Organization and Basis of Presentation (Policies)	6 Months Ended
Jun. 30, 2025
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Reverse Stock Split	Reverse Stock Split On October 18, 2024, the Company effected a 1-for-8.6103 reverse stock split of its issued and outstanding shares of common stock. Upon the effectiveness of the reverse stock split, (i) all shares of outstanding common stock were adjusted; (ii) the conversion ratio of the convertible preferred stock was adjusted; (iii) the number of shares of common stock for which each outstanding option to purchase common stock is exercisable were adjusted; and (iv) the exercise price of each outstanding option to purchase common stock was adjusted. All of the outstanding common stock (including shares of common stock subject to the Company’s options and as converted for the outstanding convertible preferred stock), share prices, exercise prices and per share amounts contained in the condensed financial statements have been retroactively adjusted to reflect this reverse stock split for all periods presented. The par value per share and the authorized number of shares of common stock and preferred stock were not adjusted as a result of the reverse stock split. The number of authorized shares did not change.
Initial Public Offering	Initial Public Offering In October 2024, the Company completed its initial public offering (“IPO”), pursuant to which it issued and sold an aggregate of 18.4 million shares of its common stock at the IPO price of $18.00 per share, including 2.4 million shares that were issued to the underwriters pursuant to the full exercise of their option to purchase additional shares, resulting in net proceeds of $302.8 million, net of total offering costs of $28.4 million. Immediately prior to the closing of the IPO, the Company’s then outstanding convertible preferred stock automatically converted into 22,839,774 shares of common stock. Following the closing of the IPO, no shares of convertible preferred stock were authorized or outstanding. In connection with the completion of the IPO, on October 28, 2024, the Company’s certificate of incorporation was amended and restated to (i) authorize 500.0 million shares of common stock, par value $0.001 per share, which eliminates all references to the previously existing series of convertible preferred stock; and (ii) authorize 10.0 million shares of undesignated preferred stock, par value $0.001 per share, that may be issued from time to time by the Company’s board of directors in one or more series.
Liquidity and Capital Resources	Liquidity and Capital Resources The accompanying unaudited condensed financial statements have been prepared assuming the Company will continue as a going concern, which assumes that the Company will realize its assets and satisfies its liabilities in the normal course of business. The Company is subject to risks inherent in operating an early-stage biotechnology business. These risks include, but are not limited to, dependence on the development of marketable products, the ability to attract, retain, and motivate qualified personnel, rapid technological changes and the rapidly evolving nature of the biotechnology industry. The Company has incurred significant losses and negative cash flows from operations since its inception and expects to incur losses as a result of its continued research and development activities. To date, none of the Company’s product candidates have been approved by the U.S. Food and Drug Administration (the “FDA”) for commercial sale and, therefore, the Company has not generated any revenue from product sales. The Company historically financed its operations primarily through the issuance of convertible promissory notes, convertible preferred stock, and most recently, through the sale of its common stock in an IPO. The Company will need additional financing to support its continuing operations and pursue its growth strategy. Until such time as the Company can generate significant revenue from product sales, if ever, it expects to finance its operations through equity offerings, debt financings or other capital sources, including collaborations with other companies or other strategic transactions. The Company may not be able to obtain funding on acceptable terms, or at all. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. For the six months ended June 30, 2025 and 2024, the Company incurred net losses of $46.3 million and $30.6 million, respectively. The Company had an accumulated deficit of $164.7 million as of June 30, 2025. The Company believes its cash, cash equivalents, and marketable securities of $379.2 million as of June 30, 2025 will be sufficient to fund its operations for, at least, 12 months from the date of issuance of the unaudited condensed financial statements.
Basis of Presentation	Basis of Presentation The accompanying unaudited interim condensed financial statements have been prepared in conformity with generally accepted accounting principles in the U.S. (“U.S. GAAP”) for interim financial information. Accordingly, they do not include all of the information and footnotes required by U.S. GAAP for complete financial statements. In the opinion of management, all adjustments (consisting only of normal recurring adjustments) that are necessary for the fair presentation of the unaudited interim condensed financial statements for the periods presented have been included. Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification and Accounting Standards Updates (“ASUs”), of the Financial Accounting Standards Board (“FASB”). The interim condensed financial statements are unaudited. The operating results presented in these unaudited interim condensed financial statements are not necessarily indicative of the results that may be expected for the full year or for any other future annual or interim period. Actual results could differ materially from estimates and assumptions. As appropriate, the Company assesses estimates each period and updates them to reflect current information, and generally reflect any changes in estimates in the period first identified. The balance sheet data at December 31, 2024 was derived from the audited financial statements included in Septerna’s Annual Report on Form 10-K for the year ended December 31, 2024 as filed with the SEC on March 27, 2025. These interim unaudited condensed financial statements should be read in conjunction with the audited financial statements and related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2024.
Significant Accounting Policies	Significant Accounting Policies During the six months ended June 30, 2025, there were no changes to the Company’s significant accounting policies as described in Note 2 of the audited financial statements for the year ended December 31, 2024.
Accounting Pronouncements Not Yet Adopted	Accounting Pronouncements Not Yet Adopted From time to time, new accounting pronouncements are issued by the FASB, under its Accounting Standards Codification (“ASC”) or other standard setting bodies, and adopted by the Company as of the specified date. In November 2024, the FASB issued ASU No. 2024-03, Income Statement—Reporting Comprehensive Income—Expense Disaggregation Disclosures (Subtopic 220-40): Disaggregation of Income Statement Expenses, an accounting standard update that requires the Company to disclose more detailed information about the types of expenses (including employee compensation, depreciation, and amortization) included in each relevant income statement expense caption. In January 2025, the FASB issued ASU 2025-0, "Income Statement—Reporting Comprehensive Income—Expense Disaggregation Disclosures (Subtopic 220-40): Clarifying the Effective Date", to clarify the effective date of ASU 2024-03. These amendments are effective for fiscal years beginning after December 15, 2026, and interim reporting periods beginning after December 15, 2027. Early adoption is permitted. The Company is currently evaluating the impact that these updates will have on its disclosures. In December 2023, the FASB issued ASU No. 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures, which enhances the transparency and decision usefulness of income tax disclosures. The standard is intended to improve income tax disclosures primarily related to the rate reconciliation and income taxes paid information. This update also includes certain other amendments to improve the effectiveness of income tax disclosures. The ASU is effective for fiscal years beginning after December 15, 2024, on a prospective basis. Early adoption and retrospective reporting are permitted. The Company is currently evaluating the impact of ASU 2023-09 on its interim condensed financial statements.

