Influence of pH on the cytotoxic activity of chlorambucil

The cytotoxic activity of chlorambucil as a function of pH was investigated in P388 tumor cells growing in static suspension culture. A decrease in extracellular pH from 7.8 to 7.2 was associated with a decrease in intracellular pH from 7.92 to 7.55. The cytotoxic potency of chlorambucil increased as the extracellular pH decreased; IC99 values were 20 and 60 microM when the extracellular pH was 7.2 and 7.8 respectively. Covalent binding to cellular macromolecules was about 1.9 times greater at pH 7.2 relative to that at pH 7.8. These results suggest that pH may be an important determinant of the oncotoxic specificity of chlorambucil, and that the cytotoxic activity of this agent could be selectively directed toward tumor cells by the selective manipulation of intracellular and extracellular pH. A potential influence of intracellular and extracellular pH on cytotoxic, mutagenic, carcinogenic, and teratogenic potencies of other chemicals is also suggested. Additionally, these investigations demonstrate the importance of carefully controlling pH throughout the drug exposure period when evaluating the relative potency of potential cytotoxic, mutagenic, carcinogenic, and teratogenic agents in cell or organ culture.