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Oxidation to a maleic anhydride derivative may be a factor in the long-term toxicity of Vioxx®, a new report suggests. This previously unknown reactivity is not shared by other Cox-2 inhibitors, such as Celebrex® and Bextra®.
Vioxx® is a weak acid, and the anion formed when it releases its acidic protein is highly reactive toward atmospheric oxygen according to laboratory studies by Harvard University chemists Leleti Rajender Reddy and E. J. Corey. The products are mainly a maleic anhydride and lesser amounts of a hydroxybutenolide.
According to the authors, the maleic anhydride has not been reported as a Vioxx® metabolite. They suggest that some of it may survive long enough in vivo to react with nucleophilic groups of biomolecules and tissues. "The consequences of this may be a low-level chronic toxicity that is cumulative and possibly dangerous over periods of many months. It is perhaps not irrelevant that the cardio toxicity of Vioxx® was not apparent from short-term (one year or less) studies," they write.
Merck's withdrawal of Vioxx® has cast doubts on other Cox-2 inhibitors in the market. However, as the authors point out, the formation of an oxygen-reactive anion is unique to Vioxx®. Their findings have been disclosed to the Food and Drug Administration.
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