EFFECTS OF INHIBITION OF PROSTAGLANDIN ENDOPEROXIDE SYNTHASE-2 IN CHRONIC GASTROINTESTINAL ULCER MODELS IN RATS

1 In the stomach, prostaglandins protect the gastric mucosa against in juries. One rate-limiting step in prostaglandin synthesis is mediated by prostaglandin endoperoxide synthase (PGHS), the target enzyme of no n-steroidal anti-inflammatory drugs (NSAIDs). Two isoforms of PGHS exi st: a constitutive (PGHS-1) and an inducible (PGHS-2) enzyme. PGHS-1 i s the major source of gastric prostaglandins under physiological condi tions. Inhibition of prostaglandin synthesis by traditional NSAIDs suc h as indomethacin and diclofenac which non-selectively inhibit both PG HS-1 and PGHS-2, causes gastric and intestinal ulceration and delays g astric ulcer healing in chronic models. It has been shown that selecti ve PGHS-2 inhibitors such as L-745,337 lphonamide-6-(2,4-difluorothio- phenyl)-1-indanone) are not ulcerogenic and do not inhibit gastro-inte stinal prostaglandin synthesis. However, minimal information is availa ble on the long-term effects of PGHS-2 inhibitors on the healing of pr eviously established gastric injuries. We assessed the cellular locali zation and expression of PGHS-1 and PGHS-2 during gastric ulcer healin g and assessed the effects of L-745,337 on previously established cryo ulcers in the rat gastric stomach. 2 PGHS-1 and PGHS-2 were located an d quantified by immunohistochemistry during experimental gastric ulcer healing. PGHS-2 immunoreactivity was only negligible in the normal ga stric wall, but after gastric ulcerations, it was strongly detected in monocytes, macrophages, fibroblasts and endothelial cells below and b etween the regenerative glands. PGHS-1 immunoreactivity detected in no rmal gastric mucosa, disappeared after gastric ulceration in the mucos a adjacent to the ulcer crater. However, it reappeared in the regenera tive glands from day 5 onwards. Thus, PGHS-1 and PGHS-2 were located a t different sites and their maximal expression followed a different ti me-sequence. 3 We assessed the effects of L-745,337, indomethacin and diclofenac on gastric ulcer healing and histological healing parameter s in rats. L-745,337, indomethacin and diclofenac dose-dependently dec reased the healing of gastric ulcers. L-745,337, indomethacin and dicl ofenac decreased epithelial cell proliferation in the ulcer margin and microvessel density in the ulcer bed on day 8 and increased the thick ness of the granulation tissue below the ulcer crater and the gap betw een both edges of the muscularis mucosae on day 15. Indomethacin and d iclofenac, but not L-745,337, decreased synthesis of 6-keto-PGF(1 alph a) and PGE(2) in tissue fragments from the stomach and terminal ileum and decreased platelet thromboxane B-2 synthesis in clotting whole blo od. 4 Dose-response curves for the inhibition of chronic gastric ulcer healing by L-745,337 (administered twice daily intragastrically) show ed an ID50 value of 1.7 mg (4.3 mu mol) kg(-1). Dose-response curves f or the inhibition of PGE(2) synthesis in inflammatory exudates in the acute carrageenin sponge rat model, showed ID50 values of 1.1 mg (3.1 mu mol) kg(-1) and 1.3 (3.3 mu mol) mg kg(-1) for indomethacin and L-7 45,337, respectively. Thus, inhibition of chronic gastric ulcer healin g by L-745,337 occurs within a potentially therapeutic dose-range. 5 I n summary, PGHS-2 is markedly accumulated after gastric ulceration in monocytes, macrophages, fibroblasts and endothelial cells in regions o f maximal repair activity. Selective inhibition of PGHS-2 by L-745,337 delayed gastric ulcer healing though interference with epithelial cel l proliferation, angiogenesis and maturation of granulation tissue in a potentially therapeutic dose range. PGHS-2-derived prostaglandins se em to have an important role in gastric ulcer healing.