OXIDATION OF A METABOLITE OF INDOMETHACIN (DESMETHYLDESCHLOROBENZOYLINDOMETHACIN) TO REACTIVE INTERMEDIATES BY ACTIVATED NEUTROPHILS, HYPOCHLOROUS ACID, AND THE MYELOPEROXIDASE SYSTEM

The use of indomethacin is associated with a relatively high incidence of adverse reactions such as agranulocytosis, Many other drugs associ ated with agranulocytosis are metabolized to reactive metabolites by a ctivated neutrophils, Therefore, we studied the oxidation of indometha cin and its metabolites by activated neutrophils, myeloperoxidase (MPO ) (the major oxidizing enzyme in neutrophils), and HOCl (the major oxi dant produced by activated neutrophils). No oxidation of indomethacin by activated neutrophils was observed. However, desmethyldeschlorobenz oylindomethacin (DMBI), a major metabolite of indomethacin, was oxidiz ed to a reactive iminoquinone that could be trapped with glutathione ( GSH) or N-acetylcysteine (NAC) to form conjugates, with MH+ ions at m/ z 511 and 367, respectively. No metabolism was detected in neutrophils that had not been activated, and the oxidation was inhibited by azide (which inhibits MPO) and by catalase (which catalyzes the breakdown o f H2O2). In reactions with HOCl, the same reactive intermediate was fo rmed; its mass spectrum, with a MH+ ion at m/z 204, was obtained by us ing a flow system in which the reactants were fed into a mixing chambe r and the products flowed directly into the mass spectrometer, The sam e GSH and NAC conjugates were also observed when DMBI was oxidized by HOCl or by the MPO system, followed by addition of GSH or NAC. NMR dat a for the NAC conjugate indicated that the sulfur was substituted in t he 4-position on the aromatic ring. The reactive intermediate generate d from DMBI by activated neutrophils may be responsible for indomethac in-induced agranulocytosis.