Interpreting the clinical significance of the differential inhibition of cyclooxygenase-1 and cyclooxygenase-2
P. Brooks,
P. Emery1,
J. F. Evans2,
H. Fenner3,
C. J. Hawkey4,
C. Patrono5,
J. Smolen6,
F. Breedveld7,
R. Day8,
M. Dougados9,
E. W. Ehrich10,
J. Gijon-Baños11,
T. K. Kvien12,
M. H. Van Rijswijk13,
T. Warner14 and
H. Zeidler15
Faculty of Health Sciences, University of Queensland, Australia,
1 Rheumatology and Rehabilitation Research Unit, Research School of Medicine, University of Leeds, UK,
2 Human Genetics Department, Merck & Co. Inc., West Point, PA, USA,
3 Swiss Federal Institute of Technology, Zurich, Switzerland,
4 Division of Gastroenterology, University Hospital, Nottingham, UK,
5 Università di Chieti `G.D'Annunzio', Cattedra di Farmacologia 1, Chieti, Italy,
6 2nd Department of Medicine, Lainz Hospital, Vienna, Austria,
7 Leiden University Hospital, Department of Rheumatology, Leiden, The Netherlands,
8 St Vincent's Hospital, Darlinghurst, NSW, Australia,
9 Institut de Rheumatologie, Hospital Cochin, Paris, France,
10 Merck & Co. Inc., Rahway, NJ, USA,
11 University Hospital, Madrid, Spain,
12 Oslo City Department of Rheumatology, Diakonhjemmet Hospital, Norway,
13 Department of Rheumatology, University Hospital, Groningen, The Netherlands,
14 Vascular Inflammation, The William Harvey Research Institute, St Bartholomew's & The Royal London School of Medicine and Dentistry, London, UK and
15 Department of Internal Medicine and Rheumatology, Medizinische Hochschule Hanover, Germany
Correspondence to:
P. Brooks, University of Queensland, Edith Cavell Building, Royal Brisbane Hospital, Herston, Qld. 4029, Australia.
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Abstract
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The International Consensus Meeting on the Mode of Action of COX-2 Inhibition (ICMMAC) brought together 17 international experts in arthritis, gastroenterology and pharmacology on 56 December 1997. The meeting was convened to provide a definition of COX-2 specificity and to consider the clinical relevance of COX-2-specific agents. These compounds are a new class of drugs that specifically inhibit the enzyme COX-2 while having no effect on COX-1 across the whole therapeutic dose range. The objectives of the meeting were to review the currently available data regarding the roles and biology of COX-1 and COX-2, and to foster a consensus definition on COX-2 specificity. At the present time, no guidelines exist for the in vitro and in vivo assessment of COX specificity, and it was felt that consensus discussion might clarify some of these issues. The meeting also reviewed recent clinical data on COX-2-specific inhibitors. The following article reflects discussion at this meeting and provides a consensus definition of COX-2-specific inhibitors.
KEY WORDS: Cyclooxygenase, COX-2-specific inhibition, COX-1, COX-2/COX-1 ratios, Osteoarthritis, Acute pain, NSAIDs, Gastrointestinal safety
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The significance of COX-1 inhibition by NSAIDs
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Mechanism of action of NSAIDs
The first convincing hypothesis for the mechanism of action of non-steroidal anti-inflammatory drugs (NSAIDs) suggested that they act through the inhibition of prostaglandin (PG) synthesis [1, 2].
All NSAIDs were believed to act by inhibiting a single enzyme [i.e. PGH-synthase; cyclooxygenase (COX)]. The COX enzyme has two catalytic sites: cyclooxygenase and peroxidase [3]. Arachidonic acid is converted to prostaglandin G2 (PGG2 ) within the COX site, which is reduced to prostaglandin H2 (PGH2 ) by the peroxidase site, before conversion to the biologically important derivatives prostaglandin E2 (PGE2 ), prostacyclin, thromboxane A2 (TXA2 ) and prostaglandin F2
(PGF2
) throughout the cell [3].
PGE2 is a major mediator of the inflammatory response and lowers nociceptor thresholds, thereby potentiating the effects of agents that cause pain (e.g. bradykinin and histamine) [1, 4, 5]. PGE2 is also a pyretic agent and contributes to the fever associated with infections [1, 4, 5]. Inhibition of the synthesis of these PGs is believed to account for the clinical efficacy of the NSAIDs.
