shall have the meanings ascribed to such terms in the Merger Agreement. On August 28, 2026, the parties entered into Amendment No. 1 to the Merger Agreement (the “Amendment”). The Amendment amends the Merger Agreement to, among other things, (i) clarify certain definitions and provisions, including in relation to the Company Pre-Closing Financing; (ii) add mechanics for the treatment of Serapha restricted stock unit awards (“Serapha RSUs”), providing that each Serapha RSU (whether vested or u…

Entry into a Material Definitive Agreement. Merger Agreement On June 22, 2026, Boundless Bio, Inc., a Delaware corporation (Boundless Bio), entered into an Agreement and Plan of Merger and Reorganization (the Merger Agreement) with Serapha Bio, Inc., a Delaware corporation (Serapha), and Boulder Merger Sub Corp., a Delaware corporation and wholly owned subsidiary of Boundless Bio (Merger Sub). Upon the terms and subject to the satisfaction of the conditions described in the Merger Agreement,…

Unregistered Sales of Equity Securities. To the extent required by this Item, the information included in

The filing appears to be a placeholder or incomplete, with no specific management change details provided.

Changes in Control of Registrant. To the extent required by this Item, the information included in

Costs Associated with Exit or Disposal Activities. On June 23, 2026, Boundless Bio, in connection with signing of the Merger Agreement, announced a workforce reduction of approximately 75% of Boundless Bio’s workforce. Boundless Bio expects to incur one-time costs of approximately $3.0 million to $5.0 million in connection with the workforce reduction primarily related to one-time termination benefits (some of which are contractual), including severance and healthcare related benefits. Boundl…

Results of Operations and Financial Condition. On May 8, 2026, Boundless Bio, Inc. (the Company) issued a press release announcing its financial results for the quarter ended March 31, 2026. A copy of the press release is attached hereto as Exhibit 99.1 and is incorporated herein by reference. In accordance with General Instruction B.2 of Form 8-K, the information in this Current Report on Form 8-K, including Exhibit 99.1, shall not be deemed “filed” for purposes of Section 18 of the Securiti…

Termination of a material definitive agreement. On April 13, 2026, Boundless Bio, Inc. (the "Company") entered into an Agreement for Termination of Lease and Voluntary Surrender of Premises (the "Lease Termination Agreement") with ARE-10933 North Torrey Pines, LLC ("Landlord"), pursuant to which the Company and Landlord agreed to terminate that certain Lease Agreement dated as of December 20, 2021, as amended by that certain First Amendment to Lease dated as of November 1, 2024 (as amended, t…

by reference. SIGNATURES Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized. BOUNDLESS BIO, INC. Date: April 13, 2026 By: /s/ Jessica Oien Name: Jessica Oien Title: Chief Legal Officer and Corporate Secretary

Director — Christine Brennan, Ph.D.: Dr. Brennan resigned to take a new role at Johnson & Johnson Development Corporation.

Results of Operations and Financial Condition. On March 9, 2026, Boundless Bio, Inc. (the Company) issued a press release announcing its financial results for the fourth quarter and full year ended December 31, 2025. A copy of the press release is attached hereto as Exhibit 99.1 and is incorporated herein by reference. In accordance with General Instruction B.2 of Form 8-K, the information in this Current Report on Form 8-K, including Exhibit 99.1, shall not be deemed “filed” for purposes of…

Results of Operations and Financial Condition. On November 5, 2025, Boundless Bio, Inc. (the Company) issued a press release announcing its financial results for the quarter ended September 30, 2025. A copy of the press release is attached hereto as Exhibit 99.1 and is incorporated herein by reference. In accordance with General Instruction B.2 of Form 8-K, the information in this Current Report on Form 8-K, including Exhibit 99.1, shall not be deemed “filed” for purposes of Section 18 of the…

