Biodistributions, myelosuppression, and toxicities in mice treated with an anti-CD45 antibody labeled with the alpha-emitting radionuclides bismuth-213 or astatine-211

Abstract

We previously investigated the potential of targeted radiotherapy using a bismuth-213 ((213)Bi)-labeled anti-CD45 antibody to replace total body irradiation as conditioning for hematopoietic cell transplantation in a canine model. Although this approach allowed sustained marrow engraftment, limited availability, high cost, and short half-life of (213)Bi induced us to investigate an alternative alpha-emitting radionuclide, astatine-211 ((211)At), for the same application. Biodistribution and toxicity studies were conducted with conjugates of the anti-murine CD45 antibody 30F11 with either (213)Bi or (211)At. Mice were injected with 2 to 50 muCi on 10 microg or 20 muCi on 2 or 40 microg of 30F11 conjugate. Biodistribution studies showed that the spleen contained the highest concentration of radioactivity, ranging from 167 +/- 23% to 417 +/- 109% injected dose/gram (% ID/g) after injection of the (211)At conjugate and 45 +/- 9% to 166 +/- 11% ID/g after injection of the (213)Bi conjugate. The higher concentrations observed for (211)At-labeled 30F11 were due to its longer half-life, which permitted better localization of isotope to the spleen before decay. (211)At was more effective at producing myelosuppression for the same quantity of injected radioactivity. All mice injected with 20 or 50 muCi (211)At, but none with the same quantities of (213)Bi, had lethal myeloablation. Severe reversible acute hepatic toxicity occurred with 50 muCi (213)Bi, but not with lower doses of (213)Bi or with any dose of (211)At. No renal toxicity occurred with either radionuclide. The data suggest that smaller quantities of (211)At-labeled anti-CD45 antibody are sufficient to achieve myelosuppression and myeloablation with less nonhematologic toxicity compared with (213)Bi-labeled antibody.

Figure 1. ²¹³Bi- and ²¹¹At-labeled rat anti-mouse anti-CD45 antibody 30F11 conjugate (²¹³Bi- and ²¹¹At-MAb) biodistributions

Tissue biodistributions were obtained in mice to assess MAb targeting to spleen, which has high concentrations of CD45-containing cells, and to determine the effect of varying the quantity of MAb on tissue concentrations. The graphs of the tissue concentrations, expressed as percent injected dose / gram (%ID/g), for studies that employed 2 μg (black bars), 10 μg (white bars) or 40 μg (gray bars) of MAb are shown. Data were obtained at 15, 45, 90 and 180 min after injection of ²¹³Bi-MAb and at 1, 3, 7 and 24 h after injection of ²¹¹At-MAb. Data were obtained from groups of 5 mice per time point and were plotted as average values ± one standard deviation. Values plotted for injections of 2 and 40 μg quantities of MAb were obtained from single experiments (5 mice per time point), whereas values plotted for 10 μg were averaged from 3 (²¹³Bi) or 4 (²¹¹At) separate experiments (total of 15−20 mice per time point) because the biodistributions of MAb labeled with a specific radionuclide differed only when the quantities of MAb were different. Note that the y-axis maximum is 200 %ID/g for ²¹³Bi-MAb and 500 %ID/g for ²¹¹At-MAb.

Figure 2. Myelosupression with varying amounts (2, 10, 20, 50 μCi) of ²¹³Bi-MAb or ²¹¹At-MAb

Peripheral blood counts were monitored in collected blood at 180 min and then weekly for ²¹³Bi and at 24 h and then weekly for ²¹¹At. White blood cell (WBC) counts, hemoglobin (Hb) levels and platelet (Plt) counts were monitored up to 8 weeks after injection. The displayed data were obtained from mice treated with 2, 10, 20 and 50 μCi of ²¹³Bi or ²¹¹At on 10 μg of MAb. The dashed lines indicate normal ranges (mean ± 2 standard deviations) which were calculated with data obtained from 42 untreated female BALB/c mice.

Figure 3. Hepatic and renal toxicity with varying amounts (2, 10, 20, 50 μCi) of ²¹³Bi-MAb ²¹¹At-MAb

Hepatic and renal toxicities were monitored in sera from peripheral blood collected at 180 min (²¹³Bi) or 24 h (²¹¹At) after injection, and then weekly. Liver toxicity was assessed by monitoring the enzymes aspartate aminotransferase (AST) and alanine aminotransferase (ALT), and total bilirubin (T-Bil). Renal toxicity was assessed by monitoring the blood urea nitrogen (BUN) and creatinine (Cre). The monitoring was performed up to 8 weeks after injection. The displayed data were obtained from mice treated with 2, 10, 20 and 50 μCi of ²¹³Bi or ²¹¹At on 10 μg of MAb. The dashed lines indicate normal ranges (mean ± 2 standard deviations) which were calculated with data obtained from 42 untreated female BALB/c mice.

Figure 4. Pathological changes in the spleen and bone marrow after injection of ²¹¹At-MAb

Pathological examination of bone marrow from femurs (Panel A) and the spleens (Panel B) was performed on mice sacrificed 24, 48 h, 1, 2, 4 weeks after injection of 10 μCi/10 μg ²¹¹At-MAb. Pathological examination of the bone marrow from femur or sternum was conducted on necropsy samples on day 8 and day 10 after 20 μCi/40 μg or 50 μCi/10 μg ²¹¹At-MAb were administered, respectively (Panel C; row a) and the spleens on day 8 and day 11 after 20 μCi/40 μg or 50 μCi/10 μg ²¹¹At-MAb were administered, respectively (Panel C; row b).