Paradoxical effects of Auger electron-emitting 111In-DTPA-NLS-CSL360 radioimmunoconjugates on hCD45+ cells in the bone marrow and spleen of leukemia-engrafted NOD/SCID or NRG mice

Introduction111In-DTPA-NLS-CSL360 radioimmunoconjugates (RIC) recognize the overexpression of the interleukin-3 receptor α-subchain (CD123) relative to the β-subchain (CD131) on leukemia stem cells (LSC). Our aim was to study Auger electron radioimmunotherapy (RIT) of acute myeloid leukemia (AML) with 111In-DTPA-NLS-CSL360 in non-obese diabetic severe combined immunodeficiency (NOD/SCID) mice or NOD-Rag1nullIL2rγnull (NRG) mice engrafted with CD123+ human AML-5 cells.MethodsThe toxicity of three doses of 111In-DTPA-NLS-CSL360 (3.3–4.8 MBq; 11–15 μg each) injected i.v. every two weeks was studied in non-engrafted NOD/SCID or NRG mice pre-treated with 200 cGy of γ-radiation required for AML engraftment. Engraftment efficiency of (1–5) × 106 cells AML-5 cells inoculated i.v. into NOD/SCID or NRG mice was assessed by flow cytometric analysis for human CD45+ (hCD45+) cells in the bone marrow (BM) and spleen. AML-5 engrafted mice were treated with two or three doses (3.7 MBq; 10 μg each) every two weeks of 111In-DTPA-NLS-CSL360, non-specific 111In-DTPA-NLS-hIgG, unlabeled CSL360 (10 μg) or normal saline. The percentage of hCD45+ cells in the BM and spleen were measured at one week after completion of treatment.Results111In-DTPA-NLS-CSL360 in combination with 200 cGy of γ-radiation caused an initial transient decrease in body weight in NOD/SCID but not in NRG mice. There were no hematological, liver or kidney toxicities. The spleen exhibited 13-fold lower engraftment efficiency than the BM in NOD/SCID mice inoculated with 1 × 106 cells but both organs were highly (>85%) engrafted in NRG mice. Unexpectedly, 111In-DTPA-NLS-CSL360 or non-specific 111In-DTPA-NLS-hIgG caused a paradoxical 1.5-fold increase (P < 0.0001) in the proportion of hCD45+ cells in the BM of NOD/SCID mice compared to normal saline treated mice. 111In-DTPA-NLS-CSL360 reduced hCD45+ cells in the spleen by 3.0-fold compared to 111In-DTPA-NLS-hIgG (P = 0.0015) but the proportion of hCD45+ cells was not significantly different than in normal saline treated mice. Unlabeled CSL360 decreased the percentage of hCD45+ cells in the BM (P = 0.004) or spleen (P = 0.007) in NOD/SCID mice by 1.6-fold and 2.5-fold, respectively. 111In-DTPA-NLS-CSL360 or unlabeled CSL360 did not decrease the proportion of hCD45+ cells in the BM or spleen of NRG mice, due to a much higher leukemic burden.Conclusion111In-DTPA-NLS-CSL360 and 111In-DTPA-NLS-hIgG caused a paradoxical increase in the proportion of hCD45+ cells in the BM of NOD/SCID mice. This may be due to a priming effect on the BM niche that promotes expansion of engrafted hCD45+ cells, analogous to γ-radiation required for AML engraftment. There appears to be a competition between this effect and the cytotoxic effects of the Auger electrons on leukemia cells. The effectiveness of 111In-DTPA-NLS-CSL360 on reducing hCD45+ cells in the BM or spleen of NOD/SCID and NRG mice was dependent on the leukemic burden.Advances in knowledge and implications for patient careThis study demonstrates for the first time a paradoxical radiation priming effect of RIT on enhancing the hCD45+ cell population in the BM and spleen of NOD/SCID or NRG mice. Our results have important implications for preclinical evaluation of radioimmunotherapies for patients with AML.