the following (in thousands): September 30, 2020 December 31, 2019 Accrued employee compensation and benefits $ 1,524 $ 991 Accrued external research and development expenses 1,715 1,565 Accrued professional and consulting fees 1,048 744 Other 66 26 $ 4,353 $ 3,326

Equity

Equity	9 Months Ended
	Sep. 30, 2020
Equity [Abstract]
Equity	7 . Equity Common Shares On June 19, 2020, the Company effected a one-for-5.339 reverse share split of its issued and outstanding common shares and a proportional adjustment to the existing conversion ratios for each class of the Company’s Preferred Shares and Preferred Exchangeable Shares. Accordingly, all share and per share amounts for all periods presented in the accompanying condensed consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse share split and adjustment of the preferred share conversion ratios. On June 30, 2020, the Company closed its IPO of common shares and issued and sold 12,500,000 shares of common shares at a public offering price of $17.00 per share, resulting in net proceeds of approximately $193.1 million after deducting underwriting fees and offering costs. Upon the closing of the IPO, all outstanding voting and non-voting common shares were converted to a single class of common shares authorized by the Company’s articles of the corporation, as amended and restated. As of September 30, 2020, the Company’s articles of the corporation, as amended and restated, authorized the Company to issue unlimited common shares, each with no par value per share. Each common share entitles the holder to one vote on all matters submitted to a vote of the Company’s shareholders. Common shareholders are entitled to receive dividends, if any, as may be declared by the board of directors. Through September 30, 2020, no cash dividends had been declared or paid by the Company. Convertible Preferred Shares Prior to the IPO, the Company has issued Class A convertible preferred shares (the “Class A preferred shares”) and Class B convertible preferred shares (the “Class B preferred shares” and, together with the Class A preferred shares, the “Preferred Shares”). As of December 31, 2019, the Company’s articles of the corporation, as amended and restated, authorized the Company to issue an aggregate of 132,207,290 Preferred Shares, respectively, each with no par value per share. In March 2019, the Company completed its first closing of its Class B preferred shares and issued and sold 30,207,129 Class B preferred shares at a price of $1.5154 per share for gross proceeds of $45.8 million (the “2019 Preferred Share Financing”). In January 2020, the Company executed the First Amendment to the Class B Subscription Agreement (“Amended Class B Subscription Agreement”) whereby the Canada Pension Plan Investment Board (“CPP”) agreed to purchase an aggregate of $20.0 million of Class B preferred shares, at a price of $1.5154 per share, in two tranches. In January 2020, the Company issued and sold to CPP 6,598,917 Class B preferred shares, resulting in gross proceeds of $10.0 million (the “Additional Class B Closing”). The Company incurred issuance costs of $0.1 million in connection with this transaction. The rights and preferences of the Class B preferred shares sold under the Additional Class B Closing are the same as the rights and preferences of the Class B preferred shares issued and sold by the Company in March 2019. Accordingly, under the terms of the Amended Class B Subscription Agreement, upon the earlier occurrence of a specified development or specified regulatory milestone, CPP was obligated to purchase an additional 6,598,917 Class B preferred shares at a price of $1.5154 per share. The Company concluded that these rights or obligations of CPP to participate in the Milestone Financing of Class B preferred shares met the definition of a freestanding financial instrument that was required to be recorded as a liability at fair value as (i) the instruments are legally detachable and separately exercisable from the Class B preferred shares and (ii) the rights will require the Company to transfer assets upon future closings of the Class B preferred shares. Upon the Additional Class B Closing in January 2020, the Company recorded an additional liability for the preferred share tranche right of $1.1 million and a corresponding reduction to the carrying value of the Class B preferred shares. In May 2020, the Company achieved the specified regulatory milestone associated with the Class B preferred share tranche right, which triggered the requirement of the Class B shareholders to participate in the Milestone Financing. Upon closing of the Milestone Financing on June 2, 2020, the Company issued and sold 36,806,039 Class B preferred shares at a price of $1.5154 per share for aggregate proceeds of $55.8 million. The Class B preferred share tranche right liability was settled in connection with the achievement of the regulatory milestone associated with the Class B preferred share tranche right. Specifically, the fair value of the Class B preferred share tranche right liability was remeasured for the last time as of the Milestone Financing closing date, resulting in the Company recognizing a loss in the consolidated statement of operations and comprehensive loss for the nine months ended September 30, 2020 of $32.7 million for the change in the fair value of the tranche right liability between December 31, 2019 and June 2, 2020. Immediately thereafter, the balance of the Class B preferred share tranche right liability of $39.6 million was reclassified to Class B convertible preferred shares in an amount of $35.3 million and to non-controlling interest in Fusion Pharmaceuticals (Ireland) Limited in an amount of $4.3 million on the consolidated balance sheet. Upon the closing of the IPO, the Company converted the then outstanding Class A and Class B preferred shares into common shares at a conversion ratio of 5.339 Preferred Share to one common share. Preferred Exchangeable Shares and Special Voting Shares In connection with each issuance and sale of its Class A preferred shares and Class B preferred shares, the Company’s Ireland subsidiary, Fusion Pharmaceuticals (Ireland) Limited, issued and sold Class A and Class B preferred exchangeable shares (together, the “Preferred Exchangeable Shares”) to investors. Simultaneously with the issuance and sale of the Preferred Exchangeable Shares, the Company issued and sold its Class A and Class B special voting shares (together, the “Special Voting Shares”) to the same investors. Prior to the IPO, the Company’s Ireland subsidiary’s amended constitution authorized it to issue an aggregate of 28,874,378 Preferred Exchangeable Shares and 29,747,987 Preferred Exchangeable Shares, respectively, with a par value of $0.001 per share. Prior to the IPO, the Company’s articles of the corporation, as amended and restated, authorized the Company to issue an aggregate of 28,874,378 Special Voting Shares and 29,747,987 Special Voting Shares, respectively, with a cash redemption value of $0.000001 per share. In March 2019, in connection with the first closing of Class B preferred shares, as described above, the Company’s Ireland subsidiary issued and sold 4,437,189 Class B preferred exchangeable shares at a price of $1.5154 per share and the Company issued and sold 4,437,189 Class B special voting shares at a price of $0.000001 per share for aggregate gross proceeds of $6.7 million (the “2019 Preferred Exchangeable Share Financing”). In May 2020, the Company achieved the specified regulatory milestone associated with the Class B preferred share tranche right, which triggered the requirement of the Class B shareholders to participate in the Milestone Financing. Upon closing of the Milestone Financing on June 2, 2020, the Company issued and sold 4,437,189 Class B special voting shares at a price of $0.000001 per share and the Company’s Ireland subsidiary issued and sold 4,437,189 Class B preferred exchangeable shares at a price of $1.5154 per share, for aggregate gross proceeds of $6.7 million. Upon the closing of the IPO, the Company converted all of the outstanding Class A and Class B preferred exchangeable shares and Special Voting Shares into Class A and Class B preferred shares on a one-for-one basis then converted the Class A and Class B preferred shares into common shares at a conversion ratio of 5.339 Preferred Share to one common share. Warrants In January 2020, in conjunction with the Company’s execution of the Amended Class B Subscription Agreement, the Company issued to the existing holders of Class B convertible preferred shares (excluding the investor in the Additional Class B Closing in January 2020) warrants to purchase 3,126,391 Class B convertible preferred shares, at an exercise price of $1.5154 per share, and Fusion Pharmaceuticals (Ireland) Limited issued to the existing holders of Class B preferred exchangeable shares warrants to purchase 873,609 Class B preferred exchangeable shares, at an exercise price of $1.5154 per share (collectively the “Preferred Share Warrants”). If the warrants to purchase Class B preferred exchangeable shares are exercised, at that same time, the shareholder is obligated to purchase from the Company an equal number of Class B special voting shares at a price of $0.000001 per share. The Preferred Share Warrants were issued for no consideration, and the specified exercise prices of each warrant are subject to adjustment for share dividends, share splits, combination or other similar recapitalization transactions as provided under the terms of the warrants. The Preferred Share Warrants were immediately exercisable and expire two years from the date of issuance or upon the earlier occurrence of specified qualifying events, which include the consummation of a Deemed Liquidation Event and the closing of a qualifying share sale (as defined in the articles of the corporation, as amended and restated). Upon the closing of a qualified public offering, on specified terms, all outstanding warrants to purchase Class B convertible preferred shares of the Company and warrants to purchase Class B preferred exchangeable shares of Fusion Pharmaceutics (Ireland) Limited will become warrants to purchase common shares of the Company. Upon issuance of the Preferred Share Warrants in January 2020, the Company recorded on its consolidated balance sheet a preferred share warrant liability of $1.4 million, equal to the issuance-date fair value of the Preferred Share Warrants, as well as a corresponding decrease of $1.3 million to additional paid-in capital, reducing that to zero, and an increase of $0.1 million to accumulated deficit for the remainder. The issuance of the Preferred Share Warrants was treated as a deemed dividend to existing preferred shareholders for purposes of the Company’s calculation of net loss per share attributable to common shareholders, and, as such, the aggregate value of the dividend to existing preferred shareholders was deducted from the Company’s net loss when computing net loss per share attributable to common shareholders (see Note 11). The Company remeasures the fair value of the liability associated with the Preferred Share Warrants at each reporting date (see Note 3) and records any adjustments as a component of other income (expense) in the consolidated statements of operations and comprehensive loss. Upon the closing of the IPO, the warrants to purchase 3,126,391 of its convertible preferred shares and warrants to purchase 873,609 preferred exchangeable shares of Fusion Pharmaceuticals (Ireland) Limited were converted into warrants to purchase 749,197 shares of the Company’s common shares at an exercise price of $8.10 per share. As a result, the warrant liability was remeasured a final time on the closing date of the IPO and reclassified to shareholders’ equity (deficit) as the warrants qualify for equity classification (see Note 3). For the nine months ended September 30, 2020, the Company recognized a loss of $6.4 million as a component of other income (expense) in the condensed consolidated statement of operations and comprehensive loss to reflect an increase in fair value of the Preferred Share Warrant. On August 17, 2020, a holder of common share warrants exercised 97,381 common share warrants through a cashless exercise and the Company issued 38,340 common shares with the remaining 59,041 warrants being cancelled to settle the exercise price. As of September 30, 2020, 651,816 common share warrants remained outstanding.

