Isomeric Mono-, Di-, and Tri-Bromobenzo-1H-Triazoles as Inhibitors of Human Protein Kinase CK2α

To further clarify the role of the individual bromine atoms of 4,5,6,7-tetrabromotriazole (TBBt), a relatively selective inhibitor of protein kinase CK2, we have examined the inhibition (IC50) of human CK2α by the two mono-, the four di-, and the two tri- bromobenzotriazoles relative to that of TBBt. Halogenation of the central vicinal C(5)/C(6) atoms proved to be a key factor in enhancing inhibitory activity, in that 5,6-di-Br2Bt and 4,5,6-Br3Bt were almost as effective inhibitors as TBBt, notwithstanding their marked differences in pKa for dissociation of the triazole proton. The decrease in pKa on halogenation of the peripheral C(4)/C(7) atoms virtually nullifies the gain due to hydrophobic interactions, and does not lead to a decrease in IC50. Molecular modeling of structures of complexes of the ligands with the enzyme, as well as QSAR analysis, pointed to a balance of hydrophobic and electrostatic interactions as a discriminator of inhibitory activity. The role of halogen bonding remains debatable, as originally noted for the crystal structure of TBBt with CK2α (pdb1j91). Finally we direct attention to the promising applicability of our series of well-defined halogenated benzotriazoles to studies on inhibition of kinases other than CK2.