Synthesis and characterisation of a radioligand for positron emission tomography imaging of the 5-HT 1A receptor

Abstract

Dysfunction of the central 5-HT system has been implicated in affective disorders such as anxiety and depression. However, one factor which has restricted the progress of investigations into possible changes in brain levels of 5-HT in affective disorders is the limited tools available to estimate in vivo 5-HT levels in the human brain. The in vivo imaging technique Positron Emission Tomography (PET) could address this issue. This thesis describes the chemical synthesis and biological characterisation of the 5-HT1A receptor PET ligand 4-(2’-methoxy-)phenyl-1-[2’-(N-2”-pyridinyl-)4-[18F]fluorobenzamido-]ethyl piperazine (4-[18F]MPPF), with the aim of determining whether this might be a suitable ligand to estimate in vivo 5-HT levels in the brain. Development of a routine radiosynthesis starting with low activity was performed. Successful 4-[18F]MPPF production was achieved using a combination of the Eckert and Zielgler ModularLab and a microwave reactor via a fluorodenitration reaction of 4-(2’-methoxy-)phenyl-1-[2’-(N-2”-pyridinyl-)4-nitrobenzamido-]ethyl piperazine (4-MPPNO2) with fluorine-18. A radiochemical yield of 33.5% was achieved with a total synthesis time of 60 minutes. PET and autoradiography studies in rats revealed 4-[18F]MPPF specifically bound in areas rich in 5-HT 1A receptors and binding could be blocked by pre-treatment with cold 4-MPPF. The ability of the 5-HT releasing agent fenfluramine to increase the concentration of endogenous 5-HT in the brain and displace radioligands from the 5-HT 1A receptor was examined in parallel with PET, in vitro and ex vivo autoradiography, and microdialysis studies. The microdialysis studies revealed that fenfluramine (3 mg/kg) increased 5-HT levels to approximately 400% of basal levels, reaching a maximum concentration of 1 x 10-8 M 5-HT (in hippocampus). PET and autoradiography studies indicated that 4-[18F]MPPF was not displaced using this dose of fenfluramine. These studies demonstrate that the Eckert and Zielgler ModularLab is an effective radiosynthesis platform for the synthesis of 4-[18F]MPPF for use in PET and 4-[18F]MPPF is a suitable PET ligand for the examination of the distribution of 5-HT 1A receptors in the rat brain. However, this study suggests that 4-[18F]MPPF may not be suitable for use in examining possible changes in 5-HT in the brain in affective disorders.