Non-P-glycoprotein-mediated multidrug-resistant C-A120 cells that overexpressed multidrug resistance protein (MRP) were 10.8- and 29.6-fold more resistant to 7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin(CPT-11) and SN-38, respectively, than parental KB-3-1 cells. To see whether MRP is involved in CPT-11 and SN-38 resistance, MRP cDNA was transfected into KB-3-1 cells. The transfectant, KB/MRP, which overexpressed MRP, was resistant to both CPT-II and SN-38. 2-[4-Diphenylmethyl)-1-piperazinyl]ethyl-5-(trans-4,6dimethyl-1,3,2-dioxaphosphorinan-2-yl)-2,6-dimethy nitrophenyl)-3-pyridinecarboxylate P-oxide (PAK-104P) and MK571,which reversed drug resistance in MRP overexpressing multidrug-resistant cells, significantly increased the sensitivity of C-A120 and KB/MRP cells, but not of KB-3-1 cells, to CPT-11 and SN-38. The accumulation of both CPT-11 and SN-38 in C-A120 and KB/MRP cells was lower than that in KB-3-1 cells. The treatment with 10 mu M PAK-104P increased the accumulation of CPT-II and SN-38 in C-A120 and KB/MRP cells to a level similar to that found in KB-3-1 cells. The ATP-dependent efflux of CPT-11 and SN-38 from C-A120 and KB/MRP cells was inhibited by PAK-104P. DNA topoisomerase I expression, activity, and sensitivity to SN-38 were similar in the three cell lines. Furthermore, the conversion of CPT-II to SN-38 in KB-3-1 and C-A120 cell lines was similar. These findings suggest that MRP transports CPT-11 and SN-38 and is involved in resistance to CPT-11 and SN-38 and that PAK-104P reverses the resistance to CPT-11 and SN-38 in tumors that overexpress MRP.