Orchard Therapeutics (ORTX) 10-Q 13 Nov 23 2023 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2023	Nov. 09, 2023
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2023
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q3
Entity Registrant Name	ORCHARD THERAPEUTICS PLC
Current Fiscal Year End Date	--12-31
Entity Central Index Key	0001748907
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Common Stock, Shares Outstanding		227,610,439
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Incorporation, State or Country Code	X0
Entity Address, Address Line One	245 Hammersmith Road
Entity Address, City or Town	London
Entity Address, Postal Zip Code	W6 8PW
City Area Code	44
Local Phone Number	(0) 203 808-8286
Document Quarterly Report	true
Document Transition Report	false
Entity File Number	001-38722
Entity Emerging Growth Company	false
Entity Tax Identification Number	00-0000000
Entity Address, Country	GB
Title of 12(b) Security	American Depositary Shares, each representing tenordinary shares, nominal value £0.10 per share
Trading Symbol	ORTX
Security Exchange Name	NASDAQ

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 23,469	$ 68,424
Marketable securities	101,944	75,326
Accounts receivable, net	6,187	8,467
Inventory	6,638	3,400
Prepaid expenses and other current assets	7,451	6,586
Research and development tax credit receivable	8,234	5,942
Total current assets	153,923	168,145
Non-current assets:
Operating lease right-of-use-assets	20,001	22,774
Property and equipment, net	7,131	8,138
Research and development tax credit receivable	3,372
Restricted cash	4,215	4,215
Intangible assets, net	3,321	3,560
Other assets	10,303	12,075
Total non-current assets	48,343	50,762
Total assets	202,266	218,907
Current liabilities:
Accounts payable	3,661	9,318
Accrued expenses and other current liabilities	29,995	34,437
Deferred revenue	742	959
Operating lease liabilities	5,548	6,424
Notes payable	9,429	9,429
Total current liabilities	49,375	60,567
Notes payable, long-term	16,162	22,991
Deferred revenue, net of current portion	10,345	10,315
Operating lease liabilities, net of current portion	16,681	19,246
PIPE Warrant liabilities	21,068
Other long-term liabilities	8,377	7,524
Total liabilities	122,008	120,643
Commitments and contingencies (see Note 17)
Shareholders’ equity:
Ordinary shares (voting and non-voting), GBP 0.10 par value; Most recent authority to allot up to a maximum nominal value of GBP 33,556,809 and GBP 13,023,851.50 of shares at September 30, 2023 and December 31, 2022, respectively; Issued and outstanding - 227,273,209 and 126,947,225 ordinary shares at September 30, 2023 and December 31, 2022, respectively.	29,463	16,419
Additional paid-in capital	994,108	956,711
Accumulated other comprehensive income	22,609	26,018
Accumulated deficit	(965,922)	(900,884)
Total shareholders’ equity	80,258	98,264
Total liabilities and shareholders’ equity	$ 202,266	$ 218,907

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - GBP (£)	Sep. 30, 2023	Jun. 22, 2023	Mar. 10, 2023	Dec. 31, 2022
Statement of Financial Position [Abstract]
Ordinary Shares, Par Value	£ 0.10	£ 0.1	£ 0.1	£ 0.10
Ordinary Shares, Authorized	£ 33,556,809			£ 13,023,851.5
Ordinary Shares, Issued	227,273,209			126,947,225
Ordinary Shares, Outstanding	227,273,209			126,947,225

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Total revenue	$ 6,302	$ 5,777	$ 14,853	$ 15,669
Costs and operating expenses:
Cost of product revenue	1,745	1,645	4,301	4,338
Research and development	14,553	18,103	47,241	68,302
Selling, general and administrative	11,609	11,496	33,736	38,525
Total costs and operating expenses	27,907	31,244	85,278	111,165
Loss from operations	(21,605)	(25,467)	(70,425)	(95,496)
Other income (expense):
Interest income	1,505	404	3,925	686
Interest expense	(932)	(799)	(2,864)	(2,146)
Changes in fair value of PIPE Warrant and PIPE Unit liabilities	(8,801)		3,256
Other income (expense), net	(4,510)	(22,787)	2,059	(47,066)
Total other income (expense), net	(12,738)	(23,182)	6,376	(48,526)
Net loss before income taxes	(34,343)	(48,649)	(64,049)	(144,022)
Income tax benefit (expense)	(976)	1,084	(989)	1,245
Net loss	$ (35,319)	$ (47,565)	$ (65,038)	$ (142,777)
Net loss per ordinary share , basic	$ (0.15)	$ (0.37)	$ (0.35)	$ (1.12)
Net loss per ordinary share , diluted	$ (0.15)	$ (0.37)	$ (0.35)	$ (1.12)
Weighted average ordinary shares outstanding, basic	228,388,561	128,132,092	186,811,772	127,895,426
Weighted average ordinary shares outstanding, diluted	228,388,561	128,132,092	186,811,772	127,895,426
Other comprehensive income:
Foreign currency translation adjustment	$ 3,978	$ 21,459	$ (3,523)	$ 44,504
Unrealized gain (loss) on marketable securities	79	84	114	(177)
Total other comprehensive income (loss)	4,057	21,543	(3,409)	44,327
Total comprehensive loss	(31,262)	(26,022)	(68,447)	(98,450)
Product Revenue, Net [Member]
Total revenue	5,559	5,377	12,744	14,217
Collaboration Revenue [Member]
Total revenue	$ 743	$ 400	$ 2,109	$ 1,452

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Cash flows from operating activities:
Net loss attributable to ordinary shareholders	$ (65,038)	$ (142,777)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,950	2,020
Share-based compensation	8,558	12,084
Non-cash interest expense	266	279
Amortization of provision on loss contract	(2,900)	(276)
(Accretion) amortization of (discount) premium on marketable securities	(2,658)	66
Deferred income taxes	1,125	2,318
Change in fair value of PIPE Warrant and PIPE Unit liabilities	(3,256)
Unrealized foreign currency and other non-cash adjustments	(3,177)	46,215
Changes in operating assets and liabilities:
Accounts receivable	2,397	(3,058)
Inventory	(3,274)	11,315
Research and development tax credit receivable	(5,764)	(303)
Prepaid expenses, other current assets and other assets	(245)	4,059
Operating leases, right-of-use assets	3,356	7,171
Accounts payable, accrued expenses and other liabilities	(8,261)	(178)
Deferred revenue	(286)	(4,420)
Operating lease liabilities	(4,026)	1,582
Net cash used in operating activities	(81,233)	(63,903)
Cash flows from investing activities:
Proceeds from sales and maturities of marketable securities	122,822	169,360
Purchases of marketable securities	(146,671)	(71,206)
Purchases of property and equipment	(609)	(6,030)
Net cash provided by (used in) investing activities	(24,458)	92,124
Cash flows from financing activities:
Proceeds from employee equity plans, net of taxes withheld	228	118
Payment of taxes on restricted stock vesting	(179)
Proceeds from the issuance of units in private placement	68,000
Payment of placement agent fees and offering costs in connection with private placement	(889)
Repayment of notes payable	(7,071)
Net cash provided by financing activities	60,089	118
Effect of exchange rate changes on cash, cash equivalents, and restricted cash	647	(3,500)
Net decrease in cash, cash equivalents and restricted cash	(44,955)	24,839
Cash, cash equivalents, and restricted cash, beginning of period	72,639	60,178
Cash, cash equivalents, and restricted cash, end of period	27,684	85,017
Supplemental disclosure of cash flow information:
Lease assets obtained in exchange for new/modified operating lease liabilities		4,912
Non-cash adjustments to operating lease right-of-use assets and liabilities	447	530
Cash paid for interest	$ 2,340	$ 1,843

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Ordinary Shares	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at Dec. 31, 2021	$ 209,950	$ 16,253	$ 940,675	$ 3,246	$ (750,224)
Balance, Shares at Dec. 31, 2021		125,674,095
Share-based compensation expense	4,660		4,660
Exercise of share options	(1)	$ 28	(29)
Exercise of share options, Shares		222,381
Vesting of restricted stock units, net of shares withheld for taxes	(3)	$ 1	(4)
Vesting of restricted stock units, net of shares withheld for taxes, shares		3,217
Ordinary shares issued as part of a consulting agreement	1	$ 1
Ordinary shares issued as part of consulting agreement, Shares		5,252
Foreign currency translation	5,595			5,595
Unrealized loss on available for sale debt securities	(260)			(260)
Net loss attributable to ordinary shareholders	(44,296)				(44,296)
Balance at Mar. 31, 2022	175,646	$ 16,283	945,302	8,581	(794,520)
Balance, Shares at Mar. 31, 2022		125,904,945
Balance at Dec. 31, 2021	209,950	$ 16,253	940,675	3,246	(750,224)
Balance, Shares at Dec. 31, 2021		125,674,095
Foreign currency translation	44,504
Unrealized loss on available for sale debt securities	(177)
Net loss attributable to ordinary shareholders	(142,777)
Balance at Sep. 30, 2022	123,703	$ 16,370	952,761	47,573	(893,001)
Balance, Shares at Sep. 30, 2022		126,572,749
Balance at Mar. 31, 2022	175,646	$ 16,283	945,302	8,581	(794,520)
Balance, Shares at Mar. 31, 2022		125,904,945
Share-based compensation expense	3,956		3,956
Exercise of share options	1	$ 24	(23)
Exercise of share options, Shares		175,153
Issuance of ESPP shares	136	$ 46	90
Issuance of ESPP shares, Shares		356,115
Foreign currency translation	17,450			17,450
Unrealized loss on available for sale debt securities	(1)			(1)
Net loss attributable to ordinary shareholders	(50,916)				(50,916)
Balance at Jun. 30, 2022	146,272	$ 16,353	949,325	26,030	(845,436)
Balance, Shares at Jun. 30, 2022		126,436,213
Share-based compensation expense	3,468		3,468
Exercise of share options		$ 15	(15)
Exercise of share options, Shares		115,551
Vesting of restricted stock units, net of shares withheld for taxes	(15)	$ 2	(17)
Vesting of restricted stock units, net of shares withheld for taxes, shares		20,985
Foreign currency translation	21,459			21,459
Unrealized loss on available for sale debt securities	84			84
Net loss attributable to ordinary shareholders	(47,565)				(47,565)
Balance at Sep. 30, 2022	123,703	$ 16,370	952,761	47,573	(893,001)
Balance, Shares at Sep. 30, 2022		126,572,749
Balance at Dec. 31, 2022	98,264	$ 16,419	956,711	26,018	(900,884)
Balance, Shares at Dec. 31, 2022		126,947,225
Share-based compensation expense	3,512		3,512
Exercise of share options	41	$ 43	(2)
Exercise of share options, Shares		332,209
Vesting of restricted stock units, net of shares withheld for taxes	(166)	$ 40	(206)
Vesting of restricted stock units, net of shares withheld for taxes, shares		310,025
Sale of voting and non-voting ordinary shares, net of allocated issuance costs	23,073	$ 7,369	15,704
Sale of voting and non-voting ordinary shares, net of allocated issuance costs, Shares		56,666,900
Foreign currency translation	(5,731)			(5,731)
Unrealized loss on available for sale debt securities	74			74
Net loss attributable to ordinary shareholders	(17,412)				(17,412)
Balance at Mar. 31, 2023	101,655	$ 23,871	975,719	20,361	(918,296)
Balance, Shares at Mar. 31, 2023		184,256,359
Balance at Dec. 31, 2022	$ 98,264	$ 16,419	956,711	26,018	(900,884)
Balance, Shares at Dec. 31, 2022		126,947,225
Exercise of share options, Shares	370,430
Foreign currency translation	$ (3,523)
Unrealized loss on available for sale debt securities	114
Net loss attributable to ordinary shareholders	(65,038)
Balance at Sep. 30, 2023	80,258	$ 29,463	994,108	22,609	(965,922)
Balance, Shares at Sep. 30, 2023		227,273,209
Balance at Mar. 31, 2023	101,655	$ 23,871	975,719	20,361	(918,296)
Balance, Shares at Mar. 31, 2023		184,256,359
Share-based compensation expense	2,578		2,578
Issuance of ESPP shares	169	$ 59	110
Issuance of ESPP shares, Shares		455,550
Vesting of restricted stock units, net of shares withheld for taxes		$ 1	(1)
Vesting of restricted stock units, net of shares withheld for taxes, shares		3,790
Sale of voting and non-voting ordinary shares, net of allocated issuance costs	18,761	$ 5,525	13,236
Sale of voting and non-voting ordinary shares, net of allocated issuance costs, Shares		42,500,000
Foreign currency translation	(1,770)			(1,770)
Unrealized loss on available for sale debt securities	(39)			(39)
Net loss attributable to ordinary shareholders	(12,307)				(12,307)
Balance at Jun. 30, 2023	109,047	$ 29,456	991,642	18,552	(930,603)
Balance, Shares at Jun. 30, 2023		227,215,699
Share-based compensation expense	2,468		2,468
Exercise of share options	18	$ 5	13
Exercise of share options, Shares		38,250
Vesting of restricted stock units, net of shares withheld for taxes	(13)	$ 2	(15)
Vesting of restricted stock units, net of shares withheld for taxes, shares		19,260
Foreign currency translation	3,978			3,978
Unrealized loss on available for sale debt securities	79			79
Net loss attributable to ordinary shareholders	(35,319)				(35,319)
Balance at Sep. 30, 2023	$ 80,258	$ 29,463	$ 994,108	$ 22,609	$ (965,922)
Balance, Shares at Sep. 30, 2023		227,273,209

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Shareholders' Equity (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2023	Mar. 31, 2023
Statement of Stockholders' Equity [Abstract]
Sale of voting and non-voting ordinary shares, net of allocated issuance costs	$ 953	$ 889

