Vol. 26, Issue 7, 676-680, July 1998

Oxidation of a Metabolite of Indomethacin (Desmethyldeschlorobenzoylindomethacin) to Reactive Intermediates by Activated Neutrophils, Hypochlorous Acid, and the Myeloperoxidase System

C. Ju and J. P. Uetrecht

Faculties of Pharmacy (C.J., J.P.U.) and Medicine (J.P.U.), University of Toronto

The use of indomethacin is associated with a relatively high incidence of adverse reactions such as agranulocytosis. Many other drugs associated with agranulocytosis are metabolized to reactive metabolites by activated neutrophils. Therefore, we studied the oxidation of indomethacin and its metabolites by activated neutrophils, myeloperoxidase (MPO) (the major oxidizing enzyme in neutrophils), and HOCl (the major oxidant produced by activated neutrophils). No oxidation of indomethacin by activated neutrophils was observed. However, desmethyldeschlorobenzoylindomethacin (DMBI), a major metabolite of indomethacin, was oxidized 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 of H₂O₂). In reactions with HOCl, the same reactive intermediate was formed; its mass spectrum, with a MH⁺ ion at m/z 204, was obtained by using a flow system in which the reactants were fed into a mixing chamber and the products flowed directly into the mass spectrometer. The same 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 data for the NAC conjugate indicated that the sulfur was substituted in the 4-position on the aromatic ring. The reactive intermediate generated from DMBI by activated neutrophils may be responsible for indomethacin-induced agranulocytosis.