Structure−Activity Studies and Analgesic Efficacy of N-(3-Pyridinyl)-Bridged Bicyclic Diamines, Exceptionally Potent Agonists at Nicotinic Acetylcholine Receptors

Applications of Isosteres of Piperazine in the Design of Biologically Active Compounds: Part 1

Piperazine and homopiperazine are well-studied heterocycles in drug design that have found gainful application as scaffolds and terminal elements and for enhancing the aqueous solubility of a molecule. The optimization of drug candidates that incorporate these heterocycles in an effort to refine potency, selectivity, and developability properties has stimulated the design and evaluation of a wide range of bioisosteres that can offer advantage. In this review, we summarize the design and application of bioisosteres of piperazine and homopiperazine that have almost exclusively been in the drug design arena. While there are ∼100 approved drugs that incorporate a piperazine ring, only a single marketed agricultural product is built on this heterocycle. As part of the review, we discuss some of the potential reasons underlying the relatively low level of importance of this heterocycle to the design of agrochemicals and highlight the potential opportunities for their use in contemporary research programs.

The piperazine scaffold for novel drug discovery efforts: the evidence to date

INTRODUCTION

. Piperazine is a structural element present in drugs belonging to various chemical classes and used for numerous different therapeutic applications; it has been considered a privileged scaffold for drug design.

AREAS COVERED

The authors have searched examples of piperazine-containing compounds among drugs recently approved by the FDA, and in some research fields (nicotinic receptor modulators, compounds acting against cancer and bacterial multi-drug resistance), looking in particular to the design behind the insertion of this moiety.

EXPERT OPINION

Piperazine is widely used due to its peculiar characteristics, such as solubility, basicity, chemical reactivity, and conformational properties. This moiety has represented an important tool to modulate pharmacokinetic and pharmacodynamic properties of drugs.

3,6-diazabicyclo[3.3.1]heptanes Induces Apoptosis and Arrests Cell Cycle in Prostate Cancer Cells

Background

Prostate cancer, non-cutaneous malignant tumor, is the second common cause of cancer related mortalities in American men and is responsible for 13% of deaths related to cancer. The present study investigated the anti-cancer effects of 3,6-diazabicyclo[3.3.1]heptane on LNCaP and PC3 prostate cancer cells in vitro and on tumor growth in vivo in BALB/C nude mice.

Material/Methods

Reduction of cell viability by 3,6-diazabicyclo[3.3.1]heptane was evaluated by sulphorhodamine-B staining and apoptosis onset using annexin V and propidium iodide (PI) staining. The 2′,7′-dichlorofluorescein-diacetate stain was used for assessment of reactive oxygen species (ROS) formation while as western blotting for analysis of protein expression.

Results

The viability of LNCaP and PC3 cells was reduced significantly (P<0.05) by 3,6-diazabicyclo[3.3.1]heptane in dose-based manner. At 30 μM of 3,6-diazabicyclo[3.3.1]heptane the viability of LNCaP and PC3 cells was reduced to 32 and 28%, respectively. The 3,6-diazabicyclo[3.3.1]heptane treatment increased apoptosis in LNCaP cells to 43.31% at 30 μM. The cell cycle in LNCaP cells was arrested in G1 phase on treatment with 3,6-diazabicyclo[3.3.1]heptane. The expression of cyclin D1 and p21 proteins was significantly increased by 3,6-diazabicyclo[3.3.1]heptane in LNCaP and PC3 cells. The growth of prostate tumor was also suppressed in vivo in mice by 3,6-diazabicyclo[3.3.1]heptane treatment.

Conclusions

In summary, the study demonstrated that LNCaP and PC3 prostate cancer cell viability is suppressed by 3,6-diazabicyclo[3.3.1]heptane treatment. The suppression of prostate cancer cell viability by 3,6-diazabicyclo[3.3.1]heptane involves apoptosis induction, cell cycle arrest and upregulation of p21 expression. Therefore, 3,6-diazabicyclo[3.3.1]heptane can be a potential chemotherapeutic agent for prostate cancer.

