Comparative evaluation of positron emission tomography radiotracers for imaging the norepinephrine transporter: (S,S) and (R,R) enantiomers of reboxetine analogs ([ 11 C]methylreboxetine, 3-Cl-[ 11 C]methylreboxetine and [ 18 F]fluororeboxetine), (R)-[ 11 C]nisoxetine, [ 11 C]oxaprotiline and [ 11 C]lortalamine Yu-Shin Ding, Kuo-Shyan Lin, Jean Logan, Helene Benveniste and Pauline Carter Chemistry and Medical Departments, Brookhaven National Laboratory, Upton, New York, USA Abstract We have synthesized and evaluated several new ligands for imaging the norepinephrine transporter (NET) system in baboons with positron emission tomography (PET). Ligands possessing high brain penetration, high affinity and selectivity, appropriate lipophilicity (log P ¼ 1.0–3.5), high plasma free fraction and reasonable stability in plasma were selected for further studies. Based on our characterization studies in baboons, including 11 C-labeled (R)-nisoxetine (Nis), oxaprotiline (Oxap), lortalamine (Lort) and new analogs of methylreboxe- tine (MRB), in conjunction with our earlier evaluation of 11 C and 18 F derivatives of reboxetine, MRB and their individual (R,R) and (S,S) enantiomers, we have identified the superiority of (S,S)-[ 11 C]MRB and the suitability of MRB analogs [(S,S)-[ 11 C]MRB > (S,S)-[ 11 C]3-Cl-MRB > (S,S)-[ 18 F]fluoro- reboxetine] as potential NET ligands for PET. In contrast, Nis, Oxap and Lort displayed high uptake in striatum (higher than in thalamus). The use of these ligands is further limited by high non-specific binding and relatively low specific signal, as is characteristic of many earlier NET ligands. Thus, to our knowledge (S,S)-[ 11 C]MRB remains by far the most promising NET ligand for PET studies. Keywords: lortalamine, nisoxetine, norepinephrine transpor- ter, oxaprotiline, positron emission tomography, reboxetine. J. Neurochem. (2005) 94, 337–351. The involvement of the norepinephrine transporter (NET) in the pathophysiology and treatment of attention deficit hyper- activity disorder (ADHD), substance abuse, neurodegenera- tive disorders (e.g. Alzheimer’s disease and Parkinson’s disease) and depression has long been recognized. However, many of these important findings have resulted from studies in vitro using post-mortem tissues (Ohama and Ikuta 1976; Carlsson 1979; Tejani-Butt et al. 1993; Jellinger 1997; Klimek et al. 1997; Eshleman et al. 1999; Macey et al. 2003). Up to now, these results have never been verified by in vivo methods because brain imaging of the NET in living systems has been hampered by the lack of suitable radiolig- ands. The fact that all three monoamine transporters [dop- amine transporter (DAT), NET and serotonin transporter (SERT)] are involved in various neurological and psychiatric diseases (Zahniser and Doolen 2001) further emphasizes the need to develop suitable NET ligands so that researchers will be able to determine the contributions of each monoamine transporter system to specific CNS disorders. Norepinephrine transport in sympathetic nerves of peri- pheral tissues has been labeled with (+/–)-[ 11 C]norepinephrine (Fowler et al. 1974; Farde et al. 1994), [ 18 F]fluoronorepi- nephrine (Ding et al. 1991), [ 131 I]MIBG(metaiodobenzyl- guanidine) (Wieland et al. 1981) and [ 11 C]hydroxyephedrine (Raffel and Wieland 1999); however, this class of tracers is Received January 20, 2005; revised manuscript received March 11, 2005; accepted March 16, 2005. Address correspondence and reprint requests to Yu-Shin Ding, PhD, Chemistry and Medical Departments, Brookhaven National Laboratory, Upton, New York 11973, USA. E-mail: ding@bnl.gov Abbreviations used: ADHD, attention deficit hyperactivity disorder; CBF, cerebral blood flow; DASB, N,N-dimethyl-2-(2-amino-cyanophe- nylthio) benzylamine; DAT, dopamine transporter; DMF, dimethylformate; DV, distribution volume; DVR, distribution volume ratio; EOB, end of bombardment; FRB, fluororeboxetine; Lort, lortal- amine; MRB, methylreboxetine; NET, norepinephrine transporter; Nis, nisoxetine; Oxap, oxaprotiline; PET, positron emission tomography; PPB, plasma protein binding; ROI, region of interest; SERT, serotonin transporter; S/N, signal to noise; TMS, tetramethylsilane. Journal of Neurochemistry , 2005, 94, 337–351 doi:10.1111/j.1471-4159.2005.03202.x Ó 2005 International Society for Neurochemistry, J. Neurochem. (2005) 94, 337–351 337