The
tyrphostin AG957 (NSC 654705) inhibits p210bcr/abl, the transforming
kinase responsible for most cases of
chronic myelogenous leukemia (CML). The present studies were performed to determine the fate of AG957-treated cells and assess the selectivity of
AG957 for CML myeloid progenitors. When K562 cells (derived from a patient with
blast crisis CML) were treated with
AG957, dose- and time-dependent p210bc/abl down-regulation was followed by mitochondrial release of
cytochrome c, activation of
caspase-9 and
caspase-3, and apoptotic morphological changes. These apoptotic changes were inhibited by transfection with
cDNA encoding dominant negative
caspase-9 but not dominant-negative FADD or blocking anti-Fas
antibodies. In additional experiments, a 24-h
AG957 exposure caused dose-dependent inhibition of K562 colony formation in soft
agar. To extend these studies to clinical samples of CML, peripheral blood mononuclear cells from 10 chronic phase CML patients and normal controls were assayed for the growth of hematopoietic colonies in vitro in the presence of increasing concentrations of
AG957. These assays demonstrated selectivity of
AG957 for CML progenitors, with median IC50s (CML versus normal) of 7.3 versus >20 microM
AG957 in granulocyte colony-forming cells (P < 0.001), 5.3 versus >20 microM in granulocyte/macrophage colony-forming cells (P < 0.05), and 15.5 versus > 20 microM in erythroid colony-forming cells (P > 0.05). The adamantyl
ester of
AG957 (
NSC 680410) down-regulated p210bcr/abl in K562 cells and inhibited granulocyte colony formation in CML specimens at lower concentrations without enhanced toxicity in normal progenitors. These observations not only demonstrate that AG957-induced p210bcr/abl down-regulation is followed by activation of the
cytochrome c/Apaf-1/
caspase-9 pathway but also indicate that this class of
kinase inhibitor exhibits selectivity worthy of further evaluation.