What is the difference between cox1 and cox2 inhibitors

COX-2 has a TATA box and a number of transcriptional elements that are common in highly regulated genes and can be activated by pro-inflammatory mediators [ 16 , 18 ]. Each dimer has three independent folding units: an epidermal growth factor-like domain, a membrane-binding domain and an enzymatic domain [ 20 ]. Within the enzymatic domain, there is the peroxidase catalytic site and a separate, but adjacent, site for COX activity at the apex of a long, hydrophobic channel [ 20 ].

The isoforms also have similar mechanisms for the metabolism of arachidonic acid [ 23 ]. There are a number of structural differences between COX-1 and COX-2, some of which may contribute to inhibitor specificity [ 21 , 24 ]. The most significant difference between the two isoforms is the size and shape of the inhibitor binding sites within the COX active site. COX-2 has been shown to be inhibited by compounds that occupy this additional pocket [ 24 ]. An important difference between COX enzymes is in their expression and induction patterns Table 1 [ 25 , 26 ].

Tissue localization studies under basal physiological conditions have found expression of COX-1 in virtually all tissues, whereas COX-2 appears to be restricted to the kidney, brain, testicles and tracheal epithelial cells [ 8 , 9 , 17 , 25 , 27 — 29 ]. Although COX-1 predominates in the gut [ 25 ], a certain amount of COX-2 has also been detected in the surface mucous cells of rats [ 30 ] and in human small intestine [ 27 ].

The COX enzymes show distinct patterns of induction Table 1. COX-2 can be upregulated fold in macrophages, monocytes, synoviocytes, chondrocytes, fibroblasts and endothelial cells by various stimuli during the inflammatory process [ 13 , 17 , 22 , 31 , 32 ].

In contrast, COX-1 activity is unaffected or increased only marginally 2- to 4-fold [ 33 ]. COX-2 has been detected in the synovial tissue of patients with OA, but not in synovial tissue from normal patients [ 35 ].

The distribution of the two isoforms within joints has been investigated in patients with arthritis. COX-2 may also have a physiological role in certain tissues Fig. The prostaglandins produced by COX-2 may be involved in signalling in the brain [ 43 ], renal perfusion and glomerular haemodynamics [ 9 ], uterine function [ 44 ], responses to shear stress in the vasculature [ 45 ] and the physiology of embryonic membranes [ 46 ].

Treatment with NSAIDs is associated with adverse effects, and because of the number of prescriptions issued, there are many episodes [ 47 ]. The most common of these adverse effects involves the GI tract. As a class, the NSAIDs represent a major risk for morbidity and mortality from GI disturbances, perforation, ulcers and bleeding, and consequently represent a significant socio-economic burden [ 48 , 49 ].

There have also been case reports that suggested a causal relationship between the use of NSAIDs and the onset of congestive heart failure [ 53 ]. NSAID inhibition of prostaglandin biosynthesis can also interfere with platelet function and result in bleeding complications [ 54 ]. An increasing body of clinical evidence has established that inhibition of COX-1 results in adverse events such as GI irritation and damage, platelet dysfunction and bronchospasm.

Relative inhibition of COX-1 and COX-2 has been estimated through the development of a wide variety of in vitro assay systems. The results obtained for meloxicam in various test systems illustrate how the values can vary for a single agent Table 3.

Thus, depending on the assay, this agent's selectivity for COX-2 can vary fold. The variability between assays is a consequence of many factors. One reason is the variety of species e. These include:. Incubation time COXspecific inhibitors are almost exclusively time-dependent inhibitors.

A drug may inhibit COX by competing with arachidonic acid and, therefore, the arachidonate concentration can be important in determining the apparent IC It is also possible that the IC 50 of the drug may be higher than the plasma concentrations of the drug that are achieved clinically. Protein binding of test agents may be very different and there may be large differences in the concentration of free drug between in vitro and in vivo conditions [ 58 ]. Assays that use protein-free solutions are not representative of the environment in vivo [ 58 ].

Therefore, discrepancies observed between assays for a particular NSAID can be accounted for by differences in methodology involving one or more of the previously mentioned variables [ 56 ].

It is recommended that results from in vitro be used only as a guide to the relative in vivo selectivity of different NSAIDs studied in the same assay system. Valid comparisons cannot be made between studies using different assay systems [ 56 , 59 ].

There is, therefore, a need for a single assay system to be identified which allows direct comparison between compounds. The ideal system should use human isoforms, whole cells, endogenous substrate, and allow testing of each isoform separately [ 60 ].

The properties that make it the most appropriate system available include the following:. It can monitor the biochemical efficacy of cyclooxygenase inhibitors in blood samples obtained from patients or subjects following administration of test drugs ex vivo. Thus, inhibition can be assessed under physiologically relevant conditions e. It involves clinically relevant target cells i.

It is easily performed with limited sample manipulation, which facilitates standardization. Prostanoids are synthesized from endogenous arachidonic acid, thereby removing variation that may arise from adding exogenous substrate. It was, therefore, agreed by the ICMMAC participants that the human whole-blood assay system should be adopted as the standard method for assessing differential inhibition of COX isoforms.