Share-Based Compensation

Share-Based Compensation	9 Months Ended
	Sep. 30, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-Based Compensation	8 . Share-based Compensation 2020 Stock Option and Incentive Plan On June 18, 2020, the Company’s board of directors adopted the 2020 Stock Option and Incentive Plan (the “2020 Plan”), which became effective on June 24, 2020. The 2020 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock units, restricted stock awards, unrestricted stock awards, cash-based awards and dividend equivalent rights to the Company’s officers, employees, non-employee directors and consultants. The number of shares initially reserved for issuance under the 2020 Plan was 4,273,350, which shall be cumulatively increased on January 1, 2021 and each January 1 thereafter by 4% of the number of the Company’s common shares outstanding on the immediately preceding December 31 or such lesser number of shares determined by the Company’s compensation committee of the board of directors. The common shares underlying any awards that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by the Company prior to vesting, satisfied without the issuance of shares, expire or are otherwise terminated (other than by exercise) under the 2020 Plan and the 2017 Plan will be added back to the common shares available for issuance under the 2020 Plan. The total number of common shares reserved for issuance under the 2020 Plan was 4,273,350 shares as of September 30, 2020. As of September 30, 2020, 2,933,366 shares, remained available for future grant under the 2020 Plan. Shares that are expired, forfeited, canceled or otherwise terminated without having been fully exercised will be available for future grant under the 2020 Plan. 2017 Equity Incentive Plan The Company’s 2017 Equity Incentive Plan (the “2017 Plan”) provides for the Company to grant incentive stock options or nonqualified stock options, restricted share awards and restricted share units to employees, officers, directors and non-employee consultants of the Company. The total number of common shares reserved for issuance under the 2017 Plan was 0 and 4,700,393 shares as of September 30, 2020 and December 31, 2019, respectively. As of September 30, 2020 and December 31, 2019, 0 shares and 1,598,512 shares, respectively, remained available for future grant under the 2017 Plan. Shares that are expired, forfeited, canceled or otherwise terminated without having been fully exercised will be available for future grant under the 2020 Plan. 2020 Employee Share Purchase Plan On June 18, 2020, the Company’s board of directors adopted the 2020 Employee Share Purchase Plan (the “ESPP”), which became effective on June 24, 2020. A total of 450,169 common shares were reserved for issuance under this plan. In addition, the number of common shares that may be issued under the ESPP will automatically increase on January 1, 2021 and each January 1 thereafter by the lesser of (i) 900,338 common shares, (ii) 1% of the number of the Company’s common shares outstanding on the immediately preceding December 31 and (iii) such lesser number of shares as determined by the Company’s compensation committee of the board of directors. As of September 30, 2020, no shares were issued under the ESPP. Stock Option Valuation The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected share volatility based on the historical volatility of a publicly traded set of 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. For options with service-based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to non-employee consultants 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 and does not expect to pay any cash dividends in the foreseeable future. The following table presents, on a weighted-average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted: Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Risk-free interest rate 0.33 % 1.58 % 0.70 % 1.58 % Expected term (in years) 6.0 5.9 6.0 5.9 Expected volatility 67.0 % 64.5 % 65.5 % 64.5 % Expected dividend yield 0 % 0 % 0 % 0 % Stock Options The following table summarizes the Company’s stock option activity since December 31, 2019: Number of Shares Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term Aggregate Intrinsic Value (in years) (in thousands) Outstanding as of December 31, 2019 3,045,301 $ 1.66 8.4 $ 2,112 Granted 2,346,645 11.50 Forfeited/cancelled (125,480 ) 5.63 Outstanding as of September 30, 2020 5,266,466 $ 5.95 8.4 $ 37,696 Vested and expected to vest as of September 30, 2020 5,266,466 $ 5.95 8.4 $ 37,696 Options exercisable as of September 30, 2020 1,810,276 $ 1.51 7.0 $ 18,699 The aggregate intrinsic value of options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common shares for those options that had exercise prices lower than the fair value of the Company’s common shares. There were no stock options exercised during the nine months ended September 30, 2020 and 2019. The weighted-average grant-date fair value of stock options granted during the nine months ended September 30, 2020 and 2019 was $11.54 and $2.33 per share, respectively. Share-based Compensation Share-based compensation expense was classified in the consolidated statements of operations and comprehensive loss as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Research and development expenses $ 288 $ 37 $ 506 $ 110 General and administrative expenses 953 101 1,519 218 $ 1,241 $ 138 $ 2,025 $ 328 As of September 30, 2020, total unrecognized share-based compensation expense related to unvested share-based awards was $15.8 million, which is expected to be recognized over a weighted-average period of 3.0 years.

License Agreements and Asset Ac

License Agreements and Asset Acquisitions	9 Months Ended
	Sep. 30, 2020
License Agreement [Abstract]
License Agreements and Asset Acquisitions	9 . License Agreements and Asset Acquisitions License Agreement with the Centre for Probe Development and Commercialization Inc. In November 2015, the Company entered into a license agreement with the Centre for Probe Development and Commercialization Inc. (“CPDC”), a related party (see Note 13) (the “CPDC Agreement”). Under the agreement, the Company was granted an exclusive, sublicensable, nontransferable, worldwide license under CPDC’s patent rights related to CPDC’s radiopharmaceutical linker technology to develop, market, make, use and sell certain products for all disease indications and uses in humans, whether diagnostic or therapeutic. The Company has the right to grant sublicenses of its rights. The CPDC Agreement was amended in 2017; however, there were no material changes to the terms of the CPDC Agreement. Also in 2017, the Company entered into a second license agreement with CPDC, under which the Company was granted an exclusive, sublicensable, worldwide license under CPDC’s patent rights related to certain CPDC radiopharmaceutical linker technology to develop, market, make, use and sell certain products for all disease indications and uses in humans. The Company has the right to grant sublicenses of its rights. Under all agreements with CPDC, the Company has no obligations to make any milestone payments or to pay any royalties or annual maintenance fees to CPDC. During the three and nine months ended September 30, 2020 and 2019, the Company did not make any payments to CPDC or recognize any research and development expenses under the license agreements with CPDC. License Agreement with ImmunoGen, Inc. In December 2016, the Company entered into a license agreement with ImmunoGen, Inc. (“ImmunoGen”) (the “ImmunoGen Agreement”). Under the agreement, the Company was granted an exclusive, sublicensable, worldwide license under ImmunoGen’s patent rights to use, develop, manufacture and commercialize any radiopharmaceutical conjugate that includes a certain compound and any resulting commercialized products. The Company has the right to grant sublicenses of its rights. Under the ImmunoGen Agreement, the Company paid an upfront fee of $0.2 million to ImmunoGen. In addition, the Company is obligated to make aggregate milestone payments to ImmunoGen of up to $15.0 million upon the achievement of specified development and regulatory milestones and of up to $35.0 million upon the achievement of specified sales milestones. The Company is also obligated to pay tiered royalties of a low to mid single-digit percentage based on annual net sales by the Company and any of its affiliates and sublicensees. Royalties will be paid by the Company on a country-by-country basis beginning upon the first commercial sale in such country until ten years following the date of the first commercial sale in the United States and five years following the date of the first commercial sale in all non-U.S. countries. In addition, the Company is responsible for all costs and expenses incurred related to the development, manufacture, regulatory approval and commercialization of all licensed products. Prior to regulatory approval of a licensed product in any country, the Company has the right to terminate the agreement upon 90 days’ prior written notice to ImmunoGen. Upon receipt of its first regulatory approval of a licensed product in any country, the Company has the right to terminate the agreement upon 180 days’ prior written notice to ImmunoGen. If the Company or ImmunoGen fails to comply with any of its obligations or otherwise breaches the agreement, the other party may terminate the agreement. The ImmunoGen Agreement expires upon the expiration date of the last-to-expire royalty term. During the nine months ended September 30, 2019, the Company made a payment to ImmunoGen of $0.5 million upon the achievement of a specified development milestone and recognized this amount as research and development expense in its consolidated statements of operations and comprehensive loss. During the three and nine months ended September 30, 2020 and the three months ended September 30, 2019, the Company did not make any payments to ImmunoGen or recognize any research and development expenses under the ImmunoGen Agreement. License Agreements with Janssen Biotech, Inc. First Janssen Agreement In February 2017, the Company entered into a license agreement with Janssen Biotech, Inc. (“Janssen”) (the “Janssen Agreement”). Under the agreement (the “First Janssen Agreement”), the Company was granted an exclusive, sublicensable, worldwide license under Janssen’s patent rights to research, develop and commercialize licensed products containing the specified compound for certain alpha-emitting therapeutic and nuclear imaging uses in humans and animals. The Company was also granted non-exclusive, worldwide licenses under specified other patent rights for specified uses. The Company had the right to terminate the First Janssen Agreement upon written notice to Janssen. In March 2019, the Company terminated the First Janssen Agreement upon written notice to Janssen. During the three and nine months ended September 30, 2019, the Company did not make any payments to Janssen or recognize any research and development expenses under the First Janssen Agreement. Second Janssen Agreement Simultaneously with entering into the First Janssen Agreement, in February 2017, the Company entered into a second agreement with Janssen (the “Second Janssen Agreement”). Under the agreement, the Company was granted an exclusive, sublicensable, worldwide license under Janssen’s patent rights to research, develop and commercialize licensed products containing the specified compound for certain alpha-emitting therapeutic and nuclear imaging uses in humans and animals. The Company was also granted non-exclusive, worldwide licenses under specified other patent rights for specified uses. The Company had the right to terminate the Second Janssen Agreement upon written notice to Janssen. In January 2020, the Company terminated the Second Janssen Agreement upon written notice to Janssen. During the three and nine months ended September 30, 2020 and 2019, the Company did not make any payments to Janssen or recognize any research and development expenses under the Second Janssen Agreement. Research and License Agreement with Isogenica Ltd. In April 2018, the Company entered into a research and license agreement with Isogenica Ltd. (“Isogenica”) (the “Isogenica Agreement”). Under the agreement, the Company was granted a non-exclusive, sublicensable, worldwide license under Isogenica’s intellectual property and other technology to develop and commercialize a specified compound of antibody-like molecules that bind to targets with high affinity for all therapeutic uses or a product containing such compound. At the same time, the Company granted Isogenica a non-exclusive, non-sublicensable license to certain of Fusion’s intellectual property for research purposes. The initial term of the license was one year, and the Company had the option to extend the term of the Isogenica Agreement beyond the one-year period, which was exercised in April 2019. The Company was not entitled to any payments from Isogenica for use of the license of its intellectual property granted to Isogenica. The Company had the right to terminate the Isogenica Agreement upon written notice to Isogenica. In January 2020, the Company terminated the Isogenica Agreement upon written notice. During the nine months ended September 30, 2019, the Company made payments of £0.6 million (equivalent to $0.7 million at the time of payment) to Isogenica. During the three and nine months ended September 30, 2020 and the three months ended September 30, 2019, the Company did not make any payments to Isogenica. During the three months ended September 30, 2019 the company recognized £0.1 million (equivalent to $0.1 million) related to amortization of the annual license fee. During the nine months ended September 30, 2019, the Company exercised its option to extend the term of the agreement and recognized £0.4 million (equivalent to $0.6 million at the time of the payments) as research and development expense in the consolidated statements of operations and comprehensive loss, primarily related to the option exercise fee and amortization of the annual license fee. The Company did not recognize any research and development expenses during the three and nine months ended September 30, 2020. Asset Acquisition from and License Agreement with MediaPharma S.r.l. In May 2019, the Company and MediaPharma S.r.l. (“MediaPharma”) entered into an asset acquisition and license agreement. Under the agreement, the Company purchased all right, title and interest to MediaPharma’s, and any of its affiliates’ and sublicensees’, patents to perform research and to develop, manufacture and commercialize a specified antibody that binds to targets for the prevention, treatment and diagnosis of all diseases and conditions. The Company accounted for this purchase as an asset acquisition. At the same time, the Company granted MediaPharma an exclusive, fully paid, worldwide, sublicensable license to use the specified compound for research, development, manufacturing and commercialization of a bispecific antibody drug conjugate, but not for use as a radiopharmaceutical. In connection with the asset acquisition, the Company paid an upfront fee of $0.2 million to MediaPharma. In addition, the Company is obligated to make aggregate milestone payments to MediaPharma of up to $1.5 million upon the achievement of specified development milestones and of up to $23.0 million upon the achievement of specified sales milestones. The Company is also obligated to pay royalties of a low single-digit percentage based on annual net sales by the Company. Royalties will be paid by the Company on a country-by-country basis beginning upon the first commercial sale in such country and will expire, on a country-by-country basis, upon the earlier of (i) eight years from the first commercial sale of a licensed product in such country, (ii) the date upon which all issued patents under the agreement have expired or (iii) the date upon which a product highly similar in composition to the licensed product and having no clinically meaningful differences is sold or marketed for sale in such country by a third party. The Company is not entitled to any payments from MediaPharma for use of the license to the specified compound granted to MediaPharma. During the nine months ended September 30, 2019, the Company incurred the upfront fee due in connection with the asset agreement and subsequently made payment of and recognized $0.2 million as research and development expense in the consolidated statements of operations and comprehensive loss. During the three and nine months ended September 30, 2020 and the three months ended September 30, 2019, the Company did not make any payments to MediaPharma or recognize any research and development expenses under the MediaPharma Agreement. Asset Acquisition from Rainier Therapeutics, Inc. and License Agreement with Genentech, Inc. In March 2020 (the “Closing”), the Company and Rainier Therapeutics, Inc. (“Rainier”) entered into an asset acquisition agreement (the “Rainier Agreement”). Under the agreement, the Company purchased all right, title and interest to Rainier’s, and any of its affiliates’ and sublicensees’, patents and other tangible and intangible assets to perform research and to develop, manufacture and commercialize a specified compound of antibody molecules that bind to targets for the prevention, treatment and diagnosis of all diseases and conditions only using such compound as an antibody drug conjugate. The Company concluded to account for this purchase as an asset acquisition as substantially all of the fair value of the gross assets acquired was concentrated in a single identifiable asset, the license rights. In connection with the asset acquisition, the Company paid an upfront fee of $1.0 million to Rainier and recognized this amount as research and development expense in the consolidated statement of operations and comprehensive loss during the three months ended March 31, 2020, as the IPR&D acquired had no alternative future use as of the acquisition date. Unless the Rainier Agreement is terminated pursuant to its terms, which termination may not occur later than eight months following the Closing (the “Outside Date”), the Company is obligated to pay Rainier an additional amount of $3.5 million and to issue 313,359 of the Company’s non-voting common shares on the Outside Date. If the Rainier Agreement is not terminated by the Outside Date, the Company is also obligated to make aggregate milestone payments to Rainier of up to $22.5 million and to issue up to 156,679 of the Company’s non-voting common shares upon the achievement of specified development and regulatory milestones and of up to $42.0 million upon the achievement of specified sales milestones. In the event the Company enters into a transaction with a non-affiliated party relating to the license or sale of substantially all the Company’s rights to develop the specified compound of antibody molecules, the Company will be obligated to pay Rainier a specified percentage of the revenue from such transaction, in an amount ranging from 10% to 30%, based on how long after the Closing the transaction takes place. The Rainier Agreement may be terminated at any time prior to the Outside Date upon 30 days’ notice by the Company to Rainier or upon the mutual written consent of both parties. If the agreement is terminated prior to the Outside Date, the Company’s payment obligations, except for the non-refundable upfront fee of $1.0 million, become void and the Company will cease to have any rights to the antibody or the Genentech License Agreement (as defined below). On October 8, 2020, the Company and Rainier entered into a first amendment to the Rainier Agreement (the “Amended Rainier Agreement”) to extend certain terms of the Rainier Agreement. Specifically, the Outside Date, was amended such that termination may not occur later than eleven months following the Closing, or February 10, 2021 (the “Revised Outside Date”) (see Note 15). During the three months ended September 30, 2020, the Company did not make any payments to Rainier or recognize any research and development expenses under the Rainier License Agreement. During the nine months ended September 30, 2020, the Company paid an upfront fee of $1.0 million to Rainier and recognized this as an expense as noted above. In connection with the Rainier Agreement, in March 2020, the Company was assigned all of Rainier’s rights and obligations under an exclusive license agreement between BioClin Therapeutics, Inc. and Genentech, Inc. (“Genentech”) (the “Genentech License Agreement”). Pursuant to the Genentech License Agreement, the Company has an exclusive, worldwide, sublicensable license to make, use, research, develop, sell and import certain intellectual property and technology of Genentech relating to the a specified antibody and any mutant antibody thereof (the “Licensed Antibodies”), including any products that contain a Licensed Antibody as an active ingredient (the “Products”), for all human uses. Pursuant to the Genentech License Agreement, the Company is obligated to use commercially reasonable efforts to develop and commercialize at least one Product and the Company is solely responsible for the costs associated with the development, manufacturing, regulatory approval and commercialization of any Products. The manufacture of the antibody by any third-party contract manufacturing organization must be approved in advance by Genentech. Additionally, Genentech retains the right to use the Licensed Antibodies solely to research and develop molecules other than the Licensed Antibodies. Under Genentech License Agreement, the Company is obligated to make aggregate milestone payments to Genentech of up to $44.0 million upon the achievement of specified sales milestones. The Company is also obligated to pay to Genentech tiered royalties of a mid to high single-digit percentage based on annual net sales by the Company, and any of its affiliates and sublicensees, for the specified compound of antibody molecules and of a mid to high single-digit percentage based on annual net sales by the Company, and any of its affiliates and sublicensees, for any other compound containing mutant antibody molecules of the specified compound. In addition, the Company is obligated to pay to Genentech royalties of a low single-digit percentage based on quarterly net sales in any country in which the specified compound is not covered by a valid patent claim, and those sales will not be subject to the tiered royalties described above. All royalties may be reduced if the Company obtains a license under a third-party patent that includes the specified compound. Royalties will be paid by the Company on a country-by-country basis beginning upon the first commercial sale in such country until the later of (i) ten years following the date of the first commercial sale of a Product or (ii) the date the specified compound is no longer covered by an enforceable patent. Upon the expiration of the royalty term, the Company will have a fully paid-up license. The Company has the right to terminate the Genentech License Agreement upon written notice to Genentech if the Company determines in its sole discretion that development or commercialization of Products is not economically or scientifically feasible or appropriate. In addition, if the Company or Genentech fails to comply with any of its obligations or otherwise breaches the agreement, the other party may terminate the agreement. The Genentech License Agreement expires on the date on which all obligations under the agreement related to milestone payments or royalties have passed or expired. During the three and nine months ended September 30, 2020, the Company did not make any payments to Genentech or recognize any research and development expenses under the Genentech License Agreement. Master Services and License Agreement with Yumab GmbH On May 15, 2020, the Company entered into a master services and license agreement with Yumab GmbH (“Yumab”) (the “Yumab Agreement”). Under the agreement, Yumab will assist the Company in discovering and developing certain antibodies from certain cell lines owned by Yumab. The Company plans to use the discovered antibodies in preclinical and clinical development. Under the Yumab Agreement, the Company is obligated to pay for services performed as defined in work orders under the agreement. In addition, the Company is obligated to make aggregate milestone payments to Yumab of up to $3.9 million upon the achievement of specified development and regulatory milestones. During the three and nine months ended September 30, 2020, the Company recognized $0.2 million as research and development expense in the consolidated statements of operations and comprehensive loss under the Yumab Agreement.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Income Taxes	10 . Income Taxes The Company is domiciled in Canada and is primarily subject to taxation in that country. During the three and nine months ended September 30, 2020, the Company recorded no income tax benefits for the net operating losses incurred or for the research and development tax credits generated in Canada and Ireland in each period due to its uncertainty of realizing a benefit from those items. During the three months ended September 30, 2020, the Company recorded a tax benefit of $0.2 million as a result of the Company claiming a U.S. research and development tax credit to partially offset the current U.S. tax liability. During the three months ended September 30, 2019, the Company recorded a tax provision of $0.2 million primarily related to income tax obligations of its operating company in the U.S., which generates a profit for tax purposes. During the nine months ended September 30, 2020 and 2019, the Company recorded a tax provision of less than $0.1 million and $0.2 million, respectively, primarily related to income tax obligations of its operating company in the U.S., which generates a profit for tax purposes. The Company’s tax provision and the resulting effective tax rate for interim periods is determined based upon its estimated annual effective tax rate (“AETR”), adjusted for the effect of discrete items arising in that quarter. The impact of such inclusions could result in a higher or lower effective tax rate during a particular quarter, based upon the mix and timing of actual earnings or losses versus annual projections. In each quarter, the Company updates its estimate of the annual effective tax rate, and if the estimated annual tax rate changes, a cumulative adjustment is made in that quarter. For the three and nine months ended September 30, 2020 and 2019, the Company excluded Canada and Ireland from the calculation of the AETR as the Company anticipates an ordinary loss in these jurisdictions for which no tax benefit can be recognized. The Company has evaluated the positive and negative evidence bearing upon its ability to realize its deferred tax assets, which primarily consist of net operating loss carryforwards. The Company has considered its history of cumulative net losses in Canada and Ireland, estimated future taxable income and prudent and feasible tax planning strategies and has concluded that it is more likely than not that the Company will not realize the benefits of its Canadian and Irish deferred tax assets. As a result, as of September 30, 2020 and December 31, 2019, the Company has recorded a full valuation allowance against its net deferred tax assets in Canada and Ireland. On March 27, 2020, the Coronavirus Aid, Relief and Economic Security (“CARES”) Act was passed by the U.S. Congress and signed into law by the President of the U.S. The CARES Act, among other things, includes certain provisions for individuals and corporations; however, these benefits do not impact the Company’s income tax provision.

