title: Update on cyclooxygenase inhibitors: has a third COX isoform entered the fray?
abstract: It has been more than 30 years since Sir John Vane first reported that the pharmacological actions of aspirin-like drugs could be explained by their ability to inhibit cyclooxygenase (COX). Since then, a second isoform of COX, named COX-2, has been discovered and highly selective inhibitors of this isoform have been marketed. Most recently, a splice variant of COX-1 mRNA, retaining intron 1, and given the names COX-3, COX-1b or COX-1v, has been described. Non-selective NSAIDs such as ibuprofen and naproxen, which inhibit both COX-1 and COX-2, have proven highly effective and safe in the short-term management of acute pain. Highly selective COX-2 inhibitors including celecoxib, rofecoxib, valdecoxib, lumiracoxib, and etoricoxib were developed with the hope of significantly reducing the serious gastrointestinal toxicities associated with chronic high-dose NSAID use. While long-term studies demonstrated that rofecoxib and lumiracoxib reduced the incidence of GI perforations, ulcerations and bleeds by approximately 60% compared to non-selective NSAIDs, recent reports also demonstrated that the chronic use of rofecoxib and celecoxib in arthritis and colorectal polyp patients, and the short-term use of parecoxib and valdecoxib in patients who had undergone coronary artery bypass surgery, resulted in a significant increase in serious cardiovascular events, including myocardial infarction and stroke compared to naproxen or placebo. COX-3 mRNA has been isolated in many tissues including canine and human cerebral cortex, human aorta, and rodent cerebral endothelium, heart, kidney and neuronal tissues. In transfected insect cells, canine COX-3 protein is expressed and was selectively inhibited by acetaminophen. However, in humans and rodents an acetaminophen sensitive COX-3 protein is not expressed because the retention of intron-1 adds 94 and 98 nucleotides to the COX-3 mRNA structure respectively. Since the genetic code is a triplicate code (3 nucleotides to form one amino acid), the retention of the intron in both species results in a frame shift in the RNA message and the production of a truncated protein with a completely different amino acid sequence than COX-1 or COX-2 lacking acetaminophen sensitivity. Advances made through a combination of basic molecular biological and pharmacological techniques, and well designed randomized controlled clinical trials have demonstrated that the apparent gastrointestinal advantage of selective COX-2 inhibitors appears to be outweighed by their potential for cardiovascular toxicity and that acetaminophen's analgesic and antipyretic effects do not involve the inhibition of the COX-1 splice variant protein, putative COX-3.
authors.affiliation: Department of Oral Surgery and Pharmacology, University of Pennsylvania School of Dental Medicine, Philadelphia, PA 19104-6030, USA. evhersh@pobox.upenn.edu
authors.forename: Elliot V
authors.initials: EV
authors.lastname: Hersh
authors.forename: Edward T
authors.initials: ET
authors.lastname: Lally
authors.forename: Paul A
authors.initials: PA
authors.lastname: Moore
mesh_terms: D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000009:adverse effects / Q000737:chemistry / Q000494:pharmacology*; D052246:Cyclooxygenase 2 Inhibitors / Q000009:adverse effects / Q000737:chemistry / Q000494:pharmacology; D016861:Cyclooxygenase Inhibitors / Q000009:adverse effects / Q000737:chemistry / Q000494:pharmacology*; D006801:Humans; D011451:Prostaglandin-Endoperoxide Synthases / Q000187:drug effects* / Q000235:genetics
chemical_list: D000894:Anti-Inflammatory Agents, Non-Steroidal; D052246:Cyclooxygenase 2 Inhibitors; D016861:Cyclooxygenase Inhibitors; D011451:Prostaglandin-Endoperoxide Synthases; C466421:cyclooxygenase-3