This article presents a general discussion on what has been achieved so far and on the possible future developments of targeted alpha (α)-particle therapy (TAT). Clinical applications and potential benefits of TAT are addressed as well as the drawbacks, such as the limited availability of relevant radionuclides. Alpha-particles have a particular advantage in targeted therapy because of their high potency and specificity. These features are due to their densely ionizing track structure and short path length. The most important consequence, and the major difference compared with the more widely used β−-particle emitters, is that single targeted cancer cells can be killed by self-irradiation with α-particles. Several clinical trials on TAT have been reported, completed, or are on-going: four using 213Bi, two with 211At, two with 225Ac, and one with 212Pb/212Bi. Important and conceptual proof-of-principle of the therapeutic advantages of α-particle therapy has come from clinical studies with 223Ra-dichloride therapy, showing clear benefits in castration-resistant prostate cancer.

and 2 Cyclotron and PET Unit, KF-3982, Rigshospitalet, Copenhagen, Denmark 211 At-labeled tumor-specific antibodies have long been considered for the treatment of disseminated cancer. However, the limited availability of the nuclide and the poor efficacy of labeling procedures at clinical activity levels present major obstacles to their use. This study evaluated a procedure for the direct astatination of antibodies for the production of clinical activity levels. Methods: The monoclonal antibody trastuzumab was conjugated with the reagent N-succinimidyl-3-(trimethylstannyl)benzoate, and the immunoconjugate was labeled with astatine. Before astatination of the conjugated antibody, the nuclide was activated with N-iodosuccinimide. The labeling reaction was evaluated in terms of reaction time, volume of reaction solvent, immunoconjugate concentration, and applied activity. The quality of the astatinated antibodies was determined by in vitro analysis and biodistribution studies in nude mice. Results: The reaction proceeded almost instantaneously, and the results indicated a low dependence on immunoconjugate concentration and applied activity. Radiochemical labeling yields were in the range of 68%281%, and a specific radioactivity of up to 1 GBq/mg could be achieved. Stability and radiochemical purity were equal to or better than those attained with a conventional 2-step procedure. Dissociation constants for directly astatinated, conventionally astatinated, and radioiodinated trastuzumab were 1.0 6 0.06 (mean 6 SD), 0.44 6 0.06, and 0.29 6 0.02 nM, respectively. The tissue distribution in non-tumor-bearing nude mice revealed only minor differences in organ uptake relative to that obtained with the conventional method. Conclusion: The direct astatination procedure enables the high-yield production of astatinated antibodies with radioactivity in the amounts required for clinical applications. Among the isotopes of the heaviest element in the halogen group, 211 At has attracted interest as a prospective candidate for endoradiotherapeutic applications because of its physicochemical characteristics (1). Unlike most commonly medically applied therapeutic radionuclides that decay through medium-to high-energy b-emission, leading to low-linear-energy-transfer radiation with particle ranges of 1-10 mm, 211 At decays through a-emission, depositing high-linear-energy-transfer radiation in a microvolume corresponding to a mean a-particle range of ;65 mm. When bound to a tumor-specific substance, this radiation can be effective in the destruction of disseminated microtumors, that is, micrometastases, as has been demonstrated in several preclinical studies (2-5). The preclinical work has resulted in 2 phase I studies, a study of the treatment of malignant gliomas at

α-Emitting radionuclides deposit a large amount of energy within a few cell diameters and may be particularly effective for radioimmunotherapy targeting minimal residual disease (MRD). To evaluate this hypothesis, (211)At-labeled 1F5 monoclonal antibody (mAb) (anti-CD20) was studied in both bulky lymphoma tumor xenograft and MRD animal models. Superior treatment responses to (211)At-labeled 1F5 mAb were evident in the MRD setting. Lymphoma xenograft tumor-bearing animals treated with doses of up to 48 µCi of (211)At-labeled anti-CD20 mAb ([(211)At]1F5-B10) experienced modest responses (0% cures but two- to threefold prolongation of survival compared with negative controls). In contrast, 70% of animals in the MRD lymphoma model demonstrated complete eradication of disease when treated with (211)At-B10-1F5 at a radiation dose that was less than one-third (15 µCi) of the highest dose given to xenograft animals. Tumor progression among untreated control animals in both models was uniformly lethal. After 130 days, no significant renal or hepatic toxicity was observed in the cured animals receiving 15 µCi of [(211)At]1F5-B10. These findings suggest that α-emitters are highly efficacious in MRD settings, where isolated cells and small tumor clusters prevail.

Purpose: Alpha-emitting radionuclides exhibit a potential advantage for cancer treatments because they release large amounts of ionizing energy over a few cell diameters (50-80 µm), causing localized, irreparable double-strand DNA breaks that lead to cell death. Radioimmunotherapy (RIT) approaches using monoclonal antibodies labeled with α emitters may thus inactivate targeted cells with minimal radiation damage to surrounding tissues. Tools are needed to visualize and quantify the radioactivity distribution and absorbed doses to targeted and nontargeted cells for accurate dosimetry of all treatment regimens utilizing α particles, including RIT and others (e.g., Ra-223), especially for organs and tumors with heterogeneous radionuclide distributions. The aim of this study was to evaluate and characterize a novel single-particle digital autoradiography imager, the ionizing-radiation quantum imaging detector (iQID) camera, for use in α-RIT experiments. Methods: The iQID camera is a scintillator-based radiation detection system that images and identifies charged-particle and gamma-ray/x-ray emissions spatially and temporally on an eventby-event basis. It employs CCD-CMOS cameras and high-performance computing hardware for real-time imaging and activity quantification of tissue sections, approaching cellular resolutions. In this work, the authors evaluated its characteristics for α-particle imaging, including measurements of intrinsic detector spatial resolutions and background count rates at various detector configurations and quantification of activity distributions. The technique was assessed for quantitative imaging of astatine-211 ( 211 At) activity distributions in cryosections of murine and canine tissue samples. Results: The highest spatial resolution was measured at ∼20 µm full width at half maximum and the α-particle background was measured at a rate as low as (2.6 ± 0.5) × 10 −4 cpm/cm 2 (40 mm diameter detector area). Simultaneous imaging of multiple tissue sections was performed using a large-area iQID configuration (ø 11.5 cm). Estimation of the 211 At activity distribution was demonstrated at mBq/µg-levels. Conclusions: Single-particle digital autoradiography of α emitters has advantages over traditional film-based autoradiographic techniques that use phosphor screens, in terms of spatial resolution, sensitivity, and activity quantification capability. The system features and characterization results presented in this study show that the iQID is a promising technology for microdosimetry, because it provides necessary information for interpreting alpha-RIT outcomes and for predicting the therapeutic efficacy of cell-targeted approaches using α emitters. C 2015 American Association of Physicists in Medicine. [http://dx

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