A preliminary investigation using unnatural amino acids to stabilise peptide radiotracers

Peptide-based radiotracers are a valuable tool for diagnostic and therapeutic applications in nuclear medicine. However, some peptide-based radiotracers exhibit low metabolic stability and are rapidly degraded by proteolytic enzymes in vivo. This rapid degradation contributes to insufficient accumulation and high background radioactivity, ultimately leading to poor diagnostic and therapeutic capabilities. Previous work has shown that peptide-based radiotracers with unnatural amino acids linking the radioisotope and the peptide can improve the metabolic stability of the peptide. A promising unnatural amino acid linker that has demonstrated improvement in metabolic stability of peptides, but is still relatively underexplored, is the β-amino acid. A series of aliphatic and aromatic β-amino acids and their isostere conjugates, linked via their carboxyl groups to the amine group of glycine ethyl ester, were synthesised to assess their comparative metabolic stability. A range of synthetic strategies were performed to obtain the conjugates, which were fully characterised by nuclear magnetic resonance spectroscopy and mass spectrometry. Preliminary in vitro incubation studies with porcine liver esterase (as a model of human esterase) provided some evidence for the aromatic β-amino acid-glycine ethyl ester conjugate being more stable than its α-amino acid analogue, but further studies are needed for more conclusive results.