mechanism of action of non-steroidal anti-rheumatic drugs

1

Mechanism of action of non-steroidal anti- rheumatic drugs

M I C H A E L F O R R E S T P E T E R M. B R O O K S

Inflammation is a highly complex process involving interactions between many mediators and cells (Figure 1). The essential purpose of inflammation can be regarded as a homeostatic process designed to detect the presence of injurious or potentially injurious stimuli within the tissues, to respond to such stimuli by mounting appropriate defensive measures aimed at removing these stimuli and finally to restore normal structure and function. These homeostatic processes are likely to be invoked continually throughout our lifespan, yet as a result of their efficiency we are rarely aware of their presence. When these processes are excessively or inappropriately stimulated we become aware of an inflammatory response and may choose to seek exogenous assistance in the form of pharmacological therapy to reduce the magnitude of that response.

The inflammatory process has been described clinically in terms of five cardinal signs: heat, redness, swelling, pain and loss of function. One could add to this list the accumulation of leukocytes at an inflammatory lesion which, although not obvious on clinical examination, is clearly a feature common to all inflammatory disease. Although these cardinal signs may adequately describe the gross symptoms of inflammation, it is clear that many and varied phenomena underlie individual inflammatory conditions. A corollary to this statement is that a single drug is unlikely to cure an inflammatory disease in which multiple perturbations of mediator and cellular systems are involved. Paradoxically, single drug therapy utilizing non-steroidal anti-inflammatory drugs (NSAIDs) is common medical management of inflammatory joint disease and does provide considerable relief of these symptoms. The reason for this paradox is that the symptoms of inflammation can to a large extent be understood in terms of a single class of inflammatory mediators, the prostaglandins, whose generation from membrane-derived fatty acids (principally arachidonic acid) can be modified with NSAIDs. However, these drugs appear to have relatively little effect on the accumulation of leukocytes in inflammation and, furthermore, the underlying disease process continues relatively~unabated. The purpose of this chapter will be to review the evidence that NSAIDs may control the symptoms of

Bailli~re's ClinicaIRheurnatology--Vol. 2, No. 2, August 1988 275

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MECHANISM OF ACTION 277

inflammation by preventing the generation of proinflammatory prostaglandins from arachidonic acid and that they also have additional effects. These effects include inhibition of the generation of lipoxygenase-derived metabolites of arachidonic acid, inhibition of polymorphonuclear leukocyte (PMNL) function and modulation of immune responses. Ironically this last effect of the NSAIDs may serve to heighten certain immune-mediated inflammatory responses.

ARACHIDONIC ACID METABOLISM

Arachidonic acid is a 20 carbon unsaturated fatty acid derived from the phospholipid of cell membranes by the actions of various phospholipase enzymes. The precise mechanism by which phospholipases release arachidonic acid is incompletely understood but appears to vary with both the cell type studied and the agents used to stimulate release (Bell et al, 1979; Hsueh et al, 1981; Clark et al, 1986). Inhibition of phospholipase, viainduction of the peptide lipocortin (previously referred to as macrocortin and lipomodulin) is one of the processes by which steroids exert their anti-inflammatory activity (Blackwell et al, 1980; Hirata et al, 1980). Arachidonic acid, in its unesterified form, can be metabolized by all mammalian cell types with the exception of mature erythrocytes. There are two major pathways for the metabolism of arachidonic acid (Figure 2). The cyclo-oxygenase enzyme system converts arachidonic acid to prostaglandins (for reviews see Samuelsson et al, 1978; Needleman et al, 1986) while lipoxygenases convert arachidonic acid to monohydroperoxy acids (HPETEs), monohydroxy acids (HETEs) and dihydroxy acids (diHETEs; leukotrienes B~, C4, D4 and E4) (for reviews see Needleman et al, 1986; Parker, 1987). Recent studies have demonstrated a new series of metabolites derived from oxygenation of arachidonic acid at the C4 position. These are known as lipoxins and seem to have biological activity in early studies (Samuelsson, 1985).

The cyclo-oxygenase enzyme initially generates two unstable endoperoxides (prostaglandin G2 and prostaglandin H2) which are further metabolized via isomerases and reductases to the stable prostaglandins E2, D2 and F2~ or metabolized via either prostacyclin synthetase to prostacyclin (Moncada et al, 1976) or thromboxane synthetase to thromboxane A2 (Needleman et al, 1976). Following liberation of free arachidonic acid the spectrum of metabolites produced is in large part determined by the profile of enzymes available within particular cell types (Samuelsson, 1983). For example, endothelial ceils produce predominantly prostacyclin whereas platelets generate thromboxane A2 (see Moncada and Vane, 1979).

