²¹¹At-Anti-CD45 Radioimmunotherapy Can Replace TBI Prior to Haploidentical Bone Marrow Transplantation and Yield Long-Term Hematopoietic Engraftment

Haploidentical bone marrow transplantation (haplo-BMT) is both clinically effective and widely available because related-donors can be identified for nearly all recipients. Despite the curative promise of this approach, many patients with hematologic malignancies will relapse after haplo-BMT and more effective preparative regimens are necessary. We have shown that anti-CD45 radioimmunotherapy (RIT) delivers high-doses of radiation to hematolymphoid organs while minimizing the radiation exposure to non-targeted tissues. The efficacy of beta-emitting radionuclides may be limited by their relatively low decay energies (0.66 – 2.3 MeV). We have thus investigated the higher energy alpha-emitter astatine-211 (²¹¹At) (average decay energy of 6.8 MeV), for targeted anti-CD45 radioimmunotherapy (RIT) in lieu of total body irradiation (TBI) prior to haploidentical BMT in a murine leukemia model to decrease relapse rates.

Groups of five B6SJLF1/J mice (allotype H2-D^b) received escalated activities (20, 30 or 40 μCi) of ²¹¹At-anti-CD45 antibody [100 μg (0.67 nmol) of B10-30F11] given by tail vein injections on day -2 in place of TBI prior to BMT. Animals received cyclophosphamide (CY; 200 mg/kg/day) on days –3, –2, or –1, and +2 for graft-versus-host disease prophylaxis, either alone, or with fludarabine (FLU; 100 mg/kg/day) for 4 days starting day -6. Transplanted mice received 1.5 × 10⁷ haploidentical bone marrow cells from CB6F1/J mice (allotype H2-D^d) on day 0. Peripheral blood from recipient mice was then assayed monthly by flow cytometry to measure chimerism as the percentage of donor (H2-D^d) circulating CD8⁺ cells.

The highest activity delivered of 40 µCi ²¹¹At-anti-CD45 RIT was uniformly lethal without BMT rescue, whereas 60% of transplanted mice at this dose survived to assessment at 1 month. Mice treated with 30 µCi of ²¹¹At-anti-CD45 RIT with pre- and post-transplant CY and without TBI or FLU, had high levels of engraftment with an average of 83.7 ± 5.8% donor CD8⁺ cells 1 month after haploidentical BMT (Table 1). The addition of FLU to ²¹¹At-anti-CD45 RIT with CY did not significantly improve chimerism levels, with mean donor CD8⁺ cells in mice treated with 40 µCi ²¹¹At-anti-CD45 RIT of 64.5 ± 41.6% compared to 60.0 ± 13.9% in the absence of FLU (p=0.8668). In addition, mice that received 30 µCi ²¹¹At-anti-CD45 RIT and pre-transplant CY on either day –3, –2, or –1 showed mean donor CD8⁺ cells of 83.7 ± 5.8%, 49.9 ± 29.6% and 55.0 ± 46.2% 1 month after haploidentical-BMT, respectively. Importantly, chimerism levels remained stable 2 months after haploidentical BMT with mean donor CD8⁺ cells of 80.4 ± 16.6%, 47.0 ± 37.7% and 63.2 ± 10.7% in mice treated with 30 µCi ²¹¹At-anti-CD45 RIT and pre-transplant CY on day –3, –2, and –1, respectively. Engraftment using 40 or 30 µCi ²¹¹At-anti-CD45 RIT was comparable to using 850 or 1000 cGy TBI (mean donor CD8⁺ cells of 70.2 ± 18.8% and 60.0 ± 4.6%, respectively) prior to haploidentical BMT. RIT alone without any chemotherapy was insufficient to facilitate clinically relevant rates of donor engraftment, as mice treated with 30 µCi ²¹¹At-anti-CD45 RIT and no FLU, CY or TBI had 15.9 ± 7.1% mean donor CD8⁺ cells 1 month after haploidentical BMT.

These results suggest that ²¹¹At-anti-CD45 RIT prior to haploidentical BMT with pre– and post–transplant CY can result in high levels of donor hematopoietic cell engraftment in the absence of TBI and FLU. This conditioning regimen may be less toxic and more effective at preventing relapse than TBI-based approaches due to the high linear energy transfer of the alpha emissions, or the high decay energy of targeted ²¹¹At deposited over its short effective path-length. On-going studies are assessing the efficacy and toxicity associated with ²¹¹At-anti-CD45 RIT compared to a TBI-based haploidentical BMT using a syngeneic murine leukemia model.