Structure-activity relationship studies of highly selective inhibitors of the dopamine transporter: N-benzylpiperidine analogues of 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine

Evidence for noncompetitive modulation of substrate-induced serotonin release

Prior work indicated that serotonin transporter (SERT) inhibitors competitively inhibit substrate-induced [(3)H]5-HT release, producing rightward shifts in the substrate-dose response curve and increasing the EC(50) value without altering the E(max). We hypothesized that this finding would not generalize across a number of SERT inhibitors and substrates, and that the functional dissociation constant (Ke) of a given SERT inhibitor would not be the same for all tested substrates. To test this hypothesis, we utilized a well-characterized [(3)H]5-HT release assay that measures the ability of a SERT substrate to release preloaded [(3)H]5-HT from rat brain synaptosomes. Dose-response curves were generated for six substrates (PAL-287 [naphthylisopropylamine], (+)-fenfluramine, (+)-norfenfluramine, mCPP [meta-chlorophenylpiperazine], (±)-MDMA, 5-HT) in the absence and presence of a fixed concentration of three SERT inhibitors (indatraline, BW723C86, EG-1-149 [4-(2-(benzhydryloxy)ethyl)-1-(4-bromobenzyl)piperidine oxalate]). Consistent with simple competitive inhibition, all SERT inhibitors increased the EC(50) value of all substrates. However, in many cases a SERT inhibitor decreased the E(max) value as well, indicating that in the presence of the SERT inhibitor the substrate became a partial releaser. Moreover, the Ke values of a given SERT inhibitor differed among the six SERT substrates, indicating that each inhibitor/substrate combination had a unique interaction with the transporter. Viewed collectively, these findings suggest that it may be possible to design SERT inhibitors that differentially regulate SERT function.

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.

Pharmacophore design and database searching for selective monoamine neurotransmitter transporter ligands

Neuronal monoamine transporters (MATs) are involved in the pathophysiology and treatment of mental health conditions such as depression, attention deficit hyperactivity disorder, substance abuse and neurodegenerative disorders including Alzheimer's disease and Parkinson's disease. Various structural classes of compounds have been synthesized and tested in vitro for activity against transporters of three monoamine signaling molecules: noradrenaline (NET); serotonin (SERT) and dopamine (DAT). We have developed and validated a number of pharmacophore models describing the interaction of two classes of compounds with each of these three MATs. These pharmacophores explain the selectivity of binding to the MATs for various compound classes and have been used to search in silico databases for novel, potentially selective ligands. These ligands, after confirmation of their activities, will provide tools for investigating the function of MATs as well as the potential for new therapeutic agents in mental health applications. The database searches also retrieved close analogues of known MAT ligands, further validating the approach.

Restoration of 3,4-methylenedioxymethamphetamine-induced 5-HT depletion by the administration of l-5-hydroxytryptophan

BACKGROUND

3,4-Methylenedioxymethamphetamine (MDMA) causes persistent decreases in brain 5-HT content and 5-HT transporter (SERT) binding, with no detectable changes in SERT protein. Such data suggest that MDMA impairs 5-HT transmission but leaves 5-HT nerve terminals intact. To further test this hypothesis, we carried out two types of experiments in rats exposed to high-dose MDMA. First, we examined the effects of MDMA on SERT binding and function using different in vitro assay conditions. Next, we treated rats with the 5-HT precursor, l-5-hydroxytryptophan (5-HTP), in an attempt to restore MDMA-induced depletions of 5-HT.

METHODS

Rats received three i.p. injections of saline or MDMA (7.5 mg/kg), one injection every 2 h. Rats in one group were decapitated, and brain tissue was assayed for SERT binding and [(3)H]5-HT uptake under conditions of normal (100 or 126 mM) and low (20 mM) NaCl concentration. Rats from another group received saline or 5-hydroxytryptophan/benserazide (5-HTP-B), each drug at 50 mg/kg i.p., and were killed 2 h later.

RESULTS

MDMA reduced SERT binding to 10% of control when assayed in 100 mM NaCl, but this reduction was only 55% of control in 20 mM NaCl. MDMA decreased immunoreactive 5-HT in caudate and hippocampus to about 35% of control. Administration of 5-HTP-B to MDMA-pretreated rats significantly increased the 5-HT signal toward normal levels in caudate (85% of control) and hippocampus (66% of control).

