BET proteins, particularly BRD4, are key drivers of oncogenic transcription in solid tumors. First-generation BET inhibitors such as JQ1, molibresib, and birabresib demonstrated effectiveness in preclinical studies but did not succeed clinically due to limited efficacy, dose-limiting thrombocytopenia, and the development of resistance through mechanisms like isoform switching and compensatory signaling pathways. Clinical responses were most notable in NUT carcinoma and castration-resistant prostate cancer.
Next-generation strategies include BD2-selective inhibitors, PROTAC degraders that target BET proteins for degradation, and combination therapies with PARP inhibitors, androgen receptor antagonists, or immune checkpoint blockade. These approaches aim to address the shortcomings of earlier agents by improving selectivity and reducing toxicity while enhancing anti-tumor activity.
BET proteins (BRD2, BRD3, BRD4, BRDT) interact with acetylated chromatin through two bromodomains (BD1 and BD2). In solid tumors where transcriptional networks are complex compared to hematological cancers, inhibiting BET alone has proven less effective. Preclinical compounds displaced BRD4 from chromatin and reduced MYC expression but faced challenges when translated into clinical trials due to short half-lives of drugs and acquired resistance.
Innovations such as BD2-selective inhibitors like ABBV-744 minimize thrombocytopenia while maintaining anti-tumor effects. PROTACs including ARV-771 and MZ1 offer deeper suppression by degrading BET proteins entirely. Bivalent inhibitors such as AZD5153 engage both bromodomains for higher affinity binding. Additional methods under investigation involve dual inhibition strategies or disrupting phase separation at super-enhancers driven by BRD4.
Combination therapies have shown promise: pairing BET inhibitors with PARP inhibitors exploits DNA repair defects; combining them with androgen receptor antagonists benefits castration-resistant prostate cancer; immune checkpoint blockade combinations demonstrate preclinical potential, though toxicity remains a concern. Trials involving molibresib or birabresib confirmed activity in NUT carcinoma but required intermittent dosing to manage side effects. Some combinations like ZEN-3694 plus enzalutamide or talazoparib have produced early signs of efficacy despite several trials being terminated due to low single-agent activity or pharmacokinetic issues.
Ongoing priorities include developing more selective agents that degrade targets efficiently; identifying biomarkers such as MYC amplification or BRD4 dependency for patient selection; designing mechanism-based drug combinations; optimizing dosing schedules to reduce hematological toxicity; and focusing on rare responsive tumors like NUT carcinoma.
