Radiohalogenated nonnatural amino acids as PET and SPECT tumor imaging agents

Radiohalogenated α-amino acids are a diverse and useful class tumor imaging agents suitable for positron emission tomography and single photon emission computed tomography. These tracers target the increased rates of amino acid transport exhibited by many tumor cells. The most established clinical use for radiolabeled amino acids is imaging primary and recurrent gliomas, and there is growing evidence that they may also be useful for other oncologic applications, including neuroendocrine tumors and prostate cancer. This review focuses on the synthesis, radiolabeling, and preclinical evaluation of three series of nonnatural radiohalogenated amino acids: alicyclic, α,α-dialkyl, and 1H-[1,2,3]triazole amino acids which target system L, system A, and cationic amino acid transport systems, respectively.