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.