Tetrabromobenzotriazole (TBBt) and tetrabromobenzimidazole (TBBz) as selective inhibitors of protein kinase CK2: Evaluation of their effects on cells and different molecular forms of human CK2

The development of selective cell-permeable inhibitors of protein kinase CK2 has represented an important advance in the field. However, it is important to not overlook the existence of discrete molecular forms of CK2 that arise from the presence of distinct isozymic forms, and the existence of the catalytic CK2 subunits as free subunits and in complexes with the regulatory CK2β subunits and, possibly, other proteins. This review examines two recently developed, and presently widely applied, CK2 inhibitors, 4,5,6,7- tetrabromobenzotriazole (TBBt) and the related 4,5,6,7-tetrabromobenzimidazole (TBBz), the latter of which was previously shown to discriminate between different molecular forms of CK2 in yeast. We have shown, by spectrophotometric titration, that TBBt, with a pK_a ≃ 5, exists in solution at physiological pH almost exclusively (>99%) as the monoanion; whereas TBBz, with a pK_a ≃ 9, is predominantly (>95%) in the neutral form, both of obvious relevance to their modes of binding. In vitro, TBBt inhibits different forms of CK2 with K_i values ranging from 80 to 210 nM. TBBz better discriminates between CK2 forms, with K_i values ranging from 70 to 510 nM. Despite their general similar in vitro activities, TBBz is more effective than TBBt in inducing apoptosis and, to a lesser degree, necrosis, in transformed human cell lines. Finally, development of shRNA strategies for the selective knockdown of the CK2α and CK2α′ isoforms reinforces the foregoing results, indicating that inhibition of CK2 leads to attenuation of proliferation.