Tuning the Selectivity of Monoamine Transporter Inhibitors by the Stereochemistry of the Nitrogen Lone Pair

Solution- and solid-state conformations of C(α)-alkyl analogues of methylphenidate (Ritalin) salts: avoidance of gauche(+)gauche(-) interactions

Alkyl analogues of methylphenidate (Ritalin) salts are slow onset, long duration dopamine reuptake inhibitors with a potential use as a cocaine abuse pharmacotherapy. X-ray crystallographic studies and nuclear magnetic resonance (NMR) investigations strongly suggest that avoidance of sterically unfavorable gauche(-)gauche(+) orientations effectively influences both the C(α)-alkyl side chain conformation and the formation of a predominant rotamer about the CH-CH bond ligating piperidine and C(Ar)R moieties. The favored CH-CH rotamer in D(2)O and in CD(2)Cl(2) of the pharmacologically interesting i-Bu and CH(2)-cyc-Pnt (RS,RS)-salts has the same antiperiplanar arrangement that was found in the crystal structures, although there clearly is a fast equilibrium involving smaller amounts of synclinal partners. While the rotamer in the (RS,SR)-i-Bu HCl crystal structure exhibits a synclinal orientation for the vicinal pair of adjacent methine protons, the weighted time-averaged arrangement for these protons becomes almost completely antiperiplanar when the crystals are dissolved in D(2)O. Increased steric congestion around the CH-CH bond in the analogous N-methyl tertiary ammonium salts seems to augment the quantity of the preferred rotamer within the mixture. The stereochemistry of the species observed via NMR seems to arise from specific combinations of N-methyl orientation and avoidance of sterically unfavorable gauche(-)gauche(+) arrangements.

Synthesis and monoamine uptake inhibition of conformationally constrained 2beta-carbomethoxy-3beta-phenyl tropanes

A series of 2beta-carbomethoxy-3beta-phenyl tropanes with conformationally constrained nitrogen substituents were synthesized as potential selective dopamine transporter ligands. These novel compounds were examined for their monoamine uptake inhibition potency at the human dopamine transporter (hDAT), the human serotonin transporter (hSERT) and the human noradrenalin transporter (hNET), stably expressed in human embryonic kidney cells (HEK). A SAR-study was conducted to determine the contribution of extended, 4-fluorinated, conformationally constrained C4 chains at the tropane nitrogen to human monoamine transporter affinity and selectivity.

The stereochemistry of N-methyl and aryl substituents determine the biological activities of 3-aryl-8-methyl-8-azabicyclo[3.2.1]oct-2,3-enes

Aryl substituted tropanes and their 2,3-ene analogs are highly selective inhibitors of monoamine uptake. The solution structures of a series of aryl tropanes were determined using NMR spectroscopy and molecular modeling to identify conformational preferences that may determine the overall activity. The majority of these analogs undergo nitrogen inversion, and the rate of interconversion between the axial and equatorial N-methyl conformers is fast on the NMR timescale at room temperature but slow between 217 and 243 K allowing us to determine the thermodynamic parameters of interconversion using dynamic and magnetization transfer NMR. The biological activities correlate strongly with the nature and the orientation of the aryl group. The relative orientation of the N-methyl further modulates the activity by directly influencing the ligand interaction in the protein binding pocket and/or by forcing a favorable orientation for the aryl substituent to fit in the binding pocket.

Synthesis and monoamine transporter affinity of front bridged tricyclic 3beta-(4'-halo or 4'-methyl)phenyltropanes bearing methylene or carbomethoxymethylene on the bridge to the 2beta-position

A series of front bridged tricyclic 3beta-(4'-halo or 4'-methyl)phenyltropanes bearing methylene or carbomethoxymethylene on the bridge to the 2beta-position was synthesized, and their binding affinities were determined in cells transfected to express human norepinephrine transporter (NET), serotonin transporter (SERT), and dopamine transporter (DAT) via competition binding assays. All compounds studied in this series exhibit a moderate to high potency at all three transporters with SERT or DAT selectivity. 3beta-(4'-iodo)phenyltropane bearing methylene on the bridge to the 2beta-position (24) presents a particularly attractive pharmacological profile, with very high SERT affinity (K(i) = 0.09 nM) and selectivity versus NET (65-fold) and DAT (94-fold).

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.

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.

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.

