acetylcholinesterase inhibitors as alzheimer therapy from nerve toxins to neuroprotection

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European Journal of Medicinal Chemistry 70 (2013) 165e188

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European Journal of Medicinal Chemistry

journal homepage: http: / /www.elsevier .com/locate/ejmech

Invited review

Acetylcholinesterase inhibitors as Alzheimer therapy: From nervetoxins to neuroprotection

Manjinder Singh, Maninder Kaur, Hitesh Kukreja, Rajan Chugh, Om Silakari,Dhandeep Singh*

Pharmaceutical Chemistry Research Lab, Department of Pharmaceutical Sciences and Drug Research, Punjabi University, Patiala, Punjab 147002, India

a r t i c l e i n f o

Article history:Received 14 May 2013Received in revised form24 September 2013Accepted 28 September 2013Available online 6 October 2013

Keywords:AcetylcholineAcetylcholinesteraseAlzheimer’s diseaseNerve gases

* Corresponding author. Tel.: þ91 9814412412.E-mail address: [email protected] (D. Sin

0223-5234/$ e see front matter � 2013 Elsevier Mashttp://dx.doi.org/10.1016/j.ejmech.2013.09.050

a b s t r a c t

Acetylcholinesterase is a member of the a/b hydrolase protein super family, with a significant role inacetylcholine-mediated neurotransmission. Research in the modulators of AChEs has moved from apotent poison (Sarin, Soman) in war times to the potent medicine (physostigmine) in peaceful times.Natural anti-AChE includes carbamates, glycoalkaloids, anatoxins derived from green algae; syntheticanti-AChE includes highly poisonous organophosphates used as nerve gases and insecticides. Recently,the role of anti-AChE was reassessed from neurotoxins to neuron-protective in the diseases characterizedby impaired acetylcholine-mediated neurotransmission like Alzheimer’s disease (AD). So, the AChE hasbeen proven to be the most viable therapeutic target for the symptomatic treatment of AD. This reviewarticle gives a spectrum of strategies to design AChE inhibitors used in the Alzheimer therapy.

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1. Introduction

Research is motivated and fundedmostly for the short term goalswhich vary with time. Acetylcholinesterase (AChE) inhibitor physo-stigmine was used in glaucoma in 1876 by physician Dr LudwigLaqueur, unknowingly about themechanismof action of the drug [1].Later on AChE inhibitors were utilized as pesticide and most of theresearch was dedicated to its selective pesticidal action [2]. With theeruption of war the selectivity of these agents was rather misused todevelop Sarin as the first nerve gas and later on during thewarwholeresearch funding was concentrated on the nerve gases and findingtheir antidotes. Hence, AChE inhibitors mainly phosphates as nervegas and oximes as antidotes came into the existence [3e5]. The postwar era saw the scarcity of food and hence the fundingwent into cropgrowth and crop protection (pest management). The average popu-lation age was less and growth was the need of the hour, hence thisera saw the development of pesticides for crops. Further, this era sawthe development of biological drugs (immunization) for longevity asmost of deaths accounted due to infections (bacterial, malaria etc.)and many synthetic agents came into existence [6]. The majorbreakthrough in the research came with the advent of recombinantDNA technology and view of looking at a disease changed to a mo-lecular level [7]. With an average population growing older the

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prevalence of disorders of old age increased and hence in the presentera the major funding is targeted at older age disorders and henceAlzheimer research came to front-stage. With concentration on Alz-heimer, the history was revisited by Dr. Graeber in 1997 to study andcharacterize the disease [8]. AlongwithAChE inhibitors, several othertherapies are also used for the management of Alzheimer’s diseaseincluding Tau-based therapies, dealing with oxidative stress, target-ing cellular Ca2þ handling, anti-inflammatory therapy, amyloid tar-geted strategies (b-Secretase inhibitors and g-Secretasemodulators).Among these therapies, inhibitionofb-Secretase causes the reductionofAb level alongwithblockageof all harmfuldownstreamsteps in thepathogenesis of AD whereas g-Secretase modulators (GSMs) havebeen shown to selectively lower Ab42 production without affectingtotal Ab levels [9,10]. Although targeting amyloid seems to be favor-able strategy but no BACE inhibitors orGSMshave reachedmarket tilldate. With the approval of an acetylcholinesterase inhibitor i.e.Tacrine as an agent for Alzheimer by US-FDA, major funding andresearch was concentrated in this thrust area [11]. This review high-lights journey through time in the research of therapies targeted to-wards Alzheimer with special emphasis on AChE inhibitors.

2. Alzheimer’s disease

Dementia is a loss of brain function that occurs with certaindiseases. Dementia usually first appears as forgetfulness. Dementiasymptoms include difficulty with many areas of mental function

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Fig. 1. Schematic representation of journey of Alzheimer’s disease.

Fig. 2. Factors involved in Alzheimer’s disease progression.

M. Singh et al. / European Journal of Medicinal Chemistry 70 (2013) 165e188166

including language