Flavonoids with Gastroprotective Activity

Peptic ulcers are a common disorder of the entire gastrointestinal tract that occurs mainly in the stomach and the proximal duodenum. This disease is multifactorial and its treatment faces great difficulties due to the limited effectiveness and severe side effects of the currently available drugs. The use of natural products for the prevention and treatment of different pathologies is continuously expanding throughout the world. This is particularly true with regards to flavonoids, which represent a highly diverse class of secondary metabolites with potentially beneficial human health effects that is widely distributed in the plant kingdom and currently consumed in large amounts in the diet. They display several pharmacological properties in the gastroprotective area, acting as anti-secretory, cytoprotective and antioxidant agents. Besides their action as gastroprotectives, flavonoids also act in healing of gastric ulcers and additionally these polyphenolic compounds can be new alternatives for suppression or modulation of peptic ulcers associated with H. pylori. In this review, we have summarized the literature on ninety-five flavonoids with varying degrees of antiulcerogenic activity, confirming that flavonoids have a therapeutic potential for the more effective treatment of peptic ulcers.


Introduction
Peptic ulcers are a common disorder of the entire gastrointestinal tract [1]. They occur mainly in the stomach and the proximal duodenum. They can also occur in the esophagus, jejunum and gastric anastamotic site [2]. A peptic ulcer results from an imbalance between some endogenous aggressive factor(s) [hydrochloric acid, pepsin, refluxed bile, leukotrienes, reactive oxygen species (ROS)] and cytoprotective factors, which include the function of the mucus-bicarbonate barrier, surface active phospholipids, prostaglandins (PGs), mucosal blood flow, cell renewal and migration, nonenzymatic and enzymatic antioxidants and some growth factors [3][4][5][6]. The pathogenesis of gastric ulcers remains widespread, it is multifactorial disease where diverse factors such as a stressful lifestyle, alcohol consumption, use of steroidal and nonsteroidal antiinflammatory drugs (NSAIDs) and drugs which stimulate gastric acid and pepsin secretion, Helicobacter pylori infections, smoking, lower socio-economic status and family history all represent significant risk factors that may contribute to increasing gastric damage [3]. The prevention or cure of peptic ulcers is one of the most important challenges confronting medicine nowadays, as it is certainly a major human illness affecting nearly 8 to 10 % of the global population [7], and of these 5% suffer from gastric ulcers [3]. The prevalence of this disease is higher in men than in women [8].
Although recent advances in our understanding have highlighted the multifactorial pathogenesis of peptic ulcers, secretion of gastric acid is still recognized as a central component of this disease, therefore the main therapeutic target is the control of this secretion using antacids, H 2 receptor blockers like ranitidine, famotidine, anticholinergics like pirenzepin, telezipine or proton pump blockers like omeprazole, lansoprazole, etc. [9]. However, gastric ulcer therapy faces nowadays a major drawback because most of the drugs currently available in the market show limited efficacy against gastric diseases and are often associated with severe side effects [10,11].
In this context, the use of medicinal plants is in continuous expansion all over the world for the prevention and treatment of different pathologies, and natural products are recovering space and importance in the pharmaceutical industry as inspiring sources of new potentially bioactive molecules [12]. Clinical research has confirmed the efficacy of several plants for the treatment of gastroduodenal diseases [13,14]. The medicinal properties of many plants are attributed mainly to the presence of flavonoids, but they may be also influenced by other organic and inorganic compounds such as coumarins, alkaloids, terpenoids, tannins, phenolic acids and antioxidant micronutrients, e.g., Cu, Mn, Zn [15,16].
Flavonoids represent a highly diverse class of secondary metabolites comprising about 9,000 structures that have been identified to date. They constitute the largest and most important group of polyphenolic compounds in plants. These compounds are found in all vascular plants as well as in some mosses [17,18]. The term flavonoid is used to describe plant pigments, mostly derived from benzo-γ-pyrone, which is synonymous with chromone (rings A and C in Figure 1) [19,20]. All flavonoids derive their 15-carbon skeletons (C6-C3-C6) from two basic metabolites, malonyl-CoA and p-coumaroyl-CoA. Their crucial biosynthetic reaction is the condensation of three molecules of malonyl-CoA with one molecule of p-coumaroyl-CoA to give a chalcone intermediate [21]. Chalcones act as the precursors for the vast range of flavonoid derivatives found throughout the plant kingdom. Most contain a six-membered heterocyclic ring, formed by Michael-type nucleophilic attack of a phenol group on to the unsaturated ketone giving a flavanone [22]. The first committed step of the flavonoid pathway is catalyzed by chalcone synthase (CHS; see Scheme 1). Chalcones can then be converted into aurones, a subclass of flavonoids found in certain plant species. Beyond CHS, the next step shared by most of the flavonoid biosynthesis pathways is catalyzed by chalcone isomerase (CHI), which catalyzes a stereospecific ring closure isomerization step to form the 2S-flavanones. The flavanones may represent the most important branching point in flavonoid metabolism, because isomerization of these compounds yields the others class of flavonoids [23]. However, the chemical synthesis is carried out mostly by cyclization and condensation of hydroxyacetophenone.
