We have developed a drug-resistant cell line (RIF/Ptr1) (R) from the murine radiation-induced fibrosarcoma (RIF-1) also designated Pts or (S). This subline has been characterized previously by an increased resistance to cisplatin (cis-diamminedichloroplatinum(II) or CDDP), lowered intracellular CDDP concentrations, and elevated intracellular glutathione (GSH) levels (1.4-2.5-fold) but unaltered formation of CDDP-DNA interstrand cross-links. In this work, we have shown that RIF/Ptr1 cells were also resistant to carmustine (BCNU) and X-irradiation. Neither cell line had P-glycoprotein 170. The intrastrand CDDP-DNA adduct level was proportional to the concentration of intracellular CDDP. The oxygen consumption, ATP level, and glycolysis were similar in both cell lines. The cisplatin influx and efflux showed that the RIF/Ptr1 cells had lower drug influx and higher drug efflux compared to RIF-1. We conclude that the major difference between the cisplatin sensitive and cisplatin-resistant cells in this model is the regulation of cisplatin transport probably at the cell membrane level suggesting that a membrane active transport system other than P-glycoprotein 170 is involved. Whether glutathione is linked to the putative membrane transporter needs further investigation.

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