Abstract 1019: P300/CBP inhibitor CCS1477 targets 22Rv1 prostate tumor AR and c-myc gene expression in vivo

Background: Histone acetyl transferases E1A binding protein (p300) and CREB binding protein (CBP) are known co-activators of several key transcription factors that contribute to tumor progression including HIF1a, BRCA-1, p53, c-myc and androgen receptor (AR). A large proportion of AR regulated gene expression has been shown to be dependent on p300 either through direct regulation of AR interaction with promoters of AR regulated genes or subsequent histone modification events. Both p300 and CBP are highly expressed in advanced prostate cancer and androgen deprivation leads to upregulation of both proteins. CCS1477 is a potent, selective inhibitor of the bromodomain in CBP/p300 that has been shown to inhibit prostate tumor cell proliferation in vitro and tumor growth in vivo. Methods: 22Rv1 prostate tumor cells that express both AR and AR variants were transplanted in nude mice. Established tumors were treated with CCS1477, 20mg per kg, once daily p.o. for 28 days. CCS1477 treatment resulted in virtually complete inhibition of tumor growth that was maintained up to 24 days after cessation of treatment by which time tumor recurrence was evident. Tumors were excised from vehicle and CCS1477 treated animals at day 7, day 28, and day 52; mRNA was isolated and gene expression analysis was carried using Affymetrix Clarion D microarrays. Differential gene expression based on fold change (FC) >1.5 and FDR-adjusted p-value Results: Although ~1.5 fold downregulation of AR was maintained from day 7 to day 52, downregulation of AR target genes ETS2, TMPRSS2 and NKX3.1 recovered after treatment cessation. Similarly, expression of c-myc was significantly reduced at day 7 (-2.7 FC) and recovered by day 28. Of note, among the top downregulated genes were CIART and BHLHE40 circadian clock regulated genes that provide negative feedback loops that in conjunction with downregulation of AR and c-myc would disrupt circadian gene regulation. VEGFA mRNA that was downregulated >1.5 fold at all time points, together with c-myc and p300 are all under circadian regulation. Expression of the histone demethylase KDM3A was reduced >1.5 fold at all time points. KDM3A is known to function as an AR coactivator of key AR target genes including NKX3.1 and c-myc. Conclusions: Gene expression analysis of CCS1477 treated prostate tumors suggests an underlying mechanism involving the inhibition of key drivers of prostate cancer progression including AR and c-myc and a network of interacting pathways. Citation Format: Paul Elvin, Neil Pegg, Simone Daminelli, Izabela Eden, Barbara Young, Amy Prosser, Jenny Worthington, Nigel Brooks. P300/CBP inhibitor CCS1477 targets 22Rv1 prostate tumor AR and c-myc gene expression in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1019.