Auger Electron Radioimmunotherapeutic Agent Specific for the CD123+/CD131− Phenotype of the Leukemia Stem Cell Population

Our aim was to construct and characterize 111In-nuclear translocation sequence (NLS)-7G3, an Auger electron–emitting radioimmunotherapeutic agent that preferentially recognizes the expression of CD123 (interleukin-3 receptor [IL-3R] α-subchain) in the absence of CD131 (IL-3R β-subchain) displayed by leukemia stem cells. Methods: Monoclonal antibody 7G3 was modified with 13-mer peptides [CGYGPKKKRKVGG] harboring the NLS of SV-40 large T-antigen and with diethylenetriaminepentaacetic acid for labeling with 111In. Immunoreactivity was evaluated in a competition radioligand binding assay and by flow cytometry. Nuclear localization of 111In-NLS-7G3 was studied by cell fractionation in CD123+/CD131− acute myelogenous leukemia (AML)-3, -4, and -5 cells or in primary AML or normal leukocytes. Micro-SPECT was performed in nonobese diabetic (NOD)/severe combined immune deficient (SCID) mice engrafted subcutaneously with Raji-CD123 tumors or with disseminated AML-3 or -5 cells. The cytotoxicity of 111In-NLS-7G3 on AML-5 cells was studied after 7 d in culture by trypan blue dye exclusion. DNA damage was assessed using the γ-H2AX assay. Results: NLS-7G3 exhibited preserved CD123 immunoreactivity (affinity, 4.6 nmol/L). Nuclear importation of 111In-NLS-7G3 in AML-3, -4, or -5 cells was specific and significantly higher than unmodified 111In-7G3 and was greater in primary AML cells than in normal leukocytes. Rapid elimination of 111In-NLS-7G3 in NOD/SCID mice prevented imaging of subcutaneous Raji-CD123 tumors. This phenomenon was Fc-dependent and IgG2a isotype–specific and was overcome by the preadministration of excess IgG2a or using 111In-NLS-7G3 F(ab′)2 fragments. AML-3 and -5 cells were engrafted into the bone marrow or spleen or at extramedullary sites in NOD/SCID mice. Micro-SPECT/CT with 111In-NLS-7G3 F(ab′)2 showed splenic involvement, whereas foci of disease were seen in the spine or femur or at extramedullary sites in the brain and lymph nodes using 111In-NLS-7G3 IgG2a. The viability of AML-5 cells was reduced by exposure in vitro to 111In-NLS-7G3; this reduction was associated with an increase in unrepaired DNA double-strand breaks. Conclusion: 111In-NLS-7G3 is a promising novel Auger electron–emitting radioimmunotherapeutic agent for AML aimed at the leukemia stem cell population. Micro-SPECT/CT was useful for visualizing the engraftment of leukemia in NOD/SCID mice.

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