PKC412, a FLT3 Inhibitor, Effectively Induces Apoptosis of Leukemia Cells with 11q23 Translocation Particularly Having D835 Mutation.

About 30% cases of acute myeloid leukemia (AML) show FLT3 mutations such as internal tandem duplications within the juxtamembrane domain and point mutations within the kinase domain (D835 et al), which result in constitutive activation of FLT3 kinase activity. Although it was recently shown that acute lymphoblastic leukemia (ALL) with 11q23 translocation, often found in infantile leukemia with very poor prognosis, express FLT3 at high levels, few studies have been performed regarding the biological implication of FLT3 in leukemia with 11q23 translocation. We recently found that 2 of 6 cell lines with 11q23 translocation had D835 mutation in FLT3 (KOCL-33 with 11; 19 translocation and B-precursor phenotype, and KOCL-48 with 4; 11 translocation and monocytoid phenotype). In the present study, we evaluated the in vitro inhibitory effects of PKC412 (a kinase inhibitor of FLT3) using leukemic cell lines with or without D835 mutation. 3H-thymidine uptakes were inhibited by PKC412 at 50–200 nM very markedly in two 11q23-cell lines with D835 mutation (>90% inhibition), moderately (50–80% inhibition) in 11q23-cell lines without FLT3 mutation (n=5), but only modestly (<50% inhibition) in other B-precursor cell lines without FLT3 mutation (n=5). Two 11q23-cell lines with D835 mutation were effectively induced into apoptosis by PKC412, which was preceded by a prompt disappearance of constitutive phosphorylation of STAT5 before PKC412 treatment. The RNase protection assay revealed a decrease in mRNA of Bfl-1 which is known to be up-regulated by the NFkB activity, but showed no changes in mRNA of apoptosis-related molecules such as Bcl-2, Bcl-xl, Bad, Fas, FasL, and TRAIL. These results suggest that PKC412 is a potentially useful agent for the treatment of leukemia with 11q23 translocation, particularly having FLT3 mutation.