2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine (ADAM): an improved serotonin transporter ligand

Synthesis of N,N-dimethyl-2-(2-amino-4-[18F]fluorophenylthio)benzylamine as a serotonin transporter imaging agent

Two 403U76 analogs, N,N-dimethyl-2-(2-nitro-4-bromophenylthio)benzylamine (4) and N,N-dimethyl-2-(2,4-dinitrophenylthio)benzylamine (8) were prepared in multi-steps synthesis as precursors for the synthesis of a new serotonin transporter imaging agent, N,N-dimethyl-2-(2-amino-4-[18F]fluorophenylthio)benzylamine (12a). Reaction of 2,5-dibromonitrobenzene (1) with 2-thio-N,N-dimethylbenzamide gave N N-dimethyl-2-(2-nitro-4-bromophenylthio)benzamide (3). N,N-Dimethyl-2-(2,4-dinitrophenylthio)benzamide (6) was synthesized similarly from the reaction of 2-bromo-1,5-dinitrobenzene (2) with 2-thio-N,N-dimethylbenzamide. Reduction of 3 and 6 with BH(3)/THF gave benzylamines 4 and 8 along with their amine boranes 5 and 7. Nucleophilic substitution of 4 or 8 with K[18F]/Kryptofix 2.2.2 in DMSO at 120 degrees C followed by reduction with NaBH(4)- Cu(OAc)(2) in EtOH at 78 degrees C and purification with HPLC gave compound 12a in approximately 5-10% yield in a synthesis time of 150min from EOB. A preliminary biodistribution study in rats showed that the uptake of compound 12a in rat brain was high (approximately 1%/g) and the ratio of the uptake of compound 12a in hypothalamus (a serotonin transporter-rich area) and cerebellum (a serotonin-transporter-devoid area) was 6/1 at 1h post-injection. These results suggest that compound 12a may be a potential new serotonin transporter PET imaging agent.

Pharmacological characterization of N,N-dimethyl-2-(2-amino-4-methylphenyl thio)benzylamine as a ligand of the serotonin transporter with high affinity and selectivity

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. [3H]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 (Kd, 60 pM; Bmax, 543 fmol/mg of protein). Competition studies showed few inhibitory effect of nisoxetine (Ki = 270 nM), no inhibitory effect of desipramine or 1-[2-(diphenylmethoxy) ethyl]-4-(3-phenylpropyl)piperazine (GBR 12935) (Ki >1000 nM), and strong effect of paroxetine (Ki = 0.32 nM) and citalopram (Ki = 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 [3H]MADAM parallels the localization of serotonin transporters and is prevented by known inhibitors of them. The high affinity and selectivity of [3H]MADAM for the serotonin transporter show that it is very valuable for studies using in vitro approaches. The high selectivity and low nonspecific binding of [3H]MADAM on the postmortem human brain, together with preliminary in vivo results with [11C]MADAM, is a new argument for future use of this ligand in in vivo studies of the distribution, pharmacology, and pathophysiology of the serotonin transporter in the human brain with positron emission tomography.

Radiopharmaceuticals for single-photon emission computed tomography brain imaging

In the past 10 years, significant progress on the development of new brain-imaging agents for single-photon emission computed tomography has been made. Most of the new radiopharmaceuticals are designed to bind specific neurotransmitter receptor or transporter sites in the central nervous system. Most of the site-specific brain radiopharmaceuticals are labeled with (123)I. Results from imaging of benzodiazepine (gamma-aminobutyric acid) receptors by [(123)I]iomazenil are useful in identifying epileptic seizure foci and changes of this receptor in psychiatric disorders. Imaging of dopamine D2/D3 receptors ([(123)I]iodobenzamide and [(123)I]epidepride) and transporters [(123)I]CIT (2-beta-carboxymethoxy-3-beta(4-iodophenyl)tropane) and [(123)I]FP-beta-CIT (N-propyl-2-beta-carboxymethoxy-3-beta(4-iodophenyl)-nortropane has proven to be a simple but powerful tool for differential diagnosis of Parkinson's and other neurodegenerative diseases. A (99m)Tc-labeled agent, [(99m)Tc]TRODAT (technetium, 2-[[2-[[[3-(4-chlorophenyl)-8-methyl-8-azabicyclo [3,2,1]oct-2-yl]methyl](2-mercaptoethyl)amino]ethyl]amino] ethanethiolato(3-)]oxo-[1R-(exo-exo)]-), for imaging dopamine transporters in the brain has been successfully applied in the diagnosis of Parkinson's disease. Despite the fact that (123)I radiopharmaceuticals have been widely used in Japan and in Europe, clinical application of (123)I-labeled brain radiopharmaceuticals in the United States is limited because of the difficulties in supplying such agents. Development of (99m)Tc agents will likely extend the application of site-specific brain radiopharmaceuticals for routine applications in aiding the diagnosis and monitoring treatments of various neurologic and psychiatric disorders.

