The effect of both hyperthermia and verapamil on cytotoxicity and transport of melphalan was studied in a pleiotropic drugresistant Chinese hamster ovary cell line (<tex-math>${\rm CH}^{{\rm R}}{\rm C}5$</tex-math>) and in the drug-sensitive parent line (AuxB1). The <tex-math>${\rm CH}^{{\rm R}}{\rm C}5$</tex-math> cell line was selected for resistance to colchicine but is also cross-resistant to other drugs including melphalan. Verapamil (10 μM) increased melphalan cytotoxicity in drug-resistant cells but not in drug-sensitive cells. Hyperthermia (40 to 45°C) increased melphalan cytotoxicity in both cell lines. In drug-resistant but not drug-sensitive cells, melphalan cytotoxicity was increased further when verapamil was combined with hyperthermia (40 to 45°C). The increased cytotoxicity caused by verapamil in drug-resistant cells was accompanied by alterations in membrane permeability to melphalan. The cellular uptake of melphalan after 15 min increased in the presence of verapamil (7 to 30 μM) at 37 and 42°C. When verapamil (10 μM) was present, the rate of efflux of melphalan from <tex-math>${\rm CH}^{{\rm R}}{\rm C}5$</tex-math> cells decreased by almost 40% at 37°C. The rate of efflux was increased at 42°C relative to 37°C, but with verapamil the rate decreased to that obtained at 37°C in <tex-math>${\rm CH}^{{\rm R}}{\rm C}5$</tex-math> cells. In drug-sensitive cells, verapamil (≤50 μM) did not affect either uptake or efflux of melphalan. These findings suggest that verapamil could be beneficial by increasing the effectiveness of melphalan in the elimination of multidrug-resistant cells. The combination of hyperthermia and verapamil could be especially advantageous by increasing melphalan cytotoxicity in a localized target region.

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