Abstract
Alzheimer’s disease is a neurological disorder, which is the most common form of dementia affecting mostly elderly people. The number of patients is still rapidly increasing which results in negative impacts on population. The loss of cholinergic transmission is one of the signs of Alzheimer’s disease; it is connected with cognition impairment and memory loss. Therefore, some drug therapies are based on restoring the cholinergic function. Therefore, drug therapies are based on the cholinergic hypothesis. Acetylcholinesterase inhibitors such as donepezil, galantamine, and rivastigmine are nowadays in medical use. Tacrine possesses a simple structure and strong activity, but despite these features, it was withdrawn from the market due to its side effects. Due to the multifactorial etiology of Alzheimer’s disease, researchers have investigated multi-target-directed ligands based on tacrine structure to achieve better treatment efficacy. To broaden the therapeutic profile of the ligands, tacrine is bound to numerous moieties with various applications. The hybrids can be obtained in three ways: first, by conjugating; second, by fusing; third, by merging single molecules. The hybrids can target numerous disease pathways simultaneously. This is a promising concept, which will create multiple drugs that will minimize the symptoms of diseases in elderly populations. In this review, we will be describing differently synthesized series of tacrine compounds which were divided based on their biological activities.
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Introduction
Alzheimer’s disease (AD) is an incurable, progressive neurodegenerative disorder which is characterized at the beginning by episodic and semantic memory loss and cognitive impairment. As the disease progresses, one may experience a decline in language functions, depression, apathy, and anxiety. In later stages, psychotic features and personality changes may develop (Hugo and Ganguli 2014; Lehrner et al. 2017). It is expected that, by 2030, 65.7 million, and, by 2050, 115.4 million people will be affected by this disease (Prince et al. 2013). Despite many years of research, the main cause of the disease is unknown and only symptomatic treatment is used. Pathological changes result in a loss of cholinergic neurons, decreased level of neurotransmitter acetylcholine (ACh), increased β-amyloid (Aβ) and tau-protein deposition, and dyshomeostasis of bio-metals and oxidative stress (Mullan 1993; Querfurth and LaFerla 2010). There are a few hypotheses, which could explain the AD pathophysiology (such as the cholinergic, Aβ, or tau cascade hypotheses). Nowadays, Aβ and tau cascade hypotheses are considered as more important than the cholinergic one. The cholinergic hypothesis was defined over 30 years ago as the first AD hypothesis. The damage of the cholinergic neurons is observed in the hippocampus, amygdala, frontal cortex, and other structures responsible for learning, memory, or conscious awareness. The main changes observed in cholinergic neurons are choline uptake, impaired acetylcholine release, and deficits in the expression of nicotinic and muscarinic receptors (Bartus et al. 1982; Terry and Buccafusco 2003). The second hypothesis is the Aβ one. The basis to this hypothesis was given by Glenner and Wong 1984. They investigated the cerebrovascular amyloid derived from patients with Down syndrome (Glenner and Wong 1984). Aβ is a peptide consisting of 37–43 amino acids in which isoforms 1–40 and 1–42 are the most common. The Aβ1–42 is considered as the one with the highest toxicity to neurons. Aβ derives from the amyloid precursor protein which is cleaved by several enzymes—beta-site amyloid precursor protein-cleaving enzyme 1 (BACE-1), a β-secretase, and γ-secretase. The imbalance in production and Aβ aggregation and its accumulation in the brain may be one of the factors that causes AD (Grøntvedt et al. 2018; Haass and Selkoe 2007; Sanabria-Castro et al. 2017; Tanzi and Bertram 2005). Next hypothesis regards neurofibrillary tangles which occur during AD. The tangles are mostly consisted of hyper-phosphorylated and aggregated form of tau. Tangles may also occur as soluble protein in axons and do not cause any toxicity. Their main role is to stabilize microtubules. However, hyper-phosphorylated tau is insoluble, makes aggregates, and causes neurotoxicity. This results in cognition impairment (Khlistunova et al. 2006; Oddo et al. 2006; Querfurth and LaFerla 2010). The oxidative stress is mostly caused by Aβ, which is a generator of reactive oxygen species by causing mitochondria damaged (Good et al. 1996; Hensley et al. 1994). Aβ blocks mitochondrial enzymes (Reddy and Beal 2008). In results, electron transport, ATP production and mitochondrial membrane potential are being impaired and damaged mitochondria easily release free radicals (Good et al. 1996). In addition, higher levels of metal ions (iron, copper, and zinc) are associated with reactive oxygen species (ROS) damage and neurodegeneration. Iron ions increase tau phosphorylation and aggregation (Lovell et al. 1998; Yamamoto et al. 2004). The calcium regulation is in imbalanced in neurodegenerative disorders. The higher concentration of intracellular calcium leads to Aβ formation, its aggregation, and induction of apoptosis. Aβ by itself also increases calcium imbalance (Isaacs et al. 2006; Mullan 1993; Small 2009).
The most important risk factor is old age where AD is mostly diagnosed in people above 65. Comorbidities such as stroke, hypertension, heart diseases, hypercholesterolemia, smoking, and genetic factors have also their impacts (Alves et al. 2010; Cataldo et al. 2010; Hugo and Ganguli 2014). Currently, the most popular treatment is the inhibition of acetylcholinesterase (AChE)—which hydrolyzes the ACh to choline and acetate. The low levels of ACh result in the cognitive impairment and memory problems that occur decades after the beginning of the neurodegenerative process (Francis et al. 1999; Siegfried 1993). Treatment only alleviates the symptoms and barely stops the progression of disease. AChE inhibitors (AChEI) are donepezil, rivastigmine and galanthamine, which are used to treat mild-to-moderate AD. The N-methyl-d-aspartate (NMDA) receptor antagonist—memantine—is also used in symptomatic treatment.
