Tumor Targeting of 211At-Labeled Antibody under Sodium Ascorbate Protection against Radiolysis

Astatine-211 and actinium-225: two promising nuclides in targeted alpha therapy

Nuclear medicine therapy offers a promising approach for tumor treatment, as the energy emitted during radionuclide decay causes irreparable damage to tumor cells. Notably, α-decay exhibits an even more significant destructive potential. By conjugating α-nuclides with antibodies or small-molecule inhibitors, targeted alpha therapy (TAT) can enhance tumor destruction while minimizing toxic side effects, making TAT an increasingly attractive antineoplastic strategy. Astatine-211 ( 211At) and actinium-225 ( 225Ac) have emerged as highly effective agents in TAT due to their exceptional physicochemical properties and biological effects. In this review, we highlight the applications of 211At-/ 225Ac-radiopharmaceuticals, particularly in specific tumor targets, such as prostate-specific membrane antigen (PSMA) in prostate cancers, cluster of differentiation (CD) in hematological malignancies, human epidermal growth factor receptor-2 (HER2) in ovarian cancers, and somatostatin receptor (SSTR) in neuroendocrine tumors. We synthesize the progress from preclinical and clinical trials to provide insights into the promising potential of 211At-/ 225Ac-radiopharmaceuticals for future treatments.

Strong Affinity between Astatine and Silver: An Available Approach to Anchoring 211At in Nanocarrier for Locoregional Oncotherapy

Recently, 211At-related endoradiotherapy has emerged as an important oncotherapy strategy. Conjugating 211At with a nanocarrier provides a vital candidate for radionuclide therapy of various malignant tumors. In this study, we proposed utilizing the intrinsically high affinity of heavy halogens and sulfhydryl compounds for metallic silver to achieve highly efficient conjugation between 211At and Ag-based nanoparticles in a simple way. 211At@Ag-PEG-FA was obtained via a one-pot assembly of 211At, Ag, and SH-PEG-FA in extremely high radiolabeling yield (>95%) within 15 min and maintained excellent stability in simulated physiochemical media. Additionally, the prepared 211At@Ag-PEG-FA demonstrated specific binding to the breast cancer cell line (4T1), with a high endocytosis rate and low reflux, leading to significant cell growth inhibition. 211At@Ag-PEG-FA exhibits an excellent antitumor effect that completely suppressed tumor growth during the first week, effectively prolonging the median survival of mice to 44 days, relative to 18 days in the control group. All of the mice exhibited minimal side effects from 211At@Ag-PEG-FA in the experiment, indicating its acceptable biosafety. Our work shows that the strong affinity of Ag can be utilized to produce radioactivated nanomedicines with excellent stability and high efficiency, which also provides some valuable insights for the 211At radiolabeling of general compounds.

Development and Utility of an Imaging System for Internal Dosimetry of Astatine-211 in Mice

In targeted radionuclide therapy, determining the absorbed dose of the ligand distributed to the whole body is vital due to its direct influence on therapeutic and adverse effects. However, many targeted alpha therapy drugs present challenges for in vivo quantitative imaging. To address this issue, we developed a planar imaging system equipped with a cadmium telluride semiconductor detector that offers high energy resolution. This system also comprised a 3D-printed tungsten collimator optimized for high sensitivity to astatine-211, an alpha-emitting radionuclide, and adequate spatial resolution for mouse imaging. The imager revealed a spectrum with a distinct peak for X-rays from astatine-211 owing to the high energy resolution, clearly distinguishing these X-rays from the fluorescent X-rays of tungsten. High collimator efficiency (4.5 × 10-4) was achieved, with the maintenance of the spatial resolution required for discerning mouse tissues. Using this system, the activity of astatine-211 in thyroid cancer tumors with and without the expression of the sodium iodide symporter (K1-NIS/K1, respectively) was evaluated through in vivo imaging. The K1-NIS tumors had significantly higher astatine-211 activity (sign test, p = 0.031, n = 6) and significantly decreased post-treatment tumor volume (Student's t-test, p = 0.005, n = 6). The concurrent examination of intratumor drug distribution and treatment outcome could be performed with the same mice.