Anti-CD45 Pretargeted Radioimmunotherapy Prior to Bone Marrow Transplantation without Total Body Irradiation Facilitates Engraftment From Haploidentical Donors and Prolongs Survival in a Disseminated Murine Leukemia Model

In many cases the only curative treatment option for patients with advanced leukemias may be hematopoietic cell transplantation (HCT), which is often associated with toxicities.  Despite HCT, patients still relapse while others will not have the option of HCT due to the unavailability of an HLA-matched donor.  We aim to overcome these hurdles using anti-CD45 pretargeted radioimmunotherapy (PRIT) in lieu of total body irradiation (TBI) for haploidentical bone marrow transplantation (BMT).  B6SJLF1/J mice were given 10⁵ syngeneic myeloid leukemia cells followed by injection of anti-CD45 antibody (30F11; 140 µg) conjugated to streptavidin (SAv).  Eight hours later a biotinylated synthetic clearing agent (CA) (50 µg) was injected, followed by ⁹⁰Y-labeled DOTA- (2 µg) 2 hours later.  This strategy resulted in excellent localization of radioactivity in spleen [38.1 ± 7.3 percent of the injected dose of radioactivity per gram of organ (% ID/g)] and bone marrow (BM; 3.4 ± 1.1% ID/g), with minimal uptake in non-target organs (kidneys, 0.70 ± 0.13% ID/g; lungs, 0.3 ± 0.1% ID/g) 24 hours after radiobiotin injection.  In separate BMT studies, mice were treated with and without fludarabine (FLU) (100 mg/kg/day) on days -8 to -4, and/or cyclophosphamide (CY; 200 mg/kg/day) on days -2 and +2, and 30F11-SAv (140 µg) followed by CA (50 µg) and 400-800 µCi of ⁹⁰Y-DOTA-biotin three days prior to infusion of 15x10⁶ BM cells from haploidentical donor mice (CB6F1/J, H-2D^d).  In mice transplanted without TBI but using 800 µCi of ⁹⁰Y-DOTA-biotin, day 28 flow cytometry analysis detected up to 12% donor CD8+ cells, with no reduction in levels of chimerism in the absence of FLU or CY.  Subsequently, mice with disseminated syngeneic leukemia treated with the PRIT approach in the absence of FLU and TBI showed an improvement in median survival (OS) compared to untreated leukemic mice (see FIGURE).  Mice treated with 400-800 µCi of ⁹⁰Y-DOTA-biotin had a median OS of at least 50 days compared to a median OS of 23 days in untreated control mice.  Forty percent of mice given 800 µCi of ⁹⁰Y-DOTA-biotin died early from complications from BM aplasia.  These results suggest that anti-CD45 PRIT can localize radiation effectively to BM and spleen, and when used in conjunction with haploidentical BMT without TBI or FLU, can facilitate engraftment and lead to improvements in OS in a disseminated murine leukemia model.