Search

Me-QNB (N-methyl-quinuclidin-3-yl benzilate or N-methyl-quinuclidin-3-yl diphenylhydroxy acetate) is a hydrophilic, non-metabolized and highly specific muscarinic acetylcholinergic antagonist. Using this quaternary ammonium derivative of QNB, labelled with carbon-11, a positron-emitting isotope (half-life : 20.4 minutes), the potential for quantification of myocardial muscarinic receptors in vivo using the high-resolution, sensitive and quantitative imaging technique PET (positron emission tomography) was previously demonstrated in dogs and validated in humans. In this paper, the radiosynthesis of carbon-11-labelled Me-QNB is investigated and oriented towards the preparation of multi milliCuries of radiotracer. Typically, using no-carrier-added [11C]methyl triflate as the alkylating agent and 0.64 mg (1.89 µmol) of QNB as precursor for labelling at 100°C for 1 minute lead to a 48.5% +/−10% (15 runs) decay-corrected radiochemical yield (based on [11C]methyl triflate). 183 mCi (+/−39) of [11C]Me-QNB ([11C]-1) could be synthesized in only 27 to 28 minutes after EOB and occasionally, up to 340 mCi of [11C]Me-QNB ([11C]-1) were obtained, corresponding to a 85% decay-corrected yield. The associated decay-corrected specific radioactivities obtained were 2658 mCi/µmol (+/−971) at EOB. Copyright © 2001 John Wiley & Sons, Ltd.

[Tetrazoyl- 11 C]LY202157 8 was prepared via a three step synthesis from ethyl (3S,4aR,6S,8aR)-6-bromomethyl-2-methoxycarbonyl-1,2,3,4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylate 4. This bromo precursor was reacted with [ 11 C]hydrogen cyanide affording the corresponding [ 11 C]nitrile. Conversion to the tetrazole was achieved by treatment with azidotributyltin followed by hydrolysis with 6N hydrochloric acid at 200°C. After HPLC purification and analytical HPLC control, more than 370 MBq (10 mCi) of [tetrazoyl- 11 C] LY202157 were obtained after an overall 60 minute synthesis time with 38% yield (EOB) and specific activity of 25.9 GBq/μmol (700 mCi/μmol). Ex vivo biological studies showed that the [tetrazoyl- 11 C] LY202157 did not cross the brain blood barrier.

The BTCP derivative 1-11-(2-benzo[b]thiophenyl) cyclohexyl]-4-(2-hydroxyethyl) piperazine 2 showed, in vitro, high affinity and selectivity for the Dopamine transporter. In order to evaluate the potential of such a compound as an imaging tool for studying the dopaminergic system by Positron Emission Tomography (PET) the cold fluoroethyl BTCP piperazine 6 was synthesized. After checking the biological activity of the cold compound 6, the [ 18 F] analogue 7 was synthesized. The radiosynthesis was carried out by the nucleophilic substitution of 1-[1-(2-benzo[b]thiophenyl) cyclohexyl]-4-(2-chloroethyl) piperazine 5 with cyclotron-produced n.c.a. 18 F - , obtained by the (p,n) reaction on 18 O enriched water.

In an attempt to develop a specific probe of serotonin 5-HT3 receptors, various methods have been investigated to synthesize a radioiodinated derivative of the potent 5-HT3 antagonist zacopride. The direct iodination of 5-dechloro-zacopride 2 was performed using NaI in the presence of either chloramine T or iodo-beads. Irreproducible results or/and low yields were obtained by these methods. A third approach using a less oxidative medium allowed the synthesis of iodo-zacopride 4 from 2 with N-iodo-succinimide or with NaI and N-chlorosuccinimide. Using this third condition, high yield was obtained (49%). However, the radioiodinated compound was contaminated by some “cold” zacopride also formed during the reaction. A two step synthesis was successful to eliminate “cold” zacopride but in lower yield (2-10%). Thus, 4-amino-2-methoxy-benzoate 5 was iodinated and then coupled with 3-aminoquinuclidine 8 to give 5-iodo-zacopride 4. Radioactive synthesis was carried out in the same condition to give 5-[1251]-iodo-zacopride 1 with a yield of 98%. The two enantiomers R-and S-5-[1251]-iodo-zacopride were synthesized by direct iodination.

After verification of the biological activity of 3-iodo-PCP by binding studies to the N-methyl D-aspartate gated channel, 3-[125I]iodo-PCP was prepared by reaction of sodium iodide with a triazene precursor. The radiochemical yield was optimized and after HPLC purification, 3-[125I]iodo-PCP was obtained with a high radiochemical purity.

The synthesis of a photosensitive iodinated derivative of tetrabenazine, which should be able to lead to the identification of the vesicular catecholamine transporter of adrenal medulla by photoaffinity, is described in detail. The biological activity of this iodinated ligand was found to be significantly lower than that of non iodinated ligands.