Net Loss per Share

Net Loss per Share	9 Months Ended
	Sep. 30, 2020
Earnings Per Share [Abstract]
Net Loss per Share	1 1 . Net Loss per Share Net Loss per Share Attributable to Common Shareholders Basic and diluted net loss per share attributable to common shareholders was calculated as follows (in thousands, except share and per share amounts): Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Numerator: Net loss $ (9,993 ) $ (594 ) $ (64,948 ) $ (7,983 ) Dividends paid to preferred shareholders in the form of warrants issued — — (1,382 ) — Net loss attributable to common shareholders $ (9,993 ) $ (594 ) $ (66,330 ) $ (7,983 ) Denominator: Weighted-average common shares outstanding—basic and diluted 41,682,797 1,929,555 15,422,375 1,910,695 Net loss per share attributable to common shareholders —basic and diluted $ (0.24 ) $ (0.31 ) $ (4.30 ) $ (4.18 ) The Company’s potentially dilutive securities, which include stock options, convertible preferred shares, preferred exchangeable shares and common share warrants, have been excluded from the computation of diluted net loss per share 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: Nine Months Ended September 30, 2020 2019 Options to purchase common shares 5,266,466 2,687,413 Convertible preferred shares (as converted to common shares) — 13,696,513 Preferred exchangeable shares (as converted to convertible preferred shares and then to common shares) — 4,577,106 Warrants to purchase common shares 651,816 — 5,918,282 20,961,032

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	1 2 . Commitments and Contingencies Operating Leases In January 2018, the Company entered into an operating lease for office space in Boston, Massachusetts, which was to expire in July 2023 with no renewal options. In July 2020, the Company paid $0.1 million to terminate this lease, effective immediately. In August 2018, the Company entered into an operating lease for office space in Hamilton, Ontario, Canada. This lease was amended in September 2020 (“New Lease Commencement Date”) with a new expiry date in August 2023. The lease can be renewed for an additional period of five years at the Company’s option, and if so renewed, can be terminated during the option period upon twelve months written notice any time after the fifth anniversary of the New Lease Commencement Date. In October 2019, the Company entered into an operating lease for office space in Boston, Massachusetts, which expires September 2025 and has no renewal options. In connection with entering into this lease agreement, the Company issued a letter of credit of $1.5 million, which is classified as restricted cash (non-current) on the consolidated balance sheets as of September 30, 2020 and December 31, 2019. The lease agreements include payment escalations, rent holidays and other lease incentives, which are accrued or deferred as appropriate such that rent expense for each lease is recognized on a straight-line basis over the respective lease term, recording deferred rent for rent expense incurred but not yet paid. Rent expense was $0.3 million and $0.8 million for the three and nine months ended September 30, 2020, respectively. Rent expense was $0.1 million and $0.2 million for the three and nine months ended September 30, 2019, respectively. Future minimum lease payments due under operating leases as of September 30, 2020 are as follows (in thousands): Year Ending December 31, 2020 (three months) $ 274 2021 1,117 2022 1,147 2023 1,177 2024 1,208 Thereafter 1,592 $ 6,515 Manufacturing Commitments In January 2019, and as amended in September 2020, the Company entered into an agreement with CPDC, a related party (see Note 13), to manufacture clinical trial materials. As of September 30, 2020, the Company had non-cancelable minimum purchase commitments under the agreement totaling $0.8 million over the following twelve months. In February 2019, the Company entered into an agreement with a third-party contract manufacturing organization to manufacture clinical trial materials. As of September 30, 2020, the Company had non-cancelable minimum purchase commitments under the agreement totaling $0.8 million over the following twelve months. 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 certain of its executive 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 is not currently aware of any indemnification claims and has not accrued any liabilities related to such obligations in its condensed consolidated financial statements as of September 30, 2020 or December 31, 2019. Legal Proceedings The Company is not a party to any litigation and does not have contingency reserves established for any litigation liabilities. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses as incurred the costs related to such legal proceedings.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2020
Related Party Transactions [Abstract]
Related Party Transactions	1 3 . Related Party Transactions The Company has entered into license agreements with CPDC, a principal shareholder of the Company (see Note 9). In addition, the Company has entered into a Master Services Agreement and a Supply Agreement with CPDC, under which CPDC provides services to the Company related to preclinical and manufacturing services, administrative support services and access to laboratory facilities. In connection with the Supply Agreement, the Company is obligated to pay CPDC an amount of $0.2 million per quarter, or $0.8 million in the aggregate per year, plus fees for materials, packaging and distribution of products supplied to the Company, unless the agreement is terminated by the Company. The Company recognized expenses in connection with the services performed under the Master Services Agreement and the Supply Agreement in the consolidated statements of operations and comprehensive loss as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Research and development expenses $ 141 $ 326 $ 780 $ 785 General and administrative expenses 23 16 50 59 $ 164 $ 342 $ 830 $ 844 During the three and nine months ended September 30, 2020, the Company made payments to CPDC in connection with the services described above of $0.2 million and $1.0 million, respectively. During the three and nine months ended September 30, 2019, the Company made payments to CPDC in connection with the services described above of $0.5 million and $0.8 million, respectively. Amounts due to CPDC by the Company in connection with the services described above totaled $0.1 million and $0.2 million as of September 30, 2020 and December 31, 2019, respectively, which amounts were included in accounts payable and accrued expenses on the consolidated balance sheets. In addition to costs incurred in connection with the services described above, the Company also reimbursed CPDC for purchases on the Company’s behalf from parties with which the Company did not have an account. During the three and nine months ended September 30, 2020, the Company made payments to CPDC of less than $0.1 million and $0.1 million, respectively, for reimbursement of these pass-through costs. During the three and nine months ended September 30, 2019, the Company made payments to CPDC of less than $0.1 million and $0.1 million, respectively, for reimbursement of these pass-through costs.

