We previously investigated the potential of targeted radiotherapy using a bismuth-213 (213Bi)–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 213Bi induced us to investigate an alternative α-emitting radionuclide, astatine-211 (211At), for the same application. Biodistribution and toxicity studies were conducted with conjugates of the anti-murine CD45 antibody 30F11 with either 213Bi or 211At. Mice were injected with 2 to 50 μCi on 10 μg or 20 μCi on 2 or 40 μg 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 211At conjugate and 45 ± 9% to 166 ± 11% ID/g after injection of the 213Bi conjugate. The higher concentrations observed for 211At-labeled 30F11 were due to its longer half-life, which permitted better localization of isotope to the spleen before decay. 211At was more effective at producing myelosuppression for the same quantity of injected radioactivity. All mice injected with 20 or 50 μCi 211At, but none with the same quantities of 213Bi, had lethal myeloablation. Severe reversible acute hepatic toxicity occurred with 50 μCi 213Bi, but not with lower doses of 213Bi or with any dose of 211At. No renal toxicity occurred with either radionuclide. The data suggest that smaller quantities of 211At-labeled anti-CD45 antibody are sufficient to achieve myelosuppression and myeloablation with less nonhematologic toxicity compared with 213Bi-labeled antibody. [Cancer Res 2009;69(6):2408–15]
- allogeneic hematopoietic cell transplantation (HCT)
- transplant conditioning
- astatine-211 (211At)
- bismuth-213 (213Bi)
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 γ-irradiation (TBI) while increasing specificity and efficacy, monoclonal antibodies (mAb) labeled with β-emitting radionuclides, such as 131I-labeled anti-CD45 mAb, have been investigated ( 1– 4). However, β-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 β-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 β-particles may deliver nonlethal doses to the targeted cells while causing nonspecific toxicity to surrounding normal tissues.
In contrast to β-emissions, α-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 μm). Given this favorable feature, we investigated bismuth-213 (213Bi)–labeled anti-CD45 mAb as replacement for TBI in a nonmyeloablative 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 213Bi-labeled mAbs into clinical studies, including the very high cost of the parent radionuclide to 213Bi, actinium-225 (225Ac). Furthermore, adequate quantities of 225Ac were not available for clinical studies.
Astatine-211 (211At) is an alternative α-particle–emitting radionuclide for radioimmunotherapy ( 13). 211At has a longer half-life than 213Bi (7.21 hours versus 45.6 minutes), potentially making an 211At-labeled anti-CD45 mAb more effective for targeting and killing hematopoietic cells. Based on our success with 213Bi-labeled anti-CD45 mAb in conditioning for HCT, we compared biodistributions, myelosuppression, and nonhematopoietic toxicities in mice with a mAb targeting hematopoietic tissues after radiolabeling it with either 211At or 213Bi. 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 hybridoma cell line was a gift from Dr. Irv Bernstein (Fred Hutchinson Cancer 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 213Bi-chelation reagent, isothiocyanatobenzyl-CHX-A″-DTPA (referred to as IB-CHX-A″), used to modify 30F11 was purchased from Macrocyclics. The 211At-reactive protein modification reagent, N-(15-(aminoacyldecaborate)-4,7,10-trioxatridecanyl)-3-maleimidopropionamide (referred to as ADTM), was prepared as previously described ( 15).
Radionuclides. 213Bi was obtained by elution from an 225Ac generator purchased from the U.S. Department of Energy (Oak Ridge, TN) as previously described ( 10). 211At was obtained by irradiating bismuth metal with a 28-MeV α-beam in a Scandatronix MC50 cyclotron housed in the Department of Radiation Oncology at the University of Washington. The 211At was removed from irradiated bismuth targets by dry distillation and isolated in 0.05 N NaOH as previously described ( 16).
Modification of mAb 30F11 for radiolabeling. Modification of 30F11 for labeling with 213Bi was achieved by conjugation of IB-CHX-A″ with 30F11 in 50 mmol/L HEPES buffer (pH 8.5) at room temperature for 18 h as previously described ( 10). Rigorous demetallation was conducted before conjugation with the mAb and again after the mAb conjugate was purified. Modification of 30F11 for labeling with 211At was achieved by treatment with 10 mmol/L DTT for 1 h at room temperature followed by buffer exchange into 20 mmol/L sodium phosphate at pH 6.5, containing 1 mmol/L EDTA, and then addition of 10 equivalents of ADTM in DMSO to the DTT-treated 30F11 with ADTM. After the conjugation reaction proceeded for 1 h at room temperature, the reaction mixture was eluted on a PD-10 column (Sephadex G-25) preequilibrated in PBS (pH 7.2). The fractions containing protein were pooled and concentrated in a Centricon-30 to provide the 30F11-ADTM. The 30F11-ADTM conjugates were analyzed by size-exclusion high-performance liquid chromatography (SE-HPLC) and isoelectric focusing to assess modification to the mAb.