To conduct the assay, whole-blood samples are collected in heparinized tubes to prevent clotting. As platelets do not contain nuclei, they are unable to undergo COX-2 induction. The serum is then isolated and assayed for TXB 2 [ 57 , 61 ]. The whole-blood assay for COX-1 inhibition, which assesses an agent's action on COX-1 in platelets, does not absolutely exclude inhibition of COX-1 at other sites such as the gastric mucosa.

It is important, therefore, that COX-1 inhibition in other tissues throughout the body is assessed. The standardization of in vitro or ex vivo assays for these sites is difficult, although an assay that employs gastric biopsies to measure COX inhibition in the gastric mucosa has recently been developed [ 62 ].

It would be anticipated that, in addition to no effect on COX-1 activity in the human whole-blood assay system, a COXspecific inhibitor would not inhibit COX-1 in the gastric mucosa and at other relevant sites across the therapeutic dose range. The definition of COX-2 specificity serves as a means to differentiate agents on pharmacodynamic grounds, but does not necessarily imply that COXspecific agents have an improved safety profile.

It is necessary to establish the benefits of COX-2 specificity through randomized clinical trials. It could then be possible to relate drug concentrations within a target tissue e. In healthy human volunteers, rofecoxib demonstrated potent dose- and concentration-dependent inhibition of COX-2 activity ex vivo over a 5— mg single dose range, but did not inhibit COX-1 activity even at the highest mg dose.

As discussed below, at doses of These doses are to fold lower than the mg dose which showed no evidence of COX-1 inhibition. Based on these results, rofecoxib fulfils the definition of a COXspecific inhibitor in humans.

In a purified recombinant enzyme in vitro assay system, celecoxib inhibits COX-2 [ 64 ]. Six healthy men received celecoxib mg twice daily for 5 days and as a single dose on the sixth day.

Whole-blood thromboxane B 2 levels were determined 90 min before, and 2, 4 and 12 h after the last dose on day 6. Furthermore, as discussed below, celecoxib, at or below these doses, has been shown to be effective in OA and in dental pain studies. These data demonstrate that celecoxib also fulfils the definition of COX-2 specificity in man.

In an endoscopic study, rofecoxib has been shown to be similar to placebo and to produce less mucosal damage than aspirin or ibuprofen. Changes in gastric and duodenal mucosa were evaluated using the Lanza scale [ 66 ].

Similarly encouraging results were reported following the administration of celecoxib or mg b. Celecoxib caused no more gastroduodenal mucosal damage as assessed by ulcer incidence than placebo, whereas the NSAID, naproxen mg b.

Moreover, no gastric or duodenal ulcers developed in patients receiving placebo or celecoxib, while six patients receiving naproxen developed a total of nine gastric ulcers, ranging in size from 0. There is no evidence that rofecoxib or celecoxib inhibit platelet COX-1 and prolong bleeding time [ 64 , 69 ]. Indeed, supratherapeutic doses of celecoxib mg have been shown to have no effect on platelet aggregation or bleeding time, unlike naproxen mg which resulted in statistically significant increases in both parameters [ 69 ].

In another study, celecoxib mg b. Multiple dose administration of rofecoxib mg likewise had no effect on bleeding time [ 71 ]. Rofecoxib and celecoxib both demonstrated short-term analgesic efficacy [ 72 , 73 ]. At a dose of 50 or mg, rofecoxib demonstrated similar analgesic efficacy to ibuprofen mg and was superior to placebo following third molar extraction [ 72 ]. In contrast, most patients receiving celecoxib or aspirin did not require rescue medication within the first hour.

Unexpected adverse events with either celecoxib or rofecoxib at therapeutic doses have not been reported. The potential effects of longer term COX-2 inhibition are currently being assessed in larger and longer term trials.

Indeed, most agents were developed before the existence of the two isoforms was known. It has been suggested that these agents, which exhibit greater selectivity for COX-2, will exhibit a superior GI safety profile compared with NSAIDs that are non-selective [ 56 , 81 — 83 ].

Therefore, it has been suggested that preferential inhibition of COX-2 is unlikely to lead to decreased side-effects as there is still significant inhibition of COX-1 at therapeutic doses [ 60 , 64 ]. Nimesulide shows a fold selectivity in vitro for COX-2 in the human whole-blood assays [ 84 ]. Despite this selectivity, COX-1 inhibition is demonstrable within the clinical dose range.

It is, therefore, not surprising that nimesulide is associated with the same relative risk of serious upper GI bleeding relative risk 4. Furthermore, PUBs have been reported in association with long-term meloxicam use [ 88 , 89 ]. The data with nimesulide and meloxicam thus suggest that the improvement in GI safety risk of PUBs does not reflect the COX selectivity demonstrated in in vitro studies [ 82 ].

The active metabolite of nabumetone, 6-MNA, has been reported as being between 0 and 7-fold more selective for COX-2 than COX-1, depending on which assay system has been used [ 55 , 57 , 77 , 90 ].

