Switching to anastrozole versus continued tamoxifen treatment of early breast cancer. Updated results of the Italian tamoxifen anastrozole (ITA) trial

BACKGROUND

Tamoxifen, for many years the 'gold standard' in the adjuvant setting for the management of endocrine sensitive early breast cancer, is associated with an increased risk of endometrial cancer and other life-threatening events. Moreover, many women relapse during or after tamoxifen therapy due to the development of resistance. This provided the rationale for a switching trial with anastrozole, the updated results of which are reported here.

PATIENTS AND METHODS

This trial investigated the efficacy of switching to anastrozole for women already receiving tamoxifen. After 2-3 years of tamoxifen treatment, postmenopausal, node-positive, ER-positive patients were randomized to receive either anastrozole 1 mg/day or to continue tamoxifen, 20 mg/day, giving a total duration of 5-years treatment. The primary end point was disease-free survival and secondary endpoints were event-free survival, overall survival and safety.

RESULTS

A total of 448 patients were enrolled. At a median follow-up time of 64 months (range 12-93), 63 events had been reported in the tamoxifen group compared with 39 in the anastrozole group [HR 0.57 (95% CI 0.38-0.85) P = 0.005]. Relapse-free and overall survival were also longer in the anastrozole group [HR 0.56 (95% CI 0.35-0.89) P = 0.01 and 0.56 (95% CI 0.28-1.15) P = 0.1]. However, the latter difference was not statistically significant. Overall more patients in the anastrozole group experienced at least one adverse event (209 versus 151: P = 0.000). However, numbers of patients experiencing serious adverse events were comparable (37 versus 40, respectively: P = 0.7).

CONCLUSIONS

Switching to anastrozole after the first 2-3 years of treatment was confirmed to improve event-free and relapse-free survival of postmenopausal, node-positive, ER-positive early breast cancer patients already receiving adjuvant tamoxifen.

Anastrozole as an adjuvant endocrine treatment for postmenopausal patients with breast cancer: emerging data

The Arimidex, Tamoxifen, Alone or in Combination (ATAC) trial compared the efficacy and safety of anastrozole versus tamoxifen versus the combination as initial adjuvant treatment for early breast cancer in over 9,000 postmenopausal women. Analyses at 33 and 47 months median follow-up showed that anastrozole significantly prolonged disease-free survival and time to recurrence and reduced the incidence of contralateral breast cancer compared with tamoxifen. Results of the completed treatment analysis at 68 months median follow-up confirmed the earlier findings, showing that the absolute difference in disease-free survival continued to increase beyond completion of treatment. Mature safety data from the ATAC trial show that, overall, anastrozole has a favorable safety profile compared with tamoxifen. In the absence of current data on further follow-up or the outcome of trials investigating proactive sequencing of endocrine therapies, we present a model based on several trials, including ATAC. This model suggests that using an aromatase inhibitor as initial adjuvant therapy is a better option than switching to an aromatase inhibitor after >/=2 years of tamoxifen. The relative toxicities of the three approved third-generation aromatase inhibitors, anastrozole, letrozole, and exemestane, are discussed. These data suggest that long-term safety profiles may differ between aromatase inhibitors, although comprehensive comparative data for letrozole and exemestane versus tamoxifen are lacking.

Adjuvant endocrine therapy for postmenopausal women with early breast cancer

Results from multiple clinical trials involving aromatase inhibitors have added to the knowledge base relating to endocrine therapy of postmenopausal women with hormone receptor-positive early breast cancer. In the extended adjuvant setting, data from the Austrian Breast and Colorectal Cancer Study Group 6a trial showed an advantage for anastrozole following 5 years of tamoxifen treatment, consistent with the more robust MA.17 trial that examined letrozole versus placebo following 5 years of tamoxifen treatment. The combined analysis of the Austrian Breast and Colorectal Cancer Study Group 8 trial and the German Arimidex Nolvadex 95 trial, plus the Italian Tamoxifen Anastrozole trial, have shown the advantage of switching to anastrozole over continuing the tamoxifen to complete the full 5 years of adjuvant therapy. These trials support the previously reported larger and double-blind Intergroup Exemestane Study. The Arimidex, Tamoxifen, Alone or in Combination trial now has data out to 68 months of median follow-up showing the maintenance of a significant advantage of anastrozole over tamoxifen for disease-free survival. In this initial treatment setting, the Breast International Group 1-98 trial recently showed a significant advantage for letrozole over tamoxifen. The current debate is centered on whether the optimal strategy is to give an aromatase inhibitor initially or after several years of tamoxifen treatment. Multiple important questions remain, including the predictive value of molecular markers such as progesterone receptor, the optimal duration of aromatase inhibitor therapy, the long-term adverse effects, and the relative efficacy and toxicity of the different aromatase inhibitors.

Is There a Role for Adjuvant Tamoxifen in Progesterone Receptor–Positive Breast Cancer? An In silico Clinical Trial

PURPOSE

Subset analysis from the Arimidex, Tamoxifen, Alone or in Combination trial, a major adjuvant therapy trial, suggests that progesterone receptor-negative (PR-) cases may derive greater benefit from aromatase inhibitor compared with tamoxifen than PR+ cases. We postulated that estrogen receptor-positive (ER+)/PR+ patients might do as well or better by starting on tamoxifen and later switching to an aromatase inhibitor rather than by starting on an aromatase inhibitor as initial therapy.

EXPERIMENTAL DESIGN

We constructed a computer model using retrospective data to approximate exponential failure rates for PR+ and PR- in tamoxifen-treated and tamoxifen-untreated patients, adding the assumptions that about half of patients are cured at surgery and that approximately 20% of postmenopausal ER+ early breast cancer cases are PR-. This model provided a very good approximation to the published overview data and to the clinical trials. We then used the failure rates to generate relapse times for a large number of cases (n = 50,000) for each treatment scenario.

RESULTS

In PR- cases, initial therapy with an aromatase inhibitor is superior to tamoxifen and this advantage can never be made up by switching. In PR+ cases, tamoxifen is only modestly inferior to aromatase inhibitor at the outset, and after switching to an aromatase inhibitor at 3 or 5 years the tamoxifen relapse-free survival curve catches up and then begins to surpass the aromatase inhibitor curve at 7.5 or 12 years, respectively.

DISCUSSION

Although our in silico trial is based on many assumptions, it closely approximates results of the published trials and, therefore, suggests that an in vivo comparison in ER+/PR+ patients of aromatase inhibitor versus tamoxifen followed by aromatase inhibitor may be worth considering.