Radiolabeling methods. The 30F11-CHX-A″ conjugate was radiolabeled with 213Bi in 0.3 mol/L NH4OAc (pH 4.2–4.5) for 2 to 5 min as described ( 10). The 30F11-ADTM conjugate was labeled with 211At as follows. To 100 to 200 μL of 1 to 5 mg/mL solution of 30F11-ADTM conjugate in PBS was added 2 to 100 μL of Na[211At]At and then 20 to 40 μL of chloramine-T (1 mg/mL) in H2O. The reaction was allowed to proceed for 30 s to 2 min; then, 20 to 40 μL of a 1 mg/mL solution of Na2S2O5 in H2O were added to quench the reaction. The reaction mixture was then passed over a G-25 Sephadex column (PD-10, Pharmacia) eluting with 0.9% saline ( 15). Fractions were collected and those containing protein were combined. Radiochemical yield was determined by the amount of radioactivity associated with the protein relative to the amount of radioactivity placed on the column. Radiochemical purity of the radiolabeled proteins was determined by SE-HPLC.
Animal studies. All mouse experiments were conducted under a protocol approved by the Fred Hutchinson Cancer Research Center Institutional Animal Care and Use Committee. Female BALB/c mice were obtained from The Jackson Laboratory. All reagents were administered to the BALB/c mice in a total volume of ∼200 μL via the lateral tail vein. Sets of 30 mice were injected with either 211At- or 213Bi-labeled mAb. Of those mice, 20 were sacrificed at predetermined times to obtain tissue distribution data, and the remaining 10 were evaluated over 8 wk for myelosuppression and toxicities. All mice were weighed weekly to assess for toxicity.
A total of 13 biodistribution studies were conducted. Seven experiments were conducted with [211At]30F11-ADTM (211At-mAb) and six with [213Bi]30F11-CHX-A″ (213Bi-mAb). In the experiments, tissue distributions of conjugates containing varying quantities of both radioactivity and mAb were evaluated (i.e., 2 μCi/10 μg, 10 μCi/10 μg, 50 μCi/10 μg, 6 μCi/2 μg, 20 μCi/2 μg, 20 μCi/10 μg, or 20 μCi/40 μg for labeled mAbs, and an additional experiment that had 20 μCi/10 μg for the 211At-mAb). In 213Bi (t1/2 = 45.6 min) experiments, groups of five mice were sacrificed at 15, 45, 90, and 180 min after injection, when 20%, 50%, 75%, and 94% of the radionuclide had decayed, whereas for the 211At (t1/2 = 7.21 h) experimental groups, mice were sacrificed 1, 3, 7, and 24 h after injection, when 9%, 25%, 49%, and 90% of the radionuclide had decayed.
For evaluation of tissue concentrations of radioactivity, eight tissues were examined, including muscle, lung, kidney, spleen, liver, intestine, neck, and stomach. The spleen was used as a surrogate tissue for hematopoiesis as total organ weight could be serially followed in addition to tissue concentrations of radioactivity. Bone marrows were sent for pathologic examination only. Blood samples were obtained by heart puncture immediately after sacrificing the mice. Excised tissues were blotted free of blood and weighed. The total blood volume was estimated to be 6% of body weight ( 17). The radioactivity in each tissue was measured with a gamma counter (Packard Cobra, GMI, Inc.), and counts per minute were corrected for decay of each sample from the initiation of counting. Tissue concentrations of radioactivity were expressed as percentages of the injected dose per gram (% ID/g). The calculation of % ID/g was based on standards containing 1 μL of the injected dose.
