At-211-anti-CD45 radioimmunotherapy as conditioning prior to haploidentical hematopoietic cell transplantation in mouse models

Abstract Background: Allogeneic hematopoietic cell transplantation (HCT) is a potentially curative treatment option for high-risk acute myeloid leukemia (AML) but requires intensive chemotherapy and/or total body irradiation (TBI) for its preparative conditioning regimen. However, the conditioning regimen for HCT can be rather toxic from off-target effects leading to morbidity and even death. Additionally, HCT for racial and ethnic minority patients is often limited because HCT had required the use of completely human leukocyte antigen (HLA)-matched donors. Haploidentical HCT (HaploHCT) has made HCT more accessible, wherein the donor only needs to be partially HLA-matched. Aside from the common complications of HCT such as graft-versus-host disease (GVHD), toxicity, mortality, and relapse, haploHCT requires multi-agent chemotherapy and/or high-dose TBI, which older, more frail patients cannot tolerate. To achieve the goal of universal availability and efficacy with minimal toxicity, conditioning regimens need to be simplified. One possibility is to minimize systemic chemotherapy and replace TBI with targeted radiation delivery, or radioimmunotherapy (RIT). RIT uses radiolabeled antibodies to focus radiation delivery to specific tissues while sparing normal, uninvolved organs. Because CD45 is an abundant cell surface marker on hematopoietic cells not found on non-blood tissues, anti-CD45 RIT before haploHCT is a logical approach. Previously, yttrium-90 (90Y)-anti-CD45 RIT achieved comparable levels of persistent haploidentical hematopoietic engraftment as myeloablative TBI. However, 90Y is a beta-emitter and can have negative effects to surrounding tissues due to its long path length. To further minimize toxicity risks with RIT, we sought to design an alternative conditioning regimen with astatine-211 (211At), an alpha-emitter with high radiation payload but shorter path length compared to 90Y, and hence potentially less negative effect to its surrounding tissues. With higher targeted radiation to the hematopoietic niche, we hypothesize that 211At can facilitate engraftment from haploidentical donors without TBI or systemic chemotherapy with minimal toxicity. Methods: B6SJLF1/J mice (CD45.1 positive) received 15 or 30 µCi of 211At-anti-CD45 RIT or comparator myeloablative TBI (10 Gy), with 0 or 200 mg/kg cyclophosphamide (CY) on day -3, and 9.0 x 106 haploidentical donor bone marrow cells on day 0 from CB6F1/J (CD45.1 negative) mice. All mice received standard post-transplant CY (200mg/kg) on day +3 for GVHD prophylaxis with no additional GVHD prophylaxis treatment. Peripheral blood was sampled monthly, hemolyzed and stained for leukocyte lineage (B220 for B-lymphocytes, CD3e for T-cells, CD11b for myeloid cells), and CD45.1 haplotype on day 28, 55, and 84 after HCT to assess lineage-specific donor-origin chimerism via multicolor flow cytometry. Results: 211At-anti-CD45 RIT (30 µCi), in combination with pre-CY, shows the highest and most comparable levels of donor-origin chimerism levels to myeloablative TBI (10Gy) standard approach, with an average donor-origin chimerism level of 56.54% ± 17.97% (B220), 39.51% ± 21.05% (CD3e), and 63.33% ± 22.45% (CD11b) at d +28, compared to 92.40% ± 1.95% (B220), 81.02% ± 3.30% (CD3e) and 92.12% ± 3.22% (CD11b) in the TBI group. Mice that received 30 µCi of 211At-anti-CD45 RIT alone had poor donor-origin chimerism levels at 19.69% ± 6.73%, 14.58% ± 7.53%, and 26.09% ± 13.94% for B220, CD3, and CD11b respectively. At most recent assessment, this level of chimerism persisted through d +84 for mice that received 211At-anti-CD45 RIT at 30 µCi with pre-CY, (46.68% ± 45.18%, 45.23% ± 45.43%, 49.81% ± 37.62% for B220, CD3e, and CD11b respectively), which approaches levels of the TBI group (95.38% ± 0.92%, 88.91% ± 1.70%, 94.39% ± 0.92% for B220, CD3e and CD11b respectively) at d +84. Conclusion: Optimization of the preparative regimen with 211At-anti-CD45 RIT before haploHCT facilitated persistent haploidentical donor engraftment without TBI nor multi-agent chemotherapy but required pre-CY for higher chimerism. In addition to confirming persistent chimerism to 6 months post HCT, we will next evaluate the optimized preparative regimen of 211At-anti-CD45 RIT before haploHCT in a disseminated leukemia model and confirm minimal off-target toxicity with organ function serum labs.