CONCLUSION

1) Following high-dose MDMA treatment sufficient to reduce SERT binding by 90%, a significant number of functionally intact 5-HT nerve terminals survive. 2) The degree of MDMA-induced decreases in SERT binding depends on the in vitro assay conditions. 3) 5-HTP-B restores brain 5-HT depleted by MDMA, suggesting that this approach might be clinically useful in abstinent MDMA users.

DAT/SERT selectivity of flexible GBR 12909 analogs modeled using 3D-QSAR methods

The dopamine reuptake inhibitor GBR 12909 (1-{2-[bis(4-fluorophenyl)methoxy]ethyl}-4-(3-phenylpropyl)piperazine, 1) and its analogs have been developed as tools to test the hypothesis that selective dopamine transporter (DAT) inhibitors will be useful therapeutics for cocaine addiction. This 3D-QSAR study focuses on the effect of substitutions in the phenylpropyl region of 1. CoMFA and CoMSIA techniques were used to determine a predictive and stable model for the DAT/serotonin transporter (SERT) selectivity (represented by pK(i) (DAT/SERT)) of a set of flexible analogs of 1, most of which have eight rotatable bonds. In the absence of a rigid analog to use as a 3D-QSAR template, six conformational families of analogs were constructed from six pairs of piperazine and piperidine template conformers identified by hierarchical clustering as representative molecular conformations. Three models stable to y-value scrambling were identified after a comprehensive CoMFA and CoMSIA survey with Region Focusing. Test set correlation validation led to an acceptable model, with q(2)=0.508, standard error of prediction=0.601, two components, r(2)=0.685, standard error of estimate=0.481, F value=39, percent steric contribution=65, and percent electrostatic contribution=35. A CoMFA contour map identified areas of the molecule that affect pK(i) (DAT/SERT). This work outlines a protocol for deriving a stable and predictive model of the biological activity of a set of very flexible molecules.

Synthesis of dopamine transporter selective 3-diarylmethoxymethyl-8-arylalkyl-8-azabicyclo[3.2.1]octane derivatives

A series of diarylmethoxymethyltropane-GBR hybrid analogues with all three possible stereochemical orientations at C3 were synthesized and evaluated at dopamine and serotonin transporters. The 3alpha derivatives were found to be the most potent compounds with the 3alpha-di(4-fluorophenyl)methoxymethyl-8-(3-phenylpropyl)-8-azabicyclo[3.2.1]octane 15b (Ki = 5 nM) being the most potent compound of the series. The corresponding 3-di(4-fluorophenyl)-methoxymethyl-8-(3-phenylpropyl)-8-azabicyclo[3.2.1]oct-2-ene 12b (Ki = 12 nM) was slightly less potent than the 3alpha-analogue, while the 3beta-di(4-fluorophenyl)methoxymethyl-8-(3-phenylpropyl)-8-azabicyclo[3.2.1]octane 23b (Ki = 78 nM) exhibited only modest affinity for the dopamine transporter. Only the 3alpha-analogue 15b (SERT/DAT = 48) exhibited higher SERT/DAT selectivity than GBR 12909. These results indicate that the dopamine transporter can tolerate some variability in proximity of the benzhydryl ether to the basic nitrogen atom of the tropane without loss in potency. In addition, the structure-activity data for these tropane-GBR 12909 hybrid analogues support previous findings that the stereochemical and conformational effects imparted by unsaturation at C3 are important for dopamine transporter selectivity over the serotonin transporter.

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.

Structure-activity relationships of substituted N-benzyl piperidines in the GBR series: Synthesis of 4-(2-(bis(4-fluorophenyl)methoxy)ethyl)-1-(2-trifluoromethylbenzyl)piperidine, an allosteric modulator of the serotonin transporter

A series of 4-(2-(bis(4-fluorophenyl)methoxy)ethyl)-(substituted benzyl) piperidines with substituents at the ortho and meta positions in the aromatic ring of the N-benzyl side chain were synthesized and their affinities and selectivities for the dopamine transporter (DAT), serotonin transporter (SERT), and norepinephrine transporter (NET) were determined. One analogue, 4-(2-(bis(4-fluorophenyl)methoxy)ethyl)-1-(2-trifluoromethylbenzyl)piperidine (the C(2)-trifluoromethyl substituted compound), has been found to act as an allosteric modulator of hSERT binding and function. It had little affinity for any of the transporters. Several compounds showed affinity for the DAT in the low nanomolar range and displayed a broad range of SERT/DAT selectivity ratios and very little affinity for the NET. The pharmacological tools provided by the availability of compounds with varying transporter affinity and selectivity could be used to obtain additional information about the properties a compound should have to act as a useful pharmacotherapeutic agent for cocaine addiction and help unravel the pharmacological mechanisms relevant to stimulant abuse.

