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, a 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.

This Post-Effective Amendment No. 1 relates to Registration Statement No. 333-236717 on Form S-3 (the “Registration Statement”) filed by Arena Pharmaceuticals, Inc., a Delaware corporation (the “Registrant” or the “Company”) with the Securities and Exchange Commission (“SEC”) on February 27, 2020 pertaining to the registration by the Company of 6,325,000 shares of common stock, par value $0.0001 per share, of the Company.

On March 11, 2022, pursuant to the Agreement and Plan of Merger, dated as of December 12, 2021 (the “Merger Agreement”), by and among the Company, Pfizer Inc., a Delaware corporation (“Parent”), and Antioch Merger Sub, Inc., a Delaware corporation (“Merger Sub”), Merger Sub merged with and into the Company, with the Company continuing as the surviving corporation and as a wholly owned subsidiary of Parent.

As a result of the consummation of the transactions contemplated by the Merger Agreement, the Registrant has terminated all offerings of its securities pursuant to the Registration Statement. In accordance with an undertaking made by the Registrant in the Registration Statement to remove from registration by means of a post-effective amendment any securities which remain unsold at the termination of the offering, the Registrant hereby removes and withdraws from registration all securities of the Registrant registered pursuant to the Registration Statement that remain unsold or otherwise unissued as of the date hereof. The Registration Statement is hereby amended, as appropriate, to reflect the deregistration of such securities, and the Registrant hereby terminates the effectiveness of the Registration Statement.

Pursuant to the requirements of the Securities Act of 1933, as amended, 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 this Post-Effective Amendment No. 1 to the Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of New York, State of New York on March 11, 2022.