Nature of the Business and Liqu

Nature of the Business and Liquidity	9 Months Ended
	Sep. 30, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Liquidity	1. Nature of the Business and Liquidity Orchard Therapeutics plc (the “Company” or “Orchard”) is a global gene therapy company dedicated to ending the devastation caused by genetic and other severe diseases by discovering, developing and commercializing new treatments using the curative potential of hematopoietic stem cell (HSC) gene therapy. In this approach, a patient’s own blood stem cells are genetically modified outside of the body and then reinserted, with the goal of correcting the underlying cause of disease in a single treatment. Orchard is advancing a pipeline spanning pre-clinical, clinical and commercial stage HSC gene therapies designed to address serious diseases where the burden is immense for patients, families and caregivers and current treatment options are limited or do not exist. The Company is a public limited company incorporated pursuant to the laws of England and Wales. The Company has American Depositary Shares (“ADSs”) registered with the U.S. Securities and Exchange Commission (the “SEC”). The ADSs were listed on the Nasdaq Global Select Market on October 31, 2018, and were transferred to the Nasdaq Capital Market on September 13, 2022. The Company’s ADSs each represent ten ordinary shares of the Company. Each holder of ordinary shares is entitled to one vote per ordinary share and to receive dividends when and if such dividends are recommended by the board of directors and declared by the shareholders. The Company did no t declare any dividends in 2023 or 2022. Share information presented in these unaudited condensed consolidated financial statements are presented on an ordinary share basis and not on an ADS converted basis unless otherwise indicated. Kyowa Kirin Transaction On October 5, 2023, the Company entered into a Transaction Agreement (the “Transaction Agreement”) with Kyowa Kirin Co., Ltd., a Japanese joint stock company (kabushiki kaisha) (“Kyowa Kirin”), pursuant to which, subject to the satisfaction or waiver of the conditions set forth therein, Kyowa Kirin will (either directly or via a nominee) acquire the entire issued and outstanding share capital of the Company pursuant to a court-sanctioned scheme of arrangement. Pursuant to the Transaction Agreement, at closing (the “Effective Time”), all outstanding shares of the Company (including ordinary shares, non-voting ordinary shares and ADSs) then issued and outstanding will be transferred to Kyowa Kirin (or its nominee). Holders of shares at the Effective Time will be entitled to receive (a) $ 16.00 in cash, without interest, per ADS, less certain fees assessed by the ADS depositary, or $ 1.60 in cash, without interest per ordinary share or non-voting ordinary share, plus (b) 10 contingent value rights (each, a “CVR”) per ADS, or one CVR per ordinary share or non-voting ordinary share, together representing a right to receive $ 0.10 per CVR, without interest, if regulatory approval is granted by the U.S. Food and Drug Administration (“FDA”) for the commercial marketing and sale of OTL-200 in the United States for the treatment of metachromatic leukodystrophy (“MLD”), which includes as an intended patient population both (x) pediatric patients with late infantile forms of MLD, and (y) pediatric patients with early juvenile forms of MLD, in each case that are pre-symptomatic or without clinical manifestations of the disease, on or before 11:59 p.m. Eastern Time on December 31, 2024. The transaction has been unanimously approved by both company’s Boards of Directors and is expected to close in the first quarter of 2024, subject to Orchard shareholder approval, receipt of applicable regulatory approvals and other customary closing conditions. Warrants outstanding as of the date of the Transaction Agreement will continue to be satisfied in accordance with their terms. Following the completion of the acquisition, the Company will become a wholly-owned subsidiary of Kyowa Kirin and will cease to be a Nasdaq-listed company. Recent Developments In January 2022, the Company began to generate revenue from product sales of Libmeldy™ in Europe following the approval of Libmeldy by the European Commission in December 2020 for the treatment of early onset MLD, characterized by biallelic mutations in the arylsulfatase-A ("ARSA") gene leading to a reduction of the ARSA enzymatic activity in children with (i) late infantile or early juvenile forms, without clinical manifestations of the disease, or (ii) the early juvenile form, with early clinical manifestations of the disease, who still have the ability to walk independently and before the onset of cognitive decline. On March 6, 2023, the Company entered into a Securities Purchase Agreement (“SPA” or “PIPE”) pursuant to which the Company agreed to sell ordinary shares, non-voting ordinary shares, and warrants to purchase ordinary shares or non-voting ordinary shares in an unregistered offering (the "2023 Private Placement"). The 2023 Private Placement consists of two closings. On March 10, 2023, the Company completed the first closing and issued and sold (i) 56,666,900 ordinary shares and non-voting ordinary shares, nominal value £ 0.10 per share and (ii) warrants to purchase an aggregate of 62,333,590 ordinary shares or non-voting ordinary shares, at a purchase price of $ 6.00 per unit, where each unit consists of ten shares and an accompanying warrant to purchase eleven shares . The initial closing of the 2023 Private Placement resulted in gross proceeds of $ 34.0 million. On June 22, 2023, the Company completed the second closing of the 2023 Private Placement and sold (i) 42,500,00 ordinary shares and non-voting ordinary shares, nominal value £ 0.10 per share and (ii) warrants to purchase an aggregate of 46,750,000 ordinary shares or non-voting ordinary shares, at a purchase price of $ 8.00 per unit, where each unit consists of ten shares and an accompanying warrant to purchase eleven shares . The second closing of the 2023 Private Placement resulted in gross proceeds of $ 34.0 million. Refer to Note 12 for further discussion of the 2023 Private Placement. On October 5, 2023, the Company entered into the Transaction Agreement with Kyowa Kirin as described above. Risks and Uncertainties The Company’s business is subject to risks and uncertainties common to companies in the biotechnology industry. There can be no assurance that the Company’s commercialization, research and development will be successfully completed, that adequate protection for the Company’s technology will be obtained, that any products developed will obtain necessary government regulatory approval or that any products, if approved, will be commercially viable. The Company operates in an environment of rapid technological innovation and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, consultants and service providers. Even if the Company’s product development efforts are successful in gaining regulatory approval, it is uncertain when, if ever, the Company will realize significant revenue from product sales. Liquidity Through September 30, 2023, the Company funded its operations primarily with proceeds from the sale of equity securities. The Company has also financed its operations through proceeds from the Company’s senior term facilities agreement with MidCap Financial (Ireland) Limited, research grants from the California Institute of Regenerative Medicine (“CIRM”), upfront payments from the Company’s collaboration agreement and share purchase agreement with Pharming Group N.V., proceeds from the sales of the Company's Libmeldy product, and reimbursements associated with two UK research and development tax relief programs, the Small and Medium-sized Enterprises research and development tax credit (“SME”) program and the Research and Development Expenditure (“RDEC”) program. The Company has incurred recurring losses since its inception and expects to continue to generate operating losses for the foreseeable future. The Company expects that its cash, cash equivalents, and marketable securities on hand as of September 30, 2023, of $ 125.4 million, together with expected proceeds from sales of Libmeldy, will be sufficient to fund its operations, capital expenditures and debt service payments for at least twelve months from the date of filing of this Form 10-Q. If the acquisition by Kyowa Kirin does not close, the Company will seek additional funding through private or public equity financings, debt financings, collaborations, strategic alliances and marketing, distribution or licensing arrangements. Under these circumstances, the Company may not be able to obtain financing on acceptable terms, or at all, and the Company may not be able to enter into collaborations or other arrangements. The terms of any financing may adversely affect the holdings or the rights of the Company's shareholders. If the acquisition by Kyowa Kirin does not close, the future viability of the Company is dependent on its ability to raise additional capital to finance its operations. If the Company is unable to obtain funding, the Company will be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2023
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies The Company's significant accounting policies are disclosed in the audited consolidated financial statements for the year ended December 31, 2022, and notes thereto, which are included in the Company's Annual Report on Form 10-K filed with the SEC on March 14, 2023 (the “Annual Report”). Since the date of those financial statements there have been no material changes to the Company's significant accounting policies other than those discussed below. Basis of presentation These condensed consolidated financial statements of the Company are unaudited and have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial reporting and in accordance with Regulation S-X, Rule 10-01. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (“ASC”), and Accounting Standards Update (“ASU”), of the Financial Accounting Standards Board (“FASB”). These unaudited condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements and accompanying notes included in the Company’s Annual Report. The condensed consolidated balance sheet as of December 31, 2022, was derived from audited consolidated financial statements included in the Company’s Annual Report but does not include all disclosures required by GAAP. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted from these interim financial statements. However, these interim financial statements include all adjustments, consisting only of normal recurring adjustments, which are, in the opinion of the Company’s management, necessary to fairly state the results of the interim period. The interim results are not necessarily indicative of results to be expected for the full year. Amounts reported are presented in thousands, except percentages, per share amounts or as otherwise noted. As a result, certain totals may not sum due to rounding. Principles of consolidation The accompanying unaudited condensed consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Use of estimates The preparation of unaudited 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 unaudited condensed consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these unaudited condensed consolidated financial statements include, but are not limited to, the accrual for research and development expenses, the research and development tax credit receivable, share-based compensation, collaboration agreement milestones, variable consideration in revenue recognition, operating lease assets and liabilities, valuation of PIPE Warrant (as defined in Note 12) and PIPE Unit liabilities (as defined in Note 12), and income taxes. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates. Foreign currency The financial statements of the Company’s subsidiaries with functional currencies other than the U.S. dollar are translated into U.S. dollars using period-end exchange rates for assets and liabilities, historical exchange rates for shareholders’ equity and weighted average exchange rates for operating results. Unrealized losses are driven primarily by intercompany balances denominated in currencies other than the functional currency of the entity with the intercompany balance, and typically fluctuates concurrently with fluctuations in the U.S. dollar, British pounds sterling, and Euro exchange rates. Translation gains and losses are included in accumulated other comprehensive income (loss) in shareholders’ equity. Foreign currency transaction gains and losses are included in other income (expense), net in the results of operations. The Company recorded realized and unrealized foreign currency transactio n gains of $ 3.0 million for the nine months ended September 30, 2023 , and realized and unrealized foreign currency transaction losses of $ 47.1 million for the nine months ended September 30, 2022 , which is included in other income (expense), net in the condensed consolidated statements of operations and comprehensive loss. Accounts receivable Accounts receivable arise from product revenue and amounts due from the Company's collaboration partners and have payment terms that generally require payment within 30 to 90 days. For some Libmeldy customers, our payment terms can range from 30 days to under one year . The amount from product revenue represents amounts due from customers and treatment centers in Europe, which are recorded net of reserves for trade discounts and allowances, and other incentives to the extent such amounts are payable to the customer by the Company. The Company monitors economic conditions to identify facts or circumstances that may indicate that its receivables are at risk of collection. The Company provides reserves against accounts receivable for estimated losses, if any, that may result from a customer's inability to pay based on the composition of its accounts receivable, current economic conditions, and historical credit loss activity. Amounts determined to be uncollectible are charged or written-off against the reserve. During the three and nine months ended September 30, 2023 , the Company did no t record any expected credit losses related to outstanding accounts receivable. Restricted cash Cash and cash equivalents that are restricted as to withdrawal or use under the terms of certain contractual agreements are recorded as restricted cash on the Company’s condensed consolidated balance sheets. The Company has an outstanding letter of credit for $ 3.0 million associated with a lease and is required to hold this amount in a standalone bank account, as of September 30, 2023, and December 31, 2022 . The Company is also contractually required to maintain cash collateral accounts associated with corporate credit cards and other leases in the amount of $ 1.3 million at September 30, 2023 and December 31, 2022. Cash, cash equivalents, and restricted cash were comprised of the following (in thousands): As of September 30, As of December 31, 2023 2022 Cash and cash equivalents $ 23,469 $ 68,424 Restricted cash 4,215 4,215 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 27,684 $ 72,639 Recently adopted accounting pronouncements In March 2020, the FASB issued ASU 2020-04 , Reference Rate Reform (Topic 848) : Facilitation of the Effects of Reference Rate Reform on Financial Reporting and issued two subsequent amendments: ASU 2021-01, issued in January 2021, refines the scope of ASU and clarifies some of its guidance as part of the FASB’s monitoring of global reference rate reform activities and ASU 2022-06, issued in December 2022, which extends the effective period of the ASU through December 31, 2023 (collectively, including ASU 2020-04, “ASC 848”). ASC 848 provides temporary optional expedients and exceptions to the GAAP guidance on contract modifications and hedge accounting to ease the financial reporting burdens related to the expected market transition from the London Interbank Offered Rate (“LIBOR”) and other interbank offered rates to alternative reference rates. ASC 848 is effective for all entities as of March 12, 2020, through December 31, 2024, at which time transition is expected to be complete. The Company adopted this standard as of January 1, 2023 . The application of this ASU did not have a material impact on the Company's financial position, results of operations, or cash flows.

Fair Value Measurements and Mar

Fair Value Measurements and Marketable Securities	9 Months Ended
	Sep. 30, 2023
Fair Value Disclosures [Abstract]
Fair Value Measurements and Marketable Securities	3. Fair value measurements and marketable securities Fair value measurements The following tables present information about the Company’s financial assets and liabilities that have been measured at fair value as of September 30, 2023, and December 31, 2022, and indicate hierarchy of valuation inputs utilized to determine such fair value (in thousands): Fair Value Measurements as of Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 7,285 $ — $ — $ 7,285 Total cash equivalents $ 7,285 $ — $ — $ 7,285 Marketable securities U.S. government securities $ — $ 17,462 $ — $ 17,462 U.S. treasuries — 18,726 — 18,726 Corporate bonds — 15,340 — 15,340 Commercial paper — 50,416 — 50,416 Total marketable securities $ — $ 101,944 $ — $ 101,944 Total Assets $ 7,285 $ 101,944 $ — $ 109,229 Liabilities PIPE Warrant liability $ — $ — $ 21,068 $ 21,068 Total Liabilities $ — $ — $ 21,068 $ 21,068 Fair Value Measurements as of Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 1,239 $ — $ — $ 1,239 U.S. treasuries — 6,600 — 6,600 U.S. government securities — 5,200 — 5,200 Commercial paper — 14,122 — 14,122 Total cash equivalents $ 1,239 $ 25,922 $ — $ 27,161 Marketable securities U.S. government securities $ — $ 1,984 $ — $ 1,984 Corporate bonds — 25,475 — 25,475 Commercial paper — 47,867 — 47,867 Total marketable securities $ — $ 75,326 $ — $ 75,326 Total Assets $ 1,239 $ 101,248 $ — $ 102,487 The Company classifies its money market funds as Level 1 assets since it measures fair value using quoted prices in active markets for identical assets. The Level 2 assets include commercial paper, U.S. government securities, U.S. treasuries, and corporate bonds and are valued based on quoted prices for similar assets in active markets and inputs other than quoted prices that are derived from observable market data. The Company evaluates transfers between levels at the end of each reporting period. There were no transfers between Level 1 and Level 2 assets during the periods presented. The PIPE Warrants (as defined in Note 12) and PIPE Units (as defined in Note 12) associated with the 2023 Private Placement did not meet the criteria for equity classification, and are therefore accounted for as liabilities at fair value. Refer to Note 12 for the accounting analysis of the PIPE Warrants and PIPE Units. The fair values of the PIPE Warrants and PIPE Units are based on significant unobservable inputs, which represent Level 3 measurements within the fair value hierarchy. The fair value of the PIPE Warrants was estimated using the Black-Scholes option pricing model, which was then adjusted by the weighted probability of the timing of the anticipated achievement of FDA marketing approval of OTL-200 in the United States and further adjusted by the probability of the occurrence of achievement of FDA marketing approval for the sale of OTL-200 in the United States. The fair value of the PIPE Units prior to the second closing of the 2023 Private Placement was estimated as the residual of the fair value of the share and warrant units to be sold in the second closing minus the second closing sales price of the share and warrant units to be sold. To calculate the fair value of the shares to be sold in the second closing, the Company utilized the stock price of its ADSs as of the valuation date and discounted it using a discount for lack of marketability ("DLOM") relating to the unregistered status of the shares. The second closing was completed in June 2023, upon which, the Company recorded a final fair value adjustment in the statement of operations and derecognized the value ascribed to the PIPE Units instrument on the balance sheet. The fair value of the PIPE Units at the time of the second closing was calculated by estimating the fair value of the PIPE Warrants and the unregistered shares issued in the second closing. To calculate the fair value of the shares issued in the second closing, the Company utilized the stock price of its ADSs as of the valuation date and discounted it using a DLOM relating to the unregistered status of the shares. T he significant unobservable inputs used in the valuation model to measure the PIPE Warrant and PIPE Unit liabilities are as follows at the issuance date and as of March 31, 2023: PIPE Unit Liability PIPE Warrants March 10, 2023 March 31, 2023 March 10, 2023 March 31, 2023 Black-Scholes inputs (Issuance date) (Issuance date) Anticipated common stock price (at exercise date) $ 10.14 - $ 10.46 $ 10.23 - $ 10.49 $ 10.00 - $ 10.31 $ 10.13 - $ 10.38 Expected term (in years) 1.08 - 1.84 1.08 - 1.84 1.39 - 2.14 1.33 - 2.08 Expected volatility 72.9 % - 76.0 % 71.7 % - 77.4 % 72.9 % - 76.0 % 71.7 % - 77.4 % Other significant unobservable inputs Implied probabilities of occurrence of Second Closing and FDA Approval 47.0 % - 59.0 % Discount for lack of marketability 11.9 % - 12.5 % The significant unobservable inputs used in the valuation models during the three months ended June 30, 2023, to measure the fair value of the PIPE Warrants and PIPE Units are as follows: PIPE Unit Liability PIPE Warrant Liability Valuation date June 22, 2023 (Issuance Date) June 30, 2023 Expected warrant exercise date July 30, 2024 April 30, 2025 July 30, 2024 April 30, 2025 Black-Scholes inputs Anticipated common stock price (at exercise date) $ 10.40 $ 10.07 $ 10.39 $ 10.07 Expected term (in years) 1.11 1.86 1.08 1.84 Expected volatility 75.0 % 74.8 % 75.6 % 74.5 % Other significant unobservable inputs Implied probabilities of occurrence of FDA approval 30.5 % 34.6 % Discount for lack of marketability 5.0 % 0 % The significant unobservable inputs used in the valuation models during the three months ended September 30, 2023, to measure the fair value of the PIPE Warrants and PIPE Units are as follows: PIPE Warrant Liability Valuation date September 30, 2023 Expected warrant exercise date July 30, 2024 April 30, 2025 Black-Scholes inputs Anticipated common stock price (at exercise date) $ 11.00 $ 9.50 Expected term (in years) 0.83 1.58 Expected volatility 77.6 % 71.7 % Other significant unobservable inputs Implied probabilities of occurrence of FDA approval 65.0 % Discount for lack of marketability 0 % The following table provides a rollforward of the aggregate fair values of the Company’s PIPE Warrant liability and PIPE Unit liability, for which fair values are determined using Level 3 inputs (in thousands): PIPE Unit Liability PIPE Warrant Liability Balance at December 31, 2022 $ — $ — Issuances during period 25 11,098 Change in fair value ( 9,668 ) 9,970 Settlement of PIPE Unit liability upon Second Closing 9,643 — Balance at September 30, 2023 $ — $ 21,068 Marketable securities The following tables summarize the amortized cost and fair value of the Company's available-for-sale marketable securities (in thousands): September 30, 2023 Amortized Gross Gross Credit Losses Fair Value U.S. government securities $ 17,470 $ 3 $ ( 10 ) $ — $ 17,462 U.S. treasuries 18,731 — ( 5 ) — $ 18,726 Corporate bonds 15,415 — ( 75 ) — $ 15,340 Commercial paper 50,448 — ( 32 ) — 50,416 Total $ 102,064 $ 3 $ ( 122 ) $ — $ 101,944 December 31, 2022 Amortized Gross Gross Credit Losses Fair Value U.S. government securities $ 7,188 $ 1 $ ( 6 ) $ — $ 7,183 U.S. treasuries 6,599 1 — — $ 6,600 Corporate bonds 25,656 — ( 180 ) — $ 25,476 Commercial paper 62,038 3 ( 52 ) — 61,989 Total $ 101,481 $ 5 $ ( 238 ) $ — $ 101,248 All investments in an unrealized loss position were in this position for less than 12 months. The Company evaluated its securities for potential impairment and considered the decline in market value to be primarily attributable to current economic and market conditions. Additionally, the Company does not intend to sell the securities in an unrealized loss position and does not expect it will be required to sell the securities before recovery of the unamortized cost basis. Given the Company's intent and ability to hold such securities until recovery, and the lack of a significant change in credit risk for these investments, the Company does not consider these investments to be impaired as of September 30, 2023. There were no realized gains or losses recognized on investments in the statement of operations for the three and nine months ended September 30, 2023. The following table summarizes the Company’s available-for-sale marketable securities by contractual maturity, as of September 30, 2023, and December 31, 2022 (in thousands): September 30, December 31, Maturities in one year or less $ 97,086 $ 98,277 Maturities between one and three years 4,858 2,971 Total $ 101,944 $ 101,248

