Measurement of differential inhibition of COX-1 and COX-2 and the pharmacology of selective inhibitors

Since the discovery of a second isoenzyme of cyclooxygenase (COX)-2, it has been hypothesized that the antiinflammatory effects of non-steroidal antiinflammatory drugs (NSAIDs) are dependent on their inhibition of COX-2, whereas inhibition of constitutive COX-1 is responsible for their gastric and renal side effects as well as for inhibition of platelet activation. Consequently, a large number of in vitro assays have been developed to characterize the COX-1 and COX-2 inhibitory activities of NSAIDs in order to look for compounds with preferential inhibition of COX-2. Depending on the test system, however, the experimental conditions may vary greatly, thereby affecting the outcome. These important variables include the source of enzymes COX-1 and COX-2 (human or animal); the cell system used (intact normal cells or transfected cell lines); the method of enzyme preparation (purified enzymes, microsomal or whole cell assays); the COX-2-inducing agent; the source of arachidonic acid and its concentration; the incubation time with drug, inducing agent or arachidonic acid; and the protein concentration in the medium. Depending on the test system employed for defining the IC(50) for COX-1 and COX-2 inhibition, the resultant ratio between these activities may lead to divergent and confusing comparisons. We review the various in vitro test systems available for the measurement of COX-1 and COX-2 inhibition. The use of human recombinant enzymes and the human whole blood assay are examined in detail. The relevance of these test systems to in vivo animal models of inflammation and the side effects of NSAIDs (e.g., gastric mucosal and renal damage) is assessed. Finally, the results of clinical studies on the effect of inhibiting COX-1 and COX-2 activity is summarized.     

Cyclooxygenase (COX)-2 selective inhibitors: aspirin, a dual COX-1/COX-2 inhibitor, to COX-2 selective inhibitors

Aspirin was developed as a non-steroidal anti-inflammatory drug (NSAID) in 1899. During the century after that, aspirin has been found to show its anti-inflammatory, analgesic and anti-pyretic activities by reducing prostaglandins biosynthesis through inhibition of cyclooxygenase (COX); and then COX was found to be constituted of two isoforms, constitutive COX-1 and inducible COX-2. Currently, novel NSAIDs, acting through selective inhibition of COX-2, that have efficacy as excellent as aspirin with significantly lower incidence of gastrointestinal adverse effects are available in America and some other countries, but not in Japan. Physiological and pathophysiological roles of COX-1 and COX-2 have been explained from studies in experimental animals, but there are many differences in species and diseases between animals and humans. Thus, physiological and pathophysiological roles of COX-2 were considered from the standpoint of clinical effects of the two latest COX-2 selective inhibitors, celecoxib and rofecoxib, on inflammation, pain, fever and colorectal cancer together with their adverse effects on gastrointestinal, renal and platelet functions; and the usefulness and limits of COX-2-selective inhibitors were discussed with the trends of new NSAIDs development.     

Clinical implications of COX-1 and/or COX-2 inhibition for the distal gastrointestinal tract

Side effects of the distal gastrointestinal tract after NSAID use are common and more frequent than previously recognized. Increased mucosal permeability and mucosal inflammation are often silent but appear after NSAID treatment with most dual COX inhibitors. Other clinical manifestations include: anemia, occult blood loss, malabsorption, protein-loss, ileal dysfunction, diarrhea, mucosal ulceration and strictures due to diaphragm disease. More common complications are lower gastrointestinal bleeding and perforation, which represent at least one third of all gastrointestinal complications observed with NSAID use. Studies with selective COX-2 inhibitors have shown that, in the short term, these agents do not increase mucosal permeability or induce anemia due to occult bleeding and that, when compared to dual COX inhibitors, lower gastrointestinal complications may be reduced by 50%. In order to minimize the impact of these side effects, it is important to increase the current standards of suspicion by physicians who treat these patients, since drug discontinuation may further reduce damage, and clinical experience with agents that may prevent or treat distal tract damage is very limited. From this perspective, selective COX-2 inhibitors may be the drugs of choice in the high-risk patient that needs NSAIDs. Another important area of uncertainty is the impact of NSAID use in patients with inflammatory bowel diseases. Data from different animal models of inflammatory bowel disease suggest that inhibition of both COX-1 and COX-2 derived prostaglandins affects the severity of the mucosal inflammation. However, current epidemiological and clinical data are contradictory. Since many patients.     

