Abstract
Radiolabeled amino acids, their derivatives and peptides have a broad scope of application and can be used as receptor ligands, as well as enzyme substrates for many different diseases as radiopharmaceutical tracers. Over the past few decades, the application of molecular imaging techniques such as positron emission tomography (PET) has gained considerable importance and significance in diagnosis in today’s advanced health care. Next to that, the availability of cyclotrons and state-of-the-art radiochemistry facilities has progressed the production of imaging agents enabling the preparation of many versatile PET radiotracers. Due to many favorable characteristics of radiolabeled amino acids and peptides, they can be used for tumor staging and monitoring the progress of therapy success, while aromatic amino acids can be employed as PET tracer to study neurological disorders. This review provides a comprehensive overview of radiosynthetic and enzymatic approaches towards carbon-11 amino acids, their analogues and peptides, with focus on stereoselective reactions, and reflects upon their clinical application.
Highlights
Over the past few decades, the application of molecular imaging techniques has become widely available for all fields of today’s health care
Among all classes of positron emission tomography (PET) tracers, radiolabeled amino acids (AAs), their derivatives and peptides have a broad scope of clinical applications [1]
To the best of our knowledge, we describe all currently available synthesis strategies for the radiosynthesis of [11C]AAs, their most common derivatives and peptides, with an emphasis on stereoselective synthesis
Summary
Over the past few decades, the application of molecular imaging techniques has become widely available for all fields of today’s health care. RCY 40–50 %, counted from [11C]CH3I, and 94 % ee in a total synthesis time of 50 min (Fig. 5c) [6, 49] Since these very encouraging results, even the precursor has become commercially available which maintained great interest for chiral alkylation reactions to obtain [11C]AAs. Asymmetric Synthesis of α-[11C]Methyl Tryptophan with [11C]Methyl Iodide Considerable potential in the use of Schiff base precursors has been displayed by the radiosynthesis of the unnatural α-[11C]methyl tryptophan. Neuropeptide MET-enkephalin, a 5-mer peptide, and its two metabolic fragments thereof, Gly-Phe-MET and Phe-MET, were labeled using the C-11 methylation strategy by Nagren et al S-[11C]MET-enkephalin and corresponding C-11labeled fragments were reported in a RCY of 50–80 %, after LC purification, in 35–50 min with AM ranging 0.4– 7.2 GBq μmol−1, depending on the AM of [11C]CH3I [85] Though this method of peptide radiolabeling proved to be highly reliable and high yielding, it should be noted that radiolabeling can only be conducted if a methionine residue is present in the peptidic structure. With a AM of 22–28 GBq μmol−1, biodistribution and PET imaging studies in pancreatic carcinoma-bearing mice showed good results, recommending its further use
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