ER+ breast cancers depend on ER signaling throughout disease progression, including after acquired resistance to existing endocrine agents, providing impetus for further optimization of ER-targeting agents. Fulvestrant, the current best-in-class endocrine agent, was first discovered as a 鈥減ure antiestrogen鈥, in contrast to earlier generation ER ligands that exhibit weak agonistic activity, such as tamoxifen. After its discovery as a full ER antagonist, fulvestrant was demonstrated to decrease ER protein levels through proteasome-mediated degradation. These observations led to the compelling hypothesis that elimination of ER by fulvestrant drives full suppression of ER signaling. ER degradation has thus taken center stage in efforts to identify the next generation of ER inhibitors. Here, I鈥檒l describe our learnings, and surprises 鈥 related to drug mechanism of action 鈥 as we progressed three generations of ER antagonists into clinical studies.
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