For pathologic examination, selected tissues (spleen, liver, and bone marrow) were analyzed in untreated mice and those sacrificed at 24 h, 48 h, 1 wk, 2 wk, and 1 mo after injection of 10 μCi 211At on 10 μg mAb. Necropsies were also performed to investigate the causes of death in five mice given 50 μCi 211At on 10 μg mAb (n = 4) or 20 μCi 211At on 40 μg mAb (n = 1) in which lethal toxicity occurred. Each tissue was fixed in 10% neutral buffered formaldehyde and then embedded in paraffin. Tissue sections were cut (4 μm) and stained with H&E staining by an automatic staining system (Tissue-Tek DRS 2000, Sakura Finetek U.S.A., Inc.).
Tissue radiation dose estimates. Radiation-absorbed doses were calculated for 213Bi and 211At in mouse tissues using standard methods for α-particle and β (electron) dosimetry. Using the mathematical formalism established by the Medical Internal Radiation Dose (MIRD) Committee of the Society of Nuclear Medicine ( 18), the absorbed doses (cGy) to target tissues for α-particles and electrons or β-particles were calculated from the available nuclear decay data ( 19) and biodistribution data (see Supplementary Data). The time-integrated activity (or total number of disintegrations) in each tissue was determined for each radionuclide by plotting the activity-time curve, identifying an appropriate function to represent the plotted data (by least-squares linear regression analysis), and integrating the best-fit regression curve from time 0 to infinity. The radiation-absorbed doses were then calculated using the MIRD formalism. The total absorbed dose to each tissue was calculated as the sum of the α plus electron contributions.
Myelosuppression and toxicities. Myelosuppression and nonhematologic toxicities were evaluated in 10 surviving mice from each experimental group remaining after biodistribution studies. After injection of the labeled mAb, blood samples were obtained by retro-orbital bleeding at 3 h for 213Bi or 24 h for 211At and weekly (alternating between two groups of five mice each) for a total of 8 wk. At each time point, the blood from the five mice within the group was pooled and peripheral blood counts, liver enzymes, and kidney function were monitored in collected blood. Blood was also obtained from five control mice for peripheral blood counts or for chemistry and pooled to allow sufficient blood volume. Five percent of EDTA was used as anticoagulant for peripheral blood samples. WBC counts, hemoglobin (Hb) level, and platelet counts were automatically measured by a quantitative automated hematology analyzer (the Sysmex XT2000i, Sysmex America). The analyses were conducted at the Seattle Cancer Care Alliance hematology laboratory. Serum aspartate aminotransferase (AST), alanine aminotransferase (ALT), total bilirubin, blood urea nitrogen (BUN), and creatinine were measured to evaluate liver and renal toxicities. Additionally, to determine reference ranges (mean ± 2 SDs) of WBC counts, Hb level and platelet counts in peripheral blood, and AST, ALT, total bilirubin, BUN, and creatinine, as a baseline for comparison, blood counts and chemistry data were also analyzed from 42 individual untreated female BALB/c mice. Blood chemistry analyses were conducted by the Department of Laboratory Medicine Research Testing Services at the University of Washington.
Tissue distributions. Tissue distributions were obtained for three different quantities (2, 10, and 40 μg) of 211At-mAb or 213Bi-mAb to determine which provided the most favorable biodistribution for therapy (i.e., had highest concentrations in spleen). Tissue concentrations were analyzed at four time points after injection to determine tissue radiation dose estimates. Concentrations (% ID/g) of radioactivity in selected tissues at the chosen time points after injection are shown in Fig. 1 . The spleen, which contains large numbers of CD45-containing hematopoietic cells, had the highest concentration of radioactivity in all experiments. Spleen concentrations were higher in the 211At studies than in the 213Bi studies, with the % ID/g (mean ± SD) ranging from 167 ± 23 (1 hour) to 417 ± 109 (24 hours) after injection of the 211At conjugate and ranging from 45 ± 9 (15 minutes) to 166 ± 11 (3 hours) after injection of the 213Bi conjugate. Interestingly, spleen weights obtained at euthanasia at each time point decreased dramatically from 73.9 ± 13.0 mg to 33.1 ± 3.2 mg at 24 hours after injection of 211At (P < 0.0001, unpaired t test), but no weight changes in spleens were noted after injection of 213Bi (data not shown). The spleen weight changes were reversible, as mice euthanized at later time points had normal spleen weight 2 weeks after injection of 211At. The liver contained the second highest concentrations, with study group averages ranging from 18% to 50% ID/g for 211At and 19% to 33% ID/g for 213Bi. The weights of the livers decreased slightly from 1.10 ± 0.09 g to 0.80 ± 0.12 g at 1 hour after injection in the 211At studies, but the changes were reversible. Kidney concentrations were low, with group averages ranging from 8% to 10% ID/g after 211At and 7% to 8% ID/g after 213Bi. Blood concentrations were similar between 213Bi and 211At groups and reflected the quantity of mAb injected. Of note, free 211At would be expected to accumulate in the thyroid (as measured in the neck, which contains the thyroid), lung, and stomach.
