Synthesis of Novel Nicotinic Ligands with Multimodal Action: Targeting Acetylcholine α4β2, Dopamine and Serotonin Transporters

Nicotinic acetylcholine receptors (nAChRs), serotonin transporters (SERT) and dopamine transporters (DAT) represent targets for the development of novel nicotinic derivatives acting as multiligands associated with different health conditions, such as depressive, anxiety and addiction disorders. In the present work, a series of functionalized esters structurally related to acetylcholine and nicotine were synthesized and pharmacologically assayed with respect to these targets. The synthesized compounds were studied in radioligand binding assays at α4β2 nAChR, h-SERT and h-DAT. SERT experiments showed not radioligand [³H]-paroxetine displacement, but rather an increase in the radioligand binding percentage at the central binding site was observed. Compound 20 showed K_i values of 1.008 ± 0.230 μM for h-DAT and 0.031 ± 0.006 μM for α4β2 nAChR, and [³H]-paroxetine binding of 191.50% in h-SERT displacement studies, being the only compound displaying triple affinity. Compound 21 displayed K_i values of 0.113 ± 0.037 μM for α4β2 nAChR and 0.075 ± 0.009 μM for h-DAT acting as a dual ligand. Molecular docking studies on homology models of α4β2 nAChR, h-DAT and h-SERT suggested potential interactions among the compounds and agonist binding site at the α4/β2 subunit interfaces of α4β2 nAChR, central binding site of h-DAT and allosteric modulator effect in h-SERT.

Polyester with Pendent Acetylcholine-Mimicking Functionalities Promotes Neurite Growth

Successful regeneration of nerves can benefit from biomaterials that provide a supportive biochemical and mechanical environment while also degrading with controlled inflammation and minimal scar formation. Herein, we report a neuroactive polymer functionalized by covalent attachment of the neurotransmitter acetylcholine (Ach). The polymer was readily synthesized in two steps from poly(sebacoyl diglyceride) (PSeD), which previously demonstrated biocompatibility and biodegradation in vivo. Distinct from prior acetylcholine-biomimetic polymers, PSeD-Ach contains both quaternary ammonium and free acetyl moieties, closely resembling native acetylcholine structure. The polymer structure was confirmed via (1)H nuclear magnetic resonance and Fourier-transform infrared spectroscopy. Hydrophilicity, charge, and thermal properties of PSeD-Ach were determined by tensiometer, zetasizer, differential scanning calorimetry, and thermal gravimetric analysis, respectively. PC12 cells exhibited the greatest proliferation and neurite outgrowth on PSeD-Ach and laminin substrates, with no significant difference between these groups. PSeD-Ach yielded much longer neurite outgrowth than the control polymer containing ammonium but no the acetyl group, confirming the importance of the entire acetylcholine-like moiety. Furthermore, PSeD-Ach supports adhesion of primary rat dorsal root ganglions and subsequent neurite sprouting and extension. The sprouting rate is comparable to the best conditions from previous report. Our findings are significant in that they were obtained with acetylcholine-like functionalities in 100% repeating units, a condition shown to yield significant toxicity in prior publications. Moreover, PSeD-Ach exhibited favorable mechanical and degradation properties for nerve tissue engineering application. Humidified PSeD-Ach had an elastic modulus of 76.9 kPa, close to native neural tissue, and could well recover from cyclic dynamic compression. PSeD-Ach showed a gradual in vitro degradation under physiologic conditions with a mass loss of 60% within 4 weeks. Overall, this simple and versatile synthesis provides a useful tool to produce biomaterials for creating the appropriate stimulatory environment for nerve regeneration.

Synthesis of poly(ester-carbonate) with a pendant acetylcholine analog for promoting neurite growth

The modification of biodegradable polyesters with bioactive molecules has become an important strategy for controlling neuron adhesion and neurite outgrowth in nerve regeneration. In this study we report a biodegradable poly(ester-carbonate) with a pendant acetylcholine analog, which a neurotransmitter for the enhancement of neuron adhesion and outgrowth. The acetylcholine-functionalized poly(ester-carbonate) (Ach-P(LA-ClTMC)) was prepared by copolymerizing l-lactide (LA) and 5-methyl-5-chloroethoxycarbonyl trimethylene carbonate (ClTMC), followed by quaternization with trimethylamine. The acetylcholine analog content could be modulated by changing the molar feeding fraction of ClTMC. The incorporation of the acetylcholine analog improved the hydrophilicity of the films, but the acetylcholine analog content did not significantly influence the surface morphology of the acetylcholine-functionalized films. The results of PC12 cell culture showed that the acetylcholine analog promoted cell viability and neurite outgrowth in a concentration-dependent manner. The longest length of neurite and the percentage of cells bearing neurites were obtained on the Ach-P(LA-ClTMC)-10 film. All the results indicate that the integration of the acetylcholine analog at an appropriate fraction could be an effective strategy for optimizing the existing biodegradable polyesters for nerve regeneration applications.

