Evaluation of Toxicity and Tissue Radiation Doses Obtained in Mice with An Anti-CD45 Monoclonal Antibody (mAb) Labeled with the Alpha-Emitting Radionuclides, Astatine-211 or Bismuth-213.

Background: One strategy to decrease toxicity of conditioning regimens for hematopoietic cell transplantation (HCT) has been the use of radiation targeted systemically with radionuclide-labeled mAbs. While mAb labeled with β-emitting radionuclides have shown some efficacy, a more promising alternative is the use of α-particle emitters. In contrast to β-emitting radionuclides, the high linear energy transfer and short particle range of α-particles makes them particularly attractive for killing hematopoietic cells in the blood and marrow. We initially investigated bismuth-213 (²¹³Bi)-labeled anti-CD45 mAb to replace total body irradiation (TBI) as nonmyeloablative conditioning for HCT. This conditioning successfully allowed sustained engraftment of allogeneic marrow in a canine model. However, the limited availability, very short t_1/2 of 46 min, and cost to produce ²¹³Bi place limitations in using this for clinical trials. Therefore, in this study we evaluated whether astatine-211 (²¹¹At), which has a longer half-life (t_1/2 = 7.2 h) than ²¹³Bi, gave comparable results when targeting hematopoietic cells in mice.

Methods: After injecting with either ²¹³Bi- or ²¹¹At-labeled rat anti-murine CD45 mAb 30F11, we evaluated myelosuppression and non-hematological toxicity with varying quantities of radioactivity (2, 10, 20, and 50 μCi) on 10 μg of 30F11 and 20 μCi of radioactivity on various quantities of 30F11 (2, 10 and 40 μg). We also evaluated biodistribution with 20 μCi on various quantities of 30F11.

Results: In the biodistribution studies, the highest concentrations of radioactivity were seen in the spleen. The ²¹¹At concentrations ranged from 167–417 % injected dose/gram (% ID/g) at 24 h post injection while the ²¹³Bi concentrations ranged from 45–166 % ID/g at 3 h post injection. This result suggested that the longer half-life of ²¹¹At might allow higher quantities of labeled mAb to reach the spleen before decay. Importantly, the longer half-life and higher spleen concentrations of ²¹¹At resulted in much higher radiation doses to that tissue (e.g. 29,450 cGy/50 mCi ²¹¹At administered vs. 1165 cGy/50 mCi ²¹³Bi when 10 mg of mAb was employed). Significant cytopenias were not observed in any of the ²¹³Bi groups. In contrast, lethal and irreversible myelosuppression was observed in the mice receiving 20 μCi ²¹¹At on 40 μg of 30F11 and 50 μCi ²¹¹At on 10 μg of 30F11 (minimal WBC counts 0.21 x 10³ and 0.12 x 10³/mm³, Plt counts 1.2 x 10⁴ and 0.3 x 10⁴/mm³, Hb levels 1.5 and 4.2 g/dl, respectively). In the mice receiving 20 μCi ²¹¹At on 10 μg of 30F11, significant but reversible pancytopenia was observed, with a nadir of 2 weeks after injection (minimal WBC count 0.45 x 10³/mm³, Plt count 3.3 x 10⁴/mm³, Hb level 9.9 g/dl). The pancytopenia resolved at 3 weeks after injection. The second highest concentrations both in ²¹¹At studies (18–50% ID/g) and ²¹³Bi studies (19–33%ID/g) were observed in the liver. Reversible but severe acute hepatic toxicity (maximal AST 1329 IU/L and ALT 928 IU/L) occurred 3h after 50 μCi ²¹³Bi but not after 50 μCi ²¹¹At injection. This was surprising given the fact that the estimated radiation dose given to liver after 50 mCi ²¹³Bi injection was ~200 cGy whereas the liver dose ranged from 1350–2600 cGy for 50 mCi ²¹¹At (depending on the amount of labeled mAb). No significant renal toxicity was observed with either ²¹¹At or ²¹³Bi labeled 30F11 during 8 weeks follow-up period.

Conclusion: The study results suggest that we may be able to replace low dose TBI in nonmyeloablative conditioning regimens with much lower (mCi) quantities of ²¹¹At-labeled anti-CD45 mAb without major non-hematological toxicities compared to the same mAb labeled with ²¹³Bi.