Uses and Precautions
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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.
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