Novel N-aryl nicotinamide derivatives: Taking stock on 3,6-diazabicyclo[3.1.1]heptanes as ligands for neuronal acetylcholine receptors

We designed the synthesis of a small library of 3-substituted-3,6-diazabicyclo[3.1.1]heptanes whose affinity on neuronal nicotinic receptors (nAChRs) was evaluated. Among the synthesized compounds, the 5-(3,6-diazabicyclo[3.1.1]heptane-3-yl)-N-(2-fluorophenyl)nicotinamide 43 proved to be the most interesting compound with α4β2Ki value of 10 pM and a very high α7/α4β2 selectivity. Furthermore, compounds 35, 39 and 43 elicited a selective partial agonist activity for α4β2 nAChR subtype. Finally, in this paper we also report the conclusions on the 3,6-diazabicyclo[3.1.1]heptanes as ligands for nAChRs, resulting from our consolidated structure activity relationship (SAR) studies on this template.

Pyridinyl- and pyridazinyl-3,6-diazabicyclo[3.1.1]heptane-anilines: Novel selective ligands with subnanomolar affinity for α4β2 nACh receptors

The cholinergic pathways in the central nervous system (CNS) of animals and humans are important for cognitive and behavioural functions. Until a few years ago, it was thought that the key molecules transducing the cholinergic message were the metabotropic muscarinic receptors, but it is now known that ionotropic neuronal nicotinic receptors (nAChRs) are also involved. Based on recent studies, we prepared a small library of novel 3-substituted-3,6-diazabicyclo [3.1.1]heptanes, whose binding activity and functionality have been assayed. Among the synthesized compounds, the 3-(anilino)pyridine series resulted in the most interesting compounds with α₄β₂K_i values ranging from 0.0225 nM (12g) to 2.06 nM (12o).

Crystal structure of (1S,4S)-2,5-diazo-niabi-cyclo[2.2.1]heptane dibromide

The cage of 2,5-di-aza-bicyclo-[2.2.1]heptane is frequently employed in synthetic chemistry as a rigid bicyclic counterpart of the piperazine ring. The 2,5-di-azabicyclo-[2.2.1]heptane scaffold is incorporated into a variety of compounds having pharmacological and catalytic applications. The unsubstituted parent ring of the system, 2,5-di-aza-bicyclo-[2.2.1]heptane itself, has not been structurally characterized. We herein report on the mol-ecular structure of the parent ring in (1S,4S)-2,5-diazo-niabi-cyclo-[2.2.1]heptane dibromide, C5H12N22+·2Br-. The asymmetric unit contains two crystallographically independent cages of 2,5-di-aza-bicyclo-[2.2.1]heptane. Each cage is protonated at the two nitro-gen sites. The overall charge balance is maintained by four crystallographically independent bromide ions. In the crystal, the components of the structure are linked via a complex three-dimensional network of N-H⋯Br hydrogen bonds.

Synthesis and biological evaluation of novel hybrids of highly potent and selective α4β2-Nicotinic acetylcholine receptor (nAChR) partial agonists

We previously reported the cyclopropylpyridine and isoxazolylpyridine ether scaffolds to be versatile building blocks for creating potent α4β2 nicotinic acetylcholine receptor (nAChR) partial agonists with excellent selectivity over the α3β4 subtype. In our continued efforts to develop therapeutic nicotinic ligands, seven novel hybrid compounds were rationally designed, synthesized, and evaluated in [³H]epibatidine binding competition studies. Incorporation of a cyclopropane- or isoxazole-containing side chain onto the 5-position of 1-(pyridin-3-yl)-1,4-diazepane or 2-(pyridin-3-yl)-2,5-diazabicyclo[2.2.1]heptane led to highly potent and selective α4β2* nAChR partial agonists with K_i values of 0.5-51.4 nM for α4β2 and negligible affinities for α3β4 and α7. Moreover, compounds 21, 25, and 30 maintained the functional profiles (EC₅₀ and IC₅₀ values of 15-50 nM) of the parent azetidine-containing compounds 3 and 4 in the ⁸⁶Rb⁺ ion flux assays. In vivo efficacy of the most promising compound 21 was confirmed in the mouse SmartCube^® platform and classical forced swim tests, supporting the potential use of α4β2 partial agonists for treatment of depression.

