Effects of DNA adduct structure and distribution on the mutagenicity and genotoxicity of two platinum anticancer drugs.

ABSTRACT

cis-Diamminedichloroplatinum(II) (cis-DDP) and cis,trans,cis-ammine(cyclohexylamine)-dibutyratodichloroplatinu m(IV) (ACDDP) are anticancer drugs that bind to DNA, forming replication blocking adducts. ACDDP, probably manifests its cytotoxicity through the metabolite cis-ammine(cyclohexylamine)dichloroplatinum(II) (ACDP). The biological effects of ACDP and cis-DDP were compared by studying polymerase inhibition in vitro and mutagenesis and genotoxicity in vivo in the duplex genome of bacteriophage M13mp18 replicated in Escherichia coli. cis-DDP and ACDP adducts were equally genotoxic within the statistical error limits of the analysis. Survival of genomes platinated by either drug, increased by threefold in cells pretreated with u.v. irradiation to induce the SOS functions of the host. In the M13mp18 lacZ' gene fragment, the mutagenicity of ACDP was lower than that of cis-DDP; the difference was approximately twofold at a dose of two adducts per 370 base-pair mutational target. Mutagenesis by both compounds was SOS-dependent. The structural basis for lower mutagenicity of ACDP is proposed to be its reduced reactivity at d(ApG) sites. This effect is attributed to an orientational isomerism that precludes the formation of one of two possible DNA adducts at d(ApG) residues. The types of mutations induced for both drugs were similar, but they occurred with different distributions. Both compounds induced primarily G-->T transversions at d(GpG) sites whereas G-->A transitions and A-->T transversions, many at d(ApG), d(GpNpG), and d(GpG) sites, were also well represented. The mapping of DNA adducts by DNA synthesis inhibition revealed excellent correlation between the location of DNA lesions and the sites of mutations. Analysis of the distribution of mutations and the distribution of potential platination sites revealed no sequence-dependent mutation hotspots; i.e. mutagenesis was random throughout the lacZ' region of the M13mp18 bacteriophage genome. These results offer insights into the molecular mechanism of mutagenicity of platinum anticancer drugs.