Tissue radiation dose estimates. The biodistribution data were used to estimate the tissue radiation doses when 2, 10, or 40 μg of 211At-mAb or 213Bi-mAb were administered ( Table 1A ). For comparison, the absorbed doses of 213Bi were multiplied by 9.49 (difference in half-lives) to equate the total number of 213Bi atoms to that of 211At. As a further comparison, in Table 1B, the tissue doses obtained using 50 μCi or, hypothetically, 500 μCi 213Bi (administered on the three different quantities of mAb) were compared with those obtained if 50 μCi of 211At were administered. The 500 μCi 213Bi value was an arbitrary value chosen as it would provide about the same radioactive dose as 50 μCi of 211At taking into account the 9.49× half-life factor between the two isotopes.
Myelosuppression. Reference ranges determined for the WBC count, Hb, and platelet count in an untreated cohort of mice were 4.8 ± 3.4 (×103 mm3), 16.0 ± 2.0 (g/dL), and 92.3 ± 28.4 (×104 mm3), respectively. No significant cytopenias were observed in the 213Bi groups ( Fig. 2 ). On the other hand, lethal myelosuppression was observed in all five mice receiving 20 μCi 211At on 40 μg mAb (data not shown in the figure) or 50 μCi 211At on 10 μg mAb (minimal WBC count, 0.21 × 103 and 0.12 × 103/mm3; platelet counts, 1.2 × 104 and 0.3 × 104/mm3; and Hb level, 1.5 and 4.2 g/dL, respectively). Pancytopenias started to appear 1 week after injection and were irreversible. In the mice receiving 20 μCi 211At on 10 μg mAb, significant pancytopenias were observed, with nadirs at 2 weeks after injection (minimal WBC count, 0.45 × 103/mm3). However, the pancytopenias resolved at 3 weeks after injection. In mice receiving 211At-mAb, leukopenia appeared at 24 hours after injection, except in mice receiving the lowest quantity (2 μg) of mAb. In contrast, leukopenia was not observed in mice administered 213Bi-mAb.
Hepatic toxicity. The reference ranges (mean ± 2 SDs) for AST, ALT, and total bilirubin were 129 ± 96 (units/L), 52 ± 31 (units/L), and 0.48 ± 0.27 (mg/dL), respectively, in an untreated cohort of mice. Severe but nonlethal hepatic toxicity was observed at 3 hours after injection of 50 μCi 213Bi on 10 μg mAb (maximal AST, 1,329 units/L; ALT, 928 units/L; Fig. 3A ). This hepatic toxicity resolved by day 15. In all 213Bi groups, except for mice given 2 μCi 213Bi on 2 μg mAb, mild temporary hepatic enzyme elevations were detected at 3 hours. However, the values recovered to near normal levels at 1 week. On the other hand, no significant hepatic toxicity was observed in any 211At-treated group.
Renal toxicity. The reference ranges (mean ± 2 SDs) determined for BUN and creatinine were 21.5 ± 7.8 (mg/dL) and 0.35 ± 0.17 (mg/dL), respectively, in an untreated cohort of mice. No significant renal toxicity was observed in mice administered either 213Bi or 211At ( Fig. 3B).
Tissue pathology (211At). Mice in all experimental study groups gained weight over the study period, except the group receiving 20 μCi 211At on 40 μg mAb. In the group receiving 10 μCi 211At, the bone marrow showed progressive hypocellularity, and hematopoiesis significantly decreased at 24 and 48 hours after injection ( Fig. 4A ). However, hematopoietic recovery occurred by 1 week after injection. Similarly, red pulp in the spleen significantly decreased and white pulp in the spleen became atrophic at 24 and 48 hours after injection ( Fig. 4B). In the 24-hour sample, there was diffuse necrosis of lymphoid cells intermixed with proliferating lymphocytes. The white pulp became atrophic (∼25% of normal cellularity) and red pulp was depleted down to ∼50% of normal cellularity at 24 hours. Megakaryocytes remained. However, recovery of hematopoiesis in the spleen was also observed in the specimen 1 week after injection. There were no pathologic abnormalities in the liver. In the five animals on which necropsies were performed, there were no abnormalities noted in the liver or kidney.
