VX-710 (biricodar, Incel) restores drug sensitivity to cells expressing P-glycoprotein (P-gp) and multidrug resistance-associated protein (MRP1). MRP1 is expressed in a high proportion of prostate tumors while P-gp expression is variable. Since mitoxantrone (M) and prednisone (P) are substrates for MDR transporters, we initiated a study to evaluate the safety, pharmacokinetics, and efficacy of VX-710 plus M/P in patients with hormone-refractory prostate cancer (HRPC).Eligible patients had progressive HRPC (defined as new lesions, new disease-related pain, or 50% increase in PSA within 6 weeks of entry), testosterone /=50% and 9 of the 12 patients (23% overall, 95% CI 10-35%) achieved a reduction in PSA of >/=80% that was sustained for the duration of treatment with M/P plus VX-710. The median time to PSA progression was 41 weeks (95% CI 34-68 weeks). Of the 40 patients, 15 completed treatment with stable disease and 13 had progressive disease with increasing serum PSA during study treatment. Median survival was 48 weeks for the intent-to-treat population of 40 patients.The addition of VX-710 to M/P therapy did not appear to increase the proportion of patients with significant serum PSA reductions compared to M/P alone. However, the duration of PSA response observed for the 12 PSA responders suggests that MDR inhibition may benefit some patients with HRPC. In addition to MRP1 or P-gp expression, other mechanisms of drug resistance are probably associated with the relative insensitivity of HRPC to cytotoxic therapy.