Synthesis of 2beta-acyl-3beta-(substituted naphthyl)-8-azabicyclo[3.2.1]octanes and their binding affinities at dopamine and serotonin transport sites

Synthesis and structure-activity relationship studies of 3-biaryl-8-oxabicyclo[3.2.1]octane-2-carboxylic acid methyl esters

Stille cross coupling protocols were utilized for the synthesis of 3-(biaryl)-8-oxabicyclo[3.2.1]oct-2-ene-2-carboxylic acid methyl esters, which furnished products in high yields where in some cases Suzuki coupling under the conditions utilized provided complex reaction mixture. Samarium iodide reduction of the resulting coupling products produced both of the 2β-carbomethoxy-3-biaryl-8-oxabicyclo[3.2.1]octane diastereomers and the 2α-carbomethoxy-3-biaryl-8-oxabicyclo[3.2.1]octane diastereomers. Among the series synthesized, the benzothiophene substituted compounds demonstrated significant binding profiles of inhibition of WIN 35,438 with 177-fold selectivity for DAT versus SERT.

Singular value decomposition analysis of the torsional angles of dopamine reuptake inhibitor GBR 12909 analogs: effect of force field and charges

Three-dimensional quantitative structure-activity relationship (3D-QSAR) analysis of large, flexible molecules, such as the dopamine reuptake inhibitor GBR 12909 (1), is complicated by the fact that they can take on a wide range of closely-related conformations. The first step in the analysis is to classify the conformers into groups. Over 600 conformers each of a piperazine (2) and piperidine (3) analog of 1 were generated by random search conformational analysis using the Merck Molecular Force Field (MMFF94). Singular value decomposition (SVD) was used to group the conformers of 2 and 3 by the similarity of their non-ring torsional angles. SVD uncovered subtle differences in their conformer populations due to that fact that the conformers separate along different principal components, and ultimately to the fact that different torsional angles are the chief contributors to these components. The results were compared to our previous SVD analysis (Fiorentino, et al., Journal of Computational Chemistry, 2006, 27, 609-620) of conformer populations of 2 and 3 generated by the Tripos force field and Gasteiger-Hückel charges. Except for the dominant contribution of angle B3 to principal component 8 seen with both force fields, the angles which are chiefly responsible for the grouping of the conformers of 2 and 3 are different with both force fields. This illustrates that SVD is useful in identifying unique groupings of conformers in large data sets of flexible molecules-a first step in selecting representative conformers for 3D-QSAR modeling studies.

Hierarchical clustering analysis of flexible GBR 12909 dialkyl piperazine and piperidine analogs

Pharmacophore modeling of large, drug-like molecules, such as the dopamine reuptake inhibitor GBR 12909, is complicated by their flexibility. A comprehensive hierarchical clustering study of two GBR 12909 analogs was performed to identify representative conformers for input to three-dimensional quantitative structure-activity relationship studies of closely-related analogs. Two data sets of more than 700 conformers each produced by random search conformational analysis of a piperazine and a piperidine GBR 12909 analog were studied. Several clustering studies were carried out based on different feature sets that include the important pharmacophore elements. The distance maps, the plot of the effective number of clusters versus actual number of clusters, and the novel derived clustering statistic, percentage change in the effective number of clusters, were shown to be useful in determining the appropriate clustering level. Six clusters were chosen for each analog, each representing a different region of the torsional angle space that determines the relative orientation of the pharmacophore elements. Conformers of each cluster that are representative of these regions were identified and compared for each analog. This study illustrates the utility of using hierarchical clustering for the classification of conformers of highly flexible molecules in terms of the three-dimensional spatial orientation of key pharmacophore elements.

