Synthesis and In Vivo Evaluation of Halogenated N,N-Dimethyl-2-(2′-amino-4′-hydroxymethylphenylthio)benzylamine Derivatives as PET Serotonin Transporter Ligands

Abstract

N,N-Dimethyl-2-(2′-amino-4′-hydroxymethylphenylthio)benzylamine (38), substituted on ring A, was reported to display high binding affinity and selectivity to the human brain serotonin transporter (SERT). In an attempt to explore the potential of compounds substituted on ring B of the phenylthiophenyl core structure, three derivatives of 38 were synthesized: N,N-dimethyl-2-(2′-amino-4′-hydroxymethyl-phenylthio)-5-fluorobenzylamine (35), N,N-dimethyl-2-(2′-amino-4′-hydroxymethyl-phenylthio)-5-bromobenzylamine (36), and N,N-dimethyl-2-(2′-amino-4′-hydroxymethyl-phenylthio)-5-iodobenzylamine (37). The in vitro binding studies in cells transfected with human SERT, norepinephrine transporter (NET), and dopamine transporter (DAT) showed that 35, 36, and 37 exhibited high SERT affinity with K_is (SERT) = 1.26, 0.29, and 0.31 nM (vs [³H]citalopram), respectively. [¹¹C]-(35), [¹¹C]-(36), and [¹¹C]-(37) were prepared by methylation of their monomethyl precursors 16, 17, and 18, with [¹¹C]iodomethane in 28, 11, and 14% radiochemical yields, respectively. The microPET images of [¹¹C]-(35), [¹¹C]-(36), and [¹¹C]-(37) showed high uptake in the monkey brain regions rich in SERT with peak midbrain to cerebellum ratios of 3.41, 3.24, and 3.00 at 85 min post-injection, respectively. In vivo bindings of [¹¹C]-(35), [¹¹C]-(36), and [¹¹C]-(37) were shown to be specific to the SERT as displacement with citalopram (a potent SERT ligand) reduced radioactivity in SERT-rich regions to the cerebellum level. These results suggest that [¹¹C]-(35), [¹¹C]-(36), and [¹¹C]-(37) could be potential agents for mapping human SERT by PET and radiolabeling 37 with iodine-123, which could afford the first SPECT SERT imaging agent exhibiting fast kinetics.