Benzodiazepine-induced inhibition of human malignant melanoma (M-6) cell growth

Since benzodiazepines (BZs) have been shown to inhibit the growth of some cell lines, the effects of these drugs on human melanoma (M-6) cell growth were examined. Cell growth was measured by the tetrazolium salt (MTT) assay or the Hoechst 33258 DNA assay. Diazepam, a non-selective BZ agonist, and Ro5-4864, a peripheral-type agonist, inhibited M-6 cell proliferation by 36% and 55% with EC50s of 139 microM and 107 microM respectively, after four days of treatment in culture. The central-type agonists, clonazepam and flunitrazepam, were ineffective. The antiproliferative effect of diazepam was partially reversed by treatment with phorbol 12-myristate 13-acetate (PMA). Neither PK 11195, a peripheral-type BZ receptor antagonist, nor flumazenil a central-type antagonist, blocked the effect of diazepam, indicating that these BZ receptors are not involved. The effect of PMA suggests that the antiproliferative effect of the BZs may involve inhibition of a calcium/protein kinase C-related pathway in M-6 cells.