Gastroprotective agents for the prevention of NSAID-induced gastropathy.

Over 30 million people in the world take non-steroidal anti-inflammatory drugs (NSAID's). A large percentage of these individuals will develop gastric ulcers and related complications, a condition known as "NSAID gastropathy". NSAID gastropathy differs from classic peptic ulcer disease in many ways, and traditional peptic ulcer therapy is largely ineffective in preventing NSAID-induced gastropathy. The prostaglandin misoprostol has been shown to be effective and is approved for the prevention of NSAID gastropathy. However, misoprostol has side effects that limit its general use. For this reason, considerable effort throughout the 1990's has focused on the identification of new gastroprotective molecules. Some synthetic studies have been aimed at the preparation of new prostaglandins, prostacyclin mimetics, and thromboxane antagonists. New histamine H2 receptor antagonists have also been developed which, unlike cimetidine or ranitidine, now appear to couple true gastroprotective activity with antisecretory properties. One new H2 antagonist, ebrotidine, has shown clinical utility in preventing NSAID gastropathy. Many other types of structures (flavonoids, peptides, terpenoids, xanthines, others), as well as compounds displaying certain pharmacological actions (5-hydroxytryptamine receptor binding, adrenergic receptor binding, mast cell stabilization, others) have been linked in some way to gastroprotection. This article reviews many of these recent gastroprotection findings, with emphasis on those of potential use for prevention of NSAID gastropathy.

Flavonoid-induced gastroprotection in rats: role of blood flow and leukocyte adherence.

To elucidate the mechanisms of flavonoid-induced protection against nonsteroidal anti-inflammatory drug (indomethacin)-induced acute gastric damage, the effects of 5-methoxyflavone and 5-methoxyflavanone on the gastric vasculature were compared both in vivo (using laser Doppler flowmetry in anesthetized rats) and in vitro on rat superior mesenteric arteries. The effects of the compounds on indomethacin-induced leukocyte adherence to mesenteric venules were investigated by intravital videomicroscopy. Oral 5-methoxyflavone reduced indomethacin-induced macroscopic damage by 38 to 99% (ED50 = 5.5 mg/kg). Damage was not significantly reduced by 5-methoxyflavanone. Light microscopy studies also demonstrated a reduction in damage severity. 5-Methoxyflavone, but not 5-methoxyflavanone, increased the gastric conductance significantly. The effects on isolated mesenteric arteries correlated with the effects on in vivo conductance. Finally, indomethacin-induced leukocyte adherence was inhibited to a greater extent by 5-methoxyflavone than by 5-methoxyflavanone. In conclusion, the flavonoid 5-methoxyflavone provides gastroprotection against nonsteroidal anti-inflammatory drug-induced gastric damage. A structurally similar compound, 5-methoxyflavanone, demonstrated minimal gastroprotective activity, suggesting that the double bond of 5-methoxyflavone is required for biological activity. The finding that 5-methoxyflavone (but not 5-methoxyflavanone) significantly increased gastric vascular perfusion and reduced leukocyte adherence to mesenteric venules suggests that these mechanisms may contribute to the flavonoid's gastroprotective activity.

Synthesis and biological evaluation of substituted flavones as gastroprotective agents.

Flavone (1) was found to protect against ethanol-induced gastric damage in rats; however, it is known that certain compounds in the flavone class, including flavone itself, are inducers of hepatic drug metabolizing enzymes. With the hope of identifying gastroprotective flavones that have minimal effects on drug metabolizing enzymes, we have synthesized and evaluated selected flavone analogs. Gastroprotective potency in the ethanol model was retained by methoxy substitution in the 5-position (4) and by methoxy (12) or methyl (14) substitution in the 7-position. A number of substituted analogs of the potent molecule 5-methoxyflavone (4) were also synthesized, and in many cases, these substitutions provided gastroprotective molecules. In order to assess liver enzyme induction potential, two of the gastroprotective flavones, 7-methoxyflavone (12) and 5-methoxy-4'-fluoroflavone (26), were examined for their effect on liver microsomal cytochrome P450 and 7-ethoxyresorufin O-dealkylase (CYP1A) activity. These two compounds caused minimal changes in the cytochrome P450 concentration and were considerably less potent than beta-naphthoflavone as inducers of CYP1A enzyme activity. Furthermore, following oral administration to rats, 5-methoxy-4'-fluoroflavone (26) was found to protect against indomethacin-induced gastric damage. These results indicate that, through appropriate substitution, flavones can be obtained that are gastroprotective but have minimal effects on drug-metabolizing enzymes.

Selective irreversible inhibitors of aldose reductase.

A series of 5-substituted-1,3-dioxo-1H-benz[de]isoquinoline-2(3H)-acetic acid analogues have been examined as irreversible inhibitors of aldose reductase. The 5-alpha-bromoacetamide and 5-alpha-iodoacetamide analogues 5 and 6 gave irreversible inhibition of aldose reductase while the 5-alpha-chloroacetamide analogue 3 did not show this type of inhibition. Protection studies indicate that irreversible inhibitions are occurring at the inhibitor binding site. Comparative irreversible inhibition studies with rat lens aldose reductase (RLAR) and rat kidney aldehyde reductase (RKALR) indicate that 5-alpha-haloacetamide analogues 5 and 6 are much more effective inhibitors of RLAR.

Synthesis and application of 13C enriched phenylisothiocyanate as a 13C NMR protein probe.

Phenylisothiocyanate, enriched with 13C at the isothiocyanate carbon, has been synthesized and utilized as a 13C NMR probe of proteins for the first time. The reagent has been used to label the amino groups of oxidized glutathione, and the resulting 13C NMR spectrum shows a prominent thiocarbonyl peak after a single NMR scan. The reagent is also capable of differentiating amino groups on the insulin molecule with distinct peaks corresponding to the amino groups on the A and B chains of insulin. This study illustrates the potential of using a new 13C label to functionalize amino groups of proteins and to study the labeled proteins with 13C NMR.

Synthesis and biological evaluation of irreversible inhibitors of aldose reductase.

5-Isothiocyanatoalrestatin (1b) and 5-azidoalrestatin (1c) were prepared synthetically and examined as potential affinity and photoaffinity inhibitors of rat lens aldose reductase. Both compound 1b and 1c under appropriate conditions at 10(-4) M produced a 70% irreversible inactivation of aldose reductase within 1 min. The enzyme could, in part, be protected by preincubation with sorbinil 2, a known potent inhibitor of aldose reductase.