In healthy individuals, however, prostaglandins have roles in a diverse variety of physiological functions, including protection of the gastrointestinal (GI) tract (PGE2 and PGI2 ), renal homeostasis (PGE2 and PGI2 ), vascular homeostasis (PGI2 and TXA2 ), uterine function, embryo implantation and labour (PGF2
), and regulation of the sleepwake cycle (PGD2 ) and body temperature (PGE2 ) [69]. It is believed that inhibiting production of these PGs accounts for much of the toxicity of NSAIDs, in particular their tendency to induce gastric ulceration and GI bleeding.
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New understanding of mechanism of action of NSAIDs
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COX-1 and COX-2
The existence of two different COX enzymes, COX-1 and COX-2, was proposed following the observation that the glucocorticoid, dexamethasone, could inhibit the increase in COX activity induced in macrophages, but had no effect on basal production of prostaglandins [10]. In vivo administration of dexamethasone also inhibited COX induction following in vivo administration of the mitogen, lipopolysaccharide (LPS) [10]. These observations suggested that the action of both LPS and dexamethasone on prostaglandin production could be attributed to regulation of de novo cellular COX synthesis. This second COX enzyme, COX-2, was first characterized in 1991 and it has since been established that there are differences between the two isoforms that have important physiological and therapeutic implications [1115].
Molecular biology of COX-1 and COX-2
Differences between COX-1 and COX-2 are apparent in their gene sequences. The gene structures of the two isoforms dictate that COX-2 is a highly inducible `immediate-early gene', while COX-1 is constitutively expressed [16, 17]. The differences in gene sequence between the two isoforms include:
- Translational start site and signal peptide are located in exons 1 and 2 of COX-1, but condensed into a single exon in COX-2.
- COX-2 introns are smaller than COX-1 introns [16].
- 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].
- COX-2 mRNA has multiple AUUUA instability sequences dictating rapid degradation and short half-life [19].
Protein structures of COX isoforms
COX-1 and COX-2 are homodimers of an ~71 kDa monomeric unit [20, 21]. 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].
Although COX-1 and COX-2 proteins are ~60% identical [22], there are only small differences in the amino acids lining the cyclooxygenase active site [21]. 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 a secondary internal pocket off the inhibitor binding site that is not observed in COX-1. The inhibitor binding site in COX-2 is 25% larger than that in COX-1 [24]. The secondary pocket contributes significantly to the larger volume of the inhibitor binding site in COX-2, although the central channel of the binding site is also 17% larger in COX-2 than COX-1 [21]. COX-2 has been shown to be inhibited by compounds that occupy this additional pocket [24].
Location and expression of COX-1 and COX-2
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, 2729]. 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 20-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].
Upregulation of COX-2 mRNA and protein is observed in explants of osteoarthritic (OA) cartilage and, furthermore, this upregulation coincides with superinduction of PGE2 production [34]. 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. In one study in patients with OA, COX-2 mRNA was detected in the synovial blood vessel endothelium, chondrocytes and synovial lining cells, whereas COX-1 mRNA was almost exclusively detected in the synovial lining cells [36]. Another study reported a much more restricted distribution of the two isoforms in patients with OA; COX-1 and COX-2 mRNA was found only in the synovial lining and sublining [37].
The location and pattern of expression of the two isoforms suggest that COX-1 is responsible for the production of prostaglandins critical to the autocrine/paracrine responses to circulating hormones and maintenance of gastric mucosal integrity and platelet function, whereas COX-2 is responsible for the biosynthesis of inflammatory prostaglandins (Fig. 1
) [38, 39].
COX-2 may also have a physiological role in certain tissues (Fig. 1
). 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].
NSAID safety and inhibition of COX-1
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]. In the USA, the number of deaths per year due to NSAID gastropathy is estimated to be 7600 and the number of hospitalizations is 76000/year [50]. In the UK, it has been estimated that 12000 ulcer complications and 1200 deaths/year are attributable to NSAID usage [51].
Other side-effects caused by NSAIDs include a variety of renal effects, such as fluid retention which occurs in 5% of patients [52]. 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. NSAIDs inhibit both COX-1 and COX-2 [55], and it is largely the inhibition of COX-1 which is thought to contribute to their side-effect profile [55].