Chemistry and Pharmacology of a Series of Unichiral Analogues of 2-(2-Pyrrolidinyl)-1,4-benzodioxane, Prolinol Phenyl Ether, and Prolinol 3-Pyridyl Ether Designed as α4β2-Nicotinic Acetylcholine Receptor Agonists

Some unichiral analogues of 2R,2'S-2-(1'-methyl-2'-pyrrolidinyl)-7-hydroxy-1,4-benzodioxane, a potent and selective α4β2-nAChR partial agonist, were designed by opening dioxane and replacing hydroxyl carbon with nitrogen. The resulting 3-pyridyl and m-hydroxyphenyl ethers have high α4β2 affinity and good subtype selectivity, which get lost if OH is removed from phenyl or the position of pyridine nitrogen is changed. High α4β2 affinity and selectivity are also attained by meta hydroxylating the 3-pyridyl and the phenyl ethers of (S)-N-methylprolinol and the phenyl ether of (S)-2-azetidinemethanol, known α4β2 agonists, although the interaction mode of the aryloxymethylene substructure cannot be assimilated to that of benzodioxane. Indeed, the α4β2 and α3β4 functional tests well differentiate behaviors that the binding tests homologize: both the 3-hydroxyphenyl and the 5-hydroxy-3-pyridyl ether of N-methylprolinol are α4β2 full agonists, but only the latter is highly α4β2/α3β4 selective, while potent and selective partial α4β2 agonism characterizes the hydroxybenzodioxane derivative and its two opened semirigid analogues.

Synthesis of (1R,4R)-2,5-diazabicyclo[2.2.1]heptane derivatives by an epimerization–lactamization cascade reaction

An epimerization–lactamization cascade of functionalized (2S,4R)-4-aminoproline methyl esters is developed and applied in synthesizing (1R,4R)-2,5-diazabicyclo[2.2.1]heptane (DBH) derivatives.

Identification and pharmacological characterization of 3,6-diazabicyclo[3.1.1]heptane-3-carboxamides as novel ligands for the α4β2 and α6/α3β2β3 nicotinic acetylcholine receptors (nAChRs)

We have synthesized a novel series of compounds, 3,6-diazabicyclo[3.1.1]heptane-3-carboxamides, targeting both the α4β2 and α6/α3β2β3 nAChRs. Members of the obtained chemical library are partial or full agonists at both the high sensitivity (α4)2(β2)3 and α6/α3β2β3 nAChRs. 3-(Cyclopropylcarbonyl)-3,6-diazabicyclo[3.1.1]heptane (TC-8831 or compound 7 herein) demonstrated a safe in vitro pharmacological profile and the potential for reducing or preventing L-dopa-induced dyskinesias (LID) in several in vivo animal models [1-4]. In vivo metabolism studies in rat and in vitro metabolism studies in liver microsomes from human, rat, dog and monkey showed TC-8831 to be relatively stable. In vivo pharmacokinetic analysis in the rat confirmed brain penetration, with an average brain:plasma ratio of approximately 0.3 across time points from 0.5 to 4 h. Docking into homology models predicted alternative binding modes for TC-8831 and highlighted the importance of the cationic center, hydrogen-bond acceptor, and hydrophobic aliphatic features in promoting binding affinity to both nAChRs. Pharmacophore elucidation confirmed the importance of these key interactions. QSAR modeling suggested that binding affinity is primarily driven by ligand shape, relative positive charge distribution onto the molecular surface, and molecular flexibility. Of the two subtypes, ligand binding to α6β2β3 appears to be more sensitive to bulkiness and flexibility.

Antiprotozoal activity of dicationic 3,5-diphenylisoxazoles, their prodrugs and aza-analogues

Fifty novel prodrugs and aza-analogues of 3,5-bis(4-amidinophenyl)isoxazole and its derivatives were prepared. Eighteen of the 24 aza-analogues exhibited IC₅₀ values below 25 nM against Trypanosoma brucei rhodesiense or Plasmodium falciparum. Six compounds had antitrypanosomal IC₅₀ values below 10 nM. Twelve analogues showed similar antiplasmodial activities, including three with sub-nanomolar potencies. Forty-four diamidines (including 16 aza-analogues) and the 26 prodrugs were evaluated for efficacy in mice infected with T. b. rhodesiense STIB900. Six diamidines cured 4/4 mice at daily 5 mg/kg intraperitoneal doses for 4 days, giving results far superior to pentamidine and furamidine. One prodrug attained 3/4 cures at daily 25 mg/kg oral doses for 4 days.