Organization and Basis of Presentation (Policies)

6 Months Ended

Jun. 30, 2025

Organization, Consolidation and Presentation of Financial Statements [Abstract]

On October 18, 2024, the Company effected a 1-for-8.6103 reverse stock split of its issued and outstanding shares of common stock. Upon the effectiveness of the reverse stock split, (i) all shares of outstanding common stock were adjusted; (ii) the conversion ratio of the convertible preferred stock was adjusted; (iii) the number of shares of common stock for which each outstanding option to purchase common stock is exercisable were adjusted; and (iv) the exercise price of each outstanding option to purchase common stock was adjusted. All of the outstanding common stock (including shares of common stock subject to the Company’s options and as converted for the outstanding convertible preferred stock), share prices, exercise prices and per share amounts contained in the condensed financial statements have been retroactively adjusted to reflect this reverse stock split for all periods presented. The par value per share and the authorized number of shares of common stock and preferred stock were not adjusted as a result of the reverse stock split. The number of authorized shares did not change.

Initial Public Offering

In October 2024, the Company completed its initial public offering (“IPO”), pursuant to which it issued and sold an aggregate of 18.4 million shares of its common stock at the IPO price of $18.00 per share, including 2.4 million shares that were issued to the underwriters pursuant to the full exercise of their option to purchase additional shares, resulting in net proceeds of $302.8 million, net of total offering costs of $28.4 million. Immediately prior to the closing of the IPO, the Company’s then outstanding convertible preferred stock automatically converted into 22,839,774 shares of common stock.

Following the closing of the IPO, no shares of convertible preferred stock were authorized or outstanding. In connection with the completion of the IPO, on October 28, 2024, the Company’s certificate of incorporation was amended and restated to (i) authorize 500.0 million shares of common stock, par value $0.001 per share, which eliminates all references to the previously existing series of convertible preferred stock; and (ii) authorize 10.0 million shares of undesignated preferred stock, par value $0.001 per share, that may be issued from time to time by the Company’s board of directors in one or more series.

