Dissecting the respective roles of microbiota and host genetics in the susceptibility of Card9-/- mice to colitis

BACKGROUND

The etiology of inflammatory bowel disease (IBD) is unclear but involves both genetics and environmental factors, including the gut microbiota. Indeed, exacerbated activation of the gastrointestinal immune system toward the gut microbiota occurs in genetically susceptible hosts and under the influence of the environment. For instance, a majority of IBD susceptibility loci lie within genes involved in immune responses, such as caspase recruitment domain member 9 (Card9). However, the relative impacts of genotype versus microbiota on colitis susceptibility in the context of CARD9 deficiency remain unknown.

RESULTS

Card9 gene directly contributes to recovery from dextran sodium sulfate (DSS)-induced colitis by inducing the colonic expression of the cytokine IL-22 and the antimicrobial peptides Reg3β and Reg3γ independently of the microbiota. On the other hand, Card9 is required for regulating the microbiota capacity to produce AhR ligands, which leads to the production of IL-22 in the colon, promoting recovery after colitis. In addition, cross-fostering experiments showed that 5 weeks after weaning, the microbiota transmitted from the nursing mother before weaning had a stronger impact on the tryptophan metabolism of the pups than the pups' own genotype.

CONCLUSIONS

These results show the role of CARD9 and its effector IL-22 in mediating recovery from DSS-induced colitis in both microbiota-independent and microbiota-dependent manners. Card9 genotype modulates the microbiota metabolic capacity to produce AhR ligands, but this effect can be overridden by the implantation of a WT or "healthy" microbiota before weaning. It highlights the importance of the weaning reaction occurring between the immune system and microbiota for host metabolism and immune functions throughout life. A better understanding of the impact of genetics on microbiota metabolism is key to developing efficient therapeutic strategies for patients suffering from complex inflammatory disorders. Video Abstract.

Behavioral, Hormonal, Inflammatory, and Metabolic Effects Associated with FGF21-Pathway Activation in an ALS Mouse Model

In amyotrophic lateral sclerosis (ALS), motor neuron degeneration occurs simultaneously with systemic metabolic dysfunction and neuro-inflammation. The fibroblast growth factor 21 (FGF21) plays an important role in the regulation of both phenomena and is a major hormone of energetic homeostasis. In this study, we aimed to determine the relevance of FGF21 pathway stimulation in a male mouse model of ALS (mutated SOD1-G93A mice) by using a pharmacological agonist of FGF21, R1Mab1. Mice (SOD1-WT and mutant SOD1-G93A) were treated with R1Mab1 or vehicle. Longitudinal data about clinical status (motor function, body weight) and biological parameters (including hormonal, immunological, and metabolomics profiles) were collected from the first symptoms to euthanasia at week 20. Multivariate models were performed to identify the main parameters associated with R1Mab1 treatment and to link them with clinical status, and metabolic pathways involving the discriminant metabolites were also determined. A beneficial clinical effect of R1Mab1 was revealed on slow rotarod (p = 0.032), despite a significant decrease in body weight of ALS mice (p < 0.001). We observed a decrease in serum TNF-α, MCP-1, and insulin levels (p = 0.0059, p = 0.003, and p = 0.01, respectively). At 16 weeks, metabolomics analyses revealed a clear discrimination (CV-ANOVA = 0.0086) according to the treatment and the most discriminant pathways, including sphingolipid metabolism, butanoate metabolism, pantothenate and CoA biosynthesis, and the metabolism of amino acids like tyrosine, arginine, proline, glycine, serine, alanine, aspartate, and glutamate. Mice treated with R1Mab1 had mildly higher performance on slow rotarod despite a decrease on body weight and could be linked with the anti-inflammatory effect of R1Mab1. These results indicate that FGF21 pathway is an interesting target in ALS, with a slight improvement in motor function combined with metabolic and anti-inflammatory effects.

The combination of four analytical methods to explore skeletal muscle metabolomics: Better coverage of metabolic pathways or a marketing argument?

