Donor Engraftment Following α-Emitter Astatine-211 (²¹¹At) Labeled Anti-CD45 Monoclonal Antibody (mAb) Nonmyeloablative Conditioning for Dog Leukocyte Antigen (DLA)-Identical Marrow Transplantation

Abstract 76

Previously we have demonstrated that an anti-CD45 mAb labeled with the α-emitter bismuth-213 (²¹³Bi), could replace 2 Gy total body irradiation (TBI) in nonmyeloablative conditioning for dogs transplanted with marrow from DLA identical littermates (Blood, 2002; 100: 318). However, due to several obstacles, including availability and cost of ²¹³Bi, the translation of the ²¹³Bi-labeled mAb into clinical studies was not possible. In the current study, we used astatine-211 (²¹¹At) as an alternative α-particle-emitting isotope. ²¹¹At is readily available in quantities that can be scaled up for clinical trials. We also hypothesized that the longer half-life of ²¹¹At (7.2 h) compared to ²¹³Bi (46 min) would make it more effective in killing targeted hematopoietic cells (including immune cells responsible for host versus graft reactions). This was confirmed in our murine studies comparing the two isotopes (Cancer Res. 2009; 69: 2408). In this study, ²¹¹At was linked to the anti-CD45 mAb through a conjugated closo-decaborate(2-) derivative (denoted as B10). The biodistribution in the dog showed that the spleen received the highest dose of radioactivity, 0.3% injected dose per gram of organ (%ID/g) while kidney and liver received 0.03 and 0.10% ID/g, respectively. Dose finding and toxicity studies were performed in 11 dogs receiving ²¹¹At-labeled anti-CD45 mAb doses ranging between 0.07 and 0.62 mCi/kg. All dogs experienced myelosuppression but recovered from the neutrophil nadirs (range, 0–1484.8/μ L) and lymphocyte nadirs (range, 32.0–411.0/μ L) without hematopoietic cell rescue. In 6 dogs receiving ²¹¹At–labeled anti-CD45 mAb conjugated with B10 through lysine amines, effective myelosuppression without pronounced non-hematopoietic toxicities, except for transient elevation of liver transaminases, was observed within the dose range of 0.20–0.41mCi/kg ²¹¹At. Subsequently, four dogs have been transplanted with marrow from DLA-identical littermates. In those studies 0.20–0.41mCi/kg ²¹¹At-labeled anti-CD45 mAb was given on day -3 as conditioning, and immunosuppression with mycophenolate and cyclosporine administered for 28 and 35 days, respectively, following transplantation. With 17 to 26 weeks follow-up, all dogs are alive, without graft-versus-host-disease, and with maximum and final donor chimerism in mononuclear cells ranging from 36%-68% and 26%-37% respectively. There was no renal toxicity, with blood urea nitrogen and creatinine levels within the normal range, and only negligible liver toxicity with a transient increase in liver enzymes to no more than 2 times the upper limit of the normal range. Immune reconstitution was prompt with normal T and natural killer function by day 60. In conclusion, our study demonstrated that low dose immunotherapy with 0.20–0.41mCi/kg ²¹¹At-labeled anti-CD45 mAb could replace TBI in DLA-identical littermate transplantation. The ²¹¹At-labeled anti-CD45 mAb provided sufficient immunosuppression to enable stable mixed chimerism with only minimal toxicity, making it a candidate for future clinical trials of nonmyeloablative transplantation.