BET Bromodomain Inhibitors: Targeting Transcriptional Addiction

Reading the Acetyl Mark

Histone acetylation, added by acetyltransferases and removed by deacetylases, loosens chromatin and creates docking sites for bromodomain proteins. BRD4 binds these sites and recruits the positive transcription elongation factor P-TEFb, releasing paused RNA polymerase II so that transcription proceeds.

Genes controlled by large clusters of enhancers, so-called super-enhancers, are especially dependent on BRD4, which is why BET inhibition preferentially reduces expression of a subset of highly enhancer-driven genes.

Why They Were Developed

The tool compound JQ1 showed that displacing BRD4 strongly suppresses MYC transcription in models of NUT midline carcinoma, multiple myeloma and acute leukaemia. BET inhibitors also disperse the extrachromosomal-DNA hubs that some tumours use to co-amplify and co-express oncogenes.

This gave a rationale for targeting MYC indirectly, since MYC itself has been very hard to drug.

Clinical Reality

Multiple BET inhibitors have entered trials in haematological and solid cancers. Single-agent activity has generally been modest and dose-limiting toxicities, including thrombocytopenia and gastrointestinal effects, have constrained development.

Current efforts focus on combinations, more selective compounds and degrader molecules. No BET inhibitor is a standard-of-care cancer therapy at present.

Where BET Inhibition Still Looks Promising

The clearest signals have been in NUT carcinoma, a rare cancer defined by a BRD4-NUT fusion where the drug directly opposes the driver, and in myelofibrosis, where the BET inhibitor pelabresib has been combined with ruxolitinib in late-phase trials. These narrow contexts fit the general lesson that BET inhibitors work best where a tumour is unusually enhancer-addicted.

Second-generation approaches aim to widen the therapeutic window: inhibitors selective for a single bromodomain of BRD4, and BET protein degraders that remove the protein entirely and show deeper target suppression in models. Whether these overcome the thrombocytopenia that limited the first generation is still being tested.

Key Takeaways

• ·BET proteins read histone acetylation and drive transcription elongation, especially at super-enhancers.
• ·BET inhibitors suppress enhancer-driven oncogenes such as MYC and disrupt ecDNA transcription hubs.
• ·Clinical activity so far has been limited by modest single-agent effect and toxicity.