Immunophenotypic findings in acute myeloid leukemia with FLT3 internal tandem duplication

The biological characteristics and the prognostic significance of the internal tandem duplication of the FLT3 (FLT3/ITD) were investigated in a series of de novo acute myeloid leukemia (AML) patients. One hundred and fifty-six adult patients with AML were included in the study. FLT3/ITD was detected in 41 (26%) patients (FLT3/ITD(+)).

The main differences observed between the groups with and without FLT3/ITD: a higher leukocyte count, a raised percentage of a normal karyotype and a more frequent M5 FAB diagnosis in the FLT3/ITD(+) patients. As regards the immunophenotype characteristics the FLT3/ITD(+) group very often expressed monocytic markers (CD36 and CD11b) and less commonly immature markers (CD34 and CD117). A promyelocytic-like immunophenotype pattern was also detected in a minority of these patients(4/36).

The FLT3/ITD(+) patients had a shorter overall survival, a shorter event-free survival and a higher probability of relapse. Minimal residual disease (MRD) was investigated in the FLT3/ITD(+) patients using flow cytometry. This technique had a sensitivity of 62% and a specificity of 83% in relapse prediction. Minimal residual disease analysis was hampered by the low number of patients with detectable aberrant immunophenotype.

A high frequency of changes in the phenotype and/or genotype pattern between diagnosis and relapse was detected (5/6). FLT3/ITD is a frequent molecular lesion in de novo adult AML and seems to be associated with a monocytic differentiation, a high leukocyte count and a poor prognosis. Immunophenotype and genotype patterns observed at relapse suggest that the FLT3/ITD(+) blasts may be genetically unstable and prone to clonal evolution. FLT3/ITD may not be a suitable target for minimal residual disease studies.