The standard production of 211At is by irradiating stable bismuth with helium ions (alpha-particles) using cyclotron irradiation, and date back to the discovery of the nuclide in 1940 by Corson et al (1).
Although no cyclotron is available in Gothenburg collaborations with the Department of Physics Oslo University, Norway, Forschungszentrum Dresden-Rossendorf, Germany and Cyclotron and PET Unit, Rigshospitalet, Copenhagen, Denmark has provided us with 211At on a regular basis since 1994. Early on much of the work was focused on the procedures, distillation and chemistry to obtain 211At and 211At-labeled antibodies.
In 2001 a novel procedure on distillation and work up of 211At from irradiated Bi-targets was developed (2). With this method the nuclide is rapidly converted into a chemical useful form for subsequent labeling chemistry. The labeling chemistry has been based on the method first developed by Zalutsky and co-workers (3), see Figure 1 below.
With this method labeled antibodies which have been sufficient for preclinical evaluations have been produced. However, we encountered problems when taken the research into a clinical phase I study on patients with recurrent ovarian carcinoma. The levels of activity planned for the study were difficult to reach by the conventional procedure for labeling the antibody. Therefore an extensive effort was put on developing the chemistry, and in 2008 a new chemical route that substantially improves the labeling efficacy in astatination of antibodies was developed (4), see Figure 2. In this way the levels of activity required to continue into a phase II clinical study now is possible to reach.
In addition to astatine we have access to another interesting alpha -particle emitting radionuclide, Bismuth-213. It is available through a close collaboration with the Institute of Transuran Elements (ITU) Karlsruhe, Germany. They provide us with 225Ac/213Bi generators from which we can elute 213Bi in a pure chemical form for subsequent labeling to antibodies. The labeling is performed via bifunctional chelates. Several different chelates for antibody are commercially available. Generator protocols and protocols for labeling that result in very good radiochemical yields have been developed at ITU.
The half-life of 211At (7.2 h) and 213Bi (46 min) is generally too short for conventional radioimmunotherapy except for a few special applications such as blood-born or intracavitary cancer treatments, e.g. intraperitoneal (i.p.) and intrathecal (i.t.) treatments (5-7). This is due to the relatively slow in vivo distribution and slow clearance rates of radiolabelled antibodies. Most of the injected radioactivity will therefore decay before reaching its target.
In order to circumvent the unfavourable pharmacokinetics of radiolabeled antibodies, different ways of improving the distribution of the radioactivity have been suggested, employing various pretargeting techniques (8-10). With this type of technique modified antibodies are administered for pre-binding to the tumor antigens. A sufficient time is introduced, to allow non-bound antibodies to be cleared from the circulating system, or a clearing agent is administered to enhance the clearance rate before injecting the labeled effector molecule. The effector molecule recognizes a tag on the antibody and due to the small size of the effector molecule as compared to labeled antibodies, it will localize the target more rapidly and the non-bound fraction will be cleared more efficiently, thus increasing tumor uptake and lowering the dose to normal tissue.
A successful pretargeting protocol will improve the tumor-to-normal tissue absorbed dose ratio for all types of applications involving antibodies for tumor targeting together with radionuclides with short half-lives, e.g. 213Bi and 211At. In this project a pretargeting strategy including the pretargeting molecule avidin/strepavidin conjugated antibody and an effector-molecule based on biotinylated, labeled and charge modified polylysine is investigated, see Figure 3 below.
Fig. 3. Schematic structures of effector molecules. A: Labeled with 211At via N-succinimidyl 3-(trimethylstannyl)benzoate. B: labeled with 213Bi via the kelate DOTA. Different parts of the molecule are presented as, BLUE: biotin residue; GREEN/RED: radiometal-kelate- or radiohalogen-reagent residue; PURPLE: succinic acid residue following charge modification. Click image for higher resolution.