As filed with the Securities and Exchange Commission on May 3, 2011

Registration No. 333-157957

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

POST-EFFECTIVE AMENDMENT NO. 1

TO

FORM S-8

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

Molecular Insight Pharmaceuticals, Inc.

(Exact name of registrant as specified in its charter)

Molecular Insight Pharmaceuticals, Inc.

Amended and Restated 2006 Equity Incentive Plan

(Full title of the plans)

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

DEREGISTRATION OF SECURITIES

On March 13, 2009, Molecular Insight Pharmaceuticals, Inc. (the “Registrant”) filed with the Securities and Exchange Commission a registration statement on Form S-8, Registration No. 333-157957 (the “Registration Statement”) registering additional 1,002,776 shares of the Registrant’s common stock, par value $0.01 (the “Common Stock”) for issuance under the Registrant’s Amended and Restated 2006 Equity Incentive Plan (the “Plan”). The Registrant no longer issues securities under the Plan. The Registrant files this Post-Effective Amendment No. 1 to the Registration Statement to deregister, as of the effectiveness of this post-effective amendment, all shares of the Common Stock that were registered under the Registration Statement and remain unissued under the Plan.

2

SIGNATURES

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 the requirements for filing this Post-Effective Amendment No. 1 to Form S-8 and has duly caused this Post-Effective Amendment No. 1 to Form S-8 to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Boston, Commonwealth of Massachusetts, on this May 3, 2011.

Pursuant to the requirements of the Securities Act of 1933, as amended, this Post-Effective Amendment No. 1 to Form S-8 has been signed by the following persons in the capacities below on the 3^rd day of May, 2011.