Cisplatin Resistance in Ovarian Cancer Cell Lines Cross-Resistance to Diverse Drugs Is Associated with Primary

We have previously obtained, by exposure to near continuous increas ing concentrations of cisplatin, a panel of human ovarian cancer cell lines that exhibit a wide range of primary resistance to the drug (9to >400fold). These cells had strikingly increased (4to 50-fold) levels of glutathione (GSH) as compared with the drug-sensitive cells of origin (A. K. Godwin et al., Proc. Nati. Acad. Sci. USA, 89: 3070-3074, 1992). Utilizing this panel of resistant cell lines, we evaluated cross-resistance to classical alkylating agents, natural product drugs, and irradiation. We observed that cross-resistance to carboplatin paralleled that of cisplatin, culminat ing in approximately 250-fold resistance. Similarly, melphalan cross-re sistance continued to increase to >400-fold and again paralleled the pri mary cisplatin resistance. Cell lines with low to very high levels of resistance to cisplatin are 8to 850-fold resistant to the epipodophyllotoxin derivative etoposide. Cross-resistance is also observed for other natural product drugs, including Adriamycin (—80-fold), mitoxantrone (—440fold), and taxol (~40-fold). Cross-resistance to irradiation is, however, modest (<2-fold). The cells with the greatest primary resistance to cisplatin most commonly had the highest cross-resistance to the other drugs exam ined. The cross-resistance to the natural product category drugs was found not to be mediated by the products of either the multidrug resistance 1 (MDR\) or multidrug resistance-associated protein (MRP) genes based on lack of coordinate increased expression or amplification of these genes as assessed by Northern and Southern blot analyses. Furthermore, verapamil failed to markedly increase drug sensitivity. Although there was no indi cation that these natural product drug efflux pumps were operative, we observed decreased doxorubicin accumulation in these cell lines crossresistant to natural products. In addition, alternations in DNA topoisomerase II mRNA levels, which have been observed in a variety of human tumor cell lines selected in vitro for resistance to etoposide or teniposide, were not detected. Only intracellular levels of GSH correlated with crossresistance to these diverse anticancer agents and partial loss of resistance was associated with a marked decrease in glutathione levels. In the ab sence of alternative mechanisms, we speculate that the very broad clini cally relevant cross-resistance seen in this model system may, at least in part, be the direct result of GSH-mediated drug inactivation or may be due to a combination of GSH conjugation to drug and conjugate efflux mediated by the putative ATP-dependent glutathione 5-conjugate export pump.