DRUG THERAPY Drug Therapy tures of a “housekeeping” enzyme, its expression may also be regulated.11,12 The expression of both cyclo- oxygenase-1 and cyclooxygenase-2 is increased in A LASTAIR J.J. WOOD, M.D., Editor the synovia of inflamed joints and in atherosclerotic plaques.13,14 Although the catalytic activities and ter- THE COXIBS, SELECTIVE INHIBITORS tiary structures of cyclooxygenase-1 and cyclooxy- 2,15,16 OF CYCLOOXYGENASE-2 genase-2 are also remarkably similar, cyclooxy- genase-2 has a broader affinity for substrates because the hydrophobic channel leading to the active site of GARRET A. FITZGERALD, M.D., AND CARLO PATRONO, M.D. this enzyme is more accommodating. However, dis- tinguishing cyclooxygenase-1 as a constitutive enzyme and cyclooxygenase-2 as an inducible enzyme that ONSTEROIDAL antiinflammatory drugs accounts for the formation of prostanoid in disease (NSAIDs) are widely used to treat arthritis, is an oversimplification of the biologic reality. Nmenstrual pain, and headache. Although they are effective, their long-term use is limited by gas- SELECTIVE AND NONSELECTIVE trointestinal effects such as dyspepsia and abdominal INHIBITION OF CYCLOOXYGENASE pain and, less often, gastric or duodenal perforation ISOFORMS or bleeding. Development of the coxibs, a new group Three broad classes of cyclooxygenase inhibitors of antiinflammatory drugs, represents a response to have emerged: aspirin synthesized from salicylic ac- the unsatisfactory therapeutic profile of NSAIDs. Both id; indomethacin and other NSAIDs, whose chemi- groups of drugs inhibit prostaglandin G/H synthase, cal modifications were based largely on findings in the enzyme that catalyzes the transformation of arach- models of inflammation and gastric mucosal damage; idonic acid to a range of lipid mediators, termed and the first selective cyclooxygenase-2 inhibitors, the prostaglandins and thromboxanes (Fig. 1). However, coxibs (e.g., celecoxib and rofecoxib). Other selective whereas NSAIDs inhibit the two recognized forms of cyclooxygenase-2 inhibitors, such as valdecoxib17 and the enzyme, also referred to as cyclooxygenase-1 and etoricoxib,18 are being developed. cyclooxygenase-2, the coxibs are selective inhibitors Selectivity for cyclooxygenase-2 may be expressed of cyclooxygenase-2. The inhibition of cyclooxygen- at several levels. A compound may be biochemically ase-2 has been more directly implicated in ameliorat- selective for cyclooxygenase-2. Drug companies assess ing inflammation, whereas the inhibition of cycloox- selectivity by in vitro assays during screening, because ygenase-1 has been related to adverse effects in the these assays are relatively rapid and simple. However, gastrointestinal tract. Therefore, it was hoped that they do not necessarily reflect the complexity of the coxibs would be better tolerated than nonselective drug–enzyme interaction in vivo. To address this NSAIDs but equally efficacious. This review will as- concern, whole-blood assays of cyclooxygenase-iso- sess the evidence that has emerged in support of that form activity have been developed.19,20 These assays are hypothesis. based on the production of thromboxane B2 during Prostaglandin G/H synthase has both cyclooxy- blood clotting (an index of platelet cyclooxygenase- genase and hydroperoxidase activity.1 Neither coxibs 1 activity) and the production of prostaglandin E2 by nor NSAIDs inhibit the activity of hydroperoxidase. bacterial lipopolysaccharide in whole blood (an index Aspirin inhibits the activity of cyclooxygenase by ir- of monocyte cyclooxygenase-2 activity). Clinical se- reversibly acetylating a serine residue at position 529 lectivity has been based on either surrogates for clinical in the only form of the enzyme expressed in platelets, toxicity (e.g., endoscopically visualized gastroduode- cyclooxygenase-1.2 There were several early sugges- nal ulcers) or actual clinical end points at antiinflam- tions of a second form of cyclooxygenase.3-5 Analysis matory doses of an inhibitor. of unrelated genes identified one that was highly ho- There are two basic requirements to test the hy- mologous to the gene for cyclooxygenase-1.6-8 This pothesis that cyclooxygenase-2 inhibitors are better isoform, termed cyclooxygenase-2, can be up-regu- tolerated than nonselective NSAIDs but just as effi- lated by cytokines, growth factors, and tumor pro- cacious. First, the drug must not inhibit cyclooxy- moters,6-10 suggesting its relevance to inflammation genase-1 activity in clinically relevant targets (gastro- and cancer. intestinal mucosa and platelets) at therapeutic plasma Although cyclooxygenase-1 has the structural fea- concentrations. Second, the clinical end points as- sessed must reflect cyclooxygenase-1–dependent gas- From the Center for