The enzymes cyclooxygenase-1 and cyclooxygenase-2 (COX-1 and COX-2) catalyze the conversion of arachidonic acid to prostaglandin (PG) H 2 , the precursor of PGs and thromboxane. These lipid mediators play important roles in inf lammation and pain and in normal physiological functions. While there are abundant data indicating that the inducible isoform, COX-2, is important in inf lammation and pain, the constitutively expressed isoform, COX-1, has also been suggested to play a role in inf lammatory processes. To address the latter question pharmacologically, we used a highly selective COX-1 inhibitor, SC-560 (COX-1 IC 50 ‫؍‬ 0.009 M; COX-2 IC 50 ‫؍‬ 6.3 M). SC-560 inhibited COX-1-derived platelet thromboxane B 2 , gastric PGE 2 , and dermal PGE 2 production, indicating that it was orally active, but did not inhibit COX-2-derived PGs in the lipopolysaccharide-induced rat air pouch. Therapeutic or prophylactic administration of SC-560 in the rat carrageenan footpad model did not affect acute inf lammation or hyperalgesia at doses that markedly inhibited in vivo COX-1 activity. By contrast, celecoxib, a selective COX-2 inhibitor, was anti-inf lammatory and analgesic in this model. Paradoxically, both SC-560 and celecoxib reduced paw PGs to equivalent levels. Increased levels of PGs were found in the cerebrospinal f luid after carrageenan injection and were markedly reduced by celecoxib, but were not affected by SC-560. These results suggest that, in addition to the role of peripherally produced PGs, there is a critical, centrally mediated neurological component to inf lammatory pain that is mediated at least in part by COX-2.

No abstract

The enzyme cyclo-oxygenase catalyses the oxygenation of arachidonic acid, leading to the formation of prostaglandins. Recently two forms of cyclo-oxygenase have been described: a constitutive (COX-1) enzyme present in most cells and tissues, and an inducible (COX-2) isoenzyme observed in many cells in response to pro-inflammatory cytokines. Constitutive and inducible forms of human cyclo-oxygenase (hCOX-1 and hCOX-2) were cloned and expressed in insect cells, utilizing a baculovirus expression system. hCOX-1 had a specific activity of 18.8 mumol of O2/mg with a Km of 13.8 microM for arachidonate and Vmax. of 1500 nmol of O2/nmol of enzyme, whereas hCOX-2 had a specific activity of 12.2 mumol of O2/mg with a Km of 8.7 microM for arachidonate and a Vmax. of 1090 nmol of O2/nmol of enzyme. Indomethacin inhibited both hCOX-1 and hCOX-2, whereas NS-398 and Dup-697 selectively inhibited hCOX-2. Both NS-398 and Dup-697 exhibited time-dependent inactivation of hCOX-2, as did indomethacin on both enzymes. The competitive inhibitor of hCOX-1, mefenamic acid, also displayed competitive inhibition of hCOX-2. These results demonstrate the ability to generate selective non-steroidal anti-inflammatory drugs (NSAIDs), which could provide useful improvement therapeutically in the treatment of chronic inflammatory disease.

Nonsteroidal anti-inflammatory drugs (NSAIDs) currently available for clinical use inhibit both COX-1 and COX-2. This suggests that clinically useful NSAIDs inhibit pro-inflammatory prostaglandins (PGs) derived from the activity of COX-2, as well as PGs in tissues like the stomach and kidney (via COX-1). A new class of compounds has recently been developed (SC-58125) that have a high degree of selectivity for the inducible form of cyxlooxygenase (COX-2) over the constitutive form (COX-1). This unique class of compounds exhibit a timedependent irreversible inhibition of COX-2, while reversibly inhibiting COX-1. The molecular basis of this selectivity was probed by site-directed mutagenesis of the active site of COX-2. The sequence differences in the active site were determined by amino acid replacement of the COX-2 sequences based on the known crystal structure of COX-1, which revealed a single amino acid difference in the active site (valine 509 to isoleucine) and a series of differences at the mouth of the active site. Mutants with the single amino acid substitution in the active site and a combination of three changes in the mouth of the active site were made in human COX-2, expressed in insect cells and purified. The single amino acid change of valine 509 to isoleucine confers selectivity of COX-2 inhibitors in the class of SC-58125 and others of the same class (SC-236, NS-398), while commonly used NSAIDs such as indomethacin showed no change in selectivity. Substitutions of COX-1 sequences in COX-2 at the mouth of the active site of COX-2 did not change the selectivity of SC-58125. This indicates that the single amino acid substitution of isoleucine at position 509 for a valine is sufficient to confer COX-2 selectivity in this example of a diaryl-heterocycle COX inhibitor.Prostaglandin synthase catalyzes two separate reactions; the first being the cyclooxygenase function, which is the addition of molecular oxygen to arachidonic acid to form the unstable PGG 2 , 1 and the second the further conversion of PGG 2 to the more stable PGH 2 by a peroxidase function. Hence, this "cyclooxygenase" (COX) enzyme performs the critical initial reaction in the arachidonic metabolic cascade leading to the formation of the prostaglandins, thromboxane, and prostacyclin (1).Recently, a second form of the COX enzyme was isolated whose expression is inducible by cytokines and growth factors (COX-2) (2-6). This inducible COX-2 is linked to inflammatory cell types and tissues and is believed to be the target enzyme for the anti-inflammatory activity of nonsteroidal anti-inflammatory drugs (NSAIDs) (7-12). NSAIDs currently available for clinical use inhibit both COX-1 and COX-2 (13, 14). This suggests that clinically useful NSAIDs inhibit pro-inflammatory PGs derived from the activity of COX-2, as well as PGs in tissues like the stomach and kidney (via COX-1). These homeostatic PGs are linked to normal gastric and renal function (15). It is possible that a selective COX-2 inhibitor may eliminate the side effects associated with COX...

