Affinities of methylphenidate derivatives for dopamine,norepinephrine and serotonin transporters

We have synthesized several derivatives of dl-threo-methylphenidate (Ritalin) bearing substituents on the phenyl ring. IC₅₀ values for binding of these compounds to rat brain monoamine transporters were assessed using [³H]WIN 35,428 (striatal membranes, dopamine transporters, DAT), [³H]nisoxetine (frontal cortex membranes, norepinephrine transporters, NET) and [³H]paroxetine (brain stem membranes, 5HT transporters, 5HTT). Affinities (1/Ki) decreased in the order: DAT > NET ⪢ 5HTT. Substitution at the para position of dl-threo-methylphenidate generally led to retained or increased affinity for the dopamine transporter (bromo > iodo > methoxy > hydroxy). Substitution at the meta position also increased affinity for the DAT (m-bromo > methylphenidate; m-iodo-p-hydroxy > p-hydroxy). Substitution at the ortho position with bromine considerably decreased affinity. Similar IC₅₀ values for binding of o-bromomethylphenidate to the dopamine transporter were measured at 0, 22 and 37 degrees. N-Methylation of the piperidine ring of methylphenidate also considerably reduced affinity. The dl-erythro isomer of obromomethylphenidate did not bind to the DAT (IC₅₀ > 50,000 nM). Affinities at the dopamine and norepinephrine transporters for substituted methylphenidate derivatives were well correlated (r² = 0.90). Abilities of several methylphenidate derivatives to inhibit [³H]dopamine uptake in striatal synaptosomes corresponded well with inhibition of [³H]WIN 35, 428 binding. None of the compounds examined exhibited significant affinity to dopamine D1 or D2 receptors (IC₅₀ > 500 or 5,000 nM, respectively), as assessed by inhibition of binding of [³H]SCH 23390 or [¹²³I]epidepride, respectively, to striatal membranes.     

Cocaine inhibition of ligand binding at dopamine, norepinephrine and serotonin transporters: a structure-activity study

Structure-activity relationships for cocaine and analog binding at the dopamine, norepinephrine and serotonin transporters were determined. Cocaine inhibition of ligand binding to each of these sites has a stereospecific requirement for the levorotatory isomer. Binding potencies of cocaine derivatives involving N-substitution, C2 and C3 substituent modifications, however, revealed differences in structure-activity relationships for cocaine binding at the transporters. Removal of the N-methyl groups produced little change in binding potency at the dopamine transporter site but produced increases in binding potency at norepinephrine and serotonin transporter sites. Changes in structure at the C2 substituent produced changes in binding potency at the dopamine transporter which were generally similar in direction, but not necessarily in magnitude at the norepinephrine and serotonin transporters. Modifications to the C3 substituent, especially substitution of a hydroxyl moiety, produce changes in affinity at norepinephrine and serotonin transporters which are much larger than those observed at dopamine transporters. In general, our results indicate that unique structural requirements exist for each transporter site, but that cocaine binding at norepinephrine and dopamine transporters can be described by more similar structure-activity relationships than those found for the serotonin transporter. Requirements for cocaine binding to the dopamine transporter, which we have previously shown to be associated with the reinforcing effects of cocaine, include levorotatory stereospecificity, the benzene ring at C3, at least some portions of the tropane ring, and the presence of the C2 methyl ester group in the beta conformation.     

Des-keto lobeline analogs with increased potency and selectivity at dopamine and serotonin transporters

A series of des-keto lobeline analogs has been synthesized and evaluated for their ability to inhibit the dopamine transporter (DAT) and serotonin transporter (SERT) function and for their affinity for the synaptic vesicle monoamine transporter (VMAT2), as well as for alpha4beta2( *) and alpha7( *) neuronal nicotinic acetylcholine receptors (nAChRs). The enantiomers 8R-hydroxylobel-9-ene (3a) and 10S-hydroxylobel-7-ene (3c) exhibited high potency and selectivity at SERT and DAT, respectively.     