Necropsies were performed on mice that received 20 μCi 211At on 40 μg mAb or 50 μCi 211At on 10 μg mAb to investigate the cause of spontaneous death. Based on the autopsy and pathologic examination, it was likely that all the mice in these two groups injected with 211At-labeled mAb died of complications of severe pancytopenia (such as anemia or sepsis). In contrast to the group receiving 10 μCi 211At, the bone marrow cellularity remained very sparse at day 8 or 10 in groups receiving 20 to 50 μCi 211At, documenting protracted myelosuppression ( Fig. 4C, row a). At days 8 to 11, the red and white pulps of the spleen were depleted down to 25% of normal cellularity and megakaryocytes almost disappeared in the groups receiving 20 μCi 211At. In the groups receiving 50 μCi 211At, the red and white pulps were depleted down to 5% to 10% of normal cellularity and megakaryocytes were rarely observed ( Fig. 4C, row b). There was no evidence of extramedullary hematopoiesis in the liver.
The current study showed that 211At was more effective than 213Bi at producing myelosuppression for the same quantity of injected radioactivity. All mice injected with 20 or 50 μCi 211At, but none with the same quantities of 213Bi, had lethal myeloablation. Severe reversible acute hepatic toxicity occurred with the highest doses of 213Bi, but not with lower doses of 213Bi or with any dose of 211At. No renal toxicity occurred with either radionuclide. The data suggest that smaller μCi quantities of 211At-labeled anti-CD45 antibody are sufficient to achieve myelosuppression and myeloablation with less nonhematologic toxicity compared with 213Bi-labeled antibody.
Our previous study showed that the donor chimerism levels in dogs conditioned with 213Bi-labeled anti-TCRαβ mAb (CA15.9D95) were lower than those observed in dogs conditioned with 213Bi-labeled pan-hematopoietic anti-CD45 mAb (CA12.10C12; refs 10, 11, 20). The results suggested that 213Bi-labeled anti-CD45 mAb was more effective in killing host residual cells, such as natural killer cells, which are responsible for graft rejection. CD45 was an excellent target because it is ubiquitously expressed on both nonmalignant and malignant hematopoietic cells ( 21– 23). However, the short half-life of 213Bi presents a significant problem because it mandates the use of large 225Ac/213Bi generators and multiple preparations/injections per patient to obtain adequate doses of clinical materials for patient treatment. A further consideration is the fact that, at present, there is a very high cost to obtain a generator of sufficient size to conduct clinical studies. Therefore, a clinical study was not feasible at the current time using 213Bi. Additionally, the longer half-life of 211At has logistical and, potentially, therapeutic benefits.
211At is available at our institution by irradiation of a bismuth target with an α-particle beam from a cyclotron. An important consideration for initiating the investigation with 211At was the fact that recent upgrades on the cyclotron and target station used to produce 211At make it possible to prepare the quantities required for clinical studies, and this can be done at a much lower cost relative to producing 213Bi. Perhaps more importantly, the consideration for studies where 213Bi is replaced by 211At is the fact that the half-life of 211At (t1/2 = 7.21 hours) is ∼9.5 times longer than that of 213Bi (t1/2 = 45.6 minutes). This difference in half-life has some important benefits. One benefit is that there are 9.5 times the number of radioactive atoms as that of 213Bi in each mCi of 211At injected. Thus, for the same number of mCi of 211At as 213Bi, much higher doses can be obtained, or considerably lower quantities of 211At might be used to deliver a therapeutic dose. Another benefit of the longer half-life is the fact that a smaller percentage of the injected radioactivity will decay during the period of targeting hematopoietic cells, potentially resulting in more specific delivery of the radiation.
