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

Alanazi M, Nemes G, El. Yazidi M. New Quinazolin-4(3H)-One Derivatives Incorporating Isoxazole Moiety as Antioxidant Agents: Synthesis, Structural Characterization, and Theoretical DFT Mechanistic Study

Background: This research centers on the development and spectroscopic characterization of new quinazolin-4(3H)-one-isoxazole derivatives (5a-e). The aim was to investigate the regioselectivity of the 1,3-dipolar cycloaddition involving arylnitriloxides and N-propargylquinazolin-4(3H)-one, and to assess the antioxidant properties of the synthesized compounds. The synthetic approach started with the alkylation of quinazolin-4(3H)-one using propargyl bromide, followed by a 1,3-dipolar cycloaddition reaction. Methods: The structural identification of the products was performed using various spectroscopic methods, such as IR, 1H, 13C, and HMBC NMR, HRMS, and single-crystal X-ray diffraction. To further examine the regioselectivity of the cycloaddition, Density Functional Theory (DFT) calculations at the B3LYP/6-31G(d) level were employed. Additionally, the antioxidant potential of the compounds was tested in vitro using DPPH (2,2-Diphenyl-1-picrylhydrazyl)radical scavenging assays. The reaction selectively produced 3,5-disubstituted isoxazoles, with the regiochemical outcome being independent of the substituents on the phenyl ring. Results: Theoretical calculations using DFT were in agreement with the experimental results, revealing activation energies of -81.15 kcal/mol for P-1 and -77.32 kcal/mol for P-2, favoring the formation of P-1. An analysis of the Intrinsic Reaction Coordinate (IRC) confirmed that the reaction proceeded via a concerted but asynchronous mechanism. The antioxidant tests demonstrated that the synthesized compounds exhibited significant radical scavenging activity, as shown in the DPPH assay. The 1,3-dipolar cycloaddition of arylnitriloxides with N-propargylquinazolin-4(3H)-one successfully resulted in novel 3,5-disubstituted isoxazoles. Conclusions: The experimental findings were well-supported by theoretical predictions, and the antioxidant assays revealed strong activity, indicating the potential for future biological applications of these compounds.

Diastereoselective [3 + 2] Cycloaddition between Tertiary Amine N-Oxides and Substituted Alkenes to Access 7-Azanorbornanes

We have developed a diastereoselective synthesis of 43 novel 7-azanorbornanes using tertiary amine N-oxides and substituted alkenes. Our method uses an efficient [3 + 2] cycloaddition, starting from either commercially available or easily accessible precursors to generate yields up to 97% and diastereomeric ratios up to >20:1. Density functional theory (DFT) calculations were performed, suggesting that the observed diastereoselectivity is likely due to steric considerations.

The patent review of the biological activity of tropane containing compounds

INTRODUCTION

Tropane-derived medications have historically played a substantial role in pharmacotherapy. Both natural and synthetic derivatives of tropane find application in addressing diverse medical conditions. Prominent examples of tropane-based drugs include hyoscine butylbromide, recognized for its antispasmodic properties, atropine, employed as a mydriatic, maraviroc, known for its antiviral effects. trospium chloride, utilized as a spasmolytic for overactive bladder, and ipratropium, a bronchodilator.

AREAS COVERED

We compiled patents pertaining to the biological activity of substances containing tropane up to the year 2023 and categorized them according to the specific type of biological activity they exhibit. ScienceFinder, ScienceDirect, and Patent Guru were used to search for scientific articles and patent literature up to 2023.

EXPERT OPINION

Pharmaceutical researchers in academic and industrial settings have shown considerable interest in tropane derivatives. Despite this, there remains a substantial amount of work to be undertaken. A focused approach is warranted for the exploration and advancement of both natural and synthetic bioactive molecules containing tropane, facilitated through collaborative efforts between academia and industry. Leveraging contemporary techniques and technologies in medicinal and synthetic chemistry, including high throughput screening, drug repurposing,and biotechnological engineering, holds the potential to unveil novel possibilities and accelerate the drug discovery process for innovative tropane-based pharmaceuticals.

