Development of novel 5-HT1B PET radioligands

Abstract

Positron emission tomography (PET) is a useful tool for studying the central nervous system (CNS) in living subjects. PET is a non-invasive imaging technique that can visualize biochemical process such as receptor-ligand interactions and has been utilized for several neurotransmitter systems including the serotoninergic system. Serotonin receptor subtype 1B (5-HT1B) is an interesting target to study using PET and two radioligands have been developed for this purpose, [11C]AZ10419369 and [11C]P943. Both are labelled with carbon-11 and have been used in various PET studies during the last decade. One type of PET study that [11C]AZ10419369 and [11C]P943 have been used for is measuring changes in endogenous 5-HT concentrations in brain. In vitro models show that an agonist could be more sensitive to competition from endogenous 5-HT than an antagonist. [11C]AZ10419369 and [11C]P943 were both originally presented as antagonists, but [11C]AZ10419369, at least, has been shown to have agonist efficacy. Both [11C]AZ10419369 and [11C]P943 are also labelled with shorter-lived radionuclide carbon-11 (t1/2 = 20.4 min) which limits their use to facilities with a cyclotron. A fluorine-18 (t1/2 = 109.7 min) would allow more facilities to conduct PET studies on 5-HT1B.

In this thesis the aim has been to develop a full antagonist and agonist 5-HT1B PET radioligand in order to be able to assess whether their intrinsic activity affects their sensitivity towards changes in endogenous 5-HT concentrations. From a library of over 3000 compounds, provided by AstraZeneca, candidates were selected primarily with respects to their affinity, intrinsic activity and lipophilicity. Paper I describes the development of a full antagonist 5-HT1B PET radioligand labelled with carbon-11, [11C]AZ10419096, with high specific binding and sensitivity to displacement by endogenous 5-HT. Paper II describes the attempts at developing a full agonist 5-HT1B PET radioligand. Neither of the agonists selected were able to enter the brain and bind to 5-HT1B receptors in a significant amount. Because of the poor performance of the full agonists, a highly agonistic 5-HT1B PET radioligand, [11C]AZ12175002, which has previously been developed and tested in baseline PET measurements, was selected for an initial pilot study with a comparison of three PET radioligands with differing intrinsic activity in paper III. The study was able to show all three radioligands, [11C]AZ10419369, [11C]AZ10419096 and [11C]AZ12175002, are dose-dependently sensitive towards fenfluramine induced changes in endogenous 5-HT concentrations. A framework for an extended study was established with regards to dose levels and multiples number of PET measurements needed at each dose. Paper IV describes the development and initial characterization of [18F]AZ10419096, a fluorine-18 labeled version and identical structure of [11C]AZ10419096, showing high binding potential and specific binding in blocking PET measurement.

In conclusion, this thesis describes the development of novel carbon-11 labelled PET radioligands with differing intrinsic activity and the development of a fluorine-18 labelled PET radioligand for future studies of the 5-HT1B receptors in brain.