Naturally occurring and synthetic -carbolines as cholinesterase inhibitors

Discovery of novel β-carboline derivatives as selective AChE inhibitors with GSK-3β inhibitory property for the treatment of Alzheimer's disease

The natural product harmine, a representative β-carboline alkaloid from the seeds of Peganum harmala L. (Zygophyllaceae), possesses a broad spectrum of biological activities. In this study, a novel series of harmine derivatives containing N-benzylpiperidine moiety were identified for the treatment of Alzheimer's disease (AD). The results showed that all the derivatives possessed significant anti-acetylcholinesterase (AChE) activity and good selectivity over butyrylcholinesterase (BChE). In particular, compound ZLWH-23 exhibited potent anti-AChE activity (IC₅₀ = 0.27 μM) and selective BChE inhibition (IC₅₀ = 20.82 μM), as well as acceptable glycogen synthase kinase-3 (GSK-3β) inhibition (IC₅₀ = 6.78 μM). Molecular docking studies and molecular dynamics simulations indicated that ZLWH-23 could form stable interaction with AChE and GSK-3β. Gratifyingly, ZLWH-23 exhibited good selectivity for GSK-3β over multi-kinases and very low cytotoxicity towards SH-SY5Y, HEK-293T, HL-7702, and HepG2 cell lines. Importantly, ZLWH-23 displayed efficient reduction against tau hyperphosphorylation on Ser-396 site in Tau (P301L) 293T cell model. Collectively, harmine-based derivatives could be considered as possible drug leads for the development of AD therapies.

β-Carboline as a Privileged Scaffold for Multitarget Strategies in Alzheimer's Disease Therapy

The natural β-carboline alkaloids display similarities with neurotransmitters that can be favorably exploited to design bioactive and bioavailable drugs for Alzheimer's disease (AD) therapy. Several AD targets are currently and intensively being investigated, divided in different hypotheses: mainly the cholinergic, the amyloid β (Aβ), and the Tau hypotheses. To date, only symptomatic treatments are available involving acetylcholinesterase and NMDA inhibitors. On the basis of plethoric single-target structure-activity relationship studies, the β-carboline scaffold was identified as a powerful tool for fostering activity and molecular interactions with a wide range of AD-related targets. This knowledge can undoubtedly be used to design multitarget-directed ligands, a highly relevant strategy preferred in the context of multifactorial pathology with intricate etiology such as AD. In this review, we first individually discuss the AD targets of the β-carbolines, and then we focus on the multitarget strategies dedicated to the deliberate design of new efficient scaffolds.

Design, synthesis and evaluation of novel bivalent β-carboline derivatives as multifunctional agents for the treatment of Alzheimer's disease

To develop potent multi-target ligands against Alzheimer's disease (AD), a series of novel bivalent β-carboline derivatives were designed, synthesized, and evaluated. In vitro studies revealed these compounds exhibited good multifunctional activities. In particular, compounds 8f and 8g showed the good selectivity potency on BuChE inhibition (IC₅₀ = 1.7 and 2.7 μM, respectively), Aβ_1-42 disaggregation and neuroprotection. Compared with the positive control resveratrol, 8f and 8g showed better activity in inhibiting Aβ_1-42 aggregation, with inhibitory rate 82.7% and 85.7% at 25 μM, respectively. Moreover, compounds 8e, 8f and 8g displayed excellent neuroprotective activity by ameliorating the impairment induced by H₂O₂, okadaic acid (OA) and Aβ_1-42 without cytotoxicity in SH-SY5Y cells. Thus, the present study evidently showed that compounds 8f and 8g are potent multi-functional agents against AD and might serve as promising lead candidates for further development.

New 2-Aryl-9-methyl-β-carbolinium salts as Potential Acetylcholinesterase Inhibitor agents: Synthesis, Bioactivity and Structure-Activity Relationship

A series of 2-aryl-9-methyl-β-carbolinium bromides (B) were synthesized and explored for anti-acetylcholinesterase (AChE) activities in vitro, action mechanism and structure-activity relationship. All the compounds B along with their respective 3,4-dihydro intermediates (A) presented anti-AChE activity at 10 μM. Thirteen compounds B showed the excellent activity with IC₅₀ values of 0.11–0.76 μM and high selectivity toward AChE relative to butyrylcholinesterase (BChE), superior to galantamine (IC₅₀ = 0.79 μM), a selective AChE inhibitor drug. Kinetic analysis showed that the action mechanisms of both compounds B and A are a competitive inhibition model. Structure-activity relationship analyses showed that the C = N⁺ moiety is a determinant for the activity. Substituents at 6, 7 or 4′ site, the indole-N-alkyl and the aromatization of the C-ring can significantly improve the activity. Molecular docking studies showed that the compounds could combine with the active site of AChE by the π-π or cation-π action between the carboline ring and the phenyl rings of the residues, and the β-carboline moiety is embedded in a cavity surrounded by four aromatic residues of Trp86, Tyr337, Trp439 and Tyr449. The present results strongly suggest that the para-position of the D-ring should be a preferred modification site for further structural optimization design. Thus, 2-aryl-9-methyl-β-carboliniums emerged as novel and promising tool compounds for the development of new AChE inhibitor agents.

