METABOLISM OF CLOZAPINE BY HUMAN NEUTROPHILS - EVIDENCE FOR A SPECIFIC OXIDATION OF CLOZAPINE BY THE MYELOPEROXIDASE SYSTEM WITH INHIBITIONOF ENZYMATIC CHLORINATION CYCLE

The use of clozapine, an unique antipsychotic drug, raises the real pr oblem of drug-induced polymorphonuclear neutrophil cytoxicity. Clozapi ne prescription has been restricted due to a 1-2% incidence of drug-in duced agranulocytosis. The exact mechanism of this adverse effect is n ot yet known. The myeloperoxidase-hydrogen peroxide system could play a key role in the initiation of agranulocytosis. Therefore, we have in vestigated the clozapine effects on hydrogen peroxide and hypochlorous acid, evaluated the peroxidase-mediated metabolism of clozapine by ma ss spectrometry analysis because myeloperoxidase uses hydrogen peroxid e and chloride producing hypochlorous acid in its chlorination cycle, and thus could oxidise clozapine in its peroxidation cycle. First, evi dence for inhibition of hypochlorous acid production and scavenging of hydrogen peroxide by clozapine were demonstrated in vitro, in differe nt cell-free and cellular systems. Results are consistent with an inhi bition of the myeloperoxidase chlorination cycle when clozapine is oxi dised in the peroxidation cycle. Secondly, ion-spray mass spectrometry analysis allowed us to confirm clozapine oxidation by the myeloperoxi dase system. Actually, clozapine N-oxide with a m/z at 343 was formed. It could be the final step of the metabolisation of clozapine via two successive univalent oxidations mediated by peroxidase. We suggest th at generation of a free cation radical, CLZ degrees(+), was the initia l step. CLZ degrees(+) is a very reactive species and may play an impo rtant role in the onset of agranulocytosis either by direct toxicity o r via an immunological mechanism. However, this assumption does not ex clude the possible role of other metabolic ways involving, in particul ar, N-desmethylclozapine.