The AChE possesses at least two substrate-binding sites: based on the bottom of the gorge, the active site; located at the entry of the gorge, the peripheral anionic site (PAS). The active site is built of two subsides: the catalytic anionic site (CAS) where the ACh moiety is stabilized by Trp84, Glu199, and Phe330 (Sussman et al. 1991); by the second subside, the catalytic triad which is made of Ser200–His440–Glu327 which hydrolyzes ACh. The carbonyl oxygen of ACh is stabilized by oxyanion hole (made of peptic NH groups from Gly118, Gly119, and Ala201), whereas the acetyl group of ACh is connected with acyl-pocket (Trp233, Phe288, Phe290, and Phe331) of AChE (Harel et al. 1993). PAS influences on the catalytic activity and is made of the three principal amino acids—Trp279, Tyr70, and Asp72. In the surface of the gorge, the rings of 14 aromatic residues (Tyr70, Trp84, Trp114, Tyr121, Tyr130, Trp233, Trp279, Phe288, Phe290, Phe330, Phe331, Tyr334, Trp432, and Tyr442) are located (Colletier et al. 2006; Sussman et al. 1993). ACh is hydrolyzed by cholinesterases—AChE and butyrylcholinesterase (BChE). During the progression of AD, AChE level decreases, whereas BChE increases (Perry et al. 1978). The PAS of the AChE is responsible not only for hydrolyzing ACh, but also for aggregation of Aβ protein, its stabilization, and deposition. Accumulation of Aβ aggregates leads to calcium dysregulation, formation of reactive oxygen species, and neuronal cell membrane damage (Cai et al. 2011; Green and LaFerla 2008; Hardy and Selkoe 2002).
Drugs, which can act on various stages of the neurotoxic cascades, have been investigated and are called multi-target-directed ligands (MTDLs). MTDLs can target numerous pathways involved in the AD development. The hybrid comprises two or more molecules; each molecule belongs to different pharmacological and chemical classes. Single molecules are connected in three ways to obtain novel hybrids (Morphy et al. 2004; Morphy and Rankovic 2005). First, “conjugates” are MTDLs, which have two frameworks (each one possessing pharmacophore elements) and are connected by a linker group (no part of the linker should be found in the ligands). This method of obtaining MTDLs is very common and is described in detail in this review. The linker between two moieties impacts on hybrid’s inhibitory activity. In the second method, the size of the linker between frameworks decreases and ligands touch themselves by the same group in the linker. This type of strategy described and evaluated by Jerábek et al. (2017) fuses tacrine (THA) with resveratrol. In the third method, MTDLs are synthesized by merging both frameworks, where common structures combine together (Morphy et al. 2004; Morphy and Rankovic 2005). Fusing and merging strategies are recently regarded as better than conjugating, due to the fact that they receive lower molecular weight and decrease in structural complexity. These features should help hybrids in crossing the blood–brain barrier (BBB) (Rankovic 2015). Hybrids represent a new strategy to treat AD, enhancing efficacy and improving safety in regards to drugs that have only a single target. AChEI is bound to another moiety to synthesize a multifunctional hybrid, which interacts with the AChE and provides additional properties. It can result in the increase of potency (Cavalli et al. 2008; Singh et al. 2016).
In our review, we focused on THA hybrids, which were synthesized in the last years. All hybrids were divided into five groups on the basis on their biological activities: THA derivatives with cholinesterase inhibition; with cholinesterase inhibition and Aβ antiaggregation properties; with cholinesterase inhibition and antioxidant properties; with cholinesterase inhibition, antiaggregation, and antioxidant properties; with cholinesterase inhibition, antioxidant, and non-hepatotoxicity properties. We provided detailed description of IC50 values toward AChE and BChE, as well as information about type of inhibition, % inhibition of Aβ aggregation, impact of length of the linker on hybrids activities, or antioxidant assay results toward the most promising hybrid of each group. Moreover, we presented chemical structures with their modifications for all hybrids, which make easier to understand the connection between structure and activity.
Tacrine
THA 1 (Fig. 1) belongs to the group of acridine compounds and is a reversible, non-competitive AChEI. THA moiety interacts with AChE by getting stacked against Trp84. The Phe330 ring rotates to situate itself parallel to THA. THA is inserted between the Phe330 and Trp84. The nitrogen from the THA ring forms a bond with the carbonyl oxygen of His440 (Wu et al. 2017). THA was approved by the FDA in 1993 for the treatment of AD. After 5 years, it was withdrawn from the market due to its hepatotoxicity caused by mediating the stimulation of reactive oxygen species production and glutathione depletion (Sameem et al. 2017). The liver toxicity effect is related to the formation of highly reactive metabolites (especially 7-hydroxytacrine and its metabolite quinone methide) which occurs during THA’s metabolism. Reactive metabolites form covalent adducts with DNA or cellular proteins, or induce different redox cycling, which can result in liver damage and toxic effects (Patocka et al. 2008). Due to the good inhibitory activity, low molecular weight, and potential attenuates Aβ-induced neurotoxicity, THA has been widely investigated by the researchers worldwide, especially for the synthesis of MTDLs for AD treatment. Putting THA in the hybrids could improve the biological profile of THA and overcome some side effects (Meng et al. 2007; Minarini et al. 2013).
THA 1
Tacrine derivatives with cholinesterase inhibition
Tacrine-trimethoxybenzene hybrids
Elsinghorst et al. synthesized trimethoxybenzene–THA hybrids 2a–b, 3a–m (Fig. 2) with different lengths of the linker. The highest inhibitory activity was presented by hybrids with the longest spacer [3g, and 3h—electric eel AChE (eeAChE) IC50 3.25 nM, 2.73 nM, Torpedo californica AChE (TcAChE) IC50 17.2 nM, 5.99 nM, humanAChE (hAChE) IC50 5.24 nM, 5.08 nM, human BChE (hBChE) IC50 0.293 nM, and 1.38 nM, respectively, THA non-tested]. 3g turned out to be a non-competitive inhibitor of eeAChE (Elsinghorst et al. 2006).
Compounds 2a–b, 3a–m
Tacrine-gallamine hybrids
Gallamine 4 (Fig. 3) was discovered in 1947 and has been used as a muscle relaxant due to its inhibition of muscarinic receptors with a selectivity for the M2 subtype. Gallamine binds to the core region of the allosteric site of the receptor (Clark and Mitchelson 1976).
Gallamine 4 and compounds 5a–b, 6a–b, 7a–b
Elsinghorst et al. investigated novel gallamine–THA hybrids 4a–b, 5a–b, and 6a–b (Fig. 3) as two binding site inhibitors. THA bound to the CAS and gallamine to the PAS of the AChE according to the molecular modeling study. Almost all the compounds showed lower IC50 values toward AChE and BChE than THA and gallamine. The compound 4a presented the best results towards eeAChE [IC50 0.467 nM, THA IC50 300 nM, and human BChE (hBChE) IC50 1.50 nM], while the compound 4b showed the highest inhibitory activity towards hAChE (IC50 5.44 nM, THA hAChE IC50 926 nM, hBChE IC50 1.42 nM). The 6-chloro substitution in the THA moiety did not increase the inhibitory activity of the hybrids as it was observed for other hybrids. Increasing substitution of the gallamine-derived moiety diminished the inhibitory potency. The allosteric potency of hybrids was higher than the one observed for gallamine or THA. Novel compounds bound to the core and peripheral region of the allosteric site (Elsinghorst et al. 2007).
