Development of a Novel F18-PET tracer for early diagnosis of Alzheimers Dementia

Dementia represents one of the most economic and resource demanding health challenges that our society faces. The World Health Organization (WHO) estimates that by 2050, more than 115 million people will suffer from dementia worldwide. Alzheimers disease (AD) constitutes the largest subgroup. In order to reduce the social burden that the increasing numbers of patients which dementia represents, it is essential to develop effective treatment that can be given as early as possible, preferably before symptoms occur and medical intervention will still work. The development of early stage treatment hinges on improved diagnostic methods for the detection of dementia. Our project is based on the recently discovered hormonal effects on aging and dementia. GnRH (Gonadotropin Releasing Hormone) is a neuro-hormone that acts on receptors expressed by cells of the hippocampus, the brain region that is affected by AD in its initial stages. Our and other research groups' studies have shown that the expression and function of these receptors is changed by AD, and recent research suggests that this change occurs in early stages of the disease. We wish to develop a small molecule PET tracer (positron emission tomography) that will selectively bind to the GnRH receptor and make it possible to measure the receptor density in the brain and subsequently diagnose AD at an earlier stage. The project is unique and has origins in the research at Oslo University Hospital (OUH), University of Oslo (UiO) and Norwegian medical cyclotron centre AS (NMS). In addition, a quantitative autoradiography study has been performed (Manuscript under preparation) in order to investigate the presence of GnRH receptor agonist and antagonist binding sites in the forebrain of rat. In some initial autoradiographic studies we tested the effect of protease inhibition and GTP--S on the 125I buserelin binding affinity. The aim was to investigate the susceptibility of 125I buserelin binding in Hippocampus to the presence of GTP and protease inhibitors. Our results showed GTP-gamma-S and Protease inhibitor actually increase the affinity with a GTP-sensitive and Protease component. Based on these results, we are increasing our knowledge about the regulation of the GnRH receptor. So far, we have developed a set of 21 small molecule compounds (ligands). In order to assess the efficacy of these ligands, we have used a staining technique called autoradiography, where brain slices from a rat are stained with a 125I labelled standard GnRH receptor agonist (Busurelin) with high affinity for the receptor. By adding fixed amounts of our 21 ligands to the brain slices, the ligands with higher affinity for the receptor will compete with Busurelin for the receptor binding site, and less Busurelin will bind to the brain region in question. By using a photographic plate, Busurelin labelled with the radioactive 125I isotope will stain the plate in quantitative fashion depending on the amount of bound Busurelin. We also combine this technique with affinity testing. Based on these results, we identified one ligand that we believe is a potential candidate for a future GnRH receptor PET tracer. We proceeded to test this ligand by labelling it with a PET isotope (18F) and injected it in the tail vein of rats. We observed a specific PET signal in the hippocampus area of the brain. In the next step, we investigated an Alzheimer animal model (Tg2576) by the use of autoradiography to measure the GnRH receptor density in the Hippocampus at different time points (5 months and 12 months in the AD mice). We would like to use this animal model to study the effect of our lead ligand. To fulfill our project, we have now planned (in the period from February to April 2018), to validate our lead candidate in wildtype rats. If these results are again positive, we will be in a position to test our lead candidate in the AD model.

Demens er en av de største økonomiske og ressurskrevende sykdomsutfordringene som samfunnet står ovenfor. I 2050 estimerer WHO at antall demente mennesker vil overstige 115 millioner globalt, hvor Alzheimers sykdom (AD) utgjør den største undergruppen. Fo r å redusere den samfunnsmessige byrden som de økende forekomstene av demens representerer, er det viktig å utvikle effektive legemidler og behandlingsmetoder som gis så tidlig som mulig, helst før symptomer opptrer og medisinsk intervensjon gir ikke tils trekkelig effekt. Tidlig behandling vil profitere sterkt på en kobling til forbedrete diagnostiske metoder som detekterer demens i et tidlig stadium. Vårt prosjekt er basert på de nylige oppdagete hormoners effekter på aldring og demens. GnRH (Gonadotrop infrigjørende hormon) er et nevrohormon som virker også på reseptorer uttrykt av celler i hippocampus, den hjerneregionen som først berøres av AD. Våre og andre forskningsgruppers studier har vist at uttrykket og funksjon av disse reseptorene endres ved A D, og nyere forskning tyder på at denne endringen inntreffer tidlig i sykdomsforløpet. Vi ønsker å benytte PET (positron emisjons tomografi) til å måle denne endringen, for dermed å kunne diagnostisere AD på et tidligere stadium. Prosjektet er unikt og h ar utspring i forskningsmiljøene ved OUS, UIO og Norsk medisinsk syklotronsenter AS (NMS). Vi har utviklet seks forbindelser (ligander) som viser sterk og selektiv binding til GnRH-reseptorer, og forbindelsene kan merkes med kortlivede radioaktive PET iso toper. Vi jobber nå videre med å studere disse ligandene i dyremodeller for å se på fundamentale egenskaper som hjerneopptak og in vivo GnRH reseptor binding. Basert på disse resultatene vil vi optimalisere forbindelsene videre til vi når vårt overordnede mål: Utvikling av en GnRH PET-tracer som vil kunne benyttes som et nytt diagnostisk verktøy for pre-symptomatisk AD.