Modulation of melphalan cytotoxic activity in human melanoma cell lines

The aim of the present study was to potentiate the cytotoxic effects of melphalan through pharmacological and physical modulators. The combination of the cytotoxic agent with ethacrynic acid, a glutathione-S-transferase pi (GST pi) inhibitor, or topotecan, a topoisomerase I inhibitor, or mild hyperthermia was investigated. The selected cell lines exhibited variable levels of expression of GST pi, DNA topoisomerase I and heat-shock proteins. Mild hyperthermia (42 degrees C) alone potentiated melphalan cytotoxicity, especially in the two cell lines exhibiting low basal levels of HSP70 expression. The combination of the GST inhibitor with melphalan resulted in a potentiation of drug cytotoxicity only in JR8 cells, one of the two cell lines which expressed high levels of GST pi mRNA and which were the less responsive to ethacrinic acid alone. A synergistic interaction between topotecan and melphalan was observed only in the cell lines expressing low levels of topoisomerase I even if all cell lines exhibited a comparable sensitivity to this agent. The results support an involvement of GST and DNA topoisomerase in cell defense and response to the alkylating agent. However, the variable potentiation of the cytotoxic effects of melphalan achieved in different cell systems suggests that factors other than the level of expression of the modulation target are responsible of such potentiation.