Labeling Monoclonal Antibody with α-emitting 211At at High Activity Levels via a Tin Precursor.

Background: In a previous clinical study, the authors evaluated the potential of antitenascin C monoclonal antibody (mAb) 81C6 labeled with 211At via the prosthetic agent N-succinimidyl 3-[211At]astatobenzoate (SAB) for the treatment of primary brain tumors. Although encouraging results were obtained, labeling chemistry failed while attempting to escalate the dose to 370 MBq. The goal of the current study was to develop a revised procedure less susceptible to radiolysis-mediated effects on 211At labeling that would be suitable for use at higher activity levels of this α-emitter. Materials and Methods: Addition of N-chlorosuccinimide to the methanol used to remove the 211At from the cryotrap after bismuth target distillation was done to thwart radiolytic decomposition of reactive 211At and the tin precursor. A series of 11 reactions were performed to produce SAB at initial 211At activity levels of 0.31-2.74 GBq from 50 µg of N-succinimidyl 3-trimethylstannylbenzoate (Me-STB), which was then reacted with murine 81C6 mAb without purification of the SAB intermediate. Radiochemical purity, immunoreactive fraction, sterility, and apyrogenicity of the 211At-labeled 81C6 preparations were evaluated. Results: Murine 81C6 mAb was successfully labeled with 211At using these revised procedures with improved radiochemical yields and decreased overall synthesis time compared with the original clinical labeling procedure. Conclusions: With 2.74 GBq of 211At, it was possible to produce 1.0 GBq of 211At-labeled 81C6 with an immunoreactive fraction of 92%. These revised procedures permit production of 211At-labeled mAbs suitable for use at clinically relevant activity levels.

Realizing Clinical Trials with Astatine-211: The Chemistry Infrastructure.

Despite the consensus around the clinical potential of the α-emitting radionuclide astatine-211 (211At), there are only a limited number of research facilities that work with this nuclide. There are three main reasons for this: (1) Scarce availability of the nuclide. Despite a relatively large number of globally existing cyclotrons capable of producing 211At, few cyclotron facilities produce the nuclide on a regular basis. (2) Lack of a chemical infrastructure, that is, isolation of 211At from irradiated targets and the subsequent synthesis of an astatinated product. At present, the research groups that work with 211At depend on custom systems for recovering 211At from the irradiated targets. Setting up and implementing such custom units require long lead times to provide a proper working system. (3) The chemistry of 211At. Compared with radiometals there are no well-established and generally accepted synthesis methods for forming sufficiently stable bonds between 211At and the tumor-specific vector to allow for systemic applications. Herein we present an overview of the infrastructure of producing 211At radiopharmaceuticals, from target to radiolabeled product including chemical strategies to overcome hurdles for advancement into clinical trials with 211At.

Application of astatine-210: Evaluation of astatine distribution and effect of pre-injected iodide in whole body of normal rats.

We proposed use of astatine-210 in preclinical study. Astatine-210 has higher yield of production and is easier to quantify than astatine-211. We produced astatine-210 with Bi target and 40 MeV alpha beam accelerated by cyclotron, free astatine-210 was separated and injected to normal rats. Three male rats (blocking group) were injected non-radioactive iodide before injection of astatine-210. Compared with the control group, the astatine-210 accumulations in the blocking group decreased to 24% in the thyroid.

Development of a preclinical 211Rn/211At generator system for targeted alpha therapy research with 211At.

INTRODUCTION: The availability of 211At for targeted alpha therapy research can be increased by the 211Rn/211At generator system, whereby 211At is produced by 211Rn electron capture decay. This study demonstrated the feasibility of using generator-produced 211At to label monoclonal antibody (BC8, anti-human CD45) for preclinical use, following isolation from the 207Po contamination also produced by these generators (by 211Rn α-decay). METHODS: 211Rn was produced by 211Fr electron capture decay following mass separated ion beam implantation and chemically isolated in liquid alkane hydrocarbon (dodecane). 211At produced by the resulting 211Rn source was extracted in strong base (2N NaOH) and purified by granular Te columns. BC8-B10 (antibody conjugated with closo-decaborate(2-)) was labeled with generator-produced 211At and purified by PD-10 columns. RESULTS: Aqueous solutions extracted from the generator were found to contain 211At and 207Po, isolated from 211Rn. High radionuclidic purity was obtained for 211At eluted from Te columns, from which BC8-B10 monoclonal antibody was successfully labeled. If not removed, 207Po was found to significantly contaminate the final 211At-BC8-B10 product. High yield efficiencies (decay-corrected, n=3) were achieved for 211At extraction from the generator (86%±7%), Te column purification (70%±10%), and antibody labeling (76%±2%). CONCLUSIONS: The experimental 211Rn/211At generator was shown to be well-suited for preclinical 211At-based research. ADVANCES IN KNOWLEDGE: We believe that these experiments have furthered the knowledge-base for expanding accessibility to 211At using the 211Rn/211At generator system. IMPLICATIONS FOR PATIENT CARE: As established by this work, the 211Rn/211At generator has the capability of facilitating preclinical evaluations of 211At-based therapies.

