Mechanism of inhibition of myeloperoxidase by anti-inflammatory drugs

Transient and steady-state kinetics of the oxidation of substituted benzoic acid hydrazides by myeloperoxidase

Myeloperoxidase is the most abundant protein in neutrophils and catalyzes the production of hypochlorous acid. This potent oxidant plays a central role in microbial killing and inflammatory tissue damage. 4-Aminobenzoic acid hydrazide (ABAH) is a mechanism-based inhibitor of myeloperoxidase that is oxidized to radical intermediates that cause enzyme inactivation. We have investigated the mechanism by which benzoic acid hydrazides (BAH) are oxidized by myeloperoxidase, and we have determined the features that enable them to inactivate the enzyme. BAHs readily reduced compound I of myeloperoxidase. The rate constants for these reactions ranged from 1 to 3 x 10(6) M-1 s-1 (15 degrees C, pH 7.0) and were relatively insensitive to the substituents on the aromatic ring. Rate constants for reduction of compound II varied between 6.5 x 10(5) M-1 s-1 for ABAH and 1.3 x 10(3) M-1 s-1 for 4-nitrobenzoic acid hydrazide (15 degrees C, pH 7.0). Reduction of both compound I and compound II by BAHs adhered to the Hammett rule, and there were significant correlations with Brown-Okamoto substituent constants. This indicates that the rates of these reactions were simply determined by the ease of oxidation of the substrates and that the incipient free radical carried a positive charge. ABAH was oxidized by myeloperoxidase without added hydrogen peroxide because it underwent auto-oxidation. Although BAHs generally reacted rapidly with compound II, they should be poor peroxidase substrates because the free radicals formed during peroxidation converted myeloperoxidase to compound III. We found that the reduction of ferric myeloperoxidase by BAH radicals was strongly influenced by Hansch's hydrophobicity constants. BAHs containing more hydrophilic substituents were more effective at converting the enzyme to compound III. This implies that BAH radicals must hydrogen bond to residues in the distal heme pocket before they can reduce the ferric enzyme. Inactivation of myeloperoxidase by BAHs was related to how readily they were oxidized, but there was no correlation with their rate constants for reduction of compounds I or II. We propose that BAHs destroy the heme prosthetic groups of the enzyme by reducing a ferrous myeloperoxidase-hydrogen peroxide complex.

Inhibition of human lymphocyte function by organic solvents

We studied the direct effect of reactive hydroxyl precursors and inhibitors on CD4+ T-cell function. We used hydrogen peroxide plus ferrous chloride as the hydroxyl radical-generating system and di-methyl sulphourea, di-methyl sulfoxide, pyrrolidine dithiocarbonate, methanol, and ethanol, at a noncytotoxic concentration, as inhibitors. The immune parameter studies were proliferation and interleukin-2 production by peripheral blood lymphocytes stimulated with anti-CD3 antibody, phytohemagglutinin and alloantigens; proliferation, interleukin-2 production and mRNA expression of interleukin-4 and interferon gamma by allogeneic CD4+ T-cell clones stimulated with alloantigens. The results show that lymphocytes produce significant amounts of reactive oxygen species as measured by malondialdehyde produced in cultures. The hydroxyl radical-generating system did not change any of the cellular responses studied although it doubled Malondialdehyde production. Hydroxyl radical scavengers significantly inhibited all responses at doses that didn't significantly decrease malondialdehyde production. DNA analysis failed to show evidence for apoptosis.

CONCLUSION

Hydroxyl radical scavengers inhibit lymphocyte mitogenesis by a process that is independent of scavenging hydroxyl radicals.

Mechanism of inactivation of myeloperoxidase by 4-aminobenzoic acid hydrazide

Hypochlorous acid is the most powerful oxidant generated by neutrophils and is likely to contribute to the damage mediated by these inflammatory cells. The haem enzyme myeloperoxidase catalyses its production from hydrogen peroxide and chloride. 4-Aminobenzoic acid hydrazide (ABAH) is a potent inhibitor of hypochlorous acid production. In this investigation we show that, in the presence of hydrogen peroxide, ABAH irreversibly inactivates myeloperoxidase. ABAH was oxidized by myeloperoxidase, and kinetic analysis of the inactivation conformed to that for a mechanism-based inhibitor. Inactivation was exacerbated by concentrations of hydrogen peroxide greater than 50 microM and by the absence of oxygen. Hydrogen peroxide alone caused minimal inactivation. Reduced glutathione inhibited the oxidation of ABAH as well as the irreversible inhibition of myeloperoxidase. In the presence of oxygen, ABAH and hydrogen peroxide initially converted myeloperoxidase into compound III, which subsequently lost haem absorbance. In the absence of oxygen, the enzyme was converted into ferrous myeloperoxidase and its haem groups were rapidly destroyed. We propose that myeloperoxidase oxidizes ABAH to a radical that reduces the enzyme to its ferrous intermediate. Ferrous myeloperoxidase reacts either with oxygen to allow enzyme turnover, or with hydrogen peroxide to give irreversible inactivation.

Studies on the inhibitory effects of analogues of dapsone on neutrophil function in-vitro

We have compared twelve sulphone analogues of dapsone in terms of inhibition both of zymosan-mediated human neutrophil respiratory burst and inhibition of interleukin-1-stimulated neutrophil adhesion to transformed human umbilical vein endothelial cells. Overall, there was a good correlation between the respective rank orders of compound potency in the two test systems. The most effective compounds in terms of respiratory burst and adherence inhibition were the 2-nitro-4-amino-, 2-hydroxy-4-aminopropyl-, and 2-methoxy-4-aminoethyl- derivatives. In general, potency was inversely associated with lipophilicity; compounds with bulky side-chains, e.g. the 2-methyl-4-aminopentyl, 2-methyl-4-aminohexyl and the 2-hydroxymethyl-4-aminoethyl derivatives, were less potent. A 2-hydroxy-4-amino- derivative was the exception, however, with low lipophilicity and relatively low potency. All of the compounds tested showed comparable or greater inhibition in both the neutrophil-mediated assays compared with dapsone. Some of the compounds might, because of their good tissue penetration and lower toxicity than dapsone, have the potential to undergo further development.

