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- MATERIALS AND METHODS
The administration of traditional NSAIDs is associated with a three-fold increase in the risk of development of significant gastrointestinal complications, depending on previous risk factors.1–3 Cross-sectional studies indicate that gastric or duodenal ulcers are present in 10–25% of patients on long-term NSAIDs.2, 4, 5 Most NSAID-induced ulcers heal spontaneously without treatment, with or without discontinuation of NSAID therapy.6 However, a small proportion of the ulcers produced by NSAIDs progress to clinically serious events, such as a perforation or a bleeding ulcer.2, 7 The critical factors in the pathogenesis of these serious events are unknown, but the particular type and dose of NSAID taken, the location of the ulcer in relation to vessels and the effect of NSAIDs on platelet function may be important, as well as whether there is adequate healing of ulcers.8–11
One of the primary mechanisms responsible for the gastrointestinal toxicity produced by NSAIDs is believed to be suppression of endogenous gastric prostaglandin synthesis. Another is the ‘topical’ effect, which may involve a surface membrane phospholipid interaction and/or an effect to uncouple mitochondrial oxidative phosphorylation.12–16 With the identification17 of two distinct isoforms of cyclooxygenase (COX) responsible for prostaglandin synthesis, COX-1 and COX-2, their roles in the pathogenesis of NSAID-induced gastroduodenal ulcers have been debated. In normal gastric tissue, high levels of COX-1 are expressed, whereas COX-2 expression is undetectable.18 It has been suggested that, because COX-1 participates in mucosal defence, the inhibition of COX-1 may be responsible for NSAID-induced gastrointestinal toxicity. However, recent observations have called into question the precise role of COX-1 inhibition in this damage. COX-1 knockout mice do not develop lesions spontaneously, but do develop ulcers in response to NSAID administration, even though they have no COX-1 to inhibit.19 Also, administration of the selective COX-1 inhibitor, SC-560, reduces gastric prostaglandins20 without gastrointestinal lesions, while dual inhibition of COX-1 and COX-2 leads to gastrointestinal lesions in rats21 and mice.22
A role for COX-2 inhibition is now emerging in adaptive cytoprotection and epithelial integrity.23, 24 More importantly, following ulcer induction, high levels of COX-2 are evident in macrophages and other inflammatory cells along ulcer margins.18, 24 Selective inhibition of COX-2 has been shown to impair ulcer healing.18, 24 Therefore, it is possible that an important factor leading to serious events is impaired ulcer healing, mediated by local inhibition of COX-2.
Traditional NSAIDs have well-documented ulcerogenic properties in both animal models and humans. Celecoxib is a selective COX-2 inhibitor that has been associated with a low incidence of endoscopically determined ulcers25 and a reduction in serious outcomes.26 Nabumetone, a non-acidic pro-drug, is less specific for COX-2, but has also been associated with a very low incidence of gastroduodenal ulcers27 and infrequent complications (perforation and bleeding).28 In this study, we compared the effects of a traditional non-selective NSAID (indometacin), a COX-2-specific NSAID (celecoxib) and a pro-drug (nabumetone) on ulcer healing in the cryoprobe-induced ulcer model.
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- MATERIALS AND METHODS
We have shown that indometacin and the selective COX-2 inhibitor celecoxib delay healing of cryoprobe-induced gastric ulcers over a 6-day period. The same was evident with the high dose of nabumetone (160 mg/kg). In contrast, at a dose of 40 mg/kg, nabumetone did not delay healing of cryoprobe-induced gastric ulcers. Inhibition of ulcer healing with both celecoxib and indometacin was associated with a decrease in proliferative response at day 3 that had normalized by day 6. Only indometacin was associated with significant decreases in intestinal prostaglandin E levels.
A number of factors have been implicated in the pathogenesis of the damage induced by NSAIDs to the gastrointestinal tract. Somasundaram et al.37 suggest that one of the main mechanisms is a ‘topical’ insult which involves the uncoupling of mitochondrial oxidative phosphorylation, thereby beginning a cascade of events that can result in cellular damage and sometimes death.37, 38 Immediate local prostaglandin synthesis via the constitutive COX-1 enzyme normally enlists multiple defensive mechanisms, such as enhanced blood flow, mucus production and bicarbonate secretion, that help to defend against these aggressive forces. However, NSAID-induced inhibition of COX-1 prevents this immediate defence mechanism. Once the cellular injury has occurred and progressed to erosions and ulcers, a second phase of defensive mechanisms commences. A number of reparative processes are initiated and implicated in the healing of ulcers. At the ulcer margin, COX-2 messenger RNA is markedly increased23, 39 and epithelial cell proliferation increases.40 The high levels of COX-2 messenger RNA during the acute stages of gastric ulcer formation are believed to be related to the repair processes,23 as the COX-2 activity is localized to reparative cells.39
NSAIDs can interfere in the process of gastric repair at a number of points. Most traditional NSAIDs, such as indometacin, are weak organic acids and can initiate direct topical cellular damage following ingestion. In addition, their ability to inhibit COX-1, at concentrations present in the gastric mucosa, prevents the immediate prostaglandin defence mechanisms. Selective COX-2 inhibitors can, in theory, interfere with gastric ulcers by preventing the second phase of inflammatory-mediated repair processes.
