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CNS & Neurological Disorders - Drug Targets, 2014, 13, 391-401 391 Current Acetylcholinesterase-Inhibitors: A Neuroinformatics Perspective

Sibhghatulla Shaikh1,†, Anupriya Verma2,†, Saimeen Siddiqui1,†, Syed Sayeed Ahmad2, Syed Mohd. Danish Rizvi1, Shazi Shakil*,2, Deboshree Biswas1, Divya Singh1, Mohd. Haris Siddiqui2, Shahnawaz Shakil3, Shams Tabrez4 and Mohammad Amjad Kamal4

1Department of Biosciences, Integral University, Lucknow-226026, India 2Department of Bio-Engineering, Integral University, Lucknow-226026, India 3Cardinal Health 7000, Cardinal Place, Dublin OH 43017, USA 4King Fahd Medical Research Centre, King Abdulaziz University, P.O. Box 80216, Jeddah 21589, Saudi Arabia

Abstract: This review presents a concise update on the inhibitors of the neuroenzyme, acetylcholinesterase (AChE; EC 3.1.1.7). AChE is a serine protease, which hydrolyses the , into acetate and choline thereby terminating neurotransmission. Molecular interactions (mode of binding to the target enzyme), clinical applications and limitations have been summarized for each of the inhibitors discussed. Traditional inhibitors (e.g. physostigmine, tacrine, donepezil, rivastigmine etc.) as well as novel inhibitors like various physostigmine-derivatives have been covered. This is followed by a short glimpse on inhibitors derived from nature (e.g. Huperzine A and B, Galangin). Also, a discussion on ‘hybrid of pre-existing drugs’ has been incorporated. Furthermore, current status of therapeutic applications of AChE- inhibitors has also been summarized. Keywords: Acetylcholinesterase, acetylcholine, physostigmine, rivastigmine, huperzine A.

1. INTRODUCTION gravis (MG) [5], Parkinson’s disease (PD) [6] and other ‘non-classical’ activities such as cell adhesion, neurite The Nobel Prize in Physiology or Medicine 1936 was formation and network formation [7] elicited numerous shared by Sir and for their researches relevant to the medical field. findings with reference to chemical transmission of nerve impulses. Until 1921 it was believed that the transmission of nerve impulses was ‘electrical’ in nature. But this theory was not acceptable due to two reasons, one being the presence of gap between neurons and effecter organs and the other a Fig. (1). Reaction catalyzed by AChE. The enzyme hydrolyses ACh decrease in activity due to impulses from inhibitory nerves. into acetate and choline (Source: Hay, D et al. 2010 [9]). Otto Loewi proved the ‘Chemical’ nature of impulse transmission through his famous experiment of two beating 2. STRUCTURE OF ACETYLCHOLINESTERASE hearts from frogs – one connected to vagus nerve and AChE (EC 3.1.1.7) is one of the two cholinesterases, accelerator nerves; second one without any nerve more specifically a serine protease, which hydrolyses ACh connection. In this experiment he discovered the first neurotransmitter into acetate and choline and hence, neurotransmitter ‘acetylcholine’ (ACh) [1]. ACh is a neurotransmitter widely distributed in the central (and also terminates neurotransmission. Due to the presence of a common α/β fold, it is included in the α/β fold super family, peripheral, autonomic and enteric) nervous system (CNS). In works near its diffusion control rate [8] and possesses a high the CNS, ACh facilitates many functions, such as learning, ‘turn-over rate’ of 2.5x104 ACh molecules per second. memory, attention and motor control. In 1914, Sir Henry Dale suggested that an enzyme which degrades the esters of It is revealed by its crystal structure that the catalytic choline, played a role in neurotransmission within the triad, formed by serine, histidine and glutamate, is present at autonomic and somatic motor nervous systems and that this the bottom of a narrow gorge of about 20-A° size. This gorge enzyme, acetylcholinesterase (AChE), was the target of widens at the base. Catalytic triad (S200, E327 and H440 in action of the drug, physostigmine (eserine) [2]. Hence, case of Torpedo californica) is designated to ‘esteratic site’ AChE enzyme regulated the release and entry of ACh in or ‘acylation site’ or ‘A-site’. A planar array formed by these cholinergic fibers [3]. Role of AChE in neurological three residues closely resembles the catalytic triad of disorders like Alzheimer’s disease (AD) [4], Myasthenia chymotrypsin and of other serine proteases except that the glutamate is the third member rather than the asparate [9,

