18F-labeled norepinephrine transporter tracer [18F]NS12137: radiosynthesis and preclinical evaluation

Abstract

Introduction: Several psychiatric and neurodegenerative diseases are associated with malfunction of brain norepinephrine transporter (NET). However, current clinical evaluations of NET function are limited by the lack of sufficiently sensitive methods of detection. To this end, we have synthesized exo-3-[(6-[¹⁸F]fluoro-2-pyridyl)oxy]-8-azabicyclo[3.2.1]-octane ([¹⁸F]NS12137) as a radiotracer for positron emission tomography (PET) and have demonstrated that it is highly specific for in vivo detection of NET-rich regions of rat brain tissue.

Methods: We applied two methods of electrophilic, aromatic radiofluorination of the precursor molecule, exo-3-[(6-trimethylstannyl-2-pyridyl)oxy]-8-azabicyclo-[3.2.1]octane-8-carboxylate: (1) direct labeling with [¹⁸F]F₂, and (2) labeling with [¹⁸F]Selectfluor, a derivative of [¹⁸F]F₂, using post-target produced [¹⁸F]F₂. The time-dependent distribution of [¹⁸F]NS12137 in brain tissue of healthy, adult Sprague-Dawley rats was determined by ex vivo autoradiography. The specificity of [¹⁸F]NS12137 binding was demonstrated on the basis of competitive binding by nisoxetine, a known NET antagonist of high specificity.

Results: [¹⁸F]NS12137 was successfully synthesized with radiochemical yields of 3.9% ± 0.3% when labeled with [¹⁸F]F₂ and 10.2% ± 2.7% when labeled with [¹⁸F]Selectfluor. The molar activity of radiotracer was 8.8 ± 0.7 GBq/μmol with [¹⁸F]F₂ labeling and 6.9 ± 0.4 GBq/μmol with [¹⁸F]Selectfluor labeling at the end of synthesis of [¹⁸F]NS12137. Uptake of [¹⁸F]NS12137 in NET-rich areas in rat brain was demonstrated with the locus coeruleus (LCoe) having the highest regional uptake. Prior treatment of rats with nisoxetine showed no detectable [¹⁸F]NS12137 in the LCoe. Analyses of whole brain samples for radiometabolites showed only the parent compound [¹⁸F]NS12137. Uptake of ¹⁸F-radioactivity in bone increased with time.

Conclusions: The two electrophilic ¹⁸F-labeling methods proved to be suitable for synthesis of [¹⁸F]NS12137 with the [¹⁸F]Selectfluor method providing an approximate three-fold higher yield than the [¹⁸F]F₂ method. As an electrostatically neutral radiotracer [¹⁸F]NS12137 crosses the blood-brain barrier and enabled specific labeling of NET-rich regions of rat brain tissue with the highest concentration in the LCoe.

Keywords: Fluorine-18; NET; NS12137; [(18)F]Selectfluor; electrophilic substitution; norepinephrine transporter.