This disclosure relates to 1-amino-3,4-difluorocyclopentane-1-carboxylic acid, esters, derivatives, or salts having positron emitting radionuclides for use as radionuclide tracers. In certain embodiments, 1-amino-3,4-difluorocyclopentane-1-carboxylic acid is isotopically enriched with fluorine 18. In certain embodiments, the radionuclide tracer is amino-3-fluoro-4-[18F]fluorocyclopentane-1-carboxylic acid. In certain embodiments, the radionuclide tracers are useful to indicate the existence of and/or position of a tumor or cancerous cells in a subject.
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