Metabolism of clozapine by human neutrophils: evidence for a specific oxidation of clozapine by the myeloperoxidase system with inhibition of enzymatic chlorination cycle

The use of clozapine, an unique antipsychotic drug, raises the real problem of drug-induced polymorphonuclear neutrophil cytotoxicity. Clozapine prescription has been restricted due to a 1-2% incidence of drug-induced agranulocytosis. The exact mechanism of this adverse effect is not yet known. The myeloperoxidase-hydrogen peroxide system could play a key role in the initiation of agranulocytosis. Therefore, we have investigated the clozapine effects on hydrogen peroxide and hypochlorous acid, evaluated the peroxidase-mediated metabolism of clozapine by mass spectrometry analysis because myeloperoxidase uses hydrogen peroxide and chloride producing hypochlorous acid in its chlorination cycle, and thus could oxidise clozapine in its peroxidation cycle. First, evidence for inhibition of hypochlorous acid production and scavenging of hydrogen peroxide by clozapine were demonstrated in vitro, in different cell-free and cellular systems. Results are consistent with an inhibition of the myeloperoxidase chlorination cycle when clozapine is oxidised in the peroxidation cycle. Secondly, ion-spray mass spectrometry analysis allowed us to confirm clozapine oxidation by the myeloperoxidase system. Actually, clozapine N-oxide with a m/z at 343 was formed. It could be the final step of the metabolisation of clozapine via two successive univalent oxidations mediated by peroxidase. We suggest that generation of a free cation radical, CLZ(o+), was the initial step. CLZ(o+) is a very reactive species and may play an important role in the onset of agranulocytosis either by direct toxicity or via an immunological mechanism. However, this assumption does not exclude the possible role of other metabolic ways involving, in particular, N-desmethylclozapine.

Is local biotransformation the key to understanding the pharmacological activity of salicylates and gold drugs?

It is suggested that some drugs may be converted by inflammatory cells to yield active species. The transformation may be non-enzymatic, although being driven by the enzymatic production of highly reactive species which are normal products of activated leukocytes, such as singlet oxygen, hydrogen peroxide, hypochlorite, hydroxyl radical and nitric oxide. Drugs which may be transformed in this fashion are the anti-rheumatic gold complexes which may be converted either to aurocyanide or to Au(III) complexes by myeloperoxidase in polymorphonuclear leukocytes. Salicylate may also be activated by its oxidation to dihydroxybenzoates although evidence for its transformation is weaker than for the gold complexes.

Salicylic acid is a modulator of tobacco and mammalian catalases

Salicylic acid (SA) plays a key role in the establishment of resistance to microbial pathogens in many plants. The discovery that SA inhibits catalase from tobacco led us to suggest that H2O2 acts as second messenger to activate plant defenses. Detailed analyses of SA's interaction with tobacco and mammalian catalases indicate that SA acts as an electron donor for the peroxidative cycle of catalase. When H2O2 fluxes were relatively low (1 microM/min or less), SA inhibited catalase, consistent with its suggested signaling function via H2O2. However, significant inhibition was only observed at 100 microM SA or more, a level reached in infected, but not in uninfected, leaves. This inhibition was probably due to siphoning catalase into the slow peroxidative reaction. Surprisingly, SA was also able to protect catalase from inactivation by damaging levels of H2O2 (lower millimolar range), which is generally assumed to reflect accumulation of inactive ferro-oxy intermediates. SA did so by supporting or substituting for the protective function of catalase-bound NADPH. These results add new features to SA's interaction with heme enzymes and its in vivo redox properties. Thus, SA, in addition to its proposed signaling function, may also have an important antioxidant role in containing oxidative processes associated with plant defense responses.

Inhibition of LDL oxidation and myeloperoxidase dependent tyrosyl radical formation by the selective estrogen receptor modulator raloxifene (LY139481 HCL)

Cellular oxidation of protein and lipoproteins is believed to contribute to the pathology associated with both acute and chronic inflammatory processes. Enzymatic, myeloperoxidase and lipoxygenase, and non- enzymatic oxidation of low density lipoprotein, LDL, has been implicated in foam cell formation and the progression of atherosclerotic changes within the arterial wall. In the present study, the in vitro protective role of the selective estrogen receptor modulator, raloxifene, in these oxidant triggered processes has been investigated. Raloxifene, as with estrogen was observed to inhibit both copper mediated LDL oxidation as well as the cellular modification of LDL by murine peritoneal macrophages. Raloxifene was, however, a more potent inhibitor of LDL oxidation than 17 beta-estradiol. The inhibition of macrophage LDL modification by raloxifene was not due to a non-specific effect on all effector functions as phagocytosis of opsonized yeast was comparable with control macrophage cultures. In addition to the protective effects on LDL oxidation, raloxifene also inhibited tyrosyl radical formation catalyzed by myeloperoxidase. The inhibition of myeloperoxidase activity was observed for both the isolated enzyme and in phorbol ester stimulated murine peritoneal neutrophils. In contrast, raloxifene was a weaker inhibitor of horseradish peroxidase. These results demonstrate a potential protective role for raloxifene as an anti-oxidant in in vitro assays designed to evaluate oxidant mediated radical formation and tissue damage.

Dapsone in rheumatoid arthritis

Dapsone, a synthetic sulfone with chemical similarities to sulfapyridine, has been used for a number of years to treat leprosy and dermatitis herpetiformis. Recently, a number of prospective, randomized, double-blind trials have shown their success in the management of rheumatoid arthritis, with dapsone being superior to placebo and comparable to chloroquine and hydroxychloroquine. Its mode of anti-inflammatory actions in rheumatoid arthritis is not clearly understood, but modulation of neutrophil activity or inhibition of neutrophil inflammatory product formation or release appear to play a role. The major limiting side effect is hemolytic anemia, which may be mitigated through careful patient selection, conservative drug dosing, close monitoring, and possibly, concurrent administration of antioxidants or cytochrome P450 inhibitors. Methemoglobinemia is another common finding among patients receiving dapsone therapy, but rarely does it result in prominent symptoms other than transient pallor. Less common adverse events to dapsone include the idiosyncratic reactions of leukopenia and agranulocytosis, cutaneous eruptions, peripheral neuropathy, psychosis, toxic hepatitis, cholestatic jaundice, nephrotic syndrome, renal papillary necrosis, severe hypoalbuminemia without proteinuria, an infectious mononucleosis-like syndrome, and minor neurological and gastrointestinal complaints. In this report, two patients with advanced rheumatoid arthritis, who were safely and effectively treated with dapsone after failure with other second-line agents, are described and the literature is reviewed. We suggest that dapsone is an effective second-line agent in the treatment of rheumatoid arthritis.

