Inhibition of spontaneous and experimental tumor metastasis by the calcium antagonist verapamil.

Verapamil, a calcium antagonist, inhibited both experimental (IV inoculation of tumor cells) and spontaneous metastasis (SC inoculation) of the highly metastatic B16 melanoma and colon adenocarcinoma 26 cell lines. Verapamil treatment resulted in a maximum 80% inhibition of metastases, the degree of inhibition varying among the different metastatic systems. Verapamil inhibited platelet aggregation induced by these tumor cell lines, the patterns of inhibition being different for B16 melanoma and colon adenocarcinoma. The inhibition of platelet aggregation induced by tumor cells is proposed as a mechanism by which the calcium antagonist exerts its antimetastatic effect. These results, together with our previous findings that calcium antagonists can increase the cytotoxicity of drugs in tumor cells with induced or inherent drug resistance by inhibiting outward transport of the drug, indicate that calcium antagonists have potential as a new class of adjuvant agents in the field of cancer chemotherapy.