cis-1, 1-Cyclobutanedicarboxylato(2R)-2-methyl-1 4-butanediammineplatinum(II) (CI-973; NK121) is a second generation analogue of cisplatin (CDDP). This analogue caused fewer undesirable side effects in vivo than CDDP and produced a tumoricidal effect superior to that of CDDP against CDDP-resistant human tumor cells in vitro. However, the mechanisms responsible for the cross-sensitivity to CI-973 in these resistant cells have not been determined. Using a human colon carcinoma cell line (LoVo) and its CDDP-resistant counterpart (CP2.0) as study models, we found that although, on a molar basis, CI-973 was less cytotoxic than CDDP in LoVo, it partially reversed the resistance of CP2.0 cells, i.e., the magnitude of resistance was reduced from 13-fold for CDDP to 3-fold for CI-973. We investigated DNA repair as a possible mechanism for the different magnitudes of resistance of these two agents. The repair efficiency was measured by the rate of removal of drug-induced DNA interstrand cross-links (determined with the ethidium bromide fluorescence binding assay) and by unscheduled DNA synthesis. Our results showed that although the resistant CP2.0 cells were more efficient at repairing CDDP-induced DNA cross-links than LoVo cells, the rate of repair of CI-973-induced DNA adducts in CP2.0 was significantly lower than that for CDDP-DNA adducts at equi-cross-linking drug concentrations. This difference was not observed in the sensitive LoVo cells. The concomitant association of a slower rate of removal of CI-973-induced adducts with a lower magnitude of resistance to CI-973 suggests that a reduced efficiency in repairing the CI-973-induced DNA damage in CDDP-resistant CP2.0 cells may in part contribute to the sensitivity of the resistant cells to this novel CDDP analogue.

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