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Abstract: Significant improvement of in vivo stability of 211At-labeled radioimmunoconjugates achieved upon employment of a recently reported new linker, succinimidyl N-2-(4-[211At]astatophenethyl)succinamate (SAPS), prompted additional studies of its chemistry. The 211At radiolabeling of succinimidyl N-2-(4-tributylstannylphenethyl)succinamate (1) was noted to decline after storage at −15°C for greater than 6 months. Compound 1 was found to degrade via a ring closure reaction with the formation of N-2-(4-tributylstannylphenethyl)succinimide (3), and a modified procedure for the preparation of 1 was developed. The N-methyl structural analog of 1, succinimidyl N-2-(4-tributylstannylphenethyl)-N-methyl succinamate (SPEMS), was synthesized to investigate the possibility of improving the stability of reagent–protein linkage chemistry. Radiolabeling of SPEMS with 211At generates succinimidyl N-2-(4-[211At]astatophenethyl)-N-methyl succinamate (Methyl-SAPS), with yields being consistent for greater than 1 year. Radiolabelings of 1 and SPEMS with 125I generated succinimidyl N-2-(4-[125I]iodophenethyl)succinamate (SIPS) and succinimidyl N-2-(4-[125I]iodophenethyl)-N-methyl succinamate (Methyl-SIPS), respectively, and showed no decline in yields. Methyl-SAPS, SAPS, Methyl-SIPS and SIPS were conjugated to Herceptin for a comparative assessment in LS-174T xenograft-bearing mice. The conjugates of Herceptin with Methyl-SAPS or Methyl-SIPS demonstrated immunoreactivity equivalent to if not superior to the SAPS and SIPS paired analogs. The in vivo studies also revealed that the N-methyl modification resulted in a superior statinated product. [Copyright &y& Elsevier]

Abstract: A simple and rapid procedure was developed for the purification of cyclotron-produced 203Pb via the 203Tl(d,2n) 203Pb reaction. A Pb(II) selective ion-exchange resin, with commercial name Pb Resin from Eichrom Technologies, Inc., was used to purify 203Pb from the cyclotron-irradiated Tl target with excellent recovery of the enriched Tl target material. The purified 203Pb was used to radiolabel the monoclonal antibody Herceptin. The in vitro and in vivo properties of the 203Pb radioimmunoconjugate were evaluated. [Copyright &y& Elsevier]

A simple and rapid procedure was developed for purification of cyclotron produced 86Y via the 86Sr(p,n) 86Y reaction. A commercially available Sr(II) selective resin was used to separate 86Y from the cyclotron irradiated Sr(II) target with a recovery of the enriched Sr(II) target while yielding a 75–80% recovery of 86Y suitable for radiolabeling either proteins or peptides. To demonstrate the utility of this methodology, the anti-HER2 monoclonal antibody Herceptin™ was radiolabeled with the purified 86Y and compared to 111In labeled Herceptin™. The biodistribution study demonstrated that 111In-Herceptin™, while a suitable surrogate for 90Y in the major organs, did not parallel the uptake of 86Y-Herceptin™in the bone, and thus may not accurately predict the level of 90Y accumulation in the bone for clinical RIT applications. This result exemplifies the requirement of employing appropriate matched pair isotopes for imaging and therapy to insure that dosimetry considerations may be addressed accurately. [Copyright &y& Elsevier]

The studies reported herein present the first in vitro and in vivo comparison of radioimmunoconjugates (RIC) radiolabeled with 177Lu using the acyclic CHX-A″-DTPA ligand and the macrocyclic ligands, C-DOTA and PA-DOTA. The in vivo studies include pharmacokinetics and biodistribution of the formed 177Lu-labeled immunoconjugates in a tumor bearing murine model with engineered monoclonal antibody HuCC49ΔCH2. The in vitro analysis indicated that the CHX-A″ RIC was superior with respect to immunoreactivity, radiolabeling with 177Lu, and specific activity. The in vivo pharmacokinetic data by itself indicated that the Lu(III)-PA-DOTA complex may not be as stable as Lu(III) complexes with CHX-A″ or C-DOTA. All three RICs demonstrated tumor targeting of human colon carcinoma xenografts in athymic mice. In these biodistribution studies, there appears to be no overall pattern or trend of one RIC over the other two. Based on these in vitro and in vivo studies, the CHX-A″ DTPA ligand should be considered a suitable bifunctional chelate for the radiolabeling of monoclonal antibodies with 177Lu for radioimmunotherapy applications. [Copyright &y& Elsevier]

