PURPOSE To evaluate the effect of quinidine, a putative modulator of P-glycoprotein-mediated drug resistance, on the response rate and toxicity profile of epirubicin in patients with advanced breast cancer. PATIENTS AND METHODS Between 1989 and 1992, 223 eligible patients were randomized in double-blind fashion to receive epirubicin 100 mg/m2 by intravenous (i.v.) bolus and prednisolone 25 mg orally twice daily, along with either placebo or quinidine (250 mg) capsules, taken for 4 days before and 2 days after chemotherapy. Treatment was continued for a maximum of eight courses. RESULTS Ten eligible patients did not complete the first cycle of treatment. Of the remaining patients, 106 in the placebo arm received 619 courses of treatment, and 107 in the quinidine arm received 612 courses. The median cumulative dose of epirubicin in both arms was 600 mg/m2. The median quinidine level (measured before epirubicin administration in 288 courses) was 5.5 mumol/L; at this concentration, the drug partially reverses anthracycline resistance in multidrug-resistant (MDR) breast carcinoma cells in vitro. There were no statistically significant differences in hematologic or gastrointestinal toxicity between the two arms. The response rate in the placebo arm was 44% (6% complete remission [CR], 38% partial remission [PR]), and in the quinidine arm was 43% (4% CR, 39% PR). Surviving patients have been monitored for a median time of 74 weeks, and there is no significant difference in the overall or progression-free survival between the two arms. The median survival times were 59 weeks for placebo and 47 weeks for quinidine patients. The estimated relative death rate (quinidine/placebo) was 1.2 (P = .247; 95% confidence interval [CI], 0.88 to 1.63). CONCLUSION Quinidine at this dose does not significantly alter the toxicity profile, response rate, or survival after epirubicin chemotherapy in patients with advanced breast cancer. This may be due to ineffective modulation of P-glycoprotein by quinidine or the lack of expression of mdr-1 in a sufficient proportion of cells in these tumors, or alternative mechanisms underlying resistance to epirubicin.
