A human ovarian cancer cell line resistant to cis-diamminedichloroplatinum(II) (DDP) (2780CP) was compared with its DDP-sensitive parental cell line (A2780) to determine whether differences in the removal rate of DNA-bound platinum were related to resistance. Both cell lines were treated in vitro with various doses of DDP for 2 h and subsequently incubated in arginine-deficient Eagle's minimum essential medium with 2.5% dialyzed fetal bovine serum in the presence or absence of aphidicolin. After 0, 12, and 24 h, DNA was isolated from the cells and DNA-bound platinum was determined by flameless atomic absorption spectrophotometry.

Binding of platinum to DNA of either cell line was a linear function of concentration ranging from 20 to 80 µm DDP. Platinum binding was almost equal at each dose in both cell lines. 2780CP cells that were 3-fold resistant to DDP lost 30.5 to 40.1% of their total DNA-bound platinum, compared with a 1.3 to 16.1% loss for A2780 cells, 12 to 24 h after a 2-h exposure to 40 µm DDP, respectively.

Aphidicolin (3.0 µg/ml) increased the cytotoxicity in 2780CP cells by about 2-fold and caused a significant delay in the time required for platinum removal in the resistant cells (14.6 and 18.9% at 12 and 24 h).

These studies indicate that the mechanism of DDP resistance in the 2780CP cell line is related to an increased ability to remove platinum-DNA adducts, and not to a difference in initial DDP binding to DNA.

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