10]. Serine is responsible for hydrolysis of choline esters *Address correspondence to this author at the Department of Bio-engineering, through proton transfer in catalytic triad. Cationic-π site Integral University, Lucknow, 226026, India; Tel: +91-8004702899; Fax: (CAS) is present above esteratic site, where quaternary +91-522-2890809; E-mail: [email protected] ammonium of choline of ACh interacts. CAS is followed by †Equally contributing authors. PAS (Peripheral Anionic Site) or P-site which forms mouth

1871-5273/14 $58.00+.00 © 2014 Bentham Science Publishers 392 CNS & Neurological Disorders - Drug Targets, 2014, Vol. 13, No. 3 Shaikh et al. of the gorge [11]. Anciently, it was believed that PAS The molecular forms of AChE correspond to various comprises of several negatively charged amino acids due to quaternary structures and modes of anchoring of the enzyme. its preference to bind cationic ligands. However, the These forms are determined by alternate splicing of AChE gene. hypothesis was discarded [12]. The high aromatic content of AChE variants are produced by alternate splicing, each with a the deep and narrow active-site gorge of AChE is a different carboxy-terminal sequence. These carboxy-terminal remarkable feature of this enzyme. There are 14 conserved sequences determine their homologous assembly into AChE aromatic amino acids lined along the gorge of Torpedo oligomers and their heterologous association with non-catalytic californica AChE (TcAChE), namely F120, F288, F290, subunits that direct the subcellular localization of the protein. F330, F331, W84, W233, W279, W432, Y70, Y121, Y130, The three AChE variants are – the ‘synaptic’ (S) or ‘tailed’ (T), Y334, and Y442 [12]. Tryptophan 84 is critical among all ‘erythrocytic’ (E) or ‘hydrophobic’ (H) and ‘readthrough’ (R) the aromatic amino acids and its substitution with alanine AChE isoforms. AChE-S is the only type of catalytic subunit results in a 3000-fold decrease in reactivity [13]. Beside that exists in all vertebrate cholinesterases. It produces the major these sites, AChE possesses ‘Acyl pocket’ which confers forms in adult brain and muscle. In AChE-E, a glycyl bond near substrate specificity and ‘Oxyanion Hole’ which the carboxyl terminus undergoes transamidation to attach a accommodates negative oxygen ion during catalysis for glycophosphatidylinositol group to the protein, which anchors catalytic efficiency of the enzyme [14]. the mature AChE-E to the outer surface of erythrocytes. AChE- R doesn’t acquire any feature for attachment and hence, remain monomeric (G1) and soluble [15-18].

Fig. (2). Schematic view of the active-site gorge of TcAChE 2.1. Targeting AChE [Source: Hay, D et al. 2010 [9]. The inhibitors of AChE help in increasing the concentration and duration of action of acetylcholine by resisting the breakdown of ACh. Many of these inhibitors are constituted in prescription for many neurological diseases such as AD etc. Although the inhibition is needed but, ''total'' inhibition of AChE causes ‘toxicity’. Thus, AChEIs can be classified into reversible, quasi-irreversible and irreversible inhibitors on the basis of affinity of AChEIs toward AChE. Reversible inhibitors act till their concentration is sufficient and are transient in nature. Irreversible and quasi-irreversible inhibitors form covalent bonds with the enzyme and inhibit it irreversibly. Such inhibitors are rarely used as therapeutics and are mainly used as insecticides/pesticides and chemical warfare agents (nerve gases).

