Effect of verapamil on the uptake and efflux of etoposide (VP16) in both sensitive and resistant cancer cells.

The effect of calcium antagonist verapamil on the uptake and efflux of Etoposide (VP16), a semi-synthetic derivative of podophylotoxin and a broad spectrum antineoplastic agent, has been investigated and compared in sensitive (UM-UC-2) and resistant (UM-UC-9) human bladder cancer cells, and L1210 leukemia cells, by using both radioisotope (3[H]-VP16) liquid scintillation and high performance liquid chromatography assay with electrochemical detection. The uptake of VP16 was rapid in all three cell lines, showing an initial rapid linear phase followed by a second slower phase, but at steady state the ratios of intracellular to extracellular VP16 concentrations were only 0.004-0.006. No significant difference in drug uptake was observed in sensitive UM-UC-2 and resistant UM-UC-9 cells at all concentrations studied. Verapamil at a concentration of 10 microM enhanced the intracellular VP-16 levels in all sensitive and resistant cell lines. The increments were 21.5% for UM-UC-2, 11.8% for UM-UC-9, and 31.0% for L1210 cells after 30 minutes incubation with 1 microM VP16. A slower efflux of VP16 was observed in verapamil treated cells in all three cell lines. There was a small increase in the nonexchangeable components in verapamil treated cells, although only 5-10% of VP16 was retained. No peak other than that of VP16 was detected in the HPLC chromatogram of extracts from both cell pellet and influx or efflux medium.