Flavonoids belong to the recently popular phytochemicals, chemicals derived from plant material with potentially beneficial effects on human health. The therapeutic effects of many traditional medicines may be related in many cases to the presence of these polyphenols [28]. For example, a wide variety of pharmacological activities have been reported for these substances, including antiviral [29], antiallergic [30], antiplatelet [31], antiestrogenic, anticancerogenic, anti-inflammatory, antiproliferative, antiangiogenic, and antioxidant properties, and their ingestion typically produces no or very little toxicity [24]. Flavonoids were also reported to act in the gastrointestinal tract, having antispasmodic [32], anti-secretory, antidiarrhoeal [33] and antiulcer properties [34]. Considering the important role of flavonoids in the prevention or reduction of gastric lesions induced by different ulcerogenic agents, this aim of this study was to review the literature on flavonoids with gastroprotective activity. The search was carried out on Pubmed, Schifinder School, Sciency Direct and NAPRALERT (Acronym for Natural Products ALERT) the data bank of The University of Illinois in Chicago, updated to December 2007, using "anti-ulcer flavonoids" as the search term. The references found in the search were later consulted for details on the models or mechanism based bioassays used for testing flavonoids against peptic ulcers.

Flavonoids studied in models that investigate anti-ulcer activity
In this literature review, it was possible to identify ninety-five flavonoids, whose gastroprotective activities cover a full range from inactive through weak activity to active and even strong activity. Of the flavonoids found in this study, forty-two were reportedly inactive; however, this inactivity could vary widely according to the experimental model, animal, route of administration and the dose. For example, flavonols like kaempferol, robinin and dactailin showed no gastroprotective effect in experimental models of reserpine [35,36] and restraint stress-induced ulcers in mouse [35], but kaempferol at doses of 50 and 100 mg/kg showed gastroprotective activity, and when the dose was increased to 250 mg/kg, it showed no activity [37]. Similar results were found for nobeletin, a flavone, where doses of 8 and 25 mg/kg protect the gastric mucosa of the rats from injuries induced by ethanol and HCl/ethanol, respectively, but it was only weakly active at a dose of 50 mg/kg in model of aspirininduced ulcers [38]. Although many of the pharmacological and biochemical actions of flavonoids are attributed to their activities as antioxidants [39], this observed inactivity in high doses may be related to the capacity of flavonoids to act as pro-oxidants. Thus, flavonoids like quercetin, myricetin and kaempferol induce a concentration-dependent decrease of both the nuclear glutathione (GSH) content and glutathione S-transferase (GST) activity in a model system of isolated rat liver nuclei, which could lead to oxidative DNA damage [40], which in turn may be responsible for their mutagenicity and carcinogenicity; this effect may be explained by the pro-oxidant effects of this compounds [40,41]. Nevertheless, the structural features that might determine the pro-oxidant activity of these compounds are not well established.
Chalcones belong to flavonoid class with the largest number of compounds with gastroprotective activity. In this review were found thirty-eight, among which we can mention sophoradin, an isoprenyl chalcone, which is present in a Chinese crude drug (the root of Sophora subprostrata) and protects the gastric mucosa from lesions induced by pylorus-ligation and water-immersion stress [42,43]. Thirty sophoradin analogs have shown anti-ulcer effects in the same ulcer induction models. Several chalcones, all having more than one isoprenyloxyl group, exhibited high inhibitory ratios. In particular, 2',4'-dihydroxy-3'-(3-methyl-2-butenyl)-4-(3-methyl-2-butenyloxy) chalcone, 2'-hydroxy-4,4'-bis(3-methyl-2-butenyloxy) chalcone and 2'-carboxymethoxy-4,4'-bis(3-methyl-2-butenyloxy) chalcone (sofalcone), showed strong activity at a dose of 100 mg/kg, with a high percentage of inhibition of lesions (70-100%), when compared to other chalcones at the same dose and were as potent as sophoradin [42]. Sofalcone is one of these analogs that in addition to its gastroprotective effects also accelerates ulcer healing [44]. The mechanisms of action involved in gastric protection are increased gastric blood flow, stimulated synthesis of mucosubstances of the gastric mucosa [45] and increasing effects on gastric tissue PGs contents [46]. Besides its cytoprotective effects, sofalcone has a direct bactericidal effect on H. pylori, with a minimum inhibitory concentration of 55-222 µmol/L, anti-urease activity and it reduces the adhesion of this organism to gastric epithelial cells [47,48]. When outpatients with peptic ulcers and H. pylori infections were medicated for 7 d with sofalcone (100 mg thrice daily) plus the triple therapy with rabeprazole (10 mg twice daily), clarithromycin (200 mg twice daily) and amoxicillin (750 mg twice daily), sofalcone significantly increased the cure rate of H. pylori infections [49]. Therefore flavonoids can be utilized as alternative or additive agents to the current therapy in treatment of peptic ulcer induced by H. pylori infection.