N-[18F]Fluoroethylpiperidin-4ylmethyl Acetate, a Novel Lipophilic Acetylcholine Analogue for PET Measurement of Brain Acetylcholinesterase Activity

The reduction of acetylcholinesterase (AChE) activity in the brain has been measured in dementia disorders such as Alzheimer's disease and dementia with Lewy bodies using (11)C-labeled acetylcholine analogues, N-[(11)C]methylpiperidin-4-yl acetate and propionate, and positron emission tomography (PET). Our aim was to develop an (18)F-labeled acetylcholine analogue useful for brain AChE mapping with PET, since (18)F, with a longer half-life, has advantages over (11)C. In a preliminary study, a series of N-[(14)C]ethylpiperidin-3-yl or -4-ylmethanol esters (acetyl and propionyl esters) were newly designed and evaluated in vitro regarding the reactivity with and specificity to AChE using purified human enzymes, leading to a novel (18)F-labeled acetylcholine analogue, N-[(18)F]fluoroethylpiperidin-4-ylmethyl acetate. In rat experiments, the (18)F-labeled candidate showed desirable properties for PET AChE measurement: high brain uptake of the authentic ester, high AChE specificity, a moderate hydrolysis rate, and low membrane permeability (metabolic trapping) of the metabolite.

[Fluorescent-labeled lipophilic analogues of serotonin, dopamine, and acetylcholine: synthesis, mass spectrometry, and biological activity]

4,4-Difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-dodecanoyl derivatives of serotonin, dopamine, choline, and N,N-dimethylaminoethanol, with the fluorescence maximum at 512 nm (lambda(exc) 470 nm), and 4,4-difluoro-5,7-diphenyl-4-bora-3a,4a-diaza-s-indacene-3-dodecanoyl derivatives of choline and N,N-dimethylaminoethanol, with the fluorescence maximum at 554 nm (lambda(exc) 470 nm), were synthesized. These compounds yield protonated molecular ions of 100% intensity upon mass spectrometry with electrospray ionization at atmospheric pressure. The fragmentation of molecular ions under the conditions of secondary mass spectrometry mainly proceeds through the elimination of hydrogen fluoride from the fluorescent core of the molecules. Experiments on sea urchin Lytechinus variegatus embryos and larvae showed that these compounds easily penetrate into the cells and are accumulated in the cytoplasm. They do not differ in their biological activity from similar derivatives of arachidonic acid described previously and are agonists of serotonin or acetylcholine or antagonists of nicotinic acetylcholine receptors. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2004, vol. 30, no. 5; see also http: // www.maik.ru.

[Lactones. 28. EPC-synthesis, structure and pharmacology of "lactonized" and "lactamized" analogues of acetylcholine]

The enantiopure gamma-aminomethyl-gamma-butyrolactones (S)- and (R)-4a-d represent constrained analogues of acetylcholine, which were synthesized from D- or L-glutamic acid following two different routes. In addition, the corresponding lactames (S)- and (R)-10 were prepared by enantioselective synthesis. Only moderate activity was found at acetylcholine sites at the guinea pig atrium.

Design and evaluation of radioactive acetylcholine analogs for mapping brain acetylcholinesterase (AchE) in vivo

For mapping brain acetylcholinesterase (AchE) in vivo, seven radioactive acetylcholine analogs, N-[14C]methylpiperidyl-3- and 4-acetates, propionates, isobutyrates, and 3-butyrate were newly synthesized and evaluated in mice. The esters readily entered the brain and were hydrolyzed into the hydrophilic metabolite, which was trapped. In brain homogenates, the esters showed a wide range of enzymatic reactivity (about 40-fold), and high specificity for AchE (more than 82%) except the butyrate. Intra-brain distribution of the esters reflected a pattern of AchE activity.

Selective signaling via unique M1 muscarinic agonists

Rigid analogs of acetylcholine (ACh) were designed for selective actions at muscarinic receptor (mAChR) subtypes and distinct second messenger systems. AF102B, AF150, and AF151 are such rigid analogs of ACh. AF102B, AF150 and AF151 are centrally active M1 agonists. AF102B has a unique agonistic profile showing, inter alia: only part of the M1 electrophysiology of ACh and unusual binding parameters to mAChRs. AF150 and AF151 are more efficacious agonists than AF102B for M1 AChRS in rat cortex and in CHO cells stably transfected with the m1 AChR subtype. Notably, the selectivity of the new m1 agonists is reflected also by activation of select second messenger systems via distinct G-proteins. These compounds reflect a new pharmacological concept, tentatively defined as ligand-selective signaling. Thus, agonist/m1AChR complexes may activate different combinations of signaling pathways, depending on the ligand used. Rigid agonists may activate a limited repertoire of signaling systems. In various animal models for Alzheimer's disease (AD) the agonists AF102B, AF150 and AF151, exhibited positive effects on mnemomic processes and a wide safety margin. Such agonists, and especially AF102B, can be considered as a rational treatment strategy for AD.