Inventory

Inventory	9 Months Ended
	Sep. 30, 2023
Inventory Disclosure [Abstract]
Inventory	4. Inventory Inventory consists of the following (in thousands): September 30, December 31, 2023 2022 Raw materials $ 983 $ 3,193 Work in process 5,655 207 Total inventory $ 6,638 $ 3,400 The Company currently has $ 5.4 million of inventory capitalized related to Libmeldy in Europe that is currently seeking regulatory approval for certain process modifications.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	9 Months Ended
	Sep. 30, 2023
Prepaid Expense and Other Assets, Current [Abstract]
Prepaid Expenses and Other Current Assets	. Prepaid expenses and other current assets Prepaid expenses and other current assets consist of the following (in thousands): September 30, December 31, 2023 2022 Prepaid external research and development expenses $ 1,797 $ 881 Other prepayments 2,199 1,817 VAT receivable 894 1,077 Non-trade receivables 1,659 1,851 Rent deposits 902 960 Total prepaid expenses and other current assets $ 7,451 $ 6,586

Research and Development Tax Cr

Research and Development Tax Credit Receivable	9 Months Ended
	Sep. 30, 2023
Research and Development Tax Credit Receivable [Abstract]
Research and Development Tax Credit Receivable	6. Research and development tax credit receivable The following table below outlines the changes to the research and development tax credit receivable, including amounts recognized as an offset to research and development expense during the periods (amounts in thousands): Three Months Ended Nine Months Ended 2023 2022 2023 2022 Balance at beginning of period $ 10,626 $ 15,176 $ 5,942 $ 30,723 Recognition of credit claims as offset to research and development expense 1,393 1,974 5,764 7,075 Receipt of credit claims — — — ( 18,390 ) Foreign currency translation ( 413 ) ( 1,363 ) ( 100 ) ( 3,621 ) Balance at end of period $ 11,606 $ 15,787 $ 11,606 $ 15,787 The Company's tax credit receivable from the UK government was approximately $ 11.6 million as of the September 30, 2023 . During the three months ended June 30, 2023, the Company recorded $ 2.3 million of additional tax credits related to a change in estimate associated with its U.K. research and development tax credit receivable claim for fiscal year 2022 ($ 8.2 million) and estimate of its 2023 claim through September 30, 2023 ($ 3.4 million). The change in estimate was based on the results of a tax credit analysis associated with the Company’s qualified projects and research and development expenditures completed during the three months ended June 30, 2023 to finalize the 2022 U.K. tax return. This amount was recorded as an offset to research and development expense.

Intangible Assets, Net

Intangible Assets, Net	9 Months Ended
	Sep. 30, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets, Net	. Intangible assets, net Intangible assets, net of accumulated amortization, consisted of the following (in thousands): As of September 30, 2023 Cost Accumulated Amortization Net License milestones $ 4,088 $ ( 767 ) $ 3,321 Total $ 4,088 $ ( 767 ) $ 3,321 As of December 31, 2022 Cost Accumulated Amortization Net License milestones $ 4,069 $ ( 509 ) $ 3,560 Total $ 4,069 $ ( 509 ) $ 3,560 License intangibles consist of capitalized milestone payments made upon receiving regulatory approval of Libmeldy in Europe. The license intangibles are being amortized on a straight-line basis over the remaining useful life of approximately twelve years . For the three and nine months ended September 30, 2023 , amortization of intangible assets totaled $ 0.1 million and $ 0.3 million, respectively. For the three and nine months ended September 30, 2022 , amortization of intangible assets totaled $ 0.1 million and $ 0.3 million, respectively. The effect of foreign currency translation on the net carrying value of intangible assets for the three and nine months ended September 30, 2023 , was less than $ 0.1 million. The effect of foreign currency translation on the net carrying value of intangible assets for each of the three and nine months ended September 30, 2022 , was $ 0.2 million and $ 0.4 million, respectively. T he following table summarizes the estimated future amortization for intangible assets for the next five years and thereafter (in thousands): As of September 30, 2023 (remaining three months) $ 86 2024 347 2025 347 2026 347 2027 347 Thereafter 1,847 Total $ 3,321

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended
	Sep. 30, 2023
Accrued Liabilities and Other Liabilities [Abstract]
Accrued Expenses and Other Current Liabilities	8. Accrued expenses and other current liabilities Accrued expenses and other current liabilities consisted of the following (in thousands): September 30, December 31, 2023 2022 Accrued external research and development expenses $ 9,915 $ 11,230 Accrued payroll and related expenses 9,710 12,312 Accrued professional fees 4,126 2,263 Accrued other 2,334 2,647 Accrued governmental rebates 3,910 2,300 Strimvelis liability - current portion — 3,685 Total accrued expenses and other current liabilities $ 29,995 $ 34,437

Notes Payable

Notes Payable	9 Months Ended
	Sep. 30, 2023
Debt Disclosure [Abstract]
Notes Payable	. Notes payable In May 2019 , the Company entered into a senior term facilities agreement, which was amended in April 2020 (the “Original Credit Facility”) with MidCap Financial (Ireland) Limited (“MidCap Financial”), as agent, and additional lenders from time to time (together with MidCap Financial, the “Lenders”), to borrow up to $ 75.0 million in term loans. In May 2021, the Company amended and restated the Original Credit Facility (the “Amended Credit Facility”). Under the Amended Credit Facility, the Lenders agreed to make term loans available to the Company in the aggregate amount of $ 100.0 million, including increasing the principal on the initial term loan to $ 33.0 million, from $ 25.0 million. In January 2023, the Company amended and restated the credit facility to change the reference rate from the London Interbank Offered Rate ("LIBOR") to the Secured Overnight Financing Rate ("SOFR"). As of September 30, 2023 , the Company has borrowed $ 33.0 million under the Amended Credit Facility. In March 2023, the Company notified MidCap Financial of its voluntary cancellation of each Lender's remaining commitments under the Amended Credit Facility. As a result of this notice, the Company can no longer draw on the remaining $ 67.0 million that was available on the Amended Credit Facility. Additionally, MidCap agreed to waive certain restrictive covenants of the Amended Credit Facility, specifically related to restrictions on the Company's ability to dispose of intellectual property related to deprioritized products. The Company's borrowings under the Amended Credit Facility bear interest at an annual rate equal to 5.95 % plus SOFR plus a 0.10 % annual increase to the annual rate. The Company was required to make interest only payments on the term loan for 18 months following the date of the Amended Credit Facility. The term loan under the Amended Credit Facility began amortizing on the 18-month anniversary of the Amended Credit Facility (December 2022), with the Company commencing equal monthly payments of principal plus interest to the Lenders to be made in consecutive monthly installments until the loan maturity date in May 2026. In addition, a final payment of 3.5 % is due on the loan maturity date. The Company is accruing the final payment amount of $ 1.2 million associated with the Amended Credit Facility, to outstanding debt by charges to interest expense using the effective-interest method from the date of issuance through the loan maturity date. The Amended Credit Facility includes affirmative and negative covenants. The affirmative covenants include, among others, covenants requiring the Company to maintain their legal existence and governmental approvals, deliver certain financial reports, maintain insurance coverage, maintain property, pay taxes, satisfy certain requirements regarding accounts and comply with laws and regulations. The negative covenants include, among others, restrictions on the Company transferring collateral, incurring additional indebtedness, engaging in mergers or acquisitions, paying dividends or making other distributions, making investments, creating liens, amending material agreements and organizational documents, selling assets, changing the nature of the business and undergoing a change in control, subject to certain exceptions. Notes payable consist of the following (in thousands): September 30, December 31, 2023 2022 Notes payable, net of issuance costs $ 24,961 $ 31,970 Less: current portion ( 9,429 ) ( 9,429 ) Notes payable, net of current portion 15,532 22,541 Accretion related to final payment 630 450 Notes payable, long term $ 16,162 $ 22,991 As of September 30, 2023, the estimated future principal payments due are as follows (in thousands): Aggregate 2023 (remaining three months) $ 2,357 2024 9,429 2025 9,429 2026 5,084 Total 26,299 Less current portion ( 9,429 ) Less unamortized portion of final payment ( 525 ) Less unamortized debt issuance costs ( 183 ) Notes payable, long term $ 16,162 During the three months ended September 30, 2023 and 2022, the Company recognized $ 0.8 million and $ 0.8 million of interest expense related to the term loan, respectively. During the nine months ended September 30, 2023 and 2022, the Company recognized $ 2.7 million and $ 2.1 million of interest expense related to the term loan, respectively. The effective annual interest rate as of September 30, 2023, was approximately 12.62 %.

Shareholders' Equity

Shareholders' Equity	9 Months Ended
	Sep. 30, 2023
Equity [Abstract]
Shareholders' Equity	10. Shareholders ’ equity The Company ’ s ordinary shares are divided into two classes: (i) Ordinary Shares and (ii) Non-Voting Ordinary Shares. The Non-Voting Ordinary Shares have the same rights and restrictions, shall be subject to the same obligations and liabilities, and shall otherwise rank pari passu in all respects with the Ordinary Shares except as set out below: • a holder of Non-Voting Ordinary Shares shall, in relation to the Non-Voting Ordinary Shares held by such holder, have no right to receive notice of, or to attend or vote at, any general meeting of shareholders (save in relation to a variation of class rights of the Ordinary Shares); and • the Non-Voting Ordinary Shares shall be non-transferable. A holder of the Non-Voting Ordinary Shares may elect to have some or all of such holder’s Non-Voting Ordinary Shares redesignated as Ordinary Shares by providing a written notice in a form reasonably acceptable to the Company, specifying the number of Non-Voting Ordinary Shares it wishes to have redesignated as Ordinary Shares. Upon the redesignation of the Non-Voting Ordinary Shares to Ordinary Shares, such Ordinary Shares shall rank pari passu with the other Ordinary Shares of the Company in all respects. The holders of Non-Voting Shares are subject to certain ownership restrictions as described in the Securities Purchase Agreement, dated March 6, 2023 (the "Purchase Agreement"), including that no purchaser of Non-Voting Ordinary Shares may redesignate such shares if the holder would own in excess of 19.99 % of the total number of Ordinary Shares of the Company (including Ordinary Shares that may be represented by ADSs) outstanding immediately after giving effect to such redesignation. At the Company's Annual General Meeting of Shareholders in June 2023, the Company received authority to allot (1) up to a maximum nominal value of £ 15,158,359 to any new shares (including both Ordinary Shares and Non-Voting Ordinary Shares) pursuant to the Purchase Agreement (the "PIPE Authority") and (2) up to a maximum nominal value of £ 18,398,450 to any new shares (including both Ordinary Shares and Non-Voting Ordinary Shares) (the "General Authority"). The General Authority was in addition to the previous authority granted to the Board of Directors on June 16, 2021 to grant shares up to a maximum aggregate nominal amount of £ 13,023,851.50 , of which approximately £ 5,526,095.70 remains (the "2021 Authority"). As of September 30, 2023 , the Company had 45,705,326 Non-Voting Ordinary shares issued and outstanding and had 181,567,883 Ordinary Shares issued and outstanding. As of September 30, 2023 , the Company has a remaining nominal value of £ 10,908,359 it can issue in new shares under the PIPE Authority and £ 23,924,546 it can issue in any new shares under the General Authority. Holders of Ordinary Shares may convert such shares to ADSs. The ADSs are listed on The Nasdaq Capital Market, and each ADS represents ten Ordinary Shares. We also have in issue one deferred share with a nominal value of £ 4.89687 . To date, the Company has not declared any dividends. Under English law, a company’s accumulated realized profits must exceed its accumulated realized losses (on a non-consolidated basis) before dividends can be declared and paid. Therefore, the Company must have distributable profits before declaring and paying a dividend.

