Synergistic effect of prochlorperazine and dipyridamole on the cellular retention and cytotoxicity of doxorubicin.

Incubation of drug-resistant human tumor cells with a combination of prochlorperazine and dipyridamole has additive/synergistic effect on the cellular retention and cytotoxicity of doxorubicin. In patients administered a fixed dose of doxorubicin and prochlorperazine with escalating doses of dipyridamole, mean plasma levels of dipyridamole and prochlorperazine achieved were as high as 3.01 +/- 0.41 microm and 0.94 +/- 0.09 microm, respectively. Plasma samples from patients were analyzed in an in vitro assay to monitor the effect on the cellular retention of tritium-labeled daunorubicin in MDR1-transfected P388 cells. In 22 of 49 of the plasma samples analyzed, the daunorubicin in efflux blocking activity was one-half or greater than that of cells incubated with 12.5 microM verapamil, a well-known efflux blocker. These observations suggest that a combination of prochlorperazine and dipyridamole may enhance cellular doxorubicin retention by blocking efflux while reducing normal tissue toxicity and unwanted side effects in vivo.