Liquidity and Capital Resources

The accompanying unaudited condensed financial statements have been prepared assuming the Company will continue as a going concern, which assumes that the Company will realize its assets and satisfies its liabilities in the normal course of business. The Company is subject to risks inherent in operating an early-stage biotechnology business. These risks include, but are not limited to, dependence on the development of marketable products, the ability to attract, retain, and motivate qualified personnel, rapid technological changes and the rapidly evolving nature of the biotechnology industry.

The Company has incurred significant losses and negative cash flows from operations since its inception and expects to incur losses as a result of its continued research and development activities. To date, none of the Company’s product candidates have been approved by the U.S. Food and Drug Administration (the “FDA”) for commercial sale and, therefore, the Company has not generated any revenue from product sales.

The Company historically financed its operations primarily through the issuance of convertible promissory notes, convertible preferred stock, and most recently, through the sale of its common stock in an IPO. The Company will need additional financing to support its continuing operations and pursue its growth strategy. Until such time as the Company can generate significant revenue from product sales, if ever, it expects to finance its operations through equity offerings, debt financings or other capital sources, including collaborations with other companies or other strategic transactions. The Company may not be able to obtain funding on acceptable terms, or at all. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders.

For the six months ended June 30, 2025 and 2024, the Company incurred net losses of $46.3 million and $30.6 million, respectively. The Company had an accumulated deficit of $164.7 million as of June 30, 2025. The Company believes its cash, cash equivalents, and marketable securities of $379.2 million as of June 30, 2025 will be sufficient to fund its operations for, at least, 12 months from the date of issuance of the unaudited condensed financial statements.

Basis of Presentation

The accompanying unaudited interim condensed financial statements have been prepared in conformity with generally accepted accounting principles in the U.S. (“U.S. GAAP”) for interim financial information. Accordingly, they do not include all of the information and footnotes required by U.S. GAAP for complete financial statements. In the opinion of management, all adjustments (consisting only of normal recurring adjustments) that are necessary for the fair presentation of the unaudited interim condensed financial statements for the periods presented have been included. Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification and Accounting Standards Updates (“ASUs”), of the Financial Accounting Standards Board (“FASB”). The interim condensed financial statements are unaudited. The operating results presented in these unaudited interim condensed financial statements are not necessarily indicative of the results that may be expected for the full year or for any other future annual or interim period. Actual results could differ materially from estimates and assumptions. As appropriate, the Company assesses estimates each period and updates them to reflect current information, and generally reflect any changes in estimates in the period first identified.

The balance sheet data at December 31, 2024 was derived from the audited financial statements included in Septerna’s Annual Report on Form 10-K for the year ended December 31, 2024 as filed with the SEC on March 27, 2025. These interim unaudited condensed financial statements should be read in conjunction with the audited financial statements and related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2024.

Significant Accounting Policies

During the six months ended June 30, 2025, there were no changes to the Company’s significant accounting policies as described in Note 2 of the audited financial statements for the year ended December 31, 2024.

Accounting Pronouncements Not Yet Adopted

From time to time, new accounting pronouncements are issued by the FASB, under its Accounting Standards Codification (“ASC”) or other standard setting bodies, and adopted by the Company as of the specified date.

In November 2024, the FASB issued ASU No. 2024-03, Income Statement—Reporting Comprehensive Income—Expense Disaggregation Disclosures (Subtopic 220-40): Disaggregation of Income Statement Expenses, an accounting standard update that requires the Company to disclose more detailed information about the types of expenses (including employee compensation, depreciation, and amortization) included in each relevant income statement expense caption. In January 2025, the FASB issued ASU 2025-0, "Income Statement—Reporting Comprehensive Income—Expense Disaggregation Disclosures (Subtopic 220-40): Clarifying the Effective Date", to clarify the effective date of ASU 2024-03. These amendments are effective for fiscal years beginning after December 15, 2026, and interim reporting periods beginning after December 15, 2027. Early adoption is permitted. The Company is currently evaluating the impact that these updates will have on its disclosures.

In December 2023, the FASB issued ASU No. 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures, which enhances the transparency and decision usefulness of income tax disclosures. The standard is intended to improve income tax disclosures primarily related to the rate reconciliation and income taxes paid information. This update also includes certain other

amendments to improve the effectiveness of income tax disclosures. The ASU is effective for fiscal years beginning after December 15, 2024, on a prospective basis. Early adoption and retrospective reporting are permitted. The Company is currently evaluating the impact of ASU 2023-09 on its interim condensed financial statements.