Structure - activity relationships of nicotinic acetylcholine receptor agonists as potential treatments for dementia

Site-specific peak intensity ratio (SPIR) from 1D 2H/1H NMR spectra for rapid distinction between natural and synthetic nicotine and detection of possible adulteration

¹H and ²H NMR spectra of 4 natural and synthetic nicotine samples were collected in a non-quantitative way and site-specific ²H/¹H peak intensity ratio (SPIR) was calculated for 12 distinct sites of nicotine. Experimental results illustrated that the SPIRs at sites of 6, 2', and 5'β of natural nicotine were significantly different from those of the synthetic nicotine, and could be used for nicotine authentication as the measured SPIRs were indicative of the site-specific natural isotope fractionation. We demonstrated that this method could be applied to detect adulteration of natural nicotine with as low as 20% synthetic nicotine, without the need to measure the site-specific δD values, which usually required time-consuming quantitative ²H NMR and additional IRMS for the overall ²H/¹H isotopic ratio determination. The distinguishable ²H/¹H SPIRs of nicotine, which can be quickly measured by NMR in non-quantitative way, can serve as an attractive alternative tool for tobacco authentication. Graphical abstract.

Cholinergic nicotinic receptors: competitive ligands, allosteric modulators, and their potential applications

Discovery of the important role played by nicotinic acetylcholine receptors (nAChRs) in several CNS disorders has called attention to these membrane proteins and to ligands able to modulate their functions. The existence of different subtypes at multiple levels has complicated the understanding of this receptor's physiological role, but at the same time has increased the efforts to discover selective compounds in order to improve the pharmacological characterization of this kind of receptor and to make the possible therapeutical use of its modulators safer. This review focuses on the structure of new ligands for nAChRs, agonists, antagonists and allosteric modulators, and on their possible applications.

Synthesis and biological evaluation of pyridine-modified analogues of 3-(2-aminoethoxy)pyridine as novel nicotinic receptor ligands

Analogues of the potent nicotinic receptor agonist 3-(2-aminoethoxy)pyridine substituted at the 5' and 6'-positions of the pyridine ring were synthesized and tested in vitro for nicotinic receptor binding activity (displacement of [(3)H](-)cytisine from whole rat brain synaptic membranes). The substituted analogues exhibited K(i) values ranging from 0.076 to 319 nM compared to a K(i) value of 26 nM for compound 1. Among the compounds tested, 5'-vinyl-6'-chloro substituted 1 was the most potent.

Conformationally constrained nicotines: polycyclic, bridged, and spiro-annulated analogues as novel ligands for the nicotinic acetylcholine receptor

A set of novel nicotine-related, conformationally constrained compounds, including tetracyclic, bridged (4), and tricyclic, spiro-annulated (5) structures, were synthesized in a straightforward manner and optically resolved in a convenient fashion with (+)- and (-)-O,O'-di-p-toluoyltartaric acids. Absolute configurations were determined by X-ray crystallography. These compounds were evaluated for their ability to displace [(3)H]cytisine in a rat forebrain preparation and compared to (-)-nicotine. Three substances emerged with high affinity in the low nanomolar range. Moreover, one of these compounds ((+)-5b) showed not only high binding affinity (K(i) = 4.79 nM) but also significant enantioselectivity over its antipode (K(i) = 148 nM), supporting the hypothesis that conformational restraint can lead to high-affinity ligands, which are stereochemically discriminated by the nicotinic acetylcholine receptor and may feature optimum locations of the active sites of the pharmacophore.

A specific anti-aggressive effect of repeatedly administered lobeline

The effects of chronic treatments with nicotinic agonists on agonistic encounters have received little attention. The effects of repeated (for 10 days) SC administration of (-)-lobeline (9.3, 18.6 and 37.2 micromol/kg) and (-)-nicotine (0.93, 1.86 and 3.72 micromol/kg) were evaluated using the mouse isolation-induced aggression model. Individually housed OF1 male mice served as experimental animals and were confronted by 'standard opponents'. Each mouse was tested only once on the last day of the repeated drug treatment. Videotaped agonistic encounters were analysed estimating the times allocated to 11 behavioural categories. Repeated treatment with the highest dose of lobeline diminished attack behaviour without significantly increasing immobility or changing any other behavioural category involving motor activity. In contrast, nicotine did not significantly alter time allocated to any behavioural category.