2.2. Traditional AChE Inhibitors 2.2.1. Physostigmine (Eserine) The IUPAC name of Physostigmine (Eserine) is (3aR, Fig. (3). Recombinant human AChE [PDB id: 3LII]. 8aS)-1,3a,8-Trimethyl-1H, 2H, 3H, 3aH, 8H, 8aH-pyrrolo[2,

Fig. (4). Alternative splicing and molecular forms of AChE (Source: Massoulié, J et al. 1999 [16]). Current Acetylcholinesterase-Inhibitors CNS & Neurological Disorders - Drug Targets, 2014, Vol. 13, No. 3 393

3-b]indol-5-ylN-methylcarbamate. It was the first choli- (a) nesterase inhibitor that was investigated for AD. It is an alkaloid usually obtained from dried ripe seeds of Physostigma venenosum (Calabar bean) and gives primarily muscarinic effects. It also possesses capability to cross the blood-brain barrier but has a short half-life and narrow therapeutic index. Moreover, there are many side effects such as nausea, vomiting, diarrhea, headaches and dizziness associated with the drug. This drug was in use for MG, delayed gastric emptying and glaucoma. However, it was not approved for AD.

(b)

Fig. (5). Chemical structure of Physostigmine. 2.2.2. Tacrine The IUPAC name of tacrine is 1, 2, 3, 4- tetrahydroacridin-9-amine. In 1993 tacrine was approved for AD. Tacrine is a potent inhibitor of both AChE and butyrylcholinesterase (BuChE). It is a synthetic ChEI. It has limited clinical applications due to hepatotoxicity via elevation of serum alanine aminotransferase levels. It is also associated with side effects such as nausea, vomiting, dizziness, diarrhoea, seizures, and syncope. A research shows interaction of tacrine with amino acid residues Phe330 and Trp84 which lie at ‘anionic site’ of AChE [19]. 2.2.3. Donepezil The IUPAC name of donepezil is (RS)-2-[(1-benzyl-4- piperidyl) methyl]-5, 6-dimethoxy-2, 3-dihydroinden-1-one, (c) which was approved for mild to moderate AD in 1996. Donepezil is used to treat dementia associated with AD, improves mental functions such as memory, attention, reasoning ability etc. Besides acting as ChEI, donepezil is also involved in every stage associated with pathogenesis of AD such as blocking various aspects of the excitotoxic cascade induced by glutamate, mitigating the effects of oxidative stress, and reducing the expression of inflammatory cytokines. The oral bioavailability of donepezil is 100% and reaches its peak plasma concentrations in 3 to 4 hours. Its unique orientation Fig. (6). (a) Interaction of Tacrine with anionic site of AChE. (b) facilitates interactions extending from the anionic subsite of AChE-Tacrine complex [PDB id: 1ACJ] (c) Interaction of Bis- the active site, at the bottom, to the PAS, at the top, via tacrine with TcAChE [PDB id: 2CKM]. aromatic stacking interactions with conserved aromatic acid residues of TcAChE [20]. However, it does not interact group, (-)-S-3-[1-(dimethylamino) ethyl] phenol, being directly with the ‘catalytic triad’ or ‘oxyanion hole’. Human retained in the "anionic" site. A significant movement of the AChE (hAChE) shows different interactions [21]. active-site histidine (H440) away from its normal hydrogen- bonded partner, E327, was observed which resulted in 2.2.4. Rivastigmine disruption of the catalytic triad. This movement may provide an explanation for the unusually slow kinetics of reactivation The IUPAC name of Rivastigmine is (S)-3-[1- [22]. (dimethylamino) ethyl] phenyl N-ethyl-N-methylcarbamate. It was approved for AD in 2000. It inhibits both AChE and 2.2.5. Galantamine BuChE. It is used for mild to moderate AD and dementia due to PD. Due to side effects (nausea, anorexia, vomiting and The IUPAC name of Galantamine is (4aS, 6R, 8aS) - 5, diarrhea) of capsular form, it was reformulated for delivery 6, 9, 10, 11, 12- hexahydro- 3-methoxy- 11-methyl- 4aH- [1] through a transdermal patch in 2007. A crystal structure of benzofuro[3a, 3, 2-ef] [2] benzazepin- 6-ol. It is a TcAChE-rivastigmine shows that carbamyl moiety gets benzazepine derived from norbelladine. It is found in covalently linked to the active-site serine, with the leaving ‘Galanthus’ and other amaryllidaceae. Galantamine is a