The endothelium-dependent vasodilator effect of acetylcholine: characterization of the endothelial relaxing factor with inhibitors of arachidonic acid metabolism

Acetylcholine (ACh) caused concentration-dependent relaxation of strips of rabbit thoracic aorta precontracted with noradrenaline if the endothelium was intact. More than ten-fold higher concentrations of ACh also stimulated the release of prostacyclin (PGI2) and PGE2 from the strips. De-endothelialized strips released much smaller amounts of prostaglandins and contracted slightly to ACh. The endothelium-dependent vasodilation was resistant to cyclooxygenase inhibition by indomethacin, flurbiprofen and diclofenac. However, it could be reversed by six different inhibitors of lipoxygenase (nordihydroguaiaretic acid, phenidone, eicosatetraynoic acid, nafazatrom, compound BW 755C and caffeic acid). BW 755C and caffeic acid, a selective inhibitor of 5-lipoxygenase, had comparatively weak effects on the relaxation. Eicosatetraynoic acid, which probably does not inhibit C-5-lipoxygenase, completely reversed the effect of ACh. It is concluded that ACh relaxes strips of rabbit aorta by a mechanism in involving a non-cyclooxygenase metabolite of arachidonic acid of endothelial origin. This compound is probably not a product of C-5-lipoxygenase.     

Differential inhibition of cyclooxygenase-1 (COX-1) and-2 (COX-2) by NSAIDS: Consequences on anti-inflammatory activity versus gastric and renal safety

The discovery of an inducible isoform of cyclooxygenase (COX-2) requires a refinement of the theory that inhibition of cyclooxygenase activity is responsible for both therapeutic and side-effects of nonsteroidal anti-inflammatory drugs (NSAIDs). Pharmacological results with developmental compounds suggest that COX-2 is the relevant target for the therapeutic (i.e. anti-inflammatory) effects of NSAIDs, whereas gastric and renal side-effects are related to inhibition of constitutive COX-1. However a role of COX-1 in inflammation cannot be excluded. Furthermore, more research effort is needed to investigate the functional relevance of COX-2 in normal tissue.     

Denopamine,a selective beta1-receptor agonist and a new coronary vasodilator

Up until now, it has been suggested that nitrate and/or calcium channel blockers were effective against variant angina pectoris. On the other hand, it is known that about 20% of variant angina pectoris was refractory to both nitrate and calcium channel blockers. In Japan, it has been reported that denopamine, which is an oral beta1-adrenoceptor selective agonist developed by the Japanese pharmaceutical industry (Tanabe Seiyaku), is effective in those refractory cases. To date, in Japan nine cases have been recognized of patients with vasospastic angina pectoris whose symptoms were relieved by taking denopamine, including one case in which the author has had personal experience. Eight of these nine cases were refractory, and were not relieved by combined therapy using both nitrate and a calcium channel blocker. It was also documented that denopamine was effective in cases where attacks were not relieved by prazosin or magnesium, which have been documented as effective in other refractory cases. In a study of canine coronary arteries, localization of beta-adrenoceptor subtypes was documented, with the beta1-adrenoceptor predominantly found in the conduit coronary artery. In recent years it has been emphasized that the principal role of sympathetic nerves was not associated with the constrictive action of alpha-adrenoceptors, but with the coronary dilative action of beta-adrenoceptors. It would therefore be worthwhile to determine whether denopamine is able to relieve vasospastic angina pectoris in many more cases.     