Share-based Compensation

Share-based Compensation	9 Months Ended
	Sep. 30, 2023
Share-Based Payment Arrangement [Abstract]
Share-based Compensation	11 . Share-based compensation The Company maintains four equity compensation plans: the Orchard Therapeutics Limited Employee Share Option Plan with Non-Employee Sub-Plan and U.S. Sub-Plan (the “2016 Plan”), the Orchard Therapeutics plc 2018 Share Option and Incentive Plan (the “2018 Plan”), the 2018 Employee Share Purchase Plan (the “ESPP”), and the 2020 Inducemen t Equity Plan (the “Inducement Plan”). The board of directors has determined it will not make any further awards under the 2016 plan. As of September 30, 2023, there were 12,748,750 ordinary shares available for grant under the 2018 Plan, 730,500 ordinary shares available for grant under the Inducement Plan, and 1,460,590 ordinary shares available for grant under the ESPP. The number of options and restricted stock units, the weighted average grant date fair value per stock option and per share, and the weighted average exercise price in the tables below are all shown below on a per ordinary share basis. The Company’s ADSs that are listed on the NASDAQ Capital Market each represent ten ordinary shares. Share option activity Share options generally vest over a four-year period. The following table summarizes option activity for ordinary shares under the plans for the nine months ended September 30, 2023: Ordinary Shares Weighted Outstanding at December 31, 2022 16,424,167 $ 1.56 Granted 5,184,380 0.49 Exercised ( 370,430 ) 0.16 Forfeited ( 1,068,387 ) 3.32 Outstanding at September 30, 2023 20,169,730 $ 1.21 Vested and expected to vest at September 30, 2023 20,169,730 $ 1.22 Exercisable at September 30, 2023 11,303,310 $ 1.73 The total intrinsic value of options exercised was $ 0.1 million for the three and nine months ended September 30, 2023. T he weighted-average grant date fair value of ordinary share options granted during the nine months ended September 30, 2023 and 2022, was $ 0.28 and $ 0.32 per ordinary share, respectively . As of September 30, 2023, total unrecognized compensation cost related to options was $ 8.1 million. This amount is expected to be recognized over a weighted average period of 2.40 years. Restricted share units Performance-based restricted share units In April 2020, the Company granted 195,000 performance-based restricted share units (“RSUs”), representing 19,500 ADSs, with a total grant date fair value of $ 1.4 million to its Chief Executive Officer, Bobby Gaspar, M.D., Ph.D. Dr. Gaspar earns one-third of the award for each of the first three to occur of four milestones ( 65,000 RSUs are earned for each achieved milestone). The milestones relate to specific clinical and regulatory goals which have to be achieved before December 31, 2023 in order for the underl ying RSUs to be eligible to vest. Vesting for any earned shares occurs on January 2, 2024, so long as Dr. Gaspar remains continuously employed with the Company through that date. The Company determined that two of the milestones were probable of being achieved and recognized stock-compensation expense of $ 0.1 million and $ 0.9 million for the three and nine months ended September 30, 2023. Time-based restricted share units Time-based restricted share units generally vest in equal annual installments over a three-year period. Restricted share unit activity The following table summarizes award activity for the nine months ended September 30, 2023: Performance-based RSUs Time-based RSUs Total RSUs Weighted Average Grant Date Fair Value per Ordinary Share Unvested at December 31, 2022 195,000 1,868,876 2,063,876 $ 0.55 Granted — 1,720,200 1,720,200 0.46 Vested — ( 649,400 ) ( 649,400 ) 0.57 Forfeited — ( 304,836 ) ( 304,836 ) 0.46 Unvested at September 30, 2023 195,000 2,634,840 2,829,840 $ 0.55 As of September 30, 2023, total unrecognized compensation cost related to time-based RSUs was $ 0.9 million. This amount is expected to be recognized over a weighted average period of 1.86 years. As of September 30, 2023, the total unrecognized compensation cost related to performance-based RSUs for which vesting was deemed probable was $ 0.1 million, which is expected to be recognized over a period of 0.25 years. As of September 30, 2023, the total unrecognized compensation cost related to performance-based RSUs for which vesting was deemed not probable was $ 0.5 million, the timing of recognition will be dependent upon achievement of remaining milestones. Share-based compensation expense Share-based compensation expense related to share options, restricted share unit awards, and the employee stock purchase plan was classified in the unaudited condensed consolidated statements of operations and comprehensive loss as follows (in thousands): Three Months Ended Nine Months Ended 2023 2022 2023 2022 Research and development $ 966 $ 1,445 $ 3,227 $ 5,151 Selling, general and administrative 1,502 2,023 5,331 6,933 Total $ 2,468 $ 3,468 $ 8,558 $ 12,084 Total share-based compensation by award type was as follows (in thousands): Three Months Ended Nine Months Ended 2023 2022 2023 2022 Ordinary share options $ 2,203 $ 3,260 $ 7,011 $ 11,531 Restricted ordinary share units 265 208 1,547 553 Total $ 2,468 $ 3,468 $ 8,558 $ 12,084

Private Placement Transaction

Private Placement Transaction	9 Months Ended
	Sep. 30, 2023
Equity [Abstract]
Private placement transaction	12. Private placement transaction On March 6, 2023, the Company entered into the SPA with several investors, pursuant to which the Company agreed to sell, in an unregistered offering (the “2023 Private Placement”), up to an aggregate of (i) 99,166,900 ordinary shares and non-voting ordinary shares, nominal value £ 0.10 per share (collectively, the “Shares”) and (ii) warrants to purchase an aggregate of 109,083,590 ordinary shares or non-voting ordinary shares. The 2023 Private Placement consisted of two closings, referred to herein as the "First Closing" and "Second Closing". At each closing, the Shares were sold in fixed combinations with the warrants as units, with each unit consisting of 10 Shares and 1 accompanying warrant to purchase 11 shares . The warrants are contingently exercisable for 30 days following (i) the Company’s public announcement of its receipt of marketing approval of its biologics license application (“BLA”) submitted to the FDA with respect to OTL-200 (the “Vesting Event”) and (ii) receipt of shareholder approval to increase the number of authorized shares. The shareholder approval to increase the number of authorized shares has since been achieved. Each warrant will have an exercise price equal to $ 1.10 per share in the event the Vesting Event occurs on or prior to December 31, 2024, and $ 0.95 per ordinary share or non-voting ordinary share in the event the Vesting Event occurs after December 31, 2024. The warrants will expire at the conclusion of the 30 -day exercise period or on March 10, 2026, whichever is earlier. Each warrant holder has the option, in their sole discretion, to exercise the warrants for either ordinary shares or non-voting ordinary shares. On March 10, 2023, the Company completed the First Closing and issued and sold (i) 40,053,500 ordinary shares, (ii) 16,613,400 non-voting ordinary shares, and (iii) warrants to purchase an aggregate of 62,333,590 ordinary shares or non-voting ordinary shares, at a purchase price of $ 6.00 per unit, generating gross proceeds of $ 34.0 million, before deducting offering expenses of $ 0.9 million payable by the Company. On June 22, 2023, the Company completed the Second Closing and issued and sold (i) 13,408,074 ordinary shares, (ii) 29,091,926 non-voting ordinary shares, and (iii) warrants to purchase an aggregate of 46,750,000 ordinary shares or non-voting ordinary shares, at a purchase price of $ 8.00 per unit (collectively, the “PIPE Units”). The Second Closing was contingent upon (x) the Company’s announcement of its intention to file a BLA submission following receipt of the minutes from the FDA in connection with the Company’s pre-BLA meeting for OTL-200, provided such minutes do not expressly advise the Company not to proceed with a BLA submission, and (y) shareholder approval to increase the number of authorized shares. The Company generated gross proceeds of $ 34.0 million from the Second Closing before deducting offering expenses of $ 1.2 million payable by the Company. The warrants issued in the First Closing and the Second Closing of the 2023 Private Placement are referred to collectively herein as the “PIPE Warrants”. Accounting Analysis Upon execution of the SPA on March 6, 2023, the Company determined that each of the instruments to be issued in the First Closing and Second Closing did not meet the criteria for equity classification as they were not considered indexed to the Company’s stock. The Company recorded these instruments at a fair value of $ 34.0 million, which was comprised of $ 27.5 million, $ 6.4 million, and $ 0.02 million allocated to the Shares, PIPE Warrants, and PIPE Units, respectively. The Company incurred issuance costs of $ 0.5 million in connection with executing the SPA, which were expensed upon execution within other income (expense) on the unaudited condensed consolidated statements of operations and comprehensive loss. Upon the First Closing on March 10, 2023, the Company determined that the ordinary shares met the criteria for equity classification. The PIPE Warrants issued in the First Closing and the PIPE Units that were issued in the Second Closing did not meet the criteria for equity classification due to the lack of sufficient authorized and unissued shares and the price adjustment feature depending on the timing of the occurrence of a Vesting Event. The Company reclassified the ordinary shares to equity and recorded a mark-to-market gain of $ 3.6 million, which represents the change in fair value of the ordinary shares between March 6, 2023, and March 10, 2023. As a result of the First Closing, the Company recorded $ 24.0 million to equity. The Company incurred issuance costs of $ 1.1 million which were contingent on the closing of the SPA. These issuance costs were allocated between the ordinary shares, PIPE Warrants, and PIPE Units, on a relative fair value basis. Issuance costs of $ 0.9 million allocated to the ordinary shares were recognized as a discount to the ordinary shares recorded in permanent equity. Issuance costs of $ 0.2 million allocated to the PIPE Warrants and PIPE Units were expensed within other income (expense) on the unaudited condensed consolidated statement of operations and comprehensive loss. Upon the Second Closing on June 22, 2023, the PIPE Units were settled. The Company recognized a final fair value adjustment to the PIPE Units immediately prior to the Second Closing. The fair value adjustment was a gain of $ 9.7 million which was recorded within other income (expense) on the unaudited condensed consolidated statement of operations and comprehensive loss. Upon issuance of the shares and warrants under the Second Closing, the Company recorded $ 19.7 million to equity, recorded $ 4.6 million to the PIPE Warrants liability, and derecognized the PIPE Units asset of $ 9.6 million. The Company incurred issuance costs of $ 1.2 million. These issuance costs were allocated between the ordinary shares and the PIPE Warrants on a relative fair value basis. Issuance costs of $ 1.0 million allocated to the ordinary shares were recognized as a discount to the ordinary shares recorded in permanent equity. Issuance costs of $ 0.2 million allocated to the PIPE Warrants were expensed within other income (expense) on the unaudited condensed consolidated statement of operations and comprehensive loss. The PIPE Warrants are classified within non-current liabilities on the unaudited condensed consolidated balance sheets as of September 30, 2023, and will be adjusted to fair value at each subsequent balance sheet date until the warrants are reclassified to equity or settled. Changes in the fair value of the PIPE Warrants are recognized as a component of other income (expense) in the unaudited condensed consolidated statement of operations and comprehensive loss. For the three and nine months ended September 30, 2023, the Company recognized changes in fair value resulting in a loss of $ 8.8 million and a loss of $ 10.0 million, respectively, related to the PIPE Warrants. As of September 30, 2023 , warrants to purchase an aggregate of 109,083,590 ordinary shares or non-voting ordinary shares were issued and outstanding, and no ne were exercisable. The PIPE Units are no longer outstanding as of September 30, 2023. Changes in the fair value of the PIPE Units were recognized as a component of other income (expense), net in the unaudited condensed consolidated statement of operations and comprehensive loss. For the nine months ended September 30, 2023, the Company recognized changes in fair value resulting in a gain of $ 9.6 millio n related to the PIPE Units.

Restructuring Charges

Restructuring Charges	9 Months Ended
	Sep. 30, 2023
Restructuring and Related Activities [Abstract]
Restructuring Charges	. Restructuring charges On March 30, 2022, the Company announced its commitment to focus on severe neurometabolic diseases and early research programs, and to discontinue its investment in and seek strategic alternatives for the Company’s programs in rare primary immune deficiencies, including OTL-103 for treatment of Wiskott Aldrich syndrome (“WAS”), OTL-102 for treatment of X-linked chronic granulomatous disease (“X-CGD”), and Strimvelis for adenosine deaminase severe combined immunodeficiency (“ADA-SCID”). The Company recognized a one-time charge in the nine months ended September 30, 2022 of approximately $ 1.7 million, which relates to employee-related termination costs, of which $ 1.4 million and $ 0.3 million was recognized in research and development expenses and selling, general, and administrative expenses, respectively, in the Company's condensed consolidated statements of operations and comprehensive loss. Activity for the three and nine months ended September 30, 2022 is summarized as follows (amounts in thousands): Three Months Ended Nine Months Ended Balance at beginning of period $ 838 $ 6 Charged to expense — 2,481 Non-cash adjustments and foreign currency translation ( 206 ) ( 760 ) Payments made ( 632 ) ( 1,727 ) Balance at end of period $ — $ — There was no restructuring activity in the nine months ended September 30, 2023.

Product Revenue, Net

Product Revenue, Net	9 Months Ended
	Sep. 30, 2023
Revenue from Contract with Customer [Abstract]
Product Revenue, Net	14. Product revenue, net The following table presents the Company's net revenue by product (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 Libmeldy $ 5,559 $ 4,781 $ 12,744 $ 12,985 Strimvelis — 596 — 1,232 Total $ 5,559 $ 5,377 $ 12,744 $ 14,217 Activity in each of the product revenue allowance and reserve categories for Libmeldy is summarized as follows (in thousands): Trade discounts and allowances Government rebates Total Balance as of December 31, 2022 $ 4,390 $ 2,300 $ 6,690 Provision Related to sales in the current year 5,191 1,655 6,846 Credits and payments made during the period ( 8,421 ) — ( 8,421 ) Foreign currency translation — ( 45 ) ( 45 ) Balance as of September 30, 2023 $ 1,160 $ 3,910 $ 5,070 The total reserves described above are summarized as components of the Company's condensed consolidated balance sheets as follows (in thousands): As of As of Reduction of accounts receivable, net $ 1,160 $ 4,390 Component of accrued expenses and other current liabilities 3,910 2,300 Total revenue-related reserves $ 5,070 $ 6,690