Investigation of functionalized α-chloroalkyllithiums for a stereospecific reagent-controlled homologation approach to the analgesic alkaloid (-)-epibatidine

Four putative functionalized α-chloroakyllithiums RCH2CHLiCl, where R=CHCH2(18 a), CCH (18 b), CH2OBn (18 c), and CH[O(CH2)2O] (18 d), were generated in situ by sulfoxide-lithium exchange from α-chlorosulfoxides, and investigated for the stereospecific reagent-controlled homologation (StReCH) of phenethyl and 2-chloropyrid-5-yl (17) pinacol boronic esters. Deuterium labeling experiments revealed that α-chloroalkyllithiums are quenched by proton transfer from their α-chlorosulfoxide precursors and it was established that this effect compromises the yield of StReCH reactions. Use of α-deuterated α-chlorosulfoxides was discovered to ameliorate the problem by retarding the rate of acid-base chemistry between the carbenoid and its precursor. Carbenoids 18 a and 18 b showed poor StReCH efficacy, particularly the propargyl group bearing carbenoid 18 b, the instability of which was attributed to a facile 1,2-hydride shift. By contrast, 18 d, a carbenoid that benefits from a stabilizing interaction between O and Li atoms gave good StReCH yields. Boronate 17 was chain extended by carbenoids 18 a, 18 b, and 18 d in 16, 0, and 68% yield, respectively; α-deuterated isotopomers D-18 a and D-18 d gave yields of 33 and 79% for the same reaction. Double StReCH of 17 was pursued to target contiguous stereodiads appropriate for the total synthesis of (-)-epibatidine (15). One-pot double StReCH of boronate 17 by two exposures to (S)-D-18 a (≤66 % ee), followed by work-up with KOOH, gave the expected stereodiad product in 16% yield (d.r.~67:33). The comparable reaction using two exposures to (S)-D-18 d (≤90% ee) delivered the expected bisacetal containing stereodiad (R,R)-DD-48 in 40% yield (≥98% ee, d.r.=85:15). Double StReCH of 17 using (S)-D-18 d (≤90% ee) followed by (R)-D-18 d (≤90% ee) likewise gave (R,S)-DD-48 in 49% yield (≥97% ee, d.r.=79:21). (R,S)-DD-48 was converted to a dideuterated isotopomer of a synthetic intermediate in Corey's synthesis of 15.

Structure-activity studies of 7-heteroaryl-3-azabicyclo[3.3.1]non-6-enes: a novel class of highly potent nicotinic receptor ligands

The potential for nicotinic ligands with affinity for the α4β2 or α7 subtypes to treat such diverse diseases as nicotine addiction, neuropathic pain, and neurodegenerative and cognitive disorders has been exhibited clinically for several compounds while preclinical activity in relevant in vivo models has been demonstrated for many more. For several therapeutic programs, we sought nicotinic ligands with various combinations of affinity and function across both subtypes, with an emphasis on dual α4β2-α7 ligands, to explore the possibility of synergistic effects. We report here the structure-activity relationships (SAR) for a novel series of 7-heteroaryl-3-azabicyclo[3.3.1]non-6-enes and characterize many of the analogues for activity at multiple nicotinic subtypes.

Intersubunit bridge formation governs agonist efficacy at nicotinic acetylcholine α4β2 receptors: unique role of halogen bonding revealed

The α4β2 subtype of the nicotinic acetylcholine receptor has been pursued as a drug target for treatment of psychiatric and neurodegenerative disorders and smoking cessation aids for decades. Still, a thorough understanding of structure-function relationships of α4β2 agonists is lacking. Using binding experiments, electrophysiology and x-ray crystallography we have investigated a consecutive series of five prototypical pyridine-containing agonists derived from 1-(pyridin-3-yl)-1,4-diazepane. A correlation between binding affinities at α4β2 and the acetylcholine-binding protein from Lymnaea stagnalis (Ls-AChBP) confirms Ls-AChBP as structural surrogate for α4β2 receptors. Crystal structures of five agonists with efficacies at α4β2 from 21-76% were determined in complex with Ls-AChBP. No variation in closure of loop C is observed despite large efficacy variations. Instead, the efficacy of a compound appears tightly coupled to its ability to form a strong intersubunit bridge linking the primary and complementary binding interfaces. For the tested agonists, a specific halogen bond was observed to play a large role in establishing such strong intersubunit anchoring.

