Synthesis and C-11 labeling of three potent norepinephrine transporter selective ligands ((R)-nisoxetine, lortalamine, and oxaprotiline) for comparative PET studies in baboons

Three potent antidepressants, (R)-nisoxetine, lortalamine, and oxaprotiline, with high affinity and high selectivity for the norepinephrine transporter (NET) were synthesized and radiolabeled with C-11 via [11C]methylation. The reference compounds and their corresponding normethyl precursors were synthesized via multi-step synthetic approaches. The radiochemical syntheses were performed by simple alkylation of the corresponding normethyl precursors with no-carrier-added [11C]CH3I in DMF. After HPLC purification, (R)-[N-11CH3]nisoxetine, [11C]lortalamine, and [11C]oxaprotiline were obtained in 63-97% radiochemical yields, whereas (R)-[O-11CH3]nisoxetine was obtained in 23-29% radiochemical yields due to substantial formation of the undesired N-[11C]methylated byproduct (64-70%). These C-11 labeled tracers allowed us to carry out comparative studies of NET in baboons with positron emission tomography (PET) and evaluate their potential as PET tracers for imaging brain NET.     

Comparative evaluation of positron emission tomography radiotracers for imaging the norepinephrine transporter: (S,S) and (R,R) enantiomers of reboxetine analogs ([11C]methylreboxetine, 3-Cl-[11C]methylreboxetine and [18F]fluororeboxetine), (R)-[11C]nisoxetine, [11C]oxaprotiline and [11C]lortalamine

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 = 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 11C-labeled (R)-nisoxetine (Nis), oxaprotiline (Oxap), lortalamine (Lort) and new analogs of methylreboxetine (MRB), in conjunction with our earlier evaluation of 11C and 18F derivatives of reboxetine, MRB and their individual (R,R) and (S,S) enantiomers, we have identified the superiority of (S,S)-[11C]MRB and the suitability of MRB analogs [(S,S)-[11C]MRB > (S,S)-[11C]3-Cl-MRB > (S,S)-[18F]fluororeboxetine] 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)-[11C]MRB remains by far the most promising NET ligand for PET studies.     

Synthesis of high-specific-radioactivity 4- and 6-[18F]fluorometaraminol-PET tracers for the adrenergic nervous system of the heart

Fluorine-18- (t(1/2) 109.8 min) and carbon-11 (t(1/2) 20.4 min)-labeled norepinephrine analogues have been found previously to be useful positron-emission-tomography (PET) radioligands to map adrenergic nerve terminals of the heart. Metaraminol ((1R,2S)-2-amino-1-(3-hydroxyphenyl)-1-propanol) is a metabolically stable structural analogue of norepinephrine and possesses high affinity towards the norepinephrine transporter and the vesicular monoamine transporter. This paper presents the radiosynthesis of new positron-emission-tomography halogeno analogues of metaraminol labeled with high specific radioactivity. Firstly, fluorine-18-labeled 4-fluorometaraminol (4-[18F]FMR or (1R,2S)-2-amino-1-(4-[18F]fluoro-3-hydroxyphenyl)-1-propanol) and its three other stereoisomers were prepared based on the following key steps: (a) condensation of the corresponding no-carrier-added labeled fluorobenzaldehyde with nitroethane, and (b) HPLC (C18 and chiral) resolution of the diastereomeric product mixture into the four individual enantiomers. Secondly, the corresponding 6-fluoro analogues, fluorine-18-labeled 6-fluorometaraminol (6-[18F]FMR or (1R,2S)-2-amino-1-(2-[18F]fluoro-5-hydroxyphenyl)-1-propanol) and its three other enantiomers, were prepared in an analogous way. Typically, 0.48-0.55 GBq of 4-[18F]FMR and 0.14-0.15 GBq of 6-[18F]FMR could be obtained after 120-160 min total synthesis time, with a specific radioactivity of 56-106 GBq/micromol. Furthermore, the synthesis of racemic 4-fluorometaraminol and 6-fluorometaraminol as reference compounds was performed. as well as independent chiral syntheses of the optically active (1R,2S) enantiomers. For the chiral syntheses, the key step was an electrophilic fluorination with acetyl hypofluorite of (1R,2S)-configurated organometallic derivatives of metaraminol. Tissue distribution studies in rats suggested that both 4-[18F]FMR and 6-[18F]FMR display similar affinity towards the presynaptic adrenergic nerve terminal in the heart. From a practical point of view, 4-[18F]FMR appeared to be the more attractive candidate for future PET investigations, due to higher radiochemical yields.     