Inhibition of ascorbate peroxidase by salicylic acid and 2,6-dichloroisonicotinic acid, two inducers of plant defense responses

In recent years, it has become apparent that salicylic acid (SA) plays an important role in plant defense responses to pathogen attack. Previous studies have suggested that one of SA's mechanisms of action is the inhibition of catalase, resulting in elevated levels of H2O2, which activate defense-related genes. Here we demonstrate that SA also inhibits ascorbate peroxoidase (APX), the other key enzyme for scavenging H2O2. The synthetic inducer of defense responses, 2,6-dichloroisonicotinic acid (INA), was also found to be an effective inhibitor of APX. In the presence of 750 microM ascorbic acid (AsA), substrate-dependent IC50 values of 78 microM and 95 microM were obtained for SA and INA, respectively. Furthermore, the ability of SA analogues to block APX activity correlated with their ability to induce defense-related genes in tobacco and enhance resistance to tobacco mosaic virus. Inhibition of APX by SA appears to be reversible, thus differing from the time-dependent, irreversible inactivation by suicide substrates such as p-aminophenol. In contrast to APX, the guaiacol-utilizing peroxidases, which participate in the synthesis and crosslinking of cell wall components as part of the defense response, are not inhibited by SA or INA. The inhibition of both catalase and APX, but not guaiacol peroxidases, supports the hypothesis that SA-induced defense responses are mediated, in part, through elevated H2O2 levels or coupled perturbations of the cellular redox state.

Dapsone toxicity: some current perspectives

1. Dapsone is a potent anti-inflammatory and anti-parasitic compound, which is metabolised by cytochrome P-450 to hydroxylamines, which in turn cause methaemoglobinaemia and haemolysis. However, during the process of methaemoglobin formation, erythrocytes are capable of detoxifying the hydroxylamine to the parent drug, which may either reach the tissues to exert a therapeutic effect or return to the liver and be re-oxidised in a form of systemic cycling. This glutathione-dependent effect, combined with the un-ionised state of the drug at physiological pH, may contribute to its efficacy. 2. Paradoxically, other aspects of the glutathione-dependent cycling of the hydroxylamine metabolite may contribute to the major adverse reaction of the drug, agranulocytosis. Erythrocytes exposed to the metabolite and repeatedly washed may still release the hydroxylamine in sufficient concentration to kill mononuclear leucocytes in vitro. Thus, erythrocytes may be a conduit for the hydroxylamine to reach the bone marrow to covalently bind to granulocyte precursors, which may trigger an immune response in certain individuals and may lead to the potentially fatal eradication of granulocytes from the circulation. 3. Attempts to increase patient tolerance to dapsone have been most successful using a metabolic inhibitor to reduce hepatic oxidation of the drug to the hydroxylamine. Methaemoglobin formation in the presence of cimetidine was maintained at 30% below control levels for almost 3 mo, and patients' reported side effects such as headache and lethargy were significantly reduced. 4. As clinical application of new and safer dapsone analogues is years away, the use of cimetidine provides an immediate route to increasing patient compliance during dapsone therapy, especially in those maintained on dapsone dosages in excess of 200 mg/day.

Inhibition of myeloperoxidase by benzoic acid hydrazides

Myeloperoxidase is the most abundant protein in neutrophils and catalyses the conversion of H2O2 and chloride into HOCl. To help clarify the role of this enzyme in bacterial killing and inflammation, a specific and potent inhibitor needs to be identified. We have studied a series of benzoic acid hydrazides and found that in general they inhibit the peroxidation activity of myeloperoxidase with an IC50 value of less than 10 microM. The IC50 values of derivatives with substituents containing oxygen or nitrogen were related to their Hammett substituent constants. This indicates that myeloperoxidase oxidizes the hydrazide group of these compounds, and the degree to which they inhibit the enzyme is dependent on the ease of their oxidation. Unsubstituted benzoic acid hydrazide and its 4-chloro derivative were poor inhibitors of peroxidation. Thus it is likely that hydrogen-bonding of the enzyme to substituents containing oxygen or nitrogen increases the binding affinity of the hydrazides and enhances their oxidation by myeloperoxidase. 4-Aminobenzoic acid hydrazide (ABAH) was the most potent inhibitor of peroxidation. It irreversibly inhibited HOCl production by the purified enzyme, having an IC50 value of 0.3 microM. With neutrophils stimulated with opsonized zymosan or phorbol myristate acetate, ABAH inhibited HOCl production by up to 90% and the IC50 values were 16 microM and 2.2 microM respectively. In the presence of superoxide dismutase, these values decreased to 6.4 microM and 0.6 microM respectively. ABAH had no effect on superoxide radical (O2-.) production and degranulation by neutrophils, nor did it inhibit catalase or glutathione peroxidase. Thus ABAH is an effective and selective inhibitor that should be useful for determining the contribution of myeloperoxidase to oxidant-mediated reactions of neutrophils.

