Acetylation of histone H3 at lysine 27 is a well-defined marker of enhancer activity. However, the functional impact of this modification at enhancers is poorly understood. Here, we use a chemical genetics approach to acutely block the function of the cAMP response element binding protein (CREB) binding protein (CBP)/P300 bromodomain in models of hematological malignancies and describe a consequent loss of H3K27Ac specifically from enhancers, despite the continued presence of CBP/P300 at chromatin. Using this approach to dissect the role of H3K27Ac at enhancers, we identify a critical role for this modification in the production of enhancer RNAs and transcription of enhancer-regulated gene networks.

Bromodomains, small protein modules that recognize acetylated lysine on histones, play a significant role in the epigenome, where they function as "readers" that ultimately determine the functional outcome of the post-translational modification. Because the initial discovery of selective BET inhibitors have helped define the role of that protein family in oncology and inflammation, BET bromodomains have continued to garner the most attention of any other bromodomain. More recently, non-BET bromodomain inhibitors that are potent and selective have been disclosed for ATAD2, CBP, BRD7/9, BRPF, BRPF/TRIM24, CECR2, SMARCA4, and BAZ2A/B. Such novel inhibitors can be used to probe the physiological function of these non-BET bromodomains and further understanding of their role in certain disease states. Here, we provide an update to the progress in identifying selective bromodomain inhibitors and their use as biological tools, as well as our perspective on the field.

The biological role played by non-BET bromodomains remains poorly understood, and it is therefore imperative to identify potent and highly selective inhibitors to effectively explore the biology of individual bromodomain proteins. A ligand-efficient nonselective bromodomain inhibitor was identified from a 6-methyl pyrrolopyridone fragment. Small hydrophobic substituents replacing the N-methyl group were designed directing toward the conserved bromodomain water pocket, and two distinct binding conformations were then observed. The substituents either directly displaced and rearranged the conserved solvent network, as in BRD4(1) and TAF1(2), or induced a narrow hydrophobic channel adjacent to the lipophilic shelf, as in BRD9 and CECR2. The preference of distinct substituents for individual bromodomains provided selectivity handles useful for future lead optimization efforts for selective BRD9, CECR2, and TAF1(2) inhibitors.

Inhibition of the bromodomain of the transcriptional regulator CBP/P300 is an especially interesting new therapeutic approach in oncology. We recently disclosed in vivo chemical tool 1 (GNE-272) for the bromodomain of CBP that was moderately potent and selective over BRD4(1). In pursuit of a more potent and selective CBP inhibitor, we used structure-based design. Constraining the aniline of 1 into a tetrahydroquinoline motif maintained potency and increased selectivity 2-fold. Structure-activity relationship studies coupled with further structure-based design targeting the LPF shelf, BC loop, and KAc regions allowed us to significantly increase potency and selectivity, resulting in the identification of non-CNS penetrant 19 (GNE-781, TR-FRET IC = 0.94 nM, BRET IC = 6.2 nM; BRD4(1) IC = 5100 nΜ) that maintained good in vivo PK properties in multiple species. Compound 19 displays antitumor activity in an AML tumor model and was also shown to decrease Foxp3 transcript levels in a dose dependent manner.

Resistance invariably develops to antiandrogen therapies used to treat newly diagnosed prostate cancers, but effective treatments for castration-resistant disease remain elusive. Here, we report that the transcriptional coactivator CBP/p300 is required to maintain the growth of castration-resistant prostate cancer. To exploit this vulnerability, we developed a novel small-molecule inhibitor of the CBP/p300 bromodomain that blocks prostate cancer growth in vitro and in vivo. Molecular dissection of the consequences of drug treatment revealed a critical role for CBP/p300 in histone acetylation required for the transcriptional activity of the androgen receptor and its target gene expression. Our findings offer a preclinical proof of concept for small-molecule therapies to target the CBP/p300 bromodomain as a strategy to treat castrationresistant prostate cancer. Cancer Res; 77(20); 5564-75. Ó2017 AACR.