Chiral synthesis of (2S,3S)-2-(2-morpholin-2-yl-2-phenylmethoxy)phenol

Resolution of (2RS,3RS)-2-[alpha-(2-methoxymethoxyphenoxy)phenylmethyl]morpholine, 11, with (+) mandelic acid led to the formation of (+)-(2S,3S)-2-[alpha-(2-methoxymethoxyphenoxy)phenyl methyl] morpholine (11a). Compound 11 was synthesized in seven steps from (2RS,3RS)-cinnamyl alcohol-2,3-epoxide (4), with an overall yield of 17%. Cleavage of the methoxymethyl group of the Fmoc derivative 12 with catalytic amounts of p-toluenesulfonic acid in methanol afforded (+)-(2S,3S)-2-(2-morpholin-2-yl-2-phenylmethoxy)phenol 2. The synthetic utility as well as the configuration of compound 2 has been demonstrated by converting (S,S)-2-(2-morpholin-2-yl-2-phenylmethoxy)phenol 2 to (2S,3S)-2-[alpha-(2-ethoxyphenoxy)phenylmethyl]morpholine (1) and (2S,3S)-2-(2-methoxyphenoxy) benzyl)morpholine (16), two potential norepinephrine reuptake inhibitors under clinical evaluation.     

Asymmetric synthesis and preliminary evaluation of (R)- and (S)-[11C]bisoprolol, a putative beta1-selective adrenoceptor radioligand

(+/-)-1-[4-(2-Isopropoxyethoxymethyl)-phenoxy]-3-isopropylamino-2-propanol (bisoprolol) is a potent, clinically used beta(1)-adrenergic agent. (R)-(+) and (S)-(-) enantiomers of bisoprolol were labelled with carbon-11 (t(1/2)=20.4 min) as putative tracers for the non-invasive assessment of the beta(1)-adrenoceptor subtype in the human heart and brain with positron emission tomography (PET). The radiosynthesis consisted of reductive alkylation of des-iso-propyl precursor with [2-11C]acetone in the presence of sodium cyanoborohydride and acetic acid. The stereo-conservative synthesis of (R)-(+) and (S)-(-)-1-[4-(2-isopropoxyethoxymethyl)-phenoxy]-3-amino-2-propanol to be used as the precursors for the radiosynthesis of [11C]bisoprolol enantiomers was readily accomplished by the use of the corresponding chiral epoxide in three steps starting from the commercially available hydroxybenzyl alcohol. The final labelled product (either (+) or (-)-1-[4-(-isopropoxyethoxymethyl)-phenoxy]-3- [11C]isopropylamino-2-propanol) was obtained in 99% radiochemical purity in 30 min with 15+/-5% (EOS, non-decay corrected) radiochemical yield and 3.5+/-1 Ci/micromol specific radioactivity. Preliminary biological evaluation of the tracer in rats showed that about 30% of heart uptake of [11C](S)-bisoprolol is due to specific binding. The high non-specific uptake in lung might mask the heart uptake, thus precluding the use of [11C](S)-bisoprolol for heart and lung studies by PET. The remarkably high uptake of the tracer in rat brain areas rich of beta-adrenergic receptors such as pituitary (1.8+/-0.3% I.D. at 30 min) was blocked by pre-treatment with the beta-adrenergic antagonists propranolol (45%) and bisoprolol (51%, p<0.05). [11C](S)-bisoprolol deserves further evaluation in other animal models as a putative beta(1) selective radioligand for in vivo investigation of central adrenoceptors.     

Synthesis, in vitro characterization, and radiolabeling of reboxetine analogs as potential PET radioligands for imaging the norepinephrine transporter

Six new (S,S)-enantiomers of reboxetine derivatives were synthesized and their binding affinities were determined via competition binding assays in cells expressing the human norepinephrine transporter (NET), serotonin transporter (SERT) or dopamine transporter (DAT). All six compounds prepared exhibit high affinity for the NET (K(i)相似文献   

Synthesis and preliminary evaluation of (R,R)(S,S) 5-(2-(2-[4-(2-[(18)F]fluoroethoxy)phenyl]-1-methylethylamino)-1-hydroxyethyl)-benzene-1,3-diol ([(18)F]FEFE) for the in vivo visualisation and quantification of the beta2-adrenergic receptor status in lung