Results of Operations and Financial Condition. On August 5, 2025, Boundless Bio, Inc. (the Company) issued a press release announcing its financial results for the quarter ended June 30, 2025. A copy of the press release is attached hereto as Exhibit 99.1 and is incorporated herein by reference. In accordance with General Instruction B.2 of Form 8-K, the information in this Current Report on Form 8-K, including Exhibit 99.1, shall not be deemed “filed” for purposes of Section 18 of the Securi…

Other Events. On May 23, 2025, the Company announced a portfolio prioritization focused on evaluating BBI-355 and BBI-825 as a combination therapy and a new development candidate, BBI-940, for its novel Kinesin program. It also announced the extension of its expected cash runway through expected proof-of concept clinical readouts for each of its programs, as further described below. BBI-355 and BBI-825 Programs The Company has been investigating BBI-355, a novel, oral, selective CHK1 inhibito…

Costs Associated with Exit or Disposal Activities. On May 23, 2025, Boundless Bio, Inc. (the Company), in connection with the portfolio prioritization described below, announced a workforce reduction of approximately one-third of the Company’s workforce. The Company expects to incur one-time costs of approximately $1.2 million in connection with the workforce reduction primarily related to one-time termination benefits (some of which are contractual), including severance and healthcare relate…

Results of Operations and Financial Condition. On May 9, 2025, Boundless Bio, Inc. (the Company) issued a press release announcing its financial results for the quarter ended March 31, 2025. A copy of the press release is attached hereto as Exhibit 99.1 and is incorporated herein by reference. In accordance with General Instruction B.2 of Form 8-K, the information in this Current Report on Form 8-K, including Exhibit 99.1, shall not be deemed “filed” for purposes of Section 18 of the Securiti…

or 7.01 of Form 8-K or to provide disclosure pursuant to

Results of Operations and Financial Condition. On March 27, 2025, Boundless Bio, Inc. (the Company) issued a press release announcing its financial results for the fourth quarter and full year ended December 31, 2024. A copy of the press release is attached hereto as Exhibit 99.1 and is incorporated herein by reference. In accordance with General Instruction B.2 of Form 8-K, the information in this Current Report on Form 8-K, including Exhibit 99.1, shall not be deemed “filed” for purposes of…

Chief Medical Officer — Klaus Wagner, M.D., Ph.D.: Klaus Wagner is stepping down from his role as Chief Medical Officer and departing the Company.

Results of Operations and Financial Condition. On November 7, 2024, Boundless Bio, Inc. (the Company) issued a press release announcing its financial results for the quarter ended September 30, 2024. A copy of the press release is attached hereto as Exhibit 99.1 and is incorporated herein by reference. In accordance with General Instruction B.2 of Form 8-K, the information in this Current Report on Form 8-K, including Exhibit 99.1, shall not be deemed “filed” for purposes of Section 18 of the…

Chief Financial Officer and Treasurer — Jami Rubin: Ms. Rubin stepped down from her role as Chief Financial Officer and Treasurer.

The filing describes a compensation adjustment through option repricing for certain employees, including named executive officers.

Results of Operations and Financial Condition. On August 12, 2024, Boundless Bio, Inc. (the Company) issued a press release announcing its financial results for the quarter ended June 30, 2024. A copy of the press release is attached hereto as Exhibit 99.1 and is incorporated herein by reference. In accordance with General Instruction B.2 of Form 8-K, the information in this Current Report on Form 8-K, including Exhibit 99.1, shall not be deemed “filed” for purposes of Section 18 of the Secur…

Results of Operations and Financial Condition. On May 13, 2024, Boundless Bio, Inc. (the Company) issued a press release announcing its financial results for the quarter ended March 31, 2024. A copy of the press release is attached hereto as Exhibit 99.1 and is incorporated herein by reference. In accordance with General Instruction B.2 of Form 8-K, the information in this Current Report on Form 8-K, including Exhibit 99.1, shall not be deemed “filed” for purposes of Section 18 of the Securit…