Doctor of Philosophy
Luyt, Leonard G.
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.
Summary for Lay Audience
Molecular imaging is a scientific field of study that allow scientists to non-invasively visualize what the cells in our body are doing in real-time. The pictures obtained through imaging procedures, such as PET, provide valuable insight into diagnosing and tracking diseases including cancer. Some diseases carry specific biomarkers, allowing scientists to develop molecules that bind to these biomarkers, like a key fits a lock. By using small amounts of molecules containing a radioactive atom, such as fluorine-18, PET scanners can detect radioactive decay to construct images. The incorporation of fluorine-18 into the molecular probe requires the use of specific chemical reactions. This thesis discusses the development and optimization of molecular imaging probes containing fluorine-18 targeting the biomarker, GHSR, and the methods used to make these compounds.
Chapter 2 describes the chemical synthesis and fluorine-18 labelling of two prosthetic groups, which act as carrier molecules to introduce fluorine-18 into a biomolecule of interest. These prosthetic groups were used to gain access to two GHSR-targeting biomolecules, one of which was subsequently investigated in a biological system as a potential PET imaging probe.
Molecular imaging probes need to be relatively stable in the body to successfully locate and accumulate at their biomarker. Chapter 3 discusses the stability of a peptide-based GHSR imaging probe in the presence of blood and liver enzymes, which are capable of metabolizing peptide biomolecules. The chapter also demonstrates how chemical synthesis can be applied to modify the molecular probe to improve its stability without impeding its binding to the biomarker.
Chapter 4 describes the effort to directly incorporate fluorine-18 into the chemical structure of a GHSR-binding molecule. The design and execution of a series of chemical reactions in an effort to access this potential PET probe, is presented.
Drug discovery is the process by which new potential medications are developed. One technique used for discovering drugs is fragment-based drug design, where small molecules (fragments) are tested for binding to a given biomarker. The successful v fragments are then further developed into drug candidates. Chapter 5 presents the potential of modifying fragment-based drug design, by exclusively designing fragments that may contain fluorine-18, as a chemical technology to directly discover new molecular imaging probes.
Childs, Marina D., "The Development of Fluorine-18 Imaging Agents Targeting the GHSR" (2021). Electronic Thesis and Dissertation Repository. 7728.
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Available for download on Saturday, April 22, 2023