The Development of Fluorine-18 Imaging Agents Targeting the GHSR
Abstract
Molecular imaging is an interdisciplinary field of study that allows for real-time visualization of biological processes in both healthy and disease states. Fluorine-18 positron emission tomography (PET) imaging is a highly sensitive technique where molecular probes containing fluorine-18 serve to specifically locate and visualize the expression of relevant disease biomarkers. The growth hormone secretagogue receptor 1a (GHSR) is differentially expressed in cancer and cardiac pathology, making it a biomarker of interest for imaging such diseases. This thesis discusses the development and optimization of GHSR-targeting 18F-PET imaging probes as well as proposes a new imaging agent discovery technique.
Chapter 2 describes the radiofluorination of two bulky, aromatic prosthetic groups in high radiochemical yields using spirocyclic iodonium ylide (SCIDY) precursors. Subsequent conjugation of these prosthetic groups to a high-affinity peptide based on ghrelin, the endogenous ligand for the GHSR, provided access to two 18F-labelled ghrelin(1-8) analogues. One of these probes was investigated in vitro and in vivo as a potential PET tracer for targeted GHSR imaging.
Chapter 3 investigates the serum and hepatic metabolic stability of the ghrelin(1-8) 18F-PET probe. Initial in vitro stability studies on the peptide identified a metabolic soft-spot between Leu5 and Ser6. A structure-activity-stability relationship study evaluated a series of modified ghrelin(1-8) analogues, which revealed new insights into the structural importance of the residues at these positions along with a new lead candidate with improved stability and strong GHSR binding affinity.
Chapter 4 describes the effort to label a quinazolinone-based small molecule with fluorine-18 to access a new high-affinity GHSR PET imaging agent. The design and synthesis of a new quinazolinone-based SCIDY precursor for direct 18F-fluorination is presented.
Chapter 5 introduces a novel chemical technology for direct discovery of molecular imaging agents through the application of the drug discovery technique, fragment-based drug design. For the first time, a fluorine-containing fragment library was assembled where each member contains fluorine in a position readily accessible to fluorine-18. This library was screened for binding toward the GHSR resulting in the identification of two fragment hits for future optimization into GHSR-targeting PET imaging agents. The resulting candidate molecules that emerge from this research would be the first examples of using a fragment-based approach to directly discover novel molecular imaging agents.