Tandem construction of biological relevant aliphatic 5-membered N-heterocycles

Nature often uses cascade reactions in a highly stereocontrolled manner for assembly structurally diverse nitrogen-containing heterocyclic scaffolds, i.e. secondary metabolites, important for medicinal chemistry and pharmacy. Five-membered nitrogen-containing heterocycles as standalone rings, as well as spiro and polycyclic systems are pharmacophores of drugs approved in various therapies, i.a. antibacterial or antiviral, antifungal, anticancer, antidiabetic, as they target many key enzymes. Furthermore, a large number of pyrrolidine derivatives are currently considered as drug candidates. Cascade transformations, also known as domino or tandem reactions, offer straightforward methods to build N-heterocyclic libraries of the great structural variety desired for drawing SAR conclusions. The tandem transformations are often atom economic and time-saving because they are performed as the one-pot, so no need for purification after each 'virtual' step and the limited necessity of protective groups are characteristic for these processes. Thus, the same results as in classical multistep synthesis can be achieved at markedly lower costs and shorter time, which is in line with modern green chemistry rules. Great advantage of cascade reactions is often reflected in their high regio- and stereoselectivities, enabling the preparing of the heterocyclic compound better fitted to the expected target in cells. This review reveals the biological relevance of N-heterocyclic scaffolds based on saturated 5-membered rings since we showed a number of examples of approved drugs together with the recent biologically attractive leading structures of drug candidates. Next, novel cascade synthetic procedures, taking into account the structure of the reactants and reaction mechanisms, enabling to obtain biological-relevant heterocyclic frameworks with good yields and relatively high stereoselectivity, were reviewed and compared. The review covers the advances of designing biological active N-heterocycles mainly from 2018 to 2021, whereas the synthetic part is focused on the last 7 years.

Iron-catalysed enantioselective carbometalation of azabicycloalkenes

The first enantioselective carbometalation reaction of azabicycloalkenes has been achieved by iron catalysis to in situ form optically active organozinc intermediates, which are amenable to further synthetic elaborations. The observed chiral induction, along with the DFT and XAS analyses, reveals the direct coordination of the chiral phosphine ligand to the iron centre during the carbon-carbon and carbon-metal bond forming step. This new class of iron-catalysed asymmetric reaction will contribute to the synthesis and production of bioactive molecules.

Epibatidine: A Promising Natural Alkaloid in Health

Epibatidine is a natural alkaloid that acts at nicotinic acetylcholine receptors (nAChRs). The present review aims to carefully discuss the affinity of epibatidine and its synthetic derivatives, analogues to nAChRs for α4β2 subtype, pharmacokinetic parameters, and its role in health. Published literature shows a low affinity and lack of binding of epibatidine and its synthetic analogues to plasma proteins, indicating their availability for metabolism. Because of its high toxicity, the therapeutic use of epibatidine is hampered. However, new synthetic analogs endowed from this molecule have been developed, with a better therapeutic window and improved selectivity. All these aspects are also discussed here. On the other hand, many reports are devoted to structure⁻activity relationships to obtain optically active epibatidine and its analogues, and to access its pharmacological effects. Although pharmacological results are obtained from experimental studies and only a few clinical trials, new perspectives are open for the discovery of new drug therapies.

