The bromodomain and extra terminal (BET) family of bromodomain‐containing proteins (BCPs) have been the subject of extensive research over the past decade, resulting in a plethora of high‐quality chemical probes for their tandem bromodomains. In turn, these chemical probes have helped reveal the profound biological role of the BET bromodomains and their role in disease, ultimately leading to a number of molecules in active clinical development. However, the BET subfamily represents just 8/61 of the known human bromodomains, and attention has now expanded to the biological role of the remaining 53 non‐BET bromodomains. Rapid growth of this research area has been accompanied by a greater understanding of the requirements for an effective bromodomain chemical probe and has led to a number of new non‐BET bromodomain chemical probes being developed. Advances since December 2015 are discussed, highlighting the strengths/caveats of each molecule, and the value they add toward validating the non‐BET bromodomains as tractable therapeutic targets.

Non-BET bromodomain containing proteins have become attractive targets for the development of novel therapeutics targeting epigenetic pathways. To help facilitate the target validation of this class of proteins, structurally diverse small molecule ligands, and methodologies to produce selective inhibitors in a predictable fashion are in high demand.Herein we report the development and application of atypical acetyl-lysine (KAc) methyl mimetics to take advantage of the differential stability of conserved water molecules in the bromodomain binding side. Discovery of the n-butyl group as an atypical KAc methyl mimetic allowed generation of 31 (GSK6776) as a soluble, permeable and selective BRD7/9 inhibitor from a pyridazinone template. The n-butyl group was then used to enhance the bromodomain selectivity of an existing BRD9 inhibitor and to transform pan-bromodomain inhibitors into BRD7/9 selective compounds. Finally, a solvent exposed vector was defined from the pyridazinone template to enable bifunctional molecule synthesis and affinity enrichment chemoproteomic experiments were used to confirm several of the endogenous protein partners of BRD7 and BRD9 which form part of the chromatin remodelling PBAF and BAF complexes, respectively.

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Bromodomain containing proteins and the acetyl-lysine binding bromodomains contained therein are increasingly attractive targets for the development of novel epigenetic therapeutics. To help validate this target class and unravel the complex associated biology, there has been a concerted effort to develop selective small molecule bromodomain inhibitors. Herein we describe the structure-based efforts and multiple challenges encountered in optimizing a naphthyridone template into selective TAF1(2) bromodomain inhibitors which, while unsuitable as chemical probes themselves, show promise for the future development of small molecules to interrogate TAF1(2) biology. Key to this work was the introduction and modulation of the basicity of a pendant amine which had a substantial impact on not only bromodomain selectivity but also cellular target engagement.

The Cover Feature shows a CREBBP bromodomain surface as a representative non‐BET bromodomain together with high‐quality bromodomain probe molecules GNE‐781, GSK4027, and GSK8814, which exemplify the advancements made in the development of non‐BET bromodomain chemical probes since December 2015. The quality of non‐BET bromodomain chemical probes, the accompanying supportive data, and the variety of chemotypes included have advanced dramatically in recent years, enabling robust target validation and is reflected in the important milestone of the first non‐BET bromodomain inhibitor reaching human clinical trials. More information can be found in the Review by Philip G. Humphreys et al. on page 362 in Issue 4, 2019 (DOI: 10.1002/cmdc.201800738).