Cis-diamminedichloroplatinum(II) toxicity in human melanoma cells and lymphocytes as related to cellular platinum accumulation, DNA cross-linking and inhibition of DNA synthesis.

The cytotoxic effect of cis-diamminedichloroplatinum(II) (cis-DDP), as measured by a dye exclusion assay was much more pronounced in bone marrow cells and phytohemagglutinin (PHA)-stimulated lymphocytes than in a human melanoma cell line. DNA synthesis measured by incorporation of 3H-thymidine was much more sensitive to cis-DDP in PHA-stimulated lymphocytes than in melanoma cells. These differences were not caused by a difference in drug accumulation since measurements of cellular platinum content gave similar results in both cell types. The total amount of DNA cross-links and DNA interstrand cross-links induced by cis-DDP was measured with alkaline elution of DNA. In both PHA-stimulated lymphocytes and melanoma cells low total levels of DNA cross-links and DNA interstrand cross-links were found immediately after drug exposure, followed by a protracted increase in DNA cross-linking for 6-12 hours during further incubation after removal of cis-DDP. The relationship between the concentration of cis-DDP and peak levels of total DNA cross-links as well as DNA interstrand cross-links was linear in both cell types. Cis-DDP was found to induce 5.6 times higher total levels of DNA cross-links 6.1 times higher levels of DNA interstrand cross-links in PHA-stimulated lymphocytes than in melanoma cells. The incorporation of 3H-thymidine was much more reduced in PHA-stimulated lymphocytes than in melanoma cells at similar levels of DNA cross-linking. Thus, both reduced DNA cross-linking and lower effect of DNA cross-links on the DNA synthesis may contribute to the greater resistance of melanoma cells to cis-DDP.