211At-Labeled Anti-CD45 Antibody as a Nonmyeloablative Conditioning for Canine DLA-Haploidentical Stem Cell Transplantation.

The α-emitter ²¹¹At deposits a high amount of energy within a few cell diameters, resulting in irreparable DNA double-strand breaks while minimizing off-target toxicity. We investigated the use of the ²¹¹At-labeled anti-CD45 monoclonal antibody (mAb) ²¹¹At-CD45-B10 as a nonmyeloablative conditioning regimen for dog-leukocyte-antigen-haploidentical hematopoietic cell transplantation. Methods: Seventeen healthy dogs were injected with either a 0.50 (n = 14) or 0.75 (n = 3) mg/kg dose of anti-CD45 mAb labeled with ²¹¹At (8.436-23.199 MBq [0.228-0.627 mCi/kg]) on day -3. Peripheral blood stem cells from dog-leukocyte-antigen-haploidentical donors were given on day 0. Peripheral blood chimerism was calculated by polymerase chain reaction assays, and blood clearance of the radioimmunoconjugate was studied using enzyme-linked immunosorbent assay and radioactivity measurements of serial blood samples. Results: All dogs achieved donor chimerism by day 28 (range, 27%-100%). The hematopoietic engraftment rate was 100%, though engraftment durability was variable. No difference in absorbed dose to blood was seen for the 2 mAb dosing levels studied. Neutropenia (0-29 cells/μL), lymphocytopenia (36-130 cells/μL), and thrombocytopenia (1.5-9 × 10³/μL) with prompt recovery were observed. The main adverse nonhematologic event related to ²¹¹At-CD45-B10 was mild reversible transaminitis. Graft-versus-host disease was not seen. Twelve of the 17 dogs survived over 30 d, with donor chimerism ranging from 3% to 99%. Conclusion: The results suggest that nonmyeloablative conditioning with ²¹¹At-CD45-B10 could be used in haploidentical hematopoietic cell transplantation though with variable engraftment.