In mammalian systems there are three major lipoxygenase enzymes which differ in the particular double bond on the arachidonic acid molecule where enzymatic attack is initiated. These enzymes are a 5-1ipoxygenase found predominantlyin leukocytes (Borgeat et al, 1976), a 12-1ipoxygenase foundin platelets (Hamberg and Samuelsson, 1974; Nugteren, 1975) and a 15- lipoxygenase whose distribution is less well characterized but is certainly found in leukocytes, particularly eosinophils (Turk et al, 1982), and epithelial

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cells (Hunter et al, 1985). The lipoxygenase enzymes initially generate monohydroperoxy fatty acids (HPETEs) which undergo further transformations to produce monohydroxy acids (HETEs). The 5-1ipoxygenase enzyme may also generate a series of compounds called leukotrienes which contain a characteristic conjugated triene structure (Murphy et al, 1979). Leukotriene A4 is the substrate for either leukotriene A4 hydrolase to produce leukotriene B4 or glutathione-S-transferase to produce the sulphidopeptide leukotriene C4. The action of successive peptidases then metabolizes leukotriene C4 to leukotriene D4 and leukotriene E4. As with the generation of cyclo- oxygenase products there are differences in the profiles of lipoxygenase- derived arachidonic acid metabolites depending on the cell type, the species and the stimulus used. In general, leukocytes, platelets and mast cells are the major sources of lipoxygenase products, although they are also produced from epithelial cells (Hunter et al, 1985), keratinocytes (Brain et al, 1982; Grabbe et al, 1984) and blood vessels (Greenwald et al, 1979). There are reports on the synthesis of leukotriene B4 from lymphocytes (Goetzl, 1981), however this finding has not been repeated by other groups (Borgeat et al, 1985; Parker, 1987).

Role of arachidonic acid metabolites in inflammation

The view that metabolites of arachidonic acid play a role in inflammation arises from three lines of evidence. Firstly, the detection of arachidonic acid metabolites in inflammatory lesions; secondly, the ability of these purified compounds to mimic aspects of the inflammatory process and, finally, the observed effects of inhibitors of arachidonic acid metabolism on the inflammatory process.

Detection of arachidonic acid metabolites in inflammatory lesions

Prostaglandin E2, 6-oxo-prostaglandin FI~ (the stable breakdown product of prostacyclin), thromboxane B2 (the stable breakdown product of thromboxane A2) and leukotrienes B4, C4 and D4 have all been detected at relevant concentrations in a variety of both animal models of inflammation and in human inflammatory diseases (Table 1). This mass of data leaves little doubt that arachidonic acid metabolites are generated in inflammatory diseases although the relative amounts produced in particular models or human conditions varies considerably. For example, leukotriene B4 is found in high concentrations in the skin of patients with psoriasis (Brain et al, 1984) and in the synovial fluid of patients with gout, whereas its concentration in synovial fluid taken from patients with rheumatoid arthritis is low (Rae et al, 1982).

Proinflammatory properties of arachidonic acid metabolites

The proinflammatory properties of prostaglandin E2 and prostacyclin upon application in a variety of models include vasodilatation (Solomon et al, 1968; Williams, 1979), oedema formation in man and rats where the effect is


Table 1. Some inflammatory lesions in which arachidonic acid metabolites have been detected.

Lesion Reference

Animal models Rat air pouch

Adjuvant arthritis Carrageenin granuloma Rat gastric ulcer model Rabbit hydronephrosis Rabbit myocardial infarction Rat brain ischaemia Human disease Gout Rheumatoid synovial fluid Psoriatic skin lesions

Human skin after allergic challenge Inflammatory bowel disease

Cystic fibrosis sputum Asthma Acute anterior nveitis Obstructive airways disease sputum

Ohuchi et al (1984) Brooks et al (1987) Parnham et al (1978) Ch~ing et al (1976) Basso et al (1983) Okegawa et al (1983) Barst and Mullane (1985) Moskowitz et al (1984)

