Stereoselective Inhibition of Inducible Cyclooxygenase by Chiral Nonsteroidal Antiinflammatory Drugs

Abstract

The stereoselective inhibition of inducible cyclooxygenase (COX‐2) by chiral nonsteroidal antiinflammatory drugs (NSAIDs)—ketoprofen, flurbiprofen, and ketorolac—has been investigated. The activity and inhibition of COX‐2 was assessed in three different in vitro systems: guinea pig whole blood, lipopolysaccharide (LPS)‐stimulated human monocytes, and purified preparations of COX‐2 from sheep placenta. The results were compared with the inhibition of constitutive cyclooxygenase (COX‐1) in three parallel in vitro models: clotting guinea pig blood, human polymorphonuclear leukocytes, and purified COX‐1 from ram seminal vesicles. In the whole blood model, both isoenzymes were inhibited by S‐enantiomers with equal potency but S‐ketoprofen was the most active on COX‐2 (IC₅₀ = 0.024 μmol/L). In contrast, both isoenzymes were inhibited less than 40% by all three R‐enantiomers at high concentration (>1 μmol/L). The inhibition of COX by the R‐enantiomers may be attributed to contamination with the S‐enantiomers (approximately 0.5%). A significant degree of enantioselectivity in COX‐2 inhibition was also observed in intact cells. The S‐enantiomers inhibited COX‐2 from monocytes with IC₅₀ values in the range of 2 to 25 nmol/L, being 100 to 500‐fold more potent than the corresponding R‐enantiomers. Finally, S‐ketoprofen inhibited COX‐2 from sheep placenta (IC₅₀ = 5.3 μmol/L) with slightly less potency than S‐ketorolac (IC₅₀ = 0.9 μmol/L) and S‐flurbiprofen (IC₅₀ = 0.48 μmol/L), whereas the R‐enantiomers were found to be essentially inactive (IC₅₀ ≥ 80 μmol/L). It is concluded that the chiral NSAIDs studied here inhibit with comparable stereoselectivity both COX‐2 and COX‐1 isoenzymes, and that the inhibition of COX‐2 previously observed for racemic NSAIDs should be attributed almost exclusively to their S‐enantiomers.