Contribution of BET proteins to androgen (DHT)-stimulated gene expression program

Competitive inhibitors of acetyl-lysine binding to the bromodomains of the BET (bromodomain and extra terminal) family are being developed for the treatment of solid and heme malignancies. BET family member BRD4 function at enhancers/super-enhancers has been shown to sustain signal-dependent or pathogenic gene expression programs. Here we tested the hypothesis that the transcription factor drivers of castration-resistant prostate cancer (CRPC) clinical progression, including the Androgen Receptor (AR), are critically dependent on BRD4 and thus represent a sensitive solid tumor indication for the BET inhibitor ABBV-075. DHT-stimulated transcription of AR target genes was inhibited by ABBV-075 without significant effect on AR protein expression. Further, ABBV-075 disrupted DHT-stimulated recruitment of BET family member BRD4 to gene regulatory regions co-occupied by AR, including the well-established PSA and TMPRSS2 enhancers. Persistent BET inhibition disrupted the composition and function of AR occupied enhancers as measured by a reduction in AR and H3K27Ac ChIP signal and inhibition of eRNA transcription. ABBV-075 displayed potent anti-proliferative activity in multiple models of resistance to second generation anti-androgens and inhibited the activity of AR-V7 and the AR LBD gain-of-function mutations, F877L and L702H. ABBV-075 was also a potent inhibitor of MYC and the TMPRSS2-ETS fusion protein, important parallel transcription factor drivers of CRPC. A total of 8 samples were included in this representative study that includes replicates of the EtOH and DHT treated samples. Enzalutamide treatment is included as a reference compound for the DHT-activated AR pathway, and three doses of the BET inhibitor ABBV-075