Summary Weak organic acids possessing anti‐inflammatory, analgesic and antipyretic properties — commonly known as aspirin‐like drugs — have been used in equine medicine for almost 100 years. These non‐steroidal anti‐inflammatory drugs (NSAIDs) may be classified chemically into two groups; the enolic acids such as phenylbutazone and carboxylic acids like flunixin, meclofenamate and naproxen. All NSAIDs have similar and possibly identical modes of action accounting for both their therapeutic and their toxic effects. They block some part of the cyclo‐oxygenase enzyme pathway and thereby suppress the synthesis of several chemical mediators of inflammation, collectively known as eicosanoids. The available evidence indicates that some of the newer NSAIDs have a reasonable safety margin but further studies are required. The toxicity of phenylbutazone in the horse has been investigated very thoroughly in recent years and it has been shown to cause renotoxicity and, most significantly, ulceration of the gastrointestinal tract when relatively high doses are administered. Several factors may predispose towards phenylbutazone toxicity in the horse, including breed and age, but high dosage is considered to be particularly important. The absorption into, and fate within, the body of NSAIDs are considered and particular attention is drawn to the ways in which these pharmacokinetic properties relate to the drugs' toxicity and clinical efficacy. In reviewing current knowledge of the clinical pharmacology of this important group of drugs, it is hoped to provide the clinician with a rational, scientific basis for their safe and effective use in equine practice.

Summary Arachidonic acid is a polyunsaturated fatty acid covalently bound in esterified form in the cell membranes of most body cells. Following irritation or injury, arachidonic acid is released and oxygenated by enzyme systems leading to the formation of an important group of inflammatory mediators, the eicosanoids. It is now recognised that eicosanoid release is fundamental to the inflammatory process. For example, the prostaglandins and other prostanoids, products of the cyclo‐oxygenase enzyme pathway, have potent inflammatory properties and prostaglandin E2 is readily detectable in equine acute inflammatory exudates. The administration of nonsteroidal anti‐inflammatory drugs results in inhibition of prostaglandin synthesis and this explains the mode of action of agents such as phenylbutazone and flunixin. Lipoxygenase enzymes metabolise arachidonic acid to a group of non‐cyclised eicosanoids, the leukotrienes, some of which are also important inflammatory mediators. They are probably of particular importance in leucocyte‐mediated aspects of chronic inflammation. Currently available non‐steroidal antiinflammatory drugs, however, do not inhibit lipoxygenase activity. In the light of recent evidence, the inflammatory process is re‐examined and the important emerging roles of both cyclo‐oxygenase and lipoxygenase derived eicosanoids are explored. The mode of action of current and future antiinflammatory drugs offered to the equine clinician can be explained by their interference with arachidonic acid metabolism.

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The clinically recommended dose rate of phenylbutazone (4.4 mg/kg) was administered intravenously as a single dose to five Welsh Mountain ponies. Distribution of phenylbutazone and its active metabolite oxyphenbutazone into body fluids was studied by measuring concentrations in plasma, tissue-cage fluid, peritoneal fluid and acute inflammatory exudate harvested from a polyester sponge model of inflammation. The ready penetration of phenylbutazone into inflammatory exudate was demonstrated by the relatively high mean value for Cmax of 12.4 micrograms/ml occurring at a time of 4.6 h and a mean AUC0-24 of 128 microgram X h/ml. A high mean exudate:plasma AUC0-24 ratio of 0.83 was recorded. Plasma:exudate concentration ratios for phenylbutazone were initially greater than and subsequently less than one; the slower clearance from exudate was indicated by approximate t1/2 beta) values of 4.8 and 24 h for plasma and exudate, respectively. These findings may help to explain the relatively long duration of action of phenylbutazone, in spite of a plasma elimination half-life of less than 5 h. Lower values of Cmax and AUC0-24 for phenylbutazone passage into peritoneal fluid (6.3 micrograms/ml and 45 micrograms X h/ml) were recorded, and a limited number of sampling times indicated a similar degree of penetration as into tissue cage fluid. Mean concentrations of oxyphenbutazone in all fluids were lower than phenylbutazone concentrations at all times, but ready penetration of the metabolite into body fluids, especially into inflammatory exudate, occurred suggesting that oxyphenbutazone may contribute to the anti-inflammatory effect. The hyperaemia of acute inflammation and the high protein levels in inflammatory exudate may both assist passage of phenylbutazone and oxyphenbutazone into exudate.

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The presence of cyclooxygenase products of arachidonic acid metabolism in carrageenin-induced inflammatory exudate was investigated in ponies using two models. In the first model, an inflammatory response was stimulated by injecting carrageenin into subcutaneously implanted polypropylene tissue cages and exudates were collected at five predetermined times between 3 and 48 h. In the second model, exudates were harvested at 6, 12 and 24 h from carrageenin-impregnated polyester sponges which had also been inserted beneath the skin. Prostaglandin (PG) E2, thromboxane (TX) B2 and the stable breakdown-product of prostacyclin (PGI2), 6-keto-PGF1 alpha, in exudates were measured by radio-immunoassay (RIA); PGE2-like and PGF2 alpha-like activities were bioassayed following an acid-lipid extraction technique which provided a recovery rate of 78%. Agreement between RIA and bioassay was within acceptable limits. In Model 1, using RIA, mean PGE2 concentration reached 197 ng X ml-1 at 12 h decreasing to less than 12 ng X ml-1 at 24 h. Mean TXB2 and 6-keto-PGF1 alpha levels were highest at 48 h (22.3 and 34.2 ng X ml-1, respectively) after considerable fluctuations and with wide standard errors prior to this time. In the sponge model, however, PGE2 levels were surprisingly low for each group (mean 12.8 ng X ml-1 at 12 h) and TXB2 and 6-keto-PGF1 alpha were similarly lower (means of 3.3 and 8.1 ng X ml-1 respectively at 12 h). Mean total leucocyte counts and total protein concentrations were increased in both models after carrageenin stimulus. PGF2 alpha was not detected in measurable quantities in any exudate.(ABSTRACT TRUNCATED AT 250 WORDS)

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