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‐1β) 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.