Direct arylation of strong aliphatic C-H bonds

Despite the widespread success of transition-metal-catalysed cross-coupling methodologies, considerable limitations still exist in reactions at sp³-hybridized carbon atoms, with most approaches relying on prefunctionalized alkylmetal or bromide coupling partners^1,2. Although the use of native functional groups (for example, carboxylic acids, alkenes and alcohols) has improved the overall efficiency of such transformations by expanding the range of potential feedstocks^3-5, the direct functionalization of carbon-hydrogen (C-H) bonds-the most abundant moiety in organic molecules-represents a more ideal approach to molecular construction. In recent years, an impressive range of reactions that form C(sp³)-heteroatom bonds from strong C-H bonds has been reported^6,7. Additionally, valuable technologies have been developed for the formation of carbon-carbon bonds from the corresponding C(sp³)-H bonds via substrate-directed transition-metal C-H insertion⁸, undirected C-H insertion by captodative rhodium carbenoid complexes⁹, or hydrogen atom transfer from weak, hydridic C-H bonds by electrophilic open-shell species^10-14. Despite these advances, a mild and general platform for the coupling of strong, neutral C(sp³)-H bonds with aryl electrophiles has not been realized. Here we describe a protocol for the direct C(sp³) arylation of a diverse set of aliphatic, C-H bond-containing organic frameworks through the combination of light-driven, polyoxometalate-facilitated hydrogen atom transfer and nickel catalysis. This dual-catalytic manifold enables the generation of carbon-centred radicals from strong, neutral C-H bonds, which thereafter act as nucleophiles in nickel-mediated cross-coupling with aryl bromides to afford C(sp³)-C(sp²) cross-coupled products. This technology enables unprecedented, single-step access to a broad array of complex, medicinally relevant molecules directly from natural products and chemical feedstocks through functionalization at sites that are unreactive under traditional methods.

Isoxazole ring as a useful scaffold in a search for new therapeutic agents

Due to its relatively easy synthesis, isoxazole ring has been as an object of interest for chemists and pharmacologists from research groups all over the world. Its chemical modifications include both connection of isoxazole with other aromatic, heteroaromatic or non aromatic rings and substitution with different alkyl groups. Thanks to their usually low cytotoxicity, isoxazole derivatives are still popular scaffolds for the development of new agents with variable biological activities, such as antimicrobial, antiviral, anticancer, anti-inflammatory, immunomodulatory, anticonvulsant or anti-diabetic properties. This review discusses the chemical structure of recently developed isoxazole derivatives with regards to their activity and potential therapeutic use.

Synthesis of isoxazole-containing sulfonamides with potent carbonic anhydrase II and VII inhibitory properties

Two series of benzenesulfonamide containing isoxazole compounds were prepared by using conventional and microwave (MW) methods. 5-Amino-3-aryl-N-(4-sulfamoylphenyl)isoxazole-4-carboxamide derivatives were synthesized by the reaction of hydroxymoyl chlorides with 2-cyano-N-(4-sulfamoylphenyl)acetamide in the presence of triethylamine. The synthesized 5-amino isoxazoles were reacted with various benzoyl chlorides in order to obtain 5-amidoisoxazoles. The novel compounds were screened in vitro as inhibitors of four human (h) isoforms of the metalloenzyme carbonic anhydrase (CA, EC 4.2.1.1): hCA I, hCA II, hCA IV and hCA VII. The derivatives of the first series were shown to possess excellent inhibitory activity against the cytosolic isoform hCA II, an antiglaucoma drug target, with K_Is in the range of 0.5-49.3nM and hCA VII, a recently validated anti-neuropathic pain target with K_Is in the range of 4.3-51.9nM.

Synthesis of Novel Heterocyclic Imine Type UV Absorbers for Application on Cotton Based Textile Materials

Novel Schiff bases with the potential to exhibit biologically active and UV absorption properties were synthesized. Their application on cotton based textile materials was performed and the UV protection factor (UPF) values and the antibacterial features of the treated fabrics were measured.

Ligand-controlled stereodivergent 1,3-dipolar cycloaddition of azomethine ylides with 3-methyl-4-nitro-5-styrylisoxazoles

An unprecedented Ag(i)-catalyzed ligand-controlled stereodivergent 1,3-dipolar cycloaddition of azomethine ylides with 3-methyl-4-nitro-5-styrylisoxazoles has been developed to afford heterocycles bearing both methylisoxazole and pyrrolidine moieties.

Synthesis of (−)-epibatidine

An asymmetric synthesis to the dendrobatid alkaloid (−)-epibatidine has been described, featuring chiral resolution of both optically pure 7-azabicyclo[2.2.1]heptanecarboxylic acid, and subsequent transformations to (−)-epibatidine.