Abstract: Novel ligands cis-2,6-bis[N,N-bis(carboxymethyl)aminomethyl]-1-piperidineacetic acid (PIP-DTPA), cis-[(1R,11S)-6,9,15-Tris-carboxymethyl-3,6,9,15-tetraazabicyclo[9.3.1]pentadec-3-yl]-acetic acid (PIP-DOTA), cis-{2,7-bis-[bis-carboxymethyl-amino)-methyl]-azepan-1-yl}-acetic acid (AZEP-DTPA), [2-(4,7-bis-carboxymethyl-[1,4,7]triazacyclononan-1-yl-ethyl]-2-carbonylmethyl-amino]-tetraacetic acid (NETA) and [{4-carboxymethyl-7-[2-(carboxymethylamino)-ethyl]-perhydro-1,4,7-triazonin-1-yl}-acetic acid (NPTA) are investigated as potential chelators of 177Lu, 90Y, 212Pb and 213Bi for radioimmunotherapy (RIT). The new ligands are radiolabeled with 177Lu, 86/88/90Y, 203Pb and 205/6Bi, and in vitro stability and in vivo stability of the radiolabeled complexes are assessed in human serum and athymic mice, respectively. In vitro studies indicate that all radiolabeled complexes with the exception of 90Y-AZEP-DTPA are stable in serum for 5–11 days. All new ligands examined herein are found to tightly hold 177Lu in vivo. Piperidine-backboned DTPA (PIP-DTPA) complexes radiolabeled with all radioisotopes examined display excellent in vivo stability, that is, excretion without dissociation. The azepane-backboned DTPA derivative, AZEP-DTPA, appears ineffective in binding all but 177Lu in vivo. NETA and NPTA radiolabeled with 86Y or 177Lu exhibit rapid blood clearance and low organ uptakes. Significant accretion in the kidney, femur and/or liver is observed with 203Pb-labeled AZEP-DTPA, PIP-DOTA and NPTA. Both 203Pb-PIP-DOTA and 205/6Bi-PIP-DOTA result in moderate to high renal accumulation of radioactivity. NETA exhibits improved renal accumulation with respect to PIP-DOTA for 205/6Bi but also shows significant liver uptake. Of all ligands studied, only PIP-DTPA appears to effectively bind 203Pb and 205/6Bi in vivo. PIP-DTPA, PIP-DOTA, NETA and NPTA all show strong evidence of rapid blood clearance and low organ uptake for 177Lu and 90Y. Serum stability and in vivo biodistribution results suggest PIP-DTPA as a potential chelating agent with broad applicability for use in 177Lu, 90Y, 212Pb and 213Bi RIT. [Copyright &y& Elsevier]

Abstract: The syntheses, radiolabeling, antibody conjugation and in vivo evaluation of new linkers for 211At labeling of monoclonal antibodies are described. Syntheses of the N-succinimidyl esters and labeling with 211At to form succinimidyl 4-methoxymethyl-3-[211At]astatobenzoate (9) and succinimidyl 4-methylthiomethyl-3-[211At]astatobenzoate (11) from the corresponding bromo-aryl esters is reported. Previously reported succinimidyl N-{4-[211At]astatophenethyl}succinamate (SAPS) is employed as a standard of in vivo stability. Each agent is conjugated with Herceptin in parallel with their respective 125I analogue, succinimidyl 4-methoxymethyl-3-[125I]iodobenzoate (10), succinimidyl 4-methylthiomethyl-3-[125I]iodobenzoate (12) and succinimidyl N-{4-[125I]iodophenethyl}succinamate (SIPS), respectively, for comparative assessment in LS-174T xenograft-bearing mice. With 9 and 11, inclusion of an electron pair donor in the ortho position does not appear to provide in vivo stability comparable to SAPS. Variables in radiolabeling chemistry of these three agents with 211At are notable. Sequential elimination of acetic acid and oxidizing agent, N-chlorosuccinimide (NCS), from the 211At radiolabeling protocol for forming SAPS improves yield, product purity and consistency. NCS appears to be critical for the radiolabeling of 6 with 211At. Formation of 11, however, is found to require the absence of NCS. Elimination of acetic acid is found to have no effect on radiolabeling efficiency or yield for either of these reactions. [Copyright &y& Elsevier]

To exploit the fact that IL-2 receptors are expressed by T-cells responding to foreign antigens but not by resting T-cells, humanized anti-Tac (HAT) armed with alpha-emitting radionuclides 212Bi and 211At was evaluated in a cynomolgus cardiac allograft model. Control graft survival was 8.2± 0.5 days compared with 14.0±1.3 days (p<0.01) survival for monkeys treated with 212Bi labeled HAT and 26.7±2.4 days survival (p<0.001 versus controls) with 211At labeled HAT. Thus, 211At labeled HAT may have application in organ transplantation and in treatment of IL-2 receptor expressing T-cell leukemia. [Copyright &y& Elsevier]

Abstract: A method for assessing the impurity 210At in cyclotron-produced 211At via isotope dilution alpha spectrometry is presented. The activity of 210At is quantified by measuring the activity of daughter nuclide 210Po. Counting sources are prepared by spontaneous deposition of Po on a silver disc. Activity of 210At (at the time of 210Po maximum activity) is found to be 83.5±9.0Bq, corresponding to an atom ratio (210At:211At at the time of distillation) of 0.010±0.007% (). The method produces high-quality alpha spectra, with baseline alpha-peak resolution and chemical yields of greater than 85%. [Copyright &y& Elsevier]

The separation of 86Y from 86Sr was optimized by a semi-automated purification system involving the passage of the target sample through three sequential columns. The target material was dissolved in 4 N HNO3 and loaded onto a Sr-selective (Sr-Spec) column to retain the 86Sr. The yttrium was eluted with 4 N HNO3 onto the second Y-selective (RE-Spec) column with quantitative retention. The RE-Spec column was eluted with a stepwise decreasing concentration of HNO3 to wash out potential metallic impurities to a waste container. The eluate was then pumped onto an Aminex A5 column with 0.1 N HCl and finally with 3 N HCl to collect the radioyttrium in 0.6-0.8 mL with a >80% recovery. This method enabled us to decontaminate Sr by 250,000 times and label 30 μ g of DOTA-Biotin with a >95% yield. [Copyright &y& Elsevier]