Synthesis, radiolabeling and preliminary biological evaluation of radiolabeled 5-methyl-6-nitroquipazine, a potential radioligand for the serotonin transporter

5-Methyl-6-nitroquipazine, a novel analogue of the potent and selective serotonin transporter inhibitor 6-nitroquipazine was synthesized and radiolabeled with tritium and the positron emitter carbon-11. [3H]5-methyl-6-nitroquipazine was found to have a K(d)=51+/-7 pM. The high affinity and the facile labeling of [11C]5-methyl-6-nitroquipazine makes it a promising radioligand for visualization of the serotonin transporter with positron emission tomography.

Comparative evaluation in nonhuman primates of five PET radiotracers for imaging the serotonin transporters: [11C]McN 5652, [11C]ADAM, [11C]DASB, [11C]DAPA, and [11C]AFM

The recent introduction of a number of new radiotracers suitable for imaging the serotonin transporters (SERT) has radically changed the field of SERT imaging. Whereas, until recently, only one selective SERT radiotracer was available ([11C]McN 5652) for SERT imaging with positron emission tomography (PET), several new C-11-labeled radiotracers of the -dimethyl-2-(arylthio)benzylamine class have been described as appropriate imaging agents for the SERT. The aim of this study was to conduct a comparative evaluation of four of the most promising agents in this class ([11C]ADAM, [11C]DASB, [11C]DAPA, and [11C]AFM) with the reference tracer [11C]McN 5652 under standardized experimental conditions. This evaluation included in vitro measurements of affinity and lipophilicity, and in vivo PET imaging experiments in baboons. In vitro, DASB displayed significantly lower affinity for SERT than the other four tracers. In the blood, [11C]DASB and [11C]AFM display faster clearance and higher free fractions. Brain uptake was analyzed with kinetic modeling using a one-tissue compartment model and the metabolite-corrected arterial input function. The kinetic uptake of [11C]DASB was significantly faster compared with the other compounds, and the scan duration required to derive time-independent estimates of regional distribution volumes was shorter. [11C]DAPA exhibited the slowest brain kinetic. Regional-specific-to-nonspecific equilibrium partition coefficient (V3") was the highest for [11C]AFM, followed by [11C]DASB and [11C]DAPA, which in turn provided higher V3" values than [11C]ADAM and [11C]McN 5652. From these experiments, two ligands emerged as superior radiotracers that provide a significant improvement over [11C]McN 5652 for PET imaging of SERT: [11C]DASB, because it enables the measurement of SERT availability in a shorter scanning time, and [11C]AFM, because its higher signal-to-noise ratios provide a more reliable measurement of SERT availability in brain regions with relatively low density of SERT, such as in the limbic system.