Hydroarylation Reactions of N-Acylaminosubstituted Tricyclic Imides

The palladium-catalysed hydroarylation of unsaturated N-acylamino-substituted tricyclic imides provided a new stereoselective synthesis of exo-aryl(heteroaryl)-substituted tricyclic N-acylamino imides.

Evaluation of the Antioxidative Properties ofN-Acylamino-Substituted Tricyclic Imides

NewN-acylamino-substituted tricyclic imides have been screened for scavenging ability against the free radical 2,2-diphenyl-1-picryl-hydrazyl (DPPH•), chelating activity on ferrous ions, and reductive potential. The results were compared with synthetic antioxidants BHT, BHA, and Trolox. The compounds exhibited different levels of antioxidant activity in all tests.

The enantiomers of epiboxidine and of two related analogs: synthesis and estimation of their binding affinity at α4β2 and α7 neuronal nicotinic acetylcholine receptors

Epiboxidine hydrochlorides (+)-2 and (-)-2, which are the structural analogs of the antipodes of epibatidine (±)-1, as well as the enantiomeric pairs (+)-3/(-)-3 and (+)-4/(-)-4 were synthesized and tested for binding affinity at α4β2 and α7 nicotinic acetylcholine receptor (nAChR) subtypes. Final derivatives were prepared through the condensation of racemic N-Boc-7-azabicyclo[2.2.1]heptane-2-one (±)-5 with the resolving agent (R)-(+)-2-methyl-2-propanesulfinamide. The pharmacological analysis carried out on the three new enantiomeric pairs evidenced an overall negligible degree of enantioselectivity at both nAChRs subtypes, a result similar to that reported for both natural and unnatural epibatidine enantiomers at the same investigated receptor subtypes.

Identification of novel α4β2-nicotinic acetylcholine receptor (nAChR) agonists based on an isoxazole ether scaffold that demonstrate antidepressant-like activity

There is considerable evidence to support the hypothesis that the blockade of nAChR is responsible for the antidepressant action of nicotinic ligands. The nicotinic acetylcholine receptor (nAChR) antagonist, mecamylamine, has been shown to be an effective add-on in patients that do not respond to selective serotonin reuptake inhibitors. This suggests that nAChR ligands may address an unmet clinical need by providing relief from depressive symptoms in refractory patients. In this study, a new series of nAChR ligands based on an isoxazole-ether scaffold have been designed and synthesized for binding and functional assays. Preliminary structure-activity relationship (SAR) efforts identified a lead compound 43, which possesses potent antidepressant-like activity (1 mg/kg, IP; 5 mg/kg, PO) in the classical mouse forced swim test. Early stage absorption, distribution, metabolism, excretion, and toxicity (ADME-Tox) studies also suggested favorable drug-like properties, and broad screening toward other common neurotransmitter receptors indicated that compound 43 is highly selective for nAChRs over the other 45 neurotransmitter receptors and transporters tested.

Synthesis and binding affinity at α4β2 and α7 nicotinic acetylcholine receptors of new analogs of epibatidine and epiboxidine containing the 7-azabicyclo[2.2.1]hept-2-ene ring system

A group of novel racemic nicotinic ligands structurally related to epibatidine or epiboxidine [(±)-10-(±)-17] was synthesized through a palladium-catalyzed cross-coupling between the appropriate vinyl triflate and a range of organometallic heterocycles. The target compounds were evaluated for binding affinity at the α4β2 and α7 neuronal nicotinic receptors (nAChRs). The set of 3-pyridinyl derivatives (±)-10, (±)-11 and (±)-12 exhibited an affinity for the α4β2 nAChR subtype in the subnanomolar range (K(i) values of 0.20, 0.40 and 0.50nM, respectively) and behaved as α4β2 versus α7 subtype selective ligands. Interestingly, the epiboxidine-related dimethylammonium iodide (±)-17, which retained a good affinity for the α4β2 nAChR (K(i)=13.30nM), tightly bound also to the α7 subtype (K(i)=1.60nM), thus displaying a reversal of the affinity trend among the reference and new nicotinic ligands under investigation.