OBJECTIVES

Metabolomics is an emerging science based on diverse high throughput methods that are rapidly evolving to improve metabolic coverage of biological fluids and tissues. Technical progress has led researchers to combine several analytical methods without reporting the impact on metabolic coverage of such a strategy. The objective of our study was to develop and validate several analytical techniques (mass spectrometry coupled to gas or liquid chromatography and nuclear magnetic resonance) for the metabolomic analysis of small muscle samples and evaluate the impact of combining methods for more exhaustive metabolite covering.

DESIGN AND METHODS

We evaluated the muscle metabolome from the same pool of mouse muscle samples after 2 metabolite extraction protocols. Four analytical methods were used: targeted flow injection analysis coupled with mass spectrometry (FIA-MS/MS), gas chromatography coupled with mass spectrometry (GC-MS), liquid chromatography coupled with high-resolution mass spectrometry (LC-HRMS), and nuclear magnetic resonance (NMR) analysis. We evaluated the global variability of each compound i.e., analytical (from quality controls) and extraction variability (from muscle extracts). We determined the best extraction method and we reported the common and distinct metabolites identified based on the number and identity of the compounds detected with low analytical variability (variation coefficient<30%) for each method. Finally, we assessed the coverage of muscle metabolic pathways obtained.

RESULTS

Methanol/chloroform/water and water/methanol were the best extraction solvent for muscle metabolome analysis by NMR and MS, respectively. We identified 38 metabolites by nuclear magnetic resonance, 37 by FIA-MS/MS, 18 by GC-MS, and 80 by LC-HRMS. The combination led us to identify a total of 132 metabolites with low variability partitioned into 58 metabolic pathways, such as amino acid, nitrogen, purine, and pyrimidine metabolism, and the citric acid cycle. This combination also showed that the contribution of GC-MS was low when used in combination with other mass spectrometry methods and nuclear magnetic resonance to explore muscle samples.

CONCLUSION

This study reports the validation of several analytical methods, based on nuclear magnetic resonance and several mass spectrometry methods, to explore the muscle metabolome from a small amount of tissue, comparable to that obtained during a clinical trial. The combination of several techniques may be relevant for the exploration of muscle metabolism, with acceptable analytical variability and overlap between methods However, the difficult and time-consuming data pre-processing, processing, and statistical analysis steps do not justify systematically combining analytical methods.

Comparative analysis of targeted metabolomics: dominance-based rough set approach versus orthogonal partial least square-discriminant analysis

BACKGROUND

Metabolomics is an emerging field that includes ascertaining a metabolic profile from a combination of small molecules, and which has health applications. Metabolomic methods are currently applied to discover diagnostic biomarkers and to identify pathophysiological pathways involved in pathology. However, metabolomic data are complex and are usually analyzed by statistical methods. Although the methods have been widely described, most have not been either standardized or validated. Data analysis is the foundation of a robust methodology, so new mathematical methods need to be developed to assess and complement current methods. We therefore applied, for the first time, the dominance-based rough set approach (DRSA) to metabolomics data; we also assessed the complementarity of this method with standard statistical methods. Some attributes were transformed in a way allowing us to discover global and local monotonic relationships between condition and decision attributes. We used previously published metabolomics data (18 variables) for amyotrophic lateral sclerosis (ALS) and non-ALS patients.

RESULTS

Principal Component Analysis (PCA) and Orthogonal Partial Least Square-Discriminant Analysis (OPLS-DA) allowed satisfactory discrimination (72.7%) between ALS and non-ALS patients. Some discriminant metabolites were identified: acetate, acetone, pyruvate and glutamine. The concentrations of acetate and pyruvate were also identified by univariate analysis as significantly different between ALS and non-ALS patients. DRSA correctly classified 68.7% of the cases and established rules involving some of the metabolites highlighted by OPLS-DA (acetate and acetone). Some rules identified potential biomarkers not revealed by OPLS-DA (beta-hydroxybutyrate). We also found a large number of common discriminating metabolites after Bayesian confirmation measures, particularly acetate, pyruvate, acetone and ascorbate, consistent with the pathophysiological pathways involved in ALS.

CONCLUSION

DRSA provides a complementary method for improving the predictive performance of the multivariate data analysis usually used in metabolomics. This method could help in the identification of metabolites involved in disease pathogenesis. Interestingly, these different strategies mostly identified the same metabolites as being discriminant. The selection of strong decision rules with high value of Bayesian confirmation provides useful information about relevant condition-decision relationships not otherwise revealed in metabolomics data.

