Inhibition of P-glycoprotein-dependent multidrug resistance by an isoquinolinesulfonamide compound H-87 in rat ascites hepatoma AH66 cells.

The effects of an isoquinolinesulfonamide compound, H-87, on naturally acquired multidrug-resistance (MDR) in rat hepatoma AH66 cells were examined. AH66 cells were highly resistant to vinblastine, SN-38, an active camptothecin analog, adriamycin, and etoposide, compared with the sensitive variant AH66F cells. Although H-87 hardly affected the sensitivities to antitumor agents of AH66F cells, this compound completely inhibited the resistance to vinblastine, moderately inhibited the resistance to SN-38 and adriamycin and had little effect on etoposide, mitomycin C, cisplatin, and 5-fluorouracil. H-87 significantly decreased the efflux of vinblastine from the resistant cells and increased the drug accumulation. SN-38 and adriamycin also exhibited a weak but significant increase in vinblastine accumulation in AH66 cells. H-87 inhibited [3H]azidopine-photolabeling to 160 kDa P-glycoprotein in the plasma membrane of AH66 cells, as reported in acquired MDR leukemic cells. Consequently, the MDR-overcoming effect of H-87 seems to be due to its direct inhibition of the binding of antitumor agents on P-glycoprotein in the plasma membrane.