Collaboration Revenue

Collaboration Revenue	9 Months Ended
	Sep. 30, 2023
Collaborative Arrangement [Abstract]
Collaboration Revenue	15. Collaboration revenue On July 1, 2021, the Company entered into a strategic collaboration with Pharming Group N.V. (“Pharming”) to research, develop, manufacture, and commercialize OTL-105, an investigational ex vivo autologous HSC gene therapy for the treatment of hereditary angioedema (HAE), a life-threatening rare disorder that causes recurring swelling attacks in the face, throat, extremities and abdomen (the “Collaboration Agreement”). Under the terms of the Collaboration Agreement, Pharming was granted worldwide rights to OTL-105 and will be responsible for clinical development, regulatory filings and commercialization of the investigational gene therapy, including associated costs. The Company will lead the completion of pre-clinical IND-enabling activities and oversee manufacturing of OTL-105 during pre-clinical and clinical development, which will be funded by Pharming. In addition, both the Company and Pharming will explore the application of less toxic conditioning regimen for use with OTL-105 administration. The Company received an upfront payment of $ 10.0 million in cash from Pharming and will receive cost reimbursements for ongoing research, development, and manufacturing services to be provided under the Collaboration Agreement. The Company is also eligible to receive up to $ 189.5 million in development, regulatory and sales milestones as well as mid-single to low double-digit percentage royalty payments on future worldwide sales. The Company also entered into a Share Purchase Agreement with Pharming on July 1, 2021 (the “Pharming SPA”), pursuant to which the Company issued 1,227,738 ordinary shares to Pharming for total consideration of $ 7.5 million. The consideration is payment for the fair value of ordinary shares with a fair value of $ 4.1 million plus a $ 3.4 million premium on the fair value of the Company’s ordinary shares. The “Collaboration Agreement” and the “Pharming SPA” are referred to together as the “Pharming Agreements.” Accounting Analysis At the commencement of the arrangement, two units of accounting were identified, which include: 1) the issuance of 1,227,738 of the Company’s ordinary shares as part of the Pharming SPA and 2) the license and collaboration agreement, which conveys the license and provides for the Company to provide research, development, manufacturing services for OTL-105. The Pharming Agreements were entered into concurrently as part of a single commercial objective and the Company considers them a single arrangement for accounting purposes. The total upfront payments of $ 17.5 million are comprised of $ 4.1 million attributed to the equity sold to Pharming and $ 13.4 million attributed to the Collaboration Agreement. The Company accounts for the Collaboration Agreement component within the scope of ASC Topic 606, Revenue from Contracts with Customers . The Company has concluded that the conveyance of the license for the HAE program and the provision of research, development, and manufacturing services for the HAE program represent a series of distinct services that are accounted for as a single performance obligation within the Collaboration Agreement. The Company determined that the transaction price includes: the $ 13.4 million attributed to the Collaboration Agreement and the variable consideration for estimated reimbursement payments at agreed upon contractual rates to be received from Pharming for the Company’s on-going research, development, and manufacturing services. There is uncertainty as to whether the future milestone payments will ultimately be achieved. Accordingly, the Company has fully constrained the variable consideration associated with the milestones. The royalty payments were also excluded from the transaction price as the Company is not required to estimate variable consideration for the royalty payments. The Company re-evaluates the transaction price as of the end of each reporting period. The Company recognizes revenue associated with the performance obligation as the research, development, and manufacturing services are provided using an input method, based on the cumulative costs incurred compared to the total estimated costs expected to be incurred to satisfy the performance obligation. The transfer of control to the customer occurs over the time period that the research, development and manufacturing services are provided by the Company. For the three and nine months ended September 30, 2023 , the Company recognized collaboration revenue under the Collaboration Agreement of $ 0.7 million and $ 2.1 million, respectively. For the three and nine months ended September 30, 2022 , the Company recognized collaboration revenue under the Collaboration Agreement of $ 0.4 million and $ 1.5 million, respectively. As of September 30, 2023 there is $ 0.7 million and $ 10.3 million of current and long-term deferred revenue, respectively, related to the Collaboration Agreement.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2023
Income Tax Disclosure [Abstract]
Income Taxes	16. Income taxes The Company recorded an income tax expense of $ 1.0 million and an income tax benefit of $ 1.2 million for the nine months ended September 30, 2023 and 2022, respectively. The Company recorded income tax expense of $ 1.0 million and an income tax benefit of $ 1.1 million for the three months ended September 30, 2023 and 2022, respectively. The Company records no income tax benefits for the net operating losses incurred in each period in the U.K due to the uncertainty regarding the realizability of the deferred tax asset . The Company's income tax relates to its subsidiaries in Europe and the U.S. The Company's income tax computed at its effective income tax rate for the three and nine months ended September 30, 2023 and 2022 differed from income taxes computed at the U.K. statutory tax rate primarily due to the U.S. deduction for foreign derived intangible income, U.S. tax credits, share-based compensation, and provision to return adjustments recorded for the US and certain European subsidiaries.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	17. Commitments and contingencies Legal proceedings The Company is not a party to any material litigation and does not have contingency reserves established for any litigation liabilities. Manufacturing and technology development master agreement with AGC Biologics The Company is party to an Agreement with AGC Biologic S.p.A (“AGC”) pursuant to which the Company is obligated to pay AGC for a minimum product manufacturing commitment, dedicated manufacturing and development resources, and for a lease component associated with the right of use of exclusive manufacturing suites within AGC’s existing facilities. The following table outlines the current commitments associated with the agreement, as of September 30, 2023 (amounts in thousands): Due in: Product manufacturing commitments Dedicated manufacturing and development resources Exclusive transduction suites Total AGC Commitment 2023 (remaining three months) $ 486 $ 1,423 $ — $ 1,909 2024 1,946 6,450 2,162 10,558 2025 973 3,225 1,081 5,279 Total manufacturing commitments $ 3,405 $ 11,098 $ 3,243 $ 17,746 Tabular disclosure above has been translated to U.S. Dollar, from Euro, using the period end exchange rate of €1.00 to $ 1.08 . 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 senior management 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 aware of any claims under indemnification arrangements, and it has not accrued any liabilities related to such obligations in its unaudited condensed consolidated financial statements as of September 30, 2023 .

Net loss per ordinary share

Net loss per ordinary share	9 Months Ended
	Sep. 30, 2023
Earnings Per Share [Abstract]
Net Loss per Ordinary Share	18 . Net loss per ordinary share The Company is in a loss position for all periods presented. As such, the basic net loss per ordinary share is the same as the diluted net loss per ordinary share as the inclusion of all potential ordinary share equivalents outstanding would have been anti-dilutive. The following securities, presented based on amounts outstanding at each period end, are considered to be ordinary share equivalents, but were excluded from the computation of diluted net loss per ordinary share because to do so would have been anti-dilutive: September 30, 2023 2022 Share options 16,822,522 13,134,055 Unvested restricted share units 3,019,163 2,326,543 Ordinary shares to be issued upon exercise of warrants (1) 109,083,590 — 128,925,275 15,460,598 (1) The warrants included in the table are exercisable subject to future contingencies (See Note 12)

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2023
Subsequent Events [Abstract]
Subsequent Event	19 . Subsequent Event On October 5, 2023, the Company entered into a Transaction Agreement (the “Transaction Agreement”) with Kyowa Kirin Co., Ltd., a Japanese joint stock company (kabushiki kaisha) (“Kyowa Kirin”), pursuant to which, subject to the satisfaction or waiver of the conditions set forth therein, Kyowa Kirin will (either directly or via a nominee) acquire the entire issued and outstanding share capital of Orchard pursuant to a court-sanctioned scheme of arrangement. Pursuant to the Transaction Agreement, at closing (the “Effective Time”), all outstanding shares of the Company (including ordinary shares, non-voting ordinary shares and ADSs) then issued and outstanding will be transferred to Kyowa Kirin (or its nominee). Holders of shares at the Effective Time will be entitled to receive (a) $ 16.00 in cash, without interest, per ADS, less certain fees assessed by the ADS depositary, or $ 1.60 in cash, without interest per ordinary share or non-voting ordinary share, plus (b) 10 contingent value rights (each, a “CVR”) per ADS, or one CVR per ordinary share or non-voting ordinary share, together representing a right to receive $ 0.10 per CVR, without interest, if regulatory approval is granted by the FDA for the commercial marketing and sale of OTL-200 in the United States for the treatment of MLD, which includes as an intended patient population both (x) pediatric patients with late infantile forms of MLD, and (y) pediatric patients with early juvenile forms of MLD, in each case that are pre-symptomatic or without clinical manifestations of the disease, on or before 11:59 p.m. Eastern Time on December 31, 2024. The transaction has been unanimously approved by both company’s Boards of Directors and is expected to close in the first quarter of 2024, subject to Orchard shareholder approval, receipt of applicable regulatory approvals and other customary closing conditions. Warrants outstanding as of the date of the Transaction Agreement will continue to be satisfied in accordance with their terms. Following the completion of the acquisition, the Company will become a wholly-owned subsidiary of Kyowa Kirin and will cease to be a Nasdaq-listed company.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2023
Accounting Policies [Abstract]
Basis of presentation	Basis of presentation These condensed consolidated financial statements of the Company are unaudited and have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial reporting and in accordance with Regulation S-X, Rule 10-01. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (“ASC”), and Accounting Standards Update (“ASU”), of the Financial Accounting Standards Board (“FASB”). These unaudited condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements and accompanying notes included in the Company’s Annual Report. The condensed consolidated balance sheet as of December 31, 2022, was derived from audited consolidated financial statements included in the Company’s Annual Report but does not include all disclosures required by GAAP. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted from these interim financial statements. However, these interim financial statements include all adjustments, consisting only of normal recurring adjustments, which are, in the opinion of the Company’s management, necessary to fairly state the results of the interim period. The interim results are not necessarily indicative of results to be expected for the full year. Amounts reported are presented in thousands, except percentages, per share amounts or as otherwise noted. As a result, certain totals may not sum due to rounding.
Principles of consolidation	Principles of consolidation The accompanying unaudited condensed consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.
Use of estimates	Use of estimates The preparation of unaudited 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 unaudited condensed consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these unaudited condensed consolidated financial statements include, but are not limited to, the accrual for research and development expenses, the research and development tax credit receivable, share-based compensation, collaboration agreement milestones, variable consideration in revenue recognition, operating lease assets and liabilities, valuation of PIPE Warrant (as defined in Note 12) and PIPE Unit liabilities (as defined in Note 12), and income taxes. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.
Foreign currency	Foreign currency The financial statements of the Company’s subsidiaries with functional currencies other than the U.S. dollar are translated into U.S. dollars using period-end exchange rates for assets and liabilities, historical exchange rates for shareholders’ equity and weighted average exchange rates for operating results. Unrealized losses are driven primarily by intercompany balances denominated in currencies other than the functional currency of the entity with the intercompany balance, and typically fluctuates concurrently with fluctuations in the U.S. dollar, British pounds sterling, and Euro exchange rates. Translation gains and losses are included in accumulated other comprehensive income (loss) in shareholders’ equity. Foreign currency transaction gains and losses are included in other income (expense), net in the results of operations. The Company recorded realized and unrealized foreign currency transactio n gains of $ 3.0 million for the nine months ended September 30, 2023 , and realized and unrealized foreign currency transaction losses of $ 47.1 million for the nine months ended September 30, 2022 , which is included in other income (expense), net in the condensed consolidated statements of operations and comprehensive loss.
Accounts receivable	Accounts receivable Accounts receivable arise from product revenue and amounts due from the Company's collaboration partners and have payment terms that generally require payment within 30 to 90 days. For some Libmeldy customers, our payment terms can range from 30 days to under one year . The amount from product revenue represents amounts due from customers and treatment centers in Europe, which are recorded net of reserves for trade discounts and allowances, and other incentives to the extent such amounts are payable to the customer by the Company. The Company monitors economic conditions to identify facts or circumstances that may indicate that its receivables are at risk of collection. The Company provides reserves against accounts receivable for estimated losses, if any, that may result from a customer's inability to pay based on the composition of its accounts receivable, current economic conditions, and historical credit loss activity. Amounts determined to be uncollectible are charged or written-off against the reserve. During the three and nine months ended September 30, 2023 , the Company did no t record any expected credit losses related to outstanding accounts receivable.
Restricted cash	Restricted cash Cash and cash equivalents that are restricted as to withdrawal or use under the terms of certain contractual agreements are recorded as restricted cash on the Company’s condensed consolidated balance sheets. The Company has an outstanding letter of credit for $ 3.0 million associated with a lease and is required to hold this amount in a standalone bank account, as of September 30, 2023, and December 31, 2022 . The Company is also contractually required to maintain cash collateral accounts associated with corporate credit cards and other leases in the amount of $ 1.3 million at September 30, 2023 and December 31, 2022. Cash, cash equivalents, and restricted cash were comprised of the following (in thousands): As of September 30, As of December 31, 2023 2022 Cash and cash equivalents $ 23,469 $ 68,424 Restricted cash 4,215 4,215 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 27,684 $ 72,639
Recently adopted accounting pronouncements	Recently adopted accounting pronouncements In March 2020, the FASB issued ASU 2020-04 , Reference Rate Reform (Topic 848) : Facilitation of the Effects of Reference Rate Reform on Financial Reporting and issued two subsequent amendments: ASU 2021-01, issued in January 2021, refines the scope of ASU and clarifies some of its guidance as part of the FASB’s monitoring of global reference rate reform activities and ASU 2022-06, issued in December 2022, which extends the effective period of the ASU through December 31, 2023 (collectively, including ASU 2020-04, “ASC 848”). ASC 848 provides temporary optional expedients and exceptions to the GAAP guidance on contract modifications and hedge accounting to ease the financial reporting burdens related to the expected market transition from the London Interbank Offered Rate (“LIBOR”) and other interbank offered rates to alternative reference rates. ASC 848 is effective for all entities as of March 12, 2020, through December 31, 2024, at which time transition is expected to be complete. The Company adopted this standard as of January 1, 2023 . The application of this ASU did not have a material impact on the Company's financial position, results of operations, or cash flows.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2023
Accounting Policies [Abstract]
Schedule of Cash, Cash Equivalents and Restricted Cash	Cash, cash equivalents, and restricted cash were comprised of the following (in thousands): As of September 30, As of December 31, 2023 2022 Cash and cash equivalents $ 23,469 $ 68,424 Restricted cash 4,215 4,215 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 27,684 $ 72,639

Fair Value Measurements and M_2

Fair Value Measurements and Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Schedule of Cash Equivalents and Marketable Securities	The following tables present information about the Company’s financial assets and liabilities that have been measured at fair value as of September 30, 2023, and December 31, 2022, and indicate hierarchy of valuation inputs utilized to determine such fair value (in thousands): Fair Value Measurements as of Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 7,285 $ — $ — $ 7,285 Total cash equivalents $ 7,285 $ — $ — $ 7,285 Marketable securities U.S. government securities $ — $ 17,462 $ — $ 17,462 U.S. treasuries — 18,726 — 18,726 Corporate bonds — 15,340 — 15,340 Commercial paper — 50,416 — 50,416 Total marketable securities $ — $ 101,944 $ — $ 101,944 Total Assets $ 7,285 $ 101,944 $ — $ 109,229 Liabilities PIPE Warrant liability $ — $ — $ 21,068 $ 21,068 Total Liabilities $ — $ — $ 21,068 $ 21,068 Fair Value Measurements as of Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 1,239 $ — $ — $ 1,239 U.S. treasuries — 6,600 — 6,600 U.S. government securities — 5,200 — 5,200 Commercial paper — 14,122 — 14,122 Total cash equivalents $ 1,239 $ 25,922 $ — $ 27,161 Marketable securities U.S. government securities $ — $ 1,984 $ — $ 1,984 Corporate bonds — 25,475 — 25,475 Commercial paper — 47,867 — 47,867 Total marketable securities $ — $ 75,326 $ — $ 75,326 Total Assets $ 1,239 $ 101,248 $ — $ 102,487
Schedule of Significant Unobservable Inputs used in Valuation Model to Measure PIPE Warrants and PIPE Units Liabilities	T he significant unobservable inputs used in the valuation model to measure the PIPE Warrant and PIPE Unit liabilities are as follows at the issuance date and as of March 31, 2023: PIPE Unit Liability PIPE Warrants March 10, 2023 March 31, 2023 March 10, 2023 March 31, 2023 Black-Scholes inputs (Issuance date) (Issuance date) Anticipated common stock price (at exercise date) $ 10.14 - $ 10.46 $ 10.23 - $ 10.49 $ 10.00 - $ 10.31 $ 10.13 - $ 10.38 Expected term (in years) 1.08 - 1.84 1.08 - 1.84 1.39 - 2.14 1.33 - 2.08 Expected volatility 72.9 % - 76.0 % 71.7 % - 77.4 % 72.9 % - 76.0 % 71.7 % - 77.4 % Other significant unobservable inputs Implied probabilities of occurrence of Second Closing and FDA Approval 47.0 % - 59.0 % Discount for lack of marketability 11.9 % - 12.5 % The significant unobservable inputs used in the valuation models during the three months ended June 30, 2023, to measure the fair value of the PIPE Warrants and PIPE Units are as follows: PIPE Unit Liability PIPE Warrant Liability Valuation date June 22, 2023 (Issuance Date) June 30, 2023 Expected warrant exercise date July 30, 2024 April 30, 2025 July 30, 2024 April 30, 2025 Black-Scholes inputs Anticipated common stock price (at exercise date) $ 10.40 $ 10.07 $ 10.39 $ 10.07 Expected term (in years) 1.11 1.86 1.08 1.84 Expected volatility 75.0 % 74.8 % 75.6 % 74.5 % Other significant unobservable inputs Implied probabilities of occurrence of FDA approval 30.5 % 34.6 % Discount for lack of marketability 5.0 % 0 % The significant unobservable inputs used in the valuation models during the three months ended September 30, 2023, to measure the fair value of the PIPE Warrants and PIPE Units are as follows: PIPE Warrant Liability Valuation date September 30, 2023 Expected warrant exercise date July 30, 2024 April 30, 2025 Black-Scholes inputs Anticipated common stock price (at exercise date) $ 11.00 $ 9.50 Expected term (in years) 0.83 1.58 Expected volatility 77.6 % 71.7 % Other significant unobservable inputs Implied probabilities of occurrence of FDA approval 65.0 % Discount for lack of marketability 0 %
Schedule of Aggregate Fair Values of PIPE Warrant Liability and PIPE Unit Liability	The following table provides a rollforward of the aggregate fair values of the Company’s PIPE Warrant liability and PIPE Unit liability, for which fair values are determined using Level 3 inputs (in thousands): PIPE Unit Liability PIPE Warrant Liability Balance at December 31, 2022 $ — $ — Issuances during period 25 11,098 Change in fair value ( 9,668 ) 9,970 Settlement of PIPE Unit liability upon Second Closing 9,643 — Balance at September 30, 2023 $ — $ 21,068
Schedule of Amortized Cost and Fair Value of Available-for-Sale Marketable Debt Securities	The following tables summarize the amortized cost and fair value of the Company's available-for-sale marketable securities (in thousands): September 30, 2023 Amortized Gross Gross Credit Losses Fair Value U.S. government securities $ 17,470 $ 3 $ ( 10 ) $ — $ 17,462 U.S. treasuries 18,731 — ( 5 ) — $ 18,726 Corporate bonds 15,415 — ( 75 ) — $ 15,340 Commercial paper 50,448 — ( 32 ) — 50,416 Total $ 102,064 $ 3 $ ( 122 ) $ — $ 101,944 December 31, 2022 Amortized Gross Gross Credit Losses Fair Value U.S. government securities $ 7,188 $ 1 $ ( 6 ) $ — $ 7,183 U.S. treasuries 6,599 1 — — $ 6,600 Corporate bonds 25,656 — ( 180 ) — $ 25,476 Commercial paper 62,038 3 ( 52 ) — 61,989 Total $ 101,481 $ 5 $ ( 238 ) $ — $ 101,248
Schedule of Available-for-sale Marketable Securities by Contractual Maturity	The following table summarizes the Company’s available-for-sale marketable securities by contractual maturity, as of September 30, 2023, and December 31, 2022 (in thousands): September 30, December 31, Maturities in one year or less $ 97,086 $ 98,277 Maturities between one and three years 4,858 2,971 Total $ 101,944 $ 101,248