Another flavonoid that appears to exert anti-ulcer activity is monomeric leucocyanidin, a natural flavonoid and the major component present in unripe plantain banana (Musa sapientum L. var. paradisiaca). It and its synthetic analogues hydroxyethylated leucocyanidin and tetrallylleucocyanidin showed protective effects against aspirin-induced gastric erosions in a prophylactic animal model, as shown by the absence of mucosal damage and a significant reduction in the ulcer index, when added to the diet at 5 mg and 15 mg per day [50,51]. The authors concluded that these compounds may be responsible for the displayed anti-ulcer properties and they suggested that the mechanism by which the active agent present in plantain banana and its synthetic analogues protects the mucosa is mediated, at least in part, by an increase in mucus thickness [51].
Another polyphenolic compound with relevant activities is garcinol, a polyisoprenylated benzophenone derivative from Garcinia indica, which shows potent free radical scavenging activity in three kinds of free radical generating systems. In the hypoxanthine/xanthine oxidase system, emulsified garcinol suppressed superoxide anion to almost the same extent as DL-α-tocopherol by weight and also suppressed hydroxyl radical more strongly than DL-α-tocopherol in the Fenton reaction system. In the H 2 O 2 /NaOH/DMSO system, this compound suppressed superoxide anion, hydroxyl radical, and methyl radical. Orally administered garcinol prevented acute ulceration in rats induced by indomethacin (40-200 mg/kg) and water immersion stress (200 mg/kg) caused by radical formation. These results suggested that garcinol might have potential as a free radical scavenger and clinical applications as an anti-ulcer drug. Although the mechanism of its anti-ulcer activity is not yet understood, garcinol may scavenge reactive oxygen species on the surface of gastric mucosa, thus protecting cells from injury [52].
A flavonoid that has been studied in some detail is rutin (quercetin-3-rhamnosylglucoside), a natural flavone derivative. It has been reported to prevent gastric mucosal ulceration in animal models including reserpine [35], acidified ethanol [37] and absolute and 50% ethanol [34,37]. The cytoprotective effect of this flavonoid does not appear to be mediated by endogenous prostaglandins [53], but its protective effects may be mediated by endogenous platelet-activating factor (PAF), since it inhibited dose-dependently the mucosal content of PAF [37]. Another possible mechanism involves the antioxidant properties of rutin, which at a dose of 200 mg/kg has a protective effect against lesions induced by 50 % ethanol, probably by reducing the levels of lipoperoxides and increasing the activity of the antioxidant enzyme glutathione peroxidase (GSH-Px). However, no significant modifications were observed in the gastric non-protein sulfhydryl (SH) content or in the ethanol-induced leukocyte infiltrate [34].

Conclusions
Flavonoids represent a highly diverse class of secondary metabolites with potentially beneficial effects on human health. These compounds protect the gastrointestinal mucosa from lesions produced by various experimental ulcer models and against different necrotic agents. Several mechanisms of action may be involved in this protective effect. Quercetin has an anti-secretory mechanism of action. This flavonol has antihistaminic properties, thus, decreases histamine levels, as well as preventing the release of histamine from gastric mast cells and inhibiting the gastric H+/K+ proton pump, diminishing acid gastric secretion. On the other hand chalcones, in particular those with more than one isoprenyloxyl group, possess cytoprotective effects, which increase the mucosal blood flow, stimulate the synthesis of mucosubstances in the gastric mucosa and increase PGs levels. However, the most important mechanism of action responsible for the anti-ulcer activity of flavonoids is their antioxidant properties, seen in garcinol, rutin and quercetin, which involves free radical scavenging, transition metal ions chelation, inhibition of oxidizing enzymes, increase of proteic and nonproteic antioxidants and reduction of lipid peroxidation. These effects are correlated with presence in the structures of an o-dihydroxy in the ring B (catechol), and additionally a 2,3 double bond in conjugation with a 4-oxo function, as well as the presence hydroxyl groups in positions 3, 5 and 7. Besides the gastroprotective activity, sofalcone (a chalcone), quercetin and naringenin (flavanones) accelerate the healing of gastric ulcers. In addition, the two first polyphenolic compounds have anti-H. pylori activity and may be utilized as an alternative or additive agent to the current therapy. Therefore flavonoids could have an ideal more effective and less toxic therapeutic potential for the treatment of gastrointestinal diseases, particularly for peptic ulcers.