Neuroreceptor imaging in psychiatric disorders

Molecular imaging, the study of receptors, transporters and enzymes, as well as other cellular processes, has grown in recent years to be one of the most active neuroimaging areas. The application of single photon emission tomography (SPECT) and positron emission tomography (PET) techniques to the study of psychiatric illness has lead to increased understanding of disease processes as well as validated, in vivo, theories of illness etiology. Within the field of psychiatry these techniques have been applied most widely to the study of schizophrenia. Studies within schizophrenia are largely limited to either the dopamine or serotonin system. This is due in large part to the availability of suitable radiotracers as well as the current theories on the etiology of the illness. Two basic study designs are used when studying schizophrenia using molecular imaging and make up the majority of studies reviewed in this manuscript. The first type, termed "clinical studies," compares the findings from PET and SPECT studies in those with schizophrenia to normal controls in an attempt to understand the pathophysiology of the illness. The second study design, termed "occupancy studies," uses these techniques to enhance the understanding of the mechanism of action of the medications used in treating this illness. This review will focus on the findings of molecular imaging studies in schizophrenia, focusing, for the most part, on the serotonin and dopamine systems. Emphasis will be placed on how these findings and techniques are currently being used to inform the development of novel treatments for schizophrenia.

Synthesis and pharmacological characterization of a new PET ligand for the serotonin transporter: [11C]5-bromo-2-[2-(dimethylaminomethylphenylsulfanyl)]phenylamine ([11C]DAPA)

A new PET radioligand for the serotonin transporter (SERT), [11C]-5-bromo-2-[2-(dimethylaminomethylphenylsulfanyl)]phenylamine ([11C]DAPA (10), was synthesized and evaluated in vivo in rats and baboons. [11C]DAPA (10) was prepared from its monomethylamino precursor 8 by reaction with high specific activity [11C]methyl iodide. Radiochemical yield was 24 +/- 5% based on [11C]methyl iodide at end of bombardment (EOB, n = 10) and specific activity was 1,553 +/- 939 Ci/mmol at end of synthesis (EOS, n = 10). Binding assays indicated that [11C]DAPA displays high affinity (Ki 1.49 +/- 0.28 nM for hSERT) and good selectivity for the SERT in vitro. Biodistribution studies in rats indicated that [11C]DAPA enters into the brain readily and localizes in brain regions known to contain high concentrations of SERT, such as the thalamus, hypothalamus, frontal cortex and striatum. Moreover, such binding in SERT-rich regions of the brain are blocked by pretreatment with either the selective serotonin reuptake inhibitor (SSRI) citalopram and by the cold compound itself, demonstrating that [11C]DAPA binding in the rat brain is saturable and specific to SERT. Imaging experiments in baboons indicated that [11C]DAPA binding is consistent with the known distribution of SERT in the baboon brain, with highest levels of radioactivity detected in the midbrain and thalamus, intermediate levels in the hippocampus and striatum, and lower levels in the cortical regions. Pretreatment of the baboon with citalopram 10 min before radioactivity injection blocked the binding of [11C]DAPA in all brain regions that contain SERT. Kinetic analysis revealed that, in all brain regions examined, [11C]DAPA specific to nonspecific distribution volume ratios (V(3)") are higher than [11C](+)-McN 5652 and similar to [11C]DASB. In summary, [11C]DAPA appears to be a promising radioligand suitable for the visualization of SERT in vivo using PET.

PET examination of [(11)C]5-methyl-6-nitroquipazine, a radioligand for visualization of the serotonin transporter

Radiohalogenated 5-halo-6-nitroquipazine analogues have been shown to be potential radioligands for visualization of the serotonin transporter (5-HTT) with PET and SPECT. In the present study a methylated analogue, 5-methyl-6-nitroquipazine (MNQP), was radiolabeled with carbon-11 in a two step reaction via a palladium catalyzed cross coupling reaction between N-t-BOC-protected 5-tributylstannyl-6-nitroquipazine and [(11)C]methyl iodide as key step. [(11)C]MNQP was examined in the cynomolgus monkey brain with positron emission tomography (PET) and the appearance of labeled metabolites in monkey plasma was measured with gradient HPLC. Radioactivity increased continuously in all brain regions during the 90 minutes acquisition time. Highest accumulation of radioactivity was observed in the thalamus and brainstem, regions with a known high density of 5-HTT. The calculated ratios between the thalamus and brainstem to the 5-HTT poor cerebellum were 1.5 and 1.3-1.4, respectively, 80 minutes after radioligand injection. Pretreatment with citalopram prior to the PET measurement markedly reduced the binding in the thalamus and the brainstem. At 15 and 30 minutes after injection of [(11)C]MNQP approximately 90% and 60%, respectively, of radioactivity in plasma represented unchanged radioligand. The slow kinetics and moderate ratios recorded however, may limit its use as a PET radioligand for quantitative studies of the serotonin transporter with PET.