Synthesis and biological activity of a novel class nicotinic acetylcholine receptors (nAChRs) ligands structurally related to anatoxin-a

The introduction of the isoxazole ring as bioisosteric replacement of the acetyl group of anatoxin-a led to a new series of derivatives binding to nicotinic acetylcholine receptors. Bulkier substitutions than methyl at the 3 position of isoxazole were shown to be detrimental for the activity. The binding potency of the most interesting compounds with α1, α7 and α3β4 receptor subtypes, was, anyway, only at micromolar level. Moreover, differently from known derivatives with pyridine, isoxazole condensed to azabicyclo ring led to no activity.

Exploiting the facile release of trifluoroacetate for the α-methylenation of the sterically hindered carbonyl groups on (+)-sclareolide and (-)-eburnamonine

An efficient method for the α-methylenation of carbonyl groups is reported, and this transformation is accomplished by a facile elimination of trifluoroacetate during the formation of the olefin. This method represents an improvement beyond existing protocol in cases of steric hindrance, and we have demonstrated the utility of the process across a series of ketones, lactams, and lactones. Additionally, we have applied this method to produce semisynthetic derivatives of the natural products (+)-sclareolide and (-)-eburnamonine, in which the carbonyl group is proximal to bulky functional groups. Mechanistic insight is also provided from a time course of (19)F NMR. Biological evaluation of the natural-product-derived enones led to the identification of a derivative of (-)-eburnamonine with significant cytotoxicity (LC(50) = 14.12 μM) in drug-resistant MDA-MB-231 breast cancer cells.

Unique spirocyclopiperazinium salt III: Further investigation of monospirocyclopiperazinium (MSPZ) salts as potential analgesics

Two novel classes of monospirocyclopiperazinium salts were designed, synthesized, and evaluated for their in vivo analgesic activities. Some interesting structure-activity relationships are revealed: (1) Spirocyclopiperazinium moiety is favorable to improve the analgesic activity; (2) The size and conformation of spirocyclopiperazinium moiety significantly affects the analgesic activity; (3) Phenylethyl group of 3d is a crucial pharmacophore. Among the compounds synthesized, 3d exhibited the most potent activity with low toxicity. Further antinociceptive mechanism studies of 3d showed that these compounds will be a new kind of analgesics.

Aza-Prins-Pinacol Approach to 7-Azabicyclo[2.2.1]heptanes: Syntheses of (±)-Epibatidine and (±)-Epiboxidine

The syntheses of (+/-)-epibatidine and (+/-)-epiboxidine have been accomplished from commercial 2-methoxy-3,4-dihydro-2H-pyran. A recently developed aza-Prins-pinacol rearrangement was employed for the construction of the key 7-azabicyclo[2.2.1]heptane skeleton of these targets.

Central Nicotinic Receptors: Structure, Function, Ligands, and Therapeutic Potential

The growing interest in nicotinic receptors, because of their wide expression in neuronal and non-neuronal tissues and their involvement in several important CNS pathologies, has stimulated the synthesis of a high number of ligands able to modulate their function. These membrane proteins appear to be highly heterogeneous, and still only incomplete information is available on their structure, subunit composition, and stoichiometry. This is due to the lack of selective ligands to study the role of nAChR under physiological or pathological conditions; so far, only compounds showing selectivity between alpha4beta2 and alpha7 receptors have been obtained. The nicotinic receptor ligands have been designed starting from lead compounds from natural sources such as nicotine, cytisine, or epibatidine, and, more recently, through the high-throughput screening of chemical libraries. This review focuses on the structure of the new agonists, antagonists, and allosteric ligands of nicotinic receptors, it highlights the current knowledge on the binding site models as a molecular modeling approach to design new compounds, and it discusses the nAChR modulators which have entered clinical trials.

Epibatidine isomers and analogues: Structure–activity relationships

Binding affinities for a range of epibatidine isomers and analogues at the alpha4beta2 and alpha3beta4 nAChR subtypes are reported; compounds having similar N-N distances to epibatidine show similar, high potencies.

