Clinical Translation: Targeting the Estrogen Receptor.

Estrogen Receptor alpha (ERα) stands as one of the most successfully prosecuted drug targets in oncology, beginning with the approval of tamoxifen for women with ERα positive (ER+) breast cancer over 40 years ago. The field continued to advance with the development of aromatase inhibitors and the pure antiestrogen fulvestrant. With multiple endocrine therapies approved for the treatment of ER+ breast cancer, efforts to generate novel ERα-targeted therapeutics somewhat diminished in the early 2000s. Today however, there are at least eight new molecular entities targeting ERα under active clinical investigation, each with the aim of bringing further benefit to patients. This remarkable re-energizing of the field was spurred in part by the discovery of highly prevalent ERα mutations as a mechanism of resistance to standard-of-care therapies, which provided unequivocal evidence of the continued, and broad, dependence of tumors on ERα, despite relapsing after earlier lines of endocrine therapy. Re-engagement of the pharmaceutical and biotechnology industries with ERα as a drug target has been further underpinned by the impressive advances made in medicinal chemistry, enabling desirable mechanistic features - high potency full ERα antagonism - to be combined with improved drug-like properties - oral bioavailability and optimized pharmacokinetics. In this chapter, we describe the rich history and science behind the currently evolving landscape of ERα targeting in breast cancer.