7-Methoxytacrine–adamantylamine hybrids
The 7-methoxytacrine (7-MEOTA) 7 (Fig. 4) is considered an active ChE inhibitor which causes minor side effects than THA (Gazova et al. 2017; Korabecny et al. 2011). Amantadine 8 (Fig. 4) is a drug used in the treatment of Parkinson’s disease. Investigations showed that amantadine, whose analogue is memantine, was able to delay the onset and severity of dementia (Inzelberg et al. 2006; Robinson and Keating 2006).
7-Methoxytacrine 7 and Amantadine 8 and compounds 9a–g
Spilovska et al. evaluated novel heterodimers of 7-MEOTA-adamantylamine thioureas with different linker lengths 9a–g (Fig. 4). 7-MEOTA and amantadine were less potent inhibitors than THA and novel heterodimers. There was no significant difference between compounds with a longer or shorter linking chain. The best inhibitory activity toward hAChE and hBChE was demonstrated by compound 9d (IC50 value of 0.47 μM for hAChE and IC50 value of 0.11 μM for hBChE) which possessed five carbon atoms in the linker and whose potency was similar to the one presented by THA (AChE IC50 0.5 µM, BChE IC50 0.023 µM). Based on molecular modeling, this compound bound itself to the CAS and PAS of AChE (Spilovska et al. 2013).
Tacrine derivatives with cholinesterase inhibition and Aβ antiaggregation properties
Tacrine-indole heterodimers
Munoz-Ruiz et al. synthesized THA–indole hybrids 10a–y (Fig. 5) linked by the methylene tether of different length. The compounds were dual-binding site of AChE inhibitors. The linker was variously modified by the following factors, such as a propionamide chain, additional N-Me function in the central part of the methylene linker, and replacement of the amide group by a carbamate moiety like in rivastigmine. Researchers assayed on the bovine erythrocyte AChE (beAChE) inhibitory potency, hBChE, and hAChE. Almost all heterodimers showed the higher potency than THA (beAChE IC50 167 nM and hBChE IC50 24 nM). 10c (beAChE IC50 0.02 nM, hAChE IC50 70 pM, and hBChE IC50 2.9 nM) and 10d (beAChE IC50 0.06 nM, hAChE IC50 20 pM, and hBChE IC50 0.1 nM) were the most active hybrids. They contained chlorine atom in position 6, two methylene groups at the indole ring, and tether length of 6–7 methylenes. The hybrids were rather selective toward AChE than BChE. The compounds also proved ability to inhibit Aβ aggregation. The compounds 10c and 10d were strong antiaggregating molecules; they noticeably inhibited the AChE-induced Aβ aggregation (inhibition of Aβ with and without AChE at the concentration of 100 µM for 10c, 10d, THA—98%, 49%, and 7% and 96%, 65%, and 0%, respectively) (Muñoz-Ruiz et al. 2005).
Compounds 10a–y
Tacrine–donepezil hybrids
Donepezil 11 (Fig. 6) has a big structure which can form various interactions with residues of the AChE. Due to its dual-binding site character, donepezil possesses Aβ antiaggregation activity. Its benzyl moiety binds to CAS and its indanone structure connects with PAS of AChE (Kryger et al. 1999).
Donepezil 11 and compounds 12a–h
Camps et al. evaluated novel donepezil–THA hybrids 12a–h (Fig. 6). According to the molecular modeling study, they bound to the PAS (donepezil moiety), CAS (THA or 6-chlorotacrine moiety), and mid-gorge-binding sites of AChE. The length of the linker between moieties has an impact on the arrangement of the hybrids inside the gorge of the enzyme. All of the novel hybrids were very potent inhibitors of human erythrocytes AChE and human serum BChE with IC50 values in the sub and nanomolar range. The most active inhibitors of human erythrocytes, AChE, were those with the indanone system, a chlorine atom on the THA moiety, and a three-methylene linker, especially the compound 12d (IC50 0.27 nM, hBChE IC50 66.3 nM, THA hAChE IC50 205 nM, and donepezil hAChE IC50 11.6 nM). Novel hybrids were less potent toward hBChE than hAChE. All of them showed higher potency than THA. Moreover, the compounds 12c–h (at the concentration of 100 µM) presented their ability to inhibit AChE-Aβ1–40 aggregation. All the hybrids showed significant Aβ antiaggregation properties, and were more potent than THA or donepezil. The most potent toward hAChE, 12d, showed 46.1% inhibition of Aβ antiaggregation. The strongest was 12f, which inhibited Aβ antiaggregation in 65.9% (THA—only 7% inhibition) (Camps et al. 2008).
Tacrine–pyrano(3,2-c)quinoline–6-chlorotacrine hybrids
A pyrano(3,2-c)quinoline scaffold comprising 5-phenylsubstituted derivative resembles a 6-phenylphenanthridinium group in propidium 13 (Fig. 7). Therefore, this compound may be considered as AChEI (Bourne et al. 2003).
Propidium 13 and compounds 14a–j
Camps et al. synthesized and tested novel pyrano(3,2-c)quinoline–6-chlorotacrine hybrids 14a–j (Fig. 7) as potent dual-binding site AChEI connected through an amido-containing oligomethylene linker. Being a dual-binding inhibitor, 6-chlorotacrine interacted with the CAS and pyrano(3,2-c)quinoline moiety with the PAS of AChE which was evaluated in the molecular modeling study. Novel compounds exhibited inhibition potency toward hAChE (14c IC50 7.03 nM) and hBChE (14c IC50 331 nM), although their inhibitory activities (except 14c) were slightly lower than that of 6-chlorotacrine (hAChE IC50 8.32 nM and hBChE IC50 916 nM). The length of the linker and the position of the amino group had an impact on potency. Due to the inhibition of the PAS of AChE, inhibitors diminished the AChE-induced Aβ-40 aggregation (14i, 6-chlorotacrine—45.9% and 8.5% of inhibition, respectively, at the concentration of 100 µM). They might inhibit self-induced Aβ aggregation (14i, 6-chlorotacrine—49.1%, 7.1% of inhibition, respectively, at the concentration of 50 µM) and β-secretase activity (14j, 6-chlorotacrine—77.8%, not active, respectively, at the concentration of 2.5 µM) (Camps et al. 2009).