A radiometabolite study of the serotonin transporter PET radioligand [(11)C]MADAM

INTRODUCTION

(11)C]MADAM is a radioligand suitable for PET studies of the serotonin transporter (SERT). Metabolite analysis in human and non-human plasma samples using HPLC separation has shown that [(11)C]MADAM was rapidly metabolized. A possible metabolic pathway is the S-oxidation which could lead to SOMADAM and SO2MADAM. In vitro evaluation of these two potential metabolites has shown that SOMADAM exhibited a good affinity for SERT and a good selectivity for SERT over NET and DAT.

METHODS

Comparative PET imaging studies in non-human primate brain with [(11)C]MADAM and [(11)C]SOMADAM were carried out, and plasma samples were analyzed using reverse phase HPLC. We have explored the metabolism of [(11)C]MADAM in rat brain with a view to understand its possible interference for brain imaging with PET.

RESULTS

PET imaging studies in non-human primate brain using [(11)C]SOMADAM indicated that this tracer does not bind with high amounts to brain regions known to be rich in SERT. The fraction of [(11)C]SOMADAM in non-human primate plasma was approximately 5% at 4min and 1% at 15min after [(11)C]MADAM injection. HPLC analysis of brain sample after [(11)C]MADAM injection to rats demonstrated that [(11)C]SOMADAM was not detected in the brain.

CONCLUSIONS

(11)C]SOMADAM is not superior over [(11)C]MADAM as a SERT PET radioligand. Nevertheless, [(11)C]SOMADAM has been identified as a minor labeled metabolite of [(11)C]MADAM measured in monkey plasma. [(11)C]SOMADAM was not detected in rat brain.

Synthesis of tropane and nortropane analogues with phenyl substitutions as serotonin transporter ligands

The effects of structural modifications of 2 beta-carbomethoxy-3 beta-phenyl tropane analogues were evaluated on in vitro affinity to the dopamine (DAT) and serotonin (5-HTT) transporters in rat brain tissue. The introduction of a large alkyl group at the 4'-position of the phenyl ring, affording 2 beta-carbomethoxy-3 beta-(4'-alkylphenyl) tropane, diminished the affinity for the DAT whereas moderate 5-HTT affinity was obtained. The introduction of an iodine at the 3'-position of the 4'-alkylphenyl, affording 2 beta-carbomethoxy-3 beta-(3'-iodo-4'-alkylphenyl) tropane, and N-demethylation, affording 2 beta-carbomethoxy-3 beta-(3'-iodo-4'-alkylphenyl) nortropane, improved affinity and specificity for the 5-HTT. It could be assumed from these results that the combination of these three modifications of tropane structure yielded highly selective compounds for the 5-HTT. Of the new compounds synthesized, the most selective cocaine derivative, 2 beta-carbomethoxy-3 beta-(3'-iodo-4'-isopropylphenyl) nortropane (8d) labeled with iodine-123 or carbon-11, could be a potential ligand for exploration of the 5-HT transporter by SPET or PET.

A selective radiobrominated cocaine analogue for imaging of dopamine uptake sites: pharmacological evaluation and PET experiments

(E)-N-(3-bromoprop-2-enyl)-2beta-carbomethoxy-3beta-4'-tolyl -nortropane or PE2Br, an analogue of cocaine was labelled with the positron emitter 76Br (T1/2=16 h) for pharmacological evaluation in the rat and PET investigation in the monkey. [76Br]PE2Br was obtained by electrophilic substitution from the tributylstannyl precursor with radiochemical yield of 80%. In vivo biodistribution studies of [76Br]PE2Br (20 MBq/nmol) in rats showed a high uptake in the striatum (2.2% ID/g tissue at 15 min p.i.). The striatum to cerebellum radioactivity ratio was 6 at 1 hour p.i. Striatal uptake of [76Br]PE2Br was almost completely prevented by pretreatment with GBR 12909, but citalopram and maprotiline had no effect, confirming the selectivity of the radioligand for the dopamine transporter. PET imaging of the biodistribution of [76Br]PE2Br in the baboon demonstrated rapid and high uptake in the brain (5% ID at 3 min p.i.). The striatal radioactivity concentration reached a plateau at 20 min p.i. (7% ID/100 mL). The uptake in the cortex and cerebellum was very low. A significantly higher uptake in the thalamus was observed. At 1h p.i., the striatum to cerebellum ratio and thalamus to cerebellum ratio were 8 and 1.9 respectively. In competition experiments the radioactivity in the striatum and the thalamus was displaced by 5 mg/kgof cocaine and 5 mg/kg of GBR 12909, but citalopram and maprotiline had no effect. These results showed that [76Br]PE2Br is in vivo a potent and selective radioligand suitable for PET imagingof the dopamine transporter.

