Evolution of Targeted Therapy in Breast Cancer: Where Precision Medicine Began.

As we consider best practices and approaches to targeted therapy in the clinic and in terms of trial design, breast cancer can serve as a useful model for other disease types, because estrogen receptor-positive and HER2-positive breast cancer have been known entities for several decades. In this review, we provide a history of the development of anti-estrogen therapy and anti-HER2-directed therapy and we discuss our growing understanding of resistance to targeted therapy as seen through this lens. We highlight some of the recent breakthroughs that have enhanced our understanding of resistance to endocrine and anti-HER2 therapy, and we discuss some of the ongoing research in the field.