Thiophene derivatives: a new series of potent norepinephrine and serotonin reuptake inhibitors

A series of (1S,3S,6R,10S)-(Z)-9-(thienylmethylene- or substituted thienylmethylene)-7-azatricyclo[4.3.1.0(3,7)]decanes was prepared and evaluated for the ability to block dopamine, serotonin, and norepinephrine reuptake by their respective transporters. Compound 5b is a NET-selective inhibitor, 5c is a mixed NET- and SERT-selective inhibitor, while 11 is a SERT-selective inhibitor.

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.

Non-amines, drugs without an amine nitrogen, potently block serotonin transport: novel antidepressant candidates?

The serotonin transporter (SERT) is a principal site of action of therapeutic antidepressants in the brain. Without exception, these inhibitors of serotonin transport contain an amine nitrogen in their structure. We previously demonstrated that novel compounds without an amine nitrogen in their structure (non-amines), blocked dopamine transport in cells transfected with the human dopamine transporter. The present study investigated whether, in the absence of an amine nitrogen, certain non-amines bind selectively to the SERT and block the transport of serotonin. At 10 microM concentration, select non-amines displayed no, or little, affinity for 9 serotonin, 5 dopamine, 7 adrenergic, 5 muscarinic cholinergic, 3 opiate and histamine receptors. The affinities of non-amines for [(3)H]citalopram binding sites on the SERT and their potencies for blocking [(3)H]serotonin transport were measured in cloned human SERT stably or transiently expressed in HEK-293. Whether oxa- or carba-based, non-amines bound to [(3)H]citalopram-labeled sites and blocked [(3)H]serotonin transport in the low nanomolar range, at values equal to or higher than those of some conventional antidepressants. A non-amine, O-1809, was 99-fold more selective for the serotonin over the dopamine transporter. As substituents on the aromatic ring of non-amines confer high affinity for the SERT, we investigated the hypothesis that aromatic-aromatic interactions may contribute significantly to non-amine/transporter association. A SERT mutant was produced in which a highly conserved aromatic amino acid, phenylalanine 548, was replaced by an alanine (F548A). Although the affinities of several non-amines were unchanged in the mutant SERT, the affinity of imipramine was decreased, revealing possible differences in amine and non-amine binding domains on the SERT. The similar affinities of non-amines and conventional antidepressant drugs for the SERT support the view that an amine nitrogen is not essential for drugs to block serotonin transport with high affinity. Non-amines open avenues for developing a new generation of antidepressants.

Sar studies of piperidine-based analogues of cocaine. Part 3: oxadiazoles

The synthesis of novel 4beta-aryl-1-methyl-3alpha-(3-substituted-1,2,4-oxadiazol-5-yl)piperidines, bioisosteres of ester (+)-1, is described. The synthesized oxadiazoles were evaluated for their affinity to the DAT and their ability to inhibit monoamine reuptake at the DAT, NET, and 5HTT. The results show that affinity to the DAT and ability to inhibit the reuptake at the DAT, NET, and 5HTT is a function of the size of the substituent in the oxadiazole ring. (+)-(3R,4S)-4beta-(4-Chlorophenyl)-1-methyl-3alpha-(3-methyl-1,2,4-oxadiazol-5-yl)piperidine [(+)-2a], which is structurally and pharmacologically most similar to the ester (+)-1 in this series, showed at least a 2-fold longer duration of action when compared to ester (+)-1.

Molecular modeling, structure--activity relationships and functional antagonism studies of 4-hydroxy-1-methyl-4-(4-methylphenyl)-3-piperidyl 4-methylphenyl ketones as a novel class of dopamine transporter inhibitors

