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

Introduction: (111)In-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 (111)In-DTPA-NLS-CSL360 in non-obese diabetic severe combined immunodeficiency (NOD/SCID) mice or NOD-Rag1(null)IL2rγ(null) (NRG) mice engrafted with CD123(+) human AML-5 cells.

Methods: The toxicity of three doses of (111)In-DTPA-NLS-CSL360 (3.3-4.8MBq; 11-15μg each) injected i.v. every two weeks was studied in non-engrafted NOD/SCID or NRG mice pre-treated with 200cGy of γ-radiation required for AML engraftment. Engraftment efficiency of (1-5)×10(6) 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 (3.7MBq; 10μg each) every two weeks of (111)In-DTPA-NLS-CSL360, non-specific (111)In-DTPA-NLS-hIgG, unlabeled CSL360 (10μg) or normal saline. The percentage of hCD45(+) cells in the BM and spleen were measured at one week after completion of treatment.

Results: (111)In-DTPA-NLS-CSL360 in combination with 200cGy 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 1×10(6) cells but both organs were highly (>85%) engrafted in NRG mice. Unexpectedly, (111)In-DTPA-NLS-CSL360 or non-specific (111)In-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. (111)In-DTPA-NLS-CSL360 reduced hCD45(+) cells in the spleen by 3.0-fold compared to (111)In-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. (111)In-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.

Conclusion: (111)In-DTPA-NLS-CSL360 and (111)In-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 (111)In-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 Care: This 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.