The (18)F-labeled beta2-adrenergic receptor ligand (R,R)(S,S) 5-(2-(2-[4-(2-[(18)F]fluoroethoxy)phenyl]-1-methylethylamino)-1-hydroxyethyl)-benzene-1,3-diol, a derivative of the original highly selective racemic fenoterol, was synthesized in an overall radiochemical yield of 20% after 65 min with a radiochemical purity higher than 98%. The specific activity was in the range of 50-60 GBq/micromol. In vitro testing of the non-radioactive fluorinated fenoterol derivative with isolated guinea pig trachea was conducted to obtain an IC(50) value of 60 nM. Preliminary ex vivo organ distribution and in vivo experiments with positron emission tomography (PET) on guinea pigs were performed to study the biodistribution as well as the displacement of the radiotracer to prove specific binding to the beta2-receptor.     

11C-MCG: synthesis,uptake selectivity,and primate PET of a probe for glutamate carboxypeptidase II (NAALADase)

Imaging of glutamate carboxypeptidase II (GCP II), also known as N-acetylated alpha-linked L-amino dipeptidase (NAALADase), may enable study of glutamatergic transmission, prostate cancer, and tumor neovasculature in vivo. Our goal was to develop a probe for GCP II for use with positron emission tomography (PET). Radiosynthesis of 11C-MeCys-C(O)-Glu or 11C-(S)-2-[3-((R)-1-carboxy-2-methylsulfanyl-ethyl)-ureido]-pentanedioic acid (11C-MCG), an asymmetric urea and potent (Ki = 1.9 nM) inhibitor of GCP II, was performed by C-11 methylation of the free thiol. Biodistribution of 11C-MCG was assayed in mice, and quantitative PET was performed in a baboon. 11C-MCG was obtained in 16% radiochemical yield at the end of synthesis with specific radioactivities over 167 GBq/mmol (4000 Ci/mmol) within 30 min after the end of bombardment. At 30 min postinjection, 11C-MCG showed 33.0 +/- 5.1%, 0.4 +/- 0.1%, and 1.1 +/- 0.2% ID/g in mouse kidney (target tissue), muscle, and blood, respectively. Little radioactivity gained access to the brain. Blockade with unlabeled MCG or 2-(phosphonomethyl)pentanedioic acid (PMPA), another potent inhibitor of GCP II, provided sevenfold and threefold reductions, respectively, in binding to target tissue. For PET, distribution volumes (DVs) were 1.38 then 0.87 pre- and postblocker (PMPA). Little metabolism of 11C-MCG occurred in the mouse or baboon. These results suggest that 11C-MCG may be useful for imaging GCP II in the periphery.     

Norepinephrine Transporter Imaging in the Brain of a Rat Model of Depression Using Radioiodinated (2S, αS)-2-(α-(2-iodophenoxy)benzyl)morpholine

AbstractTo visualize the norepinephrine transporters (NETs) in various brain diseases, we developed radioiodinated (2S,αS)-2-(α-(2-iodophenoxy)benzyl)morpholine ((S,S)-IPBM). This radioligand achieved the basic requirements for NET imaging. In this study, we assessed the potential of radioiodinated (S,S)-IPBM as an imaging biomarker of NET to obtain diagnostic information about depression in relation to NET expression in the brain using a rat depression model. The ex vivo autoradiographic experiments using the (S,S)-[125I]IPBM showed significantly lower accumulation of radioactivity in the locus coeruleus (LC) and the anteroventricular thalamic nucleus (AVTN) of the depression group than in those of the control group. Consequently, in vitro autoradiographic experiments showed that NET maximum binding (Bmax) values in the LC and AVTN, known as NET-rich regions, were significantly decreased in the rat model of depression when compared to those of the control rats. In addition, there was an extremely good correlation between NET Bmax and (S,S)-IPBM accumulation (r = .98), an indication of radioiodinated IPBM as a quantitative NET imaging biomarker. The reduction in (S,S)-[125I]IPBM accumulation in the rat model of depression correlated with that of NET density. These results suggest that (S,S)-[123I]IPBM has potential as an imaging biomarker of NET to obtain diagnostic information about major depression.     