Inventory (Tables)

Inventory (Tables)	9 Months Ended
	Sep. 30, 2023
Inventory Disclosure [Abstract]
Schedule of Inventory	Inventory consists of the following (in thousands): September 30, December 31, 2023 2022 Raw materials $ 983 $ 3,193 Work in process 5,655 207 Total inventory $ 6,638 $ 3,400

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	9 Months Ended
	Sep. 30, 2023
Prepaid Expense and Other Assets, Current [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consist of the following (in thousands): September 30, December 31, 2023 2022 Prepaid external research and development expenses $ 1,797 $ 881 Other prepayments 2,199 1,817 VAT receivable 894 1,077 Non-trade receivables 1,659 1,851 Rent deposits 902 960 Total prepaid expenses and other current assets $ 7,451 $ 6,586

Research and Development Tax _2

Research and Development Tax Credit Receivable (Tables)	9 Months Ended
	Sep. 30, 2023
Research and Development Tax Credit Receivable [Abstract]
Summary of Amounts Recognized to Offset Research and Development Expense	The following table below outlines the changes to the research and development tax credit receivable, including amounts recognized as an offset to research and development expense during the periods (amounts in thousands): Three Months Ended Nine Months Ended 2023 2022 2023 2022 Balance at beginning of period $ 10,626 $ 15,176 $ 5,942 $ 30,723 Recognition of credit claims as offset to research and development expense 1,393 1,974 5,764 7,075 Receipt of credit claims — — — ( 18,390 ) Foreign currency translation ( 413 ) ( 1,363 ) ( 100 ) ( 3,621 ) Balance at end of period $ 11,606 $ 15,787 $ 11,606 $ 15,787

Intangible Assets, Net (Tables)

Intangible Assets, Net (Tables)	9 Months Ended
	Sep. 30, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Intangible Assets Net of Accumulated Amortization	Intangible assets, net of accumulated amortization, consisted of the following (in thousands): As of September 30, 2023 Cost Accumulated Amortization Net License milestones $ 4,088 $ ( 767 ) $ 3,321 Total $ 4,088 $ ( 767 ) $ 3,321 As of December 31, 2022 Cost Accumulated Amortization Net License milestones $ 4,069 $ ( 509 ) $ 3,560 Total $ 4,069 $ ( 509 ) $ 3,560
Schedule of Estimated Future Amortization For Intangible Assets	T he following table summarizes the estimated future amortization for intangible assets for the next five years and thereafter (in thousands): As of September 30, 2023 (remaining three months) $ 86 2024 347 2025 347 2026 347 2027 347 Thereafter 1,847 Total $ 3,321

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2023
Accrued Liabilities and Other Liabilities [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): September 30, December 31, 2023 2022 Accrued external research and development expenses $ 9,915 $ 11,230 Accrued payroll and related expenses 9,710 12,312 Accrued professional fees 4,126 2,263 Accrued other 2,334 2,647 Accrued governmental rebates 3,910 2,300 Strimvelis liability - current portion — 3,685 Total accrued expenses and other current liabilities $ 29,995 $ 34,437

Notes Payable (Tables)

Notes Payable (Tables)	6 Months Ended	9 Months Ended
	Jun. 30, 2023	Sep. 30, 2023
Debt Disclosure [Abstract]
Summary of Notes Payable		Notes payable consist of the following (in thousands): September 30, December 31, 2023 2022 Notes payable, net of issuance costs $ 24,961 $ 31,970 Less: current portion ( 9,429 ) ( 9,429 ) Notes payable, net of current portion 15,532 22,541 Accretion related to final payment 630 450 Notes payable, long term $ 16,162 $ 22,991
Summary of Estimated Future Principal Payments Due	As of September 30, 2023, the estimated future principal payments due are as follows (in thousands): Aggregate 2023 (remaining three months) $ 2,357 2024 9,429 2025 9,429 2026 5,084 Total 26,299 Less current portion ( 9,429 ) Less unamortized portion of final payment ( 525 ) Less unamortized debt issuance costs ( 183 ) Notes payable, long term $ 16,162

Share-based Compensation (Table

Share-based Compensation (Tables)	9 Months Ended
	Sep. 30, 2023
Share-Based Payment Arrangement [Abstract]
Summary of Option Activity	The following table summarizes option activity for ordinary shares under the plans for the nine months ended September 30, 2023: Ordinary Shares Weighted Outstanding at December 31, 2022 16,424,167 $ 1.56 Granted 5,184,380 0.49 Exercised ( 370,430 ) 0.16 Forfeited ( 1,068,387 ) 3.32 Outstanding at September 30, 2023 20,169,730 $ 1.21 Vested and expected to vest at September 30, 2023 20,169,730 $ 1.22 Exercisable at September 30, 2023 11,303,310 $ 1.73
Summary of Award Activity	The following table summarizes award activity for the nine months ended September 30, 2023: Performance-based RSUs Time-based RSUs Total RSUs Weighted Average Grant Date Fair Value per Ordinary Share Unvested at December 31, 2022 195,000 1,868,876 2,063,876 $ 0.55 Granted — 1,720,200 1,720,200 0.46 Vested — ( 649,400 ) ( 649,400 ) 0.57 Forfeited — ( 304,836 ) ( 304,836 ) 0.46 Unvested at September 30, 2023 195,000 2,634,840 2,829,840 $ 0.55
Share-based Compensation Expense	Share-based compensation expense related to share options, restricted share unit awards, and the employee stock purchase plan was classified in the unaudited condensed consolidated statements of operations and comprehensive loss as follows (in thousands): Three Months Ended Nine Months Ended 2023 2022 2023 2022 Research and development $ 966 $ 1,445 $ 3,227 $ 5,151 Selling, general and administrative 1,502 2,023 5,331 6,933 Total $ 2,468 $ 3,468 $ 8,558 $ 12,084 Total share-based compensation by award type was as follows (in thousands): Three Months Ended Nine Months Ended 2023 2022 2023 2022 Ordinary share options $ 2,203 $ 3,260 $ 7,011 $ 11,531 Restricted ordinary share units 265 208 1,547 553 Total $ 2,468 $ 3,468 $ 8,558 $ 12,084

Restructuring Charges (Tables)

Restructuring Charges (Tables)	9 Months Ended
	Sep. 30, 2023
Restructuring and Related Activities [Abstract]
Summary of Restructuring Charges	Three Months Ended Nine Months Ended Balance at beginning of period $ 838 $ 6 Charged to expense — 2,481 Non-cash adjustments and foreign currency translation ( 206 ) ( 760 ) Payments made ( 632 ) ( 1,727 ) Balance at end of period $ — $ —

Product Revenue, Net (Tables)

Product Revenue, Net (Tables)	9 Months Ended
	Sep. 30, 2023
Revenue from Contract with Customer [Abstract]
Disaggregated Product Revenue	The following table presents the Company's net revenue by product (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 Libmeldy $ 5,559 $ 4,781 $ 12,744 $ 12,985 Strimvelis — 596 — 1,232 Total $ 5,559 $ 5,377 $ 12,744 $ 14,217
Summary of Activity of Product Revenue Allowance and Reserve Categories	Activity in each of the product revenue allowance and reserve categories for Libmeldy is summarized as follows (in thousands): Trade discounts and allowances Government rebates Total Balance as of December 31, 2022 $ 4,390 $ 2,300 $ 6,690 Provision Related to sales in the current year 5,191 1,655 6,846 Credits and payments made during the period ( 8,421 ) — ( 8,421 ) Foreign currency translation — ( 45 ) ( 45 ) Balance as of September 30, 2023 $ 1,160 $ 3,910 $ 5,070
Summary as Components of Balance Sheets	The total reserves described above are summarized as components of the Company's condensed consolidated balance sheets as follows (in thousands): As of As of Reduction of accounts receivable, net $ 1,160 $ 4,390 Component of accrued expenses and other current liabilities 3,910 2,300 Total revenue-related reserves $ 5,070 $ 6,690

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2023
Commitments and Contingencies Disclosure [Abstract]
Schedule of Annual Commitments Associated with the Contract	The following table outlines the current commitments associated with the agreement, as of September 30, 2023 (amounts in thousands): Due in: Product manufacturing commitments Dedicated manufacturing and development resources Exclusive transduction suites Total AGC Commitment 2023 (remaining three months) $ 486 $ 1,423 $ — $ 1,909 2024 1,946 6,450 2,162 10,558 2025 973 3,225 1,081 5,279 Total manufacturing commitments $ 3,405 $ 11,098 $ 3,243 $ 17,746 Tabular disclosure above has been translated to U.S. Dollar, from Euro, using the period end exchange rate of €1.00 to $ 1.08 .

Net Loss per Ordinary Share (Ta

Net Loss per Ordinary Share (Tables)	9 Months Ended
	Sep. 30, 2023
Earnings Per Share [Abstract]
Securities Excluded in the Computation of Diluted Net Loss Per Ordinary Share	The following securities, presented based on amounts outstanding at each period end, are considered to be ordinary share equivalents, but were excluded from the computation of diluted net loss per ordinary share because to do so would have been anti-dilutive: September 30, 2023 2022 Share options 16,822,522 13,134,055 Unvested restricted share units 3,019,163 2,326,543 Ordinary shares to be issued upon exercise of warrants (1) 109,083,590 — 128,925,275 15,460,598 (1) The warrants included in the table are exercisable subject to future contingencies (See Note 12)

Nature of the Business and Li_2

Nature of the Business and Liquidity - Additional Information (Details)			9 Months Ended
	Jun. 22, 2023 USD ($) shares	Mar. 10, 2023 USD ($) shares	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Oct. 05, 2023 $ / shares	Sep. 30, 2023 £ / shares	Sep. 30, 2023 USD ($)	Jun. 22, 2023 £ / shares $ / shares shares	Mar. 10, 2023 £ / shares shares	Mar. 06, 2023 $ / shares	Dec. 31, 2022 £ / shares
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Ordinary shares voting rights description			Each holder of ordinary shares is entitled to one vote per ordinary share
Ordinary shares dividends declared | $			$ 0	$ 0
Number of ordinary shares issued and sold | shares	4,250,000	56,666,900
Nominal value of shares issued and sold | £ / shares						£ 0.10		£ 0.1	£ 0.1		£ 0.10
Warrants to purchase ordinary shares | shares								46,750,000	62,333,590
Warrants to purchase price per unit								8		6
Gross proceeds of offering | $	$ 34,000,000	$ 34,000,000
Cash, cash equivalents and marketable securities | $							$ 125,400,000
Kyowa Kirin Transaction Agreement | Subsequent Event
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Acquisition share price in cash					$ 16
Cash, Without Interest Per Ordinary Share					1.6
Right to receive in cash per CVR, without interest					$ 0.1

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Summary Of Significant Accounting Policies [Line Items]
Foreign currency transaction gain (loss)	$ 3,000,000	$ (47,100,000)
Cash collateral associated with corporate credit cards and other leases	1,300,000		$ 1,300,000
Credit losses related to outstanding accounts receivable	0
Outstanding letter of credit associated with a lease	$ 3,000,000		$ 3,000,000
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2023
Change in accounting principle, accounting standards update, immaterial effect	true
New accounting pronouncement or change in accounting principle, description	ASU 2020-04
Minimum
Summary Of Significant Accounting Policies [Line Items]
General payment terms	30 days
Maximum
Summary Of Significant Accounting Policies [Line Items]
General payment terms	90 days
Libmeldy | Minimum
Summary Of Significant Accounting Policies [Line Items]
General payment terms	30 days
Libmeldy | Maximum
Summary Of Significant Accounting Policies [Line Items]
General payment terms	1 year

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Summary Of Significant Accounting Policies [Abstract]
Cash and cash equivalents	$ 23,469	$ 68,424
Restricted cash	4,215	4,215
Total cash, cash equivalents and restricted cash	$ 27,684	$ 72,639	$ 85,017	$ 60,178