Rae et al (1982) Klickstein et al (1980) Brain et al (1984) Brain et al (1985) Bisgaard et al (1985) Gould (1975) Sharon and Stenson (1984) Cromwell et al (1981) Green et al (1974) Eakins et al (1972) O'Driscoll et al (1984)

an indirect one via the degranulation of mast cells (Crunkhorn and Willis, 1971; Takagi et al, 1987) potentiation of oedema formation (Williams and Morley, 1973; Moncada et al, 1973), potentiation of neutrophil accumulation induced by other mediators (Issekutz and Movat, 1982) and hyper- algesia (Solomon et al, 1968; Ferreira, 1972). Lipoxygenase-derived products, particularly leukotriene B4, are both chemotactic and chemokinetic for a number of cell types including neutrophils, monocytes and eosinophils (Palmer et al, 1980; Bray, 1983). Leukotriene B4 increases vascular permeability via an interaction between neutrophils and vascular endothelial cells (Wedmore and Williams, 1981) and also interacts with neutrophils to stimulate a variety of cell functions including lysosomal enzyme release, generation of oxygen-free radicals and increased cellular adhesiveness (Naccache and Sha'afi, 1983). Leukotrienes C4 and D4 have variable effects on the microcirculation, being constrictor in some species (Peck et al, 1981) yet vasodilator in others (Bisgaard et al, 1982), and increase vascular permeability in some species but not others (Ueno et al, 1981).

THE EFFECTS OF NON-STEROIDAL ANTI-INFLAMMATORY DRUGS (NSAIDs)

Effect of NSAIDs on the cyclo-oxygenase pathway of arachidonic acid metabolism

Perhaps the most compelling evidence for a role of arachidonic acid metabolites in inflammation arises from the use of inhibitors of the


generation of these products and the resultant effects on the inflammatory process. In 1971, Vane (see also Ferreira et al, 1971; Smith and Willis, 1971) demonstrated that a common property of NSAIDs was their ability to inhibit the cyclo-oxygenase enzyme responsible for the metabolism of arachidonic acid to prostaglandins. The known proinflammatory properties of prostaglandins at that time led to the conclusion that the anti-inflammatory activity of NSAIDs could be attributed to their effects on the cyclo-oxygenase enzyme. Further support for this concept arose by demonstrating significant correlations between the concentrations of drug required to inhibit the cyclo-oxygenase enzyme (both in vivo and in vitro) and the concentrations of drug required to inhibit inflammation in experimental models (Higgs et al, 1976) and clinically (Robinson et al, 1978).

An apparent paradox was noticed when it was found that prostaglandins of the E series produced only modest increases in oedema formation when injected into the skin of either guinea-pigs (Williams and Morley, 1973) or rabbits (Williams and Peck, 1977). However, in these experiments it was found that prostaglandins could produce a significant potentiation of the oedema formation induced by the permeability-increasing mediators, histamine and bradykinin. These observations have since been extended to show that synergism exists between a wide range of permeability-increasing mediators and vasodilators in inducing oedema formation (Williams et al, 1983). This phenomenon is believed to be a result of prostaglandins increasing dermal blood flow and intravenular hydrostatic pressure. This contention is supported by the finding that the synergistic effect of prostaglandins and permeability-increasing mediators is less in the peritoneal cavity of animals where basal blood flow is higher (Forrest et al, 1986). One can therefore speculate that if inflammatory oedema formation is occurring in response to the generation of a wide range of permeability-increasing mediators together with a vasodilator prostaglandin, then NSAIDs will inhibit the oedema formation through inhibition ofprostaglandin synthesis. The ability to reduce inflammatory oedema through inhibition of a single class of compounds, the prostaglandins, will clearly be more satisfactory than trying to inhibit the generation or action of many different permeability-increasing mediators.

Effect of NSAIDs on the lipoxygenase pathway of arachidonic acid metabolism

Whereas there is a clear relationship between NSAIDs and inhibition of the cyclo-oxygenase pathway of arachidonic acid metabolism, the effect of these drugs on the generation of lipoxygenase-derived arachidonic acid metabolites is less clear. For some NSAIDs such as indomethacin there is evidence that at low doses there is an increased generation of lipoxygenase products (Salmon et al, 1983; Robinson et al, 1986). The mechanism behind this potentiation is uncertain, although it has been suggested that by blocking the cyclo-oxygenase path more free arachidonic acid is available for metabolism by the lipoxygenase pathway (Salmon et al, 1983). An alternative explanation for this phenomenon is that prostaglandins normally serve to reduce the production of leukotrienes, inhibition of prostaglandin production


will therefore result in increased generation of leukotrienes. Evidence for such a mechanism is suggested by the observation that E-type prostaglandins can inhibit leukotriene B4 production in polymorphonuclear leukocytes (Ham et al, 1983) and that prostacyclin reduces the efflux of leukotriene C4 from perfused rat hearts (Karmazyn, 1987).