Geographical Information

Geographical Information	6 Months Ended
	Jun. 30, 2020
Geographic Areas Long Lived Assets [Abstract]
Geographical Information	1 4 . Geographical Information The Company has operating companies in the United States and Canada and a non-operating company in Ireland. Information about the Company’s long-lived assets, consisting solely of property and equipment, net, by geographic region was as follows (in thousands): September 30, 2020 December 31, 2019 United States $ 115 $ 127 Canada 1,866 1,145 $ 1,981 $ 1,272

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2020
Subsequent Events [Abstract]
Subsequent Events	15. Subsequent Events Amendment to Asset Acquisition Agreement with Rainier Therapeutics, Inc. On October 8, 2020, the Company and Rainier entered into the Amended Rainier Agreement (see Note 9). Pursuant to the Amended Rainier Agreement, the termination provisions of the Rainier Agreement were modified. Specifically, the Outside Date was amended such that termination may not occur later than eleven months following the Closing, or February 10, 2021. Aside from modifying the termination provisions to incorporate the Revised Outside Date, no other material terms were amended as part of the Amended Rainier Agreement. Strategic Collaboration Agreement with AstraZeneca UK Limited On October 30, 2020, the Company and AstraZeneca UK Limited (“AstraZeneca”) entered into a strategic collaboration agreement (the “AstraZeneca Agreement”) pursuant to which the Company and AstraZeneca will jointly discover, develop and commercialize next-generation alpha-emitting radiopharmaceuticals and combination therapies for the treatment of cancer globally by leveraging the Company’s Targeted Alpha Therapies, or TATs, platform and expertise in radiopharmaceuticals with AstraZeneca’s leading portfolio of antibodies and cancer therapeutics, including DNA Damage Response Inhibitors, or DDRis. Each party retains full ownership over its existing assets. For the novel TATs, the parties will utilize the Company’s Fast-Clear ® For the combination therapies, the parties will evaluate up to five (5) potential combination strategies involving the Company’s existing assets, including the Company’s lead candidate FPI-1434, in combination with certain of AstraZeneca’s existing therapeutics for the treatment of various cancers. AstraZeneca will fully fund all research and development activities for the combination strategies, until such point as the Company may opt-in to the clinical development activities. The Company has the right to opt-out of clinical development activities relating to these combination therapies. In such instance, the Company will be responsible for repaying its share of the development costs via a royalty on the additional combination sales only if its drug is approved on the basis of clinical development solely conducted by AstraZeneca, in which case the royalty payments shall also include a variable risk premium based on the number of the Company’s product candidates to have received regulatory approval at that time. Each party will have the sole right, on a country-by-country basis, to commercialize its respective contributed compound as a component of any combination therapy for which such party’s contributed compound may be commercialized under a separate marketing authorization from the other party’s contributed compound to such combination therapy. The parties will negotiate in good faith on a combination therapy-by-combination therapy basis the terms and conditions to co-commercialize any combination therapy that is to be commercialized under a single marketing authorization. During the period of time commencing with the inclusion of an available molecular target in the selection pool for development as a combination therapy and ending upon the end of the nomination period (as more fully described in the AstraZeneca Agreement) or earlier removal of such combination target from such pool, the Company will not undertake any preclinical or clinical studies combining the Company’s TAT Platform with any compound modulating the activity of such combination target. Following selection of a target under the AstraZeneca Agreement and payment of an exclusivity fee by AstraZeneca, and provided that AstraZeneca enrolls its first patient in a clinical trial as further defined in the AstraZeneca Agreement within a pre-defined period of time of such selection, the Company will not undertake any preclinical or clinical studies combining the Company’s TAT Platform with compounds modulating the same combination target for the duration of the evaluation period for such combination target, as further defined in the AstraZeneca Agreement. Within a certain time period following initiation of the evaluation period with respect to a combination target, AstraZeneca has the exclusive right to undertake, alone or in collaboration with the Company, all further clinical or preclinical combination studies with respect to a combination target by paying certain exclusivity fees. The Company received an upfront payment of $5.0 million from AstraZeneca. In addition, the Company is eligible to receive future payments of up to $40.0 million, including clinical milestones. The AstraZeneca Agreement expires on a TAT-by-TAT and combination-by-combination basis upon the later of the expiration of development and exclusivity obligations relating to such TAT or combination or, if such TAT or combination is commercialized as a product under the AstraZeneca Agreement, the expiration of the commercial life of such product. The Company and AstraZeneca can each terminate the AstraZeneca Agreement for the other party’s uncured material breach following the applicable notice period. Each of the Company and AstraZeneca may also terminate the AstraZeneca Agreement with respect to any TAT or combination product if such party determines that the continued development of such TAT or combination product is not commercially viable, or for a material safety issue with respect to such TAT or combination product.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Closing of Class B Preferred Share Financing and Settlement of Class B Preferred Share Tranche Right Liability	Closing of Class B Preferred Share Financing and Settlement of Class B Preferred Share Tranche Right Liability On May 15, 2020, the Company achieved the specified regulatory milestone associated with the Class B preferred share tranche right (see Note 7), which triggered the requirement of the Class B shareholders to participate in the Milestone Financing. Upon closing of the Milestone Financing on June 2, 2020, the Company issued and sold 36,806,039 Class B preferred shares at a price of $1.5154 per share and 4,437,189 Class B special voting shares at a price of $0.000001 per share and the Company’s Ireland subsidiary issued and sold 4,437,189 Class B preferred exchangeable shares at a price of $1.5154 per share, for aggregate gross proceeds of $62.5 million. The Class B preferred share tranche right liability (see Note 7) was settled in connection with the achievement of the regulatory milestone associated with the Class B preferred share tranche right. Specifically, the fair value of the Class B preferred share tranche right liability was remeasured for the last time as of the Milestone Financing closing date, resulting in the Company recognizing a loss in the condensed consolidated statement of operations and comprehensive loss for the nine months ended September 30, 2020 of $32.7 million for the change in the fair value of the tranche right liability between December 31, 2019 and June 2, 2020. Immediately thereafter, the balance of the Class B preferred share tranche right liability of $39.6 million was reclassified to Class B convertible preferred shares in an amount of $35.3 million and to non-controlling interest in Fusion Pharmaceuticals (Ireland) Limited in an amount of $4.3 million on the consolidated balance sheet.
Reverse Share Split	Reverse Share Split On June 19, 2020, the Company effected a one-for-5.339 reverse share split of its issued and outstanding common shares and a proportional adjustment to the existing conversion ratios for each class of the Company’s Preferred Shares (see Note 7) and Preferred Exchangeable Shares (see Note 7). Accordingly, all share and per share amounts for all periods presented in the accompanying condensed consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse share split and adjustment of the preferred share conversion ratios.
Initial Public Offering	Initial Public Offering On June 25, 2020, the Company completed an initial public offering (“IPO”) of its common shares and issued and sold 12,500,000 common shares at a public offering price of $17.00 per share, resulting in net proceeds of $193.1 million after deducting underwriting fees, and after deducting offering costs. Upon closing of the IPO, the Company’s outstanding Preferred Exchangeable Shares automatically converted into convertible preferred shares then the outstanding convertible preferred shares automatically converted into shares of common shares (see Note 7). Upon conversion of the convertible preferred shares, the Company reclassified the carrying value of the convertible preferred shares to common shares and additional paid-in capital. In addition, the warrants to purchase the Company’s Series B convertible preferred shares and warrants to purchase preferred exchangeable shares of Fusion Pharmaceuticals (Ireland) Limited were converted into warrants to purchase the Company’s common shares upon the closing of the IPO. As a result, the warrant liability was remeasured a final time on the closing date of the IPO and reclassified to shareholders’ equity (deficit) (see Note 3). In connection with the IPO on June 25, 2020, the Company filed an amended and restated articles of the corporation under laws governed by the Canada Business Corporations Act
Basis of Presentation	Basis of Presentation The accompanying condensed consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. Since inception, the Company has funded its operations primarily with proceeds from sales of its convertible preferred shares, including borrowings under a convertible promissory note, which converted into convertible preferred shares, proceeds from sales of its Ireland subsidiary’s preferred exchangeable shares, and most recently with the proceeds from the IPO completed in June 2020. 10.0 64.9 0.6 8.0 September 30, 2020 99.8
Impact of the COVID-19 Pandemic	Impact of the COVID-19 Pandemic The COVID-19 pandemic, which began in December 2019 and has spread worldwide, has caused many governments to implement measures to slow the spread of the outbreak through quarantines, travel restrictions, heightened border security and other measures. The impact of this pandemic has been, and will likely continue to be, extensive in many aspects of society, which has resulted, and will likely continue to result, in significant disruptions to the global economy as well as businesses and capital markets around the world. The future progression of the pandemic and its effects on the Company’s business and operations are uncertain. In response to public health directives and orders and to help minimize the risk of the virus to employees, the Company has taken precautionary measures, including implementing work-from-home policies for certain employees. The impact of the virus, including work-from-home policies, may negatively impact productivity, disrupt the Company’s business, and delay its preclinical research and clinical trial activities and its development program timelines, the magnitude of which will depend, in part, on the length and severity of the restrictions and other limitations on the Company’s ability to conduct its business in the ordinary course. Specifically, the Company may not be able to enroll additional patient cohorts on its planned timeline due to disruptions at its clinical trial sites and is unable to predict how the COVID-19 pandemic may affect its ability to successfully progress its clinical programs in the future. Other impacts to the Company’s business may include temporary closures of its suppliers and disruptions or restrictions on its employees’ ability to travel. Any prolonged material disruption to the Company’s employees or suppliers could adversely impact the Company’s preclinical research and clinical trial activities, financial condition and results of operations, including its ability to obtain financing. The Company is monitoring the potential impact of the COVID-19 pandemic on its business and condensed consolidated financial statements. To date, the Company has not experienced material business disruptions or incurred impairment losses in the carrying values of its assets as a result of the pandemic and it is not aware of any specific related event or circumstance that would require it to revise its estimates reflected in these condensed consolidated financial statements.
Use of Estimates	Use of Estimates The preparation of the Company’s condensed 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 condensed consolidated financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, the accrual of research and development expenses and the valuations of common shares, stock options, preferred share tranche rights and preferred share warrants. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates when there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results may differ from those estimates or assumptions.
Unaudited Interim Financial Information	Unaudited Interim Financial Information The accompanying condensed consolidated balance sheet as of September 30, 2020, the condensed consolidated statement of operations and comprehensive loss, and the condensed consolidated statement of non-controlling interest, convertible preferred shares and shareholders’ equity (deficit) for the three and nine months ended September 30, 2020 and 2019, and the condensed consolidated statement of cash flows for the nine months ended September 30, 2020 and 2019 are unaudited. The unaudited interim condensed 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 September 30, 2020 and the results of its operations for three and nine months ended September 30, 2020 and 2019 and its cash flows for the nine months ended September 30, 2020 and 2019. The financial data and other information disclosed in these notes related to the three and nine months ended September 30, 2020 and 2019 are also unaudited. The results for the three and nine months ended September 30, 2020 are not necessarily indicative of results to be expected for the year ending December 31, 2020, any other interim periods, or any future year or period. The accompanying balance sheet as of December 31, 2019 has been derived from the Company’s audited financial statements for the year ended December 31, 2019. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These unaudited interim condensed consolidated financial statements should be read in conjunction with the audited annual consolidated financial statements and notes thereto as of December 31, 2019 and for each of the three years in the period ended December 31, 2019 included in the Company’s Registration Statement on Form S-1, as amended, on file with the SEC.
Foreign Currency and Currency Translation	Foreign Currency and Currency Translation The reporting currency of the Company is the U.S. dollar. The functional currency of the Company’s operating company in Canada, operating company in the U.S. and non-operating company in Ireland is also the U.S. dollar. As a result, the Company records no cumulative translation adjustments related to translation of unrealized foreign exchange gains or losses. For the remeasurement of local currencies to the U.S. dollar functional currency of the Canadian and Irish entities, assets and liabilities are translated into U.S. dollars at the exchange rate in effect on the balance sheet date, and income items and expenses are translated into U.S. dollars at the average exchange rate in effect during the period. Resulting transaction gains (losses) are included in other income (expense), net in the consolidated statements of operations and comprehensive loss, as incurred. Adjustments that arise from exchange rate changes on transactions denominated in a currency other than the local currency are included in other income (expense), net in the consolidated statements of operations and comprehensive loss, as incurred. During the three and nine months ended September 30, 2020, the Company recorded less than ($0.1) million of foreign currency losses in the consolidated statements of operations and comprehensive loss for both periods. During the three and nine months ended September 30, 2019, the Company recorded less than ($0.1) million and $0.1 million of foreign currency gains (losses) in the consolidated statements of operations and comprehensive loss for both periods.
