UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

Boundless Bio, Inc.

(Exact Name of Registrant as Specified in Its Charter)

(Former Name or Former Address, if Changed Since Last Report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§ 230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§ 240.12b-2 of this chapter).

Emerging growth company ☒

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

Item 1.02. 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, the "Lease"), related to the Company’s laboratory and office space consisting of approximately 80,168 rentable square feet (the "Premises") in that certain building located at 10955 Alexandria Way (previously known as Building 5 at One Alexandria Square), San Diego, California, as more particularly described in the Lease. Prior to the Lease Termination Agreement, the Lease was set to expire on October 31, 2034. Pursuant to the Lease Termination Agreement, the Lease will be terminated effective as of May 31, 2026. As consideration for the Lease Termination Agreement, (i) the Company agreed to pay Landlord a lease modification payment of $10.0 million, and (ii) Landlord will draw down and retain the full amount of the Company’s security deposit in the approximate amount of $0.5 million. The effectiveness of the Lease Termination Agreement was subject to satisfaction of a condition precedent relating to Landlord leasing the Premises to a new tenant, which condition precedent has been satisfied.

The foregoing summary of the Lease Termination Agreement does not purport to be complete and is qualified in its entirety by the full text of the Lease Termination Agreement, a copy of which the Company intends to file with its Quarterly Report on Form 10-Q for the quarter ending June 30, 2026.

Item 2.03. Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant.

The disclosure included in Item 1.02 of this Current Report on Form 8-K is incorporated into this Item 2.03 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.