Purpose: Persistent androgen receptor (AR) signaling drives castration resistant prostate cancer (CRPC) and confers resistance to AR targeting therapies. Novel therapeutic strategies to overcome this are urgently required. We evaluated how bromodomain and extra-terminal (BET) protein inhibitors (BETi) abrogate aberrant AR signaling in CRPC. Experimental Design: We determined associations between BET expression, AR driven transcription and patient outcome; and the effect and mechanism by which chemical BETi (JQ1 and GSK1210151A; I-BET151) and BET family protein knockdown regulates AR-V7 expression and AR signaling in prostate cancer (PC) models. Results: Nuclear BRD4 protein expression increases significantly (p=<0.01) with castration resistance in same patient treatment naïve (median H-score; interquartile range: 100; 100-170) and CRPC (150; 110-200) biopsies, with higher expression at diagnosis associating with worse outcome (HR 3.25, 95% CI 1.50-7.01; p=<0.001). BRD2, BRD3 and BRD4 RNA expression in CRPC biopsies correlates with AR driven transcription (all p=<0.001). Chemical BETi, and combined BET family protein knockdown, reduce AR-V7 expression and AR signaling. This was not recapitulated by C-MYC knockdown. In addition, we show that BETi regulates RNA processing thereby reducing alternative splicing and AR-V7 expression. Furthermore, BETi reduce growth of PC cells and patient derived organoids with known AR mutations, AR amplification and AR-V7 expression. Finally, BETi, unlike enzalutamide, decreases persistent AR signaling and growth (p=<0.001) of a patient derived xenograft model of CRPC with AR amplification and AR-V7 expression. Conclusion: BETi merit clinical evaluation as inhibitors of AR splicing and function, with trials demonstrating their blockade in proof of mechanism pharmacodynamic studies.
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