Astex Pharmaceuticals (SUPG) 10-Q 2012 Q2 Quarterly report

QuickLinks -- Click here to rapidly navigate through this document

EXHIBIT 32.1

CERTIFICATION OF CHIEF EXECUTIVE OFFICER AND CHIEF FINANCIAL OFFICER
PURSUANT TO
18 U.S.C. SECTION 1350,
AS ADOPTED PURSUANT TO
SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

I, James S.J. Manuso, certify, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, that the Quarterly Report of Astex Pharmaceuticals, Inc. on Form 10-Q for the quarterly period ended June 30, 2012 fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934 and that information contained in such Form 10-Q fairly presents in all material respects the financial condition and results of operations of Astex Pharmaceuticals, Inc.

Date:	August 9, 2012	By:	/s/ JAMES S.J. MANUSO
		Name:	James S.J. Manuso
		Title:	Chairman and Chief Executive Officer (Principal Executive Officer)

I, Michael Molkentin, certify, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, that the Quarterly Report of Astex Pharmaceuticals, Inc. on Form 10-Q for the quarterly period ended June 30, 2012 fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934 and that information contained in such Form 10-Q fairly presents in all material respects the financial condition and results of operations of Astex Pharmaceuticals, Inc.

Date:	August 9, 2012	By:	/s/ MICHAEL MOLKENTIN
		Name:	Michael Molkentin
		Title:	Chief Financial Officer (Principal Financial and Accounting Officer)

QuickLinks

EXHIBIT 32.1