Increased DNA repair as a mechanism of acquired resistance to cis-diamminedichloroplatinum(II) in human ovarian cancer cell lines

A human ovarian cancer cell line, A2780, derived from an untreated ovarian cancer patient and relatively sensitive to cisplatin was treated by stepwise incubation with cisplatin to produce a cisplatin-resistant variant, 2780(CP). The relative abilities of these cell lines to repair cisplatin-induced damage to cellular DNA then was examined by measure of [ ³H]thymidine incorporation into normal density DNA separated from bromodeoxyuridine-substituted DNA on alkaline cesium chloride gradients. These studies revealed that primary cisplatin resistance present in 2780(CP) was associated with a near twofold-increased ability to repair damage induced by the drug under conditions where 2780(CP) was approximately 5-fold resistant to cisplatin. Aphidicolin, a specific inhibitor of DNA polymerase α, showed a dose-dependent capacity to inhibit DNA repair in this system with maximum inhibition of 63% at 4 μg/ml. It was also found that inhibition of DNA repair during and shortly after cisplatin exposure resulted in an approximately threefold increase in the cytotoxicity of cisplatin as monitored by clonogenic cell survival in the resistant but not the sensitive parental cell line.