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(a) amyloid precursor protein (APP) by reducing APP translation via interaction with the 5’- untranslated region of the APP gene. Phenserine enantiomer, Posiphen [(+)- phenserine], has weak activity as an AChE inhibitor while APP expression remains more or less equal and can be dosed much higher. (+)-Posiphen may be a promising drug, either alone or in combination with (-)-phenserine, to attenuate the progression of AD. During 2003-2004, Phase III clinical trials were conducted in which 384 AD patients were recruited. But phenserine did not achieve significant improvement on the ‘ADAS-cog scores’ compared to placebo. Subsequently, further clinical trials for AD have been abandoned [24].

(a) (b)

(b)

Fig. (7). (a) Interaction of Donepezil with TcAChE (b) TcAChE- Donepezil complex [PDB id: 1EVE]. reversible, competitive AChE inhibitor that has been used to reverse the muscular effects of gallamine triethiodide and tubocurarine, and has been studied as a treatment for AD and other CNS disorders which is sold under trade name Razadyne® and Razadyne® ER [23]. 2.2.6. Metrifonate The IUPAC name of Metrifonate (Trichlorfon) is (RS)- Dimethyl (2, 2, 2-trichloro-1-hydroxyethyl). It is a long- acting irreversible organophosphate AChEI that was originally used to treat schistosomiasis. Its potential to improve cholinergic neurotransmission via a pharmaco- logically active metabolite, DDVP (2, 2-dichlorovinyl dimethyl phosphate) led to its clinical trials for AD. But its long-term usage produced side effects such as respiratory paralysis and problems with neuromuscular transmission. Due to this, clinical trials were halted during Phase-3 [23]. Fig. (8). (a) Interaction of Donepezil with hAChE. (b) hAChE- Donepezil complex [PDB id: 4EY7]. 2.3. New AChE inhibitors 2.3.1.2. Tolserine 2.3.1. Physostigmine Derivatives Tolserine only differs from phenserine at the 2-methyl substitution on its phenylcarbamoyl moiety. It is 200-fold 2.3.1.1. Phenserine more selective against hAChE than for BuChE and is also Phenserine, a phenylcarbamate derivative of physostig- more potent than both phenserine and physostigmine. mine, is a selective, non-competitive AChE inhibitor that not Kinetics of inhibition of human AChE by Tolserine shows only inhibits ACh but also modulates the amount of beta- IC (50) value of 8.13 nM, which is a very good IC (50) Current Acetylcholinesterase-Inhibitors CNS & Neurological Disorders - Drug Targets, 2014, Vol. 13, No. 3 395 value. However, there is no report on its side effects or leaving group, bisnoreseroline, trapped at the bottom of the benefits in preclinical and clinical models [25]. aromatic enzyme gorge. This leaving group was found to interact non-covalently with Ser (200) and His (440) and (a) stacked with Trp (84) at the bottom of the gorge, giving rise to an unprecedented hydrogen-bonding contact. These interactions pointed to a dominant reversible inhibition mechanism attributable to the leaving group, bisnoreseroline [27].

(a) (b)

(c)

(b)

Fig. (9). (a) Chemical structure of Rivastigmine (b) Interaction of Rivastigmine metabolite (SAF) with TcAChE (c) TcAChE- rivastigmine complex [PDB id: 1GQR]. 2.3.1.3. Esolerine It is a tacrine derivative. A study was conducted by Zhan et al. [26] in which a series of physostigmine analogues were prepared and assessed for AChEI. Compound 17, a cyclic alkyl Fig. (10). (a) Interaction of Galantamine with AChE. (b) AChE- carbamate of esolerine was found to be a very strong AChE Galantamine complex [PDB id: 4EY6]. inhibitor which significantly favoured AChE over BuChE [26]. 2.3.1.4. Bisnorcymserine It is a derivative of drug Cymserine, which is related to physostigmine. Bisnorcymserine is highly selective for BuChE. However, a study showed that it may act as a reversible inhibitor of AChE and X-ray crystal structure exhibited only the Fig. (11). Chemical structure of Metrifonate. 396 CNS & Neurological Disorders - Drug Targets, 2014, Vol. 13, No. 3 Shaikh et al.

