Improved synthesis and metabolic stability analysis of the dopamine transporter ligand [¹⁸F]FECT☆

Abstract 

Introduction

[2′-[¹⁸F]Fluoroethyl (lR-2-exo-3-exe)-8-methyl-3-(4-chlorophenyl)-8-azabicyclo[3.2.1]-octane-2-carboxylate] ([¹⁸F]FECT) is a positron emission tomography (PET) tracer for imaging the dopamine transporter (DAT) in vivo. We report an improved radiosynthesis procedure and affinity data and have analyzed both brain tissue and plasma samples for the presence of radiometabolites as a function of time post intravenous injection of [¹⁸F]FECT to rats.

Methods

The radiosynthesis of [¹⁸F]FECT was carried out using [¹⁸F]fluoroethyltriflate ([¹⁸F]FEtOTf) as a labeling agent. The affinity of FECT for DAT was determined in vitro by binding experiments on rat striatal membranes. Three rats were injected with [¹⁸F]FECT and blood samples were collected at 1 or 3 h post injection (p.i.). Plasma was separated and analyzed using reversed-phase high-performance liquid chromatography (RP-HPLC). Similarly, cerebrum and cerebellum were isolated after sacrifice of the animals at 3 h p.i. of the tracer and homogenized. HPLC analysis was performed on extracts of both samples to examine the presence of metabolites.

Results

The radiochemical yield for [¹⁸F]FECT was 85% relative to the starting activity of [¹⁸F]FEtOTf. The inhibitory constant (K_i) of FECT for DAT was found to be 6 nM. The fraction of radioactivity corresponding to intact [¹⁸F]FECT was 93% in plasma at both 1 and 3 h p.i. and 96% in cerebrum as well as cerebellum samples at 3 h p.i.

Conclusions

FECT has a high affinity for the dopamine transporter. [¹⁸F]FECT was found to be stable in vivo and the amount of radiolabeled metabolites in plasma and brain at 3 h p.i. is negligible. Hence, [¹⁸F]FECT can be used for the in vivo quantification of DAT using PET.

Keywords: FECT, Dopamine transporter, PET, Fluorine-18, 2-¹⁸F-fluoroethyltriflate, Metabolite analysis