Development of novel PET radioligands for visualizing beta cell mass and amyloid plaques

Abstract

The aim of the thesis was twofold. The first aim was to radiolabel small molecules by using carbon-11 and fluorine-18 for visualising beta cell mass (BCM) in the pancreas by PET. Diabetes Mellitus (DM) is a chronic metabolic disorder that results from an absolute or relative lack of BCM of endocrine pancreas. The lack of an adequate non-invasive imaging PET probe prevents detailed examination of beta cell loss during onset and progression of DM as well as development of novel treatments and islets transplantation progress. The second aim of the thesis was to radiolabel peptide molecules with fluorine-18 to visualise beta amyloid in Alzheimer’s disease (AD) brain. AD is a chronic, progressive neurodegenerative disorder. Brain penetration study of a labelled peptide, specific for beta amyloid that can cross blood-brain-barrier (BBB), is important to gain knowledge about the fate of the molecule as a diagnostic probe.

A series of three novel radioligands for BCM imaging has been developed in this thesis. In paper I, a vesicular monoamine transporter type 2 (VMAT2) specific radioligand [18F]FE-DTBZ-d4 was synthesised in two steps. First step is the nucleophilic [18F]fluorination to produce deuterated-[18F]fluoroethylbromide followed by the O- alkylation of desmethyl-DTBZ precursor to produce [18F]FE-DTBZ-d4. The in vivo pharmacokinetics (PK) studies in pigs by PET/CT demonstrated reduced in vivo defluorination; therefore, it may be an improved potential candidate for imaging VMAT2 dense tissue i.e. islets transplantation in proximity to cortical bone structure. In Paper II, a glucokinase (GK) specific radioligand, [11C]AZ12504948, was synthesised in one step via alkylation of O-desmethyl precursor using [11C]methyl iodide. Both in vitro and in vivo (pig and monkey) studies with [11C]AZ12504948 for imaging GK in the pancreas and liver indicated low specificity. Increased target specificity is required for further progress in GK imaging using PET radioligands. In Paper III, a radioligand for G-protein coupled receptor 44 (GPR44), [11C/3H]AZ Compound X, was synthesised via S-methylation of sodium sulfinate salt in one step using [11C/3H]methyl iodide. In vitro binding of the radioligand, evaluated by autoradiography (ARG) on human and rat pancreatic tissues, confirmed higher specific binding in islets of human pancreatic tissue and no measurable binding in rat pancreas, which is devoid of GPR44. These studies indicate that the radioligand has suitable properties for beta cell imaging with high potential for further preclinical and clinical evaluation.

Three novel D-peptides were radiolabelled with fluorine-18 ([18F]ACI-87, [18F]ACI- 88, [18F]ACI-89) by using prosthetic group N-succinimidyl-4-[18F]fluorobenzoate, [18F]SFB, with epsilon (ε)-amino groups of lysine residues of peptide precursors in two steps. First step is the synthesis of [18F]SFB followed by the addition of [18F]SFB via acylation to the peptide molecule. Trimethylammonium salt [N(CH3)3+] precursor for synthesising [18F]SFB as well as the reference standard SFB were synthesised with good yields. Three 19F-peptide reference standards were also synthesised by using SFB. Preliminary ARG measurements were performed in AD and control human brains. ARG demonstrated higher radioligand uptake in the AD brain compared to age-matched control brain, which makes them potential for further use in in vivo testing by PET. However, preliminary PET (in vivo) studies in cynomolgus monkey brain, using these 18F-D-peptides, confirmed too low BBB penetration, making them unsuitable for further use as in vivo PET probes.