ESTABLISHMENT AND CHARACTERIZATION OF AN INVITRO MODEL OF ACQUIRED-RESISTANCE TO CISPLATIN IN A HUMAN TESTICULAR NONSEMINOMATOUS GERM-CELL LINE

Clinically, human testicular nonseminomatous germ cell tumors exhibit remarkable sensitivity to platinum-based chemotherapy. To define bette r the mechanistic basis for this unusual sensitivity, the biochemical determinants of platinum-induced cytotoxicity have been investigated i n a human testicular tumor cell line (GCT27) established from a previo usly untreated patient and in an in vitro derived 5.6-fold cisplatin-r esistant stable variant (GCT27cisR). Compared to 12 ovarian and 5 cerv ical human tumor cell lines, the parent GCT27 line was among the most sensitive to the cytotoxic effects of both cisplatin (dosage producing 50% inhibition, 0.2-mu-M) and carboplatin (dosage producing 50% inhib ition, 2.9-mu-M), thus reflecting clinical data. A 4-day exposure sulf orhodamine B-staining assay was used to determine that GCT27cisR was c ross-resistant to carboplatin and iproplatin and the classical bifunct ional alkylating agents melphalan and chlorambucil. Partial cross-resi stance was observed to tetraplatin, methotrexate, and mitomycin C. No cross-resistance was observed to Adriamycin, etoposide, vinblastine, b leomycin, 1-beta-D-arabinofuranosylcytosine, and 5-fluorouracil. Intra cellular cisplatin accumulation across the dose range 2.5-100-mu-M (fo r 2 h) was 1.6 +/- 0.39-fold ( mean +/- SD) greater for the parent lin e. There was no significant difference in glutathione levels between t he two lines. The acquired resistance line was 1.9-fold more resistant than the parent line to the cytotoxic effects of cadmium chloride. Th ere was no significant difference between the two lines, however, in t he total amounts of platinum bound to DNA after cisplatin exposure (25 , 50, or 100-mu-M for 2 h). The removal of total platinum adducts from DNA was significantly faster for GCT27cisR compared to the parent lin e (half-times of removal, 32 and 67 h, respectively). These data sugge st that the abnormal sensitivity of the parent testicular tumor cell l ine to platinum-containing anticancer drugs may be due predominantly t o an inherent defect in the ability of these cells to remove platinum from their DNA. This defect is apparently lost in the acquired resista nce counterpart. Reduced intracellular accumulation and increased cyto plasmic concentrations of metallothionein may also contribute, in part , to the acquisition of cisplatin resistance in this model.