Cash, Cash Equivalents and Restricted Cash	Cash, Cash Equivalents and Restricted Cash Cash and cash equivalents consist of standard checking accounts, money market accounts, and all highly liquid investments with an original maturity of three months or less at the date of purchase. As of September 30, 2020 and December 31, 2019, the Company was required to maintain a separate cash balance of $0.3 million to collateralize corporate credit cards with a bank, which was classified as restricted cash (current) on its condensed consolidated balance sheets. In connection with the Company’s lease agreement entered into in October 2019 (see Note 12), the Company maintains a letter of credit of $1.5 million for the benefit of the landlord. As of September 30, 2020 and December 31, 2019, the underlying cash balance collateralizing this letter of credit was classified as restricted cash (non-current) on its condensed consolidated balance sheets based on the release date of the restrictions of this cash. As of September 30, 2020 and 2019, the cash, cash equivalents and restricted cash of $250.5 million and $71.1 million, respectively, presented in the condensed consolidated statements of cash flows included cash and cash equivalents of $248.8 million and $69.3 million, respectively, and restricted cash of $1.7 million and $1.8 million, respectively.
Investments	Investments The Company determines the appropriate classification of its investments in debt securities at the time of purchase and re-evaluates such determination at each balance sheet date. The Company classifies its investments as current or non-current based on each instrument’s underlying maturity date. Investments with original maturities of greater than three months and less than twelve months are classified as current and are included in short-term investments in the condensed consolidated balance sheets. Investments with original maturities greater than one year from the balance sheet date are classified as non-current and are included in long-term investments in the condensed consolidated balance sheets. The Company’s investments are classified as available-for-sale, are reported at fair value and consist of U.S. government agency securities, corporate bonds, and commercial paper. Unrealized gains and losses included in other comprehensive income (loss) as a component of shareholders’ equity (deficit) until realized. Amortization and accretion of premiums and discounts are recorded in interest income (expense). Realized gains and losses on debt securities are included in other income (expense), net. If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is other than temporary and, if so, marks the investment to market on the Company’s condensed consolidated statements of operations and comprehensive loss.
Deferred Offering Costs	Deferred Offering Costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of an equity financing, these costs are recorded as a reduction of the proceeds from the offering, either as a reduction to the carrying value of the preferred exchangeable shares or convertible preferred shares or in shareholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the offering. Should an in-process equity financing be abandoned, the deferred offering costs would be expensed immediately as a charge to operating expenses in the consolidated statements of operations and comprehensive loss. The Company did not record any deferred offering costs as of September 30, 2020 or December 31, 2019. Offering costs of $4.6 million incurred during 2020 have been recorded in shareholders’ equity (deficit) as a reduction of the gross proceeds generated from the Company’s initial public offering of common shares.
Business Combinations	Business Combinations In determining whether an acquisition should be accounted for as a business combination or asset acquisition, the Company first determines whether substantially all of the fair value of the gross assets acquired is concentrated in a single identifiable asset or a group of similar identifiable assets. If this is the case, the single identifiable asset or the group of similar assets is not deemed to be a business, and is instead deemed to be an asset. If this is not the case, the Company then further evaluates whether the single identifiable asset or group of similar identifiable assets and activities includes, at a minimum, an input and a substantive process that together significantly contribute to the ability to create outputs. If so, the Company concludes that the single identifiable asset or group of similar identifiable assets and activities is a business. The Company accounts for business combinations using the acquisition method of accounting. Application of this method of accounting requires that (i) identifiable assets acquired (including identifiable intangible assets) and liabilities assumed generally be measured and recognized at fair value as of the acquisition date and (ii) the excess of the purchase price over the net fair value of identifiable assets acquired and liabilities assumed be recognized as goodwill, which is not amortized for accounting purposes but is subject to testing for impairment at least annually. Acquired in-process research and development (“IPR&D”) is recognized at fair value and initially characterized as an indefinite-lived intangible asset, irrespective of whether the acquired IPR&D has an alternative future use. Transaction costs related to business combinations are expensed as incurred. Determining the fair value of assets acquired and liabilities assumed in a business combination requires management to use significant judgment and estimates, especially with respect to intangible assets. During the measurement period, which extends no later than one year from the acquisition date, the Company may record certain adjustments to the carrying value of the assets acquired and liabilities assumed with the corresponding offset to goodwill. After the measurement period, all adjustments are recorded in the consolidated statements of operations as operating expenses or income. To date, the Company has not recorded any asset acquisitions as a business combination.
Asset Acquisitions	Asset Acquisitions The Company measures and recognizes asset acquisitions that are not deemed to be business combinations based on the cost to acquire the assets, which includes transaction costs. Goodwill is not recognized in asset acquisitions. In an asset acquisition, the cost allocated to acquire IPR&D with no alternative future use is charged to expense at the acquisition date. Contingent consideration in asset acquisitions payable in the form of cash is recognized when payment becomes probable and reasonably estimable, unless the contingent consideration meets the definition of a derivative, in which case the amount becomes part of the asset acquisition cost when acquired. Contingent consideration payable in the form of a fixed number of the Company’s own shares is measured at fair value as of the acquisition date and recognized when the issuance of the shares becomes probable. Upon recognition of the contingent consideration payment, the amount is included in the cost of the acquired asset or group of assets, or, if related to IPR&D with no alternative future use, charged to expense.
Fair Value Measurements	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 and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. Prior to the settlement of the Company’s preferred share tranche right liability and prior to the conversion of the Company’s preferred share warrant liability, these instruments were carried at fair value, determined according to Level 3 inputs in the fair value hierarchy described above (see Note 3). The Company’s cash equivalents and investments are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s amounts due for refundable investment tax credits and Canadian harmonized sales tax, accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities.
Preferred Share Tranche Right Liability and Preferred Share Warrant Liability	Preferred Share Tranche Right Liability The subscription agreements for the Company’s Class B convertible preferred shares (see Note 7) and its Ireland subsidiary’s Class B preferred exchangeable shares (see Note 7) provides investors the right, or obligates investors, to participate in subsequent offerings of Class B convertible preferred shares or Class B preferred exchangeable shares together with Class B special voting shares in the event that specified development or regulatory milestones are achieved (the “Class B preferred share tranche right liability”). The Company classifies these preferred share tranche rights as a liability on its consolidated balance sheets as each preferred share tranche right is a freestanding financial instrument that may require the Company to transfer assets upon the achievement of specified milestone events. Each preferred share tranche right liability was initially recorded at fair value upon the date of issuance of each preferred share tranche right and is subsequently remeasured to fair value at each reporting date. Changes in the fair value of the preferred share tranche right liability are recognized as a component of other income (expense) in the consolidated statement of operations and comprehensive loss. Changes in the fair value of the preferred share tranche right liability will continue to be recognized until the respective preferred share tranche right is settled upon achievement of the specified milestones or expires. On May 15, 2020, the Company achieved the specified regulatory milestone associated with the Class B preferred share tranche right (see Note 7), which triggered the requirement of the Class B shareholders to participate in the Milestone Financing. Upon closing of the Milestone Financing on June 2, 2020, the Company issued and sold 36,806,039 Class B preferred shares at a price of $1.5154 per share and 4,437,189 Class B special voting shares at a price of $0.000001 per share and the Company’s Ireland subsidiary issued and sold 4,437,189 Class B preferred exchangeable shares at a price of $1.5154 per share, for aggregate gross proceeds of $62.5 million. The Class B preferred share tranche right liability (see Note 7) was settled in connection with the achievement of the regulatory milestone associated with the Class B preferred share tranche right. Specifically, the fair value of the Class B preferred share tranche right liability was remeasured for the last time as of the Milestone Financing closing date, resulting in the Company recognizing a loss in the consolidated statement of operations and comprehensive loss for the nine months ended September 30, 2020 of $32.7 million for the change in the fair value of the tranche right liability between December 31, 2019 and June 2, 2020. Immediately thereafter, the balance of the Class B preferred share tranche right liability of $39.6 million was reclassified to Class B convertible preferred shares in an amount of $35.3 million and to non-controlling interest in Fusion Pharmaceuticals (Ireland) Limited in an amount of $4.3 million on the consolidated balance sheet. Preferred Share Warrant Liability The Company classifies warrants to purchase its convertible preferred shares and warrants to purchase preferred exchangeable shares of Fusion Pharmaceuticals (Ireland) Limited as a liability on its consolidated balance sheets as these warrants are freestanding financial instruments that may require the Company to transfer assets upon exercise (see Note 7). The preferred share warrant liability, which consists of warrants to purchase Class B convertible preferred shares of the Company and warrants to purchase Class B preferred exchangeable shares of Fusion Pharmaceuticals (Ireland) Limited, was initially recorded at fair value upon the date of issuance of each warrant and is subsequently remeasured to fair value at each reporting date. Changes in the fair value of the preferred share warrant liability are recognized as a component of other income (expense) in the consolidated statement of operations and comprehensive loss. Changes in the fair value of the preferred share warrant liability will continue to be recognized until each respective warrant is exercised, expires or qualifies for equity classification. Upon the closing of the IPO, the warrants to purchase its convertible preferred shares and warrants to purchase preferred exchangeable shares of Fusion Pharmaceuticals (Ireland) Limited were converted into warrants to purchase shares of the Company’s common shares. As a result, the warrant liability was remeasured a final time on the closing date of the IPO and reclassified to shareholders’ equity (deficit) as the warrants qualify for equity classification.
Research, Development and Manufacturing Contract Costs and Accruals	Research, Development and Manufacturing Contract Costs and Accruals The Company has entered into various research, development and manufacturing contracts with research institutions and other companies. These agreements are generally cancelable, and related costs are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research, development and manufacturing costs. When billing terms under these contracts do not coincide with the timing of when the work is performed, the Company is required to make estimates of outstanding obligations to those third parties as of period end. Any accrual estimates are based on a number of factors, including the Company’s knowledge of the progress towards completion of the research, development and manufacturing activities, invoicing to date under the contracts, communication from the research institutions and other companies of any actual costs incurred during the period that have not yet been invoiced and the costs included in the contracts. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical accrual estimates made by the Company have not been materially different from the actual costs.
Net Loss per Share	Net Loss per Share The Company follows the two-class method when computing net income (loss) per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net income (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 income (loss) per share attributable to common shareholders is computed by dividing the net income (loss) attributable to common shareholders by the weighted-average number of common shares outstanding for the period. Diluted net income (loss) attributable to common shareholders is computed by adjusting net income (loss) attributable to common shareholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common shareholders is computed by dividing the diluted net income (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 stock options, warrants and convertible preferred shares are considered potential dilutive common shares. The Company’s convertible preferred shares contractually entitle the holders of such shares to participate in dividends but do not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common shareholders, such losses are not allocated to such participating securities. In periods in which the Company reported 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 nine months ended September 30, 2020 and 2019.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements The Company qualifies as “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012 and has elected to “opt in” to the extended transition related to complying with new or revised accounting standards, which means that when a standard is issued or revised and it has different application dates for public and nonpublic companies, the Company will adopt the new or revised standard at the time nonpublic companies adopt the new or revised standard and will do so until such time that the Company either (i) irrevocably elects to “opt out” of such extended transition period or (ii) no longer qualifies as an emerging growth company. The Company may choose to early adopt any new or revised accounting standards whenever such early adoption is permitted for private companies. In February 2016, the FASB issued ASU No. 2016-02 , Leases (Topic 842) ASU No. 2018-11, Leases (Topic 842) In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments Codification Improvements to Topic 326, Financial Instruments—Credit Losses Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis	The following tables present information about the Company’s financial assets and liabilities that are measured at fair value on a recurring basis and indicates the level of the fair value hierarchy used to determine such fair values (in thousands): Fair Value Measurements as of September 30, 2020 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 28,670 $ — $ — $ 28,670 Commercial paper — 2,999 — 2,999 Corporate bonds — 1,862 — 1,862 U.S. Government agencies — 20,998 — 20,998 Investments: Commercial paper — 6,994 — 6,994 Corporate bonds — 905 — 905 U.S. Government agencies — 46,362 — 46,362 $ 28,670 $ 80,120 $ — $ 108,790 Fair Value Measurements as of December 31, 2019 Using: Level 1 Level 2 Level 3 Total Liabilities: Preferred share tranche right liability $ — $ — $ 5,741 $ 5,741 $ — $ — $ 5,741 $ 5,741
Schedule of Fair Value from Preferred Share Tranche Liability and Warrant Liability	The following table provides a roll-forward of the aggregate fair value of the Company’s preferred share tranche right liability and preferred share warrant liability, for which fair value is determined using Level 3 inputs (in thousands): Preferred Share Tranche Right Liability Preferred Share Warrant Liability Balance as of December 31, 2019 $ 5,741 $ — Initial fair value of Class B preferred share tranche right liability 1,105 — Initial fair value of Class B preferred share warrant liability — 1,382 Change in fair value of Class B preferred share tranche right liability 32,722 — Change in fair value of Class B preferred share warrant liability — 6,399 Reclassification of Class B preferred share tranche right liability upon settlement (39,568 ) — Conversion of Class B preferred shares warrants into common share warrants — (7,781 ) Balance as of September 30, 2020 $ — $ —

