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Capital structure | 12 Months Ended
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Dec. 31, 2023
Stockholders' Equity Note [Abstract] |
Capital structure | 8. Capital structure Common stock In August 2019, the Company entered into an At the Market Offering Sales Agreement (the “Sales Agreement”) with Cantor Fitzgerald and RBC Capital Markets, LLC (together, the “Sales Agents”), pursuant to which, the Company may offer and sell shares of its common stock, having an aggregate offering price of up to $125.0 million from time to time through the Sales Agents by any method that is deemed to be an “at the market offering” as defined in Rule 415(a)(4) promulgated under the Securities Act of 1933, as amended. No shares were sold pursuant to the Sales Agreement during the years ended December 31, 2023, 2022, and 2021. The remaining shares of the Company’s common stock available to be issued and sold, under the Sales Agreement, have an aggregate offering price of up to $93.0 million as of December 31, 2023. In June 2021, the Company filed a Certificate of Amendment to its Restated Certificate of Incorporation, which increased the number of authorized shares of the Company’s common stock from 125,000,000 to 250,000,000 shares. ​ In connection with the execution of the Blackstone Credit Agreement, the Company and certain entities affiliated with the Lenders (the “Purchasers”) also entered into a stock purchase agreement (the “Stock Purchase Agreement”) on the Closing Date for the sale and issuance of 1,095,290 shares of common stock (the “Shares”) to the Purchasers at a price of $45.65 per share, for an aggregate purchase price of approximately $50.0 million. The per share price represents the closing price of the Company’s common stock on the Nasdaq Global Select Market on October 26, 2022. ​ Under the Stock Purchase Agreement, the Company agreed to register the resale of the Shares on a registration statement to be filed with the Securities and Exchange Commission within 60 days of the Closing Date. The Company agreed to keep such registration statement effective for a period of six months following the Closing Date. In addition, subject to certain conditions, the Purchasers were entitled to participate in registered underwritten public offerings by the Company during such period. ​ Pursuant to the terms of the Stock Purchase Agreement, the Purchasers and certain of their controlled affiliates agreed not to, without the prior written approval of the Company and subject to specified conditions, directly or indirectly acquire shares of the Company’s outstanding common stock in excess of specified thresholds, seek or propose any acquisition of all or substantially all of the assets of the Company, seek or propose a merger or other business combination involving the Company, solicit proxies or consents with respect to any securities of the Company, seek to influence the management, board of directors or policies of the Company, or undertake other specified actions related to the potential acquisition of additional equity interests in the Company, or to encourage others to do any of the above (the “Standstill Restrictions”). The Standstill Restrictions terminated upon the termination of the Blackstone Credit Agreement. ​ The Purchasers also agreed not to sell or transfer the Shares without the prior written approval of the Company for a period of 90 days following the Closing Date, subject to certain exceptions. ​ In February 2023, the Company completed enrollment of its Phase 3 placebo-controlled clinical trial for sepiapterin for PKU. In connection with this event and pursuant to the Censa Merger Agreement, the Company paid a $30.0 million development milestone to the former Censa securityholders during the year ended December 31, 2023. The Company elected to pay this milestone in the form of shares of its common stock, less certain cash payments in accordance with the Censa Merger Agreement. Pursuant to such election, the Company issued 657,462 shares of its common stock and paid $0.4 million to the former Censa securityholders. ​ As of December 31, 2023, the Company’s number of authorized shares of common stock was 250,000,000.

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