Paracetamol (acetaminophen) is generally considered to be a weak inhibitor of the synthesis of prostaglandins (PGs). However, the in vivo effects of paracetamol are similar to those of the selective cyclooxygenase-2 (COX-2) inhibitors. Paracetamol also decreases PG concentrations in vivo, but, unlike the selective COX-2 inhibitors, paracetamol does not suppress the inflammation of rheumatoid arthritis. It does, however, decrease swelling after oral surgery in humans and suppresses inflammation in rats and mice. Paracetamol is a weak inhibitor of PG synthesis of COX-1 and COX-2 in broken cell systems, but, by contrast, therapeutic concentrations of paracetamol inhibit PG synthesis in intact cells in vitro when the levels of the substrate arachidonic acid are low (less than about 5 mumol/L). When the levels of arachidonic acid are low, PGs are synthesized largely by COX-2 in cells that contain both COX-1 and COX-2. Thus, the apparent selectivity of paracetamol may be due to inhibition of COX-2-dependent pathways that are proceeding at low rates. This hypothesis is consistent with the similar pharmacological effects of paracetamol and the selective COX-2 inhibitors. COX-3, a splice variant of COX-1, has been suggested to be the site of action of paracetamol, but genomic and kinetic analysis indicates that this selective interaction is unlikely to be clinically relevant. There is considerable evidence that the analgesic effect of paracetamol is central and is due to activation of descending serotonergic pathways, but its primary site of action may still be inhibition of PG synthesis. The action of paracetamol at a molecular level is unclear but could be related to the production of reactive metabolites by the peroxidase function of COX-2, which could deplete glutathione, a cofactor of enzymes such as PGE synthase.

Mechanism of action of paracetamol.

INFLAMMATORY reactions are characterised by hyperaemia, and exudation of plasma resulting in tissue swelling. Putative mediators of inflammation have usually been evaluated according to their ability to mimic inflammatory reactions1. Although prostaglandins were identified in inflammatory exudates2, and prostaglandin synthesis in vitro was shown to be inhibited by nonsteroid anti-inflammatory compounds3–5, prostaglandins (unlike histamine and bradykinin) were found to be poor at eliciting plasma exudation when injected into guinea pig6,7 or rabbit skin8. But, the finding that exogenous prostaglandins (notably of the E-type) potentiate plasma exudation produced by other mediators7–11 suggested that this may be the role of prostaglandins in inflammation. An alternative view has been proposed by Kuehl et al.12, who consider that since E and F prostaglandins fail fully to mimic inflammatory reactions, other products of arachidonic acid should be considered as mediators. They suggest that the unstable prostaglandin endoperoxide, PGG2, or a nonprostaglandin product of it (a free radical could be the important mediator. We have re-examined the mode of action of prostaglandins in inflammation by measuring both increased blood flow and plasma exudation. The results presented here suggest that the mediation of vasodilatation and increased vascular permeability should be considered separately. Prostaglandins (notably the E-type13,14) mediate vasodilatation (although they may have some additional mast cell degranulating activity in the rat15). It is this vasodilatation which is responsible for the potentiation of the exudation produced by other mediators. Our observations provide a new hypothesis for the microvascular mechanisms involved in the action of non-steroid anti-inflammatory compounds.

Role of prostaglandin-mediated vasodilatation in inflammation

Copper complexes of non-steroidal anti-inflammatory drugs: an opportunity yet to be realized

Paracetamol is used worldwide for its analgesic and antipyretic actions. It has a spectrum of action similar to that of NSAIDs and resembles particularly the COX-2 selective inhibitors. Paracetamol is, on average, a weaker analgesic than NSAIDs or COX-2 selective inhibitors but is often preferred because of its better tolerance. Despite the similarities to NSAIDs, the mode of action of paracetamol has been uncertain, but it is now generally accepted that it inhibits COX-1 and COX-2 through metabolism by the peroxidase function of these isoenzymes. This results in inhibition of phenoxyl radical formation from a critical tyrosine residue essential for the cyclooxygenase activity of COX-1 and COX-2 and prostaglandin (PG) synthesis. Paracetamol shows selectivity for inhibition of the synthesis of PGs and related factors when low levels of arachidonic acid and peroxides are available but conversely, it has little activity at substantial levels of arachidonic acid and peroxides. The result is that paracetamol does not suppress the severe inflammation of rheumatoid arthritis and acute gout but does inhibit the lesser inflammation resulting from extraction of teeth and is also active in a variety of inflammatory tests in experimental animals. Paracetamol often appears to have COX-2 selectivity. The apparent COX-2 selectivity of action of paracetamol is shown by its poor anti-platelet activity and good gastrointestinal tolerance. Unlike both non-selective NSAIDs and selective COX-2 inhibitors, paracetamol inhibits other peroxidase enzymes including myeloperoxidase. Inhibition of myeloperoxidase involves paracetamol oxidation and concomitant decreased formation of halogenating oxidants (e.g. hypochlorous acid, hypobromous acid) that may be associated with multiple inflammatory pathologies including atherosclerosis and rheumatic diseases. Paracetamol may, therefore, slow the development of these diseases. Paracetamol, NSAIDs and selective COX-2 inhibitors all have central and peripheral effects. As is the case with the NSAIDs, including the selective COX-2 inhibitors, the analgesic effects of paracetamol are reduced by inhibitors of many endogenous neurotransmitter systems including serotonergic, opioid and cannabinoid systems. There is considerable debate about the hepatotoxicity of therapeutic doses of paracetamol. Much of the toxicity may result from overuse of combinations of paracetamol with opioids which are widely used, particularly in USA.

