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Mamounas Sheds Light on Adjuvant Endocrine Therapy in Breast Cancer

Angelica Welch
Published: Friday, Sep 01, 2017

Terry Mamounas, MD
Terry Mamounas, MD
Recent findings have suggested that 10 years versus 5 years of adjuvant hormone therapy reduces recurrence in women with primary breast cancer; however, the topic has been widely debated among researchers.

For example, results of the NSABP B-42 clinical trial, which were presented at the 2016 San Antonio Breast Cancer Symposium, found that extending letrozole therapy in women with early-stage, hormone receptor (HR)–positive breast cancer who completed 5 years of prior hormone therapy did not yield a statistically significant improvement in either disease-free or overall survival. However, data also showed that prolonged use of the aromatase inhibitor may be beneficial in some subgroups of women with a higher risk of recurrence.

In his presentation on duration of adjuvant endocrine therapy at the PER® 16th Annual International Congress on the Future of Breast Cancer® (East), Eleftherios (Terry) P. Mamounas, MD, discussed whether 10 years was superior to the widely practiced 5 years of adjuvant hormone therapy.

In an interview with OncLive during the meeting, Mamounas, medical director of the Comprehensive Breast Program at the University of Florida Health Cancer Center, shared his insight on recent studies of adjuvant endocrine therapy and their impact on clinicians’ choice in therapy for pre- and postmenopausal women with primary breast cancer.

OncLive: What is the current logic regarding the duration of adjuvant endocrine therapy after primary breast cancer?

Mamounas: As you know, for many years, the thinking was that 5 years of tamoxifen was standard, because many clinical trials have shown that there is a benefit. However, after 5 years, the early studies showed that additional therapy up to 10 years was not better. Therefore, 1 to 5 years was considered the standard. 

The European investigators thought that perhaps longer duration of tamoxifen would be important, so they ran these big trials—the ATLAS and aTTom trials. In the early publication, it showed no significant benefit in the first 5 years, but then with longer follow-up to 15 years, both of the studies showed a significant improvement in disease-free survival (DFS) with 10 years of tamoxifen. ATLAS also saw a significant improvement in survival. After that data were disclosed, the guidelines changed and 10 years of tamoxifen became the standard for patients who are premenopausal.

At the same time, we had introduced aromatase inhibitors, which is a whole different class of drugs with lower estrogen levels for postmenopausal patients. We also had data to indicate that if you take 5 years of tamoxifen and then 5 years of an aromatase inhibitor, the outcomes are better. For patients who already are or become postmenopausal—after 5 years of tamoxifen—most will be treated with an aromatase inhibitor rather than go on to 10 years of tamoxifen, because that has shown benefit.

The last question that we had to address was, “If you started with an aromatase inhibitor, would 10 years of the aromatase inhibitor be better than 5 years?” This was addressed recently in 2 studies—one of which is the MA.17R study that was presented that the 2016 Annual ASCO Meeting. [Regarding the] significant benefit of when you take an aromatase inhibitor for 10 years versus 5 years, the majority of those patients have had tamoxifen for 5 years because they were participating in a clinical trial. 

At the 2016 San Antonio Breast Cancer Symposium, we also presented the results of the NSABP B-42 trial that recruited almost 4000 patients with postmenopausal stage I to III breast cancer. They were randomized to 5 years versus 10 years of an aromatase inhibitor-based therapy, such as letrozole. Most of these patients have had 5 years of an aromatase inhibitor previously or tamoxifen followed by an aromatase inhibitor for 5 years. In that study, we showed a borderline improvement; it did not formally meet statistical significance by definition, though the P value is .048. 

Therefore, they were a little bit of discordant results—1 very positive study and 1 that showed a borderline significant benefit. As I explained in my lecture, the endpoints that these 2 studies used—the DFS used in the MA.17R trial included only recurrences of contralateral breast cancer. Our definition of DFS in NSABP B-42 also included deaths from other causes and nonbreast malignancies, which is a more traditional definition of DFS. When the MA.17R trial looked at the same endpoint without the nonbreast malignancies or deaths from other causes, then the difference was not statistically significant; it was about a 20% reduction. 

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Clinical Vignette Series: 34th Annual Chemotherapy Foundation Symposium: Innovative Cancer Therapy for Tomorrow®Feb 28, 20182.0
34th Annual Miami Breast Cancer Conference® Clinical Case Vignette Series™May 25, 20182.0
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