Anastrozole alone or in combination with tamoxifen versus tamoxifen alone for adjuvant treatment of postmenopausal women with early-stage breast cancer: results of the ATAC (Arimidex, Tamoxifen Alone or in Combination) trial efficacy and safety update analyses

M Baum, A Buzdar, J Cuzick, J Forbes, J Houghton, A Howell, T Sahmoud
Cancer 2003 November 1, 98 (9): 1802-10

BACKGROUND: The first analysis of the ATAC (Arimidex, Tamoxifen Alone or in Combination) trial (median follow-up, 33 months) demonstrated that in adjuvant endocrine therapy for postmenopausal patients with early-stage breast cancer, anastrozole was superior to tamoxifen in terms of disease-free survival (DFS), time to recurrence (TTR), and incidence of contralateral breast cancer (CLBC). In the current article, the results of the first efficacy update, based on a median follow-up period of 47 months, are reported along with the results of an updated safety analysis, performed 7 months after the first analysis (median duration of treatment, 36.9 months).

METHODS: DFS, TTR, CLBC incidence, and safety were assessed in the same patient group as in the first analysis of the ATAC trial.

RESULTS: DFS estimates at 4 years remained significantly more favorable (86.9% vs. 84.5%, respectively) for patients receiving anastrozole compared with those receiving tamoxifen (hazard ratio [HR], 0.86; 95% confidence interval [CI], 0.76-0.99; P = 0.03). The benefit generated by anastrozole in terms of DFS was even greater in patients with hormone receptor-positive tumors (HR, 0.82; 95% CI, 0.70-0.96; P = 0.014). The HR for TTR also indicated a significant benefit for patients receiving anastrozole compared with those receiving tamoxifen (HR, 0.83; 95% CI, 0.71-0.96; P = 0.015), with additional benefit for patients with hormone receptor-positive tumors (HR, 0.78; 95% CI, 0.65-0.93; P = 0.007). CLBC incidence data also continued to favor anastrozole (odds ratio [OR], 0.62; 95% CI, 0.38-1.02; P = 0.062), and statistical significance was achieved in the hormone receptor-positive subgroup (OR, 0.56; 95% CI, 0.32-0.98; P = 0.042). The updated safety analysis also confirmed the findings of the first analysis, in that endometrial cancer (P = 0.007), vaginal bleeding and discharge (P < 0.001 for both), cerebrovascular events (P < 0.001), venous thromboembolic events (P < 0.001), and hot flashes (P < 0.001) all occurred less frequently in the anastrozole group, whereas musculoskeletal disorders and fractures (P < 0.001 for both) continued to occur less frequently in the tamoxifen group. These results indicated that the safety profile of anastrozole remained consistent.

CONCLUSIONS: After an additional follow-up period, anastrozole continues to show superior efficacy, which is most apparent in the clinically relevant hormone receptor-positive population. Furthermore, anastrozole has numerous noteworthy advantages in terms of tolerability compared with tamoxifen. These findings suggest that the benefits of anastrozole are likely to be maintained in the long term and provide further support for the status of anastrozole as a valid treatment option for postmenopausal women with hormone-sensitive early-stage breast cancer.

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