NON-STEROIDAL ANTI-INFLAMMATORY DRUGS ( NSAIDS ) , FREE RADICALS AND REACTIVE OXYGEN SPECIES ( ROS ) : A REVIEW OF LITERATURE

A wide controversy exists regarding the effect of NSAIDs on the oxidative status. Each member of these NSAIDs has been shown to act both as antioxidant or pro-oxidant in different test systems, using different concentrations and various oxidative stress-inducing agents. This review tackles this problem and tries to look for factors that might be responsible for this variation in results.


INTRODUCTION
he mechanism behind the great interindividual variation in response to nonsteroidal anti-inflammatory drugs (NSAIDs) and also behind their recent uses in chemoprevention of cancer is still poorly understood.One possible explanation might be through their effect on free radicals and reactive oxygen species and, thus, on the oxidative status. [1]Free radicals can be generated by oxidizing or reducing reactions.A free radical is defined as any species capable of independent existence that contains one or more unpaired electrons. [1]By this definition, oxygen which has two unpaired electrons is a free radical, whereas hydrogen peroxide (H 2 O 2 ), which has no unpaired electrons, is not a free radical, but classified as reactive oxygen species (ROS). [2]xcessive reactive species can deplete the endogenous antioxidant system, giving rise to a condition known as oxidative stress.During the condition of oxidative stress, the generation of free radicals can have several consequences.Free radicals can react with and cause damage to DNA, lipids and proteins. 3Examples of free radicals and ROS include: superoxide anion (O ‾ 2 ), hydroxyl radical (OH • ), hydrogen peroxide (H 2 O 2 ), nitric oxide and nitric dioxide, and peroxynitrite. [3]ological sources of free radicals and ROS Many reactive species produced by biological system are intermediate products of several enzymatic and non-enzymatic reactions that are beneficial to the body.ROS and reactive nitrogen species (RNS) are formed during normal physiological processes that occur when the cell is not under stress. [4]The endogenous sources for ROS production in human and in animals may be as follows: [5] •Mitochondrial respiratory chain (O ‾ 2).

•Inflammation and phagocytosis (O ‾ 2, OH • , H2O2, HOCl). •Xanthine oxidase (O ‾ 2). •Vascular NAD(P)H oxidase (O ‾ 2). •Cycloxygenase (LOO • ). •Free iron and copper as metal ions (OH • ). •Reaction between O ‾ 2 and NO to yield peroxinitrites. •Reaction between H2O2 and peroxinitrites (singlet oxygen). •Auto-oxidation of catecholamine. •Ischemia/reperfusion (ROS and RNS). •Prolonged severe emotional stress (ROS and RNS).
Classification of antioxidant defences Cells have complex antioxidant systems and chemical sequesters that help prevent oxidative damage caused by high free radical concentrations.These defence systems are classified as endogenous (e.g.] The role of metal ions in the induction of oxidative stress Several metals, in their reduced forms, can catalyze the production of radicals from H 2 O 2 via Fenton reaction.The transition metals of interest are those that have variable valence which allow them to undergo changes in oxidation state involving one electron.[11] Fenton reaction occurs when a mixture of H 2 O 2 and ferrous salt (Fe +2 ) reacts with many organic molecules, as was first observed by Fenton in 1894.The reactivity is most likely due to formation of the hydroxyl radical (OH•). [11]

Non-Steroidal Anti-inflammatory Drugs (NSAIDs) and Oxidative Stress
In the literature, it was surprising to find a great controversy on the effect of NSAIDs on the oxidative status.They, sometimes, act as oxidants and in other instances as antioxidants.