Structural insights into cholinesterases inhibition by harmane β-carbolinium derivatives: a kinetics-molecular modeling approach

The natural indole alkaloids, the β-carbolines, are often associated with cholinesterase inhibition, especially their quaternary salts, which frequently have higher activity than the free bases. Due to lack of information explaining this fact in the literature, the cholinesterase inhibition by the natural product harmane and its two β-carbolinium synthetic derivative salts (N-methyl and N-ethyl) was explored, together with a combination of kinetics and a molecular modeling approach. The results, mainly for the β-carbolinium salts, demonstrated a noncompetitive inhibition profile, ruling out previous findings which associated cholinesterase inhibition by β-carbolinium salts to a possible mimicking of the choline moiety of the natural substrate, acetylcholine. Molecular modeling studies corroborate this kind of inhibition through analyses of inhibitor/enzyme and inhibitor/substrate/enzyme complexes of both enzymes.

Brunneins A-C, beta-carboline alkaloids from Cortinarius brunneus

Four beta-carboline alkaloids, brunneins A-C (1-3) and 3-(7-hydroxy-9H-beta-carboline-1-yl)propanoic acid (4), were isolated from fruiting bodies of the agaricoid fungus Cortinarius brunneus. The structures of 1-3 were determined by analysis of NMR and MS data, and the structure of compound 4 was determined by comparison with published data. Brunnein A (1) exhibited very low cholinesterase inhibitory effects and no cytotoxicity.

6-Hydroxy- and 6-methoxy-beta-carbolines as acetyl- and butyrylcholinesterase inhibitors

In the course of studies directed toward the discovery of novel acetyl- and butyrylcholinesterase (AChE and BChE) inhibitors for the treatment of Alzheimer's disease, we focused on beta-carbolines (BCs). 6-Oxygenated beta-carboline and beta-carbolinium derivatives based on the serotonin template were synthesized and tested in vitro for their ability to inhibit AChE and BChE, respectively. Particularly the carbolinium salts, which can be formed by intracerebral methylation out of the tertiary-BC prodrugs, show inhibitory activity levels reaching those of galantamine, physostigmine, and rivastigmine.

Acetylcholinesterase inhibitors from plants and fungi

This review describes 183 compounds obtained from plants and fungi which have been shown to inhibit acetylcholinesterase. The mechanism of action of cholinesterase, together with the binding sites, and, where this is known, the mode of action of inhibitors is described. The relative activities of the different compounds are recorded. The strongest inhibitors are generally alkaloids although some meroterpenoids from fungi have also been found to be active and display better selectivity.

6-Hydroxymethtryptoline is naturally occurring in mammalian urine: identification by combined chiral capillary gas chromatography and high resolution mass spectrometry

The tricyclic indole derivative 6-hydroxymethtryptoline (7-hydroxy-1-methyltetrahydro-beta-carboline) was identified as a normal constituent of human and cat urine. The identification was based on the correct retention time on a chiral capillary gas chromatographic column and on selected ion monitoring of the molecular ion with the mass spectrometer set at high resolving power (RP 10,000). The quantitation utilized a 13C-labeled analog as internal standard and demonstrated levels of 135 +/- 35 and 174 +/- 42 (pmol ml-1, mean +/- SEM) for the S(-) and R(+) enantiomers, respectively, in human urine. In cat urine, the levels were 203 +/- 55 and 108 +/- 44 (pmol ml-1, mean +/- SEM) for the S(-) and R(+) enantiomers, respectively. In human urine, the compound occurred predominantly in a conjugated form while in cat urine the free compound predominated. It was concluded that 6-hydroxymethtryptoline is naturally occurring in mammalian urine and that it most likely constitutes a metabolite of endogenous methtryptoline (1-methyltetrahydro-beta-carboline).

In vitro studies on the effect of beta-carbolines on the activities of acetylcholinesterase and choline acetyltransferase and on the muscarinic receptor binding of the rat brain

Acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7) activity and muscarinic receptor binding of homogenates from several brain structures were inhibited by beta-carbolines. The inhibition was of the noncompetitive type in the case of the enzyme and of the mixed type in the case of the receptor binding. This effect was most strongly manifested by pyridoindoles(harmane, norharmane), i.e., carbolines containing an aromatic C ring than by the corresponding piperidoindoles (tetrahydroharmane, tetrahydronorharmane), i.e., those with a reduced C ring. The activity of choline acetyltransferase (acetyl-CoA:choline O-acetyltransferase, EC 2.3.1.6) was not altered. These data are further evidence of the interactions between indoleamine derivatives and the cholinergic system. The results are discussed in terms of their possible biological significance.

The influence of systemic drugs on tear constituents

Tears of patients and laboratory animals on systemic medications have been analyzed for the presence and influence of these agents on tear constituents. Antibiotic penetration into tears, drugs stimulating or retarding lacrimation, and effects on tear electrolytes, lysozyme, and immunoglobulins are reviewed from the literature. Important applications to clinical practice such as contact lens wear, general anesthesia, eye infections, and epiphora or dry eye symptoms are discussed.