Modulation of polymorphonuclear leukocyte responsiveness by copper (II)2 (niflumate)4

Antiinflammatory activities and modulations of PMNL responses produced by treatment with tetrakis-mu-2-[3-(trifluoromethyl)-phenyl]aminonicotinatodicopper (II) [Cu(II)2(niflumate)4] and niflumic acid were studied in isologous serum-induced rat pleurisy. Doses of 10 or 30 mg/kg (35 or 106 mumol/kg) of niflumic acid or Cu(II)2(niflumate)4 (8 or 23 mumol/kg) caused significant (p < 0.01) reductions in pleural exudate and number of polymorphonuclear leukocytes (PMNLs) in the exudate. While both doses of Cu(II)2(niflumate)4 produced significant dose-related reductions in both parameters, only the higher dose of niflumic acid produced a significant dose-related reduction in both parameters. Boyden chamber measurements of N-formyl-methionyl-leucyl-phenylalanine (f-MLP) chemotaxis by PMNLs incubated with 10 or 30 micrograms/ml niflumic acid (35 or 106 nmol/ml) or Cu(II)2(niflumate)4 (8 or 23 nmol/ml) were significantly (p < 0.01 to p < 0.001) decreased in dose-related fashions. Chemotaxis of PMNLs from pleuritic rats treated orally with 10 or 30 mg/kg niflumic acid or Cu(II)2(niflumate)4 was significantly (p < 0.001) inhibited by the larger dose of niflumic acid and both doses of Cu(II)2(niflumate)4. Opsonized zymosan (OZ)-stimulated chemiluminescence (CL) of PMNLs from pleuritic rats treated orally with these same doses of niflumic acid or Cu(II)2(niflumate)4 was only significantly (p < 0.05 or p < 0.01 respectively) decreased by the larger doses. Superoxide (O2-) production by these cells was significantly decreased by the larger dose of niflumic acid (p < 0.05) while both doses of Cu(II)2(niflumate)4 produced significant (p < 0.05 to p < 0.01) decreases.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of anti-inflammatory drugs on myeloperoxidase-dependent hydroxyl radical generation by human neutrophils

Neutrophils comprise a group of leukocytes that play a pivotal role in inflammation and vascular diseases like ischemia/reperfusion. These activated phagocytic cells are drawn to the site of injury, secreting superoxide and other oxidants derived from the formation of this free radical. This series of events frequently results in localized tissue damage. Surprisingly, free radical scavengers frequently offer only minimal relief. Why this is so may be due, in part, to our limited understanding of mechanisms that govern generation of free radicals in these settings. Although the metal ion-catalyzed Haber-Weiss reaction is considered the classical pathway for neutrophil-derived hydroxyl radical, an alternative mechanism, such as the myeloperoxidase-dependent pathway, may undoubtedly contribute to the formation of this free radical by stimulated neutrophils. In this study, we explored this possibility by investigating the role of different classes of anti-inflammatory drugs to ameliorate hydroxyl radical generation via the myeloperoxidase-dependent pathway. In this paper, we report that meclofenamic acid inhibited myeloperoxidase-dependent hydroxyl radical generation through scavenging of hypochlorous acid and not by direct inhibition of myeloperoxidase. The importance of these results with regard to the clinical efficacy of this anti-inflammatory compound remains to be determined as studies into the significance of myeloperoxidase-dependent hydroxyl radical formation in inflammatory tissue injury continue.

Oxidation of bromide by the human leukocyte enzymes myeloperoxidase and eosinophil peroxidase. Formation of bromamines

Myeloperoxidase and eosinophil peroxidase catalyzed the oxidation of bromide ion by hydrogen peroxide (H2O2) and produced a brominating agent that reacted with amine compounds to form bromamines, which are long-lived oxidants containing covalent nitrogen-bromine bonds. Results were consistent with oxidation of bromide to an equilibrium mixture of hypobromous acid (HOBr) and hypobromite ion (OBr-). Up to 1 mol of bromamine was produced per mole of H2O2, indicating that bromamine formation prevented the reduction of HOBr/OBr- by H2O2 and the loss of oxidizing and brominating activity. Bromamines differed from HOBr/OBr- in that bromamines reacted slowly with H2O2, were not reduced by dimethyl sulfoxide, and had absorption spectra similar to those of chloramines, but shifted 36 nm toward higher wavelengths. Mono- and di-bromo derivatives (RNHBr and RNHBr2) of the beta-amino acid taurine were relatively stable with half-lives of 70 and 16 h at pH 7, 37 degrees C. The mono-bromamine was obtained with a 200-fold excess of amine over the amount of HOBr/OBr- and the di-bromamine at a 2:1 ratio of HOBr/OBr- to the amine. In the presence of physiologic levels of both bromide (0.1 mM) and chloride (0.1 M), myeloperoxidase and eosinophil peroxidase produced mixtures of bromamines and chloramines containing 6 +/- 4% and 88 +/- 4% bromamine. In contrast, only the mono-chloramine derivative (RNHCl) was formed when a mixture of hypochlorous acid (HOCl) and hypochlorite ion (OCl-) was added to solutions containing bromide and excess amine. The rapid formation of the chloramine prevented the oxidation of bromide by HOCl/OCl-, and the chloramine did not react with bromide within 1 h at 37 degrees C. The results indicate that when enzyme-catalyzed bromide or chloride oxidation took place in the presence of an amine compound at 10 mM or higher, bromamines were not produced in secondary reactions such as the oxidation of bromide by HOCl/OCl- and the exchange of bromide with chlorine atoms of chloramines. Therefore, the amount of bromamine produced by myeloperoxidase or eosinophil peroxidase was equal to the amount of bromide oxidized by the enzyme. Bromide was preferred over chloride as the substrate for both enzymes.

Dietary fish oil supplementation alters leukocyte function and cytokine production in healthy women

The effect of low-dose fish oil supplementation on cytokines and white cell function in women was investigated. Thirty-three healthy, nonsmoking women entered the double-blind study. For 4 weeks, 2.4 g of either fish oil (n = 16) or fish oil with vitamin E (n = 17) was added daily to the subjects' otherwise unchanged diets. Venous blood samples were taken at the onset of the trial, after the supplementation period, and again after a 9-week washout period. Plasma levels of platelet-derived growth factor and myeloperoxidase were measured using immunoassays. The intracellular peroxidase content of white blood cells was measured using a staining technique. Platelet-derived growth factor levels were significantly lowered after supplementation (P < or = .05). Intracellular peroxidase was increased (P < or = .01), and extracellular myeloperoxidase levels were lowered (P < or = .05). Taken together, these results suggest that the anti-inflammatory effect of fish oil may be due at least partly to alterations in white cell function and growth factor levels.

