Abstract 1575: Novel small molecule inhibitors of p300/CBP down-regulate AR and c-Myc for the treatment of castrate resistant prostate cancer
Authors: Brooks Nigel , Pegg Neil , Worthington Jenny , Young Barbara , Prosser Amy , Lane Jordan , Taddei David , Schiewer Matthew , deLeeuw Renee , McCann Jennifer , Knusden Karen 
Published: July 2017

Abstract
Background: E1A binding protein (p300) and CREB binding protein (CBP) are two closely related, paralogue histone acetyl transferase proteins that act as transcriptional co-activators of a variety of cancer related genes. We have developed potent, selective and orally active small molecule inhibitors of the bromodomain of p300/CBP and investigated their role in regulating androgen receptor expression and function. We have also examined their role in driving synthetic lethality in tumours. Loss of function mutations in either p300 or CBP (including in significant proportions of lung and bladder tumours), can lead to a dependency on the corresponding paralogue protein.
Methods: Binding affinity to p300, CBP and BRD4 was measured in a surface plasmon resonance (SPR) assay. Potency and functional activity was demonstrated in a panel of prostate cells lines representing hormone responsive (LNCaP), hormone independent (DU145, PC3) and castrate resistant disease (22Rv1, C4-2, VCaP, LNCaP-AR) as well as wildtype (A549) and CBP deficient (H520, H1703, LK2) lung cancer cells. Combination effects of p300/CBP inhibitors with a PARP or CDK4/6 inhibitor were determined in LNCaP and C4-2 cells. Effects of p300/CBP inhibitors (and by comparison, the BET inhibitor, JQ1), on AR, AR-V7 splice variant and c-Myc protein, as well as c-Myc, KLK3 and TMPRSS2 gene expression, were assessed in 22Rv1 cells in vitro. In vivo effects on biomarkers were measured in a 22Rv1 xenograft model.
Results: CCS1357, an in vitro probe compound, binds to p300 and CBP with high affinity (Kd=4nM) and selectivity (Kd=245nM; BRD4). It is a potent inhibitor of cell proliferation in castrate resistant cell lines (IC50=100nM in LnCaP-AR; 350nM in 22Rv1) with minimal effects in hormone independent lines. CCS1357 combined with palbociclib (CDK4/6) or olaparib (PARP) in LNCaP or C4-2 cells, showed reduced cell viability compared with any of these drugs given alone. In 22RV1 cells, CCS1357 significantly down-regulated AR-FL, AR-V7 and c-Myc protein by Western, an effect not seen with JQ1 at equivalent proliferation IC50s. CCS1357 effects were reversed by the proteasome inhibitor, MG132. CCS1357 also caused a profound inhibition of c-Myc, KLK3 and TMPRSS2 genes measured by qPCR in 22Rv1 cells in vitro. A preclinical candidate (CCS1477) given as a single oral dose (30mg/kg) inhibited plasma PSA and tumour AR, AR-V7 and c-Myc in a 22Rv1 xenograft model. In the lung cancer cell lines, we observed differential sensitivity to CCS1357; CBP deficient lines were more sensitive (cell proliferation) compared with normal.
Conclusions: Taken together these data support the clinical testing of p300/CBP inhibition in patients in two settings; firstly, castrate resistant prostate cancer by down-regulating of AR, AR-SV and c-MYC expression and function; and secondly in patients with loss of function mutations in p300 or CBP by driving synthetic lethality.
Citation Format: Nigel Brooks, Neil Pegg, Jenny Worthington, Barbara Young, Amy Prosser, Jordan Lane, David Taddei, Matthew Schiewer, Renee deLeeuw, Jennifer McCann, Karen Knusden. Novel small molecule inhibitors of p300/CBP down-regulate AR and c-Myc for the treatment of castrate resistant prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1575. doi:10.1158/1538-7445.AM2017-1575