Tacrine–phenylbenzoheterocycle hybrids
Huang et al. designed phenylbenzoheterocycle–THA hybrids 15a–h (Fig. 8) connected through the carbon spacers. All the compounds presented inhibitory activity toward eeAChE and equine serum BChE (esBChE). Almost all of them showed higher potency toward eeAChE than THA (IC50 0.311 µM). The most potent inhibitor was 15b with 4-carbon linker (eeAChE IC50 0.017 µM and esBChE IC50 0.122 µM). However, various lengths of the linker did not have an impact on the inhibitory activity. The structure of the benzoheterocycle affected the inhibitor potency. The activity of benzoheterocycle has changed in direction indole > benzothiazole > benzofuran > benzoxazole. This trend was observed with AChEI, not with butyrylcholinesterase inhibitors. Compounds demonstrated Aβ aggregation inhibitory activity (15b, 15f, 15h, curcumin—51.8%, 57.6%, 62.8%, and 18.7% of the inhibition at the concentration of 20 µM, THA not tested). The molecular modeling study presented that the inhibitors interacted with the PAS (benzothiazole moiety), the CAS (THA moiety), and the gorge of the AChE (Huang et al. 2012).
Compounds 15a–h
Tacrine–flurbiprofen hybrids
Flurbiprofen 16 (Fig. 9) is a non-steroidal antiinflammatory drug (NSAID). Moreover, it was identified as a compound which lowers the production of Aβ40/42 due to its allosteric modulation of presenilin-1 (Eriksen et al. 2003).
Flurbiprofen 16 and compounds 17a–e
Chen et al. synthesized a series of THA–flurbiprofen hybrids 17a–e (Fig. 9) connected by the alkylenediamine linker. Among all hybrids, 17d, the longest linker chain, presented the best inhibitory activity toward eeAChE (IC50 19.3 nM, IC50 BChE 3.7 nM). Its result was better than those of THA (AChE IC50 61.7 nM and BChE IC50 9.0 nM). 17d demonstrated its binding to the CAS through THA moiety and interaction with the PAS of AChE through the benzene ring of flurbiprofen. Moreover, 17d was tested in the Aβ inhibition assay (31% reduction of Aβ formation at the concentration of 0.25 µM) and could inhibit the formation of Aβ due to its ability to inhibit presenilin (Chen et al. 2013).
Tacrine–levetiracetam hybrids
Levetiracetam 18 (Fig. 10) is an antiepileptic drug which improves memory performance in patients with amnestic mild cognitive impairment and also in hAPPJ20 or APP/PS1 mouse models of AD (Sanchez et al. 2012; Shi et al. 2013).
Levetiracetam 18 and compounds 19a,b and 20
Sola et al. synthesized heptamethylene-linked levetiracetam–huprine and levetiracetam–6-chlorotacrine hybrids 19a–b, 20 (Fig. 10). The hybrids showed inhibitory potency against human recombinant AChE in nanomolar range (19a IC50 3.1 nM, 19a hBChE IC50 135 nM, THA hAChE IC50 317 nM, and 6-chlorotacrine hAChE IC50 5.9 nM). Inhibition of Aβ42 was tested in the inhibition assay at the concentration of 20 μM. 20 had the best inhibition—36.4%, where THA was below 10%. The C57BL6J mice that were treated with 20, a dose of 5 mg/kg, provide (based on results from acute toxicity study) a 43% inhibition of mouse brain AChE after 10 min of administration. Aβ burden was reduced in APP/PS1 mice’s brains which treated with 20 (Aβ burden—below 0.6% area; THA—0.8% area) (Sola et al. 2015).
Tacrine–multialkoxybenzene hybrids
Zhang et al. evaluated novel series of benzoates (or phenylacetates or cinnamates)–THA hybrids 21a–n, 22 (Fig. 11). All inhibitors showed IC50 values in nanomolar range when the strongest hybrid was the compound 21c with a trimethoxyphenylacetate group (eeAChE IC50 5.63 nM, esBChE IC50 364 nM, THA eeAChE IC50 73.36 nM, and esBChE IC50 14.45 nM). Compounds confirmed the inhibitory activity of Aβ aggregation. Compound 21j with a hydroxybenzoate group exhibited the best inhibition of Aβ aggregation with IC50 17.36 nM (21c IC50 51.81 nM and THA IC50 12.21 nM). The spacer length between THA and phenyl moiety as well as the electron density on aromatic ring that was connected with THA through the acetyl group were significant factors for AChE inhibition but lower important for Aβ self-aggregation. Molecular modeling study and Lineweaver–Burk plot presented that 21c targeted both the CAS and PAS of ChEs simultaneously (Zhang et al. 2016).
Compounds 21a–n and 22
Tacrine–fluorobenzoic acid hybrids
Czarnecka et al. evaluated a novel series of THA–fluorobenzoic acid hybrids 23a–f (Fig. 12). The compounds with butyl linker had higher activity than others with ethyl or propyl tether. Among them, compound 23c showed the highest activity toward eeAChE IC50 41.37 nM and esBChE IC50 1.39 nM, higher than the reference compound THA (eeAChE IC50 80.82 nM and esBChE IC50 19.55 nM). Kinetic characterization of AChE inhibition revealed mixed-type inhibitor behavior of compound 23c. In the Aβ inhibition assay, compound 23c confirmed its inhibitory activity with 77.32% inhibition of Aβ aggregation at the concentration of 50 µM. In the molecular modeling study, it was confirmed that THA bound to the PAS of AChE and the fluorobenzoic moiety bound to the CAS of AChE (Czarnecka et al. 2017).
Compounds 23a–f
Tacrine derivatives with cholinesterase inhibition and antioxidant properties
Tacrine–melatonin hybrids
Melatonin 24 (Fig. 13) is a neurohormone which takes part in circadian rhythms of many physiological functions, e.g., sleep timing (Brzezinski 1997). It has antioxidant activity and is able to scavenge of ROS (Reiter 1998).
Melatonin 24 and compounds 25a–m
Rodriguez-Franco et al. evaluated novel melatonin–THA hybrids 25a–m (Fig. 13). Hybrids presented inhibitory activity of IC50 from subnanomolar to picomolar range. The addition of chlorine atom into the position 6 of the THA moiety improved the inhibitory activity. The most potent compound toward hAChE was 25g (IC50 of 8 pM, hBChE IC50 7.8 nM, THA hAChE IC50 350 nM, and hBChE IC50 40 nM). Their oxygen radical absorbance capacity was higher than that of melatonin. 25e was four times, when 25g was 2.5 times more active than trolox (Rodriguez-Franco et al. 2006).