Reduction of dapsone hydroxylamine to dapsone during methaemoglobin formation in human erythrocytes in vitro. IV: Implications for the development of agranulocytosis

We have studied the efflux of dapsone hydroxylamine from normal and diabetic erythrocytes by the use of a two-compartment (1 and 2) in vitro dialysis system, in order to model the in vivo blood supply to the bone marrow. When both types of erythrocytes were dialysed against mononuclear leucocytes, the hydroxylamine crossed the membrane and caused significantly greater white cell death compared with dialysis of leucocytes against untreated erythrocytes. However, in the case of both normal and diabetic cells, the presence of the glutathione depletor diethyl maleate (DEM) caused a marked reduction in movement of hydroxylamine from compartment 1 to 2. Diethyl dithiocarbamate (DDC), a methaemoglobin accelerant, caused a marked reduction in movement of hydroxylamine from erythrocytes (diabetic and normal) in compartment 1 to 2 which led to a significant reduction in white cell death compared with the absence of DDC (18.3 +/- 5.5 vs 34.8 +/- 8.1%, P < 0.05). Dapsone recovery from compartment 1 rose significantly in the presence of DDC compared with control in both erythrocyte types. In contrast, recovery of dapsone from normal erythrocytes incubated in compartment 1 was significantly reduced by the presence of DEM compared with control, although there was no difference between control and DEM-treated diabetic cells. Dapsone analysis in compartment 2 revealed a significant increase in dapsone recovery in both diabetic (11.3 +/- 1.1%) and normal (11.9 +/- 1.1%) erythrocytes in the presence of DDC compared with diabetic (3.3 +/- 0.4%) and normal control (4.8 +/- 2.0%, P < 0.001). The presence of DEM in compartment 1 caused a significant fall in dapsone recovery in compartment 2 (3.7 +/- 0.26) compared with control (4.7 +/- 0.36%, P < 0.05). Hence, dapsone hydroxylamine is capable of leeching out of normal and diabetic erythrocytes, traversing a semipermeable membrane and causing toxicity to human mononucleocyte cells in vitro. This process may be one of the first stages in immune-mediated agranulocytosis.

Salicylates for ulcerative colitis--their mode of action

Delivery of 5-aminosalicylic acid to the colon by sulphasalazine, other azo-bonded compounds and controlled-release preparations is introduced in the context of metabolism by epithelial cells and therapeutic efficacy in ulcerative colitis. Potential modes of action are then reviewed, including actions on luminal bacteria, epithelial cell surface receptors, cellular events (such as nitric oxide release or butyrate oxidation), electrolyte transport and epithelial permeability. Evidence for an influence of salicylates on circulating and lamina propria inflammatory cells is presented, as well as actions on adhesion molecules, chemotactic peptides and inflammatory mediators, such as eicosanoids, platelet-activating factor, cytokines or reactive oxygen metabolites. The precise mechanism will remain uncertain as long as the aetiology of ulcerative colitis is unknown, but a pluripotential mode of action of salicylates is an advantage when influencing the network of events that constitute chronic inflammation.

On radiation damage to normal tissues and its treatment. II. Anti-inflammatory drugs

In addition to transiently inhibiting cell cycle progression and sterilizing those cells capable of proliferation, irradiation disturbs the homeostasis effected by endogenous mediators of intercellular communication (humoral component of tissue response to radiation). Changes in the mediator levels may modulate radiation effects either by assisting a return to normality (e.g., through a rise in H-type cell lineage-specific growth factors) or by aggravating the damage. The latter mode is illustrated with reports on changes in eicosanoid levels after irradiation and on results of empirical treatment of radiation injuries with anti-inflammatory drugs. Prodromal, acute and chronic effects of radiation are accompanied by excessive production of eicosanoids (prostaglandins, prostacyclin, thromboxanes and leukotrienes). These endogenous mediators of inflammatory reactions may be responsible for the vasodilatation, vasoconstriction, increased microvascular permeability, thrombosis and chemotaxis observed after radiation exposure. Glucocorticoids inhibit eicosanoid synthesis primarily by interfering with phospholipase A2 whilst non-steroidal anti-inflammatory drugs prevent prostaglandin/thromboxane synthesis by inhibiting cyclooxygenase. When administered after irradiation on empirical grounds, drugs belonging to both groups tend to attenuate a range of prodromal, acute and chronic effects of radiation in man and animals. Taken together, these two sets of observations are highly suggestive of a contribution of humoral factors to the adverse responses of normal tissues and organs to radiation. A full account of radiation damage should therefore consist of complementary descriptions of cellular and humoral events. Further studies on anti-inflammatory drug treatment of radiation damage to normal organs are justified and desirable.

Hydrogen peroxide activates agonist-sensitive Ca(2+)-flux pathways in canine venous endothelial cells

The effect of the biological oxidant H2O2 on purinergic-receptor-stimulated Ca2+ signalling was determined in canine venous endothelial cells. H2O2 increased cytosolic free [Ca2+] ([Ca2+]i), the rate of rise of which was dose-dependently related to H2O2 concentration. The response of [Ca2+]i to H2O2 resulted in part from release of Ca2+ from internal stores. The H2O2-sensitive intracellular Ca2+ pool was characterized in cells suspended in Ca(2+)-free/EGTA buffer and stimulated in sequence with H2O2 and ionomycin or ATP. Under this condition, the rank order of apparent compartment size sensitive to each compound was ionomycin > H2O2 > ATP. Stimulation of cells with H2O2 eliminated any response of [Ca2+]i to subsequent addition of ATP. To test more directly whether H2O2 accesses the inositol trisphosphate-sensitive Ca2+ store, cells were pretreated with thapsigargin, a selective inhibitor of that store's Ca2+ pump. Release of Ca2+ from internal Ca2+ stores by H2O2 declined as the interval after thapsigargin addition increased, a finding that supports the contention that H2O2 accesses the inositol trisphosphate-sensitive Ca2+ store. H2O2-stimulated Ca2+ influx across the cell membrane was sensitive to Ni2+, La3+, and 1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl)-1H-imidazole HCl (SKF-96365), a selective inhibitor of the agonist-stimulated Ca(2+)-influx pathway. Ca2+ entry triggered by H2O2 appears to occur via the agonist-sensitive Ca2+ influx pathway. Together, these results suggest that H2O2, which is normally secreted by activated neutrophils and monocytes, may act as an intercellular messenger and stimulate Ca2+ signalling in target endothelial cells.