Possible role of endothelial thromboxane A2 in the resting tone and contractile responses to acetylcholine and arachidonic acid in canine cerebral arteries

The amount of immunoreactive thromboxane B2 (iTXB2) released from isolated canine arteries was determined by radioimmunoassay. The amount of iTXB2 released from the cerebral, coronary, mesenteric, and saphenous arteries was 47.0 +/- 7.2, 4.0 +/- 0.6, 4.9 +/- 0.5, and 2.7 +/- 0.4 pg/mg wet weight tissue/30 min, respectively. The release of iTXB2 from the cerebral artery was decreased to less than 50% by the administration of indomethacin (10(-5) M) or OKY-046 (10(-4) M), and by intimal rubbing. The release of iTXB2 was enhanced nearly twofold by the addition of arachidonic acid (AA) (10(-5) M) to the medium, but not by the addition of acetylcholine (ACh) (10(-6) M). The cerebral arterial strips maintained the resting tone, which was reduced maximally by papaverine (10(-4) M). The resting tone was also reduced dose dependently by a cyclooxygenase inhibitor (indomethacin), a thromboxane A2 (TXA2) synthetase inhibitor (OKY-046), and a TXA2 antagonist (ONO-3708). The resting tone of rubbed strips was about half that of intact strips. ACh and AA induced similar transient contractions in the cerebral artery. Contractions produced by these agents were attenuated by indomethacin (10(-7) M), aspirin (5 X 10(-5) M), OKY-046 (10(-6) M), and ONO-3708 (10(-8) M), and abolished by intimal rubbing. From these results, it is concluded that TXA2 is produced in the endothelial cells and may be involved in maintaining the resting tone and contractile response to AA in the canine cerebral artery.     

Involvement of sensory nerves in vasodilator responses to acetylcholine and potassium ions in rat hepatic artery

In the presence of ouabain (1 mM), acetylcholine and KCl (5 mM) evoked endothelium-independent relaxations in rat hepatic arteries. Treatment with capsaicin (10 microM), scopolamine (1 microM) or CGRP(8 - 37) (3 microM) prevented these relaxations. Acetylcholine-induced relaxations in intact arterial segments in the presence of indomethacin (10 microM) and N(G)-nitro-L-arginine (0.3 mM) were only partially inhibited by ouabain plus BaCl(2) (30 microM). However, ouabain plus BaCl(2) almost abolished such relaxations in capsaicin-pre-treated preparations. In arteries without endothelium, the neurosecretagogue alpha-latrotoxin (1 nM) induced complete relaxations, which were abolished by CGRP(8 - 37) or pre-treatment with capsaicin. alpha-Latrotoxin also induced a smooth muscle hyperpolarization (12+/-2 mV), which was abolished by CGRP(8 - 37). The ability of ouabain to disclose a CGRP-mediated neurogenic relaxation must be considered when this agent is used as a pharmacological tool. The results further suggest that CGRP is a nerve-derived hyperpolarizing factor in the rat hepatic artery.     

Effects of ponalrestat on depressor responses to arachidonic acid in streptozotocin-diabetic rats

1. This study measured reductions in diastolic blood pressure to arachidonic acid (AA, 0.125-2.0 mg/kg i.v.) in 14-day streptozotocin-diabetic (60 mg/kg i.v.) and control rats anaesthetized with pentobarbitone (50-70 mg/kg i.p.) having been treated acutely or chronically with saline or ponalrestat (25 mg/kg p.o. 1 hr prior to anaesthesia, or 25 mg/kg p.o. daily for 14 days). 2. Streptozotocin-treated diabetic rats displayed reduced sensitivity to depressor effects of AA (0.125-2.0 mg/kg) when compared with controls. 3. Acute treatment with ponalrestat did not change responses to AA in controls or diabetics, whereas chronic treatment resulted in a small increase in depressor responses to AA (0.125, 0.25 and 2.0 mg/kg) in diabetics but not controls. 4. The mechanism of this action of ponalrestat remains to be elucidated, although the results are compatible with the hypothesis that increased polyol pathway activity may affect responses to, or metabolism of eicosanoids.     