Synthesis and pharmacological evaluation of some (pyridyl)cyclopropylmethyl amines and their methiodides as nicotinic receptor ligands

A series of 3- and (4-pyridyl)cyclopropylmethyl amines and their quaternary ammonium derivatives have been synthesized; they can be considered as rigid analogues of nicotine. The compounds have been tested on rat cerebral cortex to measure the affinity for the central nicotinic receptor. Only the methiodides show affinity in the micromolar range. The results obtained can provide useful information on the topography of the nicotinic receptor-binding site.

Syntheses and evaluation of pyridazine and pyrimidine containing bioisosteres of (+/-)-pyrido[3.4-b]homotropane and pyrido-[3.4-b]tropane as novel nAChR ligands

Bioisosteric replacement of the pyridine pharmacophoric element in (+/-)-pyrido[3.4-b]homotropane (PHT) and pyrido[3.4-b]tropane with the pyridazine and pyrimidine nucleus resulted in hitherto unknown nAChR ligands such as 5-8. Inverse type Diels-Alder reactions constitute the key steps in the new routes to the pyridazine- or pyrimidine-annulated bioisosteres. The enantiopure (+)-2-tropinone (11) from the 'chiral pool' is transformed to the ring-expanded silyl enol ether 12 and to the enamine 15. Both proved to be highly dienophilic species in the inverse type [4+2] cycloaddition reactions with the 1,2,4,5-tetrazines 13 and 16a,b or with the 1,3,5-triazine 19 to provide the enantiopure target compounds 5-7. In the same way the racemic pyrimidine-annulated species 8 was obtained from 3-tropanone 21. The new ligands were tested for their in vitro affinity for (alpha4)2(beta2)3 and alpha7* nAChR subtype. In comparison to PHT, well known to exhibit affinity for agonist binding sites in rat brain approximately equivalent to that of (+)-anatoxin-a (1), replacement of the pyridine by the bioisosteric pyridazine resulted in 30-fold lower affinity at the (alpha4)2(beta2)3 subtype. The annulated diazinotropanes 6-8, ligands with ferruginine-like structures more or less retained the affinity of (-)-norferruginine (3) except of compound 7. Remarkably, all of the novel ligands are devoid of affinity at the alpha7* subtype.

Structure-affinity relationships of a unique nicotinic ligand: N(1)-dimethyl-N(4)-phenylpiperazinium iodide (DMPP)

DMPP is a well-known nicotinic agonist that does not fit any proposed pharmacophore for nicotinic binding and represents a unique ligand among the hundreds of nicotinic agonists studied in the past decades. A systematic modulation of the chemical structure of DMPP, aimed to establish its structure-affinity relationships, is reported. The research has allowed to identify molecules such as 11c, 13c, 14c, and 28c, with affinities for alpha(4)beta(2) receptors in the low nanomolar range, some 2 orders of magnitude lower than the lead compound. The agonistic properties of the most interesting compounds have been assessed by measuring their analgesic activity on mice (hot-plate test). Another result of the research was the identification of DMPP analogues, such as 3a (K(i) = 90 nM) and 14b (K(i) = 180 nM), that maintain affinity for the central nicotinic receptor when the ammonium function is changed into an aminic one and are therefore possible leads for drug development in neurodegenerative diseases.

Synthesis and structure-activity relationships of 5-substituted pyridine analogues of 3

In an effort to probe the steric influence of C5 substitution of the pyridine ring on CNS binding affinity, analogues of 1 substituted with a bulky moiety--such as phenyl, substituted phenyl, or heteroaryl-were synthesized and tested in vitro for neuronal nicotinic acetylcholine receptor binding affinity. The substituted analogues exhibited Ki values ranging from 0.055 to 0.69 nM compared to a Ki value of 0.15 nM for compound 1. Assessment of functional activity at subtypes of neuronal nicotinic acetylcholine receptors led to identify several agonists and antagonists.

