In vivo assessment of [¹¹C]MRB as a prospective PET ligand for imaging the norepinephrine transporter

Purpose: Antagonism of norepinephrine reuptake is now an important pharmacological strategy in the treatment of anxiety and depressive disorders, and many antidepressants have substantial potential occupancy of the norepinephrine transporter (NET) at recommended dosages. Despite the importance of understanding this transporter's role in psychiatric disease and treatment, a suitable radioligand for studying NET has been slow to emerge. (S,S)-Methylreboxetine (MRB) is among the more promising ligands recently adapted for positron emission tomography (PET), and the present study aimed to evaluate its potential for use in higher primates. Methods: Affinities for various brain targets were determined in vitro. PET studies were conducted in baboon under both test-retest and blocking conditions using 1 mg/kg nisoxetine. Results: MRB has sixfold higher affinity for NET than the serotonin transporter, and negligible affinity for other sites. PET studies in baboons showed little regional heterogeneity in binding and were minimally affected by pretreatment with the NET antagonist nisoxetine. Conclusion: Despite improvement over previous ligands for imaging NET in vivo, the low signal to noise ratio indicates [¹¹C]MRB lacks sensitivity and reliability as a PET radiotracer in humans.