Development of new brain imaging agents based upon nocaine-modafinil hybrid monoamine transporter inhibitors

11C-labeled (+)-trans-2-[[(3R,4S)-4-(4-chlorophenyl)-1-methylpiperidin-3-yl]methylsulfanyl]ethanol ([11C]5) and (+)-trans-2-[[(3R,4S)-4-(4-chlorophenyl)-1-methylpiperidin-3-yl]methylsulfanyl]-1-(piperidin-1-yl)ethanone ([11C]6) were synthesized and evaluated as new imaging agents for the norepinephrine transporter (NET). [11C]5 and [11C]6 display high affinity for the NET in vitro (Ki = 0.94 and 0.68 nM, respectively) and significant selectivity over the dopamine (DAT) and serotonin transporters (SERT). Because of their high affinity and favorable transporter selectivities we speculated that these ligands might serve as useful PET agents for imaging NET in vivo. Contrary to our expectations, both of these ligands provided brain images that were more typical of those shown by agents binding to the DAT.     

Enantioselective σ1 receptor binding and biotransformation of the spirocyclic PET tracer 1′‐benzyl‐3‐(3‐fluoropropyl)‐3H‐spiro[[2]benzofuran‐1,4′‐piperidine]

It was shown that racemic (±)‐ 2 [1′‐benzyl‐3‐(3‐fluoropropyl)‐3H‐spiro[[2]benzofuran‐1,4′‐piperidine], WMS‐1813 ] represents a promising positron emission tomography (PET) tracer for the investigation of centrally located σ₁ receptors. To study the pharmacological activity of the enantiomers of 2 , a preparative HPLC separation of (R)‐2 and (S)‐2 was performed. The absolute configuration of the enantiomers was determined by CD‐spectroscopy together with theoretical calculations of the CD‐spectrum of a model compound. In receptor binding studies with the radioligand [³H]‐(+)‐pentazocine, (S)‐2 was thrice more potent than its (R)‐configured enantiomer (R)‐2 . The metabolic degradation of the more potent (S)‐enantiomer was considerably slower than the metabolism of (R)‐2 . The structures of the main metabolites of both enantiomers were elucidated by determination of the exact mass using an Orbitrap‐LC‐MS system. These experiments showed a stereoselective biotransformation of the enantiomers of 2 . Chirality, 2011. © 2010 Wiley‐Liss, Inc.     

Synthesis, radiosynthesis and in vivo preliminary evaluation of [11C]LBT-999, a selective radioligand for the visualisation of the dopamine transporter with PET

LBT-999 (8-((E)-4-fluoro-but-2-enyl)-3beta-p-tolyl-8-aza-bicyclo[3.2.1]octane-2beta-carboxylic acid methyl ester), a cocaine derivative belonging to a new generation of highly selective dopamine transporter (DAT) ligands, and its corresponding carboxylic acid derivative, the latter used as precursor for labelling both with tritium and the positron-emitter carbon-11 (half-life: 20.38 min), were synthesized from (R)-cocaine. [(3)H]LBT-999 (>99% radiochemically pure, specific radioactivity of 3.1 TBq/mmol) was prepared from [(3)H]methyl iodide, allowing its in vitro pharmacological evaluation (K(D): 9 nM for DAT and IC(50) > 1000 nM for SERT and NET). Routine production batches of 4.5-9.0 GBq of iv injectable solutions of [(11)C]LBT-999 (with specific radioactivities ranging from 30 to 45 GBq/mumol) were prepared in 25-30 min (HPLC purification and formulation included) using the efficient methylation reagent [(11)C]methyl triflate. The preliminary in vivo pharmacological evaluation of [(11)C]LBT-999, using both biodistributions in rats and brain imaging in monkeys with positron emission tomography (PET), clearly illustrates that this ligand is an excellent candidate for quantification with PET of DAT in humans.     