Pharmacological characterization of (E)-N-(3-iodoprop-2-enyl)-2beta-carbomethoxy-3beta-(4'-methylphenyl)n ortropane as a selective and potent inhibitor of the neuronal dopamine transporter

The pharmacological properties of the iodinated derivative of cocaine (E)-N-(3-iodoprop-2-enyl)-2beta-carbomethoxy-3beta-(4'-me thylphenyl)nortropane (PE2I) were evaluated in vitro in the rat. Binding experiments on rat striatal membranes showed that PE2I selectively recognized the dopamine transporter (DAT) according to a single binding site model with high affinity (K(d) = 4 nM, B(max) = 12 pmol/mg protein). In the cortical membranes, the binding of PE2I was also selectively associated with the DAT (IC(50) for GBR 12909 = 6 nM versus more than 1000 nM for paroxetine), with similar affinity to that of the striatum. Autoradiographic experiments on rat brain sections with [(125)I]PE2I were in agreement with the localization of the DAT. In addition, PE2I was shown to be a potent inhibitor of dopamine uptake, with IC(50) values similar to those for GBR 12909 and 2beta-carbomethoxy-3beta-(4'-iodophenyl)-tropane (beta-CIT) (2-6 nM). All of these findings, combined with previously published data, support the use of PE2I as a selective and potent tool to study the DAT both in vivo and in vitro.

PET examination of three potent cocaine derivatives as specific radioligands for the serotonin transporter

Several positron emission tomography (PET) radioligands based on the aryl tropane structure have been used for studies on monoamine reuptake sites. RTI-364, RTI-330, and RTI-357 (3-beta-(4'-n-propyl-,4'-iso-propyl-, and 4'-iso-propenyl-phenyl)nortropane-2-beta-carboxylic acid methyl ester) are three recently synthesized cocaine analogues with higher affinity for the serotonin (5-HTT) than the dopamine transporter (DAT). Unlabelled RTI-364 and RTI-330 were prepared in a two-step synthesis. The key step was the addition of the appropriate propyl Grignard reagent to anhydroecgonine methyl ester. RTI-357 was prepared in a three-step synthesis with a palladium-catalyzed coupling reaction of beta-CIT and isopropenylzinc bromide as key step. Hydrolysis of the ester functions gave the carboxylic acid analogues of RTI-364, RTI-330, and RTI-357, which were labelled with 11C using [11C]methyl iodide in dimethyl formamide (DMF) and tetrabutylammonium hydroxide (TBAH) as base. All three compounds entered the monkey brain in a high degree (approximately 5-10%). There was a low uptake of [11C]RTI-364 in serotonin-rich brain areas, whereas [11C]RTI-330 and [11C]RTI-357 showed a marked uptake of radioactivity in the thalamus and the brainstem, regions known to contain serotonin transporters. Transient equilibrium was reached at 15 and 40 min for [11C]RTI-330 and [11C]RTI-357, respectively. After pretreatment with citalopram, the ratio of radioactivity in the thalamus and the brainstem to the cerebellum were markedly reduced for [11C]RTI-357 but not for [11C]RTI-330. The results indicate that [11C]RTI-357 is a potential PET radioligand for quantitation of the serotonin reuptake site.

