The human polynucleotide kinase/phosphatase (hPNKP) inhibitor A12B4C3 radiosensitizes human myeloid leukemia cells to Auger electron-emitting anti-CD123 111In-NLS-7G3 radioimmunoconjugates

Introduction: Leukemia stem cells (LSCs) are believed to be responsible for initiating and propagating acute myeloid leukemia (AML) and for causing relapse after treatment. Radioimmunotherapy (RIT) targeting these cells may improve the treatment of AML, but is limited by the low density of target epitopes. Our objective was to study a human polynucleotide kinase/phosphatase (hPNKP) inhibitor that interferes with DNA repair as a radiosensitizer for the Auger electron RIT agent, In-111-NLS-7G3, which recognizes the CD123(+)/CD131(-) phenotype uniquely displayed by LSCs. Methods: The surviving fraction (SF) of CD123(+)/CD131(-)AML-5 cells exposed to In-111-NLS-7G3 (33-266 nmols/L: 0.74 MBq/mu g) or to gamma-radiation (0.25-5 Gy) was determined by clonogenic assays. The effect of A12B4C3 (25 mu mols/L) combined with In-111-NLS-7G3 (16-66 nmols/L) or with gamma-radiation (0.25-2 Gy) on the SF of AML-5 cells was assessed. The density of DNA double-strand breaks (DSBs) in the nucleus was measured using the gamma-H2AX assay. Cellular dosimetry was estimated based on the subcellular distribution of In-111-NLS-7G3 measured by cell fractionation. Results: Binding of In-111-NLS-7G3 to AML-5 cells was reduced by 2.2-fold in the presence of an excess (1 mu M) of unlabeled NLS-7G3, demonstrating specific binding to the CD123(+)/CD131(-) epitope. In-111-NLS-7G3 reduced the SF of AML-5 cells from 86.1 +/- 11.0% at 33 nmols/L to 10.5 +/- 3.6% at 266 nmols/L Unlabeled NLS-7G3 had no significant effect on the SF. Treatment of AML-5 cells with gamma-radiation reduced the SF from 98.9 +/- 14.9% at 0.25 Gy to 0.03 +/- 01% at 5 Gy. A12B4C3 combined with In-111-NLS-7G3 (16-66 nmols/L) enhanced the cytotoxicity up to 1.7-fold compared to treatment with radioimmunoconjugates alone and was associated with a 1.6-fold increase in DNA DSBs in the nucleus. A12B4C3 enhanced the cytotoxicity of gamma-radiation (0.25-0.5 Gy) on AML-5 cells by up to 1.5-fold, and DNA DSBs were increased by 1.7-fold. Exposure to In-111-NLS-7G3 (66 nmols/L) delivered up to 0.6 Gy to AML-5 cells. Conclusions: We conclude that A12B4C3 radiosensitized AML cells to the DNA damaging effects of In-111-NLS-7G3. Combination treatment may increase the effectiveness for Auger electron Rh T of AML targeting the LSC subpopulation. (C) 2014 Elsevier Inc. All rights reserved.