Extended adjuvant endocrine therapy in hormone-receptor positive early breast cancer: Current and future evidence

• The optimal duration for adjuvant endocrine treatment of breast cancer has not yet been established.
• Current evidence for extended adjuvant endocrine therapy is based on an outdated regime of 5 years of tamoxifen.
• Multiple clinical trials are being conducted using modern AI-containing regimes.
• Prognostic markers used for decisions on chemotherapy, could also be valuable to decide on extended endocrine treatment.

The optimal duration and regimen of adjuvant hormonal therapy for premenopausal and postmenopausal patients with hormone receptor positive early breast cancer has not yet been established. This review will give an overview of published and ongoing studies concerning extended endocrine treatment. Most of the currently published studies are based on the adjuvant treatment regime of 5 years tamoxifen, which has been proven to be inferior compared to aromatase inhibitor (AI)-containing regimes. Therefore, until today, there is no clear evidence for the extension of endocrine therapy after upfront AI-based adjuvant treatment regimes. Multiple clinical trials, which will be discussed in this review, are ongoing to elucidate on this matter. We emphasize the need for tailoring of extended adjuvant endocrine treatment. The quest for predictive biomarkers, which are currently being investigated in the context of decision-making whether or not to start adjuvant chemotherapy, should be expanded to include the feasibility of extended endocrine treatment based on these markers. By tailoring the extension of endocrine treatment, overtreatment, side effects and unnecessary costs will be prevented.