The chemotherapeutic consequences of tumor pH in breast carcinoma model systems

Abstract

We hypothesize that acid-outside transplasmalemmalar pH gradients, typical of those found in drug-insensitive breast tumors, will result in drug resistance by the exclusion of weakly basic chemotherapeutic agents via ion trapping. We tested our hypothesis in vitro by evaluating pH dependent cytotoxicity of various clinically relevant chemotherapeutic drugs in the MCF-7 human breast adenocarcinoma cell line. The results of this work were consistent with our hypothesis, namely that (i) weak acids are more toxic in acidic extracellular media, (ii) weak bases are more toxic in basic extracellular media, and (iii) large lipophilic molecules are unaffected by pH change. Both doxorubicin and mitoxantrone were found to have enhanced intracellular accumulation in alkaline media, with the effect being more significant on mitoxantrone. In contrast, only mitoxantrone had a partition coefficient that was appropriately high and compatible with our ion-trapping model. Moreover, the pH dependent cytotoxicity and drug accumulation of mitoxantrone, doxorubicin and chlorambucil remained constant even in drug resistant cell lines such as the MCF-7D40 and MCF-7mitox cell lines. In an MCF-7 tumor/SCID mouse model the in vivo biodistribution of doxorubicin in alkalotic mice was evaluated. No significant difference in doxorubicin accumulation was observed in the tumor compared to controls. We evaluated the anti-tumor activity of mitoxantrone in the C3H model system and found that metabolic alkalosis significantly increased both the tumor growth delay and the log cell kill. In our MCF-7/SCID model system, doxorubicin combined with chronic metabolic alkalosis resulted in a reduction in the rate of tumor regrowth. In vivo mitoxantrone C3H mouse toxicity studies were performed with and without metabolic alkalosis. The LD50 and average weight loss per group remained constant with or without alkalosis. In conclusion, mitoxantrone has a pH-dependent behavior consistent with ion-trapping. (Abstract shortened by UMI.)