Ion-trapping theory predicts that alkalinization of tumor extracellular pH will enhance the anti-tumor activity of weak-base chemotherapeutics. We have previously demonstrated that chronic and acute treatment of tumor-bearing mice with sodium bicarbonate results in tumor-specific alkalinization of extracellular pH. Furthermore, bicarbonate pretreatment enhances the anti-tumor activity of doxorubicin and mitoxantrone in two different mouse tumor models. Previous work has indicated subtle, yet significant differences between the pH sensitivities of the biodistribution and anti-tumor efficacies of doxorubicin and mitoxantrone in vitro. The present study demonstrates that systemic alkalinization selectively enhances tumor uptake of radiolabeled mitoxantrone, but not doxorubicin. Results using these two drugs are quantitatively and qualitatively very different, and can be explained on the basis of differences in the octanol-water partition coefficients of their charged forms. These results suggest that inducing metabolic alkalosis in patients would have a positive effect on response to mitoxantrone therapy. However, the therapeutic index would not increase if sodium bicarbonate also caused increased retention of mitoxantrone in susceptible normal tissues in the host. The major dose-limiting organ systems for mitoxantrone are heart, liver, bone marrow, spleen and blood cells. Bicarbonate was found to have no significant effect on the distribution of mitoxantrone to any of these tissues except for spleen. However, neither spleen weights nor lymphocyte counts were adversely affected by NaHCO(3) pretreatment, indicating that this co-therapy does not enhance myelosuppression due to mitoxantrone therapy. These findings suggest that metabolic alkalosis would produce a net gain in mitoxantrone therapeutic index.