Singular value decomposition of torsional angles of analogs of the dopamine reuptake inhibitor GBR 12909

Analysis of large, flexible molecules, such as the dopamine reuptake inhibitor GBR 12909 (1), is complicated by the fact that they can take on a wide range of closely related conformations. The first step in the analysis is to classify the conformers into groups. Here, Singular Value Decomposition (SVD) was used to group conformations of GBR 12909 analogs by the similarity of their nonring torsional angles. The significance of the present work, the first application of SVD to the analysis of very flexible molecules, lies in the development of a novel scaling technique for circular data and in the grouping of molecular conformations using a technique that is independent of molecular alignment. Over 700 conformers each of a piperazine (2) and piperidine (3) analog of 1 were studied. Analysis of the score and loading plots showed that the conformers of 2 separate into three large groups due to torsional angles on the naphthalene side of the molecule, whereas those of 3 separate into nine groups due to torsional angles on the bisphenyl side of the molecule. These differences are due to nitrogen inversion at the unprotonated piperazinyl nitrogen of 2, which results in a different ensemble of conformers than those of 3, where no inversion is possible at the corresponding piperidinyl carbon.

Synthesis and monoamine transporter affinity of 3-aryl substituted trop-2-enes

A series of new 3-aryl-tropanes have been synthesized, and their affinities and selectivities were evaluated for monoamine transporters. (1RS)-3-(Fluoren-2-yl)-8-methyl-8-azabicyclo[3.2.1]oct-2-ene exhibited the highest affinity for the human serotonin transporter (IC(50)=14.5nM). It is also 52-fold and 230-fold selective over human dopamine and norepinephrine transporters, respectively.

Effects of long-term biogenic amine transporter blockade on receptor/G-protein coupling in rat brain

This study examines the effect of long-term elevation of brain monoamine levels on receptor/G-protein coupling by chronic administration of a highly potent tropane analog, WF-23 (2beta-propanoyl-3beta-(2-naphthyl) tropane). WF-23 blocks dopamine, serotonin and norepinephrine transporters with high affinity in vitro, and blocks transporters for at least two days following a single in vivo administration. Rats were chronically treated for 15 days with 1mg/kg WF-23, injected i.p. every two days. Receptor activation of G-proteins was determined by [35S]GTPgammaS autoradiography in brain sections for D2, 5-HT1A and alpha2-adrenergic receptors, as well as mu opioid receptors as a non-monoamine receptor control. Chronic treatment with WF-23 produced significant reductions in D2, 5-HT1A, and alpha2-adrenergic receptor-stimulated [35S]GTPgammaS binding in caudate/putamen, hippocampus and amygdala, respectively. There were no effects of WF-23 treatment on mu opioid-stimulated [35S]GTPgammaS binding. Additionally, there was no effect of WF-23 treatment on D2 receptor binding, as determined by [3H]spiperone autoradiography. These data show that chronic blockade of monoamine transporters produces specific uncoupling of receptors and G-proteins in specific brain regions in the absence of receptor downregulation.

Conformational analysis of methylphenidate: comparison of molecular orbital and molecular mechanics methods

Methylphenidate (MP) binds to the cocaine binding site on the dopamine transporter and inhibits reuptake of dopamine, but does not appear to have the same abuse potential as cocaine. This study, part of a comprehensive effort to identify a drug treatment for cocaine abuse, investigates the effect of choice of calculation technique and of solvent model on the conformational potential energy surface (PES) of MP and a rigid methylphenidate (RMP) analogue which exhibits the same dopamine transporter binding affinity as MP. Conformational analysis was carried out by the AM1 and AM1/SM5.4 semiempirical molecular orbital methods, a molecular mechanics method (Tripos force field with the dielectric set equal to that of vacuum or water) and the HF/6-31G* molecular orbital method in vacuum phase. Although all three methods differ somewhat in the local details of the PES, the general trends are the same for neutral and protonated MP. In vacuum phase, protonation has a distinctive effect in decreasing the regions of space available to the local conformational minima. Solvent has little effect on the PES of the neutral molecule and tends to stabilize the protonated species. The random search (RS) conformational analysis technique using the Tripos force field was found to be capable of locating the minima found by the molecular orbital methods using systematic grid search. This suggests that the RS/Tripos force field/vacuum phase protocol is a reasonable choice for locating the local minima of MP. However, the Tripos force field gave significantly larger phenyl ring rotational barriers than the molecular orbital methods for MP and RMP. For both the neutral and protonated cases, all three methods found the phenyl ring rotational barriers for the RMP conformers/invertamers (denoted as cte, tte, and cta) to be: cte, tte > MP > cta. Solvation has negligible effect on the phenyl ring rotational barrier of RMP. The B3LYP/6-31G* density functional method was used to calculate the phenyl ring rotational barrier for neutral MP and gave results very similar to those of the HF/6-31G* method.