Production of (R)-3-amino-3-phenylpropionic acid and (S)-3-amino-3-phenylpropionic acid from (R,S)-N-acetyl-3-amino-3-phenylpropionic acid using microorganisms having enantiomer-specific amidohydrolyzing activity

(R)-3-Amino-3-phenylpropionic acid ((R)-beta-Phe) and (S)-3-amino-3-phenylpropionic acid ((S)-beta-Phe) are key compounds on account of their use as intermediates in synthesizing pharmaceuticals. Enantiomerically pure non-natural amino acids are generally prepared by enzymatic resolution of the racemic N-acetyl form, but despite the intense efforts this method could not be used for preparing enantiomerically pure beta-Phe, because the effective enzyme had not been found. Therefore, screening for microorganisms capable of amidohydrolyzing (R,S)-N-acetyl-3-amino-3-phenylpropionic acid ((R,S)-N-Ac-beta-Phe) in an enantiomer-specific manner was performed. A microorganism having (R)-enantiomer-specific amidohydrolyzing activity and another having both (R)-enantiomer- and (S)-enantiomer-specific amidohydrolyzing activities were obtained from soil samples. Using 16S rDNA analysis, the former organism was identified as Variovorax sp., and the latter as Burkholderia sp. Using these organisms, enantiomerically pure (R)-beta-Phe (>99.5% ee) and (S)-beta-Phe (>99.5% ee) with a high molar conversion yield (67%-96%) were obtained from the racemic substrate.     

Stereoselective and species-dependent kinetics of reboxetine in mouse and rat

Reboxetine, (RS)-2-[(RS)-α-(2-ethoxyphenoxy)benzyl]morpholine methanesulphonate, is a racemic compound and consists of a mixture of the (R,R)- and (S,S)-enantiomers. In this study, brain and plasma levels of both enantiomers were determined in mice and rats after oral administration of reboxetine at doses (1.1 mg/kg, mouse; 20 mg/kg, rat) twice the respective ED₅₀ values in the antireserpine test. Plasma and brain concentrations of each enantiomer were measured up to 6 h postdosing using an HPLC method with fluorimetric detection after derivatization with a chiral agent (FLEC). In mice and rats, brain and plasma levels of the (R,R)-enantiomer were always higher than those of the (S,S)-enantiomer. After normalization for dose, the mean AUC_0-tz values of both the (R,R)- and (S,S)-enantiomers in mouse brain were about 23 and 32 times higher than in rat brain, respectively. In plasma, the corrected mean AUC_0-tz values were about 5 (R,R) and 10 (S,S) times higher in mice than in rats. These results provide evidence for the higher bioavailability and/or lower clearance of both enantiomers in mice than in rats, and for a higher penetration of both enantiomers into mouse brain compared to rat brain. © 1995 Wiley-Liss, Inc.     

Radiosynthesis of [11C]BBAC and [11C]BBPC as potential PET tracers for orexin2 receptors

Radiosynthesis of [N-methyl-(11)C](S)-N-([1,1'-biphenyl]-2-yl)-1-(2-((1-methyl-1H-benzo[d]imidazol-2-yl)thio)acetyl)pyrrolidine-2-carboxamide ([(11)C]BBAC or [(11)C]3) and [N-methyl-(11)C] (S)-N-([1,1'-biphenyl]-2-yl)-1-(3-(1-methyl-1H-benzo[d]imidazol-2-yl)propanoyl)pyrrolidine-2-carboxamide ([(11)C]BBPC or [(11)C]-4), two potential PET tracers for orexin2 receptors are described. Syntheses of non-radioactive standards 3, 4 and corresponding desmethyl precursors 1, 2 were achieved from common intermediate (S)-2-([1,1'-biphenyl]-2-yl)-1-(pyrrolidin-2-yl)ethanone. Methylation using [(11)C]CH(3)OTf in the presence of base in acetone afforded [(11)C]3 and [(11)C]4 in 30±5% yield (EOS) with >99 % radiochemical purities with a specific activity ranged from 2.5±0.5 Ci/μmol (EOB). The logP of [(11)C]3 and [(11)C]4 were determined as 3.4 and 2.8, respectively. The total synthesis time was 30 min from EOB. However, PET scans performed in a rhesus monkey did not show tracer retention or appropriate brain uptake. Hence [(11)C]3 and [(11)C]4 cannot be used as PET tracers for imaging orexin2 receptors.     

Determination of the absolute configuration of bicyclo[3.3.1]nonane-2,7-dione by circular dichroism spectroscopy and chemical correlation.

