Imaging by scintigraphy the serotonin transporter (5‐HTT) in the living human brain would be of great value in research on the pathophysiology and treatment of neuropsychiatric disorders such as depression. For that reason, and in order to obtain a selective radiotracer applicable to PET, we report here the carbon‐11 labelling of a selective 5‐HTT radioligand: N, N‐dimethyl‐2‐(2‐amino‐4‐methylphenylthio)benzylamine or MADAM in two different positions: [p‐11C‐methyl]MADAM and [N‐11C‐methyl]MADAM. The synthesis of Bu3Sn‐ADAM and N‐dimethyl‐MADAM is described. [p‐11C‐methyl]MADAM was obtained by a Stille coupling reaction between Bu3Sn‐ADAM and [11C]methyl iodide using palladium (0) as a catalyst without (Ia) or with copper chloride as a co‐catalyst (Ib). [N‐11C‐methyl]MADAM was obtained by an N‐methylation reaction between N‐demethyl‐MADAM and [11C]methyl iodide (II). The carbon‐11 incorporation yield in [p‐11C‐methyl]MADAM was 10–30% (Ia and Ib) and in [N‐11C‐methyl]MADAM was 75‐80% (II). The final product in each case was obtained in 30 min total synthesis time, including HPLC purification and with >99% radiochemical purity. Copyright © 2001 John Wiley & Sons, Ltd.

The aim of this study was to explore the potential of a new selective serotonin transporter (5-HTT) inhibitor, N,N-dimethyl-2-(2-amino-4-methylphenylthio)benzylamine (MADAM, K(i)=1.65 nM), as a PET radioligand for examination of 5-HTT in the nonhuman primate brain. MADAM was radiolabeled by an N-methylation reaction using [(11)C]methyl triflate and the binding was characterized by PET in four cynomolgus monkeys. Metabolite levels in plasma were measured by gradient high-performance liquid chromatography (HPLC). The radiochemical incorporation yield of [(11)C]MADAM was 75-80% and the specific radioactivity at the time of administration was 34-652 GBq/micromol (n=8). The highest uptake of radioactivity was observed in striatum, thalamus, mesencephalon, and the lower brainstem. Lower binding was detected in neocortex and the lowest radioactive uptake was found in the cerebellum. This distribution is in accordance with the known expression of 5-HTT in vitro. The fraction of the total radioactivity in monkey plasma representing unchanged [(11)C]MADAM was 20% at 45 min after injection, as measured by gradient HPLC. Pretreatment measurements, using unlabeled citalopram, GBR 12909, and maprotiline, as well as a displacement measurement, using unlabeled MADAM, confirmed that [(11)C]MADAM binds selectively and reversibly to 5-HTT, and support the use of the cerebellum as reference region. The present characterization of binding in the monkey brain suggests that [(11)C]MADAM is a potential PET radioligand for quantitative studies of 5-HTT binding in the human brain.

Serotonin transporter has a key-role in regulation of serotoninergic function, and is involved in numerous neurodegenerative and psychiatric disorders. To obtain an efficient radioactive ligand allowing the study of this transporter in vitro and in vivo, we synthesized a new diphenyl sulfide derivative, N,N-dimethyl-2-(2-amino-4-methylphenylthio)benzylamine or MADAM. We present here extensive pharmacological characterization of this compound.[3 H]MADAM bound to serotonin transporters with a very high affinity in vitro on rat cortical membranes, at least 2 times better than the most commonly used radioactive probes (K d , 60 pM; B max , 543 fmol/mg of protein). Competition studies showed few inhibitory effect of nisoxetine (K i ϭ 270 nM), no inhibitory effect of desipramine or 1-[2-(diphenylmethoxy) ethyl]-4-(3-phenylpropyl)piperazine (GBR 12935) (K i Ͼ1000 nM), and strong effect of paroxetine (K i ϭ 0.32 nM) and citalopram (K i ϭ 1.57 nM). Therefore, MADAM has around 1000-fold better selectivity for the serotonin transporter than for other transporters. Autoradiographic studies both on rat and postmortem human brain slices demonstrated that the distribution of

Key Words: N,N-dimethyl-2-(2-amino-4-methylphenylthio)benzylamine, serotonin transporter, carbon-1 1, PET .Imaging of the serotonin transporter (5-HTT) has potential in research on the pathophysiology and treatment of neuropsychiatric disorders (1, 2). Although several compounds have been synthesised possessing high affinity for the 5-Hm in vitro, none of these are optimal in PET and SPECT due to poor binding characteristics.Oya et al. has described a 403U76 derivative named ADAM, which is an excellent SPECT tracer for visualisation of 5-HIT (3). We have earlier reported C-1 1 labelling of ADAM for PET examination (4). Recently Houle et al. has described C-1 1 labelled methoxy (DAPP) and cyano (DASB) substituted phenylthiobenzylamine derivatives demonstrating potential for the PET visualisation of 5-HTT in the human brain (5). ADAM MADAMHere we report C-1 1 labelling of N,N-dimethyl-2-(2-amino-4-methylphenylthio)benzylamine (MADAM, 6) in two different positions which has a picomolar affinity and high selectivity for 5-HTT. MADAM ( 6 J contains three methyl substituents in two different parts of molecule: two N-methyl groups and one in p-position of the phenylring. We also report the preliminary PET evaluation and metabolite studies of [N-methyl-"CIMADAM in monkey. The N-demethyl-MADAM ( 3 J and unlabelled reference compound MADAM @) were obtained by the direct coupling of 4-bromo-3-nitrotoluene with N-methyl-2thiobenzamide and N,N-dimethyl-2-thiobenzamide (prepared using literature methods 6, 7 and €9, respectively (Fig. 2). The amide and nitro functions were reduced giving N-demethyl-MADAM (3) and MADAM (6). The synthesis of the