against AChE than against BuChE. In addition, CHF2819 is more selective for inhibition of central (brain) AChE than peripheral (heart) AChE. The crystal structure of the ganstigmine conjugate with TcAChE shows that carbamoyl moiety is involved in interaction with active site but the Fig. (12). Chemical structure of Phenserine. leaving group geneseroline is not retained in the catalytic pocket. The carbamoyl moiety forms covalent bond with active site serine and hydrogen bond with histidine (His440) via its nitrogen atom. These interactions may account for the long duration of action of ganstigmine in vivo [28, 29]. (a)

Fig. (13). Chemical structure of Tolserine [PubChem id: CID 9928397].

Fig. (14). Chemical structure of Compound 17. (a) (b)

(b)

(c)

Fig. (15). (a) Interaction of Bisnorcymserine with bisnoreseroline. (b) TcAChE- bisnoreseroline complex [PDB id: 3ZV7]. 2.3.1.5. Ganstigmine (CHF-2819) It is an orally active, geneserine (N-oxide of Fig. (16). (a) Chemical structure of Ganstigmine hydrochloride physostigmine) derived, AChE inhibitor developed for the [PubChem id: CID 56842812]. (b) Interaction of Ganstigmine with treatment of AD. This compound is 115 times more potent TcAChE. (c) TcAChE-Ganstigmine complex [PDB id: 2BAG]. Current Acetylcholinesterase-Inhibitors CNS & Neurological Disorders - Drug Targets, 2014, Vol. 13, No. 3 397

2.3.2. Tesofensine (NS2330) (a) It is a compound that increases the activity of dopamine, nor-epinephrine and ACh. Tesofensine induced decline in beta-amyloid in mice and cognitive improvement in mild AD during Phase IIA trials, but Phase IIB trials showed low activity. Currently, it is being marketed as a drug against obesity, while in 2008 the trials were discontinued [30].

(b) Fig. (17). Chemical structure of Tesofensine [Pubchem id: CID 11370864]. 2.3.3. Derived from Nature 2.3.3.1. Huperzine Huperzine A (HupA) is a Lycopodium alkaloid isolated from the Chinese medicinal herb Huperzia serrata. It is used for memory deficiency, and is greatly interesting as it acts as a highly selective, reversible, and potent AChE inhibitor. It is of 2 types – HupA and Hup B. Hup B is a natural homologue of HupA. HupA is found to be more potent than tacrine, rivastigmine, and galantamine in terms of inhibition activities, and it has the least amount of activity against BuChE. It was also hybridized with tacrine and donepezil. Tacrine derivatives were found more effective than donepezil derivatives. A pro-drug form of HupA (ZT-1) is under development as a treatment for AD. Hup B is less (c) potent and selective than Hup A, but it has higher therapeutic index and other positive benefits. Hence, a series of 16 substitued derivatives of Hup B was created. Among these compound ‘9i’ was found to be most potent and exhibited higher efficacy than Hup A, galantamine, and rivastigmine. Both Huperzines interact in similar way – Both interact with anionic site via pi-pi stacking and Trp84 and Phe330 via van der Waals or C-H pi interactions. Alpha-pyridone moieties of huperzines interact with active site via hydrogen bonding, and possibly via C-H pi interactions. Carbonyl oxygens of huperzines repel carbonyl oxygen of Gly117. Due to this the peptide bond between Gly117 and Gly118 gets flipped. This flipped conformation is stabilized by hydrogen bonding of (d) Gly117O to Gly119N and Ala201N, the other two functional elements of the three-pronged "oxyanion hole" characteristic of cholinesterases [31]. 2.3.3.2. Nelumbo Nucifera Nelumbo nucifera (Indian Lotus) is an aquatic plant that belongs to the family of Nelumbonaceae, bearing a lot of medicinal properties. It has been reported that N. Nucifera semen extract improved memory in rats with scopolamine- induced dementia through the induction of choline acetyltransferase expression and inhibition of AChE. Five compounds, namely, 1) beta-cyclogeraniol diglycoside, 2) cycloartenol, 3) p- hydroxybenzoic acid, 4) vanilloloside Fig. (18). (a) Interaction of Hup A with hAChE. (b) hAChE-Hup A and 5) 5-O-methyladenosine were isolated. Compounds complex [PDB id: 4EY5] (c) Interaction of Hup B with TcAChE. (d) TcAChE- Hup B complex [PDB id: 1GPN]. 398 CNS & Neurological Disorders - Drug Targets, 2014, Vol. 13, No. 3 Shaikh et al.

