In order to study the mechanisms responsible for resistance to CDDP, 5 human tumor cell lines were made resistant to CDDP by repeated in vitro exposures. After cloning it was found that the cell lines developed were between 3.3-fold and 17-fold more resistant to CDDP than the parental cell lines at the lC9,,. These lines were also resistant to carboplatin and tetraplatin; however, resistance to tetraplatin was lower than to the other platinum complexes. Sensitivity was also assessed to Adria, MTX, 5-FU. chlorambucil, 4-HC, 4-HIF. BCNU, Thiotepa, HN2, Mito C and L-PAM, and no consistent cross-resistance was observed. As compared with the parental lines, non-protein sulfhydryl content was elevated in 3 resistant lines, and protein sulfhydryl was elevated in all 5 lines, as was glutathione-S-transferase activity. Measurements of platinum in whole cells and nuclei after exposure of the cultures to 25 p~ CDDP for either l or 6 hr showed that nuclear levels reflected those in whole cells and that, per mg protein, platinum levels were lower in resistant cells at both time points. Formation of DNA cross-links, determined by alkaline elution, was lower in resistant cell lines than in parental cell lines, but did not correlate with the absolute cell kill observed. These results indicate that cellular resistance to CDDP often involves decreases in drug accumulation and increases in protein sulfhydryl content. Possible strategies for overcoming these mechanisms are discussed.
Cis-diamminedichloroplatinum(I1) (CDDP) is perhaps the most widely used chemotherapeutic agent for the treatment of solid human tumors (Loehrer and Einhorn, 1984). Because of its wide spectrum of activity and utility, the study of mechanisms by which neoplastic cells become resistant to CDDP is a very active area of investigation. Most studies have used murine or human cell lines in which resistance has been developed by repeated exposure either in vitro or in vivo (