Fair Value Measurements and M_3

Fair Value Measurements and Marketable Securities - Schedule of Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Cash equivalents
Total cash equivalents	$ 7,285	$ 27,161
Marketable securities
Total marketable securities	101,944	75,326
Total	109,229	102,487
Liabilities
Total Liabilities	21,068
Level 1
Cash equivalents
Total cash equivalents	7,285	1,239
Marketable securities
Total	7,285	1,239
Level 2
Cash equivalents
Total cash equivalents		25,922
Marketable securities
Total marketable securities	101,944	75,326
Total	101,944	101,248
Level 3
Liabilities
Total Liabilities	21,068
PIPE Warrant Liability
Liabilities
Total Liabilities	21,068
PIPE Warrant Liability | Level 3
Liabilities
Total Liabilities	21,068
Money Market Funds
Cash equivalents
Total cash equivalents	7,285	1,239
Money Market Funds | Level 1
Cash equivalents
Total cash equivalents	7,285	1,239
U.S. Treasuries
Cash equivalents
Total cash equivalents		6,600
Marketable securities
Total marketable securities	18,726
U.S. Treasuries | Level 2
Cash equivalents
Total cash equivalents		6,600
Marketable securities
Total marketable securities	18,726
U.S. Government Securities
Cash equivalents
Total cash equivalents		5,200
Marketable securities
Total marketable securities	17,462	1,984
U.S. Government Securities | Level 2
Cash equivalents
Total cash equivalents		5,200
Marketable securities
Total marketable securities	17,462	1,984
Corporate Bonds
Marketable securities
Total marketable securities	15,340	25,475
Corporate Bonds | Level 2
Marketable securities
Total marketable securities	15,340	25,475
Commercial Paper
Cash equivalents
Total cash equivalents		14,122
Marketable securities
Total marketable securities	50,416	47,867
Commercial Paper | Level 2
Cash equivalents
Total cash equivalents		14,122
Marketable securities
Total marketable securities	$ 50,416	$ 47,867

Fair Value Measurements and M_4

Fair Value Measurements and Marketable Securities - Additional Information (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2023	Jun. 22, 2023	Mar. 10, 2023	Mar. 06, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets transfers between level 1 and level 2	$ 0	$ 0				$ 0
Warrants to purchase ordinary shares			46,750,000	62,333,590
Realized gains or losses recognized on investments	$ 0	$ 0
Securities Purchase Agreement | 2023 Private Placement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants to purchase ordinary shares					109,083,590
Securities Purchase Agreement | 2023 Private Placement | Initial Closing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants to purchase ordinary shares				62,333,590

Fair Value Measurements and M_5

Fair Value Measurements and Marketable Securities - Schedule of Significant Unobservable Inputs used in Valuation Model to Measure PIPE Warrants and PIPE Units Liabilities (Details)	Sep. 30, 2023 $ / shares	Jun. 30, 2023 $ / shares	Jun. 22, 2023 $ / shares	Mar. 31, 2023 $ / shares	Mar. 10, 2023 $ / shares
PIPE Units Liability | Anticipated Common Stock Price (at Exercise Date) | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input, stock price				$ 10.23	$ 10.14
PIPE Units Liability | Anticipated Common Stock Price (at Exercise Date) | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input, stock price				$ 10.49	$ 10.46
PIPE Units Liability | Expected Term (in years) | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input, term				1 year 29 days	1 year 29 days
PIPE Units Liability | Expected Term (in years) | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input, term				1 year 10 months 2 days	1 year 10 months 2 days
PIPE Units Liability | Expected Volatility | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input				0.717	0.729
PIPE Units Liability | Expected Volatility | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input				0.774	0.76
PIPE Units Liability | Implied Probabilities of Occurrence of Second Closing and FDA Approval | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input				0.47
PIPE Units Liability | Implied Probabilities of Occurrence of Second Closing and FDA Approval | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input				0.59
PIPE Units Liability | Implied Probabilities of Occurrence of FDA Approval
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input			0.305
PIPE Units Liability | Discount for Lack of Marketability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input			0.05
PIPE Units Liability | Discount for Lack of Marketability | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input				0.119
PIPE Units Liability | Discount for Lack of Marketability | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input				0.125
PIPE Units Liability | Expected Warrant Exercise Date July 30, 2024 | Anticipated Common Stock Price (at Exercise Date)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input, stock price			$ 10.4
PIPE Units Liability | Expected Warrant Exercise Date July 30, 2024 | Expected Term (in years)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input, term			1 year 1 month 9 days
PIPE Units Liability | Expected Warrant Exercise Date July 30, 2024 | Expected Volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input			75
PIPE Units Liability | Expected Warrant Exercise Date April 30, 2025 | Anticipated Common Stock Price (at Exercise Date)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input, stock price			$ 10.07
PIPE Units Liability | Expected Warrant Exercise Date April 30, 2025 | Expected Term (in years)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input, term			1 year 10 months 9 days
PIPE Units Liability | Expected Warrant Exercise Date April 30, 2025 | Expected Volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input			74.8
PIPE Warrant Liability | Anticipated Common Stock Price (at Exercise Date) | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants measurement input, stock price				$ 10.13	$ 10
PIPE Warrant Liability | Anticipated Common Stock Price (at Exercise Date) | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants measurement input, stock price				$ 10.38	$ 10.31
PIPE Warrant Liability | Expected Term (in years) | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants measurement input, term				1 year 3 months 29 days	1 year 4 months 20 days
PIPE Warrant Liability | Expected Term (in years) | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants measurement input, term				2 years 29 days	2 years 1 month 20 days
PIPE Warrant Liability | Expected Volatility | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants measurement input				0.717	0.729
PIPE Warrant Liability | Expected Volatility | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants measurement input				0.774	0.76
PIPE Warrant Liability | Implied Probabilities of Occurrence of FDA Approval
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input	0.65	0.346
PIPE Warrant Liability | Discount for Lack of Marketability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Units measurement input	0	0
PIPE Warrant Liability | Expected Warrant Exercise Date July 30, 2024 | Anticipated Common Stock Price (at Exercise Date)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants measurement input, stock price	$ 11	$ 10.39
PIPE Warrant Liability | Expected Warrant Exercise Date July 30, 2024 | Expected Term (in years)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants measurement input, term	9 months 29 days	1 year 29 days
PIPE Warrant Liability | Expected Warrant Exercise Date July 30, 2024 | Expected Volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants measurement input	0.776	75.6
PIPE Warrant Liability | Expected Warrant Exercise Date April 30, 2025 | Anticipated Common Stock Price (at Exercise Date)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants measurement input, stock price	$ 9.5	$ 10.07
PIPE Warrant Liability | Expected Warrant Exercise Date April 30, 2025 | Expected Term (in years)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants measurement input, term	1 year 6 months 29 days	1 year 10 months 2 days
PIPE Warrant Liability | Expected Warrant Exercise Date April 30, 2025 | Expected Volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants measurement input	0.717	74.5

Fair Value Measurements and M_6

Fair Value Measurements and Marketable Securities - Schedule of Aggregate Fair Values of PIPE Warrant Liability and PIPE Unit Liability (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2023 USD ($)
PIPE Warrant Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at December 31, 2022	$ 0
Issuances during period	11,098
Change in fair value	9,970
Settlement of PIPE Unit liability upon Second Closing	0
Balance at March 31, 2023	21,068
PIPE Units Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at December 31, 2022	0
Issuances during period	25
Change in fair value	(9,668)
Settlement of PIPE Unit liability upon Second Closing	9,643
Balance at March 31, 2023	$ 0

Fair Value Measurements and M_7

Fair Value Measurements and Marketable Securities - Schedule of Amortized Cost and Fair Value of Available-for-Sale Marketable Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 102,064	$ 101,481
Gross Unrealized Gains	3	5
Gross Unrealized Losses	(122)	(238)
Fair Value	101,944	101,248
U.S. Government Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	17,470	7,188
Gross Unrealized Gains	3	1
Gross Unrealized Losses	(10)	(6)
Fair Value	17,462	7,183
U.S. Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	18,731	6,599
Gross Unrealized Gains		1
Gross Unrealized Losses	(5)
Fair Value	18,726	6,600
Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	15,415	25,656
Gross Unrealized Losses	(75)	(180)
Fair Value	15,340	25,476
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	50,448	62,038
Gross Unrealized Gains		3
Gross Unrealized Losses	(32)	(52)
Fair Value	$ 50,416	$ 61,989

Fair Value Measurements and M_8

Fair Value Measurements and Marketable Securities - Schedule of Available-for-sale Marketable Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Fair Value Disclosures [Abstract]
Maturities in one year or less	$ 97,086	$ 98,277
Maturities between one and three years	4,858	2,971
Total	$ 101,944	$ 101,248

Inventory - Schedule of Invento

Inventory - Schedule of Inventory (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Inventory Disclosure [Abstract]
Raw materials	$ 983	$ 3,193
Work in process	5,655	207
Total inventory	$ 6,638	$ 3,400

Inventory - Additional Informat

Inventory - Additional Information (Details) $ in Millions	Sep. 30, 2023 USD ($)
Inventory Disclosure [Abstract]
Inventory capitalized	$ 5.4

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Prepaid Expense and Other Assets, Current [Abstract]
Prepaid external research and development expenses	$ 1,797	$ 881
Other prepayments	2,199	1,817
VAT receivable	894	1,077
Non-trade receivables	1,659	1,851
Rent deposits	902	960
Total prepaid expenses and other current assets	$ 7,451	$ 6,586

Research and Development Tax _3

Research and Development Tax Credit Receivable - Summary of Amounts Recognized to Offset Research and Development Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Tax Credit Carryforward [Line Items]
Foreign currency translation			$ 3,000	$ (47,100)
United Kingdom | Research and Development Expense
Tax Credit Carryforward [Line Items]
Balance at beginning of period	$ 10,626	$ 15,176	5,942	30,723
Recognition of credit claims as offset to research and development expense	1,393	1,974	5,764	7,075
Receipt of credit claims				(18,390)
Foreign currency translation	(413)	(1,363)	(100)	(3,621)
Balance at end of period	$ 11,606	$ 15,787	$ 11,606	$ 15,787

Research and Development Tax _4

Research and Development Tax Credit Receivable - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2023	Sep. 30, 2023	Dec. 31, 2022
Tax Credit Carryforward [Line Items]
Tax credit receivable, current		$ 8,234	$ 5,942
United Kingdom
Tax Credit Carryforward [Line Items]
Tax credit receivable, current		11,600
United Kingdom | Research and Development Expense
Tax Credit Carryforward [Line Items]
Tax credit receivable, current		$ 3,400	$ 8,200
Additional income tax credits	$ 2,300

Intangible Assets, Net - Schedu

Intangible Assets, Net - Schedule of Intangible Assets Net of Accumulated Amortization (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Finite Lived Intangible Assets [Line Items]
Intangible Assets, Cost	$ 4,088	$ 4,069
Intangible Assets, Accumulated Amortized	(767)	(509)
Intangible Assets, Net	3,321	3,560
License Milestones
Finite Lived Intangible Assets [Line Items]
Intangible Assets, Cost	4,088	4,069
Intangible Assets, Accumulated Amortized	(767)	(509)
Intangible Assets, Net	$ 3,321	$ 3,560

Intangible Assets, Net - Additi

Intangible Assets, Net - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Finite Lived Intangible Assets [Line Items]
Intangible asset, useful life	12 years		12 years
Amortization of intangible assets	$ 0.1	$ 0.1	$ 0.3	$ 0.3
Effect of foreign currency translation on the net carrying value of intangible assets		$ 0.2		$ 0.4
Maximum
Finite Lived Intangible Assets [Line Items]
Effect of foreign currency translation on the net carrying value of intangible assets	$ 0.1		$ 0.1

Intangible Assets, Net - Sche_2

Intangible Assets, Net - Schedule of Estimated Future Amortization For Intangible Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Goodwill and Intangible Assets Disclosure [Abstract]
2023 (remaining three months)	$ 86
2024	347
2025	347
2026	347
2027	347
Thereafter	1,847
Intangible Assets, Net	$ 3,321	$ 3,560

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Accrued Liabilities and Other Liabilities [Abstract]
Accrued external research and development expenses	$ 9,915	$ 11,230
Accrued payroll and related expenses	9,710	12,312
Accrued professional fees	4,126	2,263
Accrued other	2,334	2,647
Accrued governmental rebates	3,910	2,300
Strimvelis liability - current portion		3,685
Total accrued expenses and other current liabilities	$ 29,995	$ 34,437

Notes Payable - Additional Info

Notes Payable - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended		9 Months Ended
	May 31, 2021	May 31, 2019	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Debt Instrument [Line Items]
Line of credit facility agreement date		May 31, 2019
Credit facility maximum borrowings	$ 100,000,000	$ 75,000,000
Credit facility, remaining borrowings			$ 67,000,000		$ 67,000,000
Line of credit facility amortization term	18 months
Line of credit facility interest payments term	18 months
Line of credit facility, frequency of payments					monthly payments
Line of credit facility, percentage of final payment	3.50%
Final payment amount	$ 1,200,000
Term Loan
Debt Instrument [Line Items]
Debt instrument, effective annual interest rate					12.62%
Debt instrument, interest expense			$ 800,000	$ 800,000	$ 2,700,000	$ 2,100,000
LIBOR
Debt Instrument [Line Items]
Debt instrument, annual interest rate	5.95%
Initial Term Loan
Debt Instrument [Line Items]
Credit facility maximum borrowings	$ 33,000,000	$ 25,000,000
Amended Credit Facility
Debt Instrument [Line Items]
Proceeds from lines of credit					$ 33,000,000
Amended Credit Facility | SOFR
Debt Instrument [Line Items]
Debt instrument, effective annual interest rate	0.10%

Notes Payable - Summary of Note

Notes Payable - Summary of Notes Payable (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Notes and Loans, Noncurrent [Abstract]
Notes payable, net of issuance costs	$ 24,961	$ 31,970
Less: current portion	(9,429)	(9,429)
Notes payable, net of current portion	15,532	22,541
Accretion related to final payment	630	450
Notes payable, long term	$ 16,162	$ 22,991

Notes Payable - Summary of Esti

Notes Payable - Summary of Estimated Future Principal Payments Due (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Long-Term Debt, Fiscal Year Maturity [Abstract]
2023 (remaining three months)	$ 2,357
2024	9,429
2025	9,429
2026	5,084
Total	26,299
Less: current portion	(9,429)
Less: unamortized portion of final payment	(525)
Less: unamortized debt issuance costs	(183)
Notes payable, long term	$ 16,162	$ 22,991

Shareholders' Equity - Addition

Shareholders' Equity - Additional Information (Details) - GBP (£)	6 Months Ended	9 Months Ended
	Jun. 30, 2023	Sep. 30, 2023	Dec. 31, 2022	Jun. 16, 2021
Class of Stock [Line Items]
Percentage rate of excess ordinary shares		19.99%
Ordinary shares, authorized		£ 33,556,809	£ 13,023,851.5
Ordinary shares, issued		227,273,209	126,947,225
Ordinary shares, outstanding		227,273,209	126,947,225
PIPE Authority
Class of Stock [Line Items]
Ordinary shares, authorized	£ 15,158,359
Remaining ordinary shares		£ 10,908,359
General Authority
Class of Stock [Line Items]
Ordinary shares, authorized	£ 18,398,450
Remaining ordinary shares		£ 23,924,546
2021 Authority
Class of Stock [Line Items]
Ordinary shares, authorized				£ 13,023,851.5
Remaining ordinary shares				£ 5,526,095.7
Ordinary Shares
Class of Stock [Line Items]
Ordinary shares, issued		181,567,883
Ordinary shares, outstanding		181,567,883
Number of deferred share	1
Nominal value of deferred share	£ 4.89687
Non-voting Ordinary Shares
Class of Stock [Line Items]
Ordinary shares, issued		45,705,326
Ordinary shares, outstanding		45,705,326

