Another promising PET radionuclide is gallium-68 (68Ga). Its parent nuclide, 68Ge, has a half-life of 271 days, and the existing generators can provide sufficient quantities of 68Ga for up to one year, resulting in a relatively inexpensive and reliable source of a positron-emitting radionuclide. In addition to PET imaging of amyloids, metal-containing agents could be used for AD visualization by the means of single-photon emission computed tomography (SPECT) and magnetic resonance imaging (MRI).
However, the development of AD imaging agents is restricted by the presence of the blood-brain barrier (BBB) which limits the substance from reaching the cerebral target. The BBB is a highly selective mechanism that controls the passage of substances from the blood into the cerebrospinal fluid and thus into the brain, and serves as the clearance path for waste metabolites of the brain. Thus, the BBB makes it difficult to develop new treatments of brain diseases, or new radiopharmaceuticals for neuroimaging of the brain.
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A few metal-based agents have demonstrated the ability to cross the BBB and bind with amyloid in the brain: 64Cu, 68Ga and 99mTc (technetium-99 m). These isotopes are significantly easier to produce than 11C and 11F, with a longer life-span. Among the variety of compounds considered in the review, the most promising results were shown by copper-based coordination compounds for PET imaging, gallium-based coordination compounds for MRI, and technetium -based coordination compounds for SPECT imaging.
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