Clozapine Promotes the Proliferation of Granulocyte Progenitors in the Bone Marrow Leading to Increased Granulopoiesis and Neutrophilia in Rats

Clozapine is an atypical antipsychotic that is limited in its use due to the risk of idiosyncratic agranulocytosis. The bone marrow is suspected to be the site of the reaction, and indirect measurements in patients suggest that neutrophil production and maturation are altered in the marrow by clozapine. Specifically, the majority of patients have elevated neutrophil counts at the start of treatment, often paired with increased serum granulocyte-colony stimulating factor (G-CSF). Employing a rat model of clozapine treatment, we set out to determine if the neutrophilia observed at the start of treatment is characteristic of G-CSF-associated bone marrow stimulation. Female Sprague-Dawley rats were treated with 30 mg/kg/day of clozapine for 10 days, and sustained neutrophilia was evident after 1 week of treatment paired with spikes in G-CSF. Within the bone marrow, clozapine was found to induce proliferation of the granulocyte progenitor colonies as measured by a methylcellulose assay. This led to elevated granulopoiesis observed by H&E and myeloperoxidase staining of bone marrow slices. Increased release of neutrophils from the marrow to the circulation was measured through 5-bromo-2'-deoxyuridine labeling in vivo, and these neutrophils appeared to be less mature based on (a) a decrease in the nuclear lobe count and (b) increased expression of surface CD62L. Furthermore, faster transit of the neutrophils through the marrow was suggested by a shift toward elevated numbers of neutrophils in the bone marrow maturation pool and increased CD11b and CD18 staining on the less mature neutrophils residing in the marrow. Taken together, these data indicate that clozapine stimulates the bone marrow to produce more neutrophils in a manner that is characteristic of endogenous G-CSF stimulation, and it is consistent with the inflammatory response observed in patients treated with clozapine.