NSAIDs as antioxidants
There are many examples in which NSAIDs act as antioxidants at different clinical and laboratory settings.In one study, ibuprofen was tested on iron-mediated oxidation of LDL. [12]he authors had shown that the properties of ibuprofen are due both to inhibition of OH• generation in Fenton reaction and to the chelation of iron ions.Zapolska-Downar and Naruszewicz [13] found that ibuprofen exerted effects such as inhibition of adhesion and transendothelial migration of leukocytes, suppressing intracellular production of reactive oxygen species and oxidative modification of LDL.In a recent study, Dabhi et al [14] concluded that ibuprofen showed antihyperlipidemic, antiatherosclerotic, antioxidant, antiinflammatory and non-ulcerogenic activity in atherosclerotic animals.Finally, Dokmeci et al [15] confirmed the protective effects of ibuprofen in the experimental testicular torsion/detorsion model in rats due to its antiinflammatory and antioxidant effects.Celecoxib, a COX-2 selective inhibitor, was shown to have the ability to inhibit lipid peroxidation (induced by ferrous salts in rat liver whole homogenate) and to scavenge hydroxyl radicals. [16]The exact mechanism by which celecoxib inhibited radical generation is not known.It may be due to direct scavenging of the peroxyl radicals or by donating reducing equivalents to the peroxyl radicals. [16]After oral administration, aspirin is rapidly converted to salicylic acid. [17]Salicylic acid can decrease oxidative stress and pro-inflammatory and potentially neoplastic prostaglandins with a concomitant increase in glutathione peroxidase activity.Salicylic acid may contribute to the protective effect of aspirin against colon cancer. [18][21][22] Acute ethanol intoxication produces widely studied hepatic metabolic disturbances, like limited gluconeogenesis, accumulation in triacylglycerides and reduction in protein synthesis. [23]Of practical interest is the finding that several NSAIDs administered to rats simultaneously with ethanol diminished some indicators of liver damage promoted by this hepatotoxic compound.[26] It has been shown that nonsteroidal anti-inflammatory drugs (NSAIDs) (aspirin, ibuprofen and naproxen) appeared to delay the onset and slow the progression of Alzheimer's disease.NSAIDs are reported to neutralize the destructive hydroxyl free radicals.In the human body, this could prevent radicalmediated neurotoxic cell death and hinder the formation of neuritic plaques in the brain. [27]ntwerpen et al [28] found that oxicams are more reactive against reactive oxygen species than nimesulide and ibuprofen.This is not so pronounced for OH • scavenging and the absence of interaction with H 2 O 2 brings no further information concerning this reactive oxygen species.More interesting is the scavenging of HOCl since a substantial difference was observed between the oxicams and nimesulide.The importance of enol groups, inhibition of polymorphonuclear leukocyte migration, decreased oxygen radical production, and inhibition of lymphocyte function are suggested mechanisms. [29]Cyclooxygenase (COX) and lipoxygenase (LOX) pathways cause generation of reactive oxygen species (peroxides and superoxide anions) and peroxides, themselves, can trigger COX and LOX activity. [30]rostaglandins are also involved in the activation of NADP(H) oxidase in neutrophils. [31]The results of Chakraborty et al [32] indicated that diclofenac sodium, ibuprofen, flurbiprofen, paracetamol (although it is an analgesic but not a NSAID), nimesulide and celecoxib exerted mild antioxidant activity.
In the study of Kadiiskaa et al [33] , after induction of oxidant stress in rats with CCl4, lipid peroxidation products measured in plasma and urine demonstrated that isoprostanes and MDA can be partially inhibited to different extents by cyclooxygenase inhibitors (indometacin and meclofenac).The lowering of isoprostane and MDA formation may be the result of the suppression of nonenzymatic lipid peroxidation rather than diminution of catalytic generation of isoprostanes or MDA by the cyclooxygenases.Fernandes et al [34] pointed out that several NSAIDs, including indomethacin, tolmetin, ketorolac, acemetacin and oxaprozin can effectively inhibit the peroxyl-radical derived from 2, 2′-azobis (2-amidinopropane) dihydrochloride in various noncellular in vitro systems.The order of the scavenging activity is acemetacin = tolmetin > ketorolac > indometacin > oxaprozin.While Parij et al [35] found that these NSAIDs possessed no prooxidant activity.Nevertheless, it has been suggested that the anti-inflammatory activity of NSAIDs may also be partly due to their ability to scavenge ROS and RNS and to inhibit the respiratory burst of neutrophils triggered by various activator agents. [34]Oktar et al [36] found that burn-induced intestinal and liver oxidative damage in rats using GSH and MPO activity as markers can be prevented by ketorolac, nimesulide and piroxicam.On the other hand, Kalapatapu et al [37] proved that ketorolac can attenuate the oxidative stress (measured by MDA and GSH) in postoperative intimal hyperplasia in a rat carotid endarterectomy.Diclofenac, indomethacin ibuprofen, naproxen and paracetamol at the therapeutic and higher concentrations, and dipyrone at high concentration exerted a statistically significant inhibition of H 2 O 2 -forced erythrocytic membrane lipid peroxidation. [38]Kamiyama et al [39] showed that acetaminophen did not increase diene conjugates in vivo and it inhibited thiobarbituric acid reactive substance production in vitro.

