CCT251921

CCT251921 (compound 109) is a potent, selective, and orally bioavailable small-molecule inhibitor of the Mediator complex-associated kinases CDK8 and CDK19. Developed through structure-based optimization of the earlier probe CCT251545, it exhibits significantly improved oral PK properties, enabling in-depth evaluation of CDK8/19 pharmacology and progression into preclinical efficacy and safety studies. Biochemically, CCT251921 inhibits CDK8 and CDK19 with IC₅₀ values of 4.9 and 2.6 nM, respectively, and displays long residence times on both kinases. Structural studies (PDB 5HBJ) confirm canonical hinge interactions and reveal an additional hydrogen bond with Asp98 enhances binding affinity and selectivity. In broad kinase and receptor profiling, CCT251921 shows remarkable selectivity, displaying minimal activity across 279 kinases and 55 receptors, ion channels, and enzymes at 1 μM concentration, with only weak inhibition of CYPs. Its measured log D (2.5) and improved solubility contribute to favorable pharmacokinetics across species, with a predicted human clearance of ~31% of liver blood flow and good oral bioavailability (F = 30–68% in preclinical species). Functionally, CCT251921 potently inhibits WNT/β-catenin signaling in human cancer cell lines with constitutively active WNT pathway in the 15–64 nM range. In vivo, oral administration (30 mg/kg) in an APC-mutant SW620 colorectal carcinoma xenograft model produced a 54.2% reduction in tumor weight by day 15, with sustained inhibition of STAT1 Ser727 phosphorylation for more than 6 hours post-dose, confirming durable target engagement. Limitations: Its equipotent inhibition of both kinases precludes dissection of individual CDK8 versus CDK19 functions. The probe targets the kinase domain rather than the full Mediator kinase module, so potential effects on complex assembly or transcriptional crosstalk remain unexplored. Transporter-mediated hepatic clearance contributes to higher in vivo clearance than microsomal assays predict, and its activity has been validated primarily in WNT-driven colorectal models, with limited assessment in other biological contexts.

CCT-251921 is an excellent probe for CDK8/CDK19 in terms of its selectivity. Its broad evaluation against 279 kinases in a Millipore kinase panel, as well as additional selectivity screens against seven CYP enzymes (with the closest off-target being CYP2C8 with an IC50 of 8.5 µM), and in the CEREP panel against 59 receptors, ion channels, and 29 enzymes (with the closest off-target being acetylcholinesterase with an IC50 of 1.3 µM), is notable. Unfortunately, neither CDK8 nor CDK19 was included in the kinase panel. A negative control substance was not provided either. However, it should also be noted that cytotoxicity was not mentioned in the original literature. In a comparative study by Chen et al. (https://doi.org/10.3390/cells8111413), systemic toxicity was reported for CCT251921, among others, but not for all CDK8/19 inhibitors per se. Therefore, it is recommended that additional CDK8/CDK19 inhibitors, such as BI-1347 and the corresponding negative control substance BI-1374, are used for target evaluation in a cellular context.