Antitumour imidazotetrazines. Part 37. Conjugation of the DNA major-groove alkylating imidazotetrazine mitozolomide to peptide motifs recognizing the minor groove.

Abstract

Methods have been developed to conjugate the antitumour imidazotetrazines mitozolomide and temozolomide to DNA minor and major groove-binding peptidic motifs by solid phase peptide synthesis. Side chain deprotection and resin cleavage steps were accomplished under acidic conditions to maintain the structural integrity of the imidazotetrazine nucleus. When mitozolomide was conjugated to the DNA minor groove-binding peptide (SPKK)2-NH2 (3) a strong preference for binding with [dA-dT]2 sequences was observed by circular dichroism studies, consistent with the construct making non-covalent interactions within the minor groove. This conjugate showed a > 100-fold DNA alkylating activity compared with the free imidazotetrazine as measured by a Taq polymerase assay. Unexpectedly, alkylation patterns of all conjugates were nearly identical to those elicited by the major groove interactive agents cisplatin and the unconjugated imidazotetrazines temozolomide and mitozolomide, indicating that covalent modification was restricted to guanine sites in the major groove of DNA irrespective of the targeting property of the peptidic ligand. The electrophilic reactive chloroethyldiazonium ion intermediate formed in the breakdown of the imidazotetrazine ring of mitozolomide (methyldiazonium ion from temozolomide) must be liberated from the DNA-bound conjugate prior to the alkylation event, and must diffuse to and react with more nucleophilic sites in the major groove.