Tacrine–mefenamic acid hybrids
Mefenamic acid 26 (Fig. 14) is a non-steroidal antiinflammatory drugs. Apart from its ability of cyclooxygenase inhibition, it interacts with AChE and decreases the occurrence of free radicals and Aβ-induced neurotoxicity (Joo et al. 2006).
Mefenamic acid 26 and compounds 27a–t, 28a–t, 29a–f, 30a–f
Bornstein et al. evaluated novel series of THA and 6-chlorotacrine–mefenamic acid hybrids 27a–t, 28a–t, 29a–f, 30a–f (Fig. 14). Hybrids were non-competitive or mixed-type inhibitors of AChE. Hybrids interacted with AChE by PAS (mefenamic acid), CAS (THA moiety), and spanning the active-site gorge through a methylene-based linker according to the molecular modeling study. Most of the hybrids presented higher inhibitory activity toward eeAChE AChE than THA (IC50 52.4 nM). Among the series of compounds (29a–f, 30a–f), hybrid 30c (IC50 AChE 0.495 nM) possessed the highest inhibitory activity towards AChE. Among the series of compounds 27a–t, 28a–t, the hybrids with 6-chlorotacrine moieties showed higher inhibitory activity than non-halogen counterparts (28m IC50 AChE 0.418 nM). Due to the fact that mefenamic acid has antiinflammatory activity, the ROS inhibition assay was performed. All compounds presented good inhibitory activity, where hybrid 28m had the best inhibition toward ROS (IC50 0.009 nM and THA IC50 183 nM) (Bornstein et al. 2011).
Tacrine–(hydroxybenzoyl-pyridone) hybrids
Chand et al. tested novel THA–(hydroxybenzoyl-pyridone) hybrids 31a–c (Fig. 15). The molecular modeling study was performed on eeAChE (BChE was not tested). The THA moiety interacted with the bottom of the gorge of the enzyme, binding to the CAS by π–π interaction with the aromatic ring of Trp84 and Phe330. The benzoyl ring of inhibitors placed at the entrance of the gorge was bound with Tyr70 and Trp279 of the PAS. It was proved that compound with an alkyl spacer of 4 methylene carbon atoms established stronger interaction with Asp275 residue than compound with two methylene carbon atoms in the linker. The eeAChE inhibitory activity test confirmed that compound 31c was the strongest inhibitor (IC50 0.57 µM) among others, but slightly weaker than THA (IC50 0.31 µM). The DPPH test showed that radical scavenging capacity of novel inhibitors was better (EC50 204–249 µM) than that of THA (EC50 > 1000 µM) (Chand et al. 2016).
Compounds 31a–c
Tacrine–1,2,3-triazole hybrids
Najafi et al. evaluated a new series of THA–1,2,3-triazole hybrids 32a–o (Fig. 16). Almost all compounds showed good inhibitory activity toward eeAChE and esBChE. Compound 32l, which had chlorine in 7-position of the acridine moiety and 4-methoxyphenyl connected to 1,2,3-triazole ring, was the most active inhibitor toward eeAChE—IC50 0.521 µM and esBChE IC50 1.853 µM; however, it was weaker than a reference drug, THA (eeAChE IC50 0.048 µM and esBChE IC50 0.010 µM). Compound 32k with chlorine in 6-position of the acridine ring and 4-methoxyphenyl connected to 1,2,3-triazole moiety showed less eeAChE inhibitory activity (IC50 0.737 µM) in comparison with the compound 32l. Molecular modeling and kinetic studies presented that compounds 32l and 32j bound both to the PAS and CAS of the AChE and BChE. The tested compounds 32d, 32j, and 32l showed no important antioxidant activities in the DPPH test. The compound 32l showed neuroprotective activity at 50 and 100 µM (cell viability 73.45% and 74.46%, respectively) against H2O2-induced cell death in PC12 neurons (Najafi et al. 2017).
Compounds 32a–o
Tacrine derivatives with cholinesterase inhibition, antiaggregation, and antioxidant properties
Tacrine–carbazole hybrids
Carvedilol 33 (Fig. 17) is a vasodilating blocker with antioxidant properties. It is used in the treatment of mild-to-moderate hypertension. Carvedilol also has a modulatory action on the NMDA receptors as a low-affinity antagonist. Its carbazole moiety has antioxidant properties and may act as an inhibitor of Aβ fibril formation (Howlett et al. 1999; Lysko et al. 1998).
Carvedilol 33 and compounds 34a–d
Rosini et al. designed and investigated novel carvedilol–THA hybrids 34a–d (Fig. 17). Compounds presented their inhibitory activity toward hAChE in the nanomolar range (compound 34c IC50 1.54 nM and hBChE IC50 189 nM) and their potency was higher than that of THA (hAChE IC50 424 nM). Hybrids were able to bind to the CAS of AChE by tetrahydroacridine moiety and PAS through carbazole moiety as to the molecular modeling study. The compounds were also able to inhibit Aβ self- and AChE aggregation (34c, THA 63.2% and 7% at 100 µM concentration; 25.5% and 5% at 10 µM concentration, respectively) and antagonized NMDA receptors. The compound 34a was capable of neuronal cell protection against ROS (IC50 23 µM; trolox IC50 49.55 µM) caused by oxidative stress (Rosini et al. 2008).
Tacrine–nimodipine hybrids
1,4-Dihydropyridines selectively inhibit L-type voltage-dependent Ca2+ channels (VDCC). Nimodipine 35 (Fig. 18) belongs to the VDCC group; therefore, it may protect cells from neuronal death caused by Ca2+ overload and focal cerebral ischemia (Cano-Abad et al. 2001; Sobrado et al. 2003). Calcium dysfunction is one of the factors which triggers the development of AD (Kruman et al. 1998; Mattson et al. 1992).
Nimodipine 35 and compounds 36a–n
Marco-Contelles et al. evaluated novel hybrids based on THA and nimodipine called tacripyrines 36a–n (Fig. 18). Compounds possessed inhibitory activity toward hAChE and hBChE. Half of them presented higher potency toward AChE than THA, although their inhibitory activity for BChE was rather weak. The best inhibitors were 36h (hAChE IC50 71 nM; hBChE IC50 > 100,000 nM), 36j,l (hAChE IC50 58 nM and 45 nM, respectively, both hBChE IC50 > 100,000 nM) compounds (THA hAChE IC50 147 nM and hBChE IC50 36 nM). Hybrids showed their potency to inhibit pro-aggregating action of AChE and self-aggregation of Aβ (the best was compound 36k—30.7% of inhibition of hAChE-induced aggregation and 34.9% of inhibition of self-aggregation at the concentration of 50 µM). This compound was also investigated in molecular modeling tests. It showed binding to the PAS, but not to the CAS of AChE. Hybrids also showed higher efficiency against free radicals than THA and nimodipine (36k—55.78% of protection; nimodypine—36.03%; THA—0%) (Marco-Contelles et al. 2009).
