Development of positron emission tomography radioligands for the dopaminergic and serotonergic neurotransmission systems

Abstract

The dopaminergic and serotonergic neurotransmission systems are of central interest in relation to several neuropsychiatric disorders. The main objectives were to develop new suitable radioligands for positron emission tomography (PET) examinations of the dopamine transporter (DAT) as well as the serotonin 5-HT1A and 5-HT2A-receptors.

Radiolabelling was performed with any of the positron emitting radionuclides carbon-11, fluorine-18 or bromine-76. Carbon-11 methylation was achieved by alkylation of a phenol group or by esterification using [11C]methyl iodide or [11C] C]methyl triflate. The latter improved the labelling results giving higher radiochemical yields and higher specific radioactivity. Another carbon-11 labelling route was acylation of an amine group with [11C]acid chloride. Fluorine-18 labelling was achieved by nucleophilic substitution and alkylation of a secondary amine with [18F]fluoropropyl bromide. Labellings with bromine-76 were performed by regioselective bromodestannylation. The in vivo distribution of radioactivity was measured in the cynomolgus monkey brain. Particular attention was given to the time course 'and identification of labelled metabolites that appears in plasma and were measured with various HPLC methods.

Two phenyltropane analogues were evaluated as radioligands for the DAT. [O-Methyl-11C]ßCIT-FE did readily reach equilibrium conditions in the striatum within the time frame of a PET measurement, and should accordingly be suitable for quantitation in applied studies. [O-Methyl-11C]ß-CIT-FP bound specifically to the DAT as demonstrated with autoradiography and PET. Radioligand binding approached equilibrium conditions at the end of the PET measurement. Due to the longer half-life of the radioinuclide [18F]ß-CIT-FP could more reliably be used to measure equilibrium conditions. A lipophilic labelled metabolite was detected in monkey plasma after i.v. injection of [O-methyl-11C]ß-CIT-FP and [O-methyl-11C]ß-CIT-FE. Importantly, no lipophilic labelled metabolite was detected after i.v. injection of [18F]ß-CIT-FP which further substantiate the advantage of using [[18F]ß-CIT-FP for quantitation of the DAT.

In search for a serotonin transporter radioligand specific binding of [76Br]5-bromo-6 nitroquipazine was shown in ex vivo rat studies. A subsequent PET study in a baboon demonstrated uptake in the thalamus and pons indicating that [76Br]5-bromo-6-nitroquipazine might have potential as a PET radioligand for the serotonin transporter.

Specific binding of the selective 5-HT1A antagonist WAY-100635 was characterized with autoradiography and found to be high in 5-HT1A receptor rich areas. Metabolite studies of [O methyl-11C]WAY-100635 revealed a significant fraction of the labelled metabolite [O-methyl-11C]WAY- 100634 which passes the blood-brain-barrier and add to the non-specific binding in brain. [Carbonyl-11C]WAY-100635 was accordingly superior to [O-methyl-11C]WAY-100635 for in vivo PET studies due to the absence of labelled WAY-100634. Specific binding to the 5-HT2A receptor in the neocortex was demonstrated in a cynomolgus monkey with the highly selective 5-HT2A antagonist [11C]MDL 100907. The neocortex to cerebellum ratio was about four at time of transient equilibrium and this radioligand has subsequently been used for human PET studies.