CYTOTOXIC EFFECT OF RB-6145 IN HUMAN TUMOR-CELL LINES - DEPENDENCE ONHYPOXIA, EXTRACELLULAR AND INTRACELLULAR PH AND DRUG UPTAKE

Low pH and hypoxia are a common feature of many solid tumours. This st udy examined the effect of these two conditions on the cytotoxic prope rties of the bifunctional agent RE 6145, the prodrug of RSU 1069. The effect of acidic pH on RE 6145 toxicity was examined in six human tumo ur cell lines under hypoxic conditions and was found to have little ef fect in HT 29, A549, U373 and HT 144 cells. Treatment was for 1 h at 3 7 degrees C, pH 6.4 or 7.4. Significant potentiation of RE 6145 toxici ty was observed in SiHa cells (enhancement ratio; ER(pH) similar to 1. 6) and in U1 cells (ER(pH) similar to 1.4). In these two cell lines th e potentiation of RE 6145 toxicity arising from hypoxia was large, wit h ER(Hyp) similar to 11 and 15 in SiHa and U1 cells respectively. SiHa cells, which show a pH effect and HT 29 cells, which do not, were cho sen for further comparative studies of drug uptake and regulation of i ntracellular pH. High-performance liquid chromatography (HPLC) determi nations of the uptake of RE 6145 and its dervatives showed that in SiH a cells, intracellular to extracellular drug concentration ratio (C-i/ C-e) at Ih was similar to 40% higher at pH 6.4 than at pH 7.4, whereas in HT 29 cells C-i/C-e was similar to 25% lower. Under conditions of acidic extracellular pH, regulation of pH was somewhat less effective in SiHa cells, where pH(i) dropped to within 0.2 pH units of the extra cellular pH over a 2.5 h treatment at pH 6.4. It seems likely that inc reased drug uptake was at least part of the basis for the observed pot entiation of RE 6145 toxicity in SiHa cells. A model which would bette r explain the results for both cell lines might also include the possi bility that low pH per se potentiates cytotoxic damage to a modest ext ent and that it is offset or augmented by altered uptake in HT 29 and SiHa cells respectively.