Priming with G-CSF effectively enhances low-dose Ara-C-induced in vivo apoptosis in myeloid leukemia cells

We investigated the role of apoptosis in chemotherapy for hematologic malignancies. Twelve consecutive patients with acute myelogenous leukemia (AML) or refractory anemia with excess of blasts in transformation (RAEB-t) who were not tolerable for standard-dose chemotherapy were treated with CAG regimen (low-dose cytosine arabinoside [Ara-C] plus aclarubicin with concurrent administration of granulocyte colony-stimulating factor [G-CSF]). Bone marrow mononuclear cells obtained before the commencement of the chemotherapy were cultured with various concentrations (0-10(-5) M) of Ara-C in the presence or absence of 10 ng/mL of G-CSF, and the resultant cell proliferation/cytotoxicity was assayed. In all but one patient, half killing concentration (LC50) of Ara-C was significantly reduced in the presence of G-CSF (by 400- and 1.45-fold, median: 21-fold). Furthermore, LC(50) values in responders assayed in the presence of 10 ng/mL of G-CSF were significantly lower than those in nonresponders (p = 0.02). In vitro killing tests using a G-CSF-dependent leukemic cell line suggested that addition of G-CSF potentiates Ara-C-induced cytotoxicity through the mechanism of apoptosis. We thus assayed apoptosis in peripheral blood leukemic cells during CAG chemotherapy by flow cytometry using 7-amino-actinomycin D. Peak percentages of apoptosis in responders were significantly higher than those in nonresponders (p = 0.02). These results collectively suggest that apoptosis plays an important role for eradicating leukemic cells by CAG chemo-therapy.