A PET imaging agent with fast kinetics: synthesis and in vivo evaluation of the serotonin transporter ligand [11C]2-[2-dimethylaminomethylphenylthio)]-5-fluorophenylamine ([11C]AFA)

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Abstract

A new serotonin transporter (SERT) ligand, [11C]2-[2-(dimethylaminomethylphenylthio)]-5-fluorophenylamine (10, [11C]AFA), was synthesized and evaluated as a candidate PET radioligand in pharmacological and pharmacokinetic studies. As a PET radioligand, AFA (8) can be labeled with either C-11 or F-18. In vitro, AFA displayed high affinity for SERT (Ki 1.46±0.15 nM) and lower affinity for norepinephrine transporter (NET, Ki 141.7±47.4 nM) or dopamine transporter (DAT, Ki >10,000 nM). [11C]AFA (10) was prepared from its monomethylamino precursor 9 by reaction with high specific activity [11C]methyl iodide. Radiochemical yield was 43±20% based on [11C]methyl iodide at end of bombardment (EOB, n = 10) and specific activity was 2,129 ± 1,369 Ci/mmol at end of synthesis (EOS, n = 10). Biodistribution studies in rats indicated that [11C]AFA accumulated in brain regions known to contain high concentrations of SERT. Binding in SERT-rich brain regions was reduced significantly by pretreatment with either the cold compound 8 or with the selective serotonin reuptake inhibitor (SSRI) citalopram, but not by the selective norepinephrine reuptake inhibitor nisoxetine, thus underlining its in vivo binding selectivity and specificity for SERT. Imaging experiments in baboons demonstrated that the uptake pattern of [11C]AFA in the baboon brain is consistent with the known distribution of SERT, with highest activity levels in the midbrain and thalamus, followed by striatum, hippocampus, and cortical regions. Activity levels in the baboon brain peaked at 15–40 min after radioligand injection, indicating a fast uptake kinetics for [11C]AFA. Pretreatment of the baboon with citalopram (4 mg/kg) significantly reduced the specific binding of [11C]AFA in all SERT-containing brain regions. Kinetic analysis revealed that the regional equilibrium specific to non-specific partition coefficients (V3″) of [11C]AFA are similar to those of [11C]McN5652, but lower than those of [11C]AFM or [11C]DASB. In summary, [11C]AFA appears to be an appropriate PET radioligand with a fast brain uptake kinetics:

Introduction

The neurotransmitter serotonin (5-HT) is believed to play an important role in the regulation of many physiological functions such as mood, appetite, sleep, pain and aggressive behavior [1]. The serotonin transporter (SERT), located on the cell bodies and terminals of the 5-HT neurons, is a marker of 5-HT innervation [2]. In the human brain, the regional density of SERT is high in the midbrain, thalamus, hypothalamus, and striatum, moderate in the structures of the limbic system (hippocampus, amygdala, and anterior cingulate cortex), low in the neocortical areas and negligible in the cerebellum [3], [4], [5], [6], [7]. Alterations of SERT densities have been noted in a number of neuropsychiatric conditions, including major depression, anxiety disorders, schizophrenia, substance abuse, alcoholism, eating disorders; and Alzheimer's and Parkinson's disease [8], [9], [10], [11], [12]. Most frequently prescribed antidepressants are selective serotonin reuptake inhibitors (SSRIs) and exert their antidepressant activity through interaction with the SERT [13], [14], [15], [16]. Many of the psychostimulants with abuse potential, such as cocaine and amphetamine, are blockers of the monoamine transporters including SERT, dopamine transporter (DAT), and norepinephrine transporter (NET) [16], [17]. Therefore, imaging the regional brain distribution of SERT in vivo provides an important tool to study the role of the 5-HT system in the pathophysiology and treatment of neuropsychiatric disorders.

