In the two decades since the elusive "slow reacting substance of anaphylaxis" (SRS-A) was identified as a product of the action of the 5-lipoxygenase enzyme on arachidonic acid, it has been well established that the leukotrienes are key mediators of both alIergy and inflammation. Their release by alIergen or other challenge has been demonstrated in the lungs of asthmatic subjects, and measurement of urinary leukotriene concentrations in such patients has been shown to be a valuable, non invasive indicator. Significant progress has been made towards the characterization of the leukotriene receptor subtypes, exemplified by the cloning of the LTB4 receptor earlier this year. Coupled with this there has been a continued elucidation of signal transduction mechanisms underlying receptor activation. Consequent upon these advances has been the development of potent antagonists of the CysLT receptor, J and both these and inhibitors of leukotriene biosynthesis have entered clinical practice in the therapy of asthma. In this clinical setting antagonists of the CysLTJ receptor have been shown to be an effective therapy in chronic asthmatics, against antigen- and exercise-induced bronchoconstriction, and in aspirin-intolerant asthmatics. The advent of this new class of agents promises to change the way in which asthmatic patients are currently treated."

The mainstay of therapy for rheumatoid disease is the non-steroid antiinflammatory drugs (NSAIDs), despite their inherent gastrointestinal toxicity and ability to cause renal damage in susceptible patients. The theory that the beneficial and toxic effects of NSAIDs stem from a reduction in prostanoid production through inhibition of cyclooxygenase implied that particular toxicities were inevitable with NSAIDs and would always be correlated with efficacy. However, over the years, it became apparent that at therapeutic doses, some NSAIDs had greater toxic side-effects than others, a fact not explained by the general theory. A significant clarification arose from the discovery that there are two distinct isoforms of COX, a constitutive enzyme (COX-I) responsible for the production of prostanoids necessary for platelet aggregation and protection of the gastric mucosa and kidney; and an inducible enzyme (COX-2) that is newly synthesized at sites of tissue damage and produces prostaglandins that manifest pathological effects. It became clear that different NSAIDs had greater or lesser effects on COX-I when used in therapeutic doses, explaining the variation in side-effects. ' The elucidation of the crystal structure of these different enzymes and the skills of medicinal chemists have led to the synthesis of new chemicals with a selectivity for the inducible enzyme, and thus with therapeutic efficacy without those toxic effects result ing from inhibition of the constitutive enzyme.

For the past 100 years the mainstay of therapy for rheumatoid arthritis (RA) has been aspirin or other drugs of the non-steroid anti-inflammatory group. In 1971 Vane pro posed that both the beneficial and toxic actions of these drugs was through inhibition of prostaglandin synthesis. The recent discovery that prostaglandins responsible for pain and other symptoms at inflammatory foci are synthesized by an inducible cyclooxygenase (COX-2) that is encoded by a gene distinct from that of the consti tutive enzyme (COX-I) provided a new target for therapy of RA. A drug that would selectively inhibit COX-2 would hopefully produce the symptomatic benefit provided by existing NSAIDs without the gastrointestinal and renal toxicity due to the inhibition of COX-I. Drugs selective for COX-2 are now available. Experimental studies have shown them to be effective with minimal toxicity, and in clinical trials gastric and renal toxicities are less. Highly selective COX-2 inhibitors, perhaps designed with knowledge of the crystal structures of COX-I and COX-2, are also available. Other experimental studies, including those in animals lacking effective genes for COX-lor COX-2 and in experimental carcinomas, suggest there is still much to be learned of the pathophysiological functions of these enzymes. The inflammatory response is a complex reaction involving many mediators that derive from white blood cells, endothelial cells and other tissues. Preliminary data have revealed that inhibitors of the cytokines and adhesion molecules that play a crucial role in the migration of white cells to inflammatory sites may be useful in RA.

In 1971, Vane proposed that the mechanism of action of the aspirin-like drugs was through their inhibition of prostaglandin biosynthesis. Since then, there has been intense interest in the interaction between this diverse group of inhibitors and the enzyme known as cyclooxygenase (COX). It exists in two isoforms, COX-l and COX-2 (discovered some 5 years ago). Over the last two decades several new drugs have reached the market based on COX-l enzyme screens. Elucidation of the three-dimensional structure of COX-l has provided a new understanding for the actions of COX inhibitors. The constitutive isoform of COX, COX-l has clear physiological functions. Its activation leads, for instance, to the production of prostacyclin which when released by the endothelium is anti-thrombogenic and anti-atherosclerotic, and in the gastric mucosa is cyto protective. COX-l also generates prostaglandins in the kidney, where they help to maintain blood flow and promote natriuresis. The inducible isoform, COX-2, was discovered through its activity being increased in a number of cells by pro inflammatory stimuli. A year or so later, COX-2 was identified as a distinct isoform encoded by a different gene from COX-I. COX-2 is induced by inflammatory stimuli and by cytokines in migratory and other cells. Thus the anti-inflammatory actions of non-steroid anti-inflammatory drugs (NSAIDs) may be due to the inhibition of COX-2, whereas the unwanted side-effects such as irritation of the stomach lining and toxic effects on the kidney are due to inhibition of the constitutive enzyme, COX-I.

In the two decades since the elusive "slow reacting substance of anaphylaxis" (SRS-A) was identified as a product of the action of the 5-lipoxygenase enzyme on arachidonic acid, it has been well established that the leukotrienes are key mediators of both alIergy and inflammation. Their release by alIergen or other challenge has been demonstrated in the lungs of asthmatic subjects, and measurement of urinary leukotriene concentrations in such patients has been shown to be a valuable, non invasive indicator. Significant progress has been made towards the characterization of the leukotriene receptor subtypes, exemplified by the cloning of the LTB4 receptor earlier this year. Coupled with this there has been a continued elucidation of signal transduction mechanisms underlying receptor activation. Consequent upon these advances has been the development of potent antagonists of the CysLT receptor, J and both these and inhibitors of leukotriene biosynthesis have entered clinical practice in the therapy of asthma. In this clinical setting antagonists of the CysLTJ receptor have been shown to be an effective therapy in chronic asthmatics, against antigen- and exercise-induced bronchoconstriction, and in aspirin-intolerant asthmatics. The advent of this new class of agents promises to change the way in which asthmatic patients are currently treated."

This volume is the proceedings of the William Harvey Research Conference held in Cannes, France, on the 20th and 21st March, 1997. It describes the present knowledge of the structures of the cyclooxygenase isoforms and the experimental and clinical effects of selective inhibitors of cyclooxygenase-2. The pathophysiological significance of the cyclooxygenase enzymes in tumorigenesis, programmed cell death, vascular disease and asthma is also covered.