A study of the enantiomers of bicyclo[3.3.1]nonane-2,7-dione, a chiral molecule containing two carbonyl chromophores, was performed. Enantiomers of this structure were obtained by HPLC resolution and the (+)-(1R,5S)-enantiomer by enantiospecific synthesis from(+)-(1S,5S)-bicyclo[3.3.1]nonane-2,6-dione. The title structure is an interesting molecule to demonstrate the validity of the octant rule. The location of the major chair-chair conformer into octants placing each chromophore into the origin of the octants led to the opposite configuration assignments. In order to prove unequivocally absolute configuration, enantiospecific synthesis of the title compound was carried out. The kinetic resolution of racemic bicyclo[3.3.1]nonane-2,6-dione using baker's yeast afforded (+)-(1S,5S)-2,6-dione. Employing a reaction sequence analogous to one developed earlier by us with racemic substrates led to carbonyl group shift giving enantiomerically pure (+)-(1R,5S)-bicyclo[3.3.1]nonane-2,7-dione. The absolute configuration of the investigated compound was established by combined use of the octant rule and chemical correlation.     

11C-Radiosynthesis and preliminary human evaluation of the disposition of the ACE inhibitor [11C]zofenoprilat

(4S)-1-[(S)-3-Mercapto-2-methylpropanoyl]-4-phenylthio-L-proline (Zofenoprilat, 2), the active metabolite of the potent ACE inhibitor Zofenopril Calcium (1), was labelled with carbon-11 (t1/2=20.4 min) to evaluate its pharmacokinetics behaviour in human body using Positron Emission Tomography (PET). [11C]2 labelling procedures were based on the use of immobilized Grignard reagent and the acylation of (S)-4-phenylthio-L-proline methyl ester (5) with 11C-labelled methacryloyl chloride, followed by a Michael addition with thiobenzoic acid. The radiochemical yield was 5-10% (EOB, decay corrected) and specific radioactivity ranged from 0.5 to 1.5 Ci/micromol (18.5-55.5 GBq/micromol). Preliminary in vivo human evaluation of [11C]2 showed that the drug accumulates in organs which express high levels of ACE, like lungs and kidneys, and in organs involved in drug metabolism such as the liver and gall bladder. Results of the distribution of [11C]2 showed a measurable concentration of the drug in the target tissues such as the kidney and to a minor extent, the heart, where it can afford organ protection.     

Pharmacokinetics of reboxetine enantiomers in the dog

Reboxetine, (RS)-2-[(RS)-α-(2-ethoxyphenoxy)benzyl]morpholine methanesulphonate, is a racemic compound and consists of a mixture of the (R,R)- and (S,S)-enantiomers. The pharmacokinetics of reboxetine enantiomers were determined in a crossover study in three male beagle dogs. Each animal received the following oral treatments, separated by 1-week washout period: 10 mg/kg reboxetine, 5 mg/kg (R,R)- and 5 mg/kg (S,S)-. Plasma and urinary levels of the reboxetine enantiomers were monitored up to 48 h post-dosing using an enantiospecific HPLC method with fluorimetric detection (LOQ: 1.1 ng/ml in plasma and 5 ng/ml in urine for each enantiomer). After reboxetine administration mean t_max was about 1 h for both enantiomers. C_max and AUC were about 1.5 times higher for the (R,R)- than for the (S,S)-enantiomer, mean values ± SD being 704 ± 330 and 427 ± 175 ng/ml for C_max and 2,876 ± 1,354 and 1,998 ± 848 ng.h/ml for AUC, respectively. No differences between the (R,R)- and (S,S)-enantiomers were observed in t½ (3.9 h). Total recovery of the two enantiomers in urine was similar, the Ae (0–48 h) being 1.3 ± 0.7 and 1.1 ± 0.7% of the enantiomer dose for the (R,R)- and the (S,S)-enantiomers, respectively. No marked differences in the main plasma pharmacokinetic parameters were found for either enantiomer on administration of the single enantiomers or reboxetine. No chiral inversion was observed after administration of the separate enantiomers, as already observed in humans. Chirality 9:303–306, 1997. © 1997 Wiley-Liss, Inc.     

