Studies towards metabolically stable Fluorine-18 labelled peptides and their efficient syntheses

Positron emission tomography (PET) is a powerful nuclear medicine imaging technique that has become invaluable for the in vivo diagnosis and staging of cancers and many other diseases. The overall aim of this project was to develop strategies for improving the metabolic stability and ease of synthesis of 18F radiolabelled peptide conjugates for use in PET. The first objective was to use model systems to determine if the substitution of an amide bond with a sulfonamide bond increases the metabolic stability of peptide conjugates. For this, N-benzoyl glycine and N-benzenesulfonyl glycine were incubated with the human carboxylesterase/amidase model enzyme pig liver esterase. Both compounds exhibited good stability, however, future work with other enzyme systems would be valuable. The second objective was to synthesise a series of bifunctional aromatic and heteroaromatic compounds that possess the good leaving group tetramethylammonium (TMA) and an activated 4-nitrophenyl ester to enable efficient fluorination and peptide conjugation. Unfortunately, the 4-nitrophenyl ester proved to be too active, making it difficult to form the TMA activated aromatic systems. As a result, a new method was developed based upon the synthesis of TMA methyl esters. Using this method, the synthesis of a TMA bearing methyl ester was successfully trialled.