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Mechanism of Cytotoxicity of Anticancer Platinum Drugs: Evidence That cis-Diamminedichloroplatinum(II) and cis-Diammine-(1,1-cyclobutanedicarboxylato)platinum(II) Differ Only in the Kinetics of Their Interaction with DNA¹

Department of Molecular Pharmacology, Institute of Cancer Research, Sutton, Surrey SM2 5PX, United Kingdom

The kinetics of the aquation reactions of cisplatin and carboplatin and their subsequent reactions with DNA, both in vitro and in vivo, have been measured. The results have been extrapolated to indicate the expected cytotoxicity of these compounds in cells obtained from human cancer patients.

Rate constants for the aquation at 37°C of cisplatin and carboplatin of 8 x 10^-5 and 7.2 x 10^-7 s^-1, respectively, were calculated from the half-life of these compounds in phosphate buffer, pH 7. This difference in their rate of activation was matched by their rates of binding to DNA. By use of a ¹⁴C-labeled ligand, carboplatin was shown to bind monofunctionally to DNA, after which there was a time-dependent formation of difunctional interstrand cross-links, formed from some of these initially monofunctional adducts. A similar, although faster, accumulation of cross-links was seen when cisplatin was bound to DNA. The loss of the ¹⁴C-CBDCA ligand of carboplatin was calculated to occur with a rate constant of 1.3 x 10^-5 s^-1 which was similar to that for the rate of formation of interstrand cross-links and faster than that for the monofunctional reaction with DNA. It was concluded therefore that the CBDCA ligand becomes a more labile leaving group once carboplatin has been monoaquated. In contrast, both chloro-ligands of cisplatin were shown to leave at similar rates. The fact that other difunctional lesions were formed to the same extent, by equal bound doses of cisplatin or carboplatin, was indicated by the unwinding of supercoiled plasmid DNA. The effects of cisplatin and carboplatin on this DNA were the same once bound to the same extent. About a 100-fold larger dose of carboplatin was, as predicted by their rates of aquation, required to produce equivalent binding to plasmid DNA.

In vivo, equal binding of the two drugs to DNA of various cell systems resulted in equal cytotoxicity. Again a much larger dose (20- to 40-fold) of carboplatin was required to produce this equal binding. In general a DNA bound platinum level of about 20 nmol/g reduced cell survival by 90%, although certain cell lines were shown to be much more sensitive to DNA bound platinum.

Similar binding values, to those above, were obtained in the DNA extracted from cells of human cancer patients treated with cisplatin. It was inferred that the cytotoxic effect of this level of platinum on DNA would be (unless the cells were of a sensitive phenotype) about 90%.

This work has shown that once bound to DNA in equal amounts both cisplatin and carboplatin cause equal difunctional lesions, interstrand cross-links, and cytotoxicity. The large differences in dose required to produce these effects were accounted for by the much faster rate of aquation of cisplatin.

¹ We are grateful to Johnson Matthey Research Centre for financial support and for grants from the Medical Research Council and the Cancer Research Campaign.

Received 7/29/85. Revised 12/13/85. Accepted 12/26/85.

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