The class III
receptor tyrosine kinase FLT3 is expressed on the blasts of >90% of patients with B-lineage acute lymphoblastic
leukemias (ALL). In addition, it is expressed at extremely high levels in ALL patients with mixed lineage
leukemia rearrangements or hyperdiploidy and is sometimes mutated in these same patients. In this report, we investigate the effects of treating ALL cell lines and primary samples with human anti-FLT3
monoclonal antibodies (mAb) capable of preventing binding of
FLT3 ligand. In vitro studies, examining the ability of two anti-FLT3 mAbs (IMC-EB10 and IMC-NC7) to affect FLT3 activation and downstream signaling in ALL cell lines and primary blasts, yielded variable results. FLT3 phosphorylation was consistently inhibited by IMC-NC7 treatment, but in some cell lines,
IMC-EB10 actually stimulated FLT3 activation, possibly as a result of antibody-mediated receptor dimerization. Through antibody-dependent, cell-mediated cytotoxicity, such an antibody could still prove efficacious against
leukemia cells in vivo. In fact,
IMC-EB10 treatment significantly prolonged survival and/or reduced engraftment of several ALL cell lines and primary ALL samples in nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. This occurred even when
IMC-EB10 treatment resulted in FLT3 activation in vitro. Moreover, fluorescence-activated cell sorting and PCR analysis of IMC-EB10-treated NOD/SCID mice surviving 150 days post-leukemic cell injection revealed that FLT3
immunotherapy reduced leukemic engraftment below the level of detection in these assays (<0.001%). Furthermore, in vivo
IMC-EB10 treatment did not select for resistant cells, because cells surviving
IMC-EB10 treatment remain sensitive to
IMC-EB10 cytotoxicity upon retransplantation. In vivo studies involving either partial depletion or activation of natural killer (NK) cells show that most of the cytotoxic effect of
IMC-EB10 is mediated through NK cells. Therefore, such an antibody, either naked or conjugated to
radioactive isotopes or
cytotoxic agents, may prove useful in the
therapy of infant ALL as well as childhood and adult ALL patients whose blasts typically express FLT3.