(±)-3,4-Methylenedioxymethamphetamine Administration to Rats Does Not Decrease Levels of the Serotonin Transporter Protein or Alter Its Distribution between Endosomes and the Plasma Membrane

We showed that the serotonin (5-HT) neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) reduces brain tissue 5-HT, decreases expression of 5-HT transporter (SERT) protein, and increases expression of glial fibrillary acidic protein (GFAP). In contrast, doses of (+/-)-3,4-methylenedioxymethamphetamine (MDMA) that decrease brain tissue 5-HT fail to alter expression of SERT or GFAP. Using a new and highly sensitive anti-SERT antibody, we determined whether MDMA alters the subcellular distribution of SERT protein by measuring SERT expression in endosomes and plasma membranes 2 weeks after MDMA administration. Rat brain tissues (caudate, cortex, and hippocampus) were collected 3 days and 2 weeks after MDMA (7.5 mg/kg i.p., every 2 h x 3 doses) or 5,7-DHT (150 microg/rat i.c.v.) administration. Representative results from cortex are as follows. At both 3 days and 2 weeks postinjection, MDMA decreased tissue 5-HT (65%) and had no effect on GFAP expression. MDMA increased heat shock protein 32 (HSP32; a marker for microglial activation) expression (30%) at 3 days, but not 2 weeks. MDMA did not alter SERT expression at either time point and did not alter SERT levels in either endosomes or plasma membranes (2 weeks). 5,7-DHT decreased tissue 5-HT (80%), increased HSP32 expression at both time points (about 50%), and increased GFAP expression at 2 weeks (40%). 5,7-DHT decreased SERT expression (33%) at 2 weeks, but not at 3 days. These findings indicate that a dosing regimen of MDMA that depletes brain 5-HT does not alter SERT protein expression or the distribution of SERT between endosomes and the plasma membrane and does not produce detectable evidence for neurotoxicity.

Studies of the Biogenic Amine Transporters. XI. Identification of a 1-[2-[Bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine (GBR12909) Analog That Allosterically Modulates the Serotonin Transporter

Previous studies identified partial inhibitors of serotonin (5-HT) transporter and dopamine transporter binding. We report here on a partial inhibitor of 5-HT transporter (SERT) binding identified among a group of 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine analogs (4-[2-[bis(4-fluorophenyl)-methoxy]ethyl]-1-(2-trifluoromethyl-benzyl)-piperidine; TB-1-099). Membranes were prepared from rat brains or human embryonic kidney cells expressing the cloned human dopamine (hDAT), serotonin (hSERT), and norepinephrine (hNET) transporters. beta-(4'-(125)Iodophenyl)tropan-2beta-carboxylic acid methyl ester ([(125)I]RTI-55) binding and other assays followed published procedures. Using rat brain membranes, TB-1-099 weakly inhibited DAT binding (K(i) = 439 nM), was inactive at NET binding ([(3)H]nisoxetine), and partially inhibited SERT binding with an extrapolated plateau ("A" value) of 20%. Similarly, TB-1-099 partially inhibited [(125)I]RTI-55 binding to hSERT with an extrapolated plateau (A value) of 14%. Upon examining the effect of increasing concentrations of TB-1-099 on the apparent K(d) and B(max) of [(125)I]RTI-55 binding to hSERT, we found that TB-1-099 decreased the B(max) in a dose-dependent manner and affected the apparent K(d) in a manner well described by a sigmoid dose-response curve. TB-1-099 increased the K(d) but not to the magnitude expected for a competitive inhibitor. In rat brain synaptosomes, TB-1-099 noncompetitively inhibited [(3)H]5-HT, but not [(3)H]dopamine, uptake. Dissociation experiments indicated that TB-1-099 promoted the rapid dissociation of a small component of [(125)I]RTI-55 binding to hSERT. Association experiments demonstrated that TB-1-099 slowed [(125)I]RTI-55 binding to hSERT in a manner unlike that of the competitive inhibitor indatraline. Viewed collectively, these results support the hypothesis that TB-1-099 allosterically modulates hSERT binding and function.

