Cytotoxic effect of RB 6145 in human tumour cell lines: dependence on hypoxia, extra- and intracellular pH and drug uptake.

Low pH and hypoxia are a common feature of many solid tumours. This study examined the effect of these two conditions on the cytotoxic properties of the bifunctional agent RB 6145, the prodrug of RSU 1069. The effect of acidic pH on RB 6145 toxicity was examined in six human tumour cell lines under hypoxic conditions and was found to have little effect in HT 29, A549, U373 and HT 144 cells. Treatment was for 1 h at 37 degrees C, pH 6.4 or 7.4. Significant potentiation of RB 6145 toxicity was observed in SiHa cells (enhancement ratio; ERpH approximately 1.6) and in U1 cells (ERpH approximately 1.4). In these two cell lines the potentiation of RB 6145 toxicity arising from hypoxia was large, with ERHyp approximately 11 and 15 in SiHa and U1 cells respectively. SiHa cells, which show a pH effect and HT 29 cells, which do not, were chosen for further comparative studies of drug uptake )nd regulation of intracellular pH. High-performance liquid chromatography (HPLC) determinations of the uptake of RB 6145 and its dervatives showed that in SiHa cells, intracellular to extracellular drug concentration ratio (Ci/Ce) at 1 h was approximately 40% higher at pH 6.4 than at pH 7.4, whereas in HT 29 cells Ci/Ce was approximately 25% lower. Under conditions of acidic extracellular pH, regulation of pH was somewhat less effective in SiHa cells, where pHi dropped to within 0.2 pH units of the extracellular pH over a 2.5 h treatment at pH 6.4. It seems likely that increased drug uptake was at least part of the basis for the observed potentiation of RB 6145 toxicity in SiHa cells. A model which would better explain the results for both cell lines might also include the possibility that low pH per se potentiates cytotoxic damage to a modest extent and that it is offset or augmented by altered uptake in HT 29 and SiHa cells respectively.