Effect of a new de-N-acetyl-lysoglycosphingolipid on chemically-induced inflammatory bowel disease: possible mechanism of action

A new, orally active de-N-acetylated lysoglycosphingolipid (WILD20) was evaluated as antiinflammatory agent using a model of chemically-induced inflammatory bowel disease (IBD) in the rat to mimic human ulcerative colitis and Chron's disease. IBD was induced by hapten trinitrobenzenesulphonic acid (TNB). WILD20, orally administered as preventive or curative, was demonstrated to be efficacious at daily dosages of 0.1-1 mg/kg for 4-5 days. Damage scores, body weight, spleen weight, colonic tissular levels of LTB4, myeloperoxidase (MPO) and malondialdehyde (MDA) are influenced and brought into parameters of normality. Histological observation demonstrated quicker healing, better repair, reduced inflammation, and poor eosinophil degranulation. The mechanisms underlying WILD20 antiinflammatory effects were investigated: whereas WILD20 fails to show a direct effect on PKC, it reduces PKC translocation to the membrane; cellular PLA2 was consequently greatly reduced through this mechanism and thought to be responsible for WILD20 efficacy towards chemically-induced IBD.

Dapsone: modes of action, toxicity and possible strategies for increasing patient tolerance

Dapsone is useful in the treatment of a number of inflammatory conditions which are characterized by neutrophil infiltration. It is the drug of choice for suppression of the symptoms of dermatitis herpetiformis, as it inhibits the process by which neutrophils leave the circulation and migrate to lesional sites. It also prevents the tissue destruction normally caused by the neutrophils' respiratory burst. Although dapsone can cause a number of serious idiosyncratic reactions, such as agranulocytosis, tolerance of the drug at higher doses is more usually determined by its haematological side-effects of methaemoglobinaemia and haemolysis. These effects are due entirely to the hepatic N-hydroxylation of dapsone to a hydroxylamine metabolite, some of which escapes from the liver and rapidly enters red cells. Attempts have been made to counteract the haemotoxic effects of the metabolite by the use of antioxidants such as vitamins E and C. Recently, the co-administration of a metabolic inhibitor such as cimetidine has been shown to reduce significantly dapsone-dependent methaemoglobinaemia, without any change in drug efficacy. It remains to be seen if this approach will be adopted clinically, to improve patient tolerance of high dapsone dosage.

Myeloperoxidase-mediated activation of xenobiotics by human leukocytes

Peripheral blood leukocytes contain a variety of enzymes that are capable of metabolising xenobiotics. The enzyme myeloperoxidase (MPO) appears to be the most important for drug metabolism. MPO is a peroxidase/oxidase and generates the powerful oxidant hypochlorous acid. MPO- or MPO-generated oxidants are capable of oxidizing a wide variety of compounds and a broad range of functional groups, especially those that contain nitrogen and sulfur. Leukocytes have a role in immune response; therefore, reactive intermediates generated by leukocyte metabolism of xenobiotics may have a role in idiosyncratic drug reactions, particularly those that are immune-mediated such as drug-induced lupus or agranulocytosis.

Anti-oxidant properties of H2-receptor antagonists. Effects on myeloperoxidase-catalysed reactions and hydroxyl radical generation in a ferrous-hydrogen peroxide system

Ulcerogenesis of the gastroduodenal mucosa is caused by the digestive action of gastric juice and initially involves an inflammatory reaction with infiltration of phagocytes. The anti-inflammatory activity of many drugs have been attributed to the inhibition of the leukocyte enzyme, myeloperoxidase (MPO). In this study, the H2-antagonists in clinical use were found to be potent inhibitors of MPO-catalysed reactions (IC50 < 3 microM) under conditions resembling those in experiments with intact neutrophils. Since peak plasma concentrations of cimetidine, ranitidine and nizatidine are well within the micromolar range, after oral therapeutic dosing, our results may be of clinical relevance. The inhibitory actions of cimetidine and nizatidine were largely due to scavenging of hypochlorous acid (HOCl), a powerful chlorinating oxidant produced in the MPO-H2O2-Cl- system. In contrast to famotidine, ranitidine was also a potent scavenger of HOCl, while both drugs inhibited MPO reversibly by converting it to compound II, which is inactive in the oxidation of Cl-. The HOCl scavenging potencies of ranitidine and nizatidine were about three times higher than that of the anti-rheumatic drug, penicillamine, which had a potency similar to that of cimetidine. The rapid HOCl scavenging ability of penicillamine is thought to contribute to its anti-inflammatory effects. Using riboflavin as a probe, the H2-antagonists were found to be inhibitors of hydroxyl radical (.OH) generated in a Fe(2+)-H2O2 reaction mixture. Spectral analyses of the interaction of iron ions with the drugs and studies with chelators, suggest that the drugs were efficient chelators of Fe2+, in addition to their .OH scavenging abilities. Since the gastrointestinal tract can contain potentially reactive iron, the simultaneous presence of H2-antagonists may help to suppress iron-driven steps in tissue damage.

Oxidation of desferrioxamine to nitroxide free radical by activated human neutrophils

