Enhancement of chlorambucil cytotoxicity by combination with flavone acetic acid in a murine tumour.

Previous studies using murine tumours have shown enhanced action of certain chemotherapeutic compounds when combined with agents that reduce tumour blood flow. In the majority of cases the compounds used were cytotoxic to the induced hypoxic cells but in this study we have investigated the relative importance of changes in tumour pH following blood flow reductions. The role of tumour pH was investigated by using combinations of the cytotoxic alkylating agent Chlorambucil (CHL) with the vascular occluding agent Flavone Acetic Acid (FAA). Chlorambucil is a weak acid (pKa = 3.7) and is concentrated within cells exposed to culture media at low pH or to the acidic microenvironment in vivo. In vitro incubations showed that greater cytotoxicity was obtained when cells were incubated at low pH and that the cytotoxicity was independent of the level of oxygenation at the time of the drug incubation. Combination of both CHL and FAA in vivo resulted in greater reductions in cell survival and growth delay than when either agent was given alone. Simultaneous administration of the two agents were very effective, potentially due to two factors; firstly, that the pH of the tumour changes during ischaemia and secondly, that the reduced blood flow potentially alters the pharmacokinetic distribution of CHL resulting in 'drug-trapping' within the tumour. Since the action of CHL is independent of the induced hypoxia which results from blood flow reduction it is suggested that the increased in vivo action is due in part to the changes in tumour pH.