The cytotoxicity of melphalan and its relationship to pH, hypoxia and drug uptake.

In the present in vitro studies we examined the effect of hypoxia and acidic pH, two important consequences of reduced blood flow in vivo, on the cytotoxicity of melphalan treatment in Chinese hamster V79-WNRE and SiHa human tumor cells. Cells were exposed to various concentrations of melphalan for 1 hr at 37 degrees C under oxic or hypoxic conditions; pH 6.6 or 7.4, and cell survival was measured. The cytotoxicity of melphalan was potentiated by both low pH and hypoxia, in both cell lines. The overall potentiation, expressed as an enhancement ratio (ER), from both hypoxia and low pH was 3.5 in V79-WNRE and 2.9 in SiHa cells. The potentiation of cell killing produced by hypoxia alone (ERHyp) ranged from 1.4 to 1.9, and was greater in V79-WNRE than in SiHa cells. The potentiation from low pH (ERpH) was approximately 2 in both cell lines. HPLC analysis showed substantial intracellular accumulation of melphalan in both cell lines. Hypoxia and reduced pH further enhanced uptake of melphalan but this was not sufficient by itself to account for the increased potentiation of cytotoxicity observed under those conditions.