Biodistributions, Myelosuppression, and Toxicities in Mice Treated with an Anti-CD45 Antibody Labeled with the A-Emitting Radionuclides Bismuth-213 or Astatine-211 Hirohisa Nakamae, 1 D. Scott Wilbur, 2 Donald K. Hamlin, 2 Monica S. Thakar, 1,3 Erlinda B. Santos, 1 Darrell R. Fisher, 6 Aimee L. Kenoyer, 1 John M. Pagel, 1,4 Oliver W. Press, 1,4,5 Rainer Storb, 1,4 and Brenda M. Sandmaier 1,4 1 Clinical Research Division, Fred Hutchinson Cancer Research Center; Departments of 2 Radiation Oncology, 3 Pediatrics, 4 Medicine, and 5 Biological Structure, University of Washington, Seattle, Washington and 6 Pacific Northwest National Laboratory, Richland, Washington Abstract We previously investigated the potential of targeted radiother- apy using a bismuth-213 ( 213 Bi)–labeled anti-CD45 antibody to replace total body irradiation as conditioning for hematopoi- etic 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 A-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 wereinjectedwith2to50 MCion10 Mgor20 MCion2or40 Mgof 30F11 conjugate. Biodistribution studies showed that the spleen contained the highest concentration of radioactivity, ranging from 167 F 23% to 417 F 109% injected dose/gram (% ID/g)afterinjectionofthe 211 Atconjugateand45 F 9%to166 F 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 MCi 211 At,butnone with the same quantities of 213 Bi, had lethal myeloablation. Severe reversible acute hepatic toxicity occurred with 50 MCi 213 Bi,butnotwithlowerdosesof 213 Biorwithanydoseof 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. [Cancer Res 2009;69(6):2408–15] Introduction Allogeneic hematopoietic cell transplantation (HCT) is a curative modality for patients with various malignant and nonmalignant hematopoietic diseases. Recently, to reduce late toxicities from total body g-irradiation (TBI) while increasing specificity and efficacy, monoclonal antibodies (mAb) labeled with h-emitting radionuclides, such as 131 I-labeled anti-CD45 mAb, have been investigated (1–4). However, h-emitting radionuclides are not optimal for killing the targeted hematopoietic cells due to their long path length and low dose rates (5–8). Owing to the long h- particle path (i.e., mean range, 0.4–5 mm; ref. 9), the majority of the emitted energy is deposited outside of the targeted hematopoietic cells. Thus, although specific targeting of hematopoietic cells may be achieved with the mAb, the h-particles may deliver nonlethal doses to the targeted cells while causing nonspecific toxicity to surrounding normal tissues. In contrast to h-emissions, a-particles are characterized by very high linear energy transfer, with most of the energy of the particles being deposited over only a few cell diameters (i.e., 40–90 Am). Given this favorable feature, we investigated bismuth-213 ( 213 Bi)– labeled anti-CD45 mAb as replacement for TBI in a nonmyeloa- blative conditioning regimen for HCT in a canine model (10–12). Although the treatment was effective in allowing successful engraftment of marrow, several pragmatic obstacles precluded translating 213 Bi-labeled mAbs into clinical studies, including the very high cost of the parent radionuclide to 213 Bi, actinium-225 ( 225 Ac). Furthermore, adequate quantities of 225 Ac were not available for clinical studies. Astatine-211 ( 211 At) is an alternative a-particle–emitting radio- nuclide for radioimmunotherapy (13). 211 At has a longer half-life than 213 Bi (7.21 hours versus 45.6 minutes), potentially making an 211 At-labeled anti-CD45 mAb more effective for targeting and killing hematopoietic cells. Based on our success with 213 Bi-labeled anti-CD45 mAb in conditioning for HCT, we compared biodis- tributions, myelosuppression, and nonhematopoietic toxicities in mice with a mAb targeting hematopoietic tissues after radio- labeling it with either 211 At or 213 Bi. The antibody, a rat anti-murine CD45 mAb, 30F11 (2, 14) used in the mouse provided a model for understanding differences between the two radionuclides. Materials and Methods Antibody and chemicals. The rat anti-murine CD45 mAb, 30F11, is an IgG2b mAb that recognizes all murine CD45 isoforms (2). The 30F11 hybridomacelllinewasagiftfromDr.IrvBernstein(FredHutchinsonCancer Research Center). The 30F11 mAb was produced by injecting the hybridoma into pristane-primed mice to generate ascites. The 30F11 mAb was purified from ascitic fluid by protein G immunoabsorption column chromatography. The protein-reactive 213 Bi-chelation reagent, isothiocyanatobenzyl-CHX-A 00 - DTPA (referred to as IB-CHX-A 00 ), used to modify 30F11 was purchased from Macrocyclics. The 211 At-reactive protein modification reagent, N -(15- (aminoacyldecaborate)-4,7,10-trioxatridecanyl)-3-maleimidopropionamide (referred to as ADTM), was prepared as previously described (15). Radionuclides. 213 Bi was obtained by elution from an 225 Ac generator purchased from the U.S. Department of Energy (Oak Ridge, TN) as Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/). Requests for reprints: Brenda M. Sandmaier, Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North, Mail Stop D1-100, P. O. Box 19024, Seattle, WA 98109-1024. Phone: 206-667-4961; Fax: 206-667-6124; E-mail: bsandmai@fhcrc.org. I2009 American Association for Cancer Research. doi:10.1158/0008-5472.CAN-08-4363 Cancer Res 2009; 69: (6). March 15, 2009 2408 www.aacrjournals.org Research Article Downloaded from http://aacrjournals.org/cancerres/article-pdf/69/6/2408/2623161/2408.pdf by guest on 21 June 2022