Time course of changes in striatal dopamine transporters and D2 receptors with specific iodinated markers in a rat model of Parkinson's disease

The time course of the loss in presynaptic dopamine transporters (DAT) and of the increase in postsynaptic dopamine D2 receptors (D2R) was studied in a rat model of Parkinson's disease. For this, in vitro autoradiographic experiments were performed in the striatum using (E)-N-(3-iodoprop-2-enyl)-2beta-carbomethoxy-3beta-(4'-methy lphenyl) nortropane (PE2I), a new single photon emission tomography (SPET) ligand for DAT, and iodobenzamide (IBZM), a SPET ligand for D2R. A significant decrease in [125I]PE2I binding was observed as early as 24 h after 6-hydroxydopamine lesion, whereas no change occurred in [125I]IBZM binding. At 48 h postlesion, PE2I binding was 50% decreased, while IBZM binding was 30% increased. Between 3 and 14 days postlesion, PE2I binding had almost totally disappeared and IBZM binding remained increased by around 40-50%. From these animal experiments, it can be assumed that PE2I would be very efficient for the detection of a reduction in the number of DAT reflecting neuronal loss, thus allowing early diagnosis of Parkinson's disease. The exploration of both DAT and D2R would improve follow-up of this disease.

Visualization of the dopamine transporter in the human brain postmortem with the new selective ligand [125I]PE2I

Using a new, 125I-labeled, selective high affinity dopamine transporter ligand, N-(3-iodoprop-2E-enyl)-2beta-carbomethoxy-3beta-(4'-methy lph enyl)nort ropane (PE2I), the distribution of the dopamine transporter was characterized in the normal postmortem human brain using whole hemisphere autoradiography. PE2I was radioiodinated to high specific radioactivity (2200 Ci/mmol, 81 GBq/micromol). PE2I binds to the dopamine transporter with high potency and, in contrast to beta-CIT, it has very low affinities for the serotonin and noradrenaline transporters. The autoradiograms showed very intense binding in basal ganglia (putamen, nucleus caudatus, nucleus accumbens) and lower binding in substantia nigra. Very low or no binding was found in other brain structures, including the neocortex or cerebellum. The labeling of human dopamine transporters with [125I]PE2I was inhibited by the dopamine transporter inhibitors GBR 12909 and beta-CIT, but not by citalopram (serotonin transporter inhibitor) or maprotiline (noradrenaline transporter inhibitor). Possibly due to the relatively high lipophilicity of the compound (theoretical log p = 4.68), it accumulated slightly in white matter. Thus, in vitro autoradiography using [125I]PE2I provided detailed qualitative and quantitative evidence that the dopamine transporter is almost exclusively localized in the basal ganglia of the human brain. Moreover, the autoradiograms indicate that [11C]PE2I and [123I]PE2I should be suitable for the in vivo visualization of the human dopamine transporter with PET or SPECT, respectively.

Toxicity, mutagenicity, and behavioral effects of beta-CIT, a ligand for dopamine transporter exploration by SPECT

The cocaine analog beta-CIT is one of the most used compounds for SPET examination of the dopamine transporter in drug abuse and Parkinson's disease. However, the toxicity of this agent has not yet been studied. We report here acute toxicity, mutagenicity, and effect on locomotor activity of beta-CIT. Acute toxicity experiments were performed in mice and rats. The LD50 values were about 20 mg and 5 mg for mice and rats, respectively. There was no sex difference. The mutagenicity was evaluated using the Ames' test. No mutagenic effect was observed for beta-CIT. Effects on locomotor activity were measured in mice using the open-field test. beta-CIT increased locomotion (+65%) when injected at a dose of 0.312 mg/kg; the maximal increase (+205%) was observed at a dose of 1.25 mg/kg; at higher doses, the effect was decreased slightly. These pharmacological findings are in agreement with an inhibitory effect of beta-CIT at the dopamine transporter. We conclude that with no mutagenic effects and LD50 more than 6 orders of magnitude higher than the routinely used doses in PET or SPET, it can be assumed that beta-CIT can be safely used as a radioligand in humans.