CNS acetylcholine receptor activity in European medicinal plants traditionally used to improve failing memory

Certain Lamiaceous and Asteraceous plants have long histories of use as restoratives of lost or declining cognitive functions in western European systems of traditional medicine. Investigations were carried out to evaluate human CNS cholinergic receptor binding activity in extracts of those European medicinal plants reputed to enhance or restore mental functions including memory. Ethanolic extracts were prepared from accessions of these plants and a number of other species related by genus. Amongst the plant extracts screened for contents able to displace [3H]-(N)-nicotine and [3H]-(N)-scopolamine from nicotinic receptors and muscarinic receptors, respectively in homogenates of human cerebral cortical cell membranes, the most potent extracts, prepared from one accession of Melissa officinalis, three Salvia species and Artemisia absinthium had IC50 concentrations of < 1 mg/ml. The displacement curves of some extracts were comparable with that of carbamylcholine chloride, a potent acetylcholine analogue. Choline, a weak nicotinic ligand (IC50 = 3 x 10(-4) M) was found in extracts of all plants studied at concentrations of 10(-6)-10(-5) M. These concentrations could not account for not more than 5% of the displacement activity observed. Some extracts displayed differential displacement at nicotinic and muscarinic acetylcholine receptors, with M. officinalis 0033 having the highest [3H]-(N)-nicotine displacement value and Salvia elegans with the highest [3H]-(N)-scopolamine displacement value. There was also considerable variation in cholinoreceptor interactions between different accessions of a single plant species. Although most plant extracts screened showed some nicotinic and muscarinic activity, only some showed dose-dependent receptor activity typical of materials with genuine cholinergic activity.

Hybridized and isosteric analogues of N1-acetyl-N4-dimethyl-piperazinium iodide (ADMP) and N1-phenyl-N4-dimethyl-piperazinium iodide (DMPP) with central nicotinic action

A series of piperazine derivatives, obtained by hybridization of N1-acetyl-N4-dimethyl-piperazinium iodide (1, ADMP) and N1-phenyl-N4-dimethyl-piperazinium iodide (3, DMPP) or of the corresponding tertiary bases (2, 4) with arecoline (5) and arecolone (6) or by isosteric substitution of the phenyl ring of DMPP, has been synthesized. Hybridization afforded compounds that, both as tertiary bases and as iodomethylates, have no affinity for the nicotinic receptor. On the contrary, isosteric substitution gave compounds that maintain affinity for the receptor; among them, two tertiary bases (37, 38), show affinity in the nanomolar range for the nicotinic receptor. The pharmacological profile of these isomeric compounds is quite interesting as they present differences in their peripheral and central effects, suggesting that they interact with different subtypes of the nicotinic receptor.

Synthesis and structure-activity data of some new epibatidine analogues

The high-pressure Diels-Alder reaction of N-carbomethyoxypyrroles and phenyl vinyl sulfone affords versatile intermediates for the palladium-catalyzed preparation of new epibatidine analogues. Structure-activity relationships of new epibatidine analogues are presented. High affinities of Ki = 0.81 and 2.6 nM for the [3H]-cytisine rat brain nicotinic acetylcholine binding sites were found for the 5-pyrimidinyl and the 5-(2-amino)-pyrimidinyl epibatidine analogues, respectively.

The 1.1 A resolution crystal structure of [Tyr15]EpI, a novel alpha-conotoxin from Conus episcopatus, solved by direct methods

Conotoxins are valuable probes of receptors and ion channels because of their small size and highly selective activity. alpha-Conotoxin EpI, a 16-residue peptide from the mollusk-hunting Conus episcopatus, has the amino acid sequence GCCSDPRCNMNNPDY(SO3H)C-NH2 and appears to be an extremely potent and selective inhibitor of the alpha3beta2 and alpha3beta4 neuronal subtypes of the nicotinic acetylcholine receptor (nAChR). The desulfated form of EpI ([Tyr15]EpI) has a potency and selectivity for the nAChR receptor similar to those of EpI. Here we describe the crystal structure of [Tyr15]EpI solved at a resolution of 1.1 A using SnB. The asymmetric unit has a total of 284 non-hydrogen atoms, making this one of the largest structures solved de novo by direct methods. The [Tyr15]EpI structure brings to six the number of alpha-conotoxin structures that have been determined to date. Four of these, [Tyr15]EpI, PnIA, PnIB, and MII, have an alpha4/7 cysteine framework and are selective for the neuronal subtype of the nAChR. The structure of [Tyr15]EpI has the same backbone fold as the other alpha4/7-conotoxin structures, supporting the notion that this conotoxin cysteine framework and spacing give rise to a conserved fold. The surface charge distribution of [Tyr15]EpI is similar to that of PnIA and PnIB but is likely to be different from that of MII, suggesting that [Tyr15]EpI and MII may have different binding modes for the same receptor subtype.