The association of COX-1 inhibition with GI side-effects has led to an intensive search for COX-2-specific compounds [56]. It is hypothesized that these agents will provide the therapeutic benefits provided by NSAIDs with a superior GI safety profile.
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Assessment of COX-2 specificity
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Following the identification of COX-1 and COX-2, the degree to which NSAIDs differentially inhibit the two isoforms has been assessed. Relative inhibition of COX-1 and COX-2 has been estimated through the development of a wide variety of in vitro assay systems.
Variability of in vitro assay systems for determining COX-2 specificity
Inhibition of COX-1 and COX-2 using in vitro assay systems is conventionally expressed in terms of the IC50 of an agent (i.e. the concentration required to inhibit 50% of COX activity). Ratios of the IC50s for COX-2 and COX-1 have been calculated to assess the differential inhibition of the isoforms; a low COX-2/COX-1 ratio implies that the agent is relatively selective for COX-2.
There is, however, a large degree of variability in the calculated COX-2/COX-1 ratios. Table 2
illustrates the range of COX-2/COX-1 ratios reported in the literature for several compounds [39].
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 30-fold. The variability between assays is a consequence of many factors. One reason is the variety of species (e.g. guinea pig, bovine, human) and tissue types that are used in the assay preparations. Several other factors can also affect the absolute IC50 values of an NSAID and, therefore, the COX-2/COX-1 ratio. These include:- Incubation time (COX-2-specific inhibitors are almost exclusively time-dependent inhibitors).
- Exogenous or endogenous substrate (arachidonic acid).
- Use of whole cells or microsomes.
- Presence or absence of plasma proteins in the medium.
Most NSAIDs show time-dependent inhibition of COX in vitro and in vivo [55, 57]. A drug may inhibit COX by competing with arachidonic acid and, therefore, the arachidonate concentration can be important in determining the apparent IC50 . It is also possible that the IC50 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.
Human whole-blood assay
The ideal system should use human isoforms, whole cells, endogenous substrate, and allow testing of each isoform separately [60]. At ICMMAC, it was proposed that the human whole-blood assay developed by Patrignani et al. [57] is currently the best assay available to assess inhibition of COX-1 and COX-2.
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.g. in the presence of plasma proteins) and at therapeutically achievable drug concentrations.
- It involves clinically relevant target cells (i.e. platelets for COX-1 and monocytes for COX-2).
- It can detect COX inhibition by active metabolites.
- 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.
In the human whole-blood assay, lipopolysaccharide (LPS)-induced PGE2 production is used as a measure of COX-2 activity in circulating monocytes. Under basal conditions, COX-2 is not detectable in blood cells, therefore LPS is added to induce COX-2 expression. To conduct the assay, whole-blood samples are collected in heparinized tubes to prevent clotting. The blood is then incubated in the presence or absence of inhibitor and in the presence of LPS for 24 h at 37°C, and the plasma isolated and assayed for PGE2 [57].
Serum thromboxane B2 (TXB2 ) is predominantly derived from platelets. As platelets do not contain nuclei, they are unable to undergo COX-2 induction. Serum TXB2 following blood coagulation reflects platelet COX-1 activity. To measure whole-blood TXB2 production, blood samples in the presence or absence of inhibitor are allowed to coagulate for 60 min at 37°C. The serum is then isolated and assayed for TXB2 [57, 61].
Definition of COX-2 specificity
The variety and variability of in vitro COX isoform assays, and the unclear relationship between COX-2/COX-1 ratios obtained using in vitro assays and clinical outcomes, have led to a new proposal for assessing the effect of an agent on the COX isoforms. The definition states that if a drug inhibits COX-2, but not COX-1, across the therapeutic dose range, using the whole-blood assays, then it is COX-2 specific. This definition of COX-2 specificity has the advantage that it is independent of the variation resulting from differences between assays for COX-1 and COX-2.
