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Previous studies have demonstrated that verapamil may overcome resistance to anthracyclines. In vitro and in vivo experiments were performed on wild-type and resistant Ehrlich ascites tumor cells. Verapamil in concentrations of 25-50 microM enhances the accumulation of daunorubicin (DNR) in resistant cells to the same level as in wild-type cells. No significant effect of verapamil on influx or nuclear binding could be demonstrated, indicating that verapamil enhances DNR uptake by blocking active drug extrusion. Exposure of cells to a high concentration of Ca2+ did not influence the effect of verapamil on DNR accumulation, suggesting a different mode of verapamil action apart from the Ca2+-blocking effect. Attempts to circumvent acquired resistance to DNR in vivo with verapamil showed that the combination of the two drugs was more toxic than DNR given alone. The LD10 of DNR was determined as 3 mg/kg and the LD10 of the combination, as 2.5 mg/kg. The therapeutic effect of verapamil at a dose of 50 mg/kg and DNR of 2.5 mg/kg increased the life span of the mice by 50%. No difference was seen in the wild-type tumor in vivo. These data lead us to conclude that verapamil can reverse DNR resistance completely, but that verapamil at non-toxic dosage only reduces DNR resistance by 50% in vivo.

Summary Eight anthracycline analogs that have been shown to modulate multidrug resistance (Friche et al., Biochem. Pharmacol., 39, 1721-1726 were tested for their inhibitory effect on the photolabelling of P-glycoprotein. We photoaffinity labelled P-glycoprotein in daunorubicin (DNR)