title: Cellular mechanisms of acetaminophen: role of cyclo-oxygenase.
abstract: Acetaminophen is one of the most commonly used drugs for the safe and effective treatment of pain and fever. Acetaminophen works by lowering cyclo-oxygenase products preferentially in the central nervous system, where oxidant stress is strictly limited. However, the precise mechanism of action for acetaminophen on cyclo-oxygenase activity is debated. Two theories prevail. First, it is suggested that acetaminophen selectively inhibits a distinct form of cyclo-oxygenase, cyclo-oxygenase-3. Second, it is suggested that acetaminophen has no affinity for the active site of cyclo-oxygenase but instead blocks activity by reducing the active oxidized form of cyclo-oxygenase to an inactive form. Here, we have used an in vitro model of cyclo-oxygenase-2 activity (A549 cells stimulated with IL-1beta) to show that acetaminophen is an effective inhibitor of cyclo-oxygenase activity in intact cells. However, acetaminophen, unlike nonsteroidal anti-inflammatory drugs (NSAIDs), cannot inhibit activity in broken cell preparations. The inhibitory effects of acetaminophen were abolished by increasing intracellular oxidation conditions with the cell-permeable hydroperoxide t-butylOOH. Similarly the inhibitory effects of the cyclo-oxygenase-2 selective inhibitor rofecoxib or the mixed cyclo-oxygenase-1/cyclo-oxygenase-2 inhibitors ibuprofen and naproxen were significant reduced by t-butylOOH. By contrast, the inhibitory effects of indomethacin or diclofenac, which also inhibit both cyclo-oxygenase-1 and cyclo-oxygenase-2, were unaffected by t-butylOOH. These observations dispel the notion that cyclo-oxygenase-3 is involved in the actions of acetaminophen and provide evidence that supports the theory that acetaminophen interferes with the oxidation state of cyclo-oxygease. Moreover, they suggest for the first time that the inhibitory effects of some NSAIDs, including the newly introduced cyclo-oxygenase-2 selective inhibitor rofecoxib, owe part of their inhibitory actions to effects on oxidation state of cyclo-oxygenase. Our data with t-butylOOH and NSAIDs illustrates an, as yet, undeveloped therapeutic window for the "cyclo-oxygenase inhibitor". Specifically, combining active site selectively with actions on enzyme oxidation state would allow for a broader range of tissue selective drugs.
authors.affiliation: Cardiothoracic Pharmacology, UCCM, Royal Brompton Hospital, NHLI, Imperial College London, UK.
authors.forename: Ruth
authors.initials: R
authors.lastname: Lucas
authors.forename: Timothy D
authors.initials: TD
authors.lastname: Warner
authors.forename: Ivana
authors.initials: I
authors.lastname: Vojnovic
authors.forename: Jane A
authors.initials: JA
authors.lastname: Mitchell
mesh_terms: D000082:Acetaminophen / Q000494:pharmacology*; D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000494:pharmacology; D001665:Binding Sites / Q000187:drug effects; D002460:Cell Line; D016861:Cyclooxygenase Inhibitors / Q000494:pharmacology*; D004008:Diclofenac / Q000494:pharmacology; D004789:Enzyme Activation / Q000187:drug effects; D006801:Humans; D007052:Ibuprofen / Q000494:pharmacology; D007213:Indomethacin / Q000494:pharmacology; D007783:Lactones / Q000494:pharmacology; D008168:Lung; D009288:Naproxen / Q000494:pharmacology; D010084:Oxidation-Reduction; D011451:Prostaglandin-Endoperoxide Synthases / Q000378:metabolism; D014176:Protein Biosynthesis / Q000187:drug effects; D013450:Sulfones / Q000494:pharmacology; D014158:Transcription, Genetic / Q000187:drug effects; D020122:tert-Butylhydroperoxide / Q000494:pharmacology
chemical_list: D000894:Anti-Inflammatory Agents, Non-Steroidal; D016861:Cyclooxygenase Inhibitors; D007783:Lactones; D013450:Sulfones; C116926:rofecoxib; D004008:Diclofenac; D000082:Acetaminophen; D009288:Naproxen; D020122:tert-Butylhydroperoxide; D011451:Prostaglandin-Endoperoxide Synthases; C466421:cyclooxygenase-3; D007052:Ibuprofen; D007213:Indomethacin