HARP Harpoon Therapeutics Inc

As filed with the Securities and Exchange Commission on March 11, 2024

Registration No. 333-275958

Registration No. 333-272064

Registration No. 333-271400

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

POST-EFFECTIVE AMENDMENT TO

FORM S-3 REGISTRATION STATEMENT NO. 333-275958

FORM S-3 REGISTRATION STATEMENT NO. 333-272064

FORM S-3/A REGISTRATION STATEMENT NO. 333-271400

UNDER

THE SECURITIES ACT OF 1933

HARPOON THERAPEUTICS, INC.

(Exact Name of Registrant as Specified in Its Charter)

Kelly E.W. Grez

Assistant Secretary

Harpoon Therapeutics, Inc.

611 Gateway Boulevard, Suite 400

South San Francisco, California 94080

(650) 443-7400

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

Catherine Dargan

Andrew Fischer

Michael Riella

Covington & Burling LLP

One CityCenter

850 Tenth Street, NW

Washington, DC 20001

(202) 662-6000

Approximate date of commencement of proposed sale to the public: Not applicable. Removal from registration of securities that were not sold pursuant to the above referenced registration statement.

If the only securities being registered on this Form are being offered pursuant to dividend or interest reinvestment plans, please check the following box. ☐

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, other than securities offered only in connection with dividend or interest reinvestment plans, check the following box. ☐

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ☐

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ☐

If this Form is a registration statement pursuant to General Instruction I.D. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box. ☐

If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box. ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

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 7(a)(2)(B) of Securities Act. ☐

DEREGISTRATION OF SECURITIES

These Post-Effective Amendments (the “Post-Effective Amendments”) relate to the following registration statements on Form S-3 (together, the “Registration Statements”), filed with the Securities and Exchange Commission (the “SEC”) by Harpoon Therapeutics, Inc., a Delaware corporation (the “Registrant”):

On March 11, 2024, pursuant to the terms of the Agreement and Plan of Merger (the “Merger Agreement”), dated as of January 7, 2024, by and among the Registrant, Merck Sharp & Dohme LLC, a New Jersey limited liability company (“Merck”), and Hawaii Merger Sub, Inc., a Delaware corporation and wholly owned subsidiary of Merck (“Merger Sub”), Merger Sub merged with and into the Registrant (the “Merger”), with the Registrant surviving the Merger as a wholly-owned subsidiary of Merck.

As a result of the Merger, the Registrant has terminated, as of the date hereof, any and all offerings of its securities pursuant to its existing registration statements under the Securities Act of 1933, as amended, including the Registration Statements. In accordance with an undertaking made by the Registrant in the Registration Statements to remove from registration, by means of a post-effective amendment, any of the securities registered under the Registration Statements that remain unissued at the termination of the offerings, by filing these Post-Effective Amendments, the Registrant hereby removes from registration all securities registered but unsold under such Registration Statements, if any, as of the date hereof, and the Registration Statements are hereby amended, as appropriate, to reflect the deregistration of such securities.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, the Registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and has duly caused these Post-Effective Amendments to the Registration Statements on Form S-3 to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Rahway, State of New Jersey, on March 11, 2024.

No other person is required to sign these Post-Effective Amendments to the Registration Statements on Form S-3 in reliance on Rule 478 of the Securities Act of 1933, as amended.