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2020
Investments Debt And Equity Securities [Abstract]
Schedule of Investments	Investments consisted of the following (in thousands): September 30, 2020 Amortized Cost Fair Value Due within one year or less $ 43,160 $ 43,159 Due after one year through three years 11,102 11,102 $ 54,262 $ 54,261
Amortized Cost and Estimated Fair Value of Investments by Contractual Maturity	As of September 30, 2020, the amortized cost and estimated fair value of investments, by contractual maturity, was as follows (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Current Non-Current Commercial paper $ 6,994 $ — $ — $ 6,994 $ 6,994 $ — Corporate bonds 905 — — 905 905 — U.S. Government agencies 46,363 1 (2 ) 46,362 35,260 11,102 $ 54,262 $ 1 $ (2 ) $ 54,261 $ 43,159 $ 11,102

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	9 Months Ended
	Sep. 30, 2020
Prepaid Expense And Other Assets Current [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following (in thousands): September 30, 2020 December 31, 2019 Prepaid clinical trial expenses $ — $ 94 Prepaid insurance 3,136 11 Prepaid software subscriptions 173 129 Income tax receivable 109 — Interest receivable 229 62 Canadian harmonized sales tax receivable 238 252 Refundable investment tax credits 315 176 Other 452 205 $ 4,652 $ 929

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2020
Payables And Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following (in thousands): September 30, 2020 December 31, 2019 Accrued employee compensation and benefits $ 1,524 $ 991 Accrued external research and development expenses 1,715 1,565 Accrued professional and consulting fees 1,048 744 Other 66 26 $ 4,353 $ 3,326

Share-Based Compensation (Table

Share-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Fair Value of Options Estimated on Date of Grant Using Black-Scholes Option Pricing Model	The following table presents, on a weighted-average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted: Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Risk-free interest rate 0.33 % 1.58 % 0.70 % 1.58 % Expected term (in years) 6.0 5.9 6.0 5.9 Expected volatility 67.0 % 64.5 % 65.5 % 64.5 % Expected dividend yield 0 % 0 % 0 % 0 %
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity since December 31, 2019: Number of Shares Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term Aggregate Intrinsic Value (in years) (in thousands) Outstanding as of December 31, 2019 3,045,301 $ 1.66 8.4 $ 2,112 Granted 2,346,645 11.50 Forfeited/cancelled (125,480 ) 5.63 Outstanding as of September 30, 2020 5,266,466 $ 5.95 8.4 $ 37,696 Vested and expected to vest as of September 30, 2020 5,266,466 $ 5.95 8.4 $ 37,696 Options exercisable as of September 30, 2020 1,810,276 $ 1.51 7.0 $ 18,699
Summary of Stock Based Compensation Expense	Share-based compensation expense was classified in the consolidated statements of operations and comprehensive loss as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Research and development expenses $ 288 $ 37 $ 506 $ 110 General and administrative expenses 953 101 1,519 218 $ 1,241 $ 138 $ 2,025 $ 328

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 30, 2020
Earnings Per Share [Abstract]
Basic and Diluted Net Loss per Share	Basic and diluted net loss per share attributable to common shareholders was calculated as follows (in thousands, except share and per share amounts): Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Numerator: Net loss $ (9,993 ) $ (594 ) $ (64,948 ) $ (7,983 ) Dividends paid to preferred shareholders in the form of warrants issued — — (1,382 ) — Net loss attributable to common shareholders $ (9,993 ) $ (594 ) $ (66,330 ) $ (7,983 ) Denominator: Weighted-average common shares outstanding—basic and diluted 41,682,797 1,929,555 15,422,375 1,910,695 Net loss per share attributable to common shareholders —basic and diluted $ (0.24 ) $ (0.31 ) $ (4.30 ) $ (4.18 )
Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share	The Company’s potentially dilutive securities, which include stock options, convertible preferred shares, preferred exchangeable shares and common share warrants, have been excluded from the computation of diluted net loss per share 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: Nine Months Ended September 30, 2020 2019 Options to purchase common shares 5,266,466 2,687,413 Convertible preferred shares (as converted to common shares) — 13,696,513 Preferred exchangeable shares (as converted to convertible preferred shares and then to common shares) — 4,577,106 Warrants to purchase common shares 651,816 — 5,918,282 20,961,032

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2020
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Rental Payments for Operating Leases	Future minimum lease payments due under operating leases as of September 30, 2020 are as follows (in thousands): Year Ending December 31, 2020 (three months) $ 274 2021 1,117 2022 1,147 2023 1,177 2024 1,208 Thereafter 1,592 $ 6,515

Related Party Transactions (Tab

Related Party Transactions (Tables)	9 Months Ended
	Sep. 30, 2020
Related Party Transactions [Abstract]
Schedule of Recognized Expenses in Connection with Services Performed Under Master Services Agreement and Supply Agreement	The Company recognized expenses in connection with the services performed under the Master Services Agreement and the Supply Agreement in the consolidated statements of operations and comprehensive loss as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Research and development expenses $ 141 $ 326 $ 780 $ 785 General and administrative expenses 23 16 50 59 $ 164 $ 342 $ 830 $ 844

Geographical Information (Table

Geographical Information (Tables)	9 Months Ended
	Sep. 30, 2020
Geographic Areas Long Lived Assets [Abstract]
Schedule of Long-lived Assets by Geographic Areas	Information about the Company’s long-lived assets, consisting solely of property and equipment, net, by geographic region was as follows (in thousands): September 30, 2020 December 31, 2019 United States $ 115 $ 127 Canada 1,866 1,145 $ 1,981 $ 1,272

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands

Jun. 30, 2020

Jun. 25, 2020

Jun. 19, 2020

Jun. 02, 2020

Jan. 31, 2020

Mar. 31, 2019

Sep. 30, 2020

Jun. 30, 2020

Mar. 31, 2020

Sep. 30, 2019

Jun. 30, 2019

Mar. 31, 2019

Sep. 30, 2020

Sep. 30, 2019

Dec. 31, 2019

Subsidiary Sale Of Stock [Line Items]

Formation and incorporation date

Dec. 31, 2014

Preferred share tranche right liability

$ 5,741

Reverse share split, description

one-for-5.339

Net loss

9,993

$ 44,733

$ 10,222

$ 594

$ 3,456

$ 3,933

64,948

$ 7,983

Accumulated deficit

$ (100)

99,818

99,818

$ 34,774

Initial Public Ooffering [Member] | Common Shares [Member]

Subsidiary Sale Of Stock [Line Items]

Shares issued and sold, date

Jun. 25, 2020

Shares issued and sold

12,500,000

12,500,000

Shares issued and sold, per share

$ 17

$ 17

$ 17

Net proceeds of common shares

$ 193,100

$ 193,100

Fusion Pharmaceuticals (Ireland) Limited [Member] | Milestone Financing [Member] | Non-Controlling Interest in Fusion Pharmaceuticals (Ireland) Limited [Member]

Subsidiary Sale Of Stock [Line Items]

Preferred share tranche right liability

$ 4,300

Preferred Class B

Subsidiary Sale Of Stock [Line Items]

Shares issued and sold, date

Jun. 2, 2020

Shares issued and sold

36,806,039

Shares issued and sold, per share

$ 1.5154

Aggregate gross proceeds

$ 55,800

10,000

$ 45,800

Preferred share tranche right liability

$ 1,100

39,600

39,600

Preferred Class B | Milestone Financing [Member]

Subsidiary Sale Of Stock [Line Items]

Loss on change in the fair value of the tranche right liability

32,700

Preferred share tranche right liability

$ 39,600

Class B Special Voting Shares [Member]

Subsidiary Sale Of Stock [Line Items]

Shares issued and sold

4,437,189

Shares issued and sold, per share

$ 0.000001

Class B Preferred Exchangeable Shares [Member] | Fusion Pharmaceuticals (Ireland) Limited [Member]

Subsidiary Sale Of Stock [Line Items]

Shares issued and sold

4,437,189

Shares issued and sold, per share

$ 1.5154

Aggregate gross proceeds

$ 62,500

Class B Convertible Preferred Shares [Member]

Subsidiary Sale Of Stock [Line Items]

Preferred share tranche right liability

$ 35,300

$ 35,300

Class B Convertible Preferred Shares [Member] | Milestone Financing [Member]

Subsidiary Sale Of Stock [Line Items]

Preferred share tranche right liability

$ 35,300

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	Jun. 02, 2020	Jun. 02, 2020	Jan. 31, 2020	Mar. 31, 2019	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Oct. 31, 2019
Summary Of Significant Accounting Policies [Line Items]
Restricted cash					$ 280,000			$ 280,000		$ 280,000
Letters of credit											$ 1,500,000
Cash, cash equivalents and restricted cash					250,500,000		$ 71,100,000	250,500,000	$ 71,100,000
Cash and cash equivalents					248,800,000		69,300,000	248,800,000	69,300,000
Restricted Cash					1,700,000		1,800,000	1,700,000	1,800,000
Deferred offering costs					$ 0			0		$ 0
Issuance of common shares upon closing of initial public offering, offering costs and underwriter fees						$ 19,447,000		$ 4,600,000
Preferred stock shares issued					0			0		73,125,790
Change in fair value of preferred share tranche right liability							3,485,000	$ (32,722,000)	3,707,000
Preferred share tranche right liability										$ 5,741,000
Preferred Class B
Summary Of Significant Accounting Policies [Line Items]
Preferred stock shares issued	36,806,039	36,806,039
Preferred stock price per share	$ 1.5154	$ 1.5154	$ 1.5154	$ 1.5154
Aggregate gross proceeds		$ 55,800,000	$ 10,000,000	$ 45,800,000
Change in fair value of preferred share tranche right liability								32,700,000
Preferred share tranche right liability			$ 1,100,000		39,600,000			39,600,000
Class B Special Voting Shares [Member]
Summary Of Significant Accounting Policies [Line Items]
Preferred stock shares issued	4,437,189	4,437,189
Preferred stock price per share	$ 0.000001	$ 0.000001
Class B Preferred Exchangable Shares [Member] | Ireland Subsidiary [Member]
Summary Of Significant Accounting Policies [Line Items]
Preferred stock shares issued	4,437,189	4,437,189
Preferred stock price per share	$ 1.5154	$ 1.5154
Aggregate gross proceeds	$ 62,500,000
Class B Convertible Preferred Shares [Member]
Summary Of Significant Accounting Policies [Line Items]
Preferred stock shares issued										132,207,290
Preferred share tranche right liability					$ 35,300,000			$ 35,300,000
Class B Convertible Preferred Shares [Member] | Ireland Subsidiary [Member]
Summary Of Significant Accounting Policies [Line Items]
Preferred stock shares issued	4,437,189	4,437,189		4,437,189	28,874,378			28,874,378		29,747,987
Preferred stock price per share	$ 0.000001	$ 0.000001		$ 1.5154	$ 0.001			$ 0.001		$ 0.001
Increase in noncontrolling interest								$ 4,300,000
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Foreign currency transaction gains (losses)					$ (100,000)		$ (100,000)	$ (100,000)	$ 100,000