Using the whole-blood assay system in human volunteers, nabumetone shows no evidence of COX-2 selectivity [ 91 ]. Limited studies suggest an incidence of gastric perforations and bleeds of up to 0. However, this improved GI safety profile may be related to pharmacological properties other than COX-2 selectivity [ 93 ]. In contrast, the newly developed agents, rofecoxib and celecoxib, are COX-2 specific. In pre-clinical and clinical studies, the lack of COX-1 inhibition at therapeutic doses achieved by COXspecific inhibitors was associated with clinical efficacy and improved GI safety.

However, the clinical profile of these inhibitors has to be defined and awaits the results of more extensive clinical trials, and post-market surveillance.

Endoscopic evaluation of healthy volunteers following administration of rofecoxib, ibuprofen, aspirin or placebo for 7 days.

Endoscopic study of the gastroduodenal effects of celecoxib, naproxen or placebo administered for 7 days to healthy volunteers. The authors would like to thank Professor Steven B. Vane JR. Inhibition of prostaglandin synthesis as a mechanism of action for the aspirin-like drugs.

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Transcriptional regulation of the rat prostaglandin endoperoxide synthase 2 gene in granulosa cells. J Biol Chem ; : —8. Outcomes data for celecoxib have also been published although differences from the combined comparator agents diclofenac and ibuprofen did not reach statistical significance.

Use of aspirin in the class study has shown that the benefits of COX-2 inhibitors may be reduced by aspirin use. Abstract By inhibiting prostaglandin synthesis, non-steroidal anti-inflammatory drugs NSAIDs cause mucosal damage, ulceration and ulcer complication throughout the gastrointestinal tract. COX-2 has been identified in fibroblasts, chondrocytes, endothelial cells, macrophages, and mesangial cells.

COX-2 is induced by exposure to various cytokines, mitogens and endotoxin, and it is up-regulated at inflammation sites. Unfortunately, this classification is now determined to be too simplistic to explain the roles of the different forms of cyclooxygenase. It now appears that COX-2 can be produced constitutively in the brain, spinal cord, kidney, lung, and other tissues.

It has even been found near gastric ulcers, and may play a role in ulcer healing. COX-2 is also involved in cellular processes including gene expression, differentiation, mitogenesis, apoptosis, bone modelling, wound healing and neoplasia. A new isoform, COX-3, has also been discovered in the brain. It appears to be a derivative of COX-1, though its function has yet to be elucidated. The prostaglandins produced in the gastrointestinal tract and the kidney that maintain mucosal integrity in the upper GI tract and renal perfusion appear to be derived from COX It is suggested that COX-2 selective NSAIDs would suppress prostaglandin synthesis at sites of inflammation but would spare constitutive prostaglandin synthesis in the GI tract and kidney.

This classification is based on inhibitory concentration IC ratios. NSAIDs act as analgesics by inhibiting COX and preventing the production of prostaglandins that sensitize the afferent nociceptors at peripheral sites of inflammation. However, there is increasing evidence that some NSAIDs have a central mechanism of action for analgesia and act synergistically with opioids. Recent work has shown that the analgesic effect of flunixin in a sheep foot rot model of pain is reversed by the administration of an opiate antagonist, naloxone.

To further complicate our understanding of their analgesic action, work with the specific enantiomers of some NSAIDs have shown the "S" enantiomers to have good cyclooxygenase inhibitory effects, while the "R" forms can have weak activity against cyclooxygenase yet still produce analgesia. The adverse effects of the NSAIDs are related to cyclooxygenase inhibition in tissues where prostaglandins are beneficial and protective. Reduction in protective prostaglandins results in blood vessels constriction and tissue necrosis in the kidney and reduction in blood flow and protective mucus production in the gastrointestinal tract results in ulcers.

NSAIDs have a higher incidence of toxicity in neonates because kidney function is not fully developed. When indicated in neonates, they should be administered at the lowest possible doses.

NSAIDs should be administered very cautiously to dehydrated animals. As they predominately distribute in extracellular water, plasma concentrations will be greater than normal in the dehydrated animal and more likely to cause toxicity. As the cat is deficient in the glucuronidation enzyme system, drugs like aspirin and acetaminophen that are eliminated by this route have prolonged elimination half-lives and high potential for toxicity.

NSAIDs such as ketoprofen that are cleared by alternate pathways can be safely used in cats. Glucuronidation also results in significant enterohepatic recirculation. This tends to be greater in dogs than in other species, such as humans. GI prostaglandins are natural inhibitors of gastric acid secretion and support mucosal blood flow. NSAID inhibition of prostaglandin biosynthesis results in increased acidity and decreases mucosal blood flow and mucous production, leading to ulcer formation.

NSAIDs should not be used in conjunction with glucocorticoids as they potentiate gastrointestinal toxicity. Misoprostol, an orally administered synthetic prostaglandin E, may be beneficial in patients at risk for ulceration.