Molecular model of the interaction between nimesulide and human cyclooxygenase-2

The cyclooxygenase-2 (COX-2) isoenzyme is a key target for COX-2-selective non-steroidal anti-inflammatory drugs (NSAIDs). An important difference in binding of nimesulide compared with non-selective NSAIDs appears to involve the amino acid at position 523 of the enzyme. Replacement of valine with isoleucine at this position provides access to a binding site that is larger in COX-2 than in COX-1. Nimesulide appears to exploit this enlarged binding site for establishing a number of favourable contacts with the enzyme that lead to selective inhibition of COX-2. We made these conclusions from a three-dimensional molecular model of the active site of human COX-2, constructed using the X-ray coordinates of COX-1 from sheep seminal vesicles and COX-2 from mouse fibroblasts as templates, with the aid of sequence alignment methods and molecular modelling techniques. The resulting model was refined, and the active site was probed for regions of steric and electrostatic complementarity for ligand binding. Docking studies were then undertaken with many different nimesulide conformers, a family of which could establish very favourable interactions with the NSAID binding site of human COX-2 by exploiting the extra space made available by the isoleucine/vaIine replacement. The stability of the resulting complexes was studied by simulating molecular dynamics.