DNA Interactions of Bifunctional Dinuclear Platinum(II) Antitumor Agents

Modifications of natural DNA in a cell‐free medium by dinuclear bisplatinum complexes with equivalent coordination spheres, represented by the general formula [{trans‐PtCl(NH3)2}2(H2N‐R‐NH2)]2+, where R is a propane or hexane, were studied by various methods of biochemical analysis or molecular biophysics. These methods include binding studies by means of differential‐pulse polarography, measurements of melting curves with the aid of absorption spectrophotometry, measurements of CD spectra, ELISA with specific antibodies that recognize DNA modified by platinum complexes, interstrand cross‐linking assay employing gel electrophoresis under denaturing conditions and mapping of DNA adducts by means of transcription assays. The results indicated that the major adduct of [{trans‐PtCl(NH3)2}2(H2N‐R‐NH2)]2+ in DNA was an interstrand cross‐link which was formed with a relatively short half‐time (≈1 h). At least some types of these interstrand cross‐links induced local denaturational changes in the DNA. The results of analyses of interactions of [{trans‐PtCl(NH3)2}2(H2N‐R‐NH2)]2+ with linear DNA at relatively higher levels of the modification could be interpreted to mean that these dinuclear platinum complexes were also capable of intrastrand‐cross‐link formation between adjacent base residues in DNA. However, these intrastrand adducts of [{trans‐PtCl(NH3)2}2(H2N‐R‐NH2)]2+ distorted DNA conformation in a way different from the DNA intrastrand adducts of cisplatin. In addition, the DNA adducts of the dinuclear platinum complexes inhibited DNA transcription in vitro. The length of the aliphatic linker chain affected the DNA‐binding mode of [{trans‐PtCl(NH3)2}2(H2N‐R‐NH2)]2+ and the resulting conformational changes in DNA. The extensive analysis of DNA interactions with [{trans‐PtCl(NH3)2}2(H2N‐R‐NH2)]2+ described in this communication has provided further experimental support for previous suggestions [Farrell, N. (1991) in Platinum and other metal coordination compounds in cancer chemotherapy (Howell, S. B., ed.) pp. 81–91, Plenum Press, New York] that the binding of the dinuclear platinum complexes modifies DNA in a way that is different from the modification by antitumor cisplatin. Thus, the results of this work are consistent with the hypothesis that platinum drugs that bind to DNA in a manner fundamentally different from that of cisplatin can exhibit altered biological properties, including a different spectrum and intensity of antitumor activity.