Three additional Type I CDK and CDK kinase inhibitors
Three additional Type I CDK8 and CDK19 kinase inhibitors have also been recently reported. Orally available 3-benzylindazole 12 was discovered by optimization of compounds with HSP90 affinity to yield potent and selective CDK8 inhibitors. Compound 12 showed strong binding activity for CDK8 with an IC50 value of 10nM in Lanthascreen TR-FRET assay. MSC2530818 (13), which was identified by the modification of an imidazothiazole compound, is also an orally available potent and selective CDK8 inhibitor with an IC50 value of 2.6 nM in Lanthascreen TR-FRET assay. Both compounds 12 and 13 demonstrated reduction of tumor growth rates in human SW620 colorectal carcinoma xenograft model. Thieno[2,3-c]pyridine third 14 was also discovered to be a Type I CDK8 and CDK19 inhibitor, with strong affinity for CDK8 indicated by an IC50 value of 1.5 nM in Lanthascreen TR-FRET assay. However, CDK8 dependent antiproliferative activity of compound 14 in HCT116 colon cancer cell lines was not observed, despite the confirmation of STAT1 phosphorylation inhibition at Ser727. The modification of the well-known kinase inhibitor Sorafenib also led to the identification of a highly potent and selective Type II CDK8 inhibitor 15, which showed high binding affinity in Lanthascreen TR-FRET assay (IC50: 17.4nM). However, compound 15 did not significantly suppress phosphorylation of STAT1 at Ser727. The 4,5-dihydroimidazolo[3′,4′:3,4]benzo[1,2-d]isothiazole derivative 16 (Fig. 3), bearing a unique [5,6,5]-fused tricyclic scaffold, was originally identified in our laboratories as a potential prophylactic or therapeutic drug for treating bone or articular diseases, possessing the ability to induce cell differentiation in osteoblasts and chondrocyte precursor cells; later reports focused on the use of these compounds to regulate muscle and fat cell differentiation. However, the target protein of this compound remained unknown for several years. Recently, this compound was screened by affinity selection-mass spectrometry (AS/MS) against a library of approximately 13,000 proteins, and found to bind specifically to CDK19. High throughput screening of an in-house compound library against CDK8 confirmed that a variety of 4,5-dihydroimidazolo[3′,4′:3,4]benzo[1,2-d]isothiazole derivatives, including 16, inhibit both CDK8 and CDK19 (IC50: 18 nM and 19 nM, respectively). These results prompted us to initiate a campaign to discover novel, potent CDK8/19 dual inhibitors for use in anticancer therapy. A docking model (shown in Fig. 3) was constructed of 16 bound to CDK8 using the program GOLD, version 5.2, and the reported co-crystal structure of CDK8 bound to a small molecule inhibitor (PDB ID: 4F6W). An amide proton and the carbonyl group of the aminocarbonyl moiety at the 4,5-dihydroimidazolo[3′,4′:3,4]benzo[1,2-d]isothiazole 6-position were suggested to be involved in interaction with the backbone CO and NH of Ala100 in the kinase hinge region of CDK8. The nitrogen atom at the N-2 position of this scaffold was implicated as being involved in a hydrogen bond with +NH3 of Lys52. Additionally, the methyl sulfide group at the 8-position of this scaffold was directed toward the front pocket region, which consists of a large hydrophobic space. On the basis of these modeling studies, a series of novel CDK8/19 dual inhibitors based on [5,6,5]-fused tricyclic scaffolds were designed (Fig. 4). An aryl group introduced into the 8-position of this scaffold was expected to occupy the hydrophobic front pocket region to improve CDK8 potency and kinase selectivity, while allowing for the modification of ADME properties. We report herein the synthesis and structure–active relationships (SARs) for a series of tricyclic 4,5-dihydrothieno[3′,4′:3,4]benzo[1,2-d]isothiazole compounds as novel CDK8/19 dual inhibitors. The optimization of this series led to the identification of 52h, which showed the antitumor efficacy in a RPMI8226 human hematopoietic and lymphoid xenograft model in mice.