Novel radiochemistry for positron emission tomography imaging

This thesis describes the development of novel radiochemistry using fluorine-18 and its subsequent application in conducting Positron Emission Tomography (PET) imaging in vivo. Publication of this work: J. Am. Chem. Soc. 2020, 142 (3), 1180 – 1185.
Chapter 1 introduces PET imaging and fluorine-18 radiochemistry, including an overview of the general considerations made in these areas. An extensive review of the library of radiolabelled fluorinating reagents available and the range of methods for radiolabelling peptides with fluorine-18 are described. The literature for synthesis and application of sodium trifluoromethanesulfinate, also known as the Langlois reagent, is also reviewed.
Chapter 2 describes the validation of a novel synthetic route for forming the Langlois reagent from sources of fluoride, difluorocarbene and sulfur dioxide. Successful synthesis of the Langlois reagent was achieved by employing potassium or caesium fluoride, a phase transfer catalyst, difluoromethylenephosphobetaine (PDFA), and DABCO-bis(sulfur dioxide) (DABSO) or a sulfur dioxide adduct of N-methylmorpholine (NMM-SO2).
This chemical strategy was successfully translated into a radiochemical setting using [18F]fluoride by extensive optimisation in stages. The identity of the [18F]Langlois reagent was confirmed analytically and a procedure was developed for its purification and isolation. Processes and reaction conditions were subsequently re-optimised to develop an automated radiosynthesis and improve the molar activity.
Chapter 3 explores the application of the [18F]Langlois reagent in labelling tyrosine and tryptophan amino acids and residues in unprotected peptides. In contrast to literature methods that generally employ large excesses of Langlois reagent, new reaction conditions were developed for reacting the Langlois reagent in substoichiometric amounts. Key to this was the addition of stoichiometric amounts of metal salts, allowing successful radiochemical translation of the reaction. A number of dipeptides and biologically active peptides were labelled using this method including human insulin, the largest unmodified peptide to date that has been labelled directly with fluorine-18. The procedure for labelling tyrosine and tryptophan containing peptides was also automated.
An in vitro competition binding study, using the tryptophan-containing peptide octreotide, was used to demonstrate that installing the trifluoromethyl group directly onto the active site of a peptide does not have a large inhibiting effect on its activity. This led us to further investigate the practical utility of this automated radiosynthesis by carrying out an in vivo PET imaging study on naïve rats using octreotide labelled with fluorine-18. The activity and molar activity of the final dose were improved such that a coherent PET signal for the tracer could be observed in naïve rats. The pharmacokinetics of labelled octreotide were in agreement with literature analogues, however further studies and improvements to molar activity are required in order for this method to find use in oncological or neurological studies.
These results are proof of concept that labelling the Langlois reagent provides a new way of studying peptides using PET imaging, which may find applications in drug development or for diagnostic imaging in the future.

Advisor: Gouverneur, V