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Cyclooxygenase enzymes (COX-1 and COX-2) catalyze the conversion of arachidonic acid to prostaglandin G2. The inhibitory activity of rapid, reversible COX inhibitors (ibuprofen, naproxen, mefenamic acid, and lumiracoxib) demonstrated a significant increase in potency and time dependence of inhibition against double tryptophan murine COX-2 mutants at the 89/90 and 89/119 positions. In contrast, the slow, time-dependent COX inhibitors (diclofenac, indomethacin, and flurbiprofen) were unaffected by those mutations. Further mutagenesis studies suggested that mutation at position 89 was principally responsible for the changes in inhibitory potency of rapid, reversible inhibitors, whereas mutation at position 90 may exert some effect on the potency of COX-2-selective diarylheterocycle inhibitors; no effect was observed with mutation at position 119. Several crystal structures with or without NSAIDs indicated that placement of a bulky residue at position 89 caused a closure of a gap at the lobby, and alteration of histidine to tryptophan at position 90 changed the electrostatic profile of the side pocket of COX-2. Thus, these two residues, especially Val-89 at the lobby region, are crucial for the entrance and exit of some NSAIDs from the COX active site.

Lumiracoxib is the first example of a marketed COX-2 inhibitor of the arylacetic acid class, and it is reported to be the most selective COXIB in vivo. However, the molecular basis of its COX-2 inhibition has not been completely defined. Using standard assays, lumiracoxib was found to be a poor inhibitor of purified ovine COX-1 and a relatively weak inhibitor of purified human COX-2. The extent of COX-2 inhibition plateaued at around 50% and suggested that the inhibitor may be reversibly bound to the enzyme. Kinetic studies with lumiracoxib demonstrated that it was a time-dependent and slowly reversible inhibitor of human COX-2 that exhibited at least two binding steps during inhibition. Derivatives of lumiracoxib were synthesized with or without the methyl group on the phenylacetic acid ring and with various substitutions on the lower aniline ring. Inhibition studies demonstrated that the methyl group on the phenylacetic acid ring is required for COX-2 selectivity. The chemical identity and position of the substituents on the lower aniline ring were important in determining the potency and extent of COX inhibition as well as COX-2 selectivity. Mutation of Ser-530 to Ala or Val-349 to Ala or Leu abolished the potent inhibition observed with wild-type human COX-2 and key lumiracoxib analogs. Interestingly, a Val-349 to Ile mutant was inhibited with equal potency to human COX-2 with 2,6-dichloro-, 2,6-dimethyl-, or 2-chloro-6-methyl-substituted inhibitors and, in the case of lumiracoxib, actually showed an increase in potency. Taken together with a recent crystal structure of a lumiracoxib-COX-2 complex, the kinetic analyses presented herein of the inhibition of mutant COX-2s by lumiracoxib allows the definition of the molecular basis of COX-2 inhibition.