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Abstract The condensation of tacrine aminopolymethylene derivatives with diethyl (ethoxymethylidene)malonate led to the new hybrid compounds—conjugates, which were the effective inhibitors of acetylcholinesterase (AChE) (IC50 up to 0.538 μM) and butyrylcholinesterase (IC50 up to 0.0314 μM). They can displace propidium iodide from peripherical anionic site of AChE at the level of the reference drug donepezil and demonstrate a weak antioxidant activity. Conjugates are of interest for further extended research as potential drugs for the Alzheimer’s disease treatment.

A series of new adamantyl and citronellyl 4,4,4-trifluoro-2-arylhydrazinylidene-3-oxo-butanoates was synthesized. The synthesized compounds in nanomolar concentrations selectively inhibit carboxylesterase, while their activity is independent of the nature and position of the substituent in the aryl fragment, which is consistent with the molecular docking results.

A series of novel conjugates of 4-amino-2,3-polymethylene quinolines and phenolic antioxidant vanillin was synthesized by the condensation of aminoquinolines with vanillin followed by reduction of imines with sodium borohydride. The conjugates effectively inhibit AChE and BChE with preferable BChE inhibition and displace propidium from the PAS AChE. Compounds with aminoalkyl spacer have preferable esterase profile being more potent cholinesterases inhibitors with lower anti-CES activity and are the most potent antioxidants in ABTS and FRAP tests.

A new series of homobivalent Dimebon analogs, bis-γ-carbolines with alkylene, phenylenedialkylene, and triazole-containing spacers, was synthesized. Doubling the γ-carboline pharmacophore increased inhibitory potency against acetylcholinesterase (AChE) compared with Dimebon, while keeping Dimebon’s anti-butyrylcholinesterase activity; therefore, leading to inversion of selectivity. Molecular docking revealed the reasons for the increased anti-AChE activity and ability to block AChE-induced aggregation of β-amyloid for bis-γ-carbolines, which became double-site inhibitors of AChE. Conjugates with ditriazole-containing spacers were the most active antioxidants in both the ABTS-test and prevention of lipid peroxidation in brain homogenates without inhibiting the mitochondrial permeability transition (MPT). Conjugates with alkylene (4a–d), phenylenedialkylene (4e), and monotriazole (8) spacers were less active as antioxidants but prevented induction of the MPT and increased the calcium retention capacity of mitochondria. Lead compound 4e showed neuroprotective potential in a cellular calcium overload model of neurodegeneration. Computational studies showed that all the bis-γ-carbolines were expected to have high values for intestinal absorption and very good blood-brain barrier permeability along with good drug-likeness. Overall, the results showed that new homobivalent Dimebon analogs exhibit an expanded spectrum of biological activity and improved pharmacological properties, making them promising candidates for further research and optimization as multitarget agents for Alzheimer’s disease treatment.

Literature data and authors’ own results on the role of serine hydrolases, acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), as drug targets for treatment of neurodegenerative diseases and carboxylesterase (CaE) inhibitors as modulators of CaE-hydrolysis of ester-containing drugs are analyzed. Today, a promising approach is the development of cholinesterase inhibitors with additional neuroprotective and disease-modifying properties. The developed esterase profile approach, that is, comparative assessment of the inhibitory activity against AChE, BChE, and CaE, can be used to evaluate both the main potential pharmacological effect and possible side effects of a new compound. Analysis of the esterase profile, in combination with computer modeling and assessment of radical-scavenging ability of the synthesized compounds and their potential ability to block AChE-induced β-amyloid aggregation revealed highly active multifunctional compounds for the treatment of Alzheimer’s disease: selective inhibitors of BChE and inhibitors of both cholinesterases without potential side effects associated with CaE inhibition. A number of effective and selective inhibitors of CaE, free from cholinergic side effects, were also found for modulation of the rate of hydrolytic metabolism and for rational use of ester-containing drugs.

