Company: DAWN
Filing Date: 2025-12-05
Form Type: SC TO-T
Source: 0001104659-25-118858
Chunk: 0

Company: Day One Biopharmaceuticals, Inc.
Filing Date: 2025-12-05
Form: SC TO-T
Chunk 0
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### UNITED STATESSECURITIES AND EXCHANGE COMMISSION

#### Washington, D.C. 20549

### SCHEDULE TO
<div align='center'>**Tender Offer Statement under Section 14(d)(1) or 13(e)(1)**</div>

#### of the Securities Exchange Act of 1934

#### MERSANA THERAPEUTICS, INC.
<div align='center'>(Name of Subject Company (issuer))</div>

#### EMERALD MERGER SUB, INC.
<div align='center'>(Offeror)</div>

#### a direct wholly owned subsidiary of

#### DAY ONE BIOPHARMACEUTICALS, INC.
<div align='center'>(Parent of Offeror) 
 (Names of Filing Persons (identifying status as offeror, issuer or other person))</div>

#### Common Stock, $0.0001 par value per share
<div align='center'>(Title of Class of Securities)</div>

#### 59045L 106
<div align='center'>(CUSIP Number of Class of Securities)</div>

#### Adam Dubow

#### General Counsel

#### Day One Biopharmaceuticals, Inc.

#### 1800 Sierra Point Parkway, Suite 200

#### Brisbane, California 94005

#### Telephone: (650) 484-0899
<div align='center'>**(Name, address, and telephone numbers of person authorized to receive notices and communications on behalf of filing persons)

Copy to:**</div>

#### Robert A. Freedman

#### David Michaels

#### Julia Forbess

#### Fenwick & West LLP

#### 401 Union Street, Floor 5

#### Seattle, WA

#### Telephone: (206) 389-4510
☐

Check the box if the filing relates solely to preliminary communications made before the commencement of a tender offer.

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

☒

Third-party tender offer subject to Rule 14d-1.

☐

Issuer tender offer subject to Rule 13e-4.

☐

Going-private transaction subject to Rule 13e-3.

☐

Amendment to Schedule 13D under Rule 13d-2.

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:

☐

Rule 13e-