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]. The assay demonstrates a correlation between the inhibitory effects of NSAIDs on gastric PGE2 synthesis and COX-1 inhibition in the whole-blood assay, but not for COX-2 inhibition in the whole-blood assay. In the assay, even NSAIDs that were more selective for COX-2 still inhibited COX-1 to such an extent so as to cause potent inhibitory effects on PGE2 synthesis at concentrations achieved in vivo [62]. It would be anticipated that, in addition to no effect on COX-1 activity in the human whole-blood assay system, a COX-2-specific 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 COX-2-specific agents have an improved safety profile. It is necessary to establish the benefits of COX-2 specificity through randomized clinical trials.
Relationship between ex vivo assays and in vivo prostaglandin production
Studies are under way to explore the relationship between ex vivo and in vivo prostaglandin inhibition for COX-2-specific compounds vis-à-vis NSAIDs. It could then be possible to relate drug concentrations within a target tissue (e.g. the joint or gastric mucosa) directly to the doseconcentration response of COX inhibition.
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COX-2-specific inhibitors
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Rofecoxib
The differential inhibition of the two COX isoforms by rofecoxib (MK-966, VIOXXTM , Merck & Co., Inc.) has been assessed in the human whole-blood assay. In healthy human volunteers, rofecoxib demonstrated potent dose- and concentration-dependent inhibition of COX-2 activity ex vivo over a 51000 mg single dose range, but did not inhibit COX-1 activity even at the highest (1000 mg) dose. In contrast, indomethacin inhibited both COX-2 and COX-1 over the 575 mg single dose range studied [63].
As discussed below, at doses of 12.5, 25 and 50 mg, MK-966 has demonstrated clinical efficacy in OA and post-surgical dental pain studies. These doses are 20- to 80-fold lower than the 1000 mg dose which showed no evidence of COX-1 inhibition. Based on these results, rofecoxib fulfils the definition of a COX-2-specific inhibitor in humans.
Celecoxib
In a purified recombinant enzyme in vitro assay system, celecoxib inhibits COX-2 [64]. Celecoxib (CelebrexTM , G. D. Searle & Co.) has been demonstrated to have no effect on whole-blood thromboxane B2 productiona measure of COX-1 activity [65]. Six healthy men received celecoxib 400 mg twice daily for 5 days and as a single dose on the sixth day. Whole-blood thromboxane B2 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.
Clinical studies of the safety and efficacy of rofecoxib and celecoxib
In an endoscopic study, rofecoxib has been shown to be similar to placebo and to produce less mucosal damage than aspirin or ibuprofen. Healthy volunteers (n=167) received rofecoxib, ibuprofen, aspirin or placebo for 7 days. Changes in gastric and duodenal mucosa were evaluated using the Lanza scale [66]. Even though the dose of rofecoxib used was 1020 times higher than a clinically effective dose, gastrointestinal damage (defined as Lanza score
2) with rofecoxib (Fig. 2
) was not statistically different from placebo (12.2% vs 8%, respectively; P>0.05) and significantly less than ibuprofen (70.6%; P<0.001) or aspirin (94.1%; P<0.001) [66].

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FIG. 2. Endoscopic evaluation of healthy volunteers following administration of rofecoxib, ibuprofen, aspirin or placebo for 7 days.
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Similarly encouraging results were reported following the administration of celecoxib (100 or 200 mg b.i.d.) to healthy volunteers (n =128). Celecoxib caused no more gastroduodenal mucosal damage (as assessed by ulcer incidence) than placebo, whereas the NSAID, naproxen (500 mg b.i.d.), caused significant upper gastrointestinal damage (Fig. 3
) [67]. 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 0.4 to 1.5 cm [68].

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FIG. 3. Endoscopic study of the gastroduodenal effects of celecoxib, naproxen or placebo administered for 7 days to healthy volunteers.
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There is no evidence that rofecoxib or celecoxib inhibit platelet COX-1 and prolong bleeding time [64, 69]. Indeed, supratherapeutic doses of celecoxib (600 mg) have been shown to have no effect on platelet aggregation or bleeding time, unlike naproxen (500 mg) which resulted in statistically significant increases in both parameters [69]. In another study, celecoxib (400 mg b.i.d.) had no effect on platelet COX-1 activity, whereas aspirin rapidly inhibited platelet aggregation [70]. Multiple dose administration of rofecoxib (375 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 500 mg, rofecoxib demonstrated similar analgesic efficacy to ibuprofen (400 mg) and was superior to placebo following third molar extraction [72]. Celecoxib has been reported to be as effective as aspirin in its analgesic effects in patients (n=200) following two or more molar extractions. Sixty per cent of patients taking placebo required rescue medication within 1 h and 90% by 3 h. In contrast, most patients receiving celecoxib or aspirin did not require rescue medication within the first hour. Furthermore, 40% of the patients receiving aspirin and 50% of patients taking celecoxib did not require rescue medication after 4 h [73].
