Increased sensitivity of isolated CD34+ cells towards clozapine-treatment

In order to establish an in vitro model of clozapine-induced agranulocytosis (CA) we compared clozapine-induced apoptosis in the promyeloid cell line HL-60 and in peripheral CD34⁺-cells isolated from stem cell donors. Clozapine induced oxidative stress and apoptosis in both cell types but beginning at different concentrations. While apoptotic HL-60 cells were detected at concentrations ≥50µM, the percentage of apoptotic CD34+cells was already increased at nearly physiological clozapine levels of 5µM. Inhibition of the superoxide dismutase by DETC significantly increased the sensitivity of HL-60 cells towards clozapine, indicating that the cell line is more resistant to oxidative stress than CD34+ cells. This resistance may be due to inactive, mutated p53 in HL-60 cells that is unable to detect DNA damage in stressed cells. While its expression is increased in CA-patients, it remains constant in HL-60 cells resulting in little activation of BAX, as detected by RT-PCR and Western blot analysis.