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Investments
Investments	$ 108,790
Liabilities:
Liabilities		$ 5,741
Preferred Share Tranche Right Liability [Member]
Liabilities:
Liabilities		5,741
Corporate Bond Securities [Member]
Cash equivalents
Cash equivalents	1,862
Investments
Investments	905
Money Market Funds [Member]
Cash equivalents
Cash equivalents	28,670
Commercial Paper [Member]
Cash equivalents
Cash equivalents	2,999
Investments
Investments	6,994
U.S Goverment agencies [Member]
Cash equivalents
Cash equivalents	20,998
Investments
Investments	46,362
Level 1
Investments
Investments	28,670
Level 1 | Money Market Funds [Member]
Cash equivalents
Cash equivalents	28,670
Level 2
Investments
Investments	80,120
Level 2 | Corporate Bond Securities [Member]
Cash equivalents
Cash equivalents	1,862
Investments
Investments	905
Level 2 | Commercial Paper [Member]
Cash equivalents
Cash equivalents	2,999
Investments
Investments	6,994
Level 2 | U.S Goverment agencies [Member]
Cash equivalents
Cash equivalents	20,998
Investments
Investments	$ 46,362
Level 3
Liabilities:
Liabilities		5,741
Level 3 | Preferred Share Tranche Right Liability [Member]
Liabilities:
Liabilities		$ 5,741

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jun. 02, 2020	Jun. 02, 2020	Jan. 31, 2020	Mar. 31, 2019	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Preferred stock, shares issued						0		73,125,790
Change in fair value of preferred share tranche right liability					$ 3,485	$ (32,722)	$ 3,707
Measurement Input, Expected Dividend Rate
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Estimated dividend yield						0.00%
Class B Preferred Shares Tranche [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Change in fair value of preferred share tranche right liability						$ 32,700
Preferred shares tranche liability	$ 35,300	$ 35,300						$ 39,600
Increase in noncontrolling interest						4,300
Preferred Class B
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Convertible preferred stock fair value per share	$ 2.47	$ 2.47	$ 1.45					$ 1.45
Preferred stock, shares issued	36,806,039	36,806,039
Preferred stock par value per share	$ 1.5154	$ 1.5154	$ 1.5154	$ 1.5154
Aggregate gross proceeds		$ 55,800	$ 10,000	$ 45,800
Change in fair value of preferred share tranche right liability						32,700
Preferred shares tranche liability						$ 39,600
Class B Special Voting Shares [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Preferred stock, shares issued	4,437,189	4,437,189
Preferred stock par value per share	$ 0.000001	$ 0.000001
Class B Preferred Exchangable Shares [Member] | Ireland Subsidiary [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Preferred stock, shares issued	4,437,189	4,437,189
Preferred stock par value per share	$ 1.5154	$ 1.5154
Aggregate gross proceeds	$ 62,500

Fair Value Measurements - Sch_2

Fair Value Measurements - Schedule of Fair Value from Preferred Share Tranche Liability and Warrant Liability (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2020USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Change in fair value of preferred share warrant liability	$ (6,399)
Preferred Share Tranche Right Liability [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Balance as of December 31, 2019	5,741
Preferred Share Tranche Right Liability [Member] | Preferred Class B
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Initial fair value of Class B preferred share tranche right liability	1,105
Change in fair value of Class B preferred share tranche right liability	32,722
Reclassification of Class B convertible preferred share and preferred exchangeable	(39,568)
Preferred Share Warrant Liability [Member] | Preferred Class B
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Initial fair value of Class B preferred share warrant liability	1,382
Change in fair value of preferred share warrant liability	6,399
Conversion of Class B preferred shares warrants into common share warrants	$ (7,781)

Investments - Schedule of Inves

Investments - Schedule of Investments (Detail) $ in Thousands	Sep. 30, 2020USD ($)
Available-for-sale securities, debt maturities, amortized cost [Abstract]
Amortized Cost, Due within one year or less	$ 43,160
Amortized Cost, Due after one year through three years	11,102
Amortized Cost, Investments	54,262
Available-for-sale securities, debt maturities, Fair Value [Abstract]
Fair Value, Due within one year or less	43,159
Fair Value, Due after one year through three years	11,102
Fair value, Investments	$ 54,261

Investments - Amortized Cost an

Investments - Amortized Cost and Estimated Fair Value of Investments by Contractual Maturity (Detail) $ in Thousands	Sep. 30, 2020USD ($)
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Investments, Amortized Cost	$ 54,262
Available-for-sale Investments, Gross Unrealized Gains	1
Available-for-sale Investments, Gross Unrealized Losses	(2)
Available-for-sale Investments, Fair Value	54,261
Available-for-sale Investments, Current	43,159
Available-for-sale Investments, Non Current	11,102
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Investments, Amortized Cost	6,994
Available-for-sale Investments, Fair Value	6,994
Available-for-sale Investments, Current	6,994
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Investments, Amortized Cost	905
Available-for-sale Investments, Fair Value	905
Available-for-sale Investments, Current	905
U.S. Government Agencies [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Investments, Amortized Cost	46,363
Available-for-sale Investments, Gross Unrealized Gains	1
Available-for-sale Investments, Gross Unrealized Losses	(2)
Available-for-sale Investments, Fair Value	46,362
Available-for-sale Investments, Current	35,260
Available-for-sale Investments, Non Current	$ 11,102

Investments - Additional Inform

Investments - Additional Information (Detail) $ in Thousands	Dec. 31, 2019USD ($)
Investments Debt And Equity Securities [Abstract]
Investments	$ 0

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Prepaid Expense And Other Assets Current [Abstract]
Prepaid clinical trial expenses		$ 94
Prepaid insurance	$ 3,136	11
Prepaid software subscriptions	173	129
Income tax receivable	109
Interest receivable	229	62
Canadian harmonized sales tax receivable	238	252
Refundable investment tax credits	315	176
Other	452	205
Prepaid expenses and other current assets	$ 4,652	$ 929

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Payables And Accruals [Abstract]
Accrued employee compensation and benefits	$ 1,524	$ 991
Accrued external research and development expenses	1,715	1,565
Accrued professional and consulting fees	1,048	744
Other	66	26
Accrued expenses	$ 4,353	$ 3,326

Equity - Additional Information

Equity - Additional Information (Detail)	Jun. 30, 2020USD ($)$ / sharesshares	Jun. 25, 2020USD ($)$ / sharesshares	Jun. 19, 2020	Jun. 02, 2020USD ($)$ / sharesshares	Jun. 02, 2020USD ($)$ / sharesshares	Jan. 31, 2020USD ($)$ / sharesshares	Mar. 31, 2019USD ($)$ / sharesshares	Jun. 30, 2020USD ($)$ / shares	Mar. 31, 2020USD ($)	Mar. 31, 2019USD ($)$ / sharesshares	Jun. 30, 2020USD ($)$ / shares	Sep. 30, 2020USD ($)$ / sharesshares	Oct. 17, 2020shares	Dec. 31, 2019USD ($)$ / sharesshares
Class Of Stock [Line Items]
Reverse share split, description			one-for-5.339
Stockholders equity note stock split conversion ratio			0.187
Common stock, no par value | $ / shares
Common stock, voting rights description												Each common share entitles the holder to one vote on all matters submitted to a vote of the Company’s shareholders.
Cash dividends												$ 0
Preferred stock, shares issued | shares												0		73,125,790
Preferred stock, no par value | $ / shares
Issuance of preferred shares and tranche right, issuance costs												$ 4,572,000
Preferred share tranche right liability														$ 5,741,000
Number of shares issuable upon exercise of outstanding warrants (in shares) | shares						749,197
Weighted-average exercise price per share (in dollars per share) | $ / shares						$ 8.10
Expiration period						2 years
Preferred share warrant liability						$ 1,400,000
Decrease in additional Paid in Capital						1,300,000
Accumulated deficit						$ 100,000						$ (99,818,000)		$ (34,774,000)
Common stock, shares issued | shares												41,702,384		1,929,555
Class B Convertible Preferred Shares [Member]
Class Of Stock [Line Items]
Stockholders equity note stock split conversion ratio												0.05339
Preferred stock, shares issued | shares														132,207,290
Preferred share tranche right liability												$ 35,300,000
Preferred shares tranche liability												$ 35,300,000
Class B Convertible Preferred Shares [Member] | Ireland Subsidiary [Member]
Class Of Stock [Line Items]
Preferred stock, shares issued | shares				4,437,189	4,437,189		4,437,189			4,437,189		28,874,378		29,747,987
Preferred stock par value per share | $ / shares				$ 0.000001	$ 0.000001		$ 1.5154			$ 1.5154		$ 0.001		$ 0.001
Increase in noncontrolling interest												$ 4,300,000
Preferred Class A
Class Of Stock [Line Items]
Preferred stock, no par value | $ / shares
Preferred Class B
Class Of Stock [Line Items]
Shares issued and sold, date					Jun. 2, 2020
Shares issued and sold | shares					36,806,039
Shares issued and sold, per share | $ / shares				$ 1.5154	$ 1.5154
Preferred stock, shares issued | shares				36,806,039	36,806,039
Preferred stock, no par value | $ / shares
Sold and issued of common stock | shares					36,806,039	6,598,917	30,207,129
Preferred stock par value per share | $ / shares				$ 1.5154	$ 1.5154	$ 1.5154	$ 1.5154			$ 1.5154
Aggregate gross proceeds					$ 55,800,000	$ 10,000,000	$ 45,800,000
Purchase an aggregate preferred share						20,000
Issuance of preferred shares and tranche right, issuance costs						100,000
Preferred share tranche right liability						$ 1,100,000						39,600,000
Loss on tranche liability contract												32,700,000
Preferred shares tranche liability												$ 39,600,000
Convertible preferred stock fair value per share | $ / shares				$ 2.47	$ 2.47	$ 1.45								$ 1.45
Preferred Class B | Canada Pension Plan Investment Board
Class Of Stock [Line Items]
Sold and issued of common stock | shares												6,598,917
Preferred stock par value per share | $ / shares												$ 1.5154
Class B Convertible Preferred Shares and Class B Preferred Share Tranche Right [Member]
Class Of Stock [Line Items]
Issuance of preferred shares and tranche right, issuance costs								$ 6,000	$ 93,000	$ 299,000
Number of shares issuable upon exercise of outstanding warrants (in shares) | shares						3,126,391
Weighted-average exercise price per share (in dollars per share) | $ / shares						$ 1.5154
Class B Preferred Exchangable Shares [Member]
Class Of Stock [Line Items]
Number of shares issuable upon exercise of outstanding warrants (in shares) | shares						873,609
Weighted-average exercise price per share (in dollars per share) | $ / shares						$ 1.5154
Class B Preferred Exchangable Shares [Member] | Ireland Subsidiary [Member]
Class Of Stock [Line Items]
Preferred stock, shares issued | shares				4,437,189	4,437,189
Preferred stock par value per share | $ / shares				$ 1.5154	$ 1.5154
Aggregate gross proceeds				$ 62,500,000
Class B Special Voting Shares [Member]
Class Of Stock [Line Items]
Preferred stock par value per share | $ / shares						$ 0.000001
Class B Special Voting Shares [Member] | Class B Convertible Preferred Shares [Member]
Class Of Stock [Line Items]
Stockholders equity note stock split conversion ratio					0.00187
Preferred stock, shares issued | shares				4,437,189	4,437,189		4,437,189			4,437,189		28,874,378		29,747,987
Preferred stock par value per share | $ / shares				$ 1.5154	$ 1.5154		$ 0.000001			$ 0.000001		$ 0.000001		$ 0.000001
Aggregate gross proceeds					$ 6,700,000		$ 6,700,000
Warrant Liability [Member]
Class Of Stock [Line Items]
Loss related to increase in fair value of the Preferred Share Warrants											$ 6,400,000
Common share warrants, exercised | shares													97,381
Common stock, shares issued | shares													38,340
Number of warrants cancelled | shares													59,041
Common share warrants remained outstanding | shares													651,816
IPO [Member] | Common Shares [Member]
Class Of Stock [Line Items]
Shares issued and sold, date		Jun. 25, 2020
Shares issued and sold | shares	12,500,000	12,500,000
Shares issued and sold, per share | $ / shares	$ 17	$ 17						$ 17			$ 17
Net proceeds of common shares	$ 193,100,000	$ 193,100,000