In a 6 week, placebo-controlled, OA study (n=672), rofecoxib 12.5, 25 and 50 mg have been shown to be more effective than placebo in the treatment of OA of the knee and hip as assessed by WOMAC pain, stiffness and disability subscales (P<0.001 vs placebo) [74].
Similarly, in a 2 week study of patients (n=293) with OA of the knee, celecoxib (40, 100 or 200 mg b.i.d.) led to a greater decrease in joint pain than placebo treatment (2030% vs 12% decrease; P<0.05) throughout the study period [67].
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.
Other compounds in development
In addition to rofecoxib and celecoxib, there are a large number of compounds currently undergoing, or which have undergone, pre-clinical or early clinical assessment of their COX-2 specificity, anti-inflammatory/analgesic efficacies and GI effects, including JTE-522 (Johnson & Johnson), RS-57067 and RS-104897 (Roche Bioscience), B-367 (Chiroscience), GR25035 (Glaxo Wellcome) and FR123626 (Fujisawa) [75, 76].
COX-1/COX-2 non-specific compounds
In the assay systems used to date, all currently available NSAIDs variably inhibit both COX-1 and COX-2 in their therapeutic dose ranges [55, 60, 7679]. Indeed, most agents were developed before the existence of the two isoforms was known. Therefore, according to the definition of COX-2 specificity, these NSAIDs are COX-1/COX-2 non-specific.
Two agents that show some degree of `preferential' COX-2 inhibition have only been introduced recently (meloxicam, nimesulide) which has made it difficult to construct a convincing case for either differential efficacy or safety [56, 80]. 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, 8183]. However, even for `preferential' COX-2 inhibitors, the drug concentrations achieved after oral dosing with therapeutic doses will also result in measurable inhibition of COX-1 activity [54]. 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 20-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 4.4) compared with non-use as other NSAIDs (e.g. indomethacin, naproxen) [85].
The efficacy and safety of meloxicam have been explored in MELISSA and SELECT, two large, short-term trials [86, 87]. In the MELISSA trial (n=~10000), meloxicam and diclofenac demonstrated similar efficacy and similar numbers of perforations, ulcers and bleeds (PUBs) [86]. Within this study, there were fewer total spontaneously reported GI adverse events with meloxicam (13% vs 19%; P<0.001) [86]. In the SELECT trial, patients (n=9286) received either meloxicam (7.5 mg) or piroxicam (20 mg). Again both treatments were associated with similar relief of the symptoms of OA and incidence of PUBs (7 vs 16; P<0.1), although the overall incidence of total GI adverse events was lower in the meloxicam group (10% vs 15%; P<0.001) [87]. 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]. These results support the important move from the concept of COX selectivity to the definition of COX-2 specificity.
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 0.95%, compared with the 24% incidence reported with NSAIDs in chronic use [92]. However, this improved GI safety profile may be related to pharmacological properties other than COX-2 selectivity [93].
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Conclusions
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The degree to which an NSAID inhibits the COX isoforms in vitro depends on the experimental assay used to assess enzyme inhibition. Variables such as incubation time with the drug, the species from which the isoform has been isolated or use of exogenous vs endogenous substrate can influence the IC50 values and, therefore, affect the COX-2/COX-1 ratio.
We recommend that:
- The human whole-blood assay be used to determine COX specificity.
- If a drug inhibits COX-2, but not COX-1, across the entire therapeutic dose range, it is COX-2 specific.
All currently available NSAIDs variably inhibit both isoforms in their therapeutic dose ranges and are, therefore, COX-1/COX-2 non-specific. 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 COX-2-specific 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.
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Acknowledgments
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The authors would like to thank Professor Steven B. Abramson for his participation in the ICMMAC meeting and contribution to this publication. The ICMMAC meeting was supported by an unrestricted grant from Merck & Co. Inc., USA.
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Submitted 2 October 1998;
revised version accepted 25 February 1999.