Anti-CD45 Radioimmunotherapy Facilitates Donor Engraftment and Prolongs Survival in the Absence of TBI Prior to Haploidentical Bone Marrow Transplantation in a Disseminated Murine Leukemia Model

Despite the curative promise of hematopoietic cell transplantation (HCT), many patients with hematologic malignancies relapse and others may not proceed to HCT due to the unavailability of a matched donor. Toxicities remain high with HCT, due in part to the administration of non-specific therapies such as total body irradiation (TBI) as part of preparative regimens. We aim to overcome these limitations by replacing TBI with anti-CD45 radioimmunotherapy (RIT) for haploidentical HCT to deliver radiation directly to leukemic cells while sparing normal organs and minimizing non-specific toxicities.

We established an initial TBI HCT regimen in B6SJLF1/J mice (H-2D^b haplotype) conditioned with fludarabine (FLU, days -6 to -2), followed by TBI (250, 500, 750 cGy; day -1). The mice then received 15 million donor (CB6F1/J, H-2D^d) BM cells (day 0), followed by cyclophosphamide (CY) for graft-versus-host disease (GvHD) prophylaxis (day +2). Subsequent RIT HCT studies involved B6SJLF1/J mice conditioned with and without fludarabine (FLU) and escalating doses (200–400 μCi) of ⁹⁰Y-anti-CD45 Ab (30F11) RIT without TBI, followed by infusion of haploidentical BM cells from CB6F1/J mice and a single dose of cyclophosphamide (CY) 2 days after HCT. Chimerism studies were performed using flow cytometric analysis to assay for engraftment of donor CD8⁺ cells. Therapeutic studies were performed in B6SJLF1/J mice given 10⁵ syngeneic leukemia cells via tail vein (day -5), followed by 200 or 400 μCi ⁹⁰Y-30F11 (day -3), and 1.5 × 10⁷ BM donor cells (day 0) and two doses of CY (days -2 and +2) without FLU.

Using this model we have demonstrated that mixed chimerism was established in mice transplanted with TBI or escalating doses (200–400 μCi) of ⁹⁰Y-30F11 RIT followed by injection of haploidentical BM donor rescue cells. TBI-based HCT showed that chimerism as determined by flow cytometric analysis for donor CD8⁺ cells was TBI dose-dependent; mice receiving ≥500 cGy were fully chimeric 4 weeks post-HCT, and persisted ≥12 months. RIT-based HCT also revealed mice with mixed chimerism, with up to 89% of donor CD8⁺ cells 1 month after HCT. Elimination of FLU from the conditioning regimen did not significantly decrease chimerism, as mice transplanted without FLU showed up to 70% donor CD8⁺ cells 1 month after HCT. Subsequent RIT experiments in B6SJLF1/J mice harboring AML were treated with escalating doses of ⁹⁰Y-30F11 prior to HCT without FLU. Mice treated with anti-CD45 RIT using 200 μCi and 400 μCi of ⁹⁰Y-30F11 had a median overall survival (OS) of 73 (p<0.001) and 107.5 (p=0.0015) days, respectively, compared to untreated leukemic control mice which had a median OS of 34 days after HCT (p values by Log rank (Mantel-Cox) testing)(Figure). Two mice in the 400 μCi-⁹⁰Y-30F11 group were euthanized on day 3 for excessive weight loss, without gross histology abnormality in kidneys or liver.

These studies suggest that anti-CD45 RIT in the absence of TBI and FLU prior to haploidentical HCT can lead to establishment of mixed chimerism. Moreover, this anti-CD45 RIT in combination with haploidentical HCT can lead to improvement in survival for mice with AML. These results suggest that clinical studies with anti-CD45 RIT in lieu of TBI and FLU in a haploidentical HCT regimen should be considered for further investigation.

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No relevant conflicts of interest to declare.