Nature of Business and Basis of Presentation	3 Months Ended
Mar. 31, 2026
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Business and Basis of Presentation	1. NATURE OF BUSINESS AND BASIS OF PRESENTATION Background Seaport Therapeutics, Inc., and its consolidated subsidiaries, Seaport or the Company, is a clinical-stage therapeutics company focused on inventing and developing medicines for patients with depression, anxiety, and other debilitating neuropsychiatric disorders using the Company's proprietary Glyph Platform. The Company's lead product candidate, GlyphAllo ("Glyph Allopregnanolone"), is a Glyphed oral prodrug of allopregnanolone, an endogenous molecule that has been clinically validated in two third-party trials in the United States for the treatment of postpartum depression, or PPD, a form of major depressive disorder, or MDD, as a rapidly acting antidepressant with anxiolytic and sleep-promoting effects. The Company's second product candidate, GlyphAgo ("Glyph Agomelatine"), is a Glyphed oral prodrug of agomelatine, a clinically validated anxiolytic and antidepressant that is approved for the treatment of generalized anxiety disorder, or GAD, in Australia and MDD in Australia and the European Union, or EU. The Company is also advancing Glyph2BLSD (Glyph 2-bromo-LSD), a Glyphed oral prodrug of the non-hallucinogenic LSD analog 2-bromo-LSD, in preclinical studies for depressive disorders, including treatment-resistant depression, or TRD, post-traumatic stress disorder, or PTSD, and headache disorders. There can be no assurance that the Company’s research and development efforts will be successfully completed, that any product candidates 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. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize significant product revenue from product sales. Reverse stock split On April 24, 2026, in anticipation of our initial public offering ("IPO"), the Company effected a 1-for-3.1407 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios of each series of the Company’s convertible preferred stock. The par value and authorized number of shares of common stock and convertible preferred stock were not adjusted as a result. All share and per share amounts for all periods presented in the accompanying condensed consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect the effect of the reverse stock split. Initial public offering During the second quarter of 2026, the Company completed its IPO, in which the Company sold an aggregate of 14,446,658 shares of its common stock, including 286,568 shares issued pursuant to the exercise of the underwriters' overallotment option, at a public offering price of $18.00 per share resulting in aggregate net proceeds of approximately $238.7 million, after deducting underwriter discounts, commissions and other estimated offering expenses. Immediately prior to the closing of the IPO, the Company's outstanding convertible preferred stock automatically converted into 36,272,475 shares of common stock. Following the closing of the IPO, no shares of convertible preferred stock were outstanding. In connection with the closing of the IPO, the Company's certificate of incorporation was amended and restated to authorize 700,000,000 shares of common stock, including 500,000,000 shares of voting common stock and 200,000,000 shares of non-voting common stock, par value $0.0001 per share and 10,000,000 shares of undesignated preferred stock, par value $0.0001 per share. Liquidity In accordance with Accounting Standards Update (“ASU”) 2014-15, Disclosure of Uncertainties About an Entity’s Ability to Continue as a Going Concern (Subtopic 205-40), the Company has evaluated whether there are conditions and events that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date the condensed consolidated financial statements are issued. The accompanying condensed consolidated financial statements have been prepared on a going-concern basis, which contemplates the realization of assets and satisfaction of liabilities in the normal course of business. Since inception, the Company has incurred significant operating losses. During the three months ended March 31, 2026 and 2025, the Company had a net loss of $25.4 million and $13.1 million, respectively. As of March 31, 2026, the Company had an accumulated deficit of $139.5 million. The Company has not generated any revenue from product sales and does not expect to generate revenue from sales of products in the near term, if at all. The Company expects to incur significant expenses and operating losses for the foreseeable future as it advances its product candidates into and through clinical development and continues to develop additional product candidates. As such, the Company expects our research and development and general and administrative costs to continue to increase significantly, including the costs associated with operating as a public company. The Company’s ability to access capital when needed is not assured and, if capital is not available to the Company when, and in the amounts needed, the Company may be required to significantly curtail, delay, or discontinue one or more of its research or development programs or the commercialization of any product candidate, or be unable to expand its operations, or otherwise capitalize on the Company’s business opportunities, as desired, which could materially harm the Company’s business, financial condition, and results of operations. To date, prior to the Company's initial public offering, the Company has funded its operations with the aggregate gross proceeds of $326.1 million from the Series A and Series B Financings. During the second quarter of 2026, the Company raised aggregate net proceeds of $238.7 million from the sale of shares of common stock in its initial public offering, after deducting underwriter discounts, commissions and other estimated offering expenses. The Company believes that its existing cash, cash equivalents and investments of $212.6 million as of March 31, 2026, together with the additional proceeds received from the IPO, will be sufficient to allow the Company to fund operations at least twelve months from the date that of these condensed consolidated financial statements are issued. Basis of Presentation The accompanying condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America, or GAAP. These condensed consolidated financial statements include the accounts of Seaport Therapeutics, Inc. and its wholly owned subsidiaries. All intercompany transactions and balances have been eliminated in consolidation. As of March 31, 2026, PureTech continued to provide various corporate services under the transition services agreement, or the TSA, which was initially set to terminate one year following the date of the Asset Transfer Agreement. In 2025, the TSA was amended to extend through July 2026 to provide ad-hoc support; however, the total value of the services are not material and the agreement was cancelled in April 2026.