Effects of oxidizing and reducing analogs of acetylcholine on neuronal nicotinic receptors

The synthesis and pharmacological characterization of dithiobisacetylcholine and dithiobis-N,N-dimethyl-4-acetylpiperazinium (two oxidizing analogs of acetylcholine), as well as those of their reduced counterparts, are described. Both the oxidizing and reducing analogs stimulate nicotinic receptors in the chick retina and block the binding of 125I-labeled neuronal bungarotoxin to retinal homogenates (IC50 values of 2 x 10(-6) to 6 x 10(-5) M). Both oxidizing compounds reverse the physiological effects of reduction by dithiothreitol on nicotinic function in intact chick retina, when applied for 2 sec (EC50 values of about 10(-5) M). This effect is selective, insofar as neither agent alters the effects of dithiothreitol treatment on receptors for N-methyl-D-aspartate. Reoxidation takes place at the disulfide located near the nicotinic receptor agonist binding site, inasmuch as reoxidation by these agents prevents affinity alkylation by bromoacetylcholine, and occupation by the competitive antagonist d-tubocurarine prevents reoxidation. Unlike thiocholine, a weak agonist with a free sulfhydryl that, paradoxically, is reported to oxidize nicotinic receptors in electroplax, the reduced forms, mercaptoacetylcholine and N,N-dimethylamino-4-mercaptoacetylpiperazinium, have no direct redox effects on retinal receptors, but they do protect the receptors against reduction by dithiothreitol.

Photoaffinity labeling of the acetylcholine transporter

The acetylcholine (AcCh) binding site in the AcCh transporter-vesamicol receptor (AcChT-VR) present in synaptic vesicles isolated from the electric organ of Torpedo was characterized. A high-affinity analogue of AcCh containing an aryl azido group, namely, cyclohexylmethyl cis-N-(4-azidophenacyl)-N-methylisonipecotate bromide (AzidoAcCh), was synthesized in nonradioactive and highly tritiated forms. AzidoAcCh was shown to be a competitive inhibitor of [3H]AcCh active transport and binding of [3H]-vesamicol to the allosteric site. The [3H]AzidoAcCh saturation curve was determined. In all cases the AcChT.AzidoAcCh complex exhibited an inhibition or dissociation constant of about 0.3 microM. Binding of [3H]AzidoAcCh was inhibited by vesamicol and AcCh. AzidoAcCh irreversibly blocked greater than 90% of the [3H]vesamicol binding sites after multiple rounds of photolysis and reequilibration with fresh ligand. Autofluorographs of synaptic vesicles photoaffinity-labeled with [3H]AzidoAcCh showed specific labeling of material exhibiting a continuous distribution from 50 to 250 kDa after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The result demonstrates that the AcChT has an unexpected structure highly suggestive of the synaptic vesicle proteoglycan.

Inhibition of acetylcholine storage by acetylcholine analogs in vitro

Forty-five acetylcholine (AcCh) analogs were chemically synthesized and characterized. They and two commercially available analogs were tested for the ability to inhibit active transport of AcCh by synaptic vesicles purified from the electric organ of Torpedo californica. A range of potencies greater than 4 orders of magnitude was found. A quaternary nitrogen and the presence of the carbonyl group are important to potency. The addition of hydrophobic groups to both ends of isonipecotic acid yielded the most potent analogs, which exhibited a nearly 1000-fold increase in potency relative to AcCh. The probable conformation of AcCh bound by the transporter has been deduced and confirmed by the synthesis of a potent rigid analog based on 2-amino-9-fluorenone. A potent analog was shown to be a competitive inhibitor with respect to AcCh, thus confirming that its site of action is the transporter active site. The structure-activity data clearly distinguish the binding site for AcCh from the site for vesamicol [(--)-(trans)-2-(4-phenylpiperidino)cyclohexanol], which is a noncompetitive inhibitor.

