Synthesis and Biological Evaluation of Enantiomerically Pure (R)- and (S)-[18F]OF-NB1 for Imaging the GluN2B Subunit-Containing NMDA Receptors

GluN2B subunit-containing N-methyl-d-aspartate (NMDA) receptors have been implicated in various neurological disorders. Nonetheless, a validated fluorine-18 labeled positron emission tomography (PET) ligand for GluN2B imaging in the living human brain is currently lacking. The aim of this study was to develop a novel synthetic approach that allows an enantiomerically pure radiosynthesis of the previously reported PET radioligands (R)-[18F]­OF-NB1 and (S)-[18F]­OF-NB1 as well as to assess their in vitro and in vivo performance characteristics for imaging the GluN2B subunit-containing NMDA receptor in rodents. A novel synthetic approach was successfully developed, which allows for the enantiomerically pure radiosynthesis of (R)-[18F]­OF-NB1 and (S)-[18F]­OF-NB1 and the translation of the probe to the clinic. While both enantiomers were selective over sigma2 receptors in vitro and in vivo, (R)-[18F]­OF-NB1 showed superior GluN2B subunit specificity by in vitro autoradiography and higher volumes of distribution in the rodent brain by small animal PET studies.