Inhibition of the 5-1ipoxygenase enzyme has been reported for a number of NSAIDs although the concentrations required are generally higher than those that are likely to be achieved therapeutically (Kitchen et al, 1985; Ku et al, 1985; Boctor et al, 1986). There also appear to be marked differences in the ability of NSAIDs to inhibit 5-1ipoxygenase depending on the species, cell type and stimulus used in different experiments (Kitchen et al, 1985). Some compounds have been produced which exhibit similar inhibitory activities against both the lipoxygenase and cyclo-oxygenase enzymes. One such compound, BW 755c (Higgs et al, 1979), was particularly effective at suppressing leukocyte infiltration in an animal model of inflammation but has subsequently proven too toxic for clinical use. Benoxaprofen was also reported to inhibit the 5-1ipoxygenase enzyme (Walker and Dawson, 1979), although this claim has been disputed (Salmon et al, 1984a,b). Again toxicity forced the withdrawal of this compound. There are also reports of NSAIDs inhibiting other lipoxygenase enzymes such as the peroxidase converting 12-HPETE to 12-HETE (Siegel et al, 1979) and of the 11- and 15-1ipoxygenase of rat neutrophils (Siegel et al, 1980).

In addition to direct effects on lipoxygenase enzymes, some NSAIDs have the ability to reduce the production of lipoxygenase products via other mechanisms. For example, aspirin and indomethacin have both been reported to show inhibitory activity against phospholipases A2 (Kaplan et al, 1978) and C (Bomalski et al, 1986; Clark et al, 1986). In addition, diclofenac reduces leukotriene production from leukocytes, not by an effect on either the lipoxygenase or phospholipase enzymes but apparently by stimulating the reincorporation of free arachidonic acid into triglycerides (Ku et al, 1986).

Anti-inflammatory effects of NSAIDs unrelated to inhibition of cyclo-oxygenase

There is an increasing body of evidence to suggcst that many of the anti- inflammatory effects of the NSAIDS arc unrelated to an inhibition of the cyclo-oxygenase enzyme system. Should this ultimately prove to be the case then a major implication is that novel drugs may be designed to exploit particular pharmacological properties of compounds with precise therapeutic activities and, hopefully, with minimal toxic side-effects.

In animal studies there was little difference in the potency of acetylsalicylic acid and sodium salicylate against carrageenin-induced paw oedema (Niemegeers et al, 1964), and yet in vitro, acetylsalicylic acid is a more potent inhibitor of the cyclo-oxygenase enzyme (Vane, 1971). Furthermore, the dose of aspirin which is required to inhibit the generation of prostaglandins is considerably less than the amounts which are used therapeutically in the treatment of inflammatory joint disease (Atkinson


and Collier, 1981). It has been suggested that aspirin may act as a pro-drug for salicylate (Henderson et al, 1986) and that both drugs inhibit inflammation as a consequence of salicylate inhibiting prostaglandin E2 formation. An alternative explanation would be that salicylate (either administered or derived from aspirin) inhibits leukocyte accumulation by an effect independent of the inhibition of cyclo-oxygenase and that the reduced cell numbers are responsible for the observed reduction in prostaglandin levels.

We have recently demonstrated (Forrest et al, 1987a) that indomethacin (2-50 mg/kg body weight) and piroxicam (3-30 mg/kg body weight) inhibit lipopolysaccharide (LPS) induced plasma leakage and polymorphonuclear leukocyte (PMNL) accumulation in the rat subcutaneous air-pouch in a dose-dependent manner. The inhibition of plasma leakage is readily reversed by administering prostaglandin E2 together with the LPS, suggesting that prostaglandins play a role in mediating plasma leakage. Prosta- glandin E2 also reversed the inhibition of PMNL accumulation in indomethacin-treated animals but not that in piroxicam-treated animals. This clearly indicates that some NSAIDs have inhibitory effects on the inflammatory process quite unrelated to an inhibition of the cyclo-oxygenase enzyme.