The [3',5']-ditritio-alpha-fluoromethyl-tyrosine 4 (specific activity 15.0 Ci/mmol) has been synthesized and used as a radioactive probe for rat neuronal tyrosine hydroxylase (TH). The route of synthesis for the preparation of 3 and 4 allowed us to not only introduce a fluorine atom into 3/4 using an inorganic source of fluorine (CsF), but also to take advantage of the high-yielding cyclization of (alpha,beta)-acetamido alcohols mediated by diethylaminosulfur trifluoride (DAST) to give the corresponding oxazolines. The distribution and metabolism of 4 have been studied in control conditions within the rat locus caeruleus (LC). Intracisternal injection of 20 microCi of 4 was followed by a rapid disappearance of 4 (t1/2 = 1.5 h) and by a specific accumulation of radioactivity into the LC anatomical limits. This was investigated each 140 microns along the caudo-rostral axis of the noradrenergic nucleus. In each anatomical interval, its distribution correlated nicely with already described caudo-rostral distribution of TH in noradrenergic cells. Thus, 4 may provide a reliable measure of TH activity in such catecholamine structures.

In an attempt to visualize in vivo the dopamine transporter and evaluate its potential as an imaging tool for monitoring dopamine fiber degeneration by positron emission tomography, the 18F-positron-emitting analogue of 3-fluoromethyl-1-[1-(2-benzothienyl)-cyclohexyl]-piperidine, [18F]BTCP, was synthetized and tested in a primate model of hemiparkinsonism. [18F]BTCP was obtained from cyclotron-produced n.c.a. [18F]fluoride (110min half-life) and by nucleophilic substitution from 3-bromomethyl-BTCP with a radiochemical yield of 6% (decay-corrected). After intravenous injection, the cerebral distribution of the radioactivity was observed mainly in cortical areas and cerebral structures enriched in catecholamine reuptake sites such as the caudate-putamen complex and the thalamus. The binding ratio, defined with respect to the cerebellum (taken as a region of non-specific binding), was highest in the thalamus (1.42), intermediate in the putamen (1.36) and lowest in the caudate nucleus (1.17), suggesting that some specific binding occurs in these regions. After saturation of dopamine and norepinephrine transporters by nomifensine, the binding ratio in the thalamus, putamen and caudate nucleus striatum remained essentially unchanged in the non-lesioned hemisphere. When comparing binding ratios between the intact and the dopamine-denervated striatum, there was a modest loss of binding in the denervated striatum, suggesting that degeneration of dopaminergic fibers could be detected using 3-[18F]fluoromethyl-BTCP. However due to a high non-specific binding in vivo, the interest of 3-[18F]fluoromethyl-BTCP to image the dopamine reuptake system in vivo appears rather limited.

International audience; The binding properties of the 125I-labeled phencyclidine derivative N-[1-(3-[125I]iodophenyl)cyclohexyl]piperidine (3-[125I]iodo-PCP), a new ligand of the N-methyl-D-aspartate (NMDA)-gated ionic channel, were investigated. Association and dissociation kinetic curves of 3-[125I]iodo-PCP with rat brain homogenates were well described by two components. About 32% of the binding was of fast association and fast dissociation, and the remaining binding was of slow association and slow dissociation. Saturation curves of 3-[125I]iodo-PCP also were well described using two binding sites: one of a high affinity (KDH = 15.8 +/- 2.3 nM) and the other of a low affinity (KDL = 250 +/- 40 nM). 3-Iodo-PCP inhibited the binding of 3-[125I]iodo-PCP with inhibition curves that were well fitted by a two-site model. The binding constants (KiH, BmaxH; KiL, BmaxL) so obtained were close to those obtained in saturation experiments. Ligands of NMDA-gated ionic channels also inhibited the binding of 3-[125I]iodo-PCP with two constants, KiH and KiL. There was a very good correlation (r = 0.987) between the affinities of these ligands to bind to NMDA-gated ionic channels and their potencies to inhibit the binding of 3-[125I]iodo-PCP with a high affinity. Moreover, the regional distribution of the high-affinity binding of 3-[125I]-iodo-PCP paralleled that of tritiated N-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP). In contrast to that of [3H] TCP, the binding of 3-[125I]iodo-PCP to well-washed rat brain membranes was fast and insensitive to glutamate and glycine.(ABSTRACT TRUNCATED AT 250 WORDS)

The synthesis of an alpha adrenergic antagonist labelled with tritium: [(2,3-3H)-1,4-benzodioxanye] WB 4101 or 2-N[2,6 dimethoxyphenoxyethye] aminomethyl 2,3-3H-1,4 benzodioxan is described. 2-cyano-1,4-benzodioxan: 1 was prepared from catechol and 2-chloroacrylonitrille and then converted to 2-cyano-1,4-benzodioxin: 2 by the successive actions of N-bromosuccinimide in CCl4 and sodium iodide in acetone. 2 was reduced with lithium aluminium hydride into 2-aminomethyl-1,4-benzodioxin: 3 and then converted to 2-[N-(2,6-dimethoxyphenoxyethyl)-aminomethyl]-1,4-benzodioxin: 4a, and its hydrochloride: 4b. The catalytic reduction of 4a, 4b with tritium in various and in presence of various catalysts was studied. The best result was obtained with Wilkinson's catalyst in benzene giving rise to [(2,3-3H)1,4-benzodioxanyl]WB 4101 with a specific activity of 60 CilmMole. 3H.NMR and MS analyses are also described.

