Reversal of MDR1-associated resistance to topotecan by PAK-200S, a new dihydropyridine analogue, in human cancer cell lines

Recent data suggest that expression of the membrane P-170-glycoprotein (P-g p) may confer resistance to the topoisomerase-I-interactive agent topotecan . The present study describes the cellular effects of a new dihydropyridine analogue, PAK-200S, on P-gp-mediated resistance to topotecan in human brea st and ovarian tumour cells. PAK-200S at a non-cytotoxic concentration of 2 .0 mu M completely reversed resistance to topotecan in P-gp-expressing MCF- 7/adr (breast) and A2780/Dx5 (ovarian) tumour cells, respectively, with no effects on parental cells. Cellular pharmacokinetic studies by reversed-pha se high-performance liquid chromatography analysis showed significantly low er cellular drug concentrations of the pharmacologically active closed-ring lactone of topotecan in multidrug-resistant cells than in parental cells. PAK-BOOS was effective in restoring the cellular lactone concentrations of topotecan in resistant MCF-7/adr cells to levels comparable to those obtain ed in parental cells. Furthermore, exposure of MCF-7/adr cells to topotecan in the presence of PAK-200S significantly increased the induction of prote in-linked DNA breaks. PAK-200S did not alter nuclear topoisomerase I-mediat ed ex vivo pBR322 DNA plasmid unwinding activity and topoisomerase-l protei n expression. These results suggest that reversal of P-gp-mediated resistan ce to topotecan by PAK-200S was related to the restoration of cellular drug concentrations of the active lactone form of topotecan rather than a direc t effect on topoisomerase-I function. (C) 1999 Cancer Research Campaign.