Clinically there are also suggestions that a common mechanism of action for NSAIDs is unlikely. This view is proposed because there are some inflammatory joint diseases which respond particularly well to a particular NSAID; for example, the use of indomethacin in gout (Smyth and Percy, 1973) and of phenylbutazone in ankylosing spondylitis (Rosenbloom et al, 1985). It seems unlikely that this variability in response to particular NSAIDs in particular diseases can be explained solely on a pharmacokinetic basis; it is more likely to be due to pharmacodynamic factors (Day and Brooks, 1987).

Arachidonic acid metabolites, NSAIDs and their effect on neutrophil functions

The polymorphonuclear leukocyte (PMNL) plays a major role in the genesis of inflammatory responses. It is the first cell type to accumulate in sub- stantial numbers in inflammatory lesions and predominantly serves the function of clearing both viable and non-viable particulate matter by the processes of phagocytosis and extracellular killing of microbial organisms. To this end the accumulation of PMNL in an inflammatory lesion is a useful defensive process. However, in inflammatory diseases the PMNL appear to be either excessively or inappropriately activated, such that their defensive roles become offensive. In this situation the cells can cause host tissue damage through the generation of reactive oxygen species and the release of lysosomal enzymes with the capacity to degrade connective tissues.

The principal biological activities of PMNL (Table 2) which become expressed during inflammation are: increased adhesiveness, allowing the cells to adhere initially to the vascular endothelium and subsequently to the connective tissue matrix; chemotaxis and chemokinesis for localizing PMNL at the inflammatory focus; phagocytosis of particulate material; the


Table 2. Biological activities of polymorphonuclear leukocytes which become expressed during inflammation.

Phagocytosis Binding of immunoglobulin and complement Degranulation and release of lysosomal enzymes Oxygen radical generation Release of proteinases which liberate peptide mediators from plasma protein precursors

(kinins, angiotensin, C5a) Release of prostaglandin E2 and leukotriene B4

generation of reactive oxygen species; the release of lysosomal enzymes and an ability to control postcapillary venular permeability to plasma proteins. There is evidence that all of these functions can be modulated by both arachidonic acid metabolites and by inhibitors of arachidonic acid metabolism (Abramson et at, 1983; Fantone, 1985).

The effect of NSAIDs on neutrophil function is complex and variable results are seen between in vivo and in vitro experiments. The reasons for this complexity are that although NSAIDs predominantly inhibit the cyclo- oxygenase pathway they also have both stimulatory and inhibitory effects on the lipoxygenase pathway, depending on the dose and the particular NSAID under consideration. Furthermore, it is apparent that some NSAIDs have effects on neutrophils quite unrelated to an effect on arachidonic acid metabolism.

Effects on neutrophil chemotaxis

In vivo effects

A number of experiments have shown NSAIDs to inhibit neutrophil accumulation in vivo during inflammatory responses initiated by a variety of stimuli (Walker et al, 1976; Ackerman et al, 1982; Shimanuki et al, 1985; Palder et al, 1986). However, an inhibition of PMNL accumulation by NSAIDs is not a general phenomenon and, indeed, some authors find an increase in cell numbers following treatment with NSAIDs (Eakins et al, 1980). These discrepancies may be explained partly on the basis of different doses of drugs being used in different models in response to different stimuli. Certainly the observation that lipoxygenase-derived arachidonic acid metabolites show potent chemotactic activity towards neutrophils both in vivo and in vitro would suggest that NSAIDs with an ability to inhibit the production of these compounds would be particularly useful at reducing cellular accumulation. Indeed Salmon et al (1983) have found that BW 755c does inhibit both leukotriene B4 production and neutrophil accumulation in carrageenin-soaked polyester sponges implanted subcutaneously in rats. Whether these effects are causally related is not clear, particularly as recent studies suggest that leukotriene B4 is not chemotactic for rat neutrophils either in vitro (Kreisle et al, 1985) or in vivo (Foster et al, 1986; Forrest, 1987b).