1-4 demonstrated good and non-competitive AChE amyloid aggregation is higher. Hence, a series of 22 inhibition whereas compounds 1-3 exhibited BuChE donepezil and AP2238 hybrids were formed in which the inhibition. Hence, compounds 1-4 can be used in AD idanone core from donepezil was linked to the phenyl-N- therapies [32]. methylbenzylamino moiety from AP2238. Compounds 15, 2.3.3.3. Himatanthus Lancifolius 21, and 22 had a 5-carbon alkyl chain with an amino moiety at one end, which gave the compounds better contact with Himatanthus lancifolius (a Brazilian species of PAS site. Derivates 21 and 22 were the most active of all the Apocynaceae) is a shrub that possesses several indole compounds [35]. alkaloids with a number of medicinal properties such as 2.4.1.2. Donepezil-Tacrine Hybrids antimicrobial effects, gastro protection, and the ability to affect the vascular and nonvascular smooth muscle A series of donepezil-tacrine hybrids were created by responsiveness. Seidl et al. [33] reported that Camps et al. [36], which were found to interact dichloromethane, and ethyl acetate fractions possess simultaneously with active, peripheral and midgorge binding significant AChE inhibitory effects. Uliene was the sites of AChE. These hybrids consisted of a unit of tacrine or significant compound present in both the fractions. However, 6-chlorotacrine which occupied the active site in a similar there is no report on humans specific to cognition. way as tacrine does and 5, 6-dimethoxy-2-[(4- piperidinyl)methyl]-1-indanone moiety of donepezil (or its 2.3.3.3. Galangin indane derivative), whose position can be adjusted along Flavonoids have been of great interest in AD research AChE’s gorge and peripheral site via suitable tether that and treatment because of their free radical scavenging connects donepezil and tacrine. Such compounds were found properties. Guo et al. [34] reported that among 21 flavonoids to inhibit both AChE and BuChE and possessed anti-beta extracted from Rhizoma Alpiniae Officinarum, Galangin amyloid aggregation [36]. exhibited highest inhibitory effect on AChE. Galangin can 2.4.1.3. Tacrine/Ferulic Acid Hybrids and Beta-Carboline be a potent treatment for AD [34]. Derivatives 2.4.1. Hybrids Two tacrine-ferulic acid hybrids (1 A, 1 B) and three As the causes and mechanisms behind most of the beta-carboline derivatives (2A, 2B, 2C) were developed. neurological disorders are multifactorial, hence, it is Tacrine hybrids showed higher inhibition than tacrine for necessary to design such drugs that would target more AChE and comparable inhibition for BuChE. Beta-carboline components of a given disease. For instance, we can develop derivatives showed inhibition as that of tacrine, but none of a drug that targets hallmarks of AD, i.e low levels of ACh, tacrine and beta-carboline derivatives improved cognition. misfolding of proteins and associated beta-amyloid Further, compound 1B worsened scopolamine-induced aggregation, oxidative stress and metal dyshomeostasis. cognition impairment in mice [37]. Hybrids of pre-existing drugs offer such opportunities. A 2.4.1.4. Tacrine-8-Hydroxyquinoline Hybrids hybrid drug has been created which acts as site-directed chelator that inhibits both AChE and monoamine oxidase Tacrine and PBT2 (an 8-hydroxyquinoline derivative) are A/B. In this hybrid, chelator M30 is complexed with well-known drugs that inhibit cholinesterases and decrease compounds (1) and (2), this complex acts as prochelator. beta-amyloid (Abeta) levels by complexation of redox-active Compound (1) is more potent while compound (2) is less metals, respectively. Fernandez-Bachiller et al. [38] potent than rivastigmine in inhibiting AChE. Also, both synthesised Tacrine-8-hydroxyquinoline hybrids which were compounds are more efficient in inhibiting monoamine more potent than tacrine in inhibition, had better CNS oxidase-A than M30. However, compound (1) is less permeability, possessed low toxicity, copper complexing and efficient than M30 in inhibiting monoamine oxidase-B. antioxidant properties [38]. When this prochelator is released in the body, it selectively 2.4.5. Synthetic Analogue - N-Alkyl-7-Methoxytacrine inhibits AChE with minimal metal binding affinity and Hydrochlorides monoamine oxidase A/B inhibition. M30 is released by AChE, thus increasing the inhibition of monoamine oxidase Tacrine was an AChEI which caused dose dependent but A/B. reversible liver toxicity. 7-methoxytacrine is an analogue of tacrine that was less toxic and pharmacologically equivalent 2.4.1.1. Donepezil and AP2238 to tacrine. A series of 7-methoxytacrine analogues were AP2238 is the first published compound to bind with prepared. Compounds 5-7 showed better inhibition of both the anionic sites of AChE. The potency against AChE is hAChE than parent drugs. Compound 5 was found to be 5 comparable to donepezil, while its ability to contrast beta- times more potent than tacrine [39].