Until recently the most widely used PET radioligand for the in vivo investigation of SERT has been [11C]McN5652 (Fig. 1) [18]. However, as an imaging agent [11C]McN5652 presents several limitations, including high levels of nonspecific binding, low specific to non-specific binding ratios, and slow brain kinetics [19], [20], [21], [22]. In the last few years a new series of SERT radiotracers has emerged from the substituted diarylsulfide class of compounds (Fig. 1). ADAM was first reported as a new high affinity SERT ligand [23]. [123I]ADAM has been shown to be an appropriate radioligand for the selective imaging of SERT using SPECT [24], [25]. Another radioligand of the same class, [11C]DASB, was introduced as a new PET ligand for SERT imaging in humans [26], [27], [28], [29]. [11C]DASB offers improvements over [11C]McN 5652, in that it displays a better signal to noise ratio and faster brain uptake kinetics [30], [31], [32]. Many other ligands in the same structural class have since been synthesized and evaluated as potential PET ligands for SERT (Fig. 1). These include [11C]DAPP, [11C]ADAM, [11C]DAPA, [11C]AFM, and [11C]MADAM [30], [33], [34], [35], [36], [37]. All these new compounds have been shown to be potent and selective serotonin transporter ligands, with varying specific to non-specific binding ratios in vivo and differing brain kinetics.

In our laboratories we have been interested in the development of radioligands that can be labeled with both F-18 and C-11, such as AFM (2-[2-(dimethylaminomethylphenylthio)]-5-fluoromethylphenylamine), AFA (2-[2-(dimethylaminomethylphenylthio)]-5-fluorophenylamine), and AFE (2-[2-(dimethylaminomethylphenylthio)]-5-fluoroethylphenylamine) [38]. We have previously reported the evaluation of [11C]AFM and demonstrated that it provides, among the available SERT ligands for PET, the highest signal to noise ratios in imaging studies in non-human primates [30], [33]. However, the kinetics of [11C]AFM in baboons is relatively slow, thus raising concerns about a potentially long scan duration in human studies. Therefore, PET SERT radioligands with potential for F-18 labeling and a kinetics faster than [11C]AFM might offer some advantages. In this paper, we report the synthesis, in vitro characterization and in vivo pharmacological and pharmacokinetic evaluation of [11C]2-[2-(dimethylaminomethylphenylthio)]-5-fluorophenylamine, or [11C]AFA. The synthesis and preliminary evaluation of the F-18 labeled counterpart, [18F]AFA (termed F-ADAM), has recently been reported by Shiue et al. [39], [40].

Section snippets

General

All reagents were purchased from commercial suppliers and used without further purification unless otherwise stated. When reactions were worked up by extraction with dichloromethane (CH2Cl2), ethyl acetate (EtOAc), or ethyl ether (Et2O), organic solutions were dried with anhydrous MgSO4 and concentrated with a rotary evaporator under reduced pressure. Reactions requiring anhydrous conditions were carried out in oven-dried glassware under an inert atmosphere of nitrogen. Anhydrous Et2O and THF

Chemistry

Synthesis of compound 8 and its radiolabeling precursor 9 is depicted in Scheme 1. Briefly, 2-chloro-5-fluoronitrobenzene 1 was reacted with thiosalicylic acid 2 in DMF to give the benzoic acid 3, which was taken to amide 4 or 5 by first converting 3 to acid chloride with thionyl chloride, then reacting the acid chloride with either N,N-dimethylamine hydrochloride or N-methylamine hydrochloride. Reduction of the amide functionality in 4 or 5 with borane-THF complex led to the benzylamines 6

Conclusion

A new PET radioligand for the serotonin transporter, [11C]AFA, was synthesized in high radiochemical yield and high specific activity. In biodistribution studies in rats and imaging studies in baboons, [11C]AFA was demonstrated to enter the brain easily and accumulate in brain regions known to be rich in serotonin transporters. In the baboons, [11C]AFA provides regional V3″ values similar to those of [11C]McN5652. However, the brain uptake kinetics of [11C]AFA is much faster than that of [11

Acknowledgements

The writers wish to thank the expert technical assistance of Van Phan, Luydmilla Savenkova, Nurat Quadri, and Chaka Peters. They would also like to thank Dr. Bryan Roth and the NIMH Psychoactive Drug Screening Program for conducting the binding assays of new compounds. Supported by the Public Health Service (NIMH/NIDA R21 MH66624-01, 1-K02-MH01603-01) and the Lieber Center for Schizophrenia Research at Columbia University.

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