Effects of long-term cyclo-oxygenase 2 selective and acid inhibition on Barrett's oesophagus

BACKGROUND: There is an overexpression of cyclo-oxygenase 2 (COX-2) in Barrett's oesophagus (BO). AIM: To determine the long-term effect of a COX-2 inhibitor on cellular mechanisms involved in BO. METHODS: A randomized controlled trial was conducted in BO patients allocated to continue the usual proton pump inhibitor (PPI) alone treatment, or PPI combined with rofecoxib (25 mg/day) for 6 months. Cell proliferation index and COX-2 expression in BO glands was determined in biopsy specimens at baseline and after treatment. Cell apoptosis, cyclin D1, p53 and vascular endothelial growth factor (VEGF) expression was also explored in a subset of patients. Student-t test and the U-Mann-Whitney test were used for quantitative and ordinal variables. RESULTS: Of 62 patients, 58 completed the study. A higher proportion of patients on rofecoxib + PPI exhibited a decrease in COX-2 expression compared to those treated with PPI alone, but cell proliferation index was not affected. Unlike PPI alone, rofecoxib + PPI was associated with an increase in the apoptotic cell index, a decrease in p53 cell staining and VEGF expression in mucosal vessels. No effect on low-grade dysplasia or cyclin D1 was observed. CONCLUSIONS: The addition of rofecoxib to PPI therapy does not affect cell proliferation index in BO cells after 6 months of therapy, but does reduce COX-2 and VEGF expression and increases cell apoptosis.     

Vasoconstrictor and vasodilator responses to various agonists in the rat perfused mesenteric arterial bed: selective inhibition by PPADS of contractions mediated via P2x-purinoceptors.

1. The effect of pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) on vasoconstrictor and/or vasodilator responses to various agonists and electrical field stimulation was investigated in the rat mesenteric arterial bed at basal tone and at tone raised by methoxamine (15-50 microM). 2. At basal tone, nucleotides produced vasoconstriction with the following rank order of potency: alpha,beta-methylene ATP >> 2-methylthio ATP > or = ATP = UTP. PPADS (0.3-10 microM) concentration-dependently antagonized alpha, beta-methylene ATP-, 2-methylthio ATP- and ATP-induced responses. UTP-, noradrenaline- and nerve-mediated (4-32 Hz) increases in perfusion pressure remained unaffected by 10 microM PPADS. 3. In raised tone preparations, nucleotides produced vasodilations, their rank order of potency being 2-methylthio ATP > ATP > UTP. Responses to 2-methylthio ATP were slightly antagonized, whereas ATP- and UTP-induced responses remained unaffected by 10 microM PPADS. In addition, acetylcholine- and adenosine-elicited relaxations were not influenced by 10 microM PPADS. 4. The present results confirm the previously described selective P2x antagonism by PPADS, this compound being ineffective at muscarinic M3- and adenosine P1-receptors as well as at alpha 1-adrenoceptors. There was some inhibition of P2y-purinoceptors but at a much higher concentration than required for inhibition of P2x-purinoceptors. 5. In addition, this study provides evidence for the ineffectiveness of PPADS at both vasoconstriction- and vasodilatation-mediating P2u-purinoceptors.     

Effect of selective inhibition of cyclooxygenase-2 on lipopolysaccharide-induced hyperalgesia

Lipopolysaccharide (LPS) is known to increase the expression and release of various pro-inflammatory mediators, including cyclooxygenase-2 (COX-2) and produce hyperalgesia. It is also well known that prostaglandins (PGs), synthesised both in the periphery and centrally by COX isoforms, play a key role in sensitisation of nociceptors and nociceptive processing. To investigate the role of COX-2 in LPS-induced hyperalgesia, parecoxib, a selective COX-2-inhibiting pro-drug, was injected intravenously 30 min before assessing hyperalgesia induced by intraperitoneal or subcutaneous administration of LPS (50 g/mouse or 25 g/paw of rat, respectively). Acetic acid-induced writhing and tail immersion assay in mice and paw withdrawal response to thermal and mechanical stimuli in rats were used to assess the effect of inhibition of COX-2 on LPSinduced hyperalgesia. Animals showed significant hyperalgesic behavior 8 h after LPS injection. Parecoxib (up to 20 mg/kg, i.v.) had no effect in the two acute nociceptive assays but showed marked antinociceptive activity in writhing and tail immersion assay in LPS-pretreated mice. Similarly, parecoxib reversed the hyperalgesia in the LPS-injected paw but not in the contralateral paw of rats. Pre-treatment with dexamethasone, an inhibitor of COX-2 expression before LPS injection significantly affected the development of hyperalgesia in both mice and rats. These findings suggest that inducible COX-2 derived PGs are involved in central nociceptive processing, which resulted in hyperalgesic behavior following LPS administration and inhibition of COX-2 or its expression attenuated LPS-induced hyperalgesia.     