Tacrine–o-hydroxy- or o-aminobenzylamine hybrids
Mao et al. designed and synthesized o-aminobenzylamine–THA hybrids 37a–g, 38a–e and o-hydroxybenzylamine-(7-chlorotacrine) hybrids 37h–n (Fig. 19). The longer the methylene linker, the better results could be obtained. Hybrids with two, three, or four carbon spacers had the lowest inhibitory activity. Regarding o-aminobenzylamine–THA hybrids, 37f possessing an eight carbon linker had the highest potency (eeAChE IC50 6.17 nM; esBChE IC50 6.25 nM) 38c, possessing a nine carbon spacer (eeAChE IC50 0.55 nM, hAChE IC50 3.52 nM, esBChE IC50 2.65 nM, THA eeAChE IC50 109 nM, and esBChE IC50 15.8 nM), which demonstrated the strongest inhibition activity toward eeAChE and hAChE. 38c exhibited good inhibition of Aβ aggregation (39.4% of inhibition at the concentration of 20 µM and curcumin 52.2% of inhibition). This inhibitor was also tested in molecular modeling study, and it occupied CAS, the mid-gorge sites, and PAS of AChE simultaneously. Most of them showed good antioxidative properties (by measuring the oxygen radical absorbance capacity of fluorescein); THA was not tested. The antioxidant activities diminished as the length of the carbon linker got longer. 43c was a moderate antioxidant (1.22 of Trolox equivalents) (Mao et al. 2012).
Compounds 37a–n, 38a–e
Tacrine–2-aminobenzothiazole hybrids
The 2-aminobenzothiazole moiety appears in many biomolecules. Its derivatives strongly interact with Aβ and inhibit the peptide aggregation (Soto-Ortega et al. 2011). They play neuroprotective roles and may be used as brain-imaging agents in Alzheimer’s disease (Alagille et al. 2011).
Keri et al. tested novel THA–2-aminobenzothiazole hybrids 39a–e (Fig. 20). Hybrids showed strong inhibitory activities toward AChE with IC50 values in the submicromolar-to-low-micromolar range, although slightly lower than THA. BChE inhibition was not determined. The geometry, length, and aromaticity of the X moiety-influenced activity. Compound 39a demonstrated the best inhibitory activity (IC50 0.34 µM and THA IC50 0.19 µM) towards eeAChE. It contained the shortest linker and a simple phenyl group attached to the BTA moiety. The presence of a methylene group or linear aliphatic Y spacer resulted in the reduction of the inhibitory activity. All the compounds were able to inhibit Aβ self-aggregation and 39b showed the strongest inhibition of the process (61.3% at the concentration of 50 µM; THA was not determined). β-Sheet secondary structure of the Aβ can interact with the 2-aminobenzothiazole moiety in 39b. A 2-aminobenzothiazole moiety composed of bi-aryl heterocyclic scaffold interacted better with an abnormal β-sheet conformation of the Aβ and, thus, provided the inhibition of the fibrilogenesis. In the DDPH, all compounds showed comparable to THA antioxidative properties (EC50 > 10,000 µM). Compounds 39d and 39e appeared to protect neuronal SHSY-5Y cells from the cell death induced by Aβ. The molecular modeling study presented that hybrids interacted with the CAS (THA moiety) and PAS (2-aminobenzothiazole moiety) of the AChE (Keri et al. 2013).
Compounds 39a–e
Tacrine–(β-carboline) hybrids
β-Carbolines are derived from the biogenic amines tryptamine and serotonin by condensation with aldehydes or R-ketoacids. They are potent and specific inhibitors of dual-specificity tyrosine phosphorylation-regulated kinase 1A; moreover, they are proved to inhibit AChE and the phosphorylation of tau protein on multiple sites associated with tau pathology in AD (Adayev et al. 2011; Frost et al. 2011).
Lan et al. evaluated a novel series of THA–(β-carboline) hybrids 40a–q (Fig. 21). The inhibitor 40c with 6 carbon atoms in the linker had the highest inhibitory activity toward eeAChE—IC50 40.6 nM and esBChE IC50 93.6 nM. Then, the series of hybrids with various moieties (modified β-carboline ring) but with the same length of tether were synthesized. Among them, the highest inhibitory activity toward eeAChE had 40l (IC50 21.6 nM and esBChE IC50 39.8 nM) with tetrahydro-β-carboline moiety when THA eeAChE IC50 260 nM and esBChE IC50 50.5 nM. The most potent inhibitory activity toward hAChE also had inhibitor 40l (IC50 63.2 nM), seven times stronger than THA (IC50 467 nM). In the kinetic study of AChE inhibition, compound 40l was a mixed-type inhibitor. Molecular modeling study showed that THA moiety of 40l bound to the CAS of AChE and the tetrahydro-β-carboline moiety interacted the PAS. Moreover, compound 40l had good inhibition of Aβ aggregation (65.8% at 20 µM; curcumin—42.3% of inhibition). In the ABTS antioxidant assay, 40l showed the most potent antioxidant activity (1.57 trolox equivalent; curcumin—1.83). In the neuroprotection assay, 40l provided cell viability of 73.67% at the concentration of 10 µM (Lan et al. 2014).
Compounds 40a–q
Tacrine hybrids with natural-based cysteine
Keri et al. tested novel hybrids with neuroprotective and antioxidant roles, hybrids conjugating THA, and 6-chlorotacrine with S-allylcysteine 41a–l (Fig. 22). The IC50 values were ranged in low-micromolar (THA derivatives) and high-nanomolar (6-chlorotacrine derivatives). 6-Chlorine atom filled the hydrophobic pocket of enzyme better and provided higher inhibitory potency. 41d showed the highest inhibitory activity toward TcAChE—IC50 0.3 µM and THA IC50 0.19 µM. The molecular modeling study presented that the spacer with three carbon atoms in the alkylene chain turned out to bind better with both the CAS and PAS of AChE. Regarding two groups of inhibitors, the length of the spacer and the cysteine moiety had only a little effect. The 6-chlorotacrine hybrids possessed the best inhibition with two carbon atoms in the linker, independently of the cysteine substituent group, allyl (41dTcAChE IC50 0.30 µM) or propargyl (41j, TcAChE IC50 0.56 µM). Similarly, for the non-substituted THA hybrids, the one with three (41bTcAChE IC50 0.88 µM) or four (41iTcAChE IC50 1.21 µM) carbon atoms, respectively, had the highest inhibitory activity. Hybrids decreased Aβ42 self-aggregation in the range of 8.5–14.7% at concentration 40 µM (41e—14.7%, THA not tested). In the DDPH test, the antioxidant activity of hybrids was better than of THA (41g, THA EC50 238 µM, > 1000 µM, respectively) (Keri et al. 2016).
