Abstract 3079: FT-6876, a potent and selective inhibitor of CBP/p300 with antitumor activity in AR-positive breast cancer

Triple negative breast cancer (TNBC) remains a high unmet medical need, commonly treated with chemotherapies. Biomarker-driven treatment strategies have led to the recent approvals of both PARP inhibitors and an immune checkpoint inhibitor for a subset of women with TNBC. Androgen receptor (AR) is expressed in a 20-30% of patients with TNBC. Preclinical studies indicate that AR in this context can substitute for estrogen receptor (ER) to drive transcription of ER target genes, supporting TNBC tumor proliferation and survival. AR-driven transcription is dependent upon multiple essential cofactors, among which are the structurally related, bromodomain (BRD) and lysine acetyl transferase containing proteins, CBP and p300. Here we report the discovery of FT-6876, a potent and selective small molecule bromodomain inhibitor of CBP/p300. Our hypothesis is that modulation of CBP/p300 with FT-6876 will alter chromatin compaction and, when in the vicinity of AR responsive genes, selectively reduce AR transcriptional activity at these loci. This should translate to growth inhibition in vitro and in vivo in AR+ breast cancer models. Consistent with this hypothesis, a rapid reduction in H3K27Ac was observed in an AR+ breast cancer cell line following treatment with FT-6876. Removal of the compound led to the restoration of this mark to baseline levels reflecting the dynamic state of histone modification in cells. High content analysis revealed that the reduction in H3K27Ac occurred in the majority of the cells. In order to have a greater understanding of the dynamic and selective impact of FT-6876 on AR/ER transcriptional activity, we used precision run-on (PRO) seq to quantify nascent RNA and infer transcription factor modulation genome-wide. GSEA analysis of nascent RNA showed a high enrichment for AR and ER pathway genes upon exposure with FT-6876. Using ChIPseq, we confirmed that the reduction of H3K27Ac at specific promoter sites was concurrent with a decrease in CBP/p300 on the chromatin and reduction in nascent RNA and eRNA. FT-6876 reduced proliferation in a time-dependent manner with optimal growth inhibition observed after 10 days drug exposure. This was maintained even after removal of the drug for several days. Across a panel of breast cancer cell lines, there was a good correlation between growth inhibition and AR expression. In vivo, FT-6876 induced tumor stasis at well tolerated doses in AR+ breast cancer models. This was associated with a reduction in H3K27Ac, AR and ER target gene modulation, and reduction in Ki67. FT-6876 is a promising new CBP/p300 bromodomain inhibitor demonstrating efficacy in preclinical models of AR+ breast cancer. Citation Format: Maureen Caligiuri, Grace L. Williams, Jennifer Castro, Linda Battalagine, Kristina Muskiewicz, Erik Wilker, Lili Yao, Shawn Schiller, Angela Toms, Ping Li, Eneida Pardo, Bradford Graves, Kenneth W. Wood, Sylvie M. Guichard. FT-6876, a potent and selective inhibitor of CBP/p300 with antitumor activity in AR-positive breast cancer [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 3079.