Abstract
Abstract Despite continued administration of antiestrogen therapies, approximately 50% of all estrogen receptor alpha (ERalpha) positive breast cancers will present new metastatic lesions. The acquisition of secondary hormone-resistant metastatic breast cancers represents a significant clinical barrier towards life-long disease free survival for the patient. Somatic mutations to the ERalpha gene (ESR1) Y537S and D538G represent a novel mechanism of acquired antiestrogen resistance because they confer hormone-free transcriptional activity and reduced selective estrogen receptor modulator (SERM) and selective estrogen receptor degrader (SERD) potency. Fulvestrant, a SERD, was the only molecule that could completely ablate mutant ERalpha activity. Unfortunately, fulvestrant possesses poor pharmacologic profiles that limit its therapeutic utility. Bazedoxifene (BZA) is a potent mixed SERM/SERD and has improved pharmacokinetics and oral bioavailability compared to fulvestrant. We show that BZA inhibits Y537S and D538G ESR1 somatic mutation transcriptional activity with a greater potency than the SERMs 4-hydroxytamoxifen (TOT) and raloxifene (RAL). Further investigations into the biophysical and structural basis for BZA action suggest that BZA increases the conformational dynamics of helix 12, a key molecular switch that governs ERalpha action resulting in SERD-like properties and improved potency against the somatic mutations compared to TOT and RAL. Citation Format: Sean W. Fanning, Venkat Dharmarajan, Christopher G. Mayne, Weiyi Toy, Kathryn E. Carlson, Teresa A. Martin, Jason Nowak, Jerome Nwachukwu, David J. Hosfield, Emad Tajkhorshid, Sarat Chandarlapaty, Patrick Griffin, Yang Shen, John A. Katzenellenbogen, Geoffrey L. Greene. Bazedoxifene inhibits ESR1 somatic mutants with improved potency compared to tamoxifene and raloxifene. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4854.
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