UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE TO

(Amendment No. 4)

Tender Offer Statement under Section 14(d)(1) or 13(e)(1)

of the Securities Exchange Act of 1934

DAY ONE BIOPHARMACEUTICALS, INC.

(Name of Subject Company (Issuer))

SERVIER DETROIT INC.

(Names of Filing Persons (Offeror))

a direct wholly owned subsidiary of

SERVIER PHARMACEUTICALS LLC

(Names of Filing Persons (Parent of Offeror))

an indirect wholly owned subsidiary of

SERVIER S.A.S.

(Names of Filing Persons (Parent of Offeror))

Common stock, $0.0001 par value per share

(Title of Class of Securities)

23954D109

(CUSIP Number of Class of Securities)

Deniz Razon

Chief Business Officer

Servier Pharmaceuticals LLC

200 Pier Four Boulevard

7th Floor

Boston, MA 02210

Telephone: (800) 807-6124

(Name, address, and telephone numbers of person authorized to receive notices and communications on behalf of filing persons)

Copy to:

Piotr Korzynski

Michael S. Pilo

Michael F. DeFranco

Michelle Carr

Baker & McKenzie LLP

300 East Randolph Street, Suite 5000

Chicago, Illinois 60601

Telephone: (312) 861-8000

Check the appropriate boxes below to designate any transactions to which the statement relates:

Check the following box if the filing is a final amendment reporting the results of the tender offer: ☒

If applicable, check the appropriate box(es) below to designate the appropriate rule provision(s) relied upon:

This Amendment No. 4 (this “Amendment”) amends and supplements the Tender Offer Statement on Schedule TO (as amended and together with any subsequent amendments and supplements thereto, the “Schedule TO”), filed by Servier S.A.S., a French société par actions simplifiée (“Servier”), Servier Pharmaceuticals LLC, a Delaware limited liability company and an indirect wholly owned subsidiary of Servier (“Parent”), and Servier Detroit Inc., a Delaware corporation and a wholly owned subsidiary of Parent (“Purchaser” and together with Servier and Parent, the “Servier Parties”), on March 26, 2026. The Schedule TO relates to the offer by the Servier Parties for all of the issued and outstanding shares of common stock, par value $0.0001 per share, of Day One Biopharmaceuticals, Inc., a Delaware corporation (the “Company”), upon the terms and subject to the conditions set forth in Offer to Purchase, the related Letter of Transmittal and the related Notice of Guaranteed Delivery, each as defined in the Schedule TO, copies of which are filed with the Schedule TO as Exhibits (a)(1)(A), (a)(1)(B) and (a)(1)(C), respectively.

Except to the extent specifically provided in this Amendment, the information set forth in the Schedule TO remains unchanged. Capitalized terms used but not defined herein have the meanings ascribed to them in the Schedule TO.

Items 1 through 9 and Item 11.

The disclosure in the Offer to Purchase and Items 1 through 9 and Item 11 of the Schedule TO, to the extent such items incorporate by reference the information contained in the Offer to Purchase, is hereby amended and supplemented by adding the following paragraphs:

“The Offer expired at one minute following 11:59 p.m., Eastern Time, on April 22, 2026 (the “Expiration Time”), and was not extended. According to the Depositary, as of the Expiration Time, 88,180,910 Shares, representing approximately 85.34% of the issued and outstanding Shares as of the Expiration Time, had been validly tendered and not properly withdrawn. As of the Expiration Time, the number of Shares validly tendered and not properly withdrawn pursuant to the Offer satisfied the Minimum Tender Condition.

Purchaser has accepted for payment, and will promptly pay for, all Shares that were validly tendered and not properly withdrawn pursuant to the Offer prior to the Expiration Time.

On April 23, 2026, the Servier Parties completed the acquisition of the Company pursuant to the terms of the Merger Agreement, through the merger of Purchaser with and into the Company in accordance with Section 251(h) of the DGCL, with the Company continuing as the surviving corporation in the Merger and thereby becoming a wholly owned subsidiary of Parent. At the Effective Time of the Merger, each issued and outstanding Share not tendered into the Offer (other than any Shares owned by the Company or the Servier Parties or held by any stockholders of the Company who were entitled to and who properly exercised appraisal rights in accordance with the DGCL) was automatically converted into the right to receive the Offer Price.

The Shares ceased to trade on Nasdaq prior to the commencement of trading on April 23, 2026, and the Company has requested that Nasdaq file a Notification of Removal from Listing and/or Registration under Section 12(b) of the Exchange Act on Form 25 to delist and deregister the Shares. Parent and the Company intend to file a certification and notice of termination of registration on Form 15 with the SEC requesting the termination of registration of the Shares under Section 12(g) of the Exchange Act and the suspension of reporting obligations under Section 13 and 15(d) of the Exchange Act with respect to the Shares and take steps to cause the termination of the registration of the Shares under the Exchange Act and suspend all of the Company’s reporting obligations under the Exchange Act as promptly as practicable.”

A copy of the press release issued by Parent on April 23, 2026, announcing the expiration and results of the Offer is attached hereto as Exhibit (a)(5)(D) and is incorporated herein by reference.”

Item 12 of the Schedule TO is hereby amended and supplemented by adding the following exhibit:

SIGNATURES

After due inquiry and to the best knowledge and belief of the undersigned, each of the undersigned certifies that the information set forth in this statement is true, complete and correct.