Compounds 41a–l
Tacrine derivatives with cholinesterase inhibition, and antioxidant and non-hepatotoxicity properties
Tacrine–ebselen hybrids
Ebselen 42 (Fig. 23) is an organoselenium compound with antiinflammatory properties, antioxidant activity, and neuroprotective effects in preclinical studies (Schewe 1995; Yamagata et al. 2008). In vivo studies revealed that selenium-deficient diet in Tg2576 transgenic mice increased the total area of Aβ plaques in comparison to mice which were put on a selenium diet (Haratake et al. 2013).
Ebselen 42 and compounds 43a–i
Mao et al. evaluated novel THA–ebselenium hybrids 43a–i (Fig. 23). Compound 43i (AChE IC50 2.55 nM and BChE IC50 2.80 nM) with 5-carbon linker presented the highest inhibitory activity toward eeAChE and esBChE when THA IC50 was 109 nM and 15.8 nM. Hybrids with shorter carbon spacer were less potent AChE and BChE inhibitors. Hybrid 43i was a mixed-type inhibitor of AChE. It interacted with the both CAS and PAS of AChE regarding to the molecular modeling study. Compounds were evaluated for their antioxidant activity. Compound 43e and 43i possessed similar hydrogen peroxide scavenging activity as ebselen (about 75%) at the concentration of 100 µM when THA had almost no activity at this concentration. In the hepatotoxicity assay (on human hepatic stellate cells), hybrid 43i at the concentration of 10 and 5 µM provided higher cell viability than THA (110–100%; THA—90%) (Mao et al. 2013).
Tacrine–trolox hybrids
The best-known natural antioxidant is vitamin E. It has capacity to absorb reactive oxygen species. To achieve a better absorption, there was synthesized a water-soluble analogue of vitamin E, trolox 44 (Fig. 24) (Scott et al. 1974). Trolox is known as a compound that protects cells against oxidative stress (Quintanilla et al. 2005; Thiratmatrakul et al. 2014).
Trolox 44, compounds 45a–u, and compounds 46a–g
Nepovimova et al. synthesized novel THA–trolox hybrids 45a–u (Fig. 24). Hybrids were tested for their inhibitory activity towards human recombinant AChE. The inhibitors block enzyme in micro and submicromolar ranges (from hAChE IC50 13.29 to 0.08 μM, from hBChE IC50 1.30 to 0.02 µM, THA hAChE IC50 0.32 µM, hBChE IC50 0.08 µM, and trolox did not inhibit AChE). 45p, 45t, and 45u presented the best inhibitory activity (hAChE IC50 0.08 µM, hBChE IC50 0.77, 0.70, 0.54 µM, respectively). Hybrids were divided into three groups, differing in the substitution on THA heterocycle. For the 7-methoxytacrine derivatives (45a–g), the IC50 values ranged in micromolar scale and were more potent (except 45d) than prototype, 7-methoxytacrine. Non-substituted hybrids provided better results than the first group. In the third group, the addition of chlorine atom into tetrahydroacridine scaffold led to increased inhibitory activity. None of novel inhibitors showed a better potency than 6-chlorotacrine. The best inhibitor, 45u (with 6-chlorotacrine, 8-carbon linker, and trolox moiety), presented the mixed-type inhibition and bound to the AChE in inverse way in comparison with the other dual-binding-site inhibitors of AChE—the trolox moiety interacted with the CAS, whereas THA binds to the PAS of the enzyme. In DDPH, 45u displayed good antioxidant activity (EC50 44.09 µM), however, slightly lower than trolox (EC50 16.20). Only compounds 45h and 45u showed Pe values above the limits (Pe 4.4 × 10−6 cm s−1, 5 × 10−6 cm s−1, respectively), which indicated that they could cross the BBB. The cytotoxicity effect on HepG2 cell line was investigated. Derivative 45u showed nontoxic properties at concentration 0.5 µM, when IC50 of THA was 19.37 µM and that of 6-chlorotacrine—7.13 µM. The metabolic assay was carried out to explain the missing in vitro hepatotoxic effect of 45u. This compound did not convert into toxic hydroxylated derivative of THA; more than 85% remained unchanged after 1 h of incubation in human liver microsomes system. In the in vivo study, 45u was evaluated for its potential acute toxicity properties and LD50 value. Conversely to the in vitro study, 45u had LD50 value above 500 mg/kg when 6-chlorotacrine only 7.5 mg/kg. It can be caused due to the differences in metabolism and pharmacokinetics in animal body in comparison only to one type of cells in the in vitro studies (Nepovimova et al. 2015).
Xie et al. synthesized novel series of THA–trolox hybrids 46a–g (Fig. 24). All compounds showed AChE inhibitory activity in nanomolar range. Compound 46d possesses the highest inhibitory activity towards hAChE (IC50 23.5 nM and THA IC50 435.1 nM) and hBChE (IC50 20.5 nM and THA IC50 23.2 nM). In the DDPH assay, hybrids showed that they were strong antioxidants (46d IC50 48.7 µM), similar to trolox (IC50 35.6 µM). Kinetic studies and molecular modeling provided information about the mixed-type inhibition by 46d. It was proved that 46d could easily cross the BBB (8.87 × 10−6 cm s−1). The hepatotoxicity test was performed in rats. Animals were treated with the highest tolerated dose of THA or equimolar dose of 46d. 46d did not change the level of liver enzymes (ALAT and ASPAT) and was comparable to control; in result, it had a little hepatotoxicity and was less toxic than THA (Xie et al. 2015).