In the current study, labeling mAb 30F11 with the two radionuclides required use of different chemical modifications that could potentially affect the tissue distribution. Further, biodistribution of the radiolabeled mAb conjugates (30F11-CHX-A″ and 30F11-ADTM) were only relevant over the period where most of the radioactivity decays, potentially making the relative biodistributions very different given the different half-lives of the two isotopes. To label 213Bi, mAb 30F11 was conjugated with a benzylisothiocyanate-cyclohexyl derivative of DTPA (IB-CHX-A″), which had been used in our previous canine conditioning studies. mAb conjugates of IB-CHX-A″ were rapidly labeled (5 minutes) in high yield (>80%) and provided good in vivo stability during the period of 213Bi decay. Stability of the label had been a problem for antibodies labeled with 211At ( 24). Although 211At-labeled benzoate esters could be used for stable labeling some mAbs, an investigation of 30F11 labeled with N-hydroxysuccinimidyl 3-[211At]astatobenzoate ( 25) indicated that it was not stable in vivo. 7 Therefore, an alternate 211At-labeling conjugate, a recently developed reagent ( 15, 26), ADTM, was used. The use of mAb-ADTM conjugates provided high radiochemical yields (70–80%) through direct labeling of 211At and provided high in vivo stability to deastatination.
The biodistribution studies showed that the much higher radiation doses delivered by 211At depleted the hematopoietic cells and accounted for the differences seen in the spleen weights. Although a large portion of the difference in radiation doses was provided by the fact that there are 9.49 times more 211At atoms per μCi administered, it seems that another factor of 2× in the dose may be due to the longer half-life of 211At, permitting more localization of mAb to the spleen before decay. Thus, from the tissue radiation dose estimates, 500 μCi of 213Bi would deliver less than half the dose to the target (spleen) of 50 μCi 211At if the radionuclides were on 10 μg mAb. As might be expected from the biodistribution and tissue dose estimates, the blood count data indicated that myelosuppression was more effective with the 211At-mAb for the same μCi amount of 213Bi-mAb.
In this study, severe hepatic toxicity appeared in the mice receiving the highest dose (50 μCi) of 213Bi-mAb, presumably due to the abundance of hematopoietic cells and Kupffer cells in the liver expressing the CD45 antigen. There may also be a nonspecific dose contribution as immunoglobulins from the bloodstream are trapped in the liver, resulting in the radiolabeled mAb being trapped even though CD45 is not expressed on hepatocytes ( 7). In the previous canine study, the dog receiving the highest dose of 8.8 mCi/kg 213Bi-labeled anti-CD45 mAb also showed marked elevation of hepatic enzymes and evidence of liver failure with the development of ascites due to toxicity from the radioimmunotherapy ( 10). Based on the present and previous data, hepatic toxicity of 213Bi-labeled anti-CD45 mAb can be considered dose limiting. The observed hepatic toxicity in the 213Bi studies was likely caused by deposition of 213Bi-labeled mAb in the liver. The nature of conjugate molecule (i.e., CHX-A″ or ADTM) to label the mAb with radionuclide might also contribute to hepatic deposition, but there are no data suggesting that either conjugate specifically localizes to liver.
The overall objective of our continuing research effort is to determine if mAbs labeled with an α-particle–emitting radionuclide can deliver a marrow-ablative dose without the other organ toxicities associated with high-dose conditioning regimens. Our earlier studies showed that 213Bi-labeled anti-CD45 mAb provided adequate myelosuppression to obtain stable chimeras in the dog model. From previous dog data, it was estimated that 1.5 to 2 mCi 213Bi/kg on 0.5 mg anti-CD45 mAb/kg would likely be required in patients to obtain stable engraftment. This study showed that 211At was more effective at myelosuppression for the same quantity of radioactivity injected than 213Bi without significant nonhematopoietic toxicity. Because there are fewer barriers to clinical studies with 211At, and encouraging results were obtained in this investigation, further studies in the dog model with 211At-labeled anti-CD45 mAb are under way.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
Grant support: NIH grants CA118940, CA015704, CA109663, and CA095448 and Frederick Kullman and Penny E. Petersen Memorial Foundations. H. Nakamae was funded by the Graduate School of Medicine, Osaka City University, Osaka, Japan. J.M. Pagel is supported by Career Development Awards from the Lymphoma Research Foundation and the Damon Runyon Cancer Foundation.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
We thank Dai Nguyen for her help in conducting the biodistribution study; Sue E. Knoblaugh and George E. Sale for pathologic evaluation; and Helen Crawford, Bonnie Larson, and Sue Carbonneau for manuscript preparation.
Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/).
↵7 Unpublished data.
- Received November 14, 2008.
- Revision received December 18, 2008.
- Accepted January 4, 2009.
- ©2009 American Association for Cancer Research.