The synthesized new binary conjugates of tetrahydro-γ-carbolines, which contained ditriazole spacers of different length, exhibited considerable anticholinesterase and antioxidant activity as well as the potential ability to block the acetylcholinesterase-induced aggregation of β-amyloid in contrast to the original prototype Dimebon. This makes the compounds promising candidates for further investigation as drugs for the treatment of Alzheimer's disease. Special attention should be given to the conjugate containing the hexamethylene intertriazole spacer, which can be considered as a leader in this series of compounds.

A comparative analysis of the esterase profile and antiradical activity of two groups of hybrid compounds, viz., tetrahydro-γ-carboline conjugates with carbazoles and tetrahydrocarbazoles (I) and carbazole conjugates with carbazoles and tetrahydrocarbazoles (II), was performed. The replacement of the tetrahydro-γ-carboline moiety (conjugates I) by the carbazole group (conjugates II) was shown to significantly reduce the ability of the compounds to inhibit butyrylcholinesterase (BChE) and scavenge free radicals. The tetrahydro-γ-carboline–tetrahydrocarbazole combination is optimal in terms of both high anti-BChE activity and free radical scavenging ability. According to molecular modeling calculations, the stronger binding of tetrahydro-γ-carboline conjugates (I) in the BChE active site compared to carbazole conjugates (II) is attributed to the ability of I to form ionic and π-cation interactions with amino acid residues lining the BChE gorge. Therefore, conjugates of tetrahydro-γ-carboline and tetrahydrocarbazole derivatives are the most promising compounds for the design of new multitarget drugs combining cognitive-stimulating and antioxidant properties.

New N-(pyridin-3-ylmethyl)-2-aminothiazolines containing various substituents at the 5 position of the thiazoline ring and the 4-tert-butylbenzyl, 4-isopropylbenzyl, or 4-fluorobenzyl moiety at the nitrogen atom of the amino group were synthesized. The inhibitory activity of the synthesized compounds against human erythrocyte acetylcholinesterase (AChE, EC 3.1.1.7), equine serum butyrylcholinesterase (BChE, EC 3.1.1.8), and porcine liver carboxylesterase (CaE, EC 3.1.1.1) was evaluated and their antioxidant properties were studied by ABTS assay. N-(Pyridin-3-ylmethyl)-2-aminothiazolines proveded to be very weak AChE inhibitors, while their inhibitory activity against BChE and CaE was structure-dependent. 2-Aminothiazolines containing the 4-tert-butylbenzyl moiety are more efficient BChE inhibitors compared to the derivatives containing the 4-isopropylbenzyl or 4-fluorobenzyl substituent. An analysis of the dependence of the esterase profile of N-(pyridin-3-ylmethyl)-2-aminothiazolines on the structure of the substituent at the 5 position of the thiazoline ring of these compounds demonstrated that the derivatives containing the iodomethyl substituent possess the highest anti-BChE activity, the compounds with R2 = H and R3 = CH2I have the optimal esterase profile. Regardless of the structure of the substituents in the benzyl moiety, all N-(pyridin-3-ylmethyl)-2-aminothiazolines containing the iodomethyl substituent at the 5 position of the thiazoline ring exhibited high radical scavenging activity comparable with that of the standard antioxidant Trolox. N-(Pyridin-3-ylmethyl)-2-aminothiazolines were shown to be a new promising class of compounds for the design of multifunctional agents for the treatment of neurodegenerative diseases.

New 5-anilino-1,2,4-thiadiazole derivatives with different substituents at position 3 were synthesized and their influence on the key binding sites of NMDA receptors, inhibitory activity against acetylcholinesterase, butyrylcholinesterase, and carboxylesterase, as well as antioxidant properties were studied. The compounds with a 2-aminopropyl fragment were found to be efficient blockers of allosteric modulator [3H]ifenprodil binding site of the NR2B-containing NMDA receptors and moderate butyrylcholinesterase inhibitors. Some compounds were found to be able to simultaneously block the [3H]ifenprodil and intrachannel [3H]MК-801 binding sites of NMDA receptors. Compounds containing a 2-aminopropenyl fragment did not exhibit activity against NMDA receptors and butyrylcholinesterase, but showed high radical-scavenging activity.