Evaluation of COX-1/COX-2 selectivity and potency of a new class of COX-2 inhibitors

A new class of selective cyclooxygenase-2 (COX-2) inhibitors has been identified by high throughput screening. Structurally distinct from previously described selective COX-2 inhibitors, these benzopyrans contain a carboxylic acid function and CF3 functionality. The compound SC-75,416 is a representative of this class. A range if in vitro and in vivo tests were employed to characterize its potency and selectivity. Using human recombinant enzymes, this compound displays a concentration that provides 50% inhibition (IC50) of 0.25 microM for COX-2 and 49.6 microM for COX-1. A mutation of the side pocket residues in COX-2 to COX-1 had little effect on potency suggesting that these inhibitors bind in a unique manner in COX-2 distinct from COX-2 inhibiting diaryl heterocycles. Using rheumatoid arthritic synovial cells stimulated with interleukin-1beta (IL-1beta) and washed platelets the compound displayed IC50 of 3 nM and 400 nM respectively. Potency and selectivity was maintained but predictably right shifted in whole blood with IC50 of 1.4 microM for lipopolysaccharide (LPS) stimulated induction of COX-2 and >200 microM for inhibition of platelet thromboxane production. SC-75,416 is 89% bioavailable and its in vivo half life is sufficient for once a day dosing. In the rat air pouch model of inflammation, the compound inhibited PGE2 production with an effective dose that provides 50% inhibition (ED50) of 0.4 mg/kg, while sparing gastric prostaglandin E2 (PGE2) production with an ED50 of 26.5 mg/kg. In a model of acute inflammation and pain caused by carrageenan injection into the rat paw, the compound reduced edema and hyperalgesia with ED50s of 2.7 and 4 mg/kg respectively. In a chronic model of arthritis the compound demonstrated an ED50 of 0.081 mg/kg and an ED(80) of 0.38 mg/kg. In a model of neuropathic pain, SC-75,416 had good efficacy. This compound's unique chemical structure and effect on COX enzyme binding and activity as well as its potency and selectivity may prove useful in treating pain and inflammation.     

Blockade of coronary reactions to arachidonic acid by glyceryl trinitrate and tranylcypromine

Isolated perfused hearts of rats or guinea pigs reacted to bolus doses of arachidonic acid (AA) with a coronary constriction followed by a protracted vasodilatation phase. Glyceryl trinitrate (GTN; 55–95 μM) produced coronary dilatation during which the AA-induced constriction remained unaltered, or even enhanced. After ‘acute tolerance’ developed by sustained GTN infusion, the constrictor phase of AA was inhibited while the vasodilation continued unaltered or slightly enhanced. Nitroprusside (Np; 5–56 μM) determined a coronary vasodilatation that persisted throughout its administration and appeared to be associated with an inhibition of the AA-induced coronary constriction. While withdrawal of Np resulted in an immediate recovery of coronary flow levels and of the reactions to AA, the blockade of AA-coronary constriction continued after GTN withdrawal. Tranylcypromine (TRC) infusion did not alter the basal coronary flow, but it produced a specific inhibition of the AA-induced coronary vasodilatation. We postulated that the blockade of the coronary constriction exerted during GTN acute tolerance would result from an inhibition of the synthesis of a constrictor metaboline (thromboxane-like substance?) formed in the coronaries through the cyclooxygenase metabolic pathway of AA.     

