A comparison of the covalent binding of clozapine and olanzapine to human neutrophils in vitro and in vivo

Covalent binding of a reactive metabolite of clozapine to neutrophils or their precursors is thought to play a role in the development of clozapine-induced agranulocytosis. Immunoblotting studies with an anti-clozapine antiserum detected covalent binding of clozapine to human neutrophils in vitro when HOCl was used to generate clozapine reactive metabolite (major clozapine adducts of 31, 49, 58, 78, 86, 126, 160, and 204 kDa). In addition, incubating neutrophils with clozapine and H2O2 (major clozapine adducts of 49 and 58 kDa) or clozapine, H2O2, and human myeloperoxidase (major clozapine adducts of 31, 49, 58, and 126 kDa) also resulted in covalent binding of clozapine to the neutrophils. The covalent binding of clozapine to neutrophils was inhibited by extracellular glutathione when HOCl, but not H2O2 was used to generate reactive metabolite. We found that the antiserum against clozapine also recognized olanzapine, an antipsychotic drug that forms a similar reactive metabolite to clozapine but has not been associated with induction of agranulocytosis. Repeating the in vitro experiments with olanzapine revealed that the major olanzapine-modified polypeptides had molecular masses of 96, 130-170, and 218 kDa. Only relatively low levels of 31, 49, and 58 kDa adducts were observed. Clozapine-modified polypeptides also were detected in neutrophils from patients being treated with clozapine. A major 58-kDa clozapine-modified polypeptide was detected in all patients tested. In contrast, no drug-modified polypeptides were detected in neutrophils from patients taking olanzapine. The differences in covalent binding exhibited by the two compounds and, in particular, the lack of olanzapine binding to human neutrophils in vivo may help to explain the difference in toxicity of these two drugs.