Alzheimer’s disease has a complex multifactorial nature; therefore, a promising approach for the development of efficient therapeutic agents is the concept of multitarget drugs, which affect several biological targets involved in the pathogenesis of the disease. We developed a synthetic algorithm for conjugating several pharmacophoric ligands acting on the key stages of pathogenesis of several neurodegenerative diseases and synthesized hybrid structures combining the γ-carboline fragment of Dimebon with carbazole and tetrahydrocarbazole moieties. Using the complex primary screening system the structures have been revealed that combine the high inhibitory activity and selectivity towards butyrylcholinesterase with the radical-scavenging activity and the ability to potentiate tubulin polymerization to microtubules with a normal structure and/or prevent mitochondrial permeability transition. The lead compound was identified for future optimization and development of new multi-target drugs against neurodegenerative diseases combining the cognitive-stimulating and neuroprotective potentials.

Hybrid γ-carboline-based compounds containing a nitrooxy group as an NO-donor were prepared. It was shown that the introduction of this group did not affect the inhibitory activity of γ-carboline pharmacophore against acetyl- and butyrylcholinesterase.

Detailed studies of properties of new 3-substituted 5-anilino-1,2,4-thiadiazoles containing different substituents at position 3 of the thiadiazole ring were carried out, in particular, their esterase profile and antioxidant properties. It was found that the presence in the molecule of 2-aminopropyl fragment determines an efficient and selective inhibition of butyrylcholinesterase as compared to acetylcholinesterase and carboxylesterase, with radical-scavenging activity being weak. The compounds containing a 2-aminopropenyl fragment possess a high radicalscavenging activity, weakly inhibit cholinesterases, and exhibit anticarboxylesterase activity. A wide spectrum of activity of 3-substituted 5-anilino-1,2,4-thiadiazoles as inhibitors of cholinesterases and highly efficient scavengers of free radicals gives a basis for the optimization of structure and development in this series of original agents for therapy of neurodegenerative diseases.

Selective carboxylesterase inhibitors are employed as modulators of hydrolytic metabolism of ester or amide-containing drugs. Using the Molecular Field Topology Analysis (MFTA), the models for the relationships between the structures and inhibitory activities of 5-halomethyl-2-aminothiazolines against acetylcholinesterase, butyrylcholinesterase, and carboxylesterase were built, the molecular design was performed, and a focused library of potentially active and selective carboxylesterase inhibitors was proposed.

Using the acylation reaction with tosyl chloride of N-aminopropyl analogues of tacrine and its cyclic homologues with different size of the aliphatic cycle (5-8), we synthesized a number of new derivatives of p-toluenesulfonamide. It is shown that the synthesized hybrid compounds of tacrine and p-toluenesulfonamide are effective inhibitors of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) with the preferential inhibition of BChE. They also displace propidium from the peripheral anionic site of the electric eel AChE (Electrophorus electricus). The characteristics of the efficiency and selectivity of cholinesterase inhibition by the test compounds were confirmed by the results of molecular docking.

A series of phosphoryl-substituted amines and amides was synthesized and their inhibition activity towards acetylcholinesterase, butyrylcholinesterase, and carboxylesterase was investigated.

Conjugates of tacrine with 1,2,4-thiadiazole derivatives were synthesized for the first time. Their esterase profile and effects on the key NMDA receptor-binding sites as well as antioxidant activity were investigated. The obtained compounds effectively inhibited cholinesterases (with a predominant effect on butyrylcholinesterase), simultaneously blocked two NMDA receptor-binding sites (allosteric and intrachannel sites, and exhibited a high radical-scavenging activity. Our study shows that the obtained compounds are promising to design drugs for the treatment of Alzheimer's disease and other multifactorial neurodegenerative diseases.