Our own experience using the rat subcutaneous air-pouch suggests that the


ability of NSAIDs to inhibit neutrophil accumulation is clearly dependent on the nature of the inflammatory stimulus used, which in turn will generate a different profile of inflammatory mediators. Thus, indomethacin (2 mg/kg i.v.) inhibits peptone-induced neutrophil accumulation by approximately 80% yet has only a marginal effect on monosodium urate (MSU) crystal- induced neutrophil accumulation (Figure 3). Despite this differential effect on cell accumulation, indomethacin inhibits plasma leakage induced by either stimulus to a similar extent (Figure 3). These results can be explained on the basis of different stimuli generating different chemotactic mediators. For example, if peptone stimulates neutrophil accumulation through activation of the complement system, then indomethacin could reduce cell accumulation by preventing plasma leakage and hence the supply of plasma-derived complement components. On the other hand, MSU crystals may be acting through the generation of a non-plasma-derived chemotactic factor. In this instance, although indomethacin has again reduced plasma leakage it has had no effect on the generation of chemotactic mediators.

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In vitro effects

As with the in vivo effect of NSAIDs on neutrophil chemotaxis, these drugs exhibit variable in vitro effects depending on the source of the cells, the nature of the chemoattractant and the method used to assess chemotaxis (Kitchen et al, 1985). Furthermore, there has been a suggestion that the


processes of chemotaxis and chemokinesis may be differentially susceptible to the effects of NSAIDs (Rivkin et al, 1976). There are reports of NSAIDs inhibiting neutrophil chemotaxis in vitro (Brown and Collins, 1977; Perianin et al, 1985; Nielsen and Webster, 1987). However, one can equally find reports demonstrating no effect of NSAIDs on chemotaxis in vitro (Perianin et al, 1984; Boot et al, 1985; Palder et al, 1986).

Effect of NSAIDs on the generation of superoxide and the release of lysosomal enzymes

An inhibition of the generation of superoxide anions and of the release of lysosomal enzymes is a property of many NSAIDs (Smolen and Weissman, 1980; Abramson et al, 1983; Minta and Williams, 1985). However, the ability of NSAIDs to inhibit these functions is clearly stimulus-dependent, such that FMLP-induced superoxide production and lysosomal enzyme release is inhibited by many NSAIDs, whereas PMA-induced production is not affected (Abramson et al, 1983; Nielsen and Webster, 1987). PMA is believed to stimulate PMNL by a direct activation of protein kinase C, whereas FMLP acts via a receptor-linked mechanism. It has been reported that NSAIDs can interfere with the binding of FMLP to this receptor, thus producing an inhibition of PMNL responses (Minta and Williams, 1985; Skubitz and Hammerschmidt, 1986). However, this is probably not the sole reason for their inhibitory action as ibuprofen can inhibit PMNL functions stimulated by a variety of ligands including FMLP, the complement derived peptide C5a and the particulate stimulus serum opsonized zymosan. This would suggest that ibuprofen was acting at some other stage in stimulus- response coupling (Nielsen and Webster, 1987).

Effect of NSAIDs on neutrophil aggregation and adhesion

The adhesion of neutrophils to each other (aggregation) and their adhesion to vascular endothelium are processes with important physiological and pathological consequences (Jacob, 1981, 1983). Although it is possible that these processes share common biochemical mechanisms, any extrapolation from drug effects on one process to the other should be treated with some caution. However, it is observed that of all the neutrophil processes influenced by NSAIDs, adherence (Nielsen and Webster, 1987) and aggregation (Kaplan et al, 1984) appear to be the most sensitive. Furthermore, inhibition of neutrophil aggregation is achieved by the majority of NSAIDs, whereas not all NSAIDs will inhibit other aspects of neutrophil activation such as lysosomal enzyme release (Abramson et al, 1983; Kaplan et al, 1984).

The accumulation of neutrophils within tissues during inflammation requires that the cells initially adhere to the vascular endothelium prior to their diapedesis through the vessel wall and subsequent migration to the inflammatory focus. One might therefore speculate that a drug which reduces neutrophil adhesion would produce a corresponding reduction in cell accumulation. This hypothesis is in part supported by the experiments of Ackerman


et al (1982) who found that both indomethacin and dexamethasone reduced the in vitro adhesion of neutrophils and the in vivo accumulation of neutrophils. However, the anti-rheumatic drug levamisole inhibited the in vitro adhesion of neutrophils but not their in vivo accumulation, suggesting that there is not a simple relationship between these two processes.

