Review: lipoxygenase inhibitors and the gut

Inhibition of antigen-induced muscle contractions by inhibitors of thromboxane pathway in rat small intestine

Rats were sensitized against egg albumin and the response of the longitudinal muscle from the proximal small intestine to the antigen was tested. Egg albumin (1-100 micrograms/ml) concentration-dependently induced a contraction of the longitudinal muscle in tissues from sensitized animals but not from nonsensitized animals. The response to the antigen was resistant to neuronal blockers like tetrodotoxin, atropine and hexamethonium. Inhibitors of thromboxane synthesis such as the cyclooxygenase inhibitor, indomethacin, the thromboxane synthase blocker, 1-benzylimidazole, or the combined cyclooxygenase/lipoxygenase/thromboxane synthase inhibitor, sulfasalazine, inhibited the contraction evoked by egg albumin. A similar concentration-dependent inhibition of the antigen response was observed with two thromboxane A2 receptor blockers, SK&F 88046 and KW-3635. None of these blockers affected the response to the muscarinic agonist, carbachol, excluding unspecific effects of the drugs on smooth muscle contractility. The effect of antigen was reduced by the mast cell stabilizing agent, quercetin, and by the histamine H1 receptor blocker, mepyramin. These drugs, however, also inhibited the response to carbachol. When contractions were stimulated directly by the stable thromboxane derivative, carbocyclic thromboxane A2, the smooth muscle proved to be more than three orders of magnitude more sensitive to this agonist of the thromboxane pathway compared to histamine. Consequently, thromboxane A2 seems to be one of the main mediators of anaphylactically induced longitudinal muscle contractions in the rat small intestine.

Eicosanoid synthesis and Helicobacter pylori associated gastritis: increase in leukotriene C4 generation associated with H. pylori colonization

The importance of pro-inflammatory leukotriene C4 in Helicobacter pylori (H. pylori) associated gastritis in man is unknown. Fresh gastric biopsy specimens from 28 dyspeptic patients were obtained: 10 showed normal antral histology with no evidence of H. pylori, the remaining 18 patients exhibited histological gastritis and were H. pylori positive as assessed by histology, culture and urease test. Twelve of these 18 patients received 240 mg twice daily colloidal bismuth subcitrate for four weeks before re-endoscopy. Gastric biopsies from H. pylori positive patients were incubated under basal and Ca(2+)-ionophore mediated conditions: Radioimmunoassay analysis of the supernatant showed basal release of prostaglandin E2 and leukotriene C4 was slightly but not significantly elevated in H. pylori positive mucosa. However in H. pylori positive mucosa there was an 85% increase in leukotriene C4 synthesis when biopsies were incubated with ionophore, compared to only 13% increase in H. pylori negative mucosa (p less than 0.02). After eradication of H. pylori by colloidal bismuth subcitrate, there was a clearance of inflammatory cell infiltrate as assessed by histology and a significant reduction in ionophore-mediated leukotriene C4 formation compared with before treatment (p less than 0.02). These results suggest that H. pylori gastritis is associated with increased capacity to generate leukotriene C4, which may amplify the damaging effects of the bacteria on gastric mucosa.

5-lipoxygenase inhibitors and their anti-inflammatory activities

A wide variety of agents have been reported as 5-LO inhibitors. The majority of the series appear to be lipophilic reducing agents, including phenols, partially saturated aromatics, and compounds containing heteroatom-heteroatom bonds. Many of these are not selective 5-LO inhibitors, but often affect CO and other LOs as well. In vivo systemic activity for many of these has been, in general, disappointing, probably because of poor bioavailability caused by lipophilicity and metabolic instability (oxidation, and conjugation of phenolic compounds). However, topically a number of agents have shown promise for skin inflammation, with Syntex's lonapalene the most advanced of these. Most results published to date appear more disappointing in the allergy/asthma field. More excitingly, a few structural types are selective 5-LO inhibitors which have shown systemic activity in vivo and in the clinic. Abbott's zileuton (136) appears to be one of the leading compounds in this category, along with other hydroxamates such as BW-A4C (129) from Burroughs-Wellcome. Recent selective non-reducing agents such as Wyeth-Ayerst's Wy-50,295 (143) and the similar ICI compounds such as ICI 216800 (145) also hold promise. The enantiospecific effects of (106) and (145) are especially interesting for the design of new inhibitors. If compounds like these validate the hypothesis that inhibition of 5-LO will have a significant anti-inflammatory effect, a redoubling of effort throughout the industry to find second- and third-generation selective agents may be expected. Part of the difficulty in interpreting and comparing the 5-LO literature is the plethora of test methods and activity criteria. As pointed out in the introduction, inhibition of product release from cells, often stimulated with A23187, has commonly been used to demonstrate 5-LO inhibition. However, this type of assay cannot be assumed to be diagnostic for 5-LO inhibition. Only if specificity for 5-LO product generation and (ideally) activity in cell-free enzymes is also shown should mechanistic interpretations be made. Recently, a new class of compounds was found at Merck which inhibited LT biosynthesis without inhibiting 5-LO, but apparently by a novel, specific mechanism. L-655,240 (169) and L-663,536 (MK-886) (170) were both active in human ISN, with IC50 values in the low micromolar range. Both also orally inhibited GPB (< 1 mg/kg). MK-886 was effective in Ascaris-induced asthma in squirrel monkeys, in rat carrageenan pleurisy, in rat Arthus pleurisy, and (topically) in guinea-pig ear oedema induced by A23187.(ABSTRACT TRUNCATED AT 400 WORDS)