Experimental Therapeutics, University of Pennsyl- trointestinal toxicity. When symptoms are the end vania, Philadelphia (G.A.F.); and the Department of Medicine and Center point, a limitation is that the dependence of cyclo- of Excellence on Aging, University of Chieti, Chieti, Italy (C.P.). Address oxygenase-1 on the signal is uncertain, and symptoms reprint requests to Dr. FitzGerald at the Department of Pharmacology, 153 Johnson Pavilion, 3620 Hamilton Walk, University of Pennsylvania, Phila- may not correlate with lesions. When endoscopically delphia, PA 19104-6084, or at [email protected]. visualized lesions are the end point, the situation is N Engl J Med, Vol. 345, No. 6 · August 9, 2001 · www.nejm.org · 433 The New England Journal of Medicine Membrane phospholipids Phospholipase A2 Diverse physical, chemical,# inflammatory, and mitogenic stimuli Arachidonic9 Coxibs acid Prostaglandin G COX Prostaglandin G Prostaglandin G/H9 2 2 Prostaglandin G/H9 synthase 19 synthase 29 (cyclooxygenase-1) HOX (cyclooxygenase-2) Prostaglandin H2 Prostaglandin H2 Tissue-specific isomerases Prostanoids Prostacyclin Thromboxane A2 Prostaglandin D2 Prostaglandin E2 Prostaglandin F2a Receptors IP TPa, TPb DP1, DP2 EP1, EP2, EP3, EP4 FPa, FPb Platelets,9 Brain, kidney,9 Uterus, airways,9 Endothelium,9 vascular smooth-9 Mast cells,9 vascular smooth-Ł vascular smooth-9 kidney,9 muscle cells,9 brain,9 muscle cells,9 muscle cells,9 platelets, brain macrophages,9 airways platelets eye kidney Figure 1. Production and Actions of Prostaglandins and Thromboxane. Arachidonic acid, a 20-carbon fatty acid containing four double bonds, is liberated from the sn2 position in membrane phospho- lipids by phospholipase A2, which is activated by diverse stimuli. Arachidonic acid is converted by cytosolic prostaglandin G/H synthases, which have both cyclooxygenase (COX) and hydroperoxidase (HOX) activity, to the unstable intermediate prostaglandin H2. The synthases are colloquially termed cyclooxygenases and exist in two forms, cyclooxygenase-1 and cyclooxygenase-2. Coxibs selectively inhibit cyclooxygenase-2. Prostaglandin H2 is converted by tissue-specific isomerases to multiple prostanoids. These bioactive lipids activate specific cell-membrane receptors of the superfamily of G-protein–coupled receptors. Some of the tissues in which individual prostanoids exert prominent effects are indicated. IP denotes prostacyclin receptor, TP thromboxane receptor, DP prostaglandin D2 receptor, EP prostaglandin E2 receptor, and FP prostaglandin F2a receptor. different. The dependence of the lesions on cycloox- The biochemical selectivity of a particular drug, as ygenase-1 has been established, but it is uncertain assessed in vitro, is critically dependent on its con- whether the finding of lesions on endoscopy is actu- centration. One can summarize such selectivity pro- ally predictive of the likelihood of serious gastroin- files by plotting the drug concentrations necessary testinal complications, such as perforation, obstruc- to inhibit the activity of cyclooxygenase-2 and cyclo- tion, and bleeding. Indeed, the hemorrhagic nature oxygenase-1 by 50 percent (Fig. 2).23-26 However, giv- of most serious gastrointestinal end points makes it en the concentration dependence of these estimates, likely that they primarily reflect the inhibition of cy- it is not useful to attempt to discriminate between clooxygenase-1 activity in platelets, rather than in gas- existing NSAIDs on the basis of small differences in tric mucosa. Finally, the low incidence of these events biochemical selectivity with the use of terms such as means that many patients must be studied for pro- “preferential cyclooxygenase-2 inhibitor.” longed periods of treatment to detect the differences How well do the results of these in vitro assays between drugs reliably.21,22 predict selectivity when the measurements are per- 434 · N Engl J Med, Vol. 345, No. 6 · August 9, 2001 · www.nejm.org DRUG THERAPY 100.00 6-MNA Naproxen Acetaminophen Ibuprofen 10.00 (µM) 50 Meloxicam 1.00 Nimesulide Indomethacin Celecoxib Rofecoxib Cyclooxygenase-2 IC 0.10 Diclofenac 0.01 0.010.10 1.00 10.00 100.00 Cyclooxygenase-1 IC50 (µM) Figure 2. Concentrations of Various Drugs Required to Inhibit the Activity of Cyclooxygenase-1 and Cy- clooxygenase-2 by 50 Percent (IC50) in Assays of Whole Blood. Each point is the mean of three or four values.23-26 Drugs plotted below the diagonal line indicating equivalence are more potent inhibitors of cyclooxygenase-2 than drugs plotted on or above the line. 6-MNA denotes 6-methoxy-2-naphthylacetic acid. formed in blood samples obtained from patients given ability, half-life, and main pathways of hepatic metab- cyclooxygenase inhibitors? Although the curves plot- olism. Differences