Discovery of the First BRD4 Second Bromodomain (BD2)-Selective Inhibitors

Pan-BD2 inhibitors have been shown to retain an antileukemia
effect
and display less dose-limiting toxicities than pan-BET inhibitors.
However, it is necessary to consider the potential off-target toxicity
associated with the inhibition of four BET BD2 proteins. To date,
no BRD4 BD2 domain selective inhibitor has been reported. Based on
our previous pan-BD2 inhibitor 12 (XY153), we successfully
identified 16o (XY221) as the first BRD4 BD2-selective
inhibitor. 16o demonstrated potent binding affinity for
BRD4 BD2 (IC50 = 5.8 nM), along with high pan-BD2 selectivity
(667-fold over BRD4 BD1) and BRD4 BD2 domain selectivity (9–32-fold
over BRD2/3/T BD2). The BRD4 BD2 selectivity of 16o was
further confirmed by the BLI assay, showing 66–144-fold selectivity
over other BET BD2 domains. 16o exhibited good liver
microsomal stability (T1/2 > 120 min)
and pharmacokinetic properties (F = 13.1%). These
data indicate that 16o may serve as a valuable candidate
for BRD4 BD2 advancing epigenetic research.