Abstract A091: Therapeutic targeting of the p300/CBP bromodomain for the treatment of castration-resistant prostate cancer
Authors: Brooks Nigel , Pegg Neil , Worthington Jenny , Young Barbara , Prosser Amy 
Published: February 2019

Abstract
Background: Sustained androgen receptor pathway activation is the hallmark of castration-resistant prostate cancer (CRPC). Therapeutic strategies for CRPC include non-ligand-binding domain targeted degradation of androgen receptor (AR) and AR variants (ARV). E1A binding protein (p300) and CREB binding protein (CBP) are two closely related histone acetyl transferase proteins that are critical transcriptional regulators of the androgen receptor. We have developed CCS1477, which is a potent, selective, and orally active small-molecule inhibitor of the bromodomain of p300/CBP, and investigated its role in regulating androgen receptor expression and function.
Methods: Binding affinity of CCS1477 to p300, CBP and BRD4 was measured in a surface plasmon resonance (SPR) assay. Potency and functional activity (proliferation, cell cycle analysis, and biomarker knockdown) was demonstrated in a panel of prostate cells lines representing hormone-responsive (LNCaP), hormone-independent (DU145, PC3), and castration-resistant disease (22Rv1, VCaP, LNCaP-AR). A JQ1 resistant cell line was generated by culturing cells in increasing doses of JQ1 over a period of approximately 9 months and used to differentiate potency and functional activity of BET vs p300/CBP inhibition in vitro. In vivo efficacy, linked to inhibition of biomarkers, was determined in 22Rv1 and LNCaP xenograft models.
Results: CCS1477 binds to p300 and CBP with high affinity (KD=1.3/1.7nM) and selectivity (KD=222nM; BRD4). It is a potent inhibitor of cell proliferation in castration-resistant cell lines (IC50= 96nM,22Rv1; 49nM,VCaP; 150nM,LNCaP-AR) with minimal effect in AR-ve lines (PC3, DU145). These inhibitory effects on cell proliferation were accompanied by a decrease in the number of cells in S phase and an increase in the number of cells in the G1/G0 phase of the cell cycle. We have developed a JQ1 resistant 22Rv1 cell line and observed a marked reduction in sensitivity (proliferation end point) to JQ1 as expected, which is accompanied by cross-resistance to a variety of other BET inhibitors, including iBET-151 and OTX-015. In contrast, and as evidence for a differentiated mechanism, sensitivity to CCS1477 is retained in this JQ1 resistant line. Inhibition of the bromodomain of p300/CBP downregulates AR-FL, AR-V7 and c-Myc protein in 22Rv1 cells by Western blot and this is accompanied by profound inhibition of c-Myc, KLK3 and TMPRSS2 genes measured by qPCR. The in vivo PK properties of CCS1477 are consistent with qd or qod oral dosing in mouse. CCS1477 dosed at 10mg/kg qd, 20mg/kg qd, or 30mg/kg qod caused complete tumor growth inhibition over 28 days in a 22Rv1 xenograft model, including extended duration in the absence of the drug for a further 24 days. This was accompanied by complete inhibition of plasma PSA and significant knockdown of tumor AR-FL, AR-V7, and c-Myc protein as well as c-Myc and TMPRSS2 mRNA expression. At 100mg/kg, a single dose of CCS1477 induces expression of cleaved PARP and when dosed orally every three days, causes tumor regression in the 22Rv1 model. Furthermore, CCS1477 results in almost complete tumor growth inhibition in a bicalutamide-resistant LNCaP xenograft model, when given alone or in combination with enzalutamide.
Conclusions: Taken together, these data support the concept that therapeutic targeting of the p300/CBP bromodomain results in a novel and differentiated approach to targeting androgen receptor pathway activation in castration-resistant prostate cancer.
Citation Format: Nigel Brooks, Neil Pegg, Jenny Worthington, Barbara Young, Amy Prosser. Therapeutic targeting of the p300/CBP bromodomain for the treatment of castration-resistant prostate cancer [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr A091.