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Resistance to cytotoxic chemotherapy is a major problem in the management of patients with metastatic breast cancer. Various data suggest P-glycoprotein-associated multidrug resistance (MDR) to be a relevant resistance mechanism in this tumor. The purpose of this study was to evaluate feasibility and activity of combining oral dexverapamil, a second-generation chemosensitizer currently in clinical development for MDR reversal, with epirubicin in patients with epirubicin-refractory high-risk metastatic breast cancer. Patients first received epirubicin alone at 120 mg/m2. In cases of clinical refractoriness, epirubicin was continued at the same dose and schedule but supplemented with oral dexverapamil. Dexverapamil was given at 300 mg every 6 h for a total of 13 doses and commenced 2 days prior to epirubicin administration. At the time of this interim analysis, 41 patients had received epirubicin alone and 20 proceeded to treatment with epirubicin plus dexverapamil. Of the 20 patients, 14 were considered evaluable for toxicity and activity. Addition of dexverapamil resulted in a significant decrease in mean heart rate and blood pressure as well as prolongation of PQ time as compared to epirubicin alone. However, these cardiovascular effects of dexverapamil were usually mild, and subjective tolerance of treatment was good. In 7/14 patients, dose escalation of dexverapamil was feasible. Dexverapamil had no effect on epirubicin toxicities and did not require reduction of the epirubicin dose. In 2/14 patients, the addition of dexverapamil to epirubicin was able to convert progressive disease and no changes respectively, into partial responses. In 3 patients with progressive disease, addition of dexverapamil temporarily prevented further tumor progression. Analyses of dexverapamil and nor-dexverapamil plasma levels, of in vitro reversal activity of patient sera containing dexverapamil, and of epirubicin pharmacokinetics without and with dexverapamil are currently in progress. Addition of oral dexverapamil to epirubicin 120 mg/m2 proved to be feasible in a multiinstitutional setting. Patient accrual is continuing to determine whether dexverapamil is capable of overcoming epirubicin refractoriness in a significant number of patients with metastatic breast cancer.