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Intangible assets and goodwill | 9 Months Ended
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Sep. 30, 2025
Goodwill and Intangible Assets Disclosure [Abstract] |
Intangible assets and goodwill | 12. Intangible assets and goodwill Definite-lived intangibles Definite-lived intangible assets consisted of the following at September 30, 2025 and December 31, 2024: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | Ending Balance at | ​ | ​ | ​ | ​ | ​ | Foreign | ​ | ​ | Ending Balance at
Definite-lived | ​ | ​ | December 31, | ​ | ​ | ​ | ​ | ​ | currency | ​ | ​ | September 30,
intangible assets, gross |  | ​ | 2024 |  | ​ | Additions |  | ​ | translation | ​ | ​ | 2025
Waylivra | ​ | ​ | 12,397 | ​ | ​ | 4,290 | ​ | ​ | 1,709 | ​ | ​ | 18,396
Tegsedi | ​ | ​ | 18,249 | ​ | ​ | 3,448 | ​ | ​ | 2,367 | ​ | ​ | 24,064
Kebilidi | ​ | ​ | 10,731 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 10,731
Upstaza | ​ | ​ | 106,937 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 106,937
Sephience | ​ | ​ | — | ​ | ​ | 282,763 | ​ | ​ | — | ​ | ​ | 282,763
Total definite-lived intangibles, gross | ​ | $ | 148,314 | ​ | $ | 290,501 | ​ | $ | 4,076 | ​ | $ | 442,891
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | Ending Balance at | ​ | ​ | ​ | ​ | ​ | Foreign | ​ | ​ | Ending Balance at
Definite-lived | ​ | ​ | December 31, | ​ | ​ | ​ | ​ | ​ | currency | ​ | ​ | September 30,
intangible assets, accumulated amortization |  | ​ | 2024 |  | ​ | Amortization |  | ​ | translation |  | ​ | 2025
Waylivra | ​ | ​ | (5,273) | ​ | ​ | (1,921) | ​ | ​ | (721) | ​ | ​ | (7,915)
Tegsedi | ​ | ​ | (5,609) | ​ | ​ | (2,774) | ​ | ​ | (791) | ​ | ​ | (9,174)
Kebilidi | ​ | ​ | (112) | ​ | ​ | (671) | ​ | ​ | — | ​ | ​ | (783)
Upstaza | ​ | ​ | (18,526) | ​ | ​ | (6,684) | ​ | ​ | — | ​ | ​ | (25,210)
Sephience | ​ | ​ | — | ​ | ​ | (3,343) | ​ | ​ | — | ​ | ​ | (3,343)
Total definite-lived intangibles, accumulated amortization | ​ | $ | (29,520) | ​ | $ | (15,393) | ​ | $ | (1,512) | ​ | $ | (46,425)
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Total definite-lived intangibles, net | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | $ | 396,466
​ Akcea is entitled to receive royalty payments subject to certain terms set forth in the Tegsedi-Waylivra Agreement related to sales of Waylivra and Tegsedi. In accordance with the guidance for an asset acquisition, the Company records royalty payments when they become payable to Akcea and increase the cost basis for the Waylivra and Tegsedi intangible assets. For the nine months ended September 30, 2025, royalty payments of $3.4 million and $4.3 million were recorded for Tegsedi and Waylivra, respectively. As of September 30, 2025, a royalty payable of $1.2 million and $0.9 million for Tegsedi and Waylivra, respectively, was recorded on the consolidated balance sheet within accounts payable and accrued expenses. Pursuant to the Censa Rights Satisfaction Agreement, in August 2025, upfront cash consideration of $225.1 million was paid to the former Censa securityholders. The upfront cash consideration was recorded as an intangible asset and is being amortized to cost of product sales over its expected useful life on a straight-line basis. Pursuant to the Censa Merger Agreement, in June 2025, a $25.0 million milestone from the Company to the former Censa securityholders was triggered when the EC granted marketing authorization to Sephience for the treatment of children and adults living with PKU. The milestone was recorded as an intangible asset and is being amortized to cost of product sales over its expected useful life on a straight-line basis. ​ Pursuant to the Censa Merger Agreement, in July 2025, a $32.5 million milestone from the Company to the former Censa securityholders was triggered when the FDA approved Sephience for the treatment of children and adults living with PKU. The milestone was recorded as an intangible asset and is being amortized to cost of product sales over its expected useful life on a straight-line basis. As of September 30, 2025, the $32.5 million milestone was recorded on the consolidated balance sheet within accounts payable and accrued expenses. ​ The former Censa securityholders may also be entitled to receive other contingent payments subject to certain terms set forth in the Censa Merger Agreement related to sales of Sephience. In accordance with the guidance for an asset acquisition, the Company will record such payments when they become payable to the former Censa securityholders and increase the cost basis for the Sephience intangible asset. For the nine months ended September 30, 2025, royalty payments of $0.2 million were recorded for Sephience. As of September 30, 2025, a royalty payable of $0.2 million for Sephience was recorded on the consolidated balance sheet within accounts payable and accrued expenses. For the three months ended September 30, 2025 and 2024, the Company recognized amortization expense of $7.5 million and $3.0 million, respectively, related to its intangible assets. For the nine months ended September 30, 2025 and 2024, the Company recognized amortization expense of $15.4 million and $57.4 million, respectively, related to its intangible assets. The estimated future amortization of the Company’s intangible assets is expected to be as follows: ​
​ | ​ | ​ | ​
​ |  | As of September 30, 2025
2025 | ​ | $ | 9,280
2026 | ​ |  | 37,098
2027 | ​ |  | 37,098
2028 | ​ |  | 37,098
2029 and thereafter | ​ |  | 275,892
Total | ​ | $ | 396,466
​ The weighted average remaining amortization period of the definite-lived intangibles as of September 30, 2025 is 12.2 years. Goodwill As a result of the Agilis Merger on August 23, 2018, the Company recorded $82.3 million of goodwill. As of September 30, 2025, there have been no changes to the balance of goodwill since the date of the Agilis Merger. Accordingly, the goodwill balance as of September 30, 2025 is $82.3 million.

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- Definition The entire disclosure for goodwill and intangible assets. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 350 -SubTopic 30 -Name Accounting Standards Codification -Publisher FASB -URI https://asc.fasb.org/350-30/tableOfContent Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 350 -SubTopic 20 -Name Accounting Standards Codification -Publisher FASB -URI https://asc.fasb.org/350-20/tableOfContent + Details
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