Rank order entropy: why one metric is not enough

The use of Quantitative Structure-Activity Relationship models to address problems in drug discovery has a mixed history, generally resulting from the misapplication of QSAR models that were either poorly constructed or used outside of their domains of applicability. This situation has motivated the development of a variety of model performance metrics (r(2), PRESS r(2), F-tests, etc.) designed to increase user confidence in the validity of QSAR predictions. In a typical workflow scenario, QSAR models are created and validated on training sets of molecules using metrics such as Leave-One-Out or many-fold cross-validation methods that attempt to assess their internal consistency. However, few current validation methods are designed to directly address the stability of QSAR predictions in response to changes in the information content of the training set. Since the main purpose of QSAR is to quickly and accurately estimate a property of interest for an untested set of molecules, it makes sense to have a means at hand to correctly set user expectations of model performance. In fact, the numerical value of a molecular prediction is often less important to the end user than knowing the rank order of that set of molecules according to their predicted end point values. Consequently, a means for characterizing the stability of predicted rank order is an important component of predictive QSAR. Unfortunately, none of the many validation metrics currently available directly measure the stability of rank order prediction, making the development of an additional metric that can quantify model stability a high priority. To address this need, this work examines the stabilities of QSAR rank order models created from representative data sets, descriptor sets, and modeling methods that were then assessed using Kendall Tau as a rank order metric, upon which the Shannon entropy was evaluated as a means of quantifying rank-order stability. Random removal of data from the training set, also known as Data Truncation Analysis (DTA), was used as a means for systematically reducing the information content of each training set while examining both rank order performance and rank order stability in the face of training set data loss. The premise for DTA ROE model evaluation is that the response of a model to incremental loss of training information will be indicative of the quality and sufficiency of its training set, learning method, and descriptor types to cover a particular domain of applicability. This process is termed a "rank order entropy" evaluation or ROE. By analogy with information theory, an unstable rank order model displays a high level of implicit entropy, while a QSAR rank order model which remains nearly unchanged during training set reductions would show low entropy. In this work, the ROE metric was applied to 71 data sets of different sizes and was found to reveal more information about the behavior of the models than traditional metrics alone. Stable, or consistently performing models, did not necessarily predict rank order well. Models that performed well in rank order did not necessarily perform well in traditional metrics. In the end, it was shown that ROE metrics suggested that some QSAR models that are typically used should be discarded. ROE evaluation helps to discern which combinations of data set, descriptor set, and modeling methods lead to usable models in prioritization schemes and provides confidence in the use of a particular model within a specific domain of applicability.

Expeditious synthesis of cis-1-methyl-2, 3,3a,4,5,9b-hexahydro-1H-pyrrolo-[3,2h]isoquinoline / [2,3-f]quinoline via azomethine ylide-alkene [3+2] cycloaddition

Expeditious syntheses of cis-1-methyl-2,3,3a,4,5,9b-hexahydro-1H-pyrrolo-[3,2h]isoquinoline /[2,3-f]]quinoline have been developed. The syntheses started with commercially available material and afforded excellent overall yields in straightforward steps. Intramolecular azomethine ylide-alkene [3+2] cycloaddition is the key step in the construction of these pyrroloisoquinoline and pyrroloquinoline scaffolds. This route is much more atom-economic than those reported in the literature and appropriate for scale-up synthesis.

Analgesic and antineuropathic drugs acting through central cholinergic mechanisms

The role of muscarinic and nicotinic cholinergic receptors in analgesia and neuropathic pain relief is relatively unknown. This review describes how such drugs induce analgesia or alleviate neuropathic pain by acting on the central cholinergic system. Several pharmacological strategies are discussed which increase synthesis and release of acetylcholine (ACh) from cholinergic neurons. The effects of their acute and chronic administration are described. The pharmacological strategies which facilitate the physiological functions of the cholinergic system without altering the normal modulation of cholinergic signals are highlighted. It is proposed that full agonists of muscarinic or nicotinic receptors should be avoided. Their activation is too intense and un-physiological because neuronal signals are distorted when these receptors are constantly activated. Good results can be achieved by using agents that are able to a) increase ACh synthesis, b) partially inhibit cholinesterase activity c) selectively block the autoreceptor or heteroreceptor feedback mechanisms. Activation of M(1) subtype muscarinic receptors induces analgesia. Chronic stimulation of nicotinic (N(1)) receptors has neuronal protective effects. Recent experimental results indicate a relationship between repeated cholinergic stimulation and neurotrophic activation of the glial derived neurotrophic factor (GDNF) family. At least 9 patents covering novel chemicals for cholinergic system modulation and pain control are discussed.

