Gdc-9545: A Pure Antiestrogen Clinical Candidate That Immobilizes The Estrogen Receptor And Profoundly Alters Chromatin Accessibility In Vivo

Estrogen Receptor-positive (ER+) breast cancer is a significant unmet medical need. Tumors expressing ER exhibit evidence of mitogenic ER signaling throughout disease progression, including after acquired resistance to existing endocrine agents. Such continued dependence on ER signaling highlights the need for next generation therapies that more effectively block ER function in tumors. Fulvestrant was first discovered as a “pure antiestrogen”, in contrast to earlier generation ER therapeutic 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 suppression of ER signaling. Importantly however, we recently demonstrated that fulvestrant and other full ER antagonists (e.g. GDC-0927) dramatically slow the intranuclear mobility of ER [Cell 178:4 (2019)]. We argue that such immobilization prevents ER function, and that increased ER turnover is a downstream consequence of immobilization, rather than a cause of ER inhibition. Here, we describe GDC-9545, our latest generation ER antagonist, currently being evaluated in clinic, that robustly immobilizes ER, and drives profound ER suppression in vivo. In the HCI-011 PDX model, we find that GDC-9545 can achieve greater ER pathway inhibition than can be achieved by tamoxifen. Intriguingly, although tamoxifen exhibits partial ER inhibition, likely through preventing recruitment of co-activators to the ER ligand binding domain, it drives increased accessibility at ~2500 chromatin sites, as determined by ATAC-seq. Chromatin regions exhibiting increased accessibility upon tamoxifen treatment are significantly enriched for the ERE motif, while a smaller number of sites exhibiting decreased accessibility upon treatment are enriched for AP-1 motifs. In contrast, GDC-9545 profoundly decreases chromatin accessibility at both ERE and AP-1 motifs, despite not fully eliminating ER protein. Notably, sites exhibiting decreased accessibility upon GDC-9545 treatment in the PDX in vivo, significantly overlap with sites displaying increased accessibility in estrogen-stimulated MCF7 cells in vitro. GDC-9545-treatment additionally alters accessibility at sites enriched for the FOXA1 motif, though unexpectedly, a sub-set of these sites exhibit increased accessibility while a distinct sub-set of sites exhibit decreased accessibility. We speculate that this particular pattern of accessibility changes may reflect redistribution of FOXA1 upon GDC-9545 treatment, and we will further explore this hypothesis. These data provide further insights into the impact of ER immobilization by the latest generation of pure antiestrogens. Citation Format: Ciara Metcalfe, Wei Zhou, Jane Guan, Robert A. Blake, Tom De Bruyn, Jennifer M. Giltnane, Ellen Ingalla, Tracy Kleinheinz, Jun Liang, Vidhi Mody, Jason Oeh, Savita Ubhayakar, Ingrid Wertz, Amy Young, Jason Zbieg, Xiaojing Wang, Marc Hafner. GDC-9545: A pure antiestrogen clinical candidate that immobilizes the estrogen receptor and profoundly alters chromatin accessibility in vivo [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3406.