Identification and initial structure-activity relationships of (R)-5-(2-azetidinylmethoxy)-2-chloropyridine (ABT-594), a potent, orally active, non-opiate analgesic agent acting via neuronal nicotinic acetylcholine receptors

New members of a previously reported series of 3-pyridyl ether compounds are disclosed as novel, potent analgesic agents acting through neuronal nicotinic acetylcholine receptors. Both (R)-2-chloro-5-(2-azetidinylmethoxy)pyridine (ABT-594, 5) and its S-enantiomer (4) show potent analgesic activity in the mouse hot-plate assay following either intraperitoneal (i.p.) or oral (p.o.) administration, as well as activity in the mouse abdominal constriction (writhing) assay, a model of persistent pain. Compared to the S-enantiomer and to the prototypical potent nicotinic analgesic agent (+/-)-epibatidine, 5 shows diminished activity in models of peripheral side effects. Structure-activity studies of analogues related to 4 and 5 suggest that the N-unsubstituted azetidine moiety and the 2-chloro substituent on the pyridine ring are important contributors to potent analgesic activity.

Synthesis and structure-activity relationships of pyridine-modified analogs of 3-[2-((S)-pyrrolidinyl)methoxy]pyridine, A-84543, a potent nicotinic acetylcholine receptor agonist

Analogs of 3-[2-((S)-pyrrolidinyl)methoxy]pyridine, (A-84543, 1) with 2-, 4-, 5-, and 6-substituents on the pyridine ring were synthesized. These analogs exhibited Ki values ranging from 0.15 to > 9,000 nM when tested in vitro for neuronal nicotinic acetylcholine receptor binding activity. Assessment of functional activity at subtypes of neuronal nicotinic acetylcholine receptors indicates that pyridine substitution can have a profound effect on efficacy at these subtypes, and several subtype-selective agonists and antagonists have been identified.

Crystal structure at 1.1 A resolution of alpha-conotoxin PnIB: comparison with alpha-conotoxins PnIA and GI

Conotoxins are small, cysteine-rich peptides isolated from the venom of Conus spp. of predatory marine snails, which selectively target specific receptors and ion channels critical to the functioning of the neuromuscular system. alpha-Conotoxins PnIA and PnIB are both 16-residue peptides (differing in sequence at only two positions) isolated from the molluscivorous snail Conus pennaceus. In contrast to the muscle-selective alpha-conotoxin GI from Conus geographus, PnIA and PnIB block the neuronal nicotinic acetylcholine receptor (nAChR). Here, we describe the crystal structure of PnIB, solved at a resolution of 1.1 A and phased using the Shake-and-Bake direct methods program. PnIB crystals are orthorhombic and belong to the space group P212121 with the following unit cell dimensions: a = 14.6 A, b = 26.1 A, and c = 29.2 A. The final refined structure of alpha-conotoxin PnIB includes all 16 residues plus 23 solvent molecules and has an overall R-factor of 14.7% (R-free of 15.9%). The crystal structures of the alpha-conotoxins PnIB and PnIA are solved from different crystal forms, with different solvent contents. Comparison of the structures reveals them to be very similar, showing that the unique backbone and disulfide architecture is not strongly influenced by crystal lattice constraints or solvent interactions. This finding supports the notion that this structural scaffold is a rigid support for the presentation of important functional groups. The structures of PnIB and PnIA differ in their shape and surface charge distribution from that of GI.

Synthesis and nicotinic activity of epiboxidine: an isoxazole analogue of epibatidine

Synthetic (+/-)-epiboxidine (exo-2-(3-methyl-5-isoxazolyl)-7-azabicyclo[2.2.1]heptane) is a methylisoxazole analog of the alkaloid epibatidine, itself a potent nicotinic receptor agonist with antinociceptive activity. Epiboxidine contains a methylisoxazolyl ring replacing the chloropyridinyl ring of epibatidine. Thus, it is also an analog of another nicotinic receptor agonist, ABT 418 ((S)-3-methyl-5-(1-methyl-2-pyrrolidinyl)isoxazole), in which the pyridinyl ring of nicotine has been replaced by the methylisoxazolyl ring. Epiboxidine was about 10-fold less potent than epibatidine and about 17-fold more potent than ABT 418 in inhibiting [3H]nicotine binding to alpha 4 beta 2 nicotinic receptors in rat cerebral cortical membranes. In cultured cells with functional ion flux assays, epiboxidine was nearly equipotent to epibatidine and 200-fold more potent than ABT 418 at alpha 3 beta 4(5) nicotinic receptors in PC12 cells. Epiboxidine was about 5-fold less potent than epibatidine and about 30-fold more potent than ABT 418 in TE671 cells with alpha 1 beta 1 gamma delta nicotinic receptors. In a hot-plate antinociceptive assay with mice, epiboxidine was about 10-fold less potent than epibatidine. However, epiboxidine was also much less toxic than epibatidine in mice.