A novel series of [3.2.1] azabicyclic biaryl ethers as alpha3beta4 and alpha6/4beta4 nicotinic receptor agonists

We report the synthesis of a series of [3.2.1]azabicyclic biaryl ethers as selective agonists of alpha3- and alpha6-containing nicotinic receptors. In particular, compound 17a from this series is a potent alpha3beta4 and alpha6/4beta4 receptor agonist in terms of both binding and functional activity. Compound 17a also shows potent in vivo activity in CNS-mediated animal models that are sensitive to antipsychotic drugs. Compound 17a may thus be a useful tool for studying the role of alpha3beta4 and alpha6/4beta4 nicotinic receptors in CNS pharmacology.

Design and synthesis of conformationally constrained N,N-disubstituted 1,4-diazepanes as potent orexin receptor antagonists

Orexins are neuropeptides that regulate wakefulness and arousal. Small molecule antagonists of orexin receptors may provide a novel therapy for the treatment of insomnia and other sleep disorders. In this Letter we describe the design and synthesis of conformationally constrained N,N-disubstituted 1,4-diazepanes as orexin receptor antagonists. The design of these constrained analogs was guided by an understanding of the preferred solution and solid state conformation of the diazepane central ring.

First-Principles Determination of Molecular Conformations of Indolizidine (-)-235B' in Solution

Indolizidine (-)-235B' is a particularly interesting natural product, as it is the currently known, most potent and subtype-selective open-channel blocker of the alpha4beta2 nicotinic acetylcholine receptor (nAChR). In the current study, extensive first-principles electronic structure calculations have been carried out in order to determine the stable molecular conformations and their relative free energies of the protonated and deprotonated states of (-)-235B' in the gas phase, in chloroform, and in aqueous solution. The (1)H and (13)C NMR chemical shifts calculated using the computationally determined dominant molecular conformation of the deprotonated state are all consistent with available experimental NMR spectra of (-)-235B' in chloroform, which suggests that the computationally determined molecular conformations are reasonable. Our computational results reveal for the first time that two geminal H atoms on carbon-3 (C3) of (-)-235B' have remarkably different chemical shifts (i.e. 3.24 and 2.03 ppm). The computational results help one to better understand and analyze the experimental (1)H NMR spectra of (-)-235B'. The finding of remarkably different chemical shifts of two C3 geminal H atoms in a certain molecular conformation of (-)-235B' may also be valuable in analysis of NMR spectra of other related ring-containing compounds. In addition, the pK(a) of (-)-235B' in aqueous solution is predicted to be ~9.7. All of the computational results provide a solid basis for future studies of the microscopic and phenomenological binding of various receptor proteins with the protonated and deprotonated structures of this unique open-channel blocker of alpha4beta2 nAChRs. This computational study also demonstrates how one can appropriately use computational modeling and spectroscopic analysis to address the structural and spectroscopic problems that cannot be addressed by experiments alone.

Neuroprotective effects of acetyl-L-carnitine on neuropathic pain and apoptosis: a role for the nicotinic receptor

Several pathologies related to nervous tissue alterations are characterized by a chronic pain syndrome defined by persistent or paroxysmal pain independent or dependent on a stimulus. Pathophysiological mechanisms related to neuropathic disease are associated with mitochondrial dysfunctions that lead to an activation of the apoptotic cascade. In a model of peripheral neuropathy obtained by the loose ligation of the rat sciatic nerve, acetyl-L-Carnitine (ALCAR; 100 mg/kg intraperitoneally [i.p.] twice daily for 14 days) was able to reduce hyperalgesia and apoptosis. In the present study, different mechanisms for the analgesic and the antineuropathic effect of ALCAR are described. The muscarinic blocker atropine (5 mg/kg i.p.) injected simultaneously with ALCAR did not antagonize the ALCAR antihyperalgesic effect on the paw-pressure test but significantly reduced the analgesic effect of ALCAR. Conversely, the antineuropathic effect of ALCAR was prevented by cotreatment with the nicotinic antagonist mecamylamine (2 mg/kg i.p. twice daily for 14 days). A pharmacological silencing of the nicotinic receptors significantly reduced the X-linked inhibitor of apoptosis protein-related protective effect of ALCAR on the apoptosis induced by ligation of the sciatic nerve. Taken together, these data highlight the relevance of nicotinic modulation in neuropathy treatment.