Synthesis of methylphenidate analogues and their binding affinities at dopamine and serotonin transport sites

The rhodium(II)-catalyzed intermolecular C-H insertion of methyl aryldiazoacetates with either N-Boc-piperidine or N-Boc-pyrrolidine followed by deprotection with trifluoroacetic acid is a very direct method for the synthesis of methylphenidate analogues. By using either dirhodium tetraacetate or dirhodium tetraprolinate derivatives as catalyst, either the racemic or enantioenriched methylphenidate analogues can be prepared. The binding affinities of the methylphenidate analogues to both the dopamine and the serotonin transporters are described. The most notable compounds are the erythro-(2-naphthyl) analogues which display high binding affinity and selectivity for the serotonin transporter.

Further Structurally Constrained Analogues of cis-(6-Benzhydrylpiperidin-3-yl)benzylamine with Elucidation of Bioactive Conformation: Discovery of 1,4-Diazabicyclo[3.3.1]nonane Derivatives and Evaluation of Their Biological Properties for the Monoamine Transporters

Our structure-activity relationship (SAR) study on piperidine analogues for monoamine transporters led to the development of a series of 3,6-disubstituted piperidine derivatives, structurally constrained versions of flexible piperidine analogues, with preferential affinity for the dopamine transporter (DAT). In our attempt to further rigidify this structure to study influence of rigidity on binding and in vivo activity, we have developed a series of 4,8-disubstituted 1,4-diazabicyclo[3.3.1]nonane derivatives. All synthesized derivatives were tested for their affinity at the DAT, serotonin transporter (SERT), and norepinephrine transporter (NET) in the brain by measuring their potency in competing for the binding of [(3)H]WIN 35, 428, [(3)H]citalopram, and [(3)H]nisoxetine, respectively. Selected compounds were also tested for their ability to inhibit uptake of [(3)H]DA. The SAR study led to the discovery of a potent lead compound (-)-S,S-10c which exhibited high affinity and selectivity for the DAT (IC(50) = 22.5 nM; SERT/DAT = 384 and NET/DAT > 444). It is interesting to note that both (-)-10c and the lead compound from the 3,6-disubstituted series (-)-2 exhibited highest activity in their (S,S) isomer indicating similar requirement of regiospecificity for maximum interaction. Overall, our current SAR results corresponded well with the results from less constrained 3,6-disubstituted versions of these molecules albeit the former class exhibited more stringent requirement in molecular structure for activity. However, the potent compounds in the current series exhibited greater selectivity for the DAT compared to their corresponding lesser constrained 3,6-disubstituted versions indicating an effect of rigidity in selective interaction with the transporter proteins. In an effort to elucidate the bioactive conformational structure of the lead molecules in the current and the 3,6-disubstituted series, a preliminary molecular modeling study was carried out where the most rigid derivative (-)-10c was used as a template structure. Compounds (-)-2 and (-)-10c exhibited stimulant activity in locomotor tests in mice in which (-)-2 exhibited a slower onset and longer duration of action compared to (-)-10c. Both compounds occasioned complete cocaine-like responding in mice trained to discriminate 10 mg/kg ip cocaine from vehicle.

