Potencies of leflunomide and HR325 as inhibitors of prostaglandin endoperoxide H synthase-1 and -2: comparison with nonsteroidal anti-inflammatory drugs

The relative anti-inflammatory activities of the immunomodulators HR325 and leflunomide, or its active metabolite A77 1726, were examined by determining potencies in vitro on prostaglandin endoperoxide H synthase (PGHS) and in vivo in rat air pouch inflammation. Nonsteroidal anti-inflammatory drugs (NSAIDs) were used as comparators. HR325 was more potent than A77 1726 as an inhibitor of PGHS in guinea pig polymorphonuclear leukocytes (IC50 = 415 and 4400 nM, respectively) and on isolated ovine PGHS-1 (IC50 = 64 and 742 microM) and PGHS-2 (IC50 = 100 and 2766 microM). In vivo, in rat carrageenan air pouch inflammation, HR325 but not leflunomide at 25 mg/kg inhibited accumulation of leukocytes (48%) and PGE2 (61%). HR325 was also more potent than A77 1726 against human peripheral blood mononuclear cell PGHS-1 [IC50 = 1.6 and 25.6 microM (thromboxane B2 production) or 1.1 and 8 microM (PGE2 production)] and lipopolysaccharide-induced PGHS-2 in human adherent peripheral blood mononuclear cells (IC50 = 435 nM and 9.5 microM) and peripheral blood polymorphonuclear leukocytes (IC50 = 91 nM and 3.2 microM). HR325 had low PGHS-2 selectivity in the human (2.5-12-fold) and was a more potent PGHS-2 inhibitor than naproxen, ibuprofen and piroxicam (28-fold). Assays using endogenous arachidonic acid as substrate yielded IC50 values for NSAIDs that were in general markedly lower than those published for assays using 10 microM substrate. With this approach, piroxicam had reasonable activity on human PGHS-2 (IC50 = 260-290 nM). Only NS398 and flufenamic acid were PGHS-2 selective in the human (90-330-fold and 37-60-fold, respectively); the other NSAIDs were either PGHS-1-selective (naproxen, ibuprofen, flurbiprofen and indomethacin) or nonselective (piroxicam and diclofenac). Inclusion of 10% human plasma reduced HR325 potency against PGHS-1 in human peripheral blood mononuclear cells approximately 32-fold (IC50 = 36 microM). Plasma protein binding further reduced HR325 potency (IC50 = 164 microM) and minimized the difference between HR325 and A77 1726 (IC50 = 292 microM) in a whole blood PGHS assay. Whether the greater activity against human PGHS-2 would allow HR325 to exhibit NSAID-like therapeutic effects in humans remains unclear.