Verapamil potentiation of doxorubicin resistance development in B16 melanoma cells both in vitro and in vivo.

The effect of the combined administration of doxorubicin (DX) and verapamil (VRP) on the induction of DX resistance of B16 melanoma cells, was investigated both in vitro and in vivo. Cells grown in the presence of increasing concentrations of DX and of 1 microM VRP, tested at several passages, were more resistant than cells grown with DX alone. The treatment of B16 melanoma bearing mice with the maximal tolerated dose of DX (12 mg kg-1 i.v.) and of VRP (25 mg kg-1 i.p.) selected a line (B16-DX. VRP) completely resistant to DX after 17 transplants, while treatment with DX alone selected a DX resistant line after 27 transplants. Lung metastases were significantly lower in the B16-DX. VRP line compared to the original B16 melanoma. The results suggest that the association of VRP with DX increases the rate of resistance development to DX.