Exploration of the dopamine transporter: in vitro and in vivo characterization of a high-affinity and high-specificity iodinated tropane derivative (E)-N-(3-iodoprop-2-enyl)-2beta-carbomethoxy-3beta-(4'-m ethylph enyl)nortropane (PE2I)

For the diagnosis and follow-up of neurodegenerative diseases, many cocaine derivatives have been proposed as radioligands to explore the dopamine transporter. As none of them have all the criteria of specificity and kinetics for human use, we have developed a new derivative, (E)-N-(3-iodoprop-2-enyl)-2beta-carbomethoxy-3beta-(4'-methy lphenyl)nortropane (PE2I), which displays promising properties. We report the characterization of PE2I in vitro on rat striatal membranes and in vivo in rats and in monkeys. PE2I had a high affinity (Kd = 0.09 +/- 0.01 nM) and high specificity for the dopamine transporter. In rats we observed a high accumulation in the striatum; by contrast, a very low fixation was measured in the cortex. Moreover, a preinjection of a saturating dose of GBR 12909 prevented the striatal accumulation of PE2I by 74%. These results confirmed the specificity of PE2I for the dopamine transporter. In vivo in monkeys, SPECT studies showed a high accumulation in striatum. Moreover, an equilibrium state was obtained 1 h after injection. PE2I seemed to be the most promising ligand for the dopamine transporter exploration by SPECT using a single-day protocol.

Pharmacokinetics and dosimetry of iodine-123 labelled PE2I in humans, a radioligand for dopamine transporter imaging

The iodine-123 labelled selective ligand N-(3-iodoprop-2E-enyl)-2-beta-carbomethoxy-3beta-(4-methylphenyl) nortropane ([123I]PE2I) was evaluated as a probe for in vivo dopamine transporter imaging in the human brain. Six healthy subjects were imaged with a high-resolution single-photon emission tomography scanner. Striatal radioactivity peaked at 1 h after injection. The background radioactivity was low. The volume of distribution in the striatum was 94+/-24 ml/ml. The results were compared with those of [123I]beta-CIT imaging. There was no significant uptake of [123I]PE2I in serotonin-rich regions such as the midbrain, hypothalamus and anterior gingulus, suggesting that in vivo binding is specific for the dopamine transporter. One main polar metabolite of [123I]PE2I was found in plasma, and the parent plasma concentration decayed rapidly. Radiation exposure to the study subject is 0.022+/-0.004 mSv/MBq (effective dose). The preliminary results suggest that [123I]PE2I is a selective SPET ligand for imaging striatal dopamine transporter density.

N-(3-lodoprop-2E-enyl)-2beta-carbomethoxy-3beta-(3',4'-dichloro phenyl)nortropane (beta-CDIT), a tropane derivative: pharmacological characterization as a specific ligand for the dopamine transporter in the rodent brain

N-(3-Iodoprop-2E-enyl)-2beta-carbomethoxy-3beta-(3',4'-dichl orophenyl)nortropane (beta-CDIT), a new iodinated tropane derivative, has been synthesized and radiolabeled with iodine. [125I]beta-CDIT was tested in vitro and ex vivo as a probe for the dopamine transporter site in the rat brain, and behavioral studies were performed in mice. Saturation studies in the striatum revealed that [125I]beta-CDIT bound to a single high-affinity site. The Kd value was 0.18 +/- 0.07 nM, and the corresponding Bmax value was 500 +/- 80 fmol/mg of protein. The pharmacological profile of specific [125I]beta-CDIT binding in the striatum was consistent with that of the dopamine transporter. In addition, competition studies in cerebral cortex regions with [3H]paroxetine and [3H]nisoxetine showed a very low affinity of beta-CDIT for the 5-hydroxytryptamine (Ki = 50 nM) and norepinephrine (Ki = 500 nM) transporters compared with beta-CIT (corresponding Ki values were 3 and 80 nM). In contrast, the competition of beta-CDIT with [3H]GBR 12935 in the striatal region (Ki = 29 nM) was of the same order of value as for beta-CIT (Ki = 27.5 nM). Behavioral experiments in mice showed that both beta-CDIT and beta-CIT induced stimulation of locomotor activity. Ex vivo autoradiographic studies in rats using [125I]beta-CDIT demonstrated high densities of [125I]beta-CDIT binding sites in areas known to be rich in dopaminergic innervation. Because of its high affinity and high selectivity for the dopamine transporter, [125I]beta-CDIT should be a valuable ligand for the exploration of the dopamine transporter with single-photon emission computed tomography.