The modern pharmacology of paracetamol: therapeutic actions, mechanism of action, metabolism, toxicity and recent pharmacological findings

Old and New Generation Lipid Mediators in Acute Inflammation and Resolution

The effects of nordihydroguairetic acid (NDGA), 3-amino-1-trifluoromethyl-)-phenyl-2-pyrazoline (BW755c), eicostatetraynoic acid (ETYA), phenidone, quercetin, and indomethacin (INDO) on the synthesis of 15-hydroxyeicosatatetraenoic acid (15-HETE) from soybean 15-lipoxygenase, leukotriene B4 (LTB4 from 5-lipoxygenase, and prostaglandin E2 (PGE2 from cyclooxygenase enzymes of rat neutrophils and mouse peritoneal macrophages were investigated. All of the drugs caused a dose-related inhibition of increased oxygen consumption by soybean 15-lipoxygenase in the presence of arachidonic acid and the rank order of potency was phenidone ≥ BW755c > ETYA > quercetin > NDGA > indomethacin. The reduction in oxygen consumption correlated with a reduction of 15-HETE formation as identified by high-performance liquid chromatography. Apart from indomethacin, these drugs were also effective against the rat neutrophil 5-lipoxygenase, although the rank order of potency did not correlate with that obtained with soybean 15-lipoxygenase. Furthermore, in both A23187-activated rat neutrophils and zymosan-activated mouse peritoneal macrophages the synthesis of prostaglandins was inhibited by all of these drugs. In the neutrophils, the rank order of potency was INDO > ETYA > BW755c > quercetin > NDGA > phenidone, whereas in mouse peritoneal macrophages, the order was INDO > ETYA > BW755c > NDGA > quercetin > phenidone. These results suggest that putative lipoxygenase inhibitors exhibit both qualitative and quantitative differences in their effects on both lipoxygenases and cyclooxygenases.

Differential effects of putative lipoxygenase inhibitors on arachidonic acid metabolism in cell-free and intact cell preparations.

1 Prostaglandins E1 and E2 but not prostaglandin F2alpha, arachidonic acid or linolenic acid, produced slight oedema when injected into the rat hindpaw. 2 Prostaglandin E1 potentiated hindpaw oedema produced by carrageenan, kaolin, bradykinin and trypsin but not that produced by 5-hydroxytryptamine (5-HT), histamine, dextran B or compound 48/80. Carrageenan- and bradykinin-induced paw oedemas were also potentiated by prostaglandin E2. Arachidonic acid potentiated responses to carrageenan and kaolin but not responses to bradykinin, trypsin, 5-HT, histamine, dextran B or compound 48/80. Linolenic acid did not potentiate hindpaw oedema induced by carrageenan. 3 Potentiation of carrageenan-induced oedema by prostaglandin E1 was not diminished by pretreatment with indomethacin, hydrocortisone or cyproheptadine. However, arachidonic acid potentiation of carrageenan oedema was reduced by pretreatment with non-steroidal anti-inflammatory drugs but not by anti-inflammatory steroids or by paracetamol. 4 The enhancement of the response to carrageenan and kaolin by prostaglandins E1, E2 and arachidonic acid is discussed in terms of kinin mediation.

On the ability of prostaglandin E1, and arachidonic acid to modulate experimentally induced oedema in the rat paw.

