Bromodomains are epigenetic reader modules that regulate gene transcription through their recognition of acetyl-lysine modified histone tails. Inhibitors of this protein-protein interaction have the potential to modulate multiple diseases as demonstrated by the profound anti-inflammatory and antiproliferative effects of a recently disclosed class of BET compounds. While these compounds were discovered using phenotypic assays, here we present a highly efficient alternative approach to find new chemical templates, exploiting the abundant structural knowledge that exists for this target class. A phenyl dimethyl isoxazole chemotype resulting from a focused fragment screen has been rapidly optimized through structure-based design, leading to a sulfonamide series showing anti-inflammatory activity in cellular assays. This proof-of-principle experiment demonstrates the tractability of the BET family and bromodomain target class to fragment-based hit discovery and structure-based lead optimization.

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The profound efficacy, yet associated toxicity of pan-BET inhibitors is well documented. The possibility of an ameliorated safety profile driven by significantly selective (>100-fold) inhibition of a subset of the eight bromodomains is enticing, but challenging given the close homology. Herein, we describe the X-ray crystal structure-directed optimization of a novel weak fragment ligand with a pan-second bromodomain (BD2) bias, to potent and highly BD2 selective inhibitors. A template hopping approach, enabled by our parallel research into an orthogonal template (15, GSK046), was the basis for the high selectivity observed. This culminated in two tool molecules, 20 (GSK620) and 56 (GSK549), which showed an anti-inflammatory phenotype in human whole blood, confirming their cellular target engagement. Excellent broad selectivity, developability, and in vivo oral pharmacokinetics characterize these tools, which we hope will be of broad utility to the field of epigenetics research.

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Pan-bromodomain and extra terminal domain (BET) inhibitors interact equipotently with the eight bromodomains of the BET family of proteins and have shown profound efficacy in a number of in vitro phenotypic assays and in vivo pre-clinical models in inflammation or oncology. A number of these inhibitors have progressed to the clinic where pharmacology-driven adverse events have been reported. To better understand the contribution of each domain to their efficacy and improve their safety profile, selective inhibitors are required. This article discloses the profile of GSK046, also known as iBET-BD2, a highly selective inhibitor of the second bromodomains of the BET proteins that has undergone extensive pre-clinical in vitro and in vivo characterization.