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By linking two or three mecamylamine or 2,2,6,6-tetramethylpiperidine (TMP) molecules together via a linear lipophilic bis-methylene linker or a specially designed conformationally restricted tris-linker, a series of bis- and tris-tertiary amine analogs has been synthesized and evaluated as potent antagonists at nAChRs mediating nicotine-evoked [ 3 H]dopamine release from rat striatal slices. Compounds 7e , 14b and 16 demonstrated high potency in decreasing nicotine-evoked [ 3 H]dopamine release (IC 50 = 2.2, 46, and 107 nM, respectively). The preliminary structure–activity data obtained with these new analogs suggest the importance of the length of the methylene linker in the bis-analog series. Such bis-tertiary amino analogs may provide a new strategy for the design of drugable ligands that have high inhibitory potency against nAChRs mediating nicotine-evoked dopamine release in striatum, which have been suggested to be target receptors of interest in the development of potential smoking cessation therapies.

Lobeline is currently being evaluated in clinical trials as a methamphetamine abuse treatment. Lobeline interacts with nicotinic receptor subtypes, dopamine transporters (DATs), and vesicular monoamine transporters (VMAT2s). Methamphetamine inhibits VMAT2 and promotes dopamine (DA) release from synaptic vesicles, resulting ultimately in increased extracellular DA. The present study generated structure-activity relationships by defunctionalizing the lobeline molecule and determining effects on [3H]dihydrotetrabenazine binding, inhibition of [3H]DA uptake into striatal synaptic vesicles and synaptosomes, the mechanism of VMAT2 inhibition, and inhibition of methamphetamine-evoked DA release. Compared with lobeline, the analogs exhibited greater potency inhibiting DA transporter (DAT) function. Saturated analogs, lobelane and nor-lobelane, exhibited high potency (Ki = 45 nM) inhibiting vesicular [3H]DA uptake, and lobelane competitively inhibited VMAT2 function. Lobeline and lobelane exhibited 67- and 35-fold greater potency, respectively, in inhibiting VMAT2 function compared to DAT function. Lobelane potently decreased (IC50 = 0.65 μM; Imax = 73%) methamphetamine-evoked DA overflow, and with a greater maximal effect compared with lobeline (IC50 = 0.42 μM, Imax = 56.1%). These results provide support for VMAT2 as a target for inhibition of methamphetamine effects. Both trans-isomers and demethylated analogs of lobelane had reduced or unaltered potency inhibiting VMAT2 function and lower maximal inhibition of methamphetamine-evoked DA release compared with lobelane. Thus, defunctionalization, cis-stereochemistry of the side chains, and presence of the piperidino N-methyl are structural features that afford greatest inhibition of methamphetamine-evoked DA release and enhancement of selectivity for VMAT2. The current results reveal that lobelane, a selective VMAT2 inhibitor, inhibits methamphetamine-evoked DA release and is a promising lead for the development of a pharmacotherapeutic for methamphetamine abuse.

A series of tetrakis-azaaromatic quaternary ammonium salts was synthesized in order to identify compounds with higher affinity and selectivity as antagonists at neuronal nicotinic receptor subtypes that mediate nicotine-evoked DA release. A high hit rate was achieved in identifying potent analogues that inhibit these nAChRs. Three tetrakis analogues, 11j, 11f and 11g, were identified as potent (IC50 = 3, 28 and 56 nM, respectively) antagonists at these receptors. These compounds represent a novel structural class of nicotinic receptor antagonists.

Back-propagation artificial neural networks (ANNs) were trained on a dataset of 104 VMAT2 ligands with experimentally measured log(1/ K i ) values. A set of related descriptors, including topological, geometrical, GETAWAY, aromaticity, and WHIM descriptors, was selected to build nonlinear quantitative structure–activity relationships. A partial least squares (PLS) regression model was also developed for comparison. The nonlinearity of the relationship between molecular descriptors and VMAT2 ligand activity was demonstrated. The obtained neural network model outperformed the PLS model in both the fitting and predictive ability. ANN analysis indicated that the computed activities were in excellent agreement with the experimentally observed values ( r 2 = 0.91, rmsd = 0.225; predictive q 2 = 0.82, loormsd = 0.316). The generated models were further tested by use of an external prediction set of 15 molecules. The nonlinear ANN model has r 2 = 0.93 and root-mean-square errors of 0.282 compared with the experimentally measured activity of the test set. The stability test of the model with regard to data division was found to be positive, indicating that the generated model is predictive. The modeling study also reflected the important role of atomic distribution in the molecules, size, and steric structure of the molecules when they interact with the target, VMAT2. The developed models are expected to be useful in the rational design of new chemical entities as ligands of VMAT2 and for directing synthesis of new molecules in the future.

A novel pyridine derivative, 3,5-bis-(1-methyl-pyrrolidin-2-yl)-pyridine, and a pair of diastereomers of 1,1'-dimethyl-[2,3']bipyrrolidinyl were isolated from the root of Nicotiana tabacum plants and identified as novel alkaloids by GC-MS analysis. The structures of these new alkaloids were confirmed by total synthesis. The affinities of these novel alkaloids, and other structurally related compounds for alpha4beta2*, alpha7* neuronal nicotinic acetylcholine receptors (nAChRs), and for nAChRs mediating nicotine-evoked dopamine release from rat striatum were also assessed. The results indicate that these compounds do not interact with alpha7* nAChRs, but inhibit [3H]nicotine binding to the alpha4beta2* nAChR subtype. The results also demonstrate that these compounds act as antagonists at nAChRs mediating nicotine-evoked dopamine release from rat striatum.

