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

Date of Report (Date of earliest event reported): June 22, 2026

Definium Therapeutics, Inc.

(Exact Name of Registrant as Specified in its Charter)

Registrant’s telephone number, including area code: (212) 220-6633

Not Applicable

(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 x

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. ¨

On June 23, 2026, Definium Therapeutics, Inc. (the “Company”) entered into an underwriting agreement (the “Underwriting Agreement”) with J.P. Morgan Securities LLC, Jefferies LLC, Leerink Partners LLC, and BofA Securities, Inc., as representatives of the several underwriters named therein (the “Underwriters”), in connection with an underwritten public offering (the “Offering”) of 20,588,236 common shares (the “Shares”) of the Company, without par value (“Common Shares”). The public offering price for the Shares is $34.00 per share. In addition, under the terms of the Underwriting Agreement, the Company granted the Underwriters an option, exercisable for 30 days, to purchase up to an additional 3,088,235 Common Shares at the same price, which was exercised by the Underwriters in full on June 24, 2026. No distribution under the Offering may occur in Canada or to a person resident in Canada except pursuant to a Canadian prospectus or prospectus exemption. The Offering is expected to close on June 25, 2026.

The gross proceeds to the Company from the Offering, including the full exercise by the Underwriters of their option to purchase additional Common Shares, are expected to be approximately $805 million. The net proceeds to the Company from the Offering, including the full exercise by the Underwriters of their option to purchase additional Common Shares, are expected to be approximately $758 million, after deducting underwriting discounts and commissions and other estimated offering expenses payable by the Company.

The Company intends to use the net proceeds from the Offering for the research and development of its product candidates, preparation activities for potential commercialization of DT120 ODT, if approved, and working capital and general corporate purposes.

The Underwriting Agreement contains customary representations, warranties and agreements by the Company, customary conditions to closing, indemnification obligations of the Company and the Underwriters, including for liabilities under the Securities Act of 1933, as amended, other obligations of the parties and termination provisions. The representations, warranties and agreements contained in the Underwriting Agreement were made only for purposes of such agreement and as of specific dates, were solely for the benefit of the parties to such agreement, and may be subject to limitations agreed upon by the contracting parties.

The Offering was made pursuant to the Company’s shelf registration statement on Form S-3 (File No. 333-280548), which was filed with the Securities and Exchange Commission on June 28, 2024 and automatically became effective upon filing, and a related base prospectus, as supplemented by a prospectus supplement.

In connection with the Underwriting Agreement, the Company and the Company’s directors and executive officers also agreed not to sell or transfer any Common Shares without first obtaining the written consent of J.P. Morgan Securities LLC, Jefferies LLC, Leerink Partners LLC, and BofA Securities, Inc., subject to certain exceptions, for 60 days after the date of the Underwriting Agreement.

The foregoing summary of the terms of the Underwriting Agreement does not purport to be complete and is qualified in its entirety by reference to the Underwriting Agreement, which is attached hereto as Exhibit 1.1, and which is incorporated herein by reference. Osler, Hoskin & Harcourt LLP, Canadian counsel to the Company, delivered an opinion as to the legality of the issuance and sale of Common Shares in the Offering, a copy of which is attached hereto as Exhibit 5.1 and is incorporated herein by reference.

The Company issued press releases announcing the launch and pricing of the Offering on June 22, 2026 and June 23, 2026, respectively. Copies of these press releases are attached hereto as Exhibit 99.1 and Exhibit 99.2, respectively, and are incorporated herein by reference.

(d)       Exhibits

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, as amended, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

ATTACHMENTS / EXHIBITS

EXHIBIT 1.1

EXHIBIT 5.1

EXHIBIT 99.1

EXHIBIT 99.2

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