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The activity of single-agent paclitaxel (Taxol; Bristol-Myers Squibb Company, Princeton, NJ) against untreated and previously treated metastatic breast cancer (documented in anthracycline-resistant disease and in extensively pretreated patients as well) has prompted investigations of the optimal doses and schedules of paclitaxel/doxorubicin combinations. With one exception, paclitaxel has been administered in either a 24- or a 3-hour infusion, while the administration times for doxorubicin vary from bolus injection to a 72-hour infusion. Results of these completed phase I and II trials are reviewed. Also reported are two ongoing European trials that have achieved promising preliminary results. In Milan, a phase I trial has achieved a preliminary response rate exceeding 90% in 30 chemotherapy-naive patients treated with an alternating schedule of paclitaxel over 3 hours and intravenous bolus doxorubicin. At doses of paclitaxel 200 mg/m2 and doxorubicin 60 mg/m2, the dose-limiting toxicity is leukopenia and mucositis. Furthermore, congestive heart failure has occurred in six patients. We are conducting a phase I/II study in minimally pretreated patients, with a 30-minute doxorubicin infusion preceding a 3-hour paclitaxel infusion every 3 weeks. Of 24 patients evaluable for response, five have achieved partial responses and three complete responses. (Another five partial and two complete responses need confirmation.) Of the two dose levels now given, all responses occurred at the higher paclitaxel/doxorubicin level, 175 and 60 mg/m2, respectively. Despite grades 3 and 4 neutropenia in 31% and 60% of courses, respectively, only six patients have been hospitalized for febrile neutropenia. Of concern, the left ventricular ejection fraction has decreased to below normal in six patients and two have developed symptomatic congestive heart failure. Whether lowering the peak doxorubicin concentration will preclude this effect, which has been observed only in the studies using short infusions of both drugs, is under investigation.