Effects of ibuprofen enantiomers and its coenzyme A thioesters on human prostaglandin endoperoxide synthases

1 Ibuprofen enantiomers and their respective coenzyme A thioesters were tested in human platelets and blood monocytes to determine their selectivity and potency as inhibitors of cyclo-oxygenase activity of prostaglandin endoperoxide synthase-1 (PGHS-1) and PGHS-2. 2 Human blood from volunteers was drawn and allowed to clot at 37 degrees C for 1 h in the presence of increasing concentrations of the test compounds (R-ibuprofen, S-ibuprofen, R-ibuprofenoyl-CoA, S-ibuprofenoyl-CoA, NS-398). Immunoreactive (ir) thromboxane B-2 (TXB2) concentrations in serum were determined by a specific EIA assay as an index of the cyclo-oxygenase activity of platelet PGHS-1. 3 Heparin-treated blood from the same donors was incubated at 37 degrees C for 24 h with the same concentrations of the test compounds in the presence of lipopolysaccharide (LPS, 10 mu g ml(-1)). The contribution of PGHS-1 was suppressed by pretreatment of the volunteers with aspirin (500 mg; 48 h before venepuncture). As a measure of LPS induced PGHS-2 activity immunoreactive prostaglandin E-2 (irPGE(2)) plasma concentrations were determined by a specific EIA assay. 4 S-ibuprofen inhibited the activity of PGHS-1 (IC50 2.1 mu M) and PGHS-2 (IC50 1.6 mu M) equally. R-ibuprofen inhibited PGHS-1 (IC50 34.9) less potently than S-ibuprofen and showed no inhibition of PGHS-2 up to 250 mu M. By contrast R-ibuprofenoyl-CoA thioester inhibited PGE(2) production from LPS-stimulated monocytes almost two orders of magnitude more potently than the generation of TXB2 (IC50 5.6 vs 219 mu M). 5 Western blotting of PGHS-2 after LPS induction of blood monocytes showed a concentration-dependent inhibition of PGHS-2 protein expression by ibuprofenoyl-CoA thioesters. 6 These data confirm that S-ibuprofen represents the active entity in the racemate with respect to cyclooxygenase activity. More importantly the data suggest a contribution of the R-enantiomer to therapeutic effects not only by chiral inversion to S-ibuprofen but also via inhibition of induction of PGHS-2 mediated by R-ibuprofenoyl-CoA thioester. 7 The data may explain why racemic ibuprofen is ranked as one of the safest non-steroidal antiinflammatory drugs (NSAIDs) so far determined in epidemiological studies.