Epibatidine and its analogues as nicotinic acetylcholine receptor agonist: an update

Epibatidine (EPB) (1), an alkaloid isolated from the skin of the Ecuadorian poison frog, Epipedobates tricolor has attracted attention because of its exceptionally powerful analgesic properties: several hundred times greater than those of morphine, and the fact that it acts at nicotine rather than opiate receptors. Although the substance is toxic, it does serve as a lead compound in the development of drugs for pain relief as well as treatment of disorders whose pathogenesis involves nicotinic receptors. In this article, isolation, synthetic methods, effect on neuronal and neuromuscular nicotinic receptors, therapeutic potential, toxicity, nicotinic pharmacophore structural modifications related issues of 1 are discussed.

Phosphorimaging detection and quantitation for isotopic ion flux assays

A 96-well-microplate-based ion flux method utilizing readily available autoradiographic phosphorimaging detection is described. Nicotinic acetylcholine receptor-mediated (22)Na influx in four cultured cell lines provided satisfactory concentration-response data for epibatidine and several other nicotinic agonists. The data were consistent with data obtained using standard 6-well assays. Assays for nicotinic-receptor-mediated (86)Rb efflux produced data similar to data obtained with the (22)Na influx assay. However, assays for (45)Ca influx were not successful, although (45)Ca was readily detected and quantified. Voltage-gated sodium channel-mediated (22)Na influx in a neuroblastoma cell line allowed assay of the effects of such sodium channel activators as batrachotoxin and a pumiliotoxin B/scorpion venom combination. Phosphorimaging detection allows for reliable beta counting of up to 1,200 simultaneous samples with excellent sensitivity and is amenable for application to high-throughput screening.

Nicotinic agonists, antagonists, and modulators from natural sources

1. Acetylcholine receptors were initially defined as nicotinic or muscarinic, based on selective activation by two natural products, nicotine and muscarine. Several further nicotinic agonists have been discovered from natural sources, including cytisine, anatoxin, ferruginine, anabaseine, epibatidine, and epiquinamide. These have provided lead structures for the design of a wide range of synthetic agents. 2. Natural sources have also provided competitive nicotinic antagonists, such as the Erythrina alkaloids, the tubocurarines, and methyllycaconitine. Noncompetitive antagonists, such as the histrionicotoxins, various izidines, decahydroquinolines, spiropyrrolizidine oximes, pseudophrynamines, ibogaine, strychnine, cocaine, and sparteine have come from natural sources. Finally, galanthamine, codeine, and ivermectin represent positive modulators of nicotinic function, derived from natural sources. 3. Clearly, research on acetylcholine receptors and functions has been dependent on key natural products and the synthetic agents that they inspired.

3-(2,5-Dihydro-1H-pyrrol-2-ylmethoxy)pyridines: synthesis and analgesic activity

We disclose an efficient procedure for the preparation of ethers of 2-substituted 2-hydroxymethylpyrroline and of 2-aminomethyl-3-pyrrolines, involving, as a key step, formation and nucleophilic ring opening of a cyclic sulfamidate. Several new analogs of epibatidine (1) and tebanicline (ABT-594, 2) were prepared and tested for analgesic activity in the mouse formalin model.

Synthesis and nicotinic acetylcholine receptor binding affinities of 2- and 3-isoxazolyl-8-azabicyclo[3.2.1]octanes

A series of epiboxidine homologues, 2- and 3-isoxazole substituted 8-azabicyclo[3.2.1]octane derivatives was synthesized and evaluated as potential ligands for neuronal nicotinic acetylcholine receptors in [(3)H]cytisine labeled rat brain. The 2beta-isoxazolyl-8-azabicyclo[3.2.1]octane 9b (K(i)=3 nM) was the most potent compound of the series with a binding affinity twice that of nicotine. The 3beta-isoxazolyl-8-azabicyclo[3.2.1]octane 15b (K(i)=148 nM) exhibited moderate affinity while the corresponding 2alpha- and 3alpha-isomers exhibited micromolar binding affinity.

