Abstract 7116: Targeting the CBP/p300 axis in lethal prostate cancer impacts DNA repair

Abstract Prostate cancer (PCa) is the 2nd leading cause of cancer related deaths in US men. PCa is an androgen dependent disease driven by the androgen receptor (AR). Currently, androgen-deprivation therapy (ADT) is the standard of care first-line therapy for metastatic PCa. Resistance to ADT uniformly leads to lethal disease, termed castration-resistant prostate cancer (CRPC). Thus, there is an unmet clinical need to identify and develop novel strategies to treat CRPC. CBP/p300 are potent co-activators for AR. High expression of CBP/p300 is associated with locally advanced disease and castration resistant function of AR, resulting in poor patient outcomes and further highlighting the need to discern the role of CBP/p300 to potentially develop therapeutic targets for precision medicine. Previous studies have relied on non-specific compounds and genetic silencing to target CBP/p300. In this study, CBP/p300 mediated bromodomain activity is targeted by CCS1477 (inobrodib), a first-in-class bromodomain inhibitor developed by Cell Centric. CCS1477 treatment demonstrated effective inhibition in growth and clonogenicity assays. Inhibition of the CBP/p300 bromodomain with CCS1477 resulted in significant downregulation of AR-FL, AR-V7, and its targets’ mRNA expression in addition to inhibition of associated factors such as c-MYC and its downstream targets in PCa cell lines as well as patient derived xenograft (PDX) models. This study shows that CBP and p300 are highly expressed and correlate closely with AR gene expression and AR activity score in primary PCa and CRPC patient samples. The clinical significance of CBP/p300 expression in PCa has been investigated via elucidation of CBP/p300 transcriptional reprogramming and its role in DNA damage response (DDR) pathways. Specifically, findings revealed that CBP/p300 bromodomain suppression sensitizes to AR-dependent DNA-repair. Transcriptional mapping identified CBP/p300 as regulators of cell proliferation and DNA repair processes, which were functionally confirmed across several PCa model systems. To assess relevance, exogenous challenge with radiation revealed that CBP/p300 are required for AR-mediated DNA repair, and CBP/p300 expression is linked to DNA repair capacity in the clinical setting. Molecular analyses revealed that CBP/p300 facilitate double-strand break (DSB) repair efficiency via homologous recombination (HR) mediated DDR. Congruently, CBP/p300 strongly correlated with HR gene expression in PCa patient tissue. These collective findings reveal that CBP/p300 govern repair of DNA DSBs by regulating HR, thus modulating genome integrity and promoting CRPC growth. These studies identify CBP/p300 as a driver of PCa tumorigenesis through coordinated control of critical transcriptional events and lay the groundwork to optimize therapeutic strategies for advanced PCa via CBP/p300 inhibition, potentially in combination with AR-directed therapies. Citation Format: Sumaira Sardar, Lakshmi Ravindranath, Christopher McNair, Saswati N. Chand, Wei Yuan, Denisa Bogdan, Jonathan Welti, Adam Sharp, Matthew Schiewer, Lisa Butler, Johann de Bono, Kris Frese, Nigel Brooks, Neil Pegg, Karen Knudsen, Ayesha A. Shafi. Targeting the CBP/p300 axis in lethal prostate cancer impacts DNA repair abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 7116.