Summary We have observed that low pH can substantially potentiate the cytotoxic effect of the bioreductive drug SR4233 in aerobic human tumour cells. No such potentiation was observed under hypoxic conditions. This pH effect might be relevant both to the therapeutic effectiveness and to the normal tissue toxicity of this new agent.The bioreductive drug 1,2,4-Benzotriazin-3-amine 1,4-Dioxide (SR4233 or WIN59075) has been shown to have a remarkably selective toxicity for hypoxic cells (Zeman et al., 1986), a property which has stimulated much interest in its potential as an adjunct to radiation and chemotherapy. It is believed that the active form of the agent is an oxygensensitive, 1-electron reduction product (Laderoute et al., 1988;Baker et al., 1988). A potentially important secondary property of SR4233 is its reported ability to act as a radiosensitiser of aerobic cells which receive a pre-or postirradiation exposure to the drug under hypoxic conditions (Zeman & Brown, 1989). These authors suggested that this characteristic might be effective in dealing with the problem of transient hypoxia known to occur in experimental tumours (Chaplin et al., 1986(Chaplin et al., , 1987. It has also been shown that hydralazine can potentiate the cytotoxic effect of SR4233 in a mouse tumour system (Brown, 1987).While there has been extensive study of the effect of hypoxia on the cytotoxicity of SR4233, there has been relatively little attention given to the influence of other microenvironmental factors which might be important to the anti-tumour potential of this drug (Tocher et al., 1990;Herman et al., 1990). In this paper we describe the influence of pH on the cytotoxic effect of SR4233 under both aerobic and hypoxic conditions. begin, a fresh stock solution of SR4233 was prepared by dissolving the drug in 37°C medium (buffered by BIS-TRIS) for 10 min, followed by sonication at 37°C for 5 min. The concentration of SR4233 was 10 mM (solubility limit is 13.5 mM). The stock solution was sterile-filtered and diluted to provide appropriate concentrations. Materials and methodsResults Figure 1 shows the effect of pH on the cytotoxicity of SR4233 in HT-29 human colon adenocarcinoma cells, for both aerobic and hypoxic conditions. The cells were incubated at 37°C for 1 h in complete medium containing 1000 LM or 40 tlM SR4233 for aerobic or hypoxic conditions, respectively. These conditions were chosen to yield approximately equivalent cell kill at pH 6.6 and they illustrate the striking potentiation of SR4233 cytotoxicity that occurs with hypoxic conditions. It can be seen that the plating efficiency of these cells, in the absence of drug, is unaffected by pH and is approximately the same under both aerobic and hypoxic conditions (range 50-86%). Under aerobic conditions, the cytotoxic effect of 1000 JSM SR4233 is strongly potentiated by low pH; the cell kill at pH 6.0 is approximately 300-fold greater than at pH 7.4. Under hypoxic treatment, however, low pH seems to have, if anything, a slightly protective effect against SR4233 c...