Structure-Guided Design and Synthesis of Pyridinone-Based Selective Bromodomain and Extra-Terminal Domain (BET)-First Bromodomain (BD1) Inhibitors.

ABSTRACT

The bromodomain and extra-terminal domain (BET) proteins are epigenetic readers, regulating transcription via two highly homologous tandem bromodomains, BD1 and BD2. Clinical development of nonselective pan-BD BET inhibitors has been challenging, partly due to dose-limiting side effects such as thrombocytopenia. This has prompted the push for domain-selective BET inhibitors to achieve a more favorable therapeutic window. We report a structure-guided drug design campaign that led to the development of a potent BD1-selective BET inhibitor, 33 (XL-126), with a Kd of 8.9 nM and 185-fold BD1/BD2 selectivity. The high selectivity was first assayed by SPR, validated by a secondary time-resolved fluorescence energy transfer assay, and further corroborated by BROMOscan (∼57-373 fold selectivity). The cocrystal of 33 with BRD4 BD1 and BD2 demonstrates the source of selectivity repulsion with His437 and lost binding with the leucine clamp. Notably, the BD1 selectivity of BET inhibitor 33 leads to both the preservation of platelets and potent anti-inflammatory efficacy.