Synthesis and properties of NBD-n-acylcholines, fluorescent analogs of acetylcholine

We have synthesized a homologous series of fluorescent analogs of acetylcholine, N-7-(4-nitrobenzo-2-oxa-1,3-diazolyl)-omega-amino-n-alkanoic acid beta (N,N,N-trialkylammonium) ethylesters (NBD-n-acylcholines) and report here on their physiological and biochemical properties. All NBD-n-acylcholines trimethylated at the cholinergic nitrogen are agonists of acetylcholine at the frog neuromuscular junction. Their potencies in depolarizing frog muscle cells decrease with decreasing chain length. The affinities of binding to the purified receptor from Electrophorus electricus also decrease with decreasing chain length with a large drop in affinity for the derivatives n = 4 and n = 3. The rate constants of association to acetylcholine receptor and to acetylcholine esterase are of the order of 10(8) M-1 S-1 and do not vary significantly with the chain length of the NBD-n-acylcholines. In contrast, the dissociation rate constants decrease with increasing chain length. The quenching of fluorescence of NBD-n-acylcholines accompanying binding to purified receptor and esterase from E. electricus appears to be due to the formation of a hydrogen bond between the omega-amino group as donor and an unidentified acceptor group in a hydrophobic pocket of the protein. With their advantageous fluorescence properties, their simple pharmacology, and their clear structure-function relationships, these compounds are useful tools for the study of cholinergic mechanisms.

Stoichiometry of the ligand-binding sites in the acetylcholine-receptor oligomer from muscle and from electric organ. Measurement by affinity alkylation with bromoacetylcholine

1. The affinity alkylation reaction of the cholinergic, depolarising ligand, bromoacetylcholine with reduced acetylcholine receptor in the membrane fragments of Torpedo marmorata and in Triton-solubilised receptor from cat denervated muscle has been studied. 2. Brief pretreatment with 100 microM bromoacetylcholine abolishes all [3H]alpha-neurotoxin binding in both cases. 3. In the receptor from each of these sources, the number of sites of specific alpha-neurotoxin binding is exactly equal to the number of sites that can be specifically alkylated by bromol[3H]acetytlcholine, at saturation of either ligand. 4. The concentration-dependence of specific bromo[3H]acetylcholine binding is found to be biphasic. A first phase can be clearly discerned in which one-half of the total specific ligand-binding sites are alkylated readily, and a second phase in which the remainder react at higher reagent concentrations. The same discrimination of two equal sets of ligand sites can be obtained by pre-blockade using low concentrations of unlabelled bromoacetylcholine followed by reaction with [3H]alpha-neurotoxin or bromo[3H]acetylcholine. 5. In both phases, a single subunit of Mr about 43000 is the sole site of specific alkylation in both Torpedo and muscle. The reasons for the appearance of two equal but distinct populations in the ligand binding sites in the receptors are discussed.

Synthesis of pyridinium analogues of acetylcholine and their interactions with intestinal muscarinic receptors

N-(beta-Acetoxyethyl)pyridinium salts were synthesized and tested for muscarinic receptor interactions by the guinea pig ileum assay. Agonist activity indicates that receptor binding is substantially retained when the ammonium group of acetylcholine is formally replaced by a pyridinium ring. Introduction of alkyl groups into the ring yields antagonists. The 4-tert-butylpyridinium derivative is proved to have an activity superior to that of the 4-methylpyridinium salt. Competitive antagonism is favored by the more hydrophobic property of the tert-butyl group. A nonpolar area is suggested to be situated in the direct vicinity of the anionic binding sites of muscarinic receptors. The interaction of hydrophobic substituents with this area determines the antimuscarinic properties of pyridinium salts.

Relationships between chemical structure and inhibition of human placental choline acetyltransferase by keto analogs of acetylcholine

Seven keto analogs of acetylcholine were synthesized and evaluated as inhibitors of human placental choline placental choline acetyltransferase. Their potencies for inhibition of horse serum cholinesterase and stimulation of cholinergic receptors in the longitudinal ileal muscle of the guinea pig were investigated. The most potent and selective inhibitor of choline acetyltransferase was (2-benzoylethyl)trimethylammonium chloride with an I50 of 3 X 10(-6) M. It exhibited considerably low activities at muscarinic and nicotinic receptors and cholinesterases. Its high potency for inhibiting choline acetyltransferase was atrributed to: (a) its cationic terminal, a site for an electron acceptor interaction; (b) an aryl moiety for hydrophobic and electron donor contributions; and (c) a positive charge on the carbon atom adjacent to the benzene ring due to the presence of the carbonyl group, which interacts with the nucleophilic residue on the enzyme.

[Cholinomimetic activity of acetylcholine and sebacinyldicholine derivatives with differing cationic group structures]

The intrinsic alpha activities and the D2 (frog, m, rectus abdominalis) concentrations were estimated for different acetylcholine and sebacinylcholine derivatives. So were also the A2 values for antagonists and the affinity constants Kc for some partial agonists. The results obtained disprove Paton's "rate-theory". The relationship between the cholinergic activity and the volume of cationic groups was studied and it could not possibly be explained by the steric hindrance alone. It is suggested that certain hydrophobic radicals of the cationic groups contact the receptor surface outside the anionic centre. Such contacts prevent the cholinoreceptor to change its conformation and thus inhibit the depolarization of the membrane. An approximate estimation of the anionic site dimensions is given.