Conclusions
The research for an effective drug against AD is still ongoing. The current treatment is only symptomatic and cannot stop the progression of disease. The promising way concerns MTDLs based on THA moiety. The THA modification leads to improve its chemical properties. In our comprehensive review, compounds are divided into the groups: THA derivatives with cholinesterase inhibition; with cholinesterase inhibition and Aβ antiaggregation properties; with cholinesterase inhibition and antioxidant properties; with cholinesterase inhibition, antiaggregation, and antioxidant properties; with cholinesterase inhibition, annd antioxidant and non-hepatotoxicity, properties; with cholinesterase inhibition, Aβ antiaggregation, and non-hepatotoxicity properties. To summarize: (1) THA moiety is important structure in MTDLs’ development. It mostly binds to CAS; however, for example in THA–trolox hybrids, THA moiety can bind also to PAS. It is possible to obtain dual-binding site hybrids, which can have good inhibitory activity. (2) The length of the spacer influences the inhibitory activity. The highest inhibition is found for hybrids with 5–9-carbon linker. (3) The substitution at 6 or 6 and 8 positions of THA ring by chlorine leads to increase inhibitory activity towards AChE. (4) The most effective inhibitors were: THA–mefenamic acid hybrid (IC50 0.495 nM with 8-carbon linker and 6-chlorotacrine moiety); THA–gallamine hybrids (IC50 0.467 nM with 5-carbon linker and THA moiety); THA–donepezil hybrids (IC50 0.27 nM with 3-carbon linker and 6-chlorotacrine moiety); THA–indole heterodimers (IC50 0.02 nM with 6-carbon linker and 6-chlorotacrine moiety); THA–o-aminobenzylamine hybrids (IC50 0.55 nM with 9-carbon linker and THA moiety); THA–benzofuran hybrids (IC50 0.86 nM with 7-carbon linker and THA moiety); THA–melatonin hybrids (IC50 8 pM with 6-carbon linker and 6,8-dichlorotacrine moiety). (5) Linking many activities of drugs together has led to the development of MTDLs. Novel hybrids possessed many properties such as: inhibition of Aβ aggregation, low cytotoxicity effect on different cell lines, low hepatotoxic properties, reduction of oxidative stress, and increase in the BBB permeability.
While talking only about advantages of MTDLs therapy, it also faces some problems. Novel hybrids might possess too high-molecular-weight mass due to two or three connected smaller moieties. Therefore, hybrids may violate Lipinski’s rule of 5 and present low CNS penetration. For novel compounds, it is always crucial to obtain good results in ADMET calculations and later in the in vivo pharmacokinetic studies. It is also important to notice the reason for the different concentrations of inhibitor in in vitro assay. In the AChE-Aβ inhibitions assays, researchers use higher concentrations of AChE, Aβ, and inhibitor than those present in human brain. The high level of AChE concentration is essential to accelerate the aggregation process of Aβ. It is important for analytical purposes. The inhibitor/AChE concentration ratio from AChE inhibition assay and from Aβ-inhibition test should be compared. The obtained values should be of the same magnitude, which can indicate that high (in µM) and low (in nM) concentrations of inhibitor give both inhibitory activities (Marco-Contelles et al. 2009). Similar situation is observed in comparison between in vivo and in vitro tests. Both assays may differ due to the differences in pharmacokinetics and metabolism, where also biotransformation is an important process, for example, in toxicity assays. Therefore, concentration of drugs used in in vitro and in vivo assays may vary—for example, like for 7-MEOTA 7, which has some toxicity for the cell in the in vitro assays, but does not show acute or chronic hepatotoxicity in the in vivo assays (Nepovimova et al. 2015). Moreover, in the in vivo assays, it is important to assess LD50 values of novel inhibitors before performing another test. It is crucial to obtain concentration that is general not toxic for animals and will enable to perform in vivo tests.
The last drug against AD, which was approved by the FDA in 2003, was memantine. Since 2003, over 200 compounds reached Phase II, but only 50 went to the Phase III. However, none of them have passed so far Phase III. It might be caused by wrong strategy, inadequate doses of novel drugs, lack of therapeutic efficacy, the serious side effects or just lack of diagnostic criteria, and markers of the disease which could set the clinical endpoints and efficacy standards. Clinical trials are conducted at late stages of AD, when therapeutic drugs are aimed at the early pathological processes. For example, antiAβ agents should be tested at first stages of disease. Moreover, the dosing needs to be more aggressive due to the large burden of pathological changes in the brain even in mild stage of AD. Some clinical trials failures have proper explanations. For example, solanezumab slowing (in around 15%) of cognitive decline, but more likely binds to monomers than to Aβ oligomers (more toxic). It caused a small yield of clinical benefit. Dimebon and R-flurbiprofen did not provide good pharmacokinetics and convincing efficacy. Currently, in III Phase are a few types of novel drugs: amyloid antibodies (antiAβ oligomers agents)—crenezumab, aducanumab and gantenerumab, acetylcholinesterase inhibitor—octohydroaminoacridine succinate, the combination of donepezil and choline alphoscerate, and the MTDLs, such as AVP-786 (NMDA antagonist and P-glycoprotein inhibitor) and TRx-00237 (inhibitor of monoamine oxidase, nitroxide production, and Tau aggregation) (Bachurin et al. 2017; Hung and Fu 2017; Stower 2018). Many novel compounds have being excessively tested in clinical trials (in 2016, antiAβ drugs were investigated in the largest number). So far, Aβ is considered as the best target for AD treatment. The Aβ hypothesis is acknowledged and inspires researchers to develop new drugs. Numerous compounds affect Aβ pathology pathway and might reduce effects (such as oxidative stress) which is caused by Aβ. These compounds focus on reducing production of Aβ (beta and gamma secretase inhibitors), increasing clearance (monoclonal antibodies, which bind to soluble Aβ or senile plaques) and preventing Aβ aggregation. Among described hybrids, almost all were tested against Aβ aggregation and showed their inhibitory properties. Therefore, MTDLs are highly promising compounds for treating AD. They might act against Aβ and toxicity which Aβ caused. Nevertheless, better understanding the pathophysiology of AD will be important to obtain compounds with advantageous properties and finally to combat the disease.
Original research on novel tacrine derivatives has been still carrying out. Since the original drafting of our review, a few important publications have appeared: Cen et al. 2018; Chioua et al. 2018; Hamulakova et al. 2018; Hepnarova et al. 2018; Hiremathad et al. 2018; Jiang et al. 2018; Li et al. 2018; Liu et al. 2018; Skibiński et al. 2017; Wu et al. 2018.
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Financial support by Grant (503/3-015-01/503-31-002) from the Medical University of Lodz and grant from the National Science Centre in Poland (Project number: 2015/19/B/NZ7/02847) is gratefully acknowledged.
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Girek, M., Szymański, P. Tacrine hybrids as multi-target-directed ligands in Alzheimer’s disease: influence of chemical structures on biological activities. Chem. Pap. 73, 269–289 (2019). https://doi.org/10.1007/s11696-018-0590-8
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DOI: https://doi.org/10.1007/s11696-018-0590-8