Synthesis of 3,6-diazabicyclo[3.1.1]heptanes as novel ligands for neuronal nicotinic acetylcholine receptors

Alpha series of novel 3,6-diazabicyclo[3.1.1]heptane derivatives 4a-f was synthesized and their affinity and selectivity towards alpha4beta2 and alpha7 nAChR subtypes were evaluated. The results of the current study revealed a number of compounds (4a, 4b and 4c) having a very high affinity for alpha4beta2 (K(i) at alpha4beta2 ranging from 0.023 to 0.056 nM) versus alpha7 nAChR subtypes; among these compounds, the 3-(6-bromopyridin-3-yl)-3,6-diazabicyclo[3.1.1]heptane 4c was found to be the most alpha7alpha4beta2 selective term in receptor binding assays (alpha7alpha4beta2=1295). Moreover, compound 4d also had high affinity for the alpha4beta2 nAChR subtype (K(i)=1.2 nM) with considerably high selectivity (alpha7/alpha4beta2=23300).

Synthesis and activity of substituted carbamates as potentiators of the alpha4beta2 nicotinic acetylcholine receptor

The synthesis and structure-activity relationship of a series of carbamate potentiators of alpha4beta2 nAChR is reported herein. These compounds were highly selective for alpha4beta2 over other nAChR subtypes. In addition, compounds increased the response of alpha4beta2 nAChRs to acetylcholine, as measured with patch-clamp electrophysiology.

Discovery and optimization of substituted piperidines as potent, selective, CNS-penetrant alpha4beta2 nicotinic acetylcholine receptor potentiators

The discovery of a series of small molecule alpha4beta2 nAChR potentiators is reported. The structure-activity relationship leads to potent compounds selective against nAChRs including alpha3beta2 and alpha3beta4 and optimized for CNS penetrance. Compounds increased currents through recombinant alpha4beta2 nAChRs, yet did not compete for binding with the orthosteric ligand cytisine. High potency and efficacy on the rat channel combined with good PK properties will allow testing of the alpha4beta2 potentiator mechanism in animal models of disease.

alpha7 nicotinic acetylcholine receptor agonist properties of tilorone and related tricyclic analogues

BACKGROUND AND PURPOSE

The alpha7 nicotinic acetylcholine receptor (nAChR) has attracted considerable interest as a target for cognitive enhancement in schizophrenia and Alzheimer's Disease. However, most recently described alpha7 agonists are derived from the quinuclidine structural class. Alternatively, the present study identifies tilorone as a novel alpha7-selective agonist and characterizes analogues developed from this lead.

EXPERIMENTAL APPROACH

Activity and selectivity were determined from rat brain alpha7 and alpha4beta2 nAChR binding, recombinant nAChR activation, and native alpha7 nAChR mediated stimulation of ERK1/2 phosphorylation in PC12 cells.

KEY RESULTS

Tilorone bound alpha7 nAChR (IC(50) 110 nM) with high selectivity relative to alpha4beta2 (IC(50) 70 000 nM), activated human alpha7 nAChR with an EC(50) value of 2.5 microM and maximal response of 67% relative to acetylcholine, and showed little agonist effect at human alpha3beta4 or alpha4beta2 nAChRs. However, the rat alpha7 nAChR maximal response was only 34%. Lead optimization led to 2-(5-methyl-hexahydro-pyrrolo[3,4-c]pyrrol-2-yl)-xanthen-9-one (A-844606) with improved binding (alpha7 IC(50) 11 nM, alpha4beta2 IC(50)>30 000 nM) and activity at both human and rat alpha7 nAChR (EC(50)s 1.4 and 2.2 microM and apparent efficacies 61 and 63%, respectively). These compounds also activated native alpha7 nAChR, stimulating ERK1/2 phosphorylation in PC12 cells.

CONCLUSIONS AND IMPLICATIONS

Tilorone, known as an interferon inducer, is a selective alpha7 nAChR agonist, suggesting utility of the fluorenone pharmacophore for the development of alpha7 nAChR selective agonists. Whether alpha7 stimulation mediates interferon induction, or whether interferon induction may influence the potential anti-inflammatory properties of alpha7 nAChR agonists remains to be elucidated.