Structure-activity studies on 2-methyl-3-(2(S)-pyrrolidinylmethoxy) pyridine (ABT-089): an orally bioavailable 3-pyridyl ether nicotinic acetylcholine receptor ligand with cognition-enhancing properties

2-Methyl-3-(2(S)-pyrrolidinylmethoxy)pyridine, ABT-089 (S-4), a member of the 3-pyridyl ether class of nicotinic acetylcholine receptor (nAChR) ligands, shows positive effects in rodent and primate models of cognitive enhancement and a rodent model of anxiolytic activity and possesses a reduced propensity to activate peripheral ganglionic type receptors. The profiles of S-4, its N-methyl analogue, and the corresponding enantiomers across several measures of cholinergic channel function in vitro and in vivo are presented, together with in vitro metabolism and in vivo bioavailability data. On the basis of its biological activities and favorable oral bioavailability, S-4 is an attractive candidate for further evaluation as a treatment for cognitive disorders.

Pharmacological characterization of a nicotinic autoreceptor in rat hippocampal synaptosomes

The modulation of [3H]ACh release by nicotinic compounds was studied in superfused rat hippocampal synaptosomes loaded with [3H]choline, (-)-Nicotine (0.1-10 microM) evoked a dose-dependent increase in [3H]ACh release; higher concentrations were less effective. Nicotine-evoked release was Ca(2+)-dependent, and blocked by the nicotinic antagonists dihydro-beta-erythroidine, mecamylamine, and pempidine. The alpha 7-selective antagonist methyllycaconitine did not inhibit nicotine-evoked release when tested at 1 microM, although at 10 microM some attenuation of the response was observed. Six agonists tested were equally efficacious in stimulating [3H]ACh release, as judged by the maximum responses, and gave the following EC50 values: (+/-)-epibatidine 0.12 microM; (+)-anatoxin-a 0.14 microM; (-)-nicotine 0.99 microM; (-)-cytisine 1.06 microM; ABT-418 2.6 microM; isoarecolone 43 microM. Each agonist generated a "bell-shaped" dose response curve, suggesting desensitisation at higher concentrations. This is supported by analysis of repetitive stimulation with (-)-nicotine and (-)-cytisine: S2/S1 ratios declined sharply with increasing concentration, whereas subsequent KC1-evoked release remained constant. These results are discussed in terms of possible nicotinic receptor subtypes that might be present on hippocampal nerve terminals.

The 1.1 A crystal structure of the neuronal acetylcholine receptor antagonist, alpha-conotoxin PnIA from Conus pennaceus

BACKGROUND

alpha-Conotoxins are peptide toxins, isolated from Conus snails, that block the nicotinic acetylcholine receptor (nAChR). The 16-residue peptides PnIA and PnIB from Conus pennaceus incorporate the same disulfide framework as other alpha-conotoxins but differ in function from most alpha-conotoxins by blocking the neuronal nAChR, rather than the skeletal muscle subtype. The crystal structure determination of PnIA was undertaken to identify structural and surface features that might be important for biological activity.

RESULTS

The 1.1 A crystal structure of synthetic PnIA was determined by direct methods using the Shake-and-Bake program. The three-dimensional structure incorporates a beta turn followed by two alpha-helical turns. The conformation is stabilised by two disulfide bridges that form the interior of the molecule, with all other side chains oriented outwards.

CONCLUSIONS

The compact architecture of the PnIA toxin provides a rigid framework for presentation of chemical groups that are required for activity. The structure is characterized by distinct hydrophobic and polar surfaces; a 16 A separation of the sole positive and negative charges (these two charged residues being located at opposite ends of the molecule); a hydrophobic region and a protruding tyrosine side chain. These features may be important for the specific interaction of PnIA with neuronal nAChR.