Synthesis of 11C-labelled (R)-OHDMI and CFMME and their evaluation as candidate radioligands for imaging central norepinephrine transporters with PET

(R)-1-(10,11-Dihydro-dibenzo[b,f]azepin-5-yl)-3-methylamino-propan-2-ol ((R)-OHDMI) and (S,S)-1-cyclopentyl-2-(5-fluoro-2-methoxy-phenyl)-1-morpholin-2-yl-ethanol (CFMME) were synthesized and found to be potent inhibitors of norepinephrine reuptake. Each was labelled efficiently in its methyl group with carbon-11 (t(1/2)=20.4 min) as a prospective radioligand for imaging brain norepinephrine transporters (NET) with positron emission tomography (PET). The uptake and distribution of radioactivity in brain following intravenous injection of each radioligand into cynomolgus monkey was examined in vivo with PET. After injection of (R)-[(11)C]OHDMI, the maximal whole brain uptake of radioactivity was very low (1.1% of injected dose; I.D.). For occipital cortex, thalamus, lower brainstem, mesencephalon and cerebellum, radioactivity ratios to striatum at 93 min after radioligand injection were 1.35, 1.35, 1.2, 1.2 and 1.0, respectively. After injection of [(11)C]CFMME, radioactivity readily entered brain (3.5% I.D.). Ratios of radioactivity to cerebellum at 93 min for thalamus, occipital cortex, region of locus coeruleus, mesencephalon and striatum were 1.35, 1.3, 1.3, 1.2 and 1.2, respectively. Radioactive metabolites in plasma were measured by radio-HPLC. (R)-[(11)C]OHDMI represented 75% of plasma radioactivity at 4 min after injection and 6% at 30 min. After injection of [(11)C]CFMME, 84% of the radioactivity in plasma represented parent at 4 min and 20% at 30 min. Since the two new hydroxylated radioligands provide only modest regional differentiation in brain uptake and form potentially troublesome lipophilic radioactive metabolites, they are concluded to be inferior to existing radioligands, such as (S,S)-[(11)C]MeNER, (S,S)-[(18)F]FMeNER-D(2) and (S,S)-[(18)F]FRB-D(4), for the study of brain NETs with PET in vivo.     

Synthesis and biodistribution of [(11)C]R116301, a promising PET ligand for central NK(1) receptors

N1-(2,6-Dimethylphenyl)-2-(4-{(2R,4S)-2-benzyl-1-[3,5-di(trifluoromethyl)[carbonyl-(11)C]benzoyl]hexahydro-4-pyridinyl}piperazino)acetamide ([(11)C]R116301) was prepared and evaluated as a potential positron emission tomography (PET) ligand for investigation of central neurokinin(1) (NK(1)) receptors. 1-Bromo-3,5-di(trifluoromethyl)benzene was converted in three steps into 3,5-di(trifluoromethyl)[carbonyl-(11)C]benzoyl chloride, which was reacted with N1-(2,6-dimethylphenyl)-2-{4-[(2R,4S)-2-benzylhexahydro-4-pyridinyl]piperazino}acetamide providing [(11)C]R116301 in 45-57% decay-corrected radiochemical yield. The total synthesis time, from end of bombardment (EOB) to the formulated product, was 35 min. Specific activity (SA) was 82-172 GBq/micromol (n=10) at the end of synthesis. N1-([4-(3)H]-2,6-Dimethylphenyl)-2-(4-{(2R,4S)-2-benzyl-1-[3,5-di(trifluoromethyl)benzoyl]hexahydro-4-pyridinyl}piperazino)acetamide ([(3)H]R116301) was also synthesized (SA: 467 GBq/mmol). The B(max) for [(3)H]R116301 measured in vitro on Chinese hamster ovary cell membranes stably transfected with the human NK(1) receptor was 19.10+/-1.02 pmol/mg protein with an apparent dissociation constant of 0.08+/-0.01 nM. Ex vivo, in vivo and in vitro autoradiography studies with [(3)H]R116301 in gerbils demonstrated a preferential accumulation of the radioactivity in the striatum, olfactory tubercule, olfactory bulb and locus coeruleus. In vivo, the biodistribution of [(11)C]R116301 in gerbils revealed that the highest initial uptake is in the lung, followed by the liver and kidney. In the brain, maximum accumulation was found in the olfactory tubercules (1.10+/-0.08 injected dose (ID)/g 20 min post injection (p.i.)) and the nucleus accumbens (1.00+/-0.12ID/g 10 min p.i.). Tissue/cerebellum concentration ratios for striatum and nucleus accumbens increased with time due to rapid uptake followed by a slow wash out (1.29 and 1.64, respectively, 30 min p.i.). A tissue to cerebellum ratio of 1.33 and 1.62 was also observed for olfactory bulb and olfactory tubercules, respectively (20 min p.i.). In summary, [(11)C]R116301 appears to be a promising radioligand suitable for the visualization of NK(1) receptors in vivo using PET.