Nature of Business and Basis of Presentation

3 Months Ended

Mar. 31, 2026

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Nature of Business and Basis of Presentation

1. NATURE OF BUSINESS AND BASIS OF PRESENTATION

Background

Seaport Therapeutics, Inc., and its consolidated subsidiaries, Seaport or the Company, is a clinical-stage therapeutics company focused on inventing and developing medicines for patients with depression, anxiety, and other debilitating neuropsychiatric disorders using the Company's proprietary Glyph Platform.

The Company's lead product candidate, GlyphAllo ("Glyph Allopregnanolone"), is a Glyphed oral prodrug of allopregnanolone, an endogenous molecule that has been clinically validated in two third-party trials in the United States for the treatment of postpartum depression, or PPD, a form of major depressive disorder, or MDD, as a rapidly acting antidepressant with anxiolytic and sleep-promoting effects.

The Company's second product candidate, GlyphAgo ("Glyph Agomelatine"), is a Glyphed oral prodrug of agomelatine, a clinically validated anxiolytic and antidepressant that is approved for the treatment of generalized anxiety disorder, or GAD, in Australia and MDD in Australia and the European Union, or EU.

The Company is also advancing Glyph2BLSD (Glyph 2-bromo-LSD), a Glyphed oral prodrug of the non-hallucinogenic LSD analog 2-bromo-LSD, in preclinical studies for depressive disorders, including treatment-resistant depression, or TRD, post-traumatic stress disorder, or PTSD, and headache disorders.

There can be no assurance that the Company’s research and development efforts will be successfully completed, that any product candidates 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. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize significant product revenue from product sales.

Reverse stock split

On April 24, 2026, in anticipation of our initial public offering ("IPO"), the Company effected a 1-for-3.1407 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios of each series of the Company’s convertible preferred stock. The par value and authorized number of shares of common stock and convertible preferred stock were not adjusted as a result. All share and per share amounts for all periods presented in the accompanying condensed consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect the effect of the reverse stock split.

Initial public offering

During the second quarter of 2026, the Company completed its IPO, in which the Company sold an aggregate of 14,446,658 shares of its common stock, including 286,568 shares issued pursuant to the exercise of the underwriters' overallotment option, at a public offering price of $18.00 per share resulting in aggregate net proceeds of approximately $238.7 million, after deducting underwriter discounts, commissions and other estimated offering expenses.

Immediately prior to the closing of the IPO, the Company's outstanding convertible preferred stock automatically converted into 36,272,475 shares of common stock. Following the closing of the IPO, no shares of convertible preferred stock were outstanding. In connection with the closing of the IPO, the Company's certificate of incorporation was amended and restated to authorize 700,000,000 shares of common stock, including 500,000,000 shares of voting common stock and 200,000,000 shares of non-voting common stock, par value $0.0001 per share and 10,000,000 shares of undesignated preferred stock, par value $0.0001 per share.

Liquidity

In accordance with Accounting Standards Update (“ASU”) 2014-15, Disclosure of Uncertainties About an Entity’s Ability to Continue as a Going Concern (Subtopic 205-40), the Company has evaluated whether there are conditions and events that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date the condensed consolidated financial statements are issued. The accompanying condensed consolidated financial statements have been prepared on a going-concern basis, which contemplates the realization of assets and satisfaction of liabilities in the normal course of business.

Since inception, the Company has incurred significant operating losses. During the three months ended March 31, 2026 and 2025, the Company had a net loss of $25.4 million and $13.1 million, respectively. As of March 31, 2026, the Company had an accumulated deficit of $139.5 million. The Company has not generated any revenue from product sales and does not expect to generate revenue from sales of products in the near term, if at all. The Company expects to incur significant expenses and operating losses for the foreseeable future as it advances its product candidates into and through clinical development and continues to develop additional product candidates. As such, the Company expects our research and development and general and administrative costs to continue to increase significantly, including the costs associated with operating as a public company. The Company’s ability to access capital when needed is not assured and, if capital is not available to the Company when, and in the amounts needed, the Company may be required to significantly curtail, delay, or discontinue one or more of its research or development programs or the commercialization of any product candidate, or be unable to expand its operations, or otherwise capitalize on the Company’s business opportunities, as desired, which could materially harm the Company’s business, financial condition, and results of operations.

To date, prior to the Company's initial public offering, the Company has funded its operations with the aggregate gross proceeds of $326.1 million from the Series A and Series B Financings. During the second quarter of 2026, the Company raised aggregate net proceeds of $238.7 million from the sale of shares of common stock in its initial public offering, after deducting underwriter discounts, commissions and other estimated offering expenses. The Company believes that its existing cash, cash equivalents and investments of $212.6 million as of March 31, 2026, together with the additional proceeds received from the IPO, will be sufficient to allow the Company to fund operations at least twelve months from the date that of these condensed consolidated financial statements are issued.

Basis of Presentation

The accompanying condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America, or GAAP. These condensed consolidated financial statements include the accounts of Seaport Therapeutics, Inc. and its wholly owned subsidiaries. All intercompany transactions and balances have been eliminated in consolidation.

As of March 31, 2026, PureTech continued to provide various corporate services under the transition services agreement, or the TSA, which was initially set to terminate one year following the date of the Asset Transfer Agreement. In 2025, the TSA was amended to extend through July 2026 to provide ad-hoc support; however, the total value of the services are not material and the agreement was cancelled in April 2026.