Many marketed pharmaceuticals are known to cause idiosyncratic agranulocytosis in humans. Similarly prinomide, an antiinflammatory drug, was associated with a low incidence of agranulocytosis (<0.3%) in clinical trials, even though chronic toxicity studies in rodents and primates showed no evidence of agranulocytosis with either prinomide or its parahydroxy metabolite, CGS 12094. To investigate mechanisms for this human specific toxicity, experiments were conducted to study the metabolism of prinomide and CGS 12094 by myeloperoxidase (MPO), a major enzyme of neutrophils and leukocyte progenitor cells. Although prinomide was not metabolized by human MPO, CGS 12094 was rapidly metabolized (>90%; 2 min); this reaction was dependent on H2O2 and MPO and was inhibited by azide. During the MPO-catalyzed metabolism of CGS 12094, reactive intermediates that irreversibly bound to protein and cysteine were generated. One of the reactive metabolites generated was identified by mass spectroscopy and trapping with cysteine as 1,4-benzoquinone, a compound implicated in the myelotoxicity associated with benzene. Thus during conditions which lead to elevated levels of H2O2 (e.g., active inflammation), CGS 12094 is rapidly metabolized by MPO to reactive intermediates that may be related to prinomide-induced agranulocytosis.