Comprehensive Head-to-Head Study between Meta-[211At]astato-benzylguanidine and with Meta-[131I]iodo-benzylguanidine in Pheochromocytoma

Pheochromocytoma (PCC) is a rare neuroendocrine tumor that is often nonfunctional in its early stages, making detection difficult during the prodromal phase. Targeted radionuclide therapy using radiopharmaceuticals has been explored for treating PCC. We aimed to evaluate and compare the therapeutic efficacy of meta-[¹³¹I]iodobenzylguanidine ([¹³¹I]MIBG) and meta-[²¹¹At]astatobenzylguanidine ([²¹¹At]MABG) in vitro and in a PCC animal model. Astatine-211 was radiolabeled with a benzylguanidine precursor, and the crude product was purified using high-performance liquid chromatography, followed by formulation to prepare an injectable [²¹¹At]MABG. [¹³¹I]MIBG was obtained from the Korea Atomic Energy Research Institute. The stability of [²¹¹At]MABG was assessed in saline and human serum with or without sodium ascorbate. In vitro experiments included evaluating cellular uptake in PC-12 cells, colony formation inhibition (clonogenic assay), and DNA damage (Comet assay). In vivo studies were conducted using PC-12 tumor-bearing mice to assess biodistribution, tumor volume reduction, survival rates, and body weight changes. Immunohistochemical analyses of tumor proliferation and apoptosis were performed using K_i-67, proliferating cell nuclear antigen (PCNA), and the terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assay. [²¹¹At]MABG was successfully synthesized with a radiochemical yield of 48.4% (decay-corrected). The radiochemical purity (RCP) was 99.3%, and the specific activity was 1.27-8.13 MBq·nmol^-1 at the end of synthesis (n = 3). In chemical stability tests of [²¹¹At]MABG, when sodium ascorbate was not added, the RCP was 76% at 24 h after production; however, when sodium ascorbate was added, its purity was 92% at the same time. In vitro uptake assays revealed that [²¹¹At]MABG exhibited higher cellular uptake in PC-12 cells compared with [²¹¹At]NaAt and [¹³¹I]MIBG, initially accumulating in the cytosol and progressively increasing over 24 h. DNA double-strand breaks were confirmed using the Comet assay. Biodistribution studies demonstrated that administration of [²¹¹At]MABG containing sodium ascorbate reduced thyroid uptake and enhanced tumor accumulation of [²¹¹At]MABG. Treatment with 0.93 MBq [²¹¹At]MABG resulted in superior tumor suppression compared with 19.25 MBq [¹³¹I]MIBG and improved survival rates. Immunohistochemical analyses confirmed decreased tumor proliferation and increased apoptosis following [²¹¹At]MABG treatment. The stability of [²¹¹At]MABG in human serum was significantly enhanced by sodium ascorbate. Even at doses 20 times lower than [¹³¹I]MIBG, [²¹¹At]MABG demonstrated superior antitumor efficacy without inducing substantial weight loss. These findings suggest that [²¹¹At]MABG may serve as a promising alternative for treating malignant PCC.