The mechanistic basis for noncompetitive ibogaine inhibition of serotonin and dopamine transporters

Ibogaine, a hallucinogenic alkaloid proposed as a treatment for opiate withdrawal, has been shown to inhibit serotonin transporter (SERT) noncompetitively, in contrast to all other known inhibitors, which are competitive with substrate. Ibogaine binding to SERT increases accessibility in the permeation pathway connecting the substrate-binding site with the cytoplasm. Because of the structural similarity between ibogaine and serotonin, it had been suggested that ibogaine binds to the substrate site of SERT. The results presented here show that ibogaine binds to a distinct site, accessible from the cell exterior, to inhibit both serotonin transport and serotonin-induced ionic currents. Ibogaine noncompetitively inhibited transport by both SERT and the homologous dopamine transporter (DAT). Ibogaine blocked substrate-induced currents also in DAT and increased accessibility of the DAT cytoplasmic permeation pathway. When present on the cell exterior, ibogaine inhibited SERT substrate-induced currents, but not when it was introduced into the cytoplasm through the patch electrode. Similar to noncompetitive transport inhibition, the current block was not reversed by increasing substrate concentration. The kinetics of inhibitor binding and dissociation, as determined by their effect on SERT currents, indicated that ibogaine does not inhibit by forming a long-lived complex with SERT, but rather binds directly to the transporter in an inward-open conformation. A kinetic model for transport describing the noncompetitive action of ibogaine and the competitive action of cocaine accounts well for the results of the present study.     

Studies on a series of milnacipran analogs containing a heteroaromatic group as potent norepinephrine and serotonin transporter inhibitors

A series of milnacipran analogs containing a heteroaromatic group were synthesized and studied as monoamine transporter inhibitors. Many compounds exhibited higher potency than milnacipran at NET and NET/SERT with no significant change in lipophilicity. For example, compound R-26f was about 10-fold more potent than milnacipran with IC(50) values of 8.7 and 26nM at NET and SERT, respectively.     

Synthesis of enantiomerically pure lysophosphatidylinositols and alkylphosphoinositols

A convenient synthesis for enantiomeric pure

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,

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(or and (or has been described. Starting from myo-inositol, penta-O-acetyl-myo-inositol was made in five steps. Then enantiomeric purification was done by a diastereomeric salts separation method, and the purity of each enantiomer was spectroscopically measured (¹⁹F-NMR). The phosphodiester was made via phosphoramidites. The enantiomeric products (>99% optical purity) of all compounds were easily obtained in large quantities (5–10 g). Synthetic phosphatidylinositol analogues of precisely defined structure and configuration are interesting tools for studying signal transduction mechanism and cell activity modulation.     

Synthesis of enantiomerically pure D-FDOC, an anti-HIV agent

The beta-D-enantiomer of FDOC (2',3'-dideoxy-5-fluoro-oxacytidine) exhibits potent anti-HIV-1 activity. It was obtained in optically pure form by employing a tandem kinetic resolution/chiral salt crystallization protocol. In addition, conditions were developed that allowed the unwanted butyrate ester of the L-enantiomer of FDOC to be racemized. This material could then be recycled in future resolutions.     

Calciumm-dependent release of endogenous serotonin,dopamine and norepinephrine from nerve endings

The release of endogenous serotonin, dopamine, norepinephrine and 5-hydroxyindoleacetic acid was studied in static incubations of synaptosome (P₂) preparations from the telencephalon of the rat. Elevated potassium medium specifically stimulated the release of the biogenic monoamines while the deaminated metabolite of serotonin was not effected. The release of the monoamines was also sensitive in part to the presence of calcium in the incubation medium.     

Bicyclo[3.2.1]octanes: synthesis and inhibition of binding at the dopamine and serotonin transporters

Herein we report the synthesis of a series of bicyclo[3.2.1]octanes and their binding characteristics at the dopamine and serotonin transporters. The data confirm that a heteroatom at position 8 of the tropane nucleus is not a prerequisite for binding since the bicyclo[3.2.1]octanes prove potent inhibitors of both transporters. Therefore the three-dimensional topology of the ligand may be more important than specific functionality with respect to stereospecific binding at the acceptor site.     

Synthesis of enantiomerically pure isotopomers of 2-phenylpropionic acids

A series of enantiomerically pure [D,(13)C]-labeled isotopomeric 2-phenylpropionic acids were efficiently synthesized using a diastereoselective alkylation and kinetic resolution strategy.     

Synthesis of new enantiomerically pure macrocycles containing phenanthroline subunits

New enantiomerically pure macrocycles have been prepared by combining 1,10-phenanthroline 2,9-dicarboxylic acid and two alpha-amino-acids linked through spacers. Different diamine linkers have been employed in order to modify the dimensions and the properties of the macrocycles whose structures have been studied by 1H and 13C NMR spectroscopy. The ability of the (L)-valine containing macrocycles to bind metal ions and phenolic molecules has been investigated by 1H NMR experiments and Molecular Mechanics calculations.     

