SABCS 2017: 2 Years of Extended Anastrozole Therapy Proved as Effective as 5 Years in Hormone Receptor–Positive Breast Cancer

Key Points

  • As of June 30, 2016, 78% of women in both trial arms were alive without recurrence.
  • 757 women experienced recurrence, relapse, or other disease-free survival events: 377 (22%) in the 2-year group and 380 (22%) in the 5-year group.
  • Bone fractures were more likely in years 3 to 5 after randomization, suggesting that a longer duration of anastrozole treatment may be a risk factor for fractures. 

Postmenopausal women with hormone receptor–positive breast cancer who took the aromatase inhibitor anastrozole for 2 years after an initial 5 years of adjuvant endocrine therapy received an equal benefit to those who took the drug for 5 additional years. The trial results suggest that a shorter duration of treatment may provide sufficient benefits while protecting women from harmful side effects, according to data from the Austrian Breast and Colorectal Cancer Study Group (ABCSG)-16 phase III trial presented by Gnant et al at the 2017 San Antonio Breast Cancer Symposium (Abstract GS3-01).

“In early-stage hormone receptor–positive breast cancer, the risk of relapse persists despite many advances in treatment,” said Michael Gnant, MD, FACS, Director and Chairman of the Department of Surgery, Comprehensive Cancer Center, at the Medical University of Vienna. “Adjuvant treatment with aromatase inhibitors has been demonstrated to improve disease-free survival of postmenopausal women with this subtype of breast cancer. However, the optimal duration of extended [aromatase inhibitor therapy] has previously been unknown. Because this treatment leads to prolonged side effects and impacts quality of life, it is important to establish how long the treatment should be given.”

ABCSG-16 Trial

In this trial, between February 2004 and June 2010, 3,484 postmenopausal women with hormone receptor–positive early-stage breast cancer were randomized in 71 centers in Austria to receive either 2 years or 5 years of extended adjuvant therapy. All had undergone an initial 5 years of adjuvant endocrine treatment—either tamoxifen or other regimens containing aromatase inhibitors.

The trial’s primary endpoint was disease-free survival. Secondary endpoints included overall survival, contralateral breast cancer, fractures, and toxicity.

As of June 30, 2016, 78% of women in both trial arms were alive without recurrence; 757 women experienced recurrence, relapse, or other disease-free survival events: 377 (22%) of women in the 2-year group and 380 (22%) of women in the 5-year group.

There was also no significant difference in overall survival or time to contralateral breast cancer. Bone fractures were more likely in years 3 to 5 after randomization, suggesting that a longer duration of anastrozole treatment may be a risk factor for fractures. 

Dr. Gnant said the trial results suggest that clinicians should consider a 2-year course of anastrozole sufficient for most patients. “I believe that these trial results should be implemented into daily practice at once,” he added. “There is simply no rationale to keep most patients on extended [aromatase inhibitor therapy] for longer than 2 years. This result can help save a lot of unnecessary side effects for many women around the world.”

Dr. Gnant cautioned that researchers cannot rule out benefits for some patients who take anastrozole for longer periods. He said future translational research using data and biomaterial from the patients in the ABCSG-16 trial could be useful to characterize potential molecular factors that influence patients’ response to anastrozole.

The content in this post has not been reviewed by the American Society of Clinical Oncology, Inc. (ASCO®) and does not necessarily reflect the ideas and opinions of ASCO®.


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