Direct photocapture of bromodomains using tropolone chemical probes

Medicinal chemistry techniques, including structure-based molecular design, fragment replacement and synthetic library enablement, were used to create potent inhibitors of bromodomain and extraterminal domain (BET) and CREB binding protein bromodomains. One inhibitor featured the homoaromatic tropolone methyl ether motif as a mimic of acetyl lysine, as confirmed by X-ray crystallography. The intrinsic photoreactivity of the tropone fragment was harnessed successfully to directly photolabel recombinant bromodomains which inspired further development of a clickable probe to assess BRD4 target engagement by (+)-JQ1 as a representative BET inhibitor in a complex proteome. The antimitotic natural product colchicine and related natural products that contain the tropolone ring system were also found to be bromodomain inhibitors, and surprisingly, our probe was shown to photolabel tubulin. These results highlight the caution that should be exercised when considering the selectivity of pharmacological agents, and photoreactive chemical probes should be assessed for their intrinsic ability to directly label their biological targets with a view to creating useful chemoproteomic tools.