Synthesis of a fluorine-18-labelled derivative of 6-nitroquipazine, as a radioligand for the in vivo serotonin transporter imaging with PET

Considerable efforts have been engaged in the design, synthesis and pharmacological characterization of radioligands for imaging the serotonin transporter, based on its implication in several neuropsychiatric diseases, such as depression, anxiety and schizophrenia. In the 5-halo-6-nitroquipazine series, the fluoro derivative has been designed for positron emission tomography (PET). The corresponding 5-iodo-, 5-bromo- and 5-chloro N-Boc-protected quipazines as labelling precursors, as well as 5-fluoro-6-nitroquipazine as a reference compound have been synthesized. 5-[(18)F]Fluoro-6-nitroquipazine has been radiolabelled with fluorine-18 (positron-emitting isotope, 109.8 min half-life) by nucleophilic aromatic substitution from the corresponding N-Boc protected 5-bromo- and 5-chloro-precursors using K[(18)F]F-K(222) complex in DMSO by conventional heating (145 degrees C, 2 min) or microwave activation (50 W, 30-45 s), followed by removal of the protective group with TFA. Typically, 15-25 mCi (5.5-9.2 GBq) of 5-[(18)F]fluoro-6-nitroquipazine (1-2 Ci/micromol or 37-72 GBq/micromol) could be obtained in 70-80 min starting from a 550-650 mCi (20.3-24.0 GBq) aliquot of a cyclotron [(18)F]F(-) production batch (2.7-3.8% non decay-corrected yield based on the starting [(18)F]fluoride). Ex vivo studies (biodistribution in rat), as well as PET imaging (in monkey) demonstrated that 5-[(18)F]fluoro-6-nitroquipazine ([(18)F]-1d) readily crossed the blood brain barrier and accumulated in the regions rich in 5-HT transporter (frontal- and posterial cortex, striata). However, the low accumulation of the tracer in the thalamus (rat and monkey) as well as the comparable displacement of the tracer observed with both citalopram, a -HT re-uptake inhibitor and maprotiline, a norepinephrine re-uptake inhibitor (rat), indicate that 5-[(18)F]fluoro-6-nitroquipazine ([(18)F]-1d) does not have the suggested potential for PET imaging of the serotin transporter (SERT).

Biodistribution study of [(123)I] ADAM in mice: correlation with whole body autoradiography

Iodine-123 labeled 2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine ([(123)I] ADAM) has been suggested as a promising serotonin transporter (SERT) imaging agent. Much research has been accomplished, mainly focusing on the SERT binding sites in the central nervous system (CNS). However, the biodistribution of [(123)I] ADAM using whole body autoradiography (WBAR) has never been previously described, to the best of our knowledge. In this study, we assayed the biodistribution of [(123)I] ADAM in tissues/organs removed from mice, and measured their radioactivity with a scintillation counter (SC). The results showed that the liver has the highest uptake. On the other hand, the WBAR clearly demonstrated that [(123)I] ADAM was bound to SERT-rich sites including those in the brain stem, lung, adrenal glands and intestinal mucosa. This radiotracer also accumulated in the liver, kidney, and thyroid. The results from both methods were compared; each has its own complementary role in the biodistribution studies. The SC method revealed the total amount of radiotracer accumulation in each organ, and the WBAR demonstrated more anatomical details of the radiotracer's distribution. The whole body distribution results of the radioligand using both methods explore the usage of this novel radioligand for most possible SERT binding sites, not only in the CNS but also in the peripheral nervous system and neuroendocrine tissues. These findings suggest that [(123)I] ADAM is a potentially useful imaging agent for SERT.