Four methods of syntheses of IDAZOXAN (RX 781094): |2-(2-imidazolinyl)-1,4-benzodioxane| labelled with tritium: 1 at high specific activity, a selective radioligand presynaptic α2- adrenoreceptor antagonist are described. 2-carbethoxy-1,4-benzodioxine 5 was treated with tritium in benzene in presence of Wilkinson's catalyst leading to 2-carbethoxy [2,3-3H]-1,4-benzodioxane: 6 spec. act. 60 Ci/m which was then condensed in toluene with a reagent made from trimethylaluminium and ethylenediamine giving rise to 1 specific activity 44 Ci/mMole. 2-(carbamido N-cyanomethyl)1,4-benzodioxine: 2 was prepared from 2-chlorocarbonyl-1,4-benzodioxine: 10 and 2-aminoacetonitrile. 2 was reduced with tritium in trifluoroacetic acid in presence of Pd/C giving 8 which, without purification was ring closed into 2 by trimethylaminium treatment. Spec. act. 35.5 Ci/mMole. When reduced with tritium in presence of Wilkinson's catalyst, 7 gave 9 spect. act. 55 Ci/mMole. After purification 9 was again reduced with tritium in trifluoroacetic acid in presence of Pd/C, or PtO2, or Rh/Al2O3 giving 8 which was cyclised into 1 by trimethylaluminium treatment in toluene. spec. act. of 1 was 41.5 - 39 Curies/mMole. 1 was purified by successive preparative thin layer chromatography. The radiochemical purity checked by TLC and PLC was better than 99%. Some observations on the rate of self radiolysis of 1 are reported.

Serotonin 5-HT1A receptors in rat hippocampal membranes were solubilized by 10 mM 3-[3-(cholamidopropyl)dimethylammonio]-1-propane sulfonate (CHAPS) and chromatographed on various gels in an attempt to design a relevant protocol for their (partial) purification. In particular, an affinity gel made of the 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) derivative 8-methoxy-2-[(N-propyl, N-butylamino)amino]tetralin (8-MeO-N-PBAT) coupled to Affigel 202 was specially developed for this purpose. First, studies of the effects of various compounds (detergents, lipids, reducing agents, sugars, etc.) on the specific binding of [3H]8-OH-DPAT and on the rate of heat-induced inactivation of solubilized 5-HT1A sites led to a buffer composed of 50 mM Tris-HCl, 50 microM dithiothreitol, 1 mM CHAPS, 10% glycerol, 0.1 mM MnCl2, and 50 micrograms/ml of cholesteryl hemisuccinate, pH 7.4, ensuring a high degree of stability of solubilized 5-HT1A sites, compatible with chromatographic analyses for 2-4 days at 4 degrees C. Adsorption and subsequent elution of [3H]8-OH-DPAT specific binding sites were found with several chromatographic gels, including wheat germ agglutinin-agarose, phenyl-Sepharose, hydroxylapatite-Ultrogel, diethylaminoethyl (DEAE)-Sepharose, and DEAE-Sephacel. Similarly, 8-MeO-N-PBAT-Affigel 202 allowed the adsorption and subsequent elution (by 1 mM 5-HT) of active 5-HT1A binding sites solubilized from rat hippocampal membranes. The two-step chromatography using 8-MeO-N-PBAT-Affigel 202 followed by wheat germ agglutinin-agarose gave a fraction enriched (by at least 400-fold) in 5-HT1A sites. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of this partially purified fraction revealed a major protein band with Mr close to 60,000.

The present knowledge of central serotonin (5-hydroxytryptamine; 5-HT) receptors largely derives from binding studies with appropriate radioli-gands. Thus it is commonly accepted that two main classes of 5-HT receptors, 5-HT1 and 5-HT2, exist in the CNS (Peroutka and Snyder, 1979). Apparently the 5-HT2 class is homogeneous and corresponds to binding sites with low (micromolar) affinity for 5-HT and most agonists, and high (nanomolar) affinity for selective antagonists. In contrast, the 5-HT1 class, with high (nanomolar) affinity for 5-HT, is heterogeneous, and four binding site subtypes, 5-HT1A, 5-HT1B, 5-HT1C and 5-HT1D, have been identified so far. Correspondence of these binding sites with authentic receptors has been demonstrated: for example, the 5-HT1A site which is negatively coupled to adenylate cyclase (De Vivo and Maayani, 1986), and the 5-HT1C site, which is positively coupled to phospholipase C in the choroid plexus (Conn et al., 1986). Finally, another class, 5-HT3 receptors, has been identified in the periphery (Richardson and Engel, 1986), and their existence within the CNS has recently been reported (see Tyers et al., this volume).