Protection of cultured malignant cells from mitoxantrone cytotoxicity by low extracellular pH: a possible mechanism for chemoresistance in vivo.

In malignant tumors the distribution of pH values is shifted to lower values (range, pH 5.8-7.4) as compared to normal tissues (range, pH 6.9-7.4) or peripheral blood (pH 7.35-7.45). We have investigated whether the cytotoxic effect of the anthracenedione anti-cancer drug mitoxantrone (MX) on malignant cells in culture is dependent on changes of extracellular pH. The clonogenic fraction of M1R rat mammary carcinoma cells was measured after exposure to MX at an extracellular pH (pHe) of 6.5-7.4. At pHe 6.8 (approximately the average pH measured in a number of malignant tumors in vivo) the clonogenic fraction of M1R cells exposed to MX (0.1 microgram/ml) only decreased to 1 X 10(-1) as compared to 2.5 X 10(-4) at pHe 7.4, corresponding to a 400-fold inhibition of MX cytotoxicity at reduced environmental pH. The H+ ion-mediated resistance of M1R cells to MX could be partially reversed by verapamil, suggesting that a reduced microenvironmental pH possibly interferes with intracellular MX accumulation. Therefore, drugs like MX may not be effective in the elimination of cells in acidic tumor areas. Moreover, investigations on anti-cancer drug activity in vitro at what is frequently referred to as 'physiological pH' may be irrelevant in terms of the cytotoxic effects of the respective agents at the pH values prevailing in malignant tissues in vivo.