Synthesis, molecular modeling, and biological studies of novel piperidine-based analogues of cocaine: evidence of unfavorable interactions proximal to the 3alpha-position of the piperidine ring

A qualitative model for the binding pocket proximal to the 3alpha-substituent of the piperidine-based monoamine transporter ligands was proposed and tested. Based on this model, a new series of druglike 3alpha-modified piperidine-based analogues of cocaine were designed, synthesized, and studied for their ability to inhibit reuptake of DA, 5-HT, and NE by the DA, 5-HT, and NE transporters. We found that the insertion of at least one additional methylene group between the piperidine ring and the polar group in the 3alpha-substituent dramatically improves the activity of the compounds that are generally inactive without this additional linker. Molecular modeling analysis showed that the more flexible 3alpha-substituents can avoid unfavorable interactions with the binding sites of DAT, SERT, and NET. The present results may have important implications for the elucidation of the structural differences between DA, 5-HT, and NE transporters and for the further design of new leads for development of cocaine abuse medication as well as certain neurological disorders such as ADHD and depression.

Synthesis and evaluation of radiolabeled analogs of the antidepressant drug zimelidine as potential SPECT-ligands for the serotonin transporter

Z-3-(4-bromophenyl)-N,N-dimethyl-3-(3-pyridinyl)-2-propen-1-amine or zimelidine (ZIM) and its first metabolite nor-zimelidine, were radioiodinated via a nonisotopic exchange, using the Cu(I)-assisted nucleophilic labeling method. To evaluate their potential as SPECT ligands for the serotonin transporter (SERT), the biodistribution of both ligands was determined and pretreatment "blocking" studies performed. Both radioligands demonstrated a good brain penetration of 0.8-1% ID/g, stable after 60 min., p.i., and a brain/blood ratio of up to 3. In vivo brain distribution did not reveal specific binding. Blocking studies by pretreatment with a known SERT ligand, had minor influence on the uptake of [(123)I]I-ZIM, between the several isolated brain regions. It may therefore be concluded that [(123)I]I-ZIM and [(123)I]I-nor-ZIM do not appear to be promising SPECT ligands for the SERT.

2,3-Disubstituted 6-azabicyclo[3.2.1]octanes as novel dopamine transporter inhibitors

A series of cis and trans 3beta-aryl-2-carbomethoxy-6-azabicyclo[3.2.1]octanes, with different substitution at the para-position of the aryl group, were synthesized and examined for reuptake inhibition at the dopamine transporter (DAT). The potency for inhibition of DA reuptake was compared with that of cocaine to determine the significance of the replacement of the 8-azabicyclo[3.2.1]octane (tropane nucleus), displayed in cocaine, for the 6-azabicyclo[3.2.1]octane (normorphan framework). This bicyclic core structure constitutes a novel chemical scaffold in DAT inhibitor design, which may provide new insights into the 3D structure of the DAT and its interaction with cocaine and DA. Among these compounds, the trans-amine series 8 were the most potent ligands at the DAT. In particular, the normorphan analogue 8c (bearing a p-chloro substituent at the beta-aryl group, IC(50)=452 nM) displayed a potency that is in the same range as cocaine (IC(50)=459 nM) itself.

Bicyclo[2.2.1]heptanes as novel triple re-uptake inhibitors for the treatment of depression

A series of substituted naphthyl containing chiral [2.2.1] bicycloheptanes were prepared utilizing asymmetric Diels-Alder chemistry. This paper describes structure-activity relationships in this series. The N-methyl 2-naphthyl analogue (16d) and its des-methyl analogue (17d) are active triple re-uptake inhibitors both in vivo and in vitro.

Dopamine transporter as target for drug development of cocaine dependence medications

Because much evidence implicates the dopamine transporter in the reinforcing effects of cocaine, development of potential medications for cocaine dependence has included the dopamine transporter as a target. The present overview covers progress in the drug development area regarding several classes of dopamine uptake inhibitors, with an emphasis on structure-activity relationships that enhance potency and selectivity at transporters for dopamine compared with those for serotonin or norepinephrine. The following categories of compounds are covered: tropane, benztropine, 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine (GBR), methylphenidate, mazindol, and phencyclidine analogs. Activity at transporters as well as on behavior is highlighted.