Human neutrophils activated by PMA were found to induce the formation of a nitroxide radical from DFO. The presence of SOD was necessary to permit the formation of the DFO radical. The inactive phorbol ester did not induce DFO radical, and DL-sphinganine suppressed the radical produced by the active phorbol ester. Other cell stimuli (Zymocel and the chemotactic peptide) also induced the formation of the DFO radical, although radical concentration was very much lower than with PMA. Participation of NO, OH or 1O2 was ruled out by the inability of NG-methyl-L-arginine, NG-nitro-L-arginine, DMSO, mannitol, histidine, and methionine to inhibit the formation of DFO radical produced by PMA-activated cells. Furthermore, PMA-activated cells did not produce detectable levels of NO2-, a stable oxidation product of NO, and D2O, which enhances the lifetime of singlet oxygen, did not modify the intensity or the lifetime of DFO radical. The involvement of cell MPO was suggested by the inhibition of the DFO radical observed after treatment with catalase or with antihuman MPO antibodies. Also, HOCl was found to induce the DFO radical in cell-free reactions, but our data indicate that the reaction leading to DFO radical formation by neutrophils involves the reduction of MPO compound II back to the active enzyme (ferric-MPO). Anti-inflammatory drugs strongly increased the DFO radical produced by activated neutrophils. On the contrary, none of these drugs was able to increase the DFO radical produced by HOCl. Histidine and methionine that inhibited the DFO radical intensity in cell-free reactions, were shown to act directly on HOCl. Experiments with MPO-H2O2 in SOD- and Cl(-)-free conditions showed the formation of DFO radical and confirmed the hypothesis of the involvement of compound II. The conversion of compound II to ferric MPO by DFO optimized the enzymatic activity of neutrophils, and in the presence of monochlorodimedon (compound II promoting agent) we measured an increased HOCl production. When DFO was modified by conjugation with hydroxyethyl starch, it lost the ability to produce the radical either by neutrophils or by MPO-H2O2 and did not increase HOCl production. The inability of these DFO derivatives to produce potentially toxic species might explain their reported lower toxicity in vivo.

Superoxide is an antagonist of antiinflammatory drugs that inhibit hypochlorous acid production by myeloperoxidase

Myeloperoxidase, the most abundant enzyme in neutrophils, catalyses the conversion of hydrogen peroxide and chloride to hypochlorous acid. This potent oxidant has the potential to cause considerable tissue damage in many inflammatory diseases. We have investigated the ability of dapsone, diclofenac, primaquine, sulfapyridine and benzocaine to inhibit hypochlorous acid production by stimulated human neutrophils. The drugs were also tested against purified myeloperoxidase using xanthine oxidase to generate hydrogen peroxide and superoxide. The inhibitory effects of the drugs on hypochlorous acid production, either by cells stimulated with phorbol myristate acetate or by myeloperoxidase and xanthine oxidase, were significantly less than those determined with myeloperoxidase and reagent hydrogen peroxide. Comparable potency was observed only when superoxide dismutase was present to remove superoxide. We also observed that with the xanthine oxidase system, inhibition of hypochlorous acid production by dapsone decreased markedly as the concentration of myeloperoxidase increased. Dapsone was a poor inhibitor of hypochlorous acid production by neutrophils stimulated with opsonized zymosan, regardless of the presence of superoxide dismutase. With this phagocytic stimulus, catalase inhibited hypochlorous acid formation by only 60%, which indicates that a substantial amount of the hypochlorous acid detected originated from within phagosomes. Thus, it is apparent that dapsone is unable to affect intraphagosomal conversion of hydrogen peroxide to hypochlorous acid. All the drugs inhibit myeloperoxidase reversibly by trapping it as its inactive redox intermediate, compound II. We propose that superoxide limits the potency of the drugs by reducing compound II back to the active enzyme. Furthermore, under conditions where the activity of myeloperoxidase exceeds that of the hydrogen peroxide-generating system, which is most likely to occur in phagosomes, partial inhibition of myeloperoxidase need not affect hypochlorous acid production. We conclude that drugs that inhibit myeloperoxidase by converting it to compound II are unlikely to be effective against hypochlorous acid-mediating tissue damage.

Peroxidation of the antimalarial drug primaquine: characterization of a benzidine-like metabolite with methaemoglobin-forming activity

1. An organic solvent-extractable product was obtained from incubations of primaquine (70 mM) with H2O2 (70mM) and horseradish peroxidase (0.5 mg/ml) in acetate buffer, pH4.2. 2. The metabolite was characterized as 5,5-di-(8-[(4-amino-1-methylbutyl)amino]-6-methoxyquinoline) by 1H-n.m.r., mass, FT-i.r. and u.v.-visible spectroscopy. 3. Incubations of rat erythrocytes with 5,5-di-(8-[(4-amino-1-methylbutyl)amino]-6-methoxyquinoline) led to the formation of methaemoglobin in a time- and metabolite concentration-dependent manner.

Biotransformation of para-aminobenzoic acid and salicylic acid by PMN

Para-aminobenzoic acid (PABA) is an essential cofactor for the production of folic acid in bacteria and has mild anti-inflammatory activity. We have recently reported that salicylic acid and benzoic acid are oxidized by stimulated granulocytes Polymorphonuclear Neutrophils (PMN). The oxidation of salicylate appears mediated by a potent oxygen metabolite generated during the respiratory burst which is dependent primarily on superoxide (O2-) for its production. These background studies with the salicylate group of drugs suggested that PABA might be similarly metabolized by PMN. In these studies, we demonstrate that PABA is metabolized by stimulated PMN. However, in contrast to the biochemical mechanism involved in the metabolism of salicylate, our scavenger studies indicate that PABA is metabolized primarily by the myeloperoxidase pathway. Our results may explain the mild anti-inflammatory actions of the drug and suggest that the degradation of PABA by PMN at an inflammatory site may limit the availability of PABA for bacterial growth.

Effect of oxidant stress on calcium signaling in vascular endothelial cells

The endothelial cell is recognized as a critical modulator of blood vessel tone and reactivity. This regulatory function of endothelial cells occurs via synthesis and release of diffusible paracrine substances which induce contraction or relaxation of adjacent vascular smooth muscle. In response to stimulation by blood-borne agonists such as bradykinin or histamine, the endothelial cell utilizes cytosolic ionic Ca2+ as a trigger in the transduction of the stimulatory signal into a paracrine response. Considerable evidence has accumulated to indicate that various forms of biologically important oxidant stress alter vascular function in an endothelium-dependent manner. Further, oxidant stress is known to alter the mechanisms which govern Ca2+ homeostasis in the endothelial cell. Recently, we have described a model in which the oxidant tert-butylhydroperoxide is utilized to examine the effects of oxidant stress on Ca(2+)-dependent signal transduction in vascular endothelial cells. In this model, three temporal phases are evident and consist of (1) inhibition of the agonist-stimulated Ca2+ influx pathway, (2) inhibition of receptor-activated release of Ca2+ from internal stores and elevation of resting cytosolic free Ca2+ concentration, and (3) progressive increase in resting cytosolic Ca2+ concentration and loss of responsiveness to agonist stimulation. In this review, the mechanisms which characterize agonist-stimulated Ca2+ signaling in vascular endothelial cells, and the effects of oxidant stress on signal transduction will be described. The mechanisms potentially responsible for oxidant-induced inhibition of Ca2+ signaling will be considered.

