The role of taxanes in the treatment of breast cancer.

The taxanes paclitaxel and docetaxel are undergoing extensive evaluation in women with breast cancer in the United States and in Europe. Their dose-limiting toxicity is neutropenia. Paclitaxel also causes peripheral neuropathy, while docetaxel can cause unpredictable and severe skin toxicity, as well as edema and effusions due to a capillary leak syndrome. Due to threshold pharmacodynamics and nonlinear pharmacokinetics, tolerability of paclitaxel is schedule dependent. Single-agent paclitaxel was very active in multiple phase II trials in patients with different numbers and types of prior chemotherapy and disease extent (20% to 60% complete plus partial responses). Effective doses ranged from 135 to 250 mg/m2. Activity was observed with all infusion schedules (1, 3, and 24 hours) and in women with anthracycline-resistant tumors (25% to 38%). The use of a 96-hour infusion schedule was very active in anthracycline-refractory patients (48%) and in women who failed short infusion taxanes. The drug is undergoing extensive evaluation in combination with doxorubicin, cyclophosphamide, cisplatin, and antimetabolites. Very promising efficacy was observed for paclitaxel by 3-hour infusion plus bolus doxorubicin (approximately 40% complete responses and 50% partial responses). The combination also caused a high incidence of clinically reversible congestive heart failure(14% to 18%). Docetaxel also has very good efficacy in breast cancer, with approximately 70% major responses in untreated patients and more than 50% in anthracycline-resistant tumors. There is no evidence that efficacy and tolerability are schedule dependent as is the case for paclitaxel. At recommended doses (100 or 75 mg/m2 by 1-hour infusion every 3 weeks), docetaxel causes a fluid retention syndrome that may affect quality of life. Its common onset after multiple cycles may limit the use of docetaxel for palliation in metastatic breast cancer. These results clearly indicate that the taxanes will become a standard component of initial chemotherapy for women with breast cancer. The definition of their actual role still requires an answer to the unresolved questions of their optimal dose and combination with other anticancer agents. Most importantly, the drugs should be prospectively evaluated in a randomized study using comparable doses and schedules to assess which of the two has the better therapeutic index in breast cancer.