Non-steroidal anti-inflammatory drugs (NSAIDs) inhibit the formation of prostaglandins by cyclo-oxygenases (COX). The discovery of a second COX isoform (COX-2) associated with inflammation led to agents that selectively inhibit COX-2, e.g. celecoxib. We evaluated the kinetics of inhibition of celecoxib and several NSAIDs. Celecoxib displays classic competitive kinetics on COX-1 (Ki=10-16 microM). An initial competitive interaction with COX-2 can also be discerned with celecoxib (Ki=11-15 microM), followed by a time-dependent interaction leading to potent inhibition, characterized as inactivation (Kinact=0.03-0.5 s-1). Half-maximal inhibition (IC50) using end-point assays reflects the competitive component on COX-1 (IC50=4-19 microM) and the inactivation component on COX-2 (IC50=0.003-0.006 microM). NSAIDs exhibit four distinct modes of COX inhibition based on kinetic behaviour: (1) competitive, e.g. ibuprofen; (2) weak binding, time-dependent, e.g. naproxen, oxicams; (3) tight binding, time-dependent, e.g. indomethacin; (4) covalent, e.g. aspirin. In addition, most NSAIDs display different kinetic behaviour for each isoform. Weakly binding inhibitors show variable behaviour in enzyme assays, with apparent inhibitory activity being markedly influenced by experimental conditions; determination of kinetic constants with this class is unreliable and IC50 values are strongly dependent on assay conditions. Although IC50 determinations are useful for structure/activity analyses, the complex and distinct mechanisms of enzyme inhibition of each COX isoform by the NSAIDs renders comparison of inhibitory activity on COX-1 and COX-2 using IC50 ratios of questionable validity.

Series of 1,2-diarylpyrroles has been synthesized and found to contain very potent and selective inhibitors of the human cyclooxygenase-2 (COX-2) enzyme. The paper describes short and practical syntheses of the target molecules utilizing the Paal-Knorr reaction. Electrophilic substitution on 1 proceeds in a regioselective fashion, and the method was used to generate a number of tetrasubstituted pyrroles. Detailed SAR on the series has been studied by modifications of the aryl rings and the substituents in the pyrrole ring. Diarylpyrrole 1 is a very potent (COX-2, IC50 = 60 nm) and selective (COX-1/COX-2 = > 1700) inhibitor whereas the isomeric 2 is completely inactive against COX-2. Modifications of the substituents on the fluorophenyl ring in 1 yields very potent inhibitors of COX-2 (IC50 = 40-80 nm) with excellent selectivity (1200 to > 2500) vs COX-1. Analog 20 containing a sulfonamide group is an excellent inhibitor of COX-2 with an IC50 of 14 nm. Tetrasubstituted pyrroles containing groups such as COCF3, SO2CF3, or CH2OAr at position 3 in the pyrrole ring give excellent inhibitors (COX-2, IC50 = 30-120 nm). In vivo testing in the carrageenan-induced paw edema model in the rat establishes that the 1,2-diarylpyrroles are orally active antiinflammatory agents. Compound 3 is the most potent inhibitor of edema with an ED50 of 4.7 mpk.

Series of 1,2-diarylimidazoles has been synthesized and found to contain highly potent and selective inhibitors of the human COX-2 enzyme. The paper describes a short synthesis of the target 1,2-diarylimidazoles starting with aryl nitriles. Different portions of the diarylimidazole (I) were modified to establish SAR. Systematic variations of the substituents in the aryl ring B have yielded very potent (IC50 = 10-100 nm) and selective (1000-12500) inhibitors of the COX-2 enzyme. The study on the influence of substituents in the imidazole ring established that a CF3 group at position 4 gives the optimum oral activity. A number of the diarylimidazoles showed excellent inhibition in the adjuvant induced arthritis model (e.g., ED50 = 0.02 mpk for 22 and 34). The diarylimidazoles are also potent inhibitors of carrageenan-induced edema (ED50 = 9-30 mpk) and hyperalgesia (ED50 = 11-40 mpk). Several orally active diarylimidazoles show no GI toxicity in the rat and mouse up to 200 mpk.

No abstract