Our studies show that indometacin and celecoxib significantly delayed ulcer healing, whereas comparable doses of nabumetone (40 mg/kg) did not, and that this correlated with the effects of the drugs on epithelial proliferation at day 3. The epithelial proliferative index was inhibited on day 3 only by indometacin and celecoxib. Previous studies33, 39 have demonstrated inhibition in epithelial proliferative activity, as assessed by the bromodeoxyuridine technique, by both non-selective COX inhibitors and by selective COX-2 inhibitors. The maximal proliferative rate appears to be early in the healing phase,39 and may explain why a difference is not seen at 6 days. In addition, it may be that the Ki-67 technique is less sensitive than the bromodeoxyuridine technique.
Indometacin decreased intestinal prostaglandins significantly, while celecoxib and nabumetone did not, suggesting that COX-1 inhibition is not the main mechanism that impairs healing of these ulcers. Furthermore, it is unlikely that the ‘topical’ effect played a major role in the delayed healing, as celecoxib and nabumetone are both non-acidic and therefore do not have this effect.41 The reason why celecoxib impaired the healing of the ulcers, while nabumetone, given at comparable anti-inflammatory doses, did not is intriguing. One possibility is that nabumetone, being a pro-drug, only inhibited gastric COX-2 in the systemic circulation after its conversion to the active component, 6-methoxy-2-naphthylacetic acid, by the liver. Celecoxib, on the other hand, being an effective COX-2 inhibitor, may have inhibited gastric COX-2 activity profoundly during absorption, when a much higher local concentration of the drug may occur, than in the systemic circulation after absorption has taken place. Alternatively, our results could be explained by a lower bioavailability of nabumetone compared with indometacin and celecoxib. However, previous studies of the metabolism of nabumetone have demonstrated that approximately 70% of a 20-mg/kg intragastric dose of nabumetone appears as 6-methoxy-2-naphthylacetic acid in the plasma of rats, suggesting that nabumetone is sufficiently bioavailable in this study at the lower dose.42 However, with administration of a very high dose (160 mg/kg), ulcer healing was delayed. It may therefore be that, at this very high dose, local concentrations of 6-methoxy-2-naphthylacetic acid, derived from the systemic circulation, increase sufficiently to inhibit COX-2 and impair ulcer healing.
Lastly, it is conceivable that delayed healing is due to the inhibition of a postulated COX-3,43 or processes that are not a result of the effect of COX inhibition during drug absorption. In this context, it is interesting that conventional NSAIDs and selective COX-2 agents impair angiogenesis,44 which is thought to play an important role in healing. It is not entirely clear whether this inhibitory effect is mediated by COX-2,45, 46 as it is only partially abolished by high prostaglandin concentrations.47 Furthermore, NSAIDs and selective COX-2 inhibitors have an antitumour action, but it is interesting that this may not be related to the COX inhibitory effects of these drugs.48, 49
One implication of our observations, namely delayed healing of pre-existing lesions as opposed to ulcerogenic effects, may be relevant to Helicobacter pylori ulcers and other inflammation within the gastrointestinal tract, such as inflammatory bowel disease. Most of such data, however, comes from experimental animals which suggests an effect on angiogenesis and/or COX-2.18, 44, 50 That conventional NSAIDs interfere with the healing of gastroduodenal ulcers33, 51 and cause relapse of inflammatory bowel disease52, 53 in humans is not in question. The mounting evidence that selective COX-2 agents delay healing of gastrointestinal inflammation and ulcers in experimental animals provides sound justification for similar studies in humans.
In conclusion, the results of these studies in the cryoprobe ulcer healing model suggest that the administration of celecoxib, a selective COX-2 inhibitor (as well as indometacin, a dual COX inhibitor), may result in significant impairment of healing of established ulcers, even though it may not initiate new ulcer formation. In contrast, nabumetone, a non-acidic pro-drug that is converted to a metabolite with both COX-1 and COX-2 inhibitory characteristics, given in similar anti-inflammatory doses, is not associated with significant impaired healing.