Production of (211)At by a vertical beam irradiation method.

We produced (211)At by irradiating the semi-sealed encapsulated Bi target with an external vertical beam. At 28.5MeV, the yield of (211)At was 22MBq/µAh (600µCi/µAh). (211)At was recovered by dry distillation, and 80% of the produced (211)At was successfully obtained in dry Na(211)At form within 2h from the end of bombardment (EOB). The radionuclidic purity of (211)At was >99% at 5h from EOB.

The ARRONAX project.

A new high-energy and high-intensity cyclotron, ARRONAX, has been set into operation in 2010. ARRONAX can accelerate both negative ions (H- and D-) and positive ions (He++ and HH+). Protons can be accelerated from 30 MeV up to 70 MeV with a maximum beam intensity of 2 × 375 µAe whereas He++ can be accelerated at 68 MeV with a maximum beam current of 70 µAe. The main fields of application of ARRONAX are radionuclide production for nuclear medicine and irradiation of inert or living materials for radiolysis and radiobiology studies. A large part of the beam time will be used to produce radionuclides for targeted radionuclide therapy (copper-67, scandium-47 and astatine-211) as well as for PET imaging (scandium-44, copper-64, strontium-82 for rubidium-82 generators and germanium-68 for gallium-68 generators). Since the beginning of the project a particular interest has been devoted to alpha-radionuclide therapy using complex ligands like antibodies and astatine-211 has been selected as a radionuclide of choice for such type of applications. Associated with appropriate carriers, all these radionuclides will respond to a maximum of unmet clinical needs.

Optimisation study of alpha-cyclotron production of At-211/Po-211g for high-LET metabolic radiotherapy purposes.

The production of no-carrier-added (NCA) alpha-emitter (211)At/(211g)Po radionuclides for high-LET targeted radiotherapy and immunoradiotherapy, through the (209)Bi(alpha,2n) reaction, together with the required wet radiochemistry and radioanalytical quality controls carried out at LASA is described, through dedicated irradiation experiments at the MC-40 cyclotron of JRC-Ispra. The amount of both the gamma-emitter (210)At and its long half-lived alpha-emitting daughter (210)Po is optimised and minimised by appropriate choice of energy and energy loss of alpha particle beam. The measured excitation functions for production of the main radioisotopic impurity (210)At-->(210)Po are compared with theoretical predictions from model calculations performed at ENEA.

Incorporation of thiosemicarbazide in Amberlite IRC-50 for separation of astatine from alpha-irradiated bismuth oxide.

A chelating resin was synthesized by incorporating thiosemicarbazide into Amberlite IRC-50, a weakly acidic polymer. Astatine radionuclides produced by alpha-irradiating bismuth oxide were separated using the newly synthesized chelating resin. The resin showed high selectivity for astatine. The adsorbed astatine was recovered using 0.1M EDTA at pH approximately 10.

Dry-distillation of astatine-211 from irradiated bismuth targets: a time-saving procedure with high recovery yields.

Astatine-211 was produced via the 209Bi(alpha,2n) 211At reaction. The radionuclide was isolated with a novel procedure employing dry-distillation of the irradiated target material. The astatine was condensed as a dry residue in a PEEK-capillary cryotrap. Distillation was completed within 1-2 min with isolation yields of 92 +/- 3%. Subsequent work-up of the nuclide resulted in final recovery yields of 79 +/- 3%.

Evaluation of an internal cyclotron target for the production of 211At via the 209Bi (alpha,2n)211 at reaction.

Astatine-211 is a 7.2 h half-life alpha-emitting radionuclide which has shown great promise for targeted radiotherapy. It is generally produced by cyclotron bombardment of bismuth metal targets with 28 MeV alpha-particles via the 209Bi(alpha,2n)211 At reaction. In order to provide 211At activity levels anticipated for clinical investigations, an internal target system has been designed and evaluated. The system has a grazing-angle configuration and leading- and trailing-edge monitors. Both aluminum and copper target backings were evaluated. With approx. 28 MeV alpha-particles, the 211At production efficiency was 41 +/- 7 MBq/microA.h, compared with 10.6 +/- 1.2 MBq/microA.h for an external target. Radionuclidic purity of 211At was high with no evidence of 210At.

Organoastatine chemistry. Astatination via electrophilic destannylation.

Substantial interest is currently focused on the alpha-emitting radiohalogen 211At, principally because of its potential use in the radiation therapy of cancer. Knowledge of organoastatine chemistry is incomplete and existing methods for its incorporation restrict the range of compounds which may be labeled. Aryl and vinyl Sn(IV) compounds are notably susceptible to substitution of tin by a variety of electrophiles. We have investigated the reaction of aryltrialkylstannanes with astatine and report here the first examples of astatodestannylation. Formation of aryl astatides proceeds rapidly, cleanly and under mild conditions. The data further elucidate aspects of astatine reactivity and suggest a general route to synthesize astatinated compounds.