Synthesis of iodinated 3beta-aryltropanes with selective binding to either the dopamine or serotonin transporters

Iodinated 3beta-aryltropanes functionalized appropriately at the 2beta-, 8- and aryl positions display selective binding to either the dopamine or serotonin transporters.     

Identification of 1S,2R-milnacipran analogs as potent norepinephrine and serotonin transporter inhibitors

A series of milnacipran analogs were synthesized and studied as monoamine transporter inhibitors, and several potent compounds with moderate lipophilicity were identified from the 1S,2R-isomers. Thus, 15l exhibited IC(50) values of 1.7nM at NET and 25nM at SERT, which were, respectively, 20- and 13-fold more potent than 1S,2R-milnacipran 1-II.     

Synthesis of 1,1,2-trisubstituted cyclopropane nucleosides in enantiomerically pure forms

Abstract

Due to the unique rigid and small steric feature of cyclopropane, cyclopropane nucleosides (CPNs) in which the ribose (deoxyribose) of nucleosides are replaced by a hydroxy-substituted cyclopropane, are of great biological interest. Novel 1,1,2-trisubstituted cyclopropane nucleosides were synthesized in enantiomerically pure forms as potential antiviral agents. In the synthesis, two cyclopropane tosylates, which were prepared from chiral cyclopropane lactones previously reported by us, were used effectively as common intermediates for the CPNs. These CPNs are also potentially useful as nucleoside units to incorporate into oligonucleotides in nucleic acids chemotherapy studies.     

Synthesis of 8-thiabicyclo[3.2.1]octanes and their binding affinity for the dopamine and serotonin transporters

Cocaine is a potent stimulant of the central nervous system. Its reinforcing and stimulant properties have been associated with inhibition of the dopamine transporter (DAT) on presynaptic neurons. In the search for medications for cocaine abuse, we have prepared 2-carbomethoxy-3-aryl-8-thiabicyclo[3.2.1]octane analogues of cocaine. We report that this class of compounds provides potent and selective inhibitors of the DAT and SERT. The selectivity resulted from reduced activity at the SERT. The 3beta-(3,4-dichlorophenyl) analogue inhibits the DAT and SERT with a potency of IC(50)=5.7 nM and 8.0 nM, respectively. The 3-(3,4-dichlorophenyl)-2,3-unsaturated analogue inhibits the DAT potently (IC(50)=4.5 nM) and selectively (>800-fold vs SERT). Biological enantioselectivity of DAT inhibition was limited for both the 3-aryl-2,3-unsaturated and the 3alpha-aryl analogues (2-fold), but more robust (>10-fold) for the 3beta-aryl analogues. The (1R)-configuration provided the eutomers.     

Affinities of dopamine analogs for monoamine granular and plasma membrane transporters: Implications for PET dopamine studies

Affinities of dopamine (DA) analogs to both granular and plasma membrane uptake transporters were measured in vitro by inhibition of [³H]DA uptake in bovine chromaffin granule ghosts and C6 glial cells transfected with cDNA for the rat presynaptic dopamine transporter, respectively. Five amines were studied: DA, 6-fluorodopamine (6FDA), m-tyramine (MTA), 6-fluoro-m-tyramine (6FMTA), and β-fluoromethylene-m-tyramine (FMMTA). Direct uptake of ¹⁸F labeled 6FDA and 6FMTA was also measured in the chromaffin granule system and compared with [³H]DA uptake. Results show that the transporter affinities of 6FDA and MTA were similar to that of DA in both transport systems while affinities of 6FMTA and FMMTA were lower. Furthermore while the direct uptake of DA and FDA in chromaffin granules were essentially identical and significantly reserpine-inhibitable, the direct uptake of 6FMTA was about 15-fold less and only minimally sensitive to reserpine pretreatment. Thus, although vesicular protection and reuptake may influence the turnover of FDA in 6-fluoroDOPA studies, they are unlikely to be important determinants of the kinetics of the slowly clearing components in studies with either 6-fluoro-m-tyrosine (6FMT) or 6-fluoro-β- fluoro-methylene-m-tyrosine (6FFMMT), the bioprecursors of 6FMTA and 6-fluoro-FMMTA, respectively. These results are consistent with the finding that the longterm component in 6FMT PET studies is 6-fluoro-hydroxyphenylacetic acid (6FHPAC), which can be explained by the lack of vesicular protection of 6FMTA from MAO oxidation.