The purpose of this study was to determine the maximal tolerable dose (MTD) of epirubicin and ADR-529 given in combination with cyclophosphamide, 5-fluorouracil, and tamoxifen. A total of 64 breast cancer patients with locally advanced disease or a first metastatic event were included. Using fixed doses of cyclophosphamide, 5-fluorouracil, and tamoxifen, cohorts of ten patients were treated with escalating doses of epirubicin and ADR-529. With the use of protocol criteria specifying evaluation after the first course, the MTD was not reached. Dose reductions carried out due to hematologic toxicity during the first four courses made it impossible to escalate doses of epirubicin beyond 80 mg/m2 given together with ADR-529 600 mg/m2. The vascular toxicity of ADR-529 necessitated central venous access in a number of patients. For phase III evaluation of ADR-529 given together with cyclophosphamide, epirubicin, 5-fluorouracil, and tamoxifen (CEF/TAM) we recommend using epirubicin/ADR-529 at 60/600 mg/m2. Together with evaluation of the cardioprotective properties of ADR-529, we recommend evaluating the impact of ADR-529 on the efficacy of cytotoxic therapy and investigating further the toxicity of ADR-529.