Fig. (19). Synthesis of N-alkyl-7-methoxy-1,2,3, 4-tetrahydoacridinamine hydrochlorides (Source: De, la et al. 2012 [40]). Current Acetylcholinesterase-Inhibitors CNS & Neurological Disorders - Drug Targets, 2014, Vol. 13, No. 3 399

2.4.6. (E)-2-(Benzo[d]Thiazol-2-yl)-3-Heteroarylacrylonitriles block, alter, or destroy the receptors for ACh at the , which prevents the muscle (E)-2 (benzo[d]thiazol-2-yl)-3-heteroarylacrylonitriles contraction from occurring. These antibodies are produced are recently developed AChE competitive inhibitors. The by body’s own immune system. The hallmark of MG is most potent compound among these exhibits an IC (50) of 64 muscle weakness that increases during periods of activity µM. Compound 7f was found to be more selective than and improves after periods of rest. Certain muscles such as galanthamine in inhibiting AChE [40]. those that control eye and eyelid movement, facial expression, chewing, talking, and swallowing are often, but not always, involved in the disorder. Medications used to treat the disorder include ChEI [neostigmine and pyridostigmine], which help improve neuromuscular Fig. (20). Chemical structure of Compound 7f. transmission and increase muscle strength. Immunosuppressive drugs such as prednisone, azathioprine, cyclosporin, mycophenolate mofetil, and tacrolimus may 2.5. Use of AChE inhibitors in Diseases also be used to improve muscle strength by suppressing the AChE exhibited a variety of roles in various diseases and production of abnormal antibodies. MG is also associated other physiological conditions, especially, neurological with the production of ‘readthrough’ (R) AChE isoforms diseases. Hence, pharmacological inhibitors of AChE are [48]. In rats with experimental autoimmune MG, inhibition important in controlling diseases that involve impaired ACh- of production of AChE-R using antisense [such as Monarsen mediated neurotransmission. (EN101)] is associated with a significant reduction in synaptic expression of AChE-R mRNA and protein, with Some of the neurological diseases are discussed: improved muscle strength and stamina and increased 2.5.1. Alzheimer’s Disease survival [49]. It is a progressive, neurodegenerative pathology that 2.5.3. Parkinson’s Disease primarily affects the elderly population, and is estimated to This disease (PD) belongs to a group of conditions called account for 50 to 80 % of dementia cases in persons over 65 motor system disorders, which are the result of the loss of years of age. But it is not just a disease of old age, since up dopamine-producing brain cells. PD usually affects people to 5 percent of people with the disease have early onset over the age of 50. It develops gradually, sometimes starting Alzheimer's (also known as younger-onset), which often with a barely noticeable tremor in just one hand. But while appears when someone is in their 40s or 50s. Cognitive tremor may be the most well-known sign of PD, the disorder dysfunction, primarily memory losses are the main also commonly causes stiffness or slowing of movement. To symptoms associated with this disease. Other features date, despite decades of intensive study, the causes of associated with the later stages of AD include language Parkinson’s remain unknown. Many experts think that the deficits, depression, behavioral problems including agitation, disease is caused by a combination of genetic and mood disturbances and psychosis. The etiology of AD is still environmental factors, which may vary from person to not clear. A lot of researches have been