Comparative inhibitory activity of etoricoxib, celecoxib, and diclofenac on COX-2 versus COX-1 in healthy subjects

We determined cyclo-oxygenase-1 and cyclo-oxygenase-2 inhibition in healthy middle-aged subjects (41-65 years) randomly assigned to four 7-day treatment sequences of etoricoxib 90 mg every day, celecoxib 200 mg twice a day, diclofenac 75 mg twice a day, or placebo in a double-blind, randomized, 4-period crossover study. Maximum inhibition of thromboxane B(2) (cyclo-oxygenase-1 activity) in clotting whole blood on day 7 (0-24 hours postdose) was the primary endpoint. Inhibition of lipopolysaccharide-induced prostaglandin E(2) in whole blood (cyclo-oxygenase-2 activity) was assessed on day 7 (0-24 hours postdose) as a secondary endpoint. Diclofenac had significantly greater maximum inhibition of thromboxane B(2) versus each comparator (P < .001); placebo 2.4% (95% confidence interval: -8.7% to 12.3%), diclofenac 92.2% (91.4% to 92.9%), etoricoxib 15.5% (6.6% to 23.5%), and celecoxib 20.2% (11.5% to 28.1%). Prostaglandin E(2) synthesis was inhibited with a rank order of potency of diclofenac > etoricoxib > celecoxib. In summary, at doses commonly used in rheumatoid arthritis, diclofenac significantly inhibits both cyclo-oxygenase-1 and cyclo-oxygenase-2, whereas etoricoxib and celecoxib significantly inhibit cyclo-oxygenase-2 and do not substantially inhibit cyclo-oxygenase-1.     

选择性COX-2抑制剂NS-398对人胃腺癌细胞增殖和凋亡的影响

目的 研究环氧化酶2(COX-2)抑制剂NS-398对胃癌培养细胞系BGC-823增殖及凋亡的影响.方法 应用MTT法检测NS-398对胃癌细胞BGC-823细胞增殖的影响;应用流式细胞术、逆转录聚合酶链反应(RT-PCR)和Western blot法检测NS-398对胃癌细胞BGC-823细胞凋亡的影响.结果 NS-398随剂量的增加及作用时间延长对胃癌细胞呈抑制作用;体外NS-398能减少BGC-823细胞株COX-2的表达,对BGC-823有细胞毒作用,可增加细胞凋亡率.结论 NS-398可抑制胃癌BGC-823细胞的增殖,促进胃癌BGC-823细胞的凋亡.其可能机制是与抑制COX-2表达有关.     

Effects of NG-nitro-L-arginine methyl ester on vasodilator responses to acetylcholine, 5''-N-ethylcarboxamidoadenosine or salbutamol in conscious rats.

1. Conscious, Long Evans rats (n = 16), chronically instrumented for the measurement of regional haemodynamics were given 3 min, randomized infusions of two doses of sodium nitroprusside (1.5 and 15 micrograms min-1), acetylcholine (0.4 and 4 micrograms min-1), 5'-N-ethylcarboxamidoadenosine (NECA; 45 and 450 ng min-1), and salbutamol (24 and 240 ng min-1) in the absence and in the presence of NG-nitro-L-arginine methyl ester (L-NAME; 1 mg kg-1 h-1), a potent inhibitor of nitric oxide biosynthesis. 2. Sodium nitroprusside caused hyperaemic vasodilatation in the mesenteric, and common carotid vascular beds. These effects were enhanced in the presence of L-NAME, as was the hypotension. 3. Acetylcholine caused hyperaemic vasodilation inp6he renal, internal carotid and common carotid vascular beds. These effects were attenuated in the presence of L-NAME, but the hypotension was unaffected. 4. NECA caused hyperaemic vasodiltation in the renal, mesenteric, hindquarters, internal carotid and common carotid vascular beds. However, only the hindquarters and internal carotid responses were diminished in the presence of L-NAME and the hypotension was unchanged. 5. Salbutamol caused hyperaemic vasodilatation in the hindquarters vascular bed only. This effect was reduced in the presence of L-NAME, but the hypotension was unchanged. 6. The results indicate marked regional variations in the sensitivity of vasodilator responses to L-NAME that can depend on the vasodilator agent chosen and the dose employed. It is clear from these findings also that measurement of mean arterial blood pressure alone cannot provide adequate information on which to judge the involvement of L-NAME-sensitive mechanisms in vasodilator responses in vivo.