Duration of Arimidex Treatment For Breast Cancer May Safely Be Shortened

Researchers find that women with postmenopausal hormone receptor (HR)-positive breast cancer may undergo shorter treatments of Arimidex, which may in turn cause less bone fractures.

Investigators with the Austria Breast and Colorectal Cancer Study Group announced today that years can safely be shaved off Arimidex (anastrozole) treatment without affecting disease-free survival (DFS) for women with postmenopausal hormone receptor (HR)-positive breast cancer.

Additionally, shorter treatment spans can significantly reduce bone fractures, a side effect of aromatase-inhibitor (AI) therapy.

Results from the ABCSG-16 phase III trial were presented at the 2017 San Antonio Breast Cancer Symposium held in San Antonio, Texas.

For years, oncologists have followed up initial AI therapy with extended therapy for up to five years. Michael Gnant, M.D., director and chairman, Department of Surgery, Comprehensive Cancer Center, Medical University of Vienna, recommended that oncologists immediately incorporate the new findings into practice by significantly reducing the use of adjuvant AI therapy following initial treatment. “There is simply no rationale to keep most patients on extended AI for longer than two years,” said Gnant, the study’s lead author.

Investigators enrolled 3,484 postmenopausal women with HR-positive early-stage breast cancer from February 2004 to June 2010. The patients, all of whom had undergone an initial five years of adjuvant endocrine treatment, were randomly assigned to two years (n = 377) or five years (n = 380) of extended adjuvant therapy. About 50 percent of the enrolled patients had been treated initially with Soltamox (tamoxifen), and the remainder with AI or any sequence of Soltamox and AI. Median age was 64 years.

The primary endpoint of was DFS, and investigators observed little difference between the two groups. At 10 years from randomization, the DFS rates were 71.1 percent for the two-year group versus 70.3 percent for the five-year group.

As of June 30, 2016, 78 percent of women in both trial arms were alive without recurrence. Recurrence, relapse, or other DFS events were equally common in both groups (22 percent). Investigators said there was no significant difference in overall survival or time to contralateral breast cancer.

For women in the five-year adjuvant therapy arm, 6.3 percent had experienced bone fractures after five years on extended AI. In the two-year group, 4.7 percent had experienced fractures by the end of the five-year cutoff. There were 71 fractures in the two-year patient group (n = 1,556) versus 98 in the five-year cohort (n = 1571). Investigators concluded that longer treatment with Arimidex may be a risk factor for fractures.

Other side-effect reductions were noted for the two-year group, including a lower incidence of arthralgia.

Ten years following randomization, 85.3 percent of patients in the two-year arm were alive versus 84.9 percent of those in the five-year arm. There was no difference between an additional two and five years of Arimidex in terms of overall survival, time to contralateral breast cancer, and time to second primary cancer, all of which were secondary endpoints in the trial.

Some patients may still benefit from taking Arimidex for longer periods, Gnant cautioned. He added that future translational research using data and biomaterial from patients in the ABCSG-16 trial could be useful in identifying potential molecular factors that influence how patients respond to Arimidex.

Patients with HR-positive breast cancer bear a significant risk of relapse and so extended AI therapy has become the norm and remains necessary, Gnant said. “More than 50 percent of relapses occur after the first five years of follow-up in this type of breast cancer.” Although ABCSG-16 found that two years of Arimidex was as good as five, Gnant said that earlier studies had established that less than two years of AI extended treatment would not produce comparable efficacy.

“We can now conclude that after five years of standard adjuvant endocrine treatment, two additional years with AIs are sufficient as extended therapy,” he said. “There is no benefit in escalating endocrine treatment beyond seven years. While not providing outcome benefits, the extension of five additional years leads to increased side effects, including fractures, and thus should be avoided.”