undertaken in recent person. Genetic factors include mutations in SNCA (alpha- years and concluded with some strong hypotheses. Some of synuclein), PINK1 (PTEN-induced putative kinase 1), them are cholinergic hypothesis, Amyloid cascade PARK2, PARK7, PLA2G6, FBXO7, and ATP13A2, which hypothesis, Tau hypothesis, Oxidative stress hypothesis, are rare cause of the disease. One particular mutation in Zinc dyshomeostasis hypothesis and Calpain-cathepsin LRRK2 (leucine-rich repeat kinase 2) has been found to be hypothesis [41-46]. Cholinergic hypothesis is the oldest one common in certain populations [50, 51]. At present, there is and it is still promising in understanding AD for designing no cure for PD. Medication used to get relief from its drugs. This hypothesis proposed that degeneration of symptoms includes levodopa combined with carbidopa cholinergic neurons in the basal forebrain and the associated (increases dopamine concentration in brain), dopamine loss of cholinergic neurotransmission in the cerebral cortex mimicking agents (bromocriptine, pramipexole, and and other areas contributed significantly to the deterioration ropinirole) and Anticholinergics that can help control tremor in cognitive function seen in patients with AD. As it has and rigidity [51]. ChEI are used to fight dementia. Some of been reported that AD is caused by multiple risk factors, ChEI used are donepezil, rivastigmine, galantamine and hence, this hypothesis is modified by certain authors. AChE tacrine [52]. Out of these Rivastigmine is the only licensed inhibitors are employed to reduce the rate at which ACh is treatment for dementia in PD [53]. broken down, thereby increasing the concentration of ACh in the brain and combating the loss of ACh caused by the death 2.5.4. Dementia with Lewy Bodies of cholinergic neurons. Current medication for AD includes use of AChE inhibitors such as Donepezil (Aricept), Dementia with Lewy bodies (DLB) or Lewy Body rivastigmine (Exelon), and galantamine (Razadyne, formerly Dementia is a form of progressive dementia that shares called Reminyl) [47]. characteristics with both AD and PD, the cause of this disease is still unclear. DLB features loss of cognitive 2.5.2. Myasthenia Gravis functions, fluctuations in attention and alertness, visual hallucinations and parkinsonian motor symptoms such as It is a chronic autoimmune neuromuscular disease loss of movement/rigidity. A lot of data show that presence characterized by varying degrees of weakness of the skeletal of lewy bodies is the reason for such symptoms. Lewy (voluntary) muscles of the body. In this disease antibodies bodies contain abnormally phosphorylated neurofilament 400 CNS & Neurological Disorders - Drug Targets, 2014, Vol. 13, No. 3 Shaikh et al. proteins aggregated with ubiquitin and alpha-synuclein. It CONFLICT OF INTEREST has been proposed that alpha-synuclein accumulation is also The authors confirm that this article content has no linked to PD, multiple system atrophy, and several other conflict of interest. disorders, which are referred to as the "synucleinopathies." In DLB, ACh neurotransmission is highly affected due to degeneration of brain-stem and basal forebrain cholinergic ACKONWLEDGEMENTS projection neurons. Treatment of DLB aims at improving Declared none. cognitive, psychiatric, and motor symptoms of the disorder. 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Received: February 13, 2013 Revised: April 17, 2013 Accepted: April 19, 2013

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PMID: 24059296