Effect of NSAIDs on neutrophil-dependent inflammatory oedema formation

In 1981 Wedmore and Williams found that permeability-increasing mediators could be grouped into those that acted directly on vascular endothelium (for example, histamine and bradykinin) and those that had an indirect effect through an interaction between neutrophils and vascular endothelium. These indirectly acting mediators all share the common property of stimulating neutrophils and include the complement-derived peptide C5a and leukotriene B4. Subsequently, Rampart and Williams (1986) have found that ibuprofen inhibits neutrophil-dependent oedema formation but not that induced by bradykinin.

Effect of prostaglandins and NSAIDs on the immune system

There is much evidence (Goodwin and Webb, 1980; Lewis, 1983; Goodwin, 1984) to support a role for prostaglandins in the immune response. This concept was proposed after observing that E-type prostaglandins inhibited a number of lymphocyte responses both in vitro (Smith et al, 1971; Morley, 1974; Gordon et al, 1976) and in vivo (Quagliata et al, 1973). That prostaglandin production during the immune response played an immunoregulatory role came from the work of Webb and Osheroff (1976) who found an increase in intrasplenic prostaglandin Fza following the injection of sheep red blood cells into mice. If mice were pretreated with indomethacin the rise in prostaglandin F2~ was abrogated, while an increase in splenic plaque- forming units was observed. This strongly suggested that prostaglandin production during the immune response was serving as a negative feedback mechanism to limit the magnitude of the immune response.

It is clear that prostaglandins exhibit many suppressive activities in cellular immune function (Table 3). One might predict therefore that NSAIDs would augment cellular immune function in vivo. This indeed has proven to be the case. For example, the delayed hypersensitivity response to Mycobacterium in guinea pigs was greatly enhanced by the simultaneous administration of indomethacin (Muscoplat et al, 1978). Furthermore, in normal human subjects indomethacin failed to enhance delayed hypersensitivity responses (Goodwin et al, 1978a) but was effective in a group of patients with depressed cellular immunity (Goodwin et al, 1978b).

The effect of pr0staglandins on the humoral immune response is mostly achieved through effects on T cell regulation (Goodwin, 1984). The activity of suppressor T cells appears to be inhibited by prostaglandin E2. Thus, stimulation of antibody from human peripheral blood mononuclear cells in response to pokeweed mitogen is inhibited by pretreatment of cell cultures


Table 3. Immunomodulatory activity of arachidonic acid metabolites.

Metabolite Action

LTB4

PGE2

PMN aggregation Enhanced C3B receptor expression on PMN Chemotaxis of PMN and eosinophils induces superoxide production

May inhibit suppressor T cells May increase mitogen-induced proliferation Inhibits lymphokine production Regulates B-cell immunoglobulin production

From Ford-Hutchinson, 1984; Goodwin et al, 1984; Kunkel and Chensue, 1984.

with NSAIDs and this effect can be reversed with physiological doses of prostaglandin E2 (Ceuppens and Goodwin, 1982a). These phenomena are abolished if the cell cultures are depleted of suppressor T cells. Comparable results are found when looking at rheumatoid factor production in peripheral blood mononuclear cells from rheumatoid arthritis patients (Ceuppens and Goodwin, 1982b).

Although NSAIDs are used so extensively in the treatment of inflammatory disease, where they play an important role in the alleviation of symptoms, it is likely that they may in fact be contributing to the chronicity of the disease. This will arise because NSAIDs inhibit the production of prostaglandins which play a role in feedback inhibition of the immune response.

CONCLUSION

There can be little doubt that metabolites of arachidonic acid play many functional roles in both inflammation and immunity. The fact that we can inhibit the generation of these compounds with relative ease, yet cannot cure inflammatory or immunologically based diseases, suggests a complex interrelationship between eicosanoids and inflammation. This complexity is compounded by both physiological and biochemical interactions between different eicosanoids, some of which are incompletely understood (Borgeat et al, 1983). It is most important that we should fully understand the biochemistry of eicosanoid metabolism, such that we may subsequently be able to understand the effects of drugs. It is feasible that our currently available drugs are interfering at an inappropriate stage in eicosanoid metabolism and that greater benefit may be derived through the use of compounds with a more selective action against particular metabolites of arachidonic acid.

It is also apparent that some of the biological effects of the NSAIDs are not related to effects on arachidonic acid metabolism. This provides the tantalizing possibility that novel anti-inflammatory drugs could be developed with effects quite unrelated to an inhibition of arachidonic acid metabolism.


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