Discovery of N‑Ethyl-4-[2-(4-fluoro-2,6-dimethyl-phenoxy)-5-(1-hydroxy-1-methyl-ethyl)phenyl]-6-methyl-7-oxo‑1H‑pyrrolo[2,3‑c]pyridine-2-carboxamide (ABBV-744), a BET Bromodomain Inhibitor with Selectivity for the Second Bromodomain

The BET family of proteins consists of BRD2, BRD3, BRD4, and BRDt. Each protein contains two distinct bromodomains (BD1 and BD2). BET family bromodomain inhibitors under clinical development for oncology bind to each of the eight bromodomains with similar affinities. We hypothesized that it may be possible to achieve an improved therapeutic index by selectively targeting subsets of the BET bromodomains. Both BD1 and BD2 are highly conserved across family members (>70% identity), whereas BD1 and BD2 from the same protein exhibit a larger degree of divergence (∼40% identity), suggesting selectivity between BD1 and BD2 of all family members would be more straightforward to achieve. Exploiting the Asp144/His437 and Ile146/Val439 sequence differences (BRD4 BD1/BD2 numbering) allowed the identification of compound 27 demonstrating greater than 100-fold selectivity for BRD4 BD2 over BRD4 BD1. Further optimization to improve BD2 selectivity and oral bioavailability resulted in the clinical development compound 46 (ABBV-744).BET蛋白家族（BET family）包含BRD2、BRD3、BRD4与BRDt四种成员。每种蛋白均含有两个独立的溴结构域（bromodomain），分别为BD1与BD2。目前处于肿瘤学临床研发阶段的BET家族溴结构域抑制剂，可与全部八种溴结构域以相似的亲和力结合。我们提出假说：通过选择性靶向BET溴结构域的特定亚群，或可获得更优的治疗指数。BD1与BD2在家族各成员间均具有高度保守性（序列同源性＞70%），而同一蛋白的BD1与BD2之间则呈现出较大的序列差异（序列同源性约40%），这提示实现所有家族成员的BD1与BD2之间的选择性将更为容易。利用BRD4 BD1/BD2编号体系下的Asp144/His437与Ile146/Val439序列差异，研究人员成功鉴定出化合物27，其对BRD4 BD2的选择性相较于BRD4 BD1提升了百倍以上。后续为优化BD2选择性与口服生物利用度所开展的研究，最终得到了临床研发化合物46（ABBV-744）。