NSAIDs as pro-oxidants
NSAIDs are known to produce various tissue damage such as gastro-intestinal damage, nephrotoxicity, and hepatotoxicity.[42] The mechanism by which NSAIDs cause injury to the gastric mucosa is mainly due to the inhibition of cyclooxygenase enzyme (COX) and suppression of prostaglandin-mediated effects on mucosal protection. [43]It has also been proposed that neutrophil and oxygen radical-dependent microvascular injuries may be important processes that lead to mucosal damage in response to administratioin of NSAIDs (ASA, diclofenac, indometacin, paracetamol and piroxicam). [40]Other hypotheses that might explain the mechanism of NSAID-induced toxicity in the small intestine involves the action of these drugs in uncoupling or inhibiting oxidative phosphorylation. [44]Basivireddy et al [45] had shown that acute indometacin poisoning produced free radical-induced damage in enterocytes, with the villous tip cells being particularly susceptible to these effects.These effects appeared to be mediated through the generation of oxygen free radicals.Increased production of these reactive molecules may result from the mitochondrial dysfunction produced by indomethacin, [46][47] infiltration of neutrophils into the mucosa in response to this drug, [48] increased activity of xanthine oxidase [49] or a combination of these factors.Long term use of low dose aspirin (ASA) has been recommended to reduce the risk of heart attack but gastric bleeding and formation of gastrointestinal ulcers are the most common adverse effects.It has been shown that, ASA damages gastric mucosa by inhibiting the synthesis of protective prostaglandins and by having direct action on the mucosa. Exposure of low-density lipoprotein (LDL) preparations to meyloperoxidase enzyme in presence of salicylates revealed that the drug could act as a catalyst of lipid oxidation in LDL . [52]In the study of Nair et al, [53] the effects of two cycloxygenase-2 (COX-2) selective inhibitors, celecoxib and nimesulide as compared to a nonselective COX inhibitor, aspirin, were studied in rat intestine.The study showed a significant increase in the lipid peroxide levels as TBAreactive substances as well as the conjugated dienes, except for celecoxib treated group which showed a decrease.Ibuprofen-induced damage to gastric mucosa after oral administration was accompanied by a significant increment in myeloperoxidase (MPO) activity (as an index of neutrophil activation), as well as lipid peroxidation (LP) levels and xanthine oxidase (XO) activity. [54]Consequently, it is concluded that the active oxygen species, derived both from XO and activated neutrophils, could play a role in the pathogenesis of gastric injury induced by ibuprofen.On the other hand, celecoxib and indomethacin both exerted a statistically significant increase of MDA content, representing a significant peroxidation. [32]Browne et al [55] investigated the pro-oxidant properties of ketorolac after evaluation of both beta-oxidation of palmitate and oxidative phosphorylation in rat hepatic mitochondria.The anti-ulcer effect of melatonin on gastric lesions caused by piroxicam was studied by Bandyopadhyay et al. [56] Increased lipid peroxidation, augmented protein oxidation and decreased glutathione content of the gastric tissue following piroxicam treatment indicated a possible involvement of oxidative stress in this NSAID-induced gastropathy. [56]Indomethacin can induce nephrotoxicity.Inhibition of PG is involved in this toxicity.In addition, inhibition of oxidative phosphorylation that can lead to a decrease in ATP production, [57] and production of ROS, resulting in oxidative stress, had also been seen as possible mechanisms in indometacin or diclofenac-induced kidney damage. [58]It has been reported that diclofenacinduced hepatotoxicity in rats was associated with the production of reactive oxygen species (ROS). [59] A number of in vitro animal models have been used to investigate the possible mechanisms of NSAID-related hepatotoxicity. Furthermore, diclofenac is more toxic to drug-metabolizing than to nonmetabolizing cell lines. [63]The in vitro cytotoxicity correlated well with the formation of 5-hydroxydiclofenac and particularly with the 5-dihydroxydiclofenac metabolite; the latter in particular can inhibit ATP synthesis. [63]tochondrial permeability transition has also been shown to be important in diclofenacinduced liver injury, resulting in generation of reactive oxygen species, mitochondrial swelling and oxidation of NADPH and protein thiols. [64]verdose of paracetamol causes hepatotoxicity, which can in severe cases lead to liver failure in experimental animals and humans. [65]Although a large percentage of the dose of paracetamol is directly conjugated with glucuronic acid or sulfate, a significant amount of paracetamol is metabolized by the cytochrome P450 system. [66]his leads to the formation of a reactive metabolite, presumably N-acetyl-pbenzoquinoneimine (NAPQI), which reacts rapidly with glutathione (GSH). [66]Thus, paracetamol metabolism causes dramatic depletion of cellular glutathione levels in the liver.If the formation of the reactive metabolite exceeds the capacity of hepatocellular glutathione, NAPQI will covalently bind to cellular proteins. [67]Paracetamol challenge (300 mg/kg, i.p) for 7 days caused a significant increase in the levels of bilirubin, liver enzymes, TBARS, and iron, while catalase activity and total protein level were reduced significantly in the serum and liver homogenate. [68][71] Interestingly, ferrous ions were released from the liver microsomes which underwent lipid peroxidation by naproxen. [72]he intracellular glutathione (GSH) decreased and the oxidized glutathione and glutathione disulfide (GSSG) increased during naproxen metabolism in the isolated rat hepatocytes. [73]It has been shown that ferrous ion released from rat liver microsomes contributes to naproxeninduced microsomal lipid peroxidation. [72]In conclusion, this controversy (summarized in Table-1) might be attributed to the different biological systems used to test the effect of these drugs, different concentrations utilized, or to the level of induction of the oxidative status.Further studies taking these factors into consideration are worth performing.