Bivalent BET Bromodomain Inhibitors Confer Increased Potency and Selectivity for BRDT via Protein Conformational Plasticity

Bromodomain and extraterminal domain (BET) proteins are
important
regulators of gene transcription and chromatin remodeling. BET family
members BRD4 and BRDT are validated targets for cancer and male contraceptive
drug development, respectively. Due to the high structural similarity
of the acetyl-lysine binding sites, most reported inhibitors lack
intra-BET selectivity. We surmised that protein–protein interactions
induced by bivalent inhibitors may differ between BRD4 and BRDT, conferring
an altered selectivity profile. Starting from nonselective monovalent
inhibitors, we developed cell-active bivalent BET inhibitors with
increased activity and selectivity for BRDT. X-ray crystallographic
and solution studies revealed unique structural states of BRDT and
BRD4 upon interaction with bivalent inhibitors. Varying spacer lengths
and symmetric vs unsymmetric connections resulted in the same dimeric
states, whereas different chemotypes induced different dimers. The
findings indicate that the increased intra-BET selectivity of bivalent
inhibitors is due to the differential plasticity of BET bromodomains
upon inhibitor-induced dimerization.