Enhancement of At-211 uptake in the thyroid gland- SPECT study in rats

1129 Background: Astatine (At) is increasingly focusing attention in theragnostics because of its similar chemical properties to iodine and of its alpha particles which have shorter range in tissue compared with beta particles. However, previous studies on the biodistribution of At-211 in rats have shown that the uptake of At-211 in thyroid gland was not as high as expected compared to extrathyroidal organs like stomach. The purpose of this study was to evaluate the uptake of At-211 solution treated with reducing agent in the thyroid gland. [Methods] Two groups of male Wistar rats (total n=6, body weight=295.2 ± 16.2g) were injected with At-211 solution through tail vein [group 1: crude At-211 solution (3.58 ± 0.65MBq), group 2: At-211 solution treated with ascorbic acid (2.72 ± 0.12MBq)]. SPECT images were acquired 3, 6, and 24 hrs after injection targeting the X-rays emitted from the daughter nuclide (Po-211). Volumes of interest were placed on the thyroid gland, the stomach and the bladder. SPECT counts (cps) were normalized by the injected dose (MBq). Radioactivity of the major organs were measured by gamma counter after euthanasia and dissection at 24 hrs. Uptakes were compared between the two groups by Mann-Whitney U test. [Results] SPECT images revealed that uptakes were significantly higher in the thyroid gland of the group 2 than those of the group 1 (p<0.05). Mean normalized counts in the thyroid gland were 0.18 ± 0.01 (3hr), 0.22 ± 0.04 (6hr), 0.12 ± 0.02 (24hr) in the group 1, and 0.39 ± 0.07 (3hr), 0.34 ± 0.03 (6hr), 0.28 ± 0.13 (24hr) in the group 2. Uptakes in the stomach or the bladder showed no significant difference between the two groups. Radioactivity measurement after dissection showed significantly higher uptakes in the thyroid gland of group 2 than those of the group 1 (p<0.05), but there was no significant difference in other organs. Counts in the thyroid gland were 3,283 ± 479 kcps/g in the group 1 and 17,080 ± 3,984 kcps/g in the group 2. [Conclusions] This study showed significant enhancement of uptake of At-211 in the thyroid gland using the solution treated with ascorbic acid compared to the crude solution, suggesting the possible application of At-211 to the targeted radionuclide therapy of the thyroid cancer as a substrate of sodium iodine symporter. Fig.1. SPECT images of group 1 and 2 at 3, 6, 24 hrs after injection Fig.2. Comparison of the mean normalized counts in the thyroid gland, the stomach and the bladder between the two groups.