Emerging role of taxanes in adjuvant and neoadjuvant therapy for breast cancer: the potential and the questions.

Adjuvant chemotherapy has gained increasing prominence in the treatment of nonmetastatic breast cancer, producing gradual improvement in the survival of these patients. The taxanes offer great hope for adding to the progress in adjuvant treatment, but data have been conflicting. Early results of multi-center trials testing the sequential addition of paclitaxel to anthracycline-based adjuvant chemotherapy have perhaps been prematurely reported, but have already made a major impact on patterns of care for node-positive and even some node-negative patients. The early dramatic improvements in CALG 9344 are fading with time, however, and have not been confirmed by a second similar trial, NSABP B-28. Moreover, it cannot be stated with certainty whether the modest improvements observed by sequential addition of paclitaxel reflect the ability of this drug to kill anthracycline-resistant cancer cells or the increased total duration and amount of treatment. By contrast, the early results of the BCIRG 001 trial suggest that combining docetaxel with doxorubicin may significantly increase survival, but these early results should be viewed with caution and do not necessarily mean that docetaxel is superior to paclitaxel. The role of neoadjuvant chemotherapy for breast cancer has also expanded over the past 2 decades, from its initial use for inoperable locally advanced breast cancer (LABC) to its current use for patients with large operable tumors to make BCT feasible. The neoadjuvant approach also has an important role in clinical trials, where it will allow more rapid comparison of treatment regimens than can be accomplished in the adjuvant setting and provides an opportunity to analyze biologic markers as predictors of response. The value of this approach, however, will ultimately depend on a clear demonstration, not yet available, that a change in therapy that increases primary tumor response will also lead to improved long-term survival. The roles of docetaxel and paclitaxel in the neoadjuvant setting has been actively investigated over the past 5 to 10 years, and exciting results are beginning to emerge. Clearly, docetaxel has potent antitumor activity against breast cancer. Several preliminary results suggest that addition of docetaxel to an anthracycline-based regimen, particularly when added sequentially, as in NASBP B-27 and the Aberdeen trial, results in higher clinical and pathologic response rates. Whether this will translate into increased long-term survival, as suggested by the early results of the Aberdeen trial, remains to be seen. Whether sequential addition of docetaxel to doxorubicin is more or less effective than combining these drugs also has not been established. The results from M.D. Anderson suggesting that paclitaxel given on a weekly schedule was more effective than the same drug given every 3 weeks are particularly intriguing, and they may help to explain why the adjuvant studies with paclitaxel given every 3 weeks have not produced more dramatic results, whereas several studies with docetaxel (also given every 3 weeks) seem so positive. It may be that paclitaxel, with activity that is highly schedule-dependent and for which cell killing is more dependent on the duration of exposure, works best when given weekly, whereas the efficacy of docetaxel depends less on scheduling. If this is the case, then weekly paclitaxel may turn out to be equally effective as docetaxel appears to be even when given every 3 weeks. Alternatively, if docetaxel is simply a more active drug, then giving docetaxel weekly may be the most effective taxane regimen. Whether routine use of weekly chemotherapy administration in the adjuvant or neoadjuvant setting is practical or not is largely subjective, but at least it appears that the toxicity of this approach is acceptable. These issues are also being addressed in ongoing trials. Finally, taxanes have produced dramatic increases in response rates in the neoadjuvant setting, but, except for the Aberdeen trial, survival benefits have not yet been shown. If, however, the high pCR rates do translate into overall survival benefits that are greater than adding taxanes to postoperative adjuvant therapy, it might suggest that, unlike other drugs, taxanes are actually more effective before surgery than after, as predicted originally based on laboratory experiments. Clearly, much work remains to be done in this area of research on breast cancer therapy.