Astatine - Uses and Precautions

Uses and Precautions

Several 211At-containing molecules and their uses

Agent	Applications
astatine-tellurium colloids	Compartmental tumors
6-astato-2-methyl-1,4-naphtaquinol diphosphate	Adenocarcinomas
211At-labeled methylene blue	Melanomas
Meta-astatobenzyl guanidine	Neuroendocrine tumors
5-astato-2'-deoxyuridine	Various
211At-labeled biotin conjugates	Various pretargeting
211At-labeled octreotide	Somatostatin receptor
211At-labeled mAbs and fragments	Various
211At-labeled bisphosphonates	Bone metastases

The newly formed astatine-211 is important in nuclear medicine. Once produced, astatine must be used quickly, as it decays with a half-life of 7.2 hours; this is, however, long enough to permit multi-step labeling strategies. Astatine-211 can be used for targeted alpha particle radiotherapy, since it decays either via emission of an alpha particle (to bismuth-207), or via electron capture (to an extremely short-lived nuclide of polonium-211, which itself undergoes further alpha decay).

In a manner similar to iodine, astatine is preferentially concentrated in the thyroid gland, although to a lesser extent. However, it tends to concentrate in the liver in the form of a radiocolloid if it is released into the systemic circulation. The principle medicinal difference between astatine-211 and iodine-131 (a radioactive iodine isotope also used in medicine) is that astatine does not emit high energy beta particles (electrons), as does iodine-131. Beta particles have considerably greater penetrating power through tissues than do the much heavier alpha particles. While an average energy alpha particle released by decay of astatine-211 can travel up to 70 µm through the surrounding tissues, an average energy beta particle emitted by iodine-131 can travel nearly 30 times as far, to about 2 mm. Thus, using astatine-211 instead of iodine-131 enables the thyroid to be dosed appropriately, while the neighboring parathyroid gland is spared. The short half-life and limited penetrating power of its radiation through tissues renders astatine generally preferable to iodine-131 when used in diagnosis as well.

Experiments in rats and monkeys, however, suggest that astatine causes much greater damage to the thyroid gland than does iodine-131, with repetitive injection of the nuclide resulting in necrosis and cell dysplasia within the gland. These experiments also suggest that astatine could cause damage to the thyroid of any organism. Early research suggested that injection of lethal quantities of astatine caused morphological changes in breast tissue (although not other tissues); however, this conclusion currently remains controversial.