Share-Based Compensation - Addi

Share-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares option, exercised	0	0
Weighted average grant-date fair value of stock options granted	$ 11.54	$ 2.33
Aggregate unrecognized share-based compensation expense	$ 15.8
Unrecognized share-based compensation expense, weighted average period expects for recognition	3 years
2020 Stock Option and incentive plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock reserved for future issuance	4,273,350
Stock incentive plan description	cumulatively increased on January 1, 2021 and each January 1 thereafter by 4% of the number of the Company’s common shares outstanding on the immediately preceding December 31 or such lesser number of shares determined by the Company’s compensation committee of the board of directors
Annual percentage increase in common stock reserved for future issuance	4.00%
Number of shares remained available for future grant	2,933,366
2017 Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock reserved for future issuance	0		4,700,393
Number of shares remained available for future grant	0		1,598,512
2020 Employee Share Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock reserved for future issuance	450,169
Stock incentive plan description	the number of common shares that may be issued under the ESPP will automatically increase on January 1, 2021 and each January 1 thereafter by the lesser of (i) 900,338 common shares, (ii) 1% of the number of the Company’s common shares outstanding on the immediately preceding December 31 and (iii) such lesser number of shares as determined by the Company’s compensation committee of the board of directors.
Annual percentage increase in common stock reserved for future issuance	1.00%
Maximum annual increase in common stock reserved for future issuance	900,338
Shares, Issued	0

Share-Based Compensation - Fair

Share-Based Compensation - Fair Value of Options Estimated on Date of Grant Using Black-Scholes Option Pricing Model (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Risk-free interest rate	0.33%	1.58%	0.70%	1.58%
Expected term (in years)	6 years	5 years 10 months 24 days	6 years	5 years 10 months 24 days
Expected volatility	67.00%	64.50%	65.50%	64.50%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%

Share-Based Compensation - Summ

Share-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares, Beginning balance	3,045,301
Number of Shares, Granted	2,346,645
Number of Shares, Forfeited/cancelled	(125,480)
Number of Shares, Ending balance	5,266,466	3,045,301
Number of Shares, Options vested and expected to vest	5,266,466
Number of Shares, Options exercisable	1,810,276
Weighted Average Exercise Price, Beginning balance	$ 1.66
Weighted Average Exercise Price, Granted	11.50
Weighted Average Exercise Price, Forfeited/cancelled	5.63
Weighted Average Exercise Price, Ending balance	5.95	$ 1.66
Weighted Average Exercise Price, Options vested and expected to vest	5.95
Weighted Average Exercise Price, Options exercisable	$ 1.51
Weighted-Average Remaining Contractual Term, Outstanding	8 years 4 months 24 days	8 years 4 months 24 days
Weighted Average Remaining Contractual Term, Options vested and expected to vest	8 years 4 months 24 days
Weighted Average Remaining Contractual Term, Options exercisable	7 years
Aggregate Intrinsic Value	$ 37,696	$ 2,112
Aggregate Intrinsic Value, Options vested and expected to vest	37,696
Aggregate Intrinsic Value, Options exercisable	$ 18,699

Share-Based Compensation - Su_2

Share-Based Compensation - Summary of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 1,241	$ 138	$ 2,025	$ 328
Research and Development Expense [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	288	37	506	110
General and Administrative Expense [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 953	$ 101	$ 1,519	$ 218

License Agreements and Asset _2

License Agreements and Asset Acquisitions - Additional Information (Detail) £ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2020USD ($)shares	Mar. 31, 2020USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2019GBP (£)	Sep. 30, 2020USD ($)shares	Sep. 30, 2019USD ($)	Sep. 30, 2019GBP (£)
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development	$ 4,529,000		$ 2,238,000		$ 12,231,000	$ 7,216,000
CPDC [Member] | Research and License Agreement [Member] | Research and Development Expense [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Milestone payment	0		0		0	0
ImmunoGen [Member] | Research and License Agreement [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront license fee paid					200,000
Research and development	0		0		0
ImmunoGen [Member] | Research and License Agreement [Member] | Second Janssen Agreement [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront license fee paid	0		0		0	0
Research and development	0		0		0	0
ImmunoGen [Member] | Research and License Agreement [Member] | Maximum [Member] | Development And Regulatory Milestone [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Performance milestone payments based on successful development	15,000,000				15,000,000
ImmunoGen [Member] | Research and License Agreement [Member] | Maximum [Member] | Sales Based Milestone [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Performance milestone payments based on successful development	35,000,000				35,000,000
ImmunoGen [Member] | Research and License Agreement [Member] | Research and Development Expense [Member] | Development And Regulatory Milestone [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront license fee paid	0		0		0	500,000
ImmunoGen [Member] | Research and License Agreement [Member] | First Janssen Agreement [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront license fee paid			0			0
Research and development			0			0
Isogenica Ltd [Member] | Research and License Agreement [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development	0				0	600,000	£ 0.4
Licensing, development and commercialization agreement, upfront payment	0		0		0	700,000	£ 0.6
Isogenica Ltd [Member] | Research and License Agreement [Member] | License Fee [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Amortization of intangible assets			100,000	£ 0.1
MediaPharma [Member] | Asset Acquisition and License Agreement [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront license fee paid					200,000
Research and development	0		$ 0		0	$ 200,000
MediaPharma [Member] | Asset Acquisition and License Agreement [Member] | Maximum [Member] | Development And Regulatory Milestone [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Performance milestone payments based on successful development	1,500,000				1,500,000
MediaPharma [Member] | Asset Acquisition and License Agreement [Member] | Maximum [Member] | Sales Based Milestone [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Performance milestone payments based on successful development	23,000,000				23,000,000
Rainier Therapeutics, Inc. [Member] | Research and Development Expense [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront license fee paid	0				1,000,000
Rainier Therapeutics, Inc. [Member] | Asset Acquisition and License Agreement [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Additional amount payable under agreement	$ 3,500,000				$ 3,500,000
Non-voting common stock, issued | shares	313,359				313,359
Non-refundable upfront license fee					$ 1,000,000
Rainier Therapeutics, Inc. [Member] | Asset Acquisition and License Agreement [Member] | Development And Regulatory Milestone [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Performance milestone payments based on successful development	$ 22,500,000				$ 22,500,000
Non-voting common stock, issued | shares	156,679				156,679
Rainier Therapeutics, Inc. [Member] | Asset Acquisition and License Agreement [Member] | Sales Based Milestone [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Performance milestone payments based on successful development	$ 42,000,000				$ 42,000,000
Rainier Therapeutics, Inc. [Member] | Asset Acquisition and License Agreement [Member] | Maximum [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Potential future payment as percentage of amount the Company receives under sublicense agreements	30.00%				30.00%
Rainier Therapeutics, Inc. [Member] | Asset Acquisition and License Agreement [Member] | Minimum [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Potential future payment as percentage of amount the Company receives under sublicense agreements	10.00%				10.00%
Rainier Therapeutics, Inc. [Member] | Asset Acquisition and License Agreement [Member] | Research and Development Expense [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront license fee paid		$ 1,000,000
Genentech, Inc. [Member] | Asset Acquisition and License Agreement [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development	$ 0				$ 0
Genentech, Inc. [Member] | Asset Acquisition and License Agreement [Member] | Maximum [Member] | Sales Based Milestone [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Performance milestone payments based on successful development	44,000,000				44,000,000
Yumab GmbH [Member] | Asset Acquisition and License Agreement [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development	200,000				200,000
Yumab GmbH [Member] | Asset Acquisition and License Agreement [Member] | Development And Regulatory Milestone [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Performance milestone payments based on successful development	$ 3,900,000				$ 3,900,000

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Operating Loss Carryforwards [Line Items]
Income tax provision	$ (185)	$ 181	$ 27	$ 213
Maximum [Member]
Operating Loss Carryforwards [Line Items]
Income tax provision			$ 100

Net Loss per Share - Basic and

Net Loss per Share - Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net loss	$ (9,993)	$ (44,733)	$ (10,222)	$ (594)	$ (3,456)	$ (3,933)	$ (64,948)	$ (7,983)
Dividends paid to preferred shareholders in the form of warrants issued							(1,382)
Net loss attributable to common shareholders	$ (9,993)			$ (594)			$ (66,330)	$ (7,983)
Denominator:
Weighted-average common shares outstanding—basic and diluted	41,682,797			1,929,555			15,422,375	1,910,695
Net loss per share attributable to common shareholders—basic and diluted	$ (0.24)			$ (0.31)			$ (4.30)	$ (4.18)

Net Loss per Share - Summary of

Net Loss per Share - Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share (Detail) - shares	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	5,918,282	20,961,032
Convertible Preferred shares [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share		13,696,513
Options to purchase common shares [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	5,266,466	2,687,413
Class B Preferred Exchangeable Shares [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share		4,577,106
Warrant Liability [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	651,816

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Jul. 31, 2020	Oct. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Non-cancelable minimum purchase commitments	$ 100
Lease expiration date	Jul. 31, 2023
Operating lease expiration period		Sep. 30, 2025			Aug. 31, 2023
Operating lease additional lease period					5 years
Operating lease termination notice period					12 months
Operating lease, option to extend by lessee					false
Restricted cash			$ 1,465		$ 1,465			$ 1,497
Lessee, operating lease, description					In August 2018, the Company entered into an operating lease for office space in Hamilton, Ontario, Canada. This lease was amended in September 2020 (“New Lease Commencement Date”) with a new expiry date in August 2023. The lease can be renewed for an additional period of five years at the Company’s option, and if so renewed, can be terminated during the option period upon twelve months written notice any time after the fifth anniversary of the New Lease Commencement Date. In October 2019, the Company entered into an operating lease for office space in Boston, Massachusetts, which expires September 2025 and has no renewal options. In connection with entering into this lease agreement, the Company issued a letter of credit of $1.5 million, which is classified as restricted cash (non-current) on the consolidated balance sheets as of September 30, 2020 and December 31, 2019.
Rent Expense			300	$ 100	$ 800	$ 200
CPDC [Member]
Defined Benefit Plan Disclosure [Line Items]
Non-cancelable minimum purchase commitments			$ 800		$ 800
Letter Of Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Restricted cash							$ 1,500	$ 1,500

Commitments and Contingencies_2

Commitments and Contingencies - Future Maturities of Lease Liabilities (Detail) $ in Thousands	Sep. 30, 2020USD ($)
Operating Leases
2020 (three months)	$ 274
2021	1,117
2022	1,147
2023	1,177
2024	1,208
Thereafter	1,592
Total lease payments	$ 6,515

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - CPDC [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Related Party Transaction [Line Items]
Payment related to supply agreement per quarter	$ 0.2		$ 0.2
Payment related to supply agreement aggregate per year	0.8		0.8
Amounts of transaction related to service	0.2	$ 0.5	1	$ 0.8
Amount due to related parties	0.1		0.1		$ 0.2
Payment for reimbursement of pass through costs			$ 0.1	$ 0.1
Maximum [Member]
Related Party Transaction [Line Items]
Payment for reimbursement of pass through costs	$ 0.1	$ 0.1

Related Party Transactions - Sc

Related Party Transactions - Schedule of Recognized Expenses in Connection with Services Performed Under Master Services Agreement and Supply Agreement (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Related Party Transaction [Line Items]
Research and development expenses	$ 4,529	$ 2,238	$ 12,231	$ 7,216
General and administrative expenses	5,790	1,922	14,105	4,864
CPDC [Member]
Related Party Transaction [Line Items]
Research and development expenses	141	326	780	785
General and administrative expenses	23	16	50	59
Total operating expenses	$ 164	$ 342	$ 830	$ 844

Geographical Information - Sche

Geographical Information - Schedule of Long-lived Assets by Geographic Areas (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Revenues From External Customers And Long Lived Assets [Line Items]
Property and equipment, net	$ 1,981	$ 1,272
United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property and equipment, net	115	127
Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property and equipment, net	$ 1,866	$ 1,145

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - Subsequent Event [Member] - USD ($)	Oct. 30, 2020	Oct. 08, 2020
Subsequent Event [Line Items]
Subsequent event, date		Oct. 8, 2020
Subsequent event, description		On October 8, 2020, the Company and Rainier entered into the Amended Rainier Agreement (see Note 9). Pursuant to the Amended Rainier Agreement, the termination provisions of the Rainier Agreement were modified. Specifically, the Outside Date was amended such that termination may not occur later than eleven months following the Closing, or February 10, 2021. Aside from modifying the termination provisions to incorporate the Revised Outside Date, no other material terms were amended as part of the Amended Rainier Agreement
AstraZeneca UK Limited [Member] | Strategic Collaboration Agreement [Member]
Subsequent Event [Line Items]
Maximum amount for milestones payments to other party	$ 145,000,000
Upfront payment received	5,000,000
Maximum amount of payments to be received for development milestones	$ 40,000,000