Synthesis and SAR of Tetracyclic Inhibitors of Protein Kinase CK2 Derived from Furocarbazole W16.

The serine/threonine kinase CK2 modulates the activity of more than 300 proteins and thus plays a crucial role in various physiological and pathophysiological processes including neurodegenerative disorders of the central nervous system and cancer. The enzymatic activity of CK2 is controlled by the equilibrium between the heterotetrameric holoenzyme CK2α2 ß2 and its monomeric subunits CK2α and CK2ß. A series of analogues of W16 ((3aR,4S,10S,10aS)-4-{[(S)-4-benzyl-2-oxo-1,3-oxazolidin-3-yl]carbonyl}-10-(3,4,5-trimethoxyphenyl)-4,5,10,10a-tetrahydrofuro[3,4-b]carbazole-1,3(3aH)-dione ((+)-3 a)) was prepared in an one-pot, three-component Levy reaction. The stereochemistry of the tetracyclic compounds was analyzed. Additionally, the chemically labile anhydride structure of the furocarbazoles 3 was replaced by a more stable imide (9) and N-methylimide (10) substructure. The enantiomer (-)-3 a (Ki =4.9 µM) of the lead compound (+)-3 a (Ki =31 µM) showed a more than sixfold increased inhibition of the CK2α/CK2ß interaction (protein-protein interaction inhibition, PPII) in a microscale thermophoresis (MST) assay. However, (-)-3 a did not show an increased enzyme inhibition of the CK2α2 ß2 holoenzyme, the CK2α subunit or the mutated CK2α' C336S subunit in the capillary electrophoresis assay. In the pyrrolocarbazole series, the imide (-)-9 a (Ki =3.6 µM) and the N-methylimide (+)-10 a (Ki =2.8 µM) represent the most promising inhibitors of the CK2α/CK2ß interaction. However, neither compound could inhibit enzymatic activity. Unexpectedly, the racemic tetracyclic pyrrolocarbazole (±)-12, with a carboxy moiety in the 4-position, displays the highest CK2α/CK2ß interaction inhibition (Ki =1.8 µM) of this series of compounds.

Unexpected CK2ß-antagonistic functionality of bisubstrate inhibitors targeting protein kinase CK2.

Protein kinase CK2, a heterotetrameric holoenzyme composed of two catalytic chains (CK2α) attached to a homodimer of regulatory subunits (CK2ß), is a target for drug development for cancer therapy. Here, we describe the tetraiodobenzimidazole derivative ARC-3140, a bisubstrate inhibitor addressing the ATP site and the substrate-binding site of CK2 with extraordinary affinity (Ki = 84 pM). In a crystal structure of ARC-3140 in complex with CK2α, three copies of the inhibitor are visible, one of them at the CK2ß interface of CK2α. Subsequent interaction studies based on microscale thermophoresis and fluorescence anisotropy changes revealed a significant impact of ARC-3140 and of its tetrabromo equivalent ARC-1502 on the CK2α/CK2ß interaction. A structural inspection revealed that ARC-3140, unlike CK2ß antagonists described so far, interferes with both sub-interfaces of the bipartite CK2α/CK2ß interaction. Thus, ARC-3140 is a lead for the further development of highly effective compounds perturbating the quaternary structure of the CK2α2ß2 holoenzyme.