Substituted amphetamines that produce long-term serotonin depletion in rat brain ("neurotoxicity") do not decrease serotonin transporter protein expression

Administration of high-dose D-fenfluramine (D-FEN) or parachloroamphetamine (PCA) produces long-lasting decreases in serotonin transporter (SERT) binding and tissue levels of serotonin (5-HT) in rat forebrain. These changes have been viewed as evidence for 5-HT neurotoxicity, but few studies have measured SERT protein levels. Thus, in the present study we determined the effect of high-dose D-FEN or PCA, administered according to a "neurotoxic" dosing regimen, on the density of SERT sites using ligand binding methods and on SERT protein levels using Western blots. Rats were sacrificed 2 days and 2 weeks after administration of drug or saline. The density of SERT was determined in homogenates of caudate and whole brain minus caudate. d-FEN and PCA decreased SERT binding by 30 to 60% in both tissues and at both time points. Similarly, D-FEN and PCA administration profoundly decreased tissue 5-HT and 5-HIAA in frontal cortex. Despite the large decreases in SERT binding and depletion of tissue 5-HT that occurred with d-FEN administration, SERT protein expression, as determined by Western blot analysis, did not change in either tissue or time point. PCA administration decreased SERT protein by about 20% only at the 2-day point in the caudate. Drug treatments did not change expression of glial fibrillary acidic protein (GFAP), a hallmark indicator of neuronal damage, in whole brain minus caudate in the 2-week group. These results support the hypothesis that D-FEN- and PCA-induced decreases in tissue 5-HT and SERT binding sites reflect neuroadaptive changes rather than neurotoxic effects.

Identification and characterization of a novel allosteric modulator (SoRI-6238) of the serotonin transporter

In the present study we describe a novel agent, SoRI-6238 (ethyl 5-amino-3-(3,4-dichlorophenyl)-1,2-dihydropyrido[3,4-b]pyrazin-7-ylcarbamate) that partially inhibits 5-HT transporter (SERT) binding and allosterically modulates SERT function. Membranes were prepared from rat brain. SoRI-6238 partially inhibited SERT binding to brain membranes with a plateau at about 40% of control. SoRI-6238 fully inhibited norepinephrine transporter (NET) and dopamine transporter (DAT) binding with IC(50) values of 12.1 microM and 5.8 microM, respectively. The apparent K(d) of [(125)I]RTI-55 binding to SERT increased, then reached a plateau with increasing concentrations of SoRI-6238. SoRI-6238 fully inhibited [(3)H]5-HT uptake, acting to decrease the V(max) (noncompetitive inhibition). In kinetic experiments, SoRI-6238 slowed the dissociation of [(125)I]RTI-55 from SERT and slowed the initial association rate. We conclude that SoRI-6238 partially inhibits SERT binding and function, most likely via an allosteric mechanism.

High-dose fenfluramine administration decreases serotonin transporter binding, but not serotonin transporter protein levels, in rat forebrain

Administration of D-fenfluramine (D-FEN) or parachloroamphetamine (PCA) can produce long-lasting decreases in serotonin transporter (SERT) binding and tissue levels of serotonin (5-HT) in rat forebrain. These changes have been viewed as evidence for 5-HT neurotoxicity, but no studies have measured SERT protein levels. In the present study, we determined the effect of high-dose D-FEN or PCA, administered according to a "neurotoxic" dosing regimen, on the density of SERT sites using ligand binding methods and on SERT protein levels using Western blots. Rats were sacrificed 2 days and 2 weeks after administration of drug or saline. The density of SERT was determined in homogenates of caudate and whole brain minus caudate. D-FEN and PCA decreased SERT binding by 30-60% in both tissues and at both time points. Similarly, D-FEN and PCA administration profoundly decreased tissue 5-HT and 5-HIAA in frontal cortex. Despite the large decreases in SERT binding and depletion of tissue 5-HT that occurred with D-FEN administration, SERT protein expression, as determined by Western blot analysis, did not change in either tissue or time point. PCA administration decreased SERT protein by about 20% only at the 2-day point in the caudate. Drug treatments did not change expression of glial fibrillary acidic protein (GFAP), a hallmark indicator of neuronal damage, in whole brain minus caudate in the 2-week group. These results support the hypothesis that decreases in tissue 5-HT and SERT binding sites induced by D-FEN and PCA reflect neuroadaptive changes, rather than neurotoxic effects.