Share-based Compensation - Addi

Share-based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 30, 2020	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average grant date fair value of share options granted				$ 0.28	$ 0.32
Total intrinsic value of share options exercised		$ 100		$ 100
Number of shares granted				1,720,200
Stock compensation expense		2,468	$ 3,468	$ 8,558	$ 12,084
RSUs vested				649,400
Award vesting period				4 years
Total unrecognized compensation cost of stock option and time-based RSUs		8,100		$ 8,100
Compensation cost expected to be recognized weighted average period				2 years 4 months 24 days
Total unrecognized compensation cost of performance-based RSUs		$ 500		$ 500
2018 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining shares available for issuance		12,748,750		12,748,750
Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining shares available for issuance		730,500		730,500
2018 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining shares available for issuance		1,460,590		1,460,590
Performance-based Restricted Share Units ("RSUs")
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation cost expected to be recognized weighted average period				3 months
Total unrecognized compensation cost of performance-based RSUs		$ 100		$ 100
Time-based Restricted Share Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares granted				1,720,200
RSUs vested				649,400
Award vesting period				3 years
Total unrecognized compensation cost of stock option and time-based RSUs		900		$ 900
Compensation cost expected to be recognized weighted average period				1 year 10 months 9 days
Chief Executive Officer | Performance-based Restricted Share Units ("RSUs")
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares granted	195,000
Total grant date fair value	$ 1,400
Vesting rights				In April 2020, the Company granted 195,000 performance-based restricted share units (“RSUs”), representing 19,500 ADSs, with a total grant date fair value of $1.4 million to its Chief Executive Officer, Bobby Gaspar, M.D., Ph.D. Dr. Gaspar earns one-third of the award for each of the first three to occur of four milestones (65,000 RSUs are earned for each achieved milestone). The milestones relate to specific clinical and regulatory goals which have to be achieved before December 31, 2023 in order for the underlying RSUs to be eligible to vest.
Chief Executive Officer | ADS
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares granted	19,500
Achievement of at Least Three of Four Milestones
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs vested				65,000
Achievement of Two Milestones
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense		$ 100		$ 900

Share-based Compensation - Summ

Share-based Compensation - Summary of Option Activity (Details) - $ / shares	9 Months Ended
	Sep. 30, 2023
Share-Based Payment Arrangement [Abstract]
Number of Options, Outstanding at December 31, 2022	16,424,167
Number of Options, Granted	5,184,380
Number of Options, Exercised	(370,430)
Number of Options, Forfeited	(1,068,387)
Number of Options, Outstanding at June 30, 2023	20,169,730
Number of Options, Vested and expected to vest at June 30, 2023	20,169,730
Number of Options, Exercisable, June 30, 2023	11,303,310
Weighted Average Exercise Price, Outstanding at December 31, 2022	$ 1.56
Weighted Average Exercise Price, Granted	0.49
Weighted Average Exercise Price, Exercised	0.16
Weighted Average Exercise Price, Forfeited	3.32
Weighted Average Exercise Price, Outstanding at June 30, 2023	1.21
Weighted Average Exercise Price, Vested and expected to vest at June 30, 2023	1.22
Weighted Average Exercise Price, Exercisable at June 30, 2023	$ 1.73

Share-based Compensation - Su_2

Share-based Compensation - Summary of Award Activity (Details)	9 Months Ended
	Sep. 30, 2023 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs, Unvested at December 31, 2022	2,063,876
RSUs, Granted	1,720,200
RSUs, Vested	(649,400)
RSUs, Forfeited	(304,836)
RSUs, Unvested at June 30, 2023	2,829,840
Weighted Average Grant Date Fair Value per Share, Unvested at December 31, 2022 | $ / shares	$ 0.55
Weighted Average Grant Date Fair Value per Share, Granted | $ / shares	0.46
Weighted Average Grant Date Fair Value per Share, Vested | $ / shares	0.57
Weighted Average Grant Date Fair Value per Share, Forfeited | $ / shares	0.46
Weighted Average Grant Date Fair Value per Share, Unvested at June 30, 2023 | $ / shares	$ 0.55
Performance-based RSUs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs, Unvested at December 31, 2022	195,000
RSUs, Unvested at June 30, 2023	195,000
Time-based RSUs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs, Unvested at December 31, 2022	1,868,876
RSUs, Granted	1,720,200
RSUs, Vested	(649,400)
RSUs, Forfeited	(304,836)
RSUs, Unvested at June 30, 2023	2,634,840

Share-based Compensation - Shar

Share-based Compensation - Share-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	$ 2,468	$ 3,468	$ 8,558	$ 12,084
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	966	1,445	3,227	5,151
Selling, General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	1,502	2,023	5,331	6,933
Ordinary Share Options
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	2,203	3,260	7,011	11,531
Restricted Ordinary Share Units
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	$ 265	$ 208	$ 1,547	$ 553

Private Placement Transaction S

Private Placement Transaction Share - Additional Information (Details) $ / shares in Units, $ in Thousands

3 Months Ended

9 Months Ended

Jun. 22, 2023 USD ($) shares

Mar. 10, 2023 USD ($) shares

Mar. 06, 2023 USD ($) shares

Sep. 30, 2023 USD ($)

Mar. 31, 2023 USD ($)

Sep. 30, 2023 USD ($)

Jan. 01, 2025 $ / shares

Dec. 31, 2024 $ / shares

Sep. 30, 2023 £ / shares

Sep. 30, 2023 USD ($) shares

Jun. 30, 2023 USD ($)

Jun. 22, 2023 £ / shares

Jun. 22, 2023 USD ($) $ / shares shares

Mar. 10, 2023 £ / shares

Mar. 10, 2023 USD ($) $ / shares shares

Mar. 06, 2023 £ / shares

Mar. 06, 2023 USD ($) shares

Dec. 31, 2022 £ / shares

Dec. 31, 2022 USD ($)

Sep. 30, 2022 USD ($)

Jun. 30, 2022 USD ($)

Mar. 31, 2022 USD ($)

Dec. 31, 2021 USD ($)

Class of Stock [Line Items]

Number of ordinary shares issued and sold | shares

4,250,000

56,666,900

Ordinary shares, nominal value | £ / shares

£ 0.10

£ 0.1

£ 0.1

£ 0.10

Warrants to purchase ordinary shares | shares

46,750,000

62,333,590

Gross proceeds of offering

$ 34,000

$ 34,000

Issuance costs

$ 889

Equity

$ 101,655

$ 80,258

$ 109,047

$ 98,264

$ 123,703

$ 146,272

$ 175,646

$ 209,950

Changes in fair value

$ (3,256)

Securities Purchase Agreement | 2023 Private Placement

Class of Stock [Line Items]

Number of ordinary shares issued and sold | shares

99,166,900

Ordinary shares, nominal value | £ / shares

£ 0.1

Warrants to purchase ordinary shares | shares

109,083,590

Warrants expiration, description

The warrants will expire at the conclusion of the 30-day exercise period or on March 10, 2026, whichever is earlier.

Warrants exercisable term

30 days

Shares sold in fixed combinaton of warrants as units

The 2023 Private Placement consisted of two closings, referred to herein as the "First Closing" and "Second Closing". At each closing, the Shares were sold in fixed combinations with the warrants as units, with each unit consisting of 10 Shares and 1 accompanying warrant to purchase 11 shares. The warrants are contingently exercisable for 30 days following (i) the Company’s public announcement of its receipt of marketing approval of its biologics license application (“BLA”) submitted to the FDA with respect to OTL-200 (the “Vesting Event”) and (ii) receipt of shareholder approval to increase the number of authorized shares.

Fair value of financial instruments

$ 34,000

Issuance costs

1,200

1,100

$ 500

Gain on mark-to-market adjustment

3,600

Equity

$ 19,700

$ 24,000

Changes in fair value

9,700

Warants outstanding | shares

109,083,590

Warrant exercisable | shares

0

Securities Purchase Agreement | 2023 Private Placement | Scenario Forecast

Class of Stock [Line Items]

Warrants to purchase price per share | $ / shares

$ 0.95

$ 1.1

Securities Purchase Agreement | 2023 Private Placement | Initial Closing

Class of Stock [Line Items]

Warrants to purchase ordinary shares | shares

62,333,590

Warrants to purchase price per share | $ / shares

$ 6

Gross proceeds of offering

34,000

Deducting offering expenses

900

Securities Purchase Agreement | 2023 Private Placement | Second Closing

Class of Stock [Line Items]

Warrants to purchase ordinary shares | shares

46,750,000

Warrants to purchase price per share | $ / shares

$ 8

Gross proceeds of offering

34,000

Deducting offering expenses

1,200

Securities Purchase Agreement | 2023 Private Placement | PIPE Warrants

Class of Stock [Line Items]

Fair value of financial instruments

6,400

Issuance costs

200

200

Warrant liability

$ 4,600

Changes in fair value

$ 8,800

$ 10,000

Securities Purchase Agreement | 2023 Private Placement | PIPE Units

Class of Stock [Line Items]

Fair value of financial instruments

20

Issuance costs

200

Derecognized assets

$ 9,600

Changes in fair value

$ 9,600

Securities Purchase Agreement | 2023 Private Placement | Ordinary Shares

Class of Stock [Line Items]

Fair value of financial instruments

$ 27,500

Issuance costs

$ 1,000

$ 900

Securities Purchase Agreement | 2023 Private Placement | Ordinary Shares | Initial Closing

Class of Stock [Line Items]

Number of ordinary shares issued and sold | shares

40,053,500

Securities Purchase Agreement | 2023 Private Placement | Ordinary Shares | Second Closing

Class of Stock [Line Items]

Number of ordinary shares issued and sold | shares

13,408,074

Securities Purchase Agreement | 2023 Private Placement | Non-voting Ordinary Shares | Initial Closing

Class of Stock [Line Items]

Number of ordinary shares issued and sold | shares

16,613,400

Securities Purchase Agreement | 2023 Private Placement | Non-voting Ordinary Shares | Second Closing

Class of Stock [Line Items]

Number of ordinary shares issued and sold | shares

29,091,926

Restructuring Charges - Additio

Restructuring Charges - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Restructuring Cost And Reserve [Line Items]
Research and development	$ 14,553,000	$ 18,103,000	$ 47,241,000	$ 68,302,000
Selling, general and administrative	$ 11,609,000	$ 11,496,000	33,736,000	38,525,000
Restructuring charges			$ 0	2,481,000
Neurometabolic Diseases and Early Research Programs
Restructuring Cost And Reserve [Line Items]
Research and development				1,400,000
Selling, general and administrative				300,000
Neurometabolic Diseases and Early Research Programs | Employee-Related Termination Costs
Restructuring Cost And Reserve [Line Items]
One-time charge				$ 1,700,000

Restructuring Charges - Summary

Restructuring Charges - Summary of Restructuring Charges (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Restructuring and Related Activities [Abstract]
Balance at beginning of period	$ 838,000		$ 6,000
Charged to expense		$ 0	2,481,000
Non-cash adjustments and foreign currency translation	(206,000)		(760,000)
Payments made	$ (632,000)		$ (1,727,000)

Product Revenue, Net - Schedule

Product Revenue, Net - Schedule of Net Revenue by Product (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Disaggregation of Revenue [Line Items]
Net revenue	$ 6,302	$ 5,777	$ 14,853	$ 15,669
Libmeldy
Disaggregation of Revenue [Line Items]
Net revenue	5,559	4,781	12,744	12,985
Strimvelis
Disaggregation of Revenue [Line Items]
Net revenue		596		1,232
Product
Disaggregation of Revenue [Line Items]
Net revenue	$ 5,559	$ 5,377	$ 12,744	$ 14,217

Product Revenue, Net - Summary

Product Revenue, Net - Summary of Activity of Product Revenue Allowance and Reserve Categories (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2023 USD ($)
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance as of December 31, 2022	$ 6,690
Provision Related to sales in the current year	6,846
Credits and payments made during the period	(8,421)
Foreign currency translation	(45)
Balance as of June 30, 2023	5,070
Trade Discounts and Allowances
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance as of December 31, 2022	4,390
Provision Related to sales in the current year	5,191
Credits and payments made during the period	(8,421)
Balance as of June 30, 2023	1,160
Government Rebates
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance as of December 31, 2022	2,300
Provision Related to sales in the current year	1,655
Foreign currency translation	(45)
Balance as of June 30, 2023	$ 3,910

Product Revenue, Net - Summar_2

Product Revenue, Net - Summary as Components of Balance Sheets (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract]
Reduction of accounts receivable, net	$ 1,160	$ 4,390
Component of accrued expenses and other current liabilities	3,910	2,300
Total revenue-related reserves	$ 5,070	$ 6,690

Collaboration Revenue - Additio

Collaboration Revenue - Additional Information (Details) - USD ($) $ in Thousands				3 Months Ended				9 Months Ended
	Jun. 22, 2023	Mar. 10, 2023	Jul. 01, 2021	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Issuance of ordinary shares	4,250,000	56,666,900
Total consideration					$ 18,761	$ 23,073
Net revenue				$ 6,302			$ 5,777	$ 14,853	$ 15,669
Strategic Collaboration Agreement with Pharming
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash upfront payment received			$ 10,000
Strategic Collaboration Agreement with Pharming | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments receivable			$ 189,500
Share Purchase Agreement with Pharming
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Issuance of ordinary shares			1,227,738
Total consideration			$ 7,500
Common stock consideration			4,100
Premium consideration			3,400
Pharming Agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total upfront payments			17,500
Upfront payments attributed to equity sold			4,100
Upfront payments attributed to research activities			13,400
Estimated reimbursement payments			$ 13,400
Collaborative Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Net revenue				700			$ 400	2,100	$ 1,500
Contract liabilities, current				700				700
Contract liabilities, long-term				$ 10,300				$ 10,300

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Income Tax Disclosure [Abstract]
Income tax benefit (expense)	$ (976)	$ 1,084	$ (989)	$ 1,245

Commitments and Contingencies -

Commitments and Contingencies - Schedule of Annual Commitments Associated with the Contract (Details) $ in Thousands	Sep. 30, 2023 USD ($)
Other Commitments [Line Items]
2023 (remaining three months)	$ 1,909
2024	10,558
2025	5,279
Total manufacturing commitments	17,746
Product Manufacturing Commitments
Other Commitments [Line Items]
2023 (remaining three months)	486
2024	1,946
2025	973
Total manufacturing commitments	3,405
Dedicated Manufacturing And Development Resources
Other Commitments [Line Items]
2023 (remaining three months)	1,423
2024	6,450
2025	3,225
Total manufacturing commitments	11,098
Exclusive Transduction Suites
Other Commitments [Line Items]
2024	2,162
2025	1,081
Total manufacturing commitments	$ 3,243

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Annual Commitments Associated with the Contract (Parenthetical) (Details)	Sep. 30, 2023
Commitments and Contingencies Disclosure [Abstract]
Exchange rate, Euro to U.S. Dollar	1.08

Net Loss per Ordinary Share - S

Net Loss per Ordinary Share - Securities Excluded in the Computation of Diluted Net Loss Per Ordinary Share (Details) - shares	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	128,925,275	15,460,598
Share Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	16,822,522	13,134,055
Unvested Restricted Shares Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	3,019,163	2,326,543
Ordinary Shares
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	109,083,590

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - Subsequent Event - Kyowa Kirin Transaction Agreement	Oct. 05, 2023 $ / shares
Subsequent Event [Line Items]
Acquisition share price in cash	$ 16
Cash without interest per ordinary share	$ 1.6
Ordinary shares transformatoin terms	all outstanding shares of the Company (including ordinary shares, non-voting ordinary shares and ADSs) then issued and outstanding will be transferred to Kyowa Kirin (or its nominee). Holders of shares at the Effective Time will be entitled to receive (a) $16.00 in cash, without interest, per ADS, less certain fees assessed by the ADS depositary, or $1.60 in cash, without interest per ordinary share or non-voting ordinary share, plus (b) 10 contingent value rights (each, a “CVR”) per ADS, or one CVR per ordinary share or non-voting ordinary share, together representing a right to receive $0.10 per CVR, without interest, if regulatory approval is granted by the FDA
Right to receive in cash per CVR, without interest	$ 0.1