N-n-Alkylpicolinium and N,N'-alkyl-bis-picolinium analogues were assessed in nicotinic receptor (nAChR) assays. The most potent and subtype-selective analogue, N,N'-dodecyl-bis-picolinium bromide (bPiDDB), inhibited nAChRs mediating nicotine-evoked [(3)H]dopamine release (IC(50)=5 nM; I(max) of 60%), and did not interact with alpha4beta2* or alpha7* nAChRs. bPiDDB represents the current lead compound for development as a tobacco use cessation agent.

A series of bis-nicotinium, bis-pyridinium, bis-picolinium, bis-quinolinium and bis-isoquinolinium compounds was evaluated for their binding affinity at nicotinic acetylcholine receptors (nAChRs) using rat brain membranes. N,N'-Decane-1,12-diyl-bis-nicotinium diiodide (bNDI) exhibited the highest affinity for [(3)H]nicotine binding sites (K(i)=330 nM), but did not inhibit [(3)H]methyllycaconitine binding (K(i)100 microM), indicative of an interaction with alpha4beta2*, but not alpha7* receptor subtypes, respectively. Also, bNDI inhibited (IC(50)=3.76 microM) nicotine-evoked (86)Rb(+) efflux from rat thalamic synaptosomes, indicating antagonist activity at alpha4beta2* nAChRs. N,N'-Dodecane-1,12-diyl-bis-quinolinium dibromide (bQDDB) exhibited highest affinity for [(3)H]methyllycaconitine binding sites (K(i)=1.61 microM), but did not inhibit [(3)H]nicotine binding (K(i)100 microM), demonstrating an interaction with alpha7*, but not alpha4beta2* nAChRs. Thus, variation of N-n-alkyl chain length together with structural modification of the azaaromatic quaternary ammonium moiety afforded selective antagonists for the alpha4beta2* nAChR subtype, as well as ligands with selectivity at alpha7* nAChRs.

A series of tertiary amine analogues derived from lead azaaromatic quaternary ammonium salts has been designed and synthesized. The preliminary structure-activity relationships of these new analogues suggest that such tertiary amine analogues, which potently inhibit nicotine-evoked dopamine release from rat striatum, represent drug-like inhibitors of α6-containing nicotinic acetylcholine receptors. The bis-tertiary amine analog 7 exhibited an IC50 of 0.95 nM, while the tris-tertiary amine analog 19 had an IC50 of 0.35 nM at nAChRs mediating nicotine-evoked dopamine release.

The analogues (IV) of lobelane are synthesized by replacing its phenyl groups with either pyridyl, indolyl, or quinolyl groups.

Replacing the phenyl groups in the structure of the VMAT2 inhibitor, lobelane with either pyridyl, quinolyl or indolyl groups affords novel analogues with improved water solubility. The synthetic methodologies reported herein also underscore the paucity of hydrogenation methods that offer selectivity in the synthesis of the different classes of heteroaromatic lobelane analogues. The quinolyl group was the only replacement for the phenyl group in lobelane that retained VMAT2 inhibition.

Based on an 85 molecule database, linear regression with different size datasets and an artificial neural network approach have been used to build mathematical relationships to fit experimentally obtained affinity values (K(i)) of a series of mono- and bis-quaternary ammonium salts from [(3)H]nicotine binding assays using rat striatal membrane preparations. The fitted results were then used to analyze the pattern among the experimental K(i) values of a set of N-n-alkylnicotinium analogs with increasing n-alkyl chain length from 1 to 20 carbons. The affinity of these N-n-alkylnicotinium compounds was shown to parabolically vary with increasing numbers of carbon atoms in the n-alkyl chain, with a local minimum for the C(4) (n-butyl) analogue. A decrease in K(i) value between C(12) and C(13) was also observed. The statistical results for the best neural network fit of the 85 experimental K(i) values are r(2) = 0.84, rmsd = 0.39; r(cv)(2) = 0.68, and loormsd = 0.56. The generated neural network model with the 85 molecule training set may also be of value for future predictions of K(i) values for new virtual compounds, which can then be identified, subsequently synthesized, and tested experimentally.

N-n-Alkylation of nicotine converts it from an agonist into an antagonist at neuronal nicotinic acetylcholine receptor subtypes mediating nicotine-evoked dopamine release. Conformationally restricted analogues exhibit both high affinity and selectivity at this site, and are able to access the brain due to their ability to act as substrates for the blood–brain barrier choline transporter.

A series of boron-containing nicotine (NIC) analogues 7-9 was synthesized and evaluated for binding to alpha4beta2 and alpha7 nicotinic receptors. Compound ACME-B inhibited [3H]methyllycaconitine binding to rat brain membranes with a similar potency compared to NIC (Ki = 2.4 and 0.77 microM, respectively), but was markedly less potent in inhibiting [3H]NIC binding when compared to NIC (Ki = 0.60 microM and 1.0 nM, respectively). Thus, tethering a two-carbon bridge between the 2-pyridyl and 3'-pyrrolidino carbons of NIC or 7 affords analogues that bind to the alpha7 receptor in a manner similar to NIC, but with a dramatic loss of affinity for the alpha4beta2 receptor.