We previously disclosed the discovery of 4-hydroxy-1-methyl-4-(4-methylphenyl)-3-piperidyl 4-methylphenyl ketone (3) as a novel class of dopamine transporter (DAT) inhibitors and showed that (+/-)-3 has a significant functional antagonism against cocaine in vitro. Our previous preliminary structure-activity relationship study led to identification of a more potent DAT inhibitor [(+/-)-4] but this compound failed to show any significant functional antagonism. To search for more potent analogues than 3 but still displaying significant functional antagonism, further SARs, molecular modeling studies and in vitro pharmacological evaluation of this novel class of DAT inhibitors were performed. Sixteen new analogues were synthesized in racemic form and evaluated as DAT inhibitors. It was found that seven new analogues are reasonably potent DAT inhibitors with K(i) values of 0.041--0.30 and 0.052--0.16 microM in [(3)H]mazindol binding and inhibition of DA reuptake. Chiral isomers of several potent DAT inhibitors were obtained through chiral HPLC separation and evaluated as inhibitors at all the three monoamine transporter sites. In general, the (-)-isomer is more active than the (+)-isomer in inhibition of DA reuptake and all the (-)-isomers are selective inhibitors at the DAT site. Evaluation of cocaine's effect on dopamine uptake in the presence and absence of (+)-3 and (-)-3 showed that (-)-3 is responsible for the functional antagonism obtained with the original lead (+/-)-3. Out of the new compounds synthesized, analogue (+/-)-20, which is 8- and 3-fold more potent than (+/-)-3 in binding and inhibition of DA reuptake, appeared to have improved functional antagonism as compared to (+/-)-3.

Synthesis and monoamine transporter affinity of 3beta-(4-(2-pyrrolyl)phenyl)-8-azabicycl

3Beta-(5-indolyl)-8-azabicyclo[3.2.1]octanes display potent binding affinity for both the dopamine and serotonin transporters, while certain 3beta-(4-(2-pyrrolyl)phenyl)-8-azabicyclo[3.2.1]octanes selectively bind to the serotonin transporter.

Synthesis and biological evaluation of 1-azabicyclo-[3.2.1]octanes: new dopamine transporter inhibitors

The synthesis and biological activity of a series of 6-substituted 1-azabicyclo[3.2.1]octanes are described. 1-Azabicyclo[3.2.1]octanes represent a new class of compounds that exhibit monoamine transporter inhibitory activity highly dependent on the overall topology and the absolute stereochemistry of the molecule.

[3 + 2]-cycloaddition of nonstabilized azomethine ylides. 10. An efficient strategy for the construction of x-Azatricyclo[m.n.0. 0]

Various new structural entities related to x-azatricyclo[m.n.0. 0(a)()(,)(b)()]alkanes are constructed by the intramolecular [3 + 2] dipolar cycloaddition of nonstabilized cyclic azomethine ylides. The ylide is generated by the sequential double desilylation of N-alkyl alpha,alpha'-bis(trimethylsilyl)cyclic amines using Ag(I)F as a one-electron oxidant.

Design, synthesis and biological evaluation of 7-azatricyclodecanes: analogues of cocaine

The synthesis and biological activity of a series of azatricyclodecane analogues of cocaine are described. All compounds studied in this series exhibit nanomolar potency and good selectivity for the serotonin transporter versus the dopamine transporter.

N-phenylalkyl-substituted tropane analogs of boat conformation with high selectivity for the dopamine versus serotonin transporter

A series of N-phenylalkyl-substituted tropane analogs of boat conformation was synthesized, and these tropanes were evaluated for their ability to inhibit high affinity uptake of dopamine (DA) and serotonin (5-HT) into striatal nerve endings (synaptosomes). Some of these compounds exhibit high affinity for the DA transporter with a 5-HT/DA transporter selectivity ratio of >50.

Synthesis of Aryl-Substituted Piperidines by Superacid Activation of Piperidones

Diarylpiperidines (8-12) may be prepared in good to excellent yields (80-99%) by the reaction of piperidones (3d-h) with benzene and the Bronsted superacid, trifluoromethanesulfonic acid (CF(3)SO(3)H, TfOH). Tropinone (6) and quinuclidone (7) also react in good yields with benzene in TfOH to give the condensation products (13 and 14). Ketal and acetal derivatives also give condensation products (8 and 24) upon reaction with C(6)H(6) in TfOH. The conversion of 3g to 11 is sensitive to both acid quantity and acid strength; a mechanism is proposed for the conversion that invokes dicationic intermediates.

Dopaminergic agents for the treatment of cocaine abuse

Cocaine is a major drug of abuse whose devastating effects have captured the attention of health officials and policy makers. Based upon the alarming health and crime-related costs associated with the use of this powerful reinforcing drug, immediate therapies are needed for the treatment of cocaine addiction. In this review, some of the small-molecule-based approaches that have been pursued in the search for such medications are highlighted. Because the pharmacological actions of cocaine stem laargely from its ability to block the dopamine transporter, many intervention strategies have focused on the dopaminergic pathway.