Synthesis and evaluation of new PET radioloigands for imaging central norepinephrine transporters

Abstract

The noradrenergic (NE), serotonergic (5-HT) and dopaminergic (DA) neurotransmission systems all have specific proteins responsible for the regulation of synaptic concentrations of neurotransmitter in the central nervous system (CNS) and in the periphery. Several reports have shown that the expression of these proteins, the monoamine transporters, within the CNS, may be altered in patients with certain neurodegenerative or neuropsychiatric disorders. Positron emission tomography (PET) is an imaging technique that enables quantitative studies in high resolution of receptor or transporter proteins inside the living human brain. At the outset of research for this thesis, PET had been used successfully in the mapping of 5-HT and DA transporters, but not NE transporters (NETs). The aim of this thesis was to develop a radioligand suitable for imaging NETs in the human brain in vivo.

This project focused on the screening of candidate NET radioligands by emission measurements in cynomolgus monkeys in vivo. Concomitant with these studies, radiometabolite analyses were performed on peripheral monkey plasma. To further characterise radioligands, in vitro autoradiography studies were performed on post mortem human brain tissue. During this screening process, nine of the most potent and selective NET inhibitors reported to date were prepared and labeled with carbon- 11 (t1/2 20.4 min) or fluorine- 18 (t1/2 109.8 min). Some improvements were also made with regards to the labelling of aryl fluoromethyl ethers and sulfides with fluorine-18, with a view to potential application in preparing new candidate NET radioligands.

Several candidate radioligands failed in the initial screening process. However, one lead compound was identified, namely (S,S)-[11C]MeNER. The regional distribution of (S,S)-[11C]MeNER in monkey brain was found to be in accord with known densities of NETs and was also shown to be specific to NET in a pre-treatment experiment. However, the binding kinetics of (S,S)-[11C]MeNER was found to be slow, which limited its utility in assessing regional NET densities in man. (S,S)-[18F]FMeNER was therefore developed as an improved analogue with a longer half-life that allowed the specific binding to reach equilibrium during the time-frame of the PET measurements. Its metabolic instability did however result in defluorination, which confounded the imaging of cortical regions. The di-deuterated analogue (S,S)-[18F]FMeNER-D2 was thus prepared, which showed a similar distribution in brain as the previously mentioned radioligands, but also displayed a reduced defluorination. In vitro autoradiography with (S,S)[18]FMeNER-D2 on post mortem human brain cryosections further demonstrated specific binding to NET. (S,S)18F]FMeNER-D2 has the potential to be the first useful radioligand for quantitative imaging of NETs in the living human brain.