Definium Therapeutics, Inc. Announces Closing of $805 Million Upsized Public Offering, Including Full Exercise of the Underwriters’ Option to Purchase Additional Shares

Definium Therapeutics, Inc. (Nasdaq: DFTX) (the “Company” or “Definium”), a late-stage clinical biopharmaceutical company developing a new generation of therapeutics intended to address underlying causes of psychiatric and neurological disorders, today announced the closing of its previously announced underwritten public offering of 23,676,471 common shares, without par value, which includes the exercise in full by the underwriters of their option to purchase 3,088,235 additional common shares, at a public offering price of $34.00 per common share. All of the shares were offered by Definium. The gross proceeds from this offering were $805 million, before deducting underwriting discounts, commissions, and other offering-related expenses.

Definium intends to use the net proceeds from this 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.

J.P. Morgan, Jefferies, Leerink Partners, and BofA Securities acted as the joint lead bookrunners for the offering, and Evercore ISI and Stifel acted as bookrunners for the offering. Oppenheimer & Co. and LifeSci Capital acted as co-lead managers for the offering. The offering closed on June 25, 2026. No distribution under the offering occurred in Canada or to a person resident in Canada.

The securities in the offering were offered by the Company pursuant to a shelf registration statement on Form S-3 (File No. 333-280548) that was filed with the Securities and Exchange Commission (“SEC”) on June 28, 2024 and became effective upon filing. The offering was made by means of a prospectus supplement and accompanying prospectus relating to the offering that form a part of the shelf registration statement. A final prospectus supplement and the accompanying prospectus relating to and describing the terms of the offering were filed with the SEC and SEDAR+ on June 24, 2026, and are available on the SEC’s website at www.sec.gov and on SEDAR+’s website at www.sedarplus.ca. Copies of the final prospectus supplement and the accompanying prospectus relating to the offering may be obtained by contacting the following: J.P. Morgan Securities LLC, c/o Broadridge Financial Solutions, 1155 Long Island Avenue, Edgewood, New York 11717 or by email at prospectus-eq_fi@jpmchase.com and postsalemanualrequests@broadridge.com J.P. Morgan Securities LLC, c/o Broadridge Financial Solutions, 1155 Long Island Avenue, Edgewood, New York 11717 or by email at prospectus-eq_fi@jpmchase.com and postsalemanualrequests@broadridge.com; Jefferies LLC by mail at Attn: Equity Syndicate Prospectus Department, 520 Madison Avenue, New York, NY 10022, by telephone at (877) 821-7388, or by email at prospectus_department@jefferies.com; Leerink Partners LLC, Attention: Syndicate Department, 53 State Street, 40th Floor, Boston, Massachusetts 02109, by telephone at (800) 808-7525 ext. 6105 or by email at syndicate@leerink.com; BofA Securities, Attention: Prospectus Department, 201 North Tryon Street, NC1-022-02-25 Charlotte, NC 28255-0001 or by email at dg.prospectus_requests@bofa.com; Evercore Group L.L.C., Attention: Equity Capital Markets, 55 East 52nd Street, 35th Floor, New York, NY 10055, by telephone at (888) 474-0200 or by email at ecm.prospectus@evercore.com; Stifel, Nicolaus & Company, Incorporated, Attention: Syndicate, One Montgomery Street, Suite 3700, San Francisco, CA 94104, by telephone at (415) 364-2720 or by emailing syndprospectus@stifel.com; Oppenheimer & Co. Inc., Attention: Syndicate Prospectus Department, 85 Broad Street, 26th Floor, New York, NY 10004, or by telephone at (212) 667-8055 or by email at EquityProspectus@opco.com and LifeSci Capital LLC, Attention: LifeSci Capital LLC at 1700 Broadway, 40th Floor, New York, New York 10019, or by email at compliance@lifescicapital.com.

This press release shall not constitute an offer to sell or the solicitation of an offer to buy any of the securities described herein, nor shall there be any sale of these securities in any state or jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such state or jurisdiction.

About Definium

The mission of Definium Therapeutics is to forge a new era of psychiatry by applying scientific rigor to psychedelics, with the goal of developing accessible treatments that unlock healing at scale. Guided by a recognition that patients deserve more than better, Definium is relentlessly advancing a new generation of therapeutics intended to address underlying causes of psychiatric and neurological disorders. By turning evidence into impact, Definium aims to change the trajectory of today’s mental health care crisis and enable a healthier future. Headquartered in New York, Definium Therapeutics trades on Nasdaq under the symbol “DFTX.”

Forward-Looking Statements

Certain statements in this press release related to the Company constitute “forward-looking information” within the meaning of applicable securities laws and are prospective in nature. Forward-looking information is not based on historical facts, but rather on current expectations and projections about future events and is therefore subject to risks and uncertainties which could cause actual results to differ materially from the future results expressed or implied by the forward-looking statements. These statements generally can be identified by the use of forward-looking words such as “will,” “may,” “should,” “could,” “intend,” “estimate,” “plan,” “anticipate,” “expect,” “believe,” “potential” or “continue,” or the negative thereof or similar variations. Forward-looking information in this press release includes, but is not limited to, statements regarding the intended use of the net proceeds from the offering. There are numerous risks and uncertainties that could cause actual results and the Company’s plans and objectives to differ materially from those expressed in the forward-looking information. These forward-looking statements are based on the Company’s current expectations, estimates, forecasts and projections about the offering, the Company’s business and the industry in which it operates and management’s beliefs and assumptions, including the non-occurrence of the risks and uncertainties that are described in its filings made with the SEC and the applicable Canadian securities regulators or other events occurring outside of its normal course of business, and are not guarantees of future performance or development and involve known and unknown risks, uncertainties and other factors that are in some cases beyond its control. Except as required by law, the Company undertakes no duty or obligation to update any forward-looking statements contained in this release as a result of new information, future events, changes in expectations or otherwise.

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