A Comprehensive Review of the Growing Landscape of Endocrine and Biologic Treatments for Hormone Receptor-positive Breast Cancer

Endocrine therapy (ET) is the backbone of treatment in early and advanced hormone receptor-positive breast cancers, as tumour growth initially relies on oestrogen receptor signalling. However, over time, disease progression often occurs due to diverse mechanisms of resistance. The understanding of such mechanisms has led to the development of many targeted treatments aimed at improving outcomes among patients with endocrine-refractory diseases. In this article, we discuss the growing landscape of novel ETs in development. We also review other developing targeted therapies, including inhibitors of the phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) pathway and cyclin-dependent kinases (CDK), as well as current clinical challenges, such as treatment combinations and sequencing.