Exhibit (a)(5)(D)

Press Release

Servier completes the acquisition of

Day One Biopharmaceuticals

Suresnes, France, April 23, 2026 – Servier, an independent international pharmaceutical group governed by a foundation, today announced the successful completion of the tender offer to acquire all of the issued and outstanding shares of common stock of Day One Biopharmaceuticals, Inc. (Nasdaq: DAWN) (“Day One”), a biopharmaceutical company dedicated to developing and commercializing targeted therapies for people of all ages with life-threatening diseases, for $21.50 per share. The acquisition strengthens Servier’s leadership in low-grade glioma and expands the Group’s position in oncology with the addition of a marketed product and pipeline focused on rare cancers.

“This acquisition represents a significant step forward for Servier as part of our 2030 ambition to strengthen our position in rare cancers, and more specifically in pediatric low-grade glioma,” said Olivier Laureau, President of Servier. “Integrating Day One’s scientific and clinical capabilities will enhance our Group’s ability to support long-term innovation and translate science into meaningful medicines for children and families affected by rare cancers.”

Day One’s portfolio includes OJEMDA^TM (tovorafenib), an FDA-approved medicine in pediatric low-grade glioma, the most common form of childhood brain tumor. Day One already markets the product in the United States and has licensed the rights outside the U.S. to Ipsen.

The transaction also strengthens Servier’s oncology pipeline from early clinical to Phase 3. In addition to tovorafenib, which is being investigated in additional indications, Day One’s pipeline includes Emi-Le (emiltatug ledadotin), a novel antibody drug conjugate (ADC) and DAY301, a targeted therapy in rare cancers.

“Welcoming Day One to Servier marks an important next chapter in how we are expanding our presence in oncology in the U.S. and strengthening our ability to deliver for patients,” said David K. Lee, Executive Vice President, USA, and CEO, Servier Pharmaceuticals. “Day One is joining us with strong science, an approved medicine, and a team that knows how to turn innovation into real outcomes for patients. This is about combining focus with execution to deliver for patients with rare cancers.”

Completion of Tender Offer and Transaction Details

The tender offer expired at one minute past 11:59 p.m., Eastern Time, on April 22, 2026, and was not further extended following satisfaction of the minimum tender condition and the other offer conditions. Promptly following the foregoing expiration, Servier accepted tendered shares for payment of the Offer Price.

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Press Release

Following its acceptance of the tendered shares, Servier Detroit Inc., a direct wholly owned subsidiary of Servier Pharmaceuticals LLC (“Parent”), which is an indirect wholly owned subsidiary of Servier S.A.S., will promptly merge with and into Day One (the “Merger”). At the effective time of the Merger, each outstanding share of common stock of Day One that has not been validly tendered (other than shares owned by Day One or Servier or by any Day One stockholders who properly perfected their appraisal rights under the DGCL – Delaware General Corporation Law) will be converted automatically into the right to receive the Offer Price. As a result of the Merger, Day One will become a wholly owned subsidiary of Parent. With completion of the Merger, the transaction will represent a total equity value of approximately $2.5 billion.

Following the close of trading on The Nasdaq Stock Market LLC (“Nasdaq”) on April 22, 2026, all shares of Day One common stock ceased trading on Nasdaq, and Servier intends promptly to cause such shares to be delisted from Nasdaq and deregistered under the Securities Exchange Act of 1934, as amended.

Contacts

Servier Group

Laura Visserias

laura.visserias.part@servier.com

Servier Pharmaceuticals (U.S.)

Paulina Bucko

paulina.bucko@servier.com

About Servier

Servier is an independent international pharmaceutical group governed by a foundation. With its governance model, the Group is committed to therapeutic progress to serve patients and integrates the patient voice at every stage of the medicine life cycle.

As a leading global player in cardiology and venous diseases, Servier aims to become a leading innovator in oncology and neurology. The Group intends to offer targeted therapeutic solutions, particularly in rare cancers and neurological diseases, and invests nearly 20% of its brand-name sales in R&D.

Headquartered in France, Servier relies on its more than 20,000 employees and a solid geographic presence with medicines distributed in more than 130 countries. In the 2024/25 financial year, the Group achieved revenues of €6.9 billion.

More information on the Group website: servier.com

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Cautionary Note Regarding Forward-Looking Statements

This press release contains “forward-looking” statements, including, but not limited to: Servier’s plans to develop and commercialize cancer therapies and its pipeline, including the ability of Emi-Le and 301 to treat ACC or other cancers, statements including words such as “believe,” “plan,” “continue,” “expect,” “will,” “develop,” “signal,” “potential,” or “ongoing” and statements in the future tense are forward-looking statements. These forward-looking statements involve risks and uncertainties, as well as assumptions, which, if they do not fully materialize or prove incorrect, could cause our results to differ materially from those expressed or implied by such forward-looking statements.

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Press Release

Forward-looking statements are subject to risks and uncertainties that may cause Servier’s actual activities or results to differ significantly from those expressed in any forward-looking statement, including risks and uncertainties in this press release, including risks related to the ability to realize the anticipated benefits of the acquisition, including the possibility that the expected benefits from the acquisition will not be realized or will not be realized within the expected time period; the risk that the businesses will not be integrated successfully; disruption from the transaction making it more difficult to maintain business and operational relationships; n significant transaction costs; unknown liabilities; Servier’s ability to develop, obtain and retain regulatory approval for or commercialize any product candidate; Servier’s ability to protect intellectual property; the potential impact of global business or macroeconomic conditions, including as a result of inflation, rising interest rates, instability in the global banking system, geopolitical conflicts and the sufficiency of Servier’s cash, cash equivalents and investments to fund its operations. These forward-looking statements speak only as of the date hereof and Servier specifically disclaims any obligation to update these forward-looking statements or reasons why actual results might differ, whether as a result of new information, future events or otherwise, except as required by law.

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