Synthesis and characterization of [¹²⁵I]2-Iodo N-[(S)-{(S)-1-methylpiperidin-2-yl}(phenyl)methyl]3-trifluoromethyl-benzamide as novel imaging probe for glycine transporter 1.

ABSTRACT
In this study, 2-iodo substituted 1-methylpiperidin-2-yl benzamide derivatives were synthesized and evaluated as candidate SPECT imaging agents for glycine transporter 1 (GlyT1). In JAR cells, which predominantly express GlyT1, 2-iodo N-[(S)-{(S)-1-methylpiperidin-2-yl}(phenyl)methyl]3-trifluoromethyl-benzamide (5) showed excellent inhibitory activity of [³H]glycine uptake (IC₅₀=2.4 nM). Saturation assay in rat cortical membranes revealed that [¹²⁵I]5 had a single high affinity binding site with a K_d of 1.54 nM and a B_max of 3.40 pmol/mg protein. In vitro autoradiography demonstrated that [¹²⁵I]5 showed consistent accumulation with GlyT1 expression. The in vitro binding was greatly inhibited by GlyT1 inhibitors but not by other site ligands, which suggested the high specific binding of [¹²⁵I]5 with GlyT1. In the biodistribution and ex vivo autoradiography studies using mice, [¹²⁵I]5 showed high blood-brain barrier permeability (1.68-2.17% dose/g at 15-60 min) and similar regional brain distribution pattern with in vitro results. In addition, pre-treatment of GlyT1 ligands resulted in significant decrease of [¹²⁵I]5 binding in the GlyT1-rich regions. This preliminary study demonstrated that radio-iodinated 5 is a promising SPECT imaging probe for GlyT1.