Oxidative metabolism of amsacrine by the neutrophil enzyme myeloperoxidase

Oxidative metabolism of the anti-cancer drug amsacrine 4'-(9-acridinylamino) methane-sulphan-m-anisidide has been suggested to account for its cytotoxicity. However, enzymes capable of oxidizing it in non-hepatic tissue have yet to be identified. A potential candidate, that may be relevant to the metabolism of amsacrine in blood and its action in myeloid leukaemias and myelosuppression, is the haem enzyme myeloperoxidase. We have found that the purified human enzyme oxidizes amsacrine to its quinone diimine, either directly or through the production of hypochlorous acid. In comparison, the 4-methyl-5-methylcarboxamide derivative of amsacrine, CI-921 9-[[2-methoxy-4[(methylsulphonyl)-amino]phenyl]amino)-N, 5-dimethyl-4-acridine carboxamide, reacted poorly with myeloperoxidase, although it was oxidized by hypochlorous acid. Detailed studies of the mechanism by which myeloperoxidase oxidizes amsacrine revealed that the semiquinone imine free radical is a likely intermediate in this reaction. Oxidation of amsacrine analogues indicated that factors other than their reduction potential determine how readily they are metabolized by myeloperoxidase. Both amsacrine and CI-921 inhibited production of hypochlorous acid by myeloperoxidase. CI-921 acted by trapping the enzyme as the inactive redox intermediate compound II. Amsacrine inhibited by a different mechanism that may involve conversion of myeloperoxidase to compound III, which is also unable to oxidize Cl-. The susceptibility of amsacrine to oxidation by myeloperoxidase indicates that this reaction may contribute to the cytotoxicity of amsacrine toward neutrophils, monocytes and their precursors.

Inhibition of the human leukocyte enzymes myeloperoxidase and eosinophil peroxidase by dapsone

Dapsone (4,4'-diaminodiphenylsulfone) is an antimicrobial substance that also has anti-inflammatory activity, which has been attributed to inhibition of the leukocyte enzyme myeloperoxidase (MPO). We observed that dapsone was a much better inhibitor of the eosinophil peroxidase (EPO) in an assay that measured peroxidase-catalyzed oxidation of tetramethylbenzidine at pH 5.4. To clarify the specificity and pH-dependence of dapsone inhibition of the purified enzymes under more physiologic conditions, we studied peroxidase-catalyzed oxidation of chloride to the antimicrobial and cytotoxic agent hypochlorous acid. Taurine was added as a trap for hypochlorous acid, to prevent inactivation of the enzymes or chlorination of dapsone by hypochlorous acid. Dapsone was much more effective as an inhibitor of both MPO and EPO when chloride rather than tetramethylbenzidine was the substrate. Inhibition of both enzymes was greater at neutral pH than at acid pH (pH 7 vs pH 5), but EPO was more sensitive to inhibition than MPO regardless of pH. Inhibition was increased by lowering chloride, raising hydrogen peroxide, or lowering the enzyme concentration. Inhibition was accompanied by irreversible loss of enzyme activity, which was correlated with loss of the heme absorption spectrum, indicating chemical modification of the enzyme active site. EPO, but not MPO, was partially protected against inactivation by adding physiologic levels of bromide along with chloride. The results suggest that dapsone could prevent MPO- and EPO-mediated tissue injury at sites where the peroxidase enzymes are secreted and diluted into the neutral pH environment of the tissue interstitial space. Dapsone might not inhibit peroxidase-mediated antimicrobial activity, which occurs at high enzyme concentrations in the acid environment of phagolysosomes.

The role of leukocyte-generated reactive metabolites in the pathogenesis of idiosyncratic drug reactions

Evidence strongly suggests that many adverse drug reactions, including idiosyncratic drug reactions, involve reactive metabolites. Furthermore, certain functional groups, which are readily oxidized to reactive metabolites, are associated with a high incidence of adverse reactions. Most drugs can probably form reactive metabolites, but a simple comparison of covalent binding in vitro is unlikely to provide an accurate indication of the relative risk of a drug causing an idiosyncratic reaction because it does not provide an indication of how efficiently the metabolite is detoxified in vivo. In addition, the incidence and nature of adverse reactions associated with a given drug is probably determined in large measure by the location of reactive metabolite formation, as well as the chemical reactivity of the reactive metabolite. Such factors will determine which macromolecules the metabolites will bind to, and it is known that covalent binding to some proteins, such as those in the leukocyte membrane, is much more likely to lead to an immune-mediated reaction or other type of toxicity. Some reactive metabolites, such as acyl glucuronides, circulate freely and could lead to adverse reactions in almost any organ; however, most reactive metabolites have a short biological half-life, and although small amounts may escape the organ where they are formed, these metabolites are unlikely to reach sufficient concentrations to cause toxicity in other organs. Many idiosyncratic drug reactions involve leukocytes, especially agranulocytosis and drug-induced lupus. We and others have demonstrated that drugs can be metabolized by activated neutrophils and monocytes to reactive metabolites. The major reaction appears to be reaction with leukocyte-generated hypochlorous acid. Hypochlorous acid is quite reactive, and therefore it is likely that many other drugs will be found that are metabolized by activated leukocytes. Some neutrophil precursors contain myeloperoxidase and the NADPH oxidase system, and it is likely that these cells can also oxidize drugs. Therefore, although there is no direct evidence, it is reasonable to speculate that reactive metabolites generated by activated leukocytes, or neutrophil precursors in the bone marrow, could be responsible for drug-induced agranulocytosis and aplastic anemia. This could involve direct toxicity or an immune-mediated reaction. These mechanisms are not mutually exclusive, and it may be that both mechanisms contribute to the toxicity, even in the same patient. In the case of drug-induced lupus, a prevalent hypothesis for lupus involves modification of class II MHC antigens.(ABSTRACT TRUNCATED AT 400 WORDS)