Syntheses and binding affinities of 6-nitroquipazine analogues for serotonin transporter. Part 2: 4-substituted 6-nitroquipazines

Eleven 4-substituted derivatives of 6-nitroquipazine were synthesized and evaluated for their abilities to displace [3H]citalopram binding to the rat cortical synaptic membranes. Among them, 4-chloro-6-nitroquipazine was shown to possess the highest binding affinity (K(i=)0.03 nM) which was approximately 6 times higher than that of 6-nitroquipazine (K(i)=0.17 nM) itself. In this paper, we describe the syntheses of 4-substituted 6-nitroquipazine derivatives, the results of corresponding biological evaluation and the SAR study.

Positron emission tomography quantification of [(11)C]-DASB binding to the human serotonin transporter: modeling strategies

[(11) C]-DASB, namely [(11) C]-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile, is a new highly selective radioligand for the in vivo visualization of the serotonin transporter (SERT) using positron emission tomography (PET). The current study evaluates different kinetic modeling strategies for quantification of [(11)C]-DASB binding in five healthy humans. Kinetic analyses of tissue data were performed with a one-tissue (1CM) and a two-tissue (2CM) compartment model. Time-activity curves were well described by a 1CM for all regions. A 2CM model with four parameters failed to converge reliably. Reliable fits of the data were obtained only if no more than three parameters were allowed to vary. However, even then, the rate constants k(3) and k(4) were estimated with poor precision. Only the ratio k(3)/k(4) was stable. Goodness of fit was not improved by using a 2CM as compared with a 1CM. The minimal study duration required to obtain stable k(3)/k(4) estimates was 80 minutes. For routine use of [(11)C]-DASB, several simplified methods using the cerebellum as a reference region to estimate nonspecific binding were also evaluated. The transient equilibrium, the linear graphical analysis, the ratio of target to reference region, and the simplified reference tissue methods all gave binding potential values consistent with those obtained with the 2CM. The suitability of [(11)C]-DASB for research on the SERT using PET is thus supported by the observations that tissue data can be described using a kinetic analysis and that simplified quantitative methods, using the cerebellum as reference, provide reliable estimates of SERT binding parameters.

Selective in vitro and in vivo binding of [(125)I]ADAM to serotonin transporters in rat brain

An improved iodinated tracer, ADAM (2-((2-((dimethylamino)methyl)- phenyl)thio)-5-iodophenylamine) for imaging serotonin transporters (SERT) with single photon emission computerized tomography (SPECT), was prepared and characterized. Scatchard analysis of saturation binding of [(125)I]ADAM to rat frontal cortical membrane homogenates gave a K(d) value of 0.15 +/- 0.03 nM and a B(max) value of 194 +/- 65 fmol/mg protein. Biodistribution of [(125)I]ADAM in rat brain after an iv injection showed a high specific binding in the regions of hypothalamus, cortex, striatum, and hippocampus, where SERT are concentrated and the specific binding peaked at 120-240 min postinjection [(hypothalamus-cerebellum)/cerebellum = 4.3 at 120 min post-iv injection]. Moreover, the specific hypothalamic uptake was blocked by pretreatment with SERT selective competing drugs, such as paroxetine and (+)McN5652, while other noncompeting drugs, such as ketanserin, raclopride, and methylphenidate, showed no effect. The radioactive material recovered from rat brain homogenates at 120 min after [(125)I]ADAM injection showed primarily the original compound (>90%), a good indication of in vivo stability in the brain tissues. Both male and female rats showed similar and comparable organ distribution pattern and regional brain uptakes. Ex vivo autoradiograms of rat brain sections (120 min after iv injection of [(125)I]ADAM) showed intense labeling in several regions (olfactory tubercle, lateral septal nucleus, hypothalamic and thalamic nuclei, globus pallidus, central gray, superior colliculus, substantia nigra, interpeduncular nucleus, dorsal and median raphes, and locus coerulus), which parallel known SERT density. These results strongly suggest that the novel tracer ADAM is superior to the congers (i.e., IDAM) reported previously. When labeled with I-123, ADAM will be an improved and useful SPECT imaging agent for SERT in the brain.