In vitro formation of DNA adducts by cisplatin, lobaplatin and oxaliplatin in calf thymus DNA in solution and in cultured human cells.

Two interesting representatives of a new generation of platinum-based cytostatic drugs that are currently being tested in clinical trials are lobaplatin [1,2-diaminomethylcyclobutane platinum(II) lactate] and oxaliplatin [1,2-diaminocyclohexane platinum (II) oxalate]. Since little is known about the DNA adduct formation of these compounds, we studied their formation in DNA in vitro in calf thymus DNA and in cells. The major adducts formed in vitro were the Pt-GG and Pt-AG intrastrand crosslinks. The latter adducts could be detected using a recently developed 32P-postlabelling method. Using both this assay and atomic absorption spectroscopy, it was shown that there is a substantially higher rate of the in vitro adduct formation by cisplatin, compared with lobaplatin and oxaliplatin. Platinum concentrations required to obtain 90% cell kill during a 2 h incubation of A2780 cells were 15 microM for cisplatin and oxaliplatin and 22 microM for lobaplatin. Using an antiserum originally raised against cisplatin-treated DNA, we were also able to detect platinum-DNA adducts induced by lobaplatin and oxaliplatin. Maximal nuclear staining for all three compounds was observed after a 4 h post-incubation period. The nuclear staining level induced by cisplatin was about 10-fold higher than after lobaplatin and oxaliplatin treatment. GG and AG adducts, measured by 32P-postlabelling, also showed maximum levels at about 4 h after treatment. Relative GG peak levels were 4:1:3 for cisplatin, lobaplatin and oxaliplatin, respectively. The ratios of GG over AG intrastrand crosslinks in the A2780 cells were not significantly different for the various compounds. In conclusion, the 32P-postlabelling technique has been shown to be appropriate for adduct analysis, not only for the classical Pt compounds cisplatin and carboplatin but also for novel platinum compounds like lobaplatin and oxaliplatin. Results indicated large differences in reactivity of the latter compounds to DNA in vitro, compared with cisplatin. This difference was smaller in cells, suggesting enhancement of adduct formation by certain cellular mechanisms and/or compounds. From these studies, no conclusions can be drawn with respect to the cytotoxicity of the different Pt-GG and Pt-AG intrastrand crosslinks formed by these compounds.