Development of Peptide-Based Positron Emission Tomography Radiopharmaceuticals with Gallium-68 and Fluorine-18

ACKR3 (Atypical Chemokine Receptor 3) plays a pivotal role as a receptor in cellular signal transduction across various diseases and cancers. Targeting and imaging ACKR3 expression in tumours facilitate early cancer detection, therapy selection, and improved patient outcomes. Positron Emission Tomography (PET) utilises positron-emitting radiotracers to visualise and quantify biological targets and molecular processes at the cellular level. PET can visualise ACKR3 expression in vivo enhances understanding of its physiological function and supports the development of targeted therapies.While many small molecules have been suggested as effective ACKR3-targeting agents, the only reported PET probe for ACKR3 is an 89Zr-labelled antibody, which necessitates further refinement for clinical use. Peptides offer promise as molecular tools for PET imaging of biological targets. In this thesis, the aim was to develop a peptide-based PET probe. Based on previous research in the Archibald group, the target peptide sequences [(Phe*-D-Ala-Gly-Phe-Met-Arg-Arg-Lys-D-Ala) and (Phe*-D-Ala-Gly-Phe-Met-Arg-Arg-Lys-D-Ala-Lys), Phe*= thioamidated phenylalanine] were identified that was predicted to bind to ACKR3 with high selectivity. Subsequently, 68Ga and 18F-labelled tracers were synthesised by using solid-phase peptide synthesis (SPPS) to assess their potential utility.Incorporating a thioamide, in which a sulphur atom replaces oxygen, into the peptide chain is expected to alter its electronic properties, reduce susceptibility to enzymatic degradation, and thereby enhance metabolic stability. The prepared precursors and non-radioactive [19F]fluorine and [natGa]gallium reference standards were evaluated for binding affinities to ACKR3 by Archibald group collaborators. The proposed structures exhibited selective binding to ACKR3, with particularly strong affinity observed for the fluorine labelled derivative (IC50=0.73nM). However, this peptide compound showed low metabolic stability. Preclinical PET/CT imaging was conducted using the 68Ga radiotracer but no tumour uptake was observed. Future efforts should focus on introducing new modifications to the peptide sequence, to improve in vivo metabolic stability, affinity and optimise both biodistribution and pharmacokinetic properties.