Copper was believed to be of therapeutic value as long ago as 1000 years s.c.; in particular, copper bracelets and foods high in copper were thought to be beneficial in treating arthritic conditions [ll. In 1945, patients with rheumatoid arthritis (RA) were shown to exhibit higher than normal serum copper levels [2]. Nevertheless, during the 1940's to early 1970's parenterally-administered copper complexes were used successfully in the treatment of arthritic conditions [3]. However, the possible acceptance of this form of therapy was impeded by the concomitant development of anti-inflammatory steroids and aspirin-like nonsteroidal anti-imflammatory drugs. It was not until 1960 that Bonta placed the clinical studies on a more scientific footing when he showed that copper compounds possessed anti-inflammatory effects in animals [4]. Sorenson confirmed and extended these findings and he opened a new chapter in the copper story by claiming that copper complexes of anti-inflammatories were indeed the active forms of these drugs [5, 6]. He also speculated that antiarthritic agents might promote tissue distribution of elevated serum copper in RA patients which was a physiological response to the inflammation (Fig. 1). The use of copper to treat RA was rationalized by suggesting that this physiological response was insufficient to quell the ongoing inflammatory processes and, consequently, exogenous help was needed. The modulatory role for copper was supported by the studies of Milanino and associates who demonstrated that exacerbation of inflammation occurred in rats on a copper-deficient diet [7]. Numerous multidisciplinary groups have subsequently re-examined the paradoxical role of copper in inflammation emphasizing the therapeutic usefulness of copper, in inflammatory conditions. Two excellent reviews [6, 8] and two books [9, 10] on the subject have recently been published covering much of this research.

The role of copper in inflammatory disorders

The development of novel anti-inflammatory drugs (AID) has been claimed to be dependent on the discovery of models of inflammation that differ from those currently used for drug screening, e.g. carrageenen paw oedema and u.v. erythema. We have thus evaluated the effect of a variety of drugs in a number of novel models of inflammation in the rat produced in the hind paw. We have utilized kaolin, zymosan, anti-rat IgG (anti-IgG) and the Reversed Passive Arthus (RPA) reaction to produce these oedema models. We found that the non-steroidal AID's, e.g. aspirin, flufenamic acid, indomethacin, naproxen, and phenylbutazone, were active in all four tests. Of the nine novel AID examined, levamisole and tetramisole demonstrated considerable activity in all four tests and dapsone was especially active in the anti-IgG and RPA tests. In contrast, the anti-rheumatic d-penicillamine was inactive in all four models. Each of the ten compounds tested which has been claimed to influence complement function, was active in the RPA but not in the kaolin model. These results are discussed in the context of the aetiology of each oedema and the suspected mode of action of the various drugs.

Comparative effects of drugs on four paw oedema models in the rat.

Several gold salts were compared in kaolin-induced rat paw oedema, u.v. erythema in guinea pigs, delayed type hypersensitivity and humoral immunity in mice, and adjuvant-induced arthritis in the rat. In the latter the additional parameters of serum gold and copper levels and lysosomal enzyme activity were determined. In addition, the in vitro inhibition of several lysosomal enzymes derived from mouse macrophages was studied. The gold compounds examined were aurothiomalate, aurothioglucose, triethylphosphine gold chloride (SK & F 36914) and its glucopyranoside derivative (SK & F D-39162), triphenylphosphine gold chloride and sodium gold chloride dihydrate. SK & F 36914 and SK & F D-39162 has significant activity after oral dosage upon paw kaolin and u.v. erythema in rats and guinea pigs, respectively. Gastric swelling also occurred. In Wistar rats, adjuvant arthritis was little affected by the gold salts but in the Lewis rats there was suppression. In both strains there was less elevation in serum copper levels with treatment by SK & F 36914 and SK & F D-39162, but not by aurothiomalate. None of the compounds had any measurable effect on delayed hypersensitivity or humoral antibody levels in mice. The in vitro activities of cathepsin B1 and cathepsin D were inhibited by all the gold compounds. Reactivity of gold compounds with glutathione and cysteine in vitro was dependent on compound solubility and the nature of the gold ligand. Considerable differences exist between the profiles of activity for the different gold salts evaluated. These observations indicate that some gold salts do possess anti-inflammatory activity with a potency similar to that of indomethacin.

A. J. Lewis

Papers

Differential effects of putative lipoxygenase inhibitors on arachidonic acid metabolism in cell-free and intact cell preparations.

On the ability of prostaglandin E1, and arachidonic acid to modulate experimentally induced oedema in the rat paw.

The role of copper in inflammatory disorders

Comparative effects of drugs on four paw oedema models in the rat.

Action of gold salts in some inflammatory and immunological models