Effect of calcium channel blockers on human CFU-GM with cytotoxic drugs.

Calcium channel blockers (CCBs) such as verapamil and nitrendipine are capable of increasing drug sensitivity in resistant murine and human tumor cells. This finding has potential value in the treatment of acquired drug resistance in human malignancies. Thus, we tested the ability of CCBs of two different structural classes to enhance the toxicity of doxorubicin (DOX), vinblastine (VBL), and vincristine (VCR) for normal myeloid and macrophage colony formation (marrow colony forming units-granulocyte-monocyte [CFU-GM]). Drug effects on colony formation from 35 normal volunteer marrows and from seven patient marrows in the recovery phase after cytotoxic chemotherapy were determined. No enhancement of toxicity was mediated by verapamil or nitrendipine when these drugs were co-incubated with the cytotoxic drugs for one hour or 24 hours before plating marrow cells in a semisolid agar system.