Design and synthesis of an irreversible dopamine-sparing cocaine antagonist

Cocaine is a powerful reinforcer and stimulant that binds to specific recognition sites associated with monoamine transporters in the mammalian brain. The search for a functional antagonist to the addictive properties of cocaine has focused on the discovery of a molecule that can inhibit cocaine binding to the dopamine transporter (DAT) but continue to allow dopamine transport by the DAT. No such dopamine-sparing cocaine antagonist has been reported and it is becoming evident that dopamine-sparing antagonism of the pharmacological effects of cocaine by a classical antagonist may not be possible. Herein we present a new concept for the design of dopamine-sparing cocaine antagonists. A unique approach is utilized to deliver an inhibitor that binds irreversibly to the DAT, then cleaves and leaves behind a small fragment attached to the DAT that blocks access by cocaine but permits dopamine transport. The design of these compounds takes advantage of a cysteinyl sulfhydryl group in the DAT. This group is hypothesized to attack the incoming inhibitor and lead to selective inhibition of the cocaine binding site while sparing dopamine transport. This concept of a mechanism based irreversible dopamine-sparing cocaine antagonist has now been demonstrated to be viable and, as example, the unsaturated 6 showed inhibition of cocaine (63%) at the DAT after 24h incubation, while at that point considerably less inhibition of dopamine is manifested (23%). In contrast, the epoxide 7 showed a greater inhibition of dopamine reuptake than cocaine binding at 24h (68% versus 18%).

Probes for the dopamine transporter: new leads toward a cocaine-abuse therapeutic--A focus on analogues of benztropine and rimcazole

In an attempt to discover a cocaine-abuse pharmacotherapeutic, extensive investigation has been directed toward elucidating the molecular mechanisms underlying the reinforcing effects of this psychostimulant drug. The results of these studies have been consistent with the inhibition of dopamine uptake, at the dopamine transporter (DAT), which results in a rapid and excessive accumulation of extracellular dopamine in the synapse as being the mechanism primarily responsible for the locomotor stimulant actions of cocaine. Nevertheless, investigation of the serotonin (SERT) and norepinephrine (NET) transporters, as well as other receptor systems, with which cocaine either directly or indirectly interacts, has suggested that the DAT is not solely responsible for the reinforcing effects of cocaine. In an attempt to further elucidate the roles of these systems in the reinforcing effects of cocaine, selective molecular probes, in the form of drug molecules, have been designed, synthesized, and characterized. Many of these compounds bind potently and selectively to the DAT, block dopamine reuptake, and are behaviorally cocaine-like in animal models of psychostimulant abuse. However, there have been exceptions noted in several classes of dopamine uptake inhibitors that demonstrate behavioral profiles that are distinctive from cocaine. Structure-activity relationships between chemically diverse dopamine uptake inhibitors have suggested that different binding interactions, at the molecular level on the DAT, as well as divergent actions at the other monoamine transporters may be related to the differing pharmacological actions of these compounds, in vivo. These studies suggest that novel dopamine uptake inhibitors, which are structurally and pharmacologically distinct from cocaine, may be developed as potential cocaine-abuse therapeutics.

An analysis of the binding of cocaine analogues to the monoamine transporters using tensor decomposition 3-d QSAR

The conformation and alignment of cocaine analogues bound to the monoamine transporter proteins were explored using the tensor decomposition 3-D QSAR method. It is proposed from these calculations that the bound conformation of these ligands to the three transporter proteins has the 3beta-aryl substituent in a conformation in which the aryl group is orthogonal or approximately orthogonal to the tropane ring. Based on these results, rigid and semi-rigid tropane analogues were designed, synthesized and their affinities for the monoamine transporters were determined.

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.