Pharmacophore Design of p38α MAP Kinase Inhibitors with Either 2,4,5-Trisubstituted or 1,2,4,5-Tetrasubstituted Imidazole Scaffold

Synthetic compounds with a tri- and tetra-substituted imidazole scaffold are known as selective inhibitors of the p38 mitogen-activated protein (MAP) kinase responsible for proinflammatory cytokine release. The scope is to review the literature describing their design, synthesis and activity studies. To date a great plethora of crystal structures of p38 in complex with small organic ligands have been published. Cocrystallized ligand information is of particular interest to our review study, i.e. ATP itself, the reference inhibitor SB203580 with its aryl-pyridinyl-imidazoles and related imidazole and pyrimidine-based derivatives. The selective inhibitors bind to the pocket of adenosine 5'-triphoshate (ATP) replacing the latter. The hydrophobic region II, however, is not occupied by the natural binder ATP, but accommodates the pyridine substituents preserving the 4-fluorophenyl ring occupation in pocket I as a prerequisite to gain higher binding selectivity and potency than the reference compound SB203580 (4-[5-(4-fluoro-phenyl)-2-(4-methanesulfinyl-phenyl)-3-himidazol-4-yl]-pyridine). Experimental and computed work is reviewed which evidence that the 2 position of the pyrimidine ring is amenable to the introduction of a side chain and the replacement of pyridine in SB203580 by a pyrimidine ring improves both inhibitory activity and selectivity for p38 over other kinases. All ligands with a pyridyl C2 side chain occupy the hydrophobic pocket II and in some cases a double hydrogen bond is reported between methionine 109 and glycine 110 of the hinge region, following an observed backbone shift. The substituted pyridine ring binds stronger than the two other side chains on the imidazole scaffold.