A new iodinated tropane derivative (beta-CDIT) for in vivo dopamine transporter exploration: comparison with beta-CIT

SPECT exploration of the dopamine transporter with tropane derivatives such as beta-CIT has already produced very valuable information in humans. However, the high affinity of this tracer for both dopamine and serotonin transporters and its slow in vivo kinetics provide the best images in humans more than 20 h after injection. In order to improve those properties, we performed structural changes in the tropane structure in the phenyl and nitrogen substituents for higher affinity and specificity and obtained a promising ligand, 2 beta-carbomethoxy-3 beta-(3',4' diclorophenyl)-8-(3-iodoprop-2E-enyl) nortropane (beta-CDIT). This iodinated ligand was characterized in vitro and in vivo in the rat in comparison with beta-CIT. In vitro competition studies revealed that beta-CIT and beta-CDIT similarly inhibited the binding of [3H]GBR 12935 (Ki = 27.5 and 29.0 nM, respectively). In contrast, competition studies with [3H]paroxetine and [3H]nisoxetine showed that beta-CDIT had a lower affinity for the serotonin transporter than beta-CIT (Ki = 50 and 3 nM, respectively) and also a lower affinity for the noradrenaline transporter than beta-CIT (Ki = 500 and 80 nM, respectively). In vivo studies in the rat showed that there was high and rapid uptake of [125I] beta-CDIT in the striatum. In addition, preinjection of GBR 12909 prevented accumulation of this ligand in the striatum by 80%, whereas only a 30% decrease was obtained for [125I] beta-CIT. It seems, therefore, that the combination of aromatic and nitrogen substitution improves the properties of tropane derivatives to provide an exclusive dopamine transporter ligand potentially usable in SPECT.

Synthesis and ligand binding of nortropane derivatives: N-substituted 2beta-carbomethoxy-3beta-(4'-iodophenyl)nortropane and N-(3-iodoprop-(2E)-enyl)-2beta-carbomethoxy-3beta-(3',4'-disubstituted phenyl)nortropane. New high-affinity and selective compounds for the dopamine transporter

Two novel series of iodinated N-substituted analogs of 2beta-carbomethoxy-3beta-(4'-iodophenyl)tropane (beta-CIT) and N-(3-iodoprop-(2E)-enyl)-2beta-carbomethoxy-3beta-(3',4'-dis ubstituted phenyl)nortropane were synthesized. They were evaluated for their inhibitory properties on dopamine (DA(T)), serotonin (5-HT(T)), and norepinephrine (NE(T)) transporters in rat brain homogenates using [3H]GBR-12935, [3H]paroxetine, and [3H]nisoxetine as specific ligands. All new N-substituted analogs of beta-CIT exhibited higher DAT selectivity over both 5-HT(T) and NE(T) than beta-CIT. Moreover compounds with the N-substituents propynyl (6), crotyl (4), 2-bromoprop-(2E)-enyl (5), and 3-iodoprop-(2E)-enyl (3d) showed similar to higher DA(T) affinities than beta-CIT (respectively 14, 15, 30, and 30 nM vs 27 nM). Compound 3d was found to be the most selective DA(T) agent of this series (5-HTT/DA(T) = 32.0 vs 0.1 for beta-CIT). The N-(3-iodoprop-(2E)-enyl) chain linked to the tropane nitrogen was therefore maintained on the tropane structure, and phenyl substitution was carried out in order to improve DA(T) affinity. K(i) values of N-(3-iodoprop-(2E)-enyl)-2beta-carbomethoxy-3beta-(3',4'-dis ubstituted phenyl)nortropanes revealed that phenyl, 4'-isopropyl, and 4'-n-propyl derivatives weakly inhibited specific binding to DA(T), whereas phenyl substitution with 4'-methyl (3c), 3',4'-dichloro (3b), and 4'-iodo (3d) yielded high-DA(T) reuptake agents with increased DA(T) selectivity compared to beta-CIT. These results demonstrate that the combination of a nitrogen and a phenyl substitution yields compounds with high affinity and selectivity for the dopamine transporter which are usable as SPECT markers for DA neurons.