Epibatidine structure–activity relationships

Epibatidine is a potent but nonselective nAChR agonist. Its biological effects appear to be mediated largely by alpha4beta2 nAChRs. Surprisingly, only a limited number of epibatidine analogues have been synthesized and evaluated in in vitro assays. Even fewer analogues have received in vivo pharmacological evaluation. In this paper, SAR studies directed toward epibatidine analogues will be reviewed.

Homoepiboxidines

Homoepiboxidine (3) and the corresponding N-methyl (4) and N-benzyl (5) derivatives were prepared from a 6beta-carbomethoxynortropane (8). Affinities and functional activities at neuromuscular, central neuronal and ganglionic-type nicotinic receptors were compared to those of epibatidine 1, and epiboxidine 2. Homoepiboxidine had equivalent affinity/activity to epiboxidine at neuromuscular, neuronal alpha4beta2, and most alpha3-containing ganglionic-type nicotinic receptors. The N-substituted derivatives showed reduced affinity/activity at most receptor subtypes. Replacement of the methylisoxazole moiety of 3 and 4 with a methyloxadiazole moiety provided analogues 6 and 7, which had greatly reduced affinity/activity in virtually all assays at nicotinic receptors. Marked analgetic activity in mice occurred at the following ip doses: epibatidine 10 microg/kg; epiboxidine 25 microg/kg; homoepiboxidine 100 microg/kg; N-methylhomoepiboxidine 100 microg/kg; the methyloxadiazole (6) 100 microg/kg. The time course at such ip doses was significantly longer for homoepiboxidine 3 with marked analgesia still manifest at 30 min post-injection. Epiboxidine and the homoepiboxidines were less toxic than epibatidine.

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.

Epibatidine: impact on nicotinic receptor research

Epibatidine is a natural product that was isolated and identified by Daly and coworkers in 1992. Since that time, it has had a profound influence on the investigation of alpha4beta2 nicotinic cholinergic (nACh) receptor pharmacophore models, and has inspired the development of novel agents with therapeutic potential in CNS disorders. Apart from acetylcholine and nicotine, probably no other agent has had as much recent impact on nACh research as has epibatidine.

3D QSAR analyses-guided rational design of novel ligands for the (alpha4)2(beta2)3 nicotinic acetylcholine receptor

Three-dimensional quantitative structure-activity relationship methods, the comparative molecular field analysis (CoMFA) and the comparative molecular similarity indices analysis (CoMSIA), were applied using a training set of 45 ligands of the (alpha4)2(beta2)3 nicotinic acetylcholine receptor (nAChR). All compounds are related to (-)-epibatidine, (-)-cytisine, (+)-anatoxin-a, and (-)-ferruginine, and additionally, novel diazabicyclo[4.2.1]nonane- and quinuclidin-2-ene-based structures were included. Their biological data have been determined by utilizing the same experimental protocol. Statistically reliable models of good predictive power (CoMFA r2 = 0.928, q2 = 0.692, no. of components = 3; CoMSIA r2 = 0.899, q2 = 0.701, no. of components = 3) were achieved. The results obtained were graphically interpreted in terms of field contribution maps. Hence, physicochemical determinants of binding, such as steric and electrostatic and, for the first time, hydrophobic, hydrogen bond donor, and hydrogen bond acceptor properties, were mapped back onto the molecular structures of a set of nAChR modulators. In particular, changes in the binding affinity of the modulators as a result of modifications in the aromatic ring systems could be rationalized by the steric, electrostatic, hydrophobic, and hydrogen bond acceptor properties. These results were used to guide the rational design of new nAChR ligands such as 48-52 and 54, which were subsequently synthesized for the first time and tested. Key steps of our synthetic approaches were successfully applied Stille and Suzuki cross-coupling reactions. Predictive r2 values of 0.614 and 0.660 for CoMFA and CoMSIA, respectively, obtained for 22 in part previously unknown ligands for the (alpha4)2(beta2)3 subtype, demonstrate the high quality of the 3D QSAR models.