S1-1: A Phase III Randomized Trial of Anastrozole Versus Anastrozole and Fulvestrant as First-Line Therapy for Postmenopausal Women with Metastatic Breast Cancer: SWOG S0226.
Authors: Mehta RS , Barlow WE , Albain KS , Vandenberg T , Dakhil SR , Tirumali NR , Lew DL , Hayes DF , Gralow JR , Livingston RB , Hortobagyi GN 
Published: June 2012

Background: Anastrozole inhibits the aromatase enzyme-induced estrogen synthesis, and fulvestrant down-regulates estrogen receptors in hormone receptor-positive breast cancer. We hypothesized that the simultaneous disruption of the ligand-receptor axis with concurrent use of these agents may be potentially additive or synergistic in first-line therapy of hormone receptor-positive metastatic breast cancer in postmenopausal women.

Materials and Methods: A total of 707 patients were randomized to either 1 mg anastrozole P.O. daily (Arm 1) or to the combination of anastrozole and fulvestrant (Arm 2). Fulvestrant was given as an intramuscular injection as follows: loading dose of 500 mg on day 0, followed by 250 mg on days 14, 28 and 250 mg maintenance monthly thereafter. The primary endpoint was progression-free survival (PFS), with a power of 90% and one-sided alpha of 0.025 to detect an expected median PFS of 10 months in Arm 1 versus 13 months in Arm 2. Randomization was stratified by adjuvant tamoxifen use with the possibility of differential benefit of fulvestrant in the two strata. Patients were encouraged to crossover to fulvestrant after progression on the anastrozole alone arm (40% did), if they were not candidates for immediate chemotherapy. Analysis of survival was by 2-sided stratified log-rank tests and Cox regression using intent-to-treat. Two interim analyses were performed with the final analysis using a 2-sided p-value of 0.04.

Results: There were 548 events (287 and 261 by arms, respectively) among 694 eligible patients (345 and 349, respectively). Overall, median PFS was 13.5 months for anastrozole and 15.0 months for the combination of fulvestrant and anastrozole (log-rank p=0.0145; HR=0.81 (95% CI 0.68−0.96). In a subset analysis of tamoxifen-naive women (60%, n=414), median PFS was 12.6 months and 17.0 months for Arms 1 and 2, respectively (log-rank p=0.0069; HR=0.74 (95% CI 0.60−0.92)), and 14.1 months and 13.5 months for Arms 1 and 2, respectively, in the 40% of tamoxifen-pretreated women (log-rank p = 0.56; HR=0.93 (95% CI 0.71−1.20)). A trend for improved overall survival (OS) in the combination arm was seen (median OS was 42 and 48.6 months based on 152 and 137 deaths respectively, log-rank p = 0.094; HR=0.82 (95% CI 0.65−1.03)). On the combination arm, three treatment-related deaths occurred: due to pulmonary embolism (2) or cerebrovascular ischemia (1); one Grade 4 thrombosis/embolism and one Grade 4 neutropenia and lymphopenia were reported. On the anastrozole arm, four patients experienced Grade 4 toxicities: thrombosis/embolism, arthralgia, thrombocytopenia, and dyspnea. In general, Grade 3 or higher toxicity was rare (12.7% vs. 14.5% for Arms 1 and 2, respectively) and did not differ significantly.

Discussion: The combination of anastrozole and fulvestrant was associated with improved PFS compared to anastrozole alone as well as a trend in OS despite substantial crossover, thus offering a new standard in the first-line treatment of hormone receptor-positive breast cancer in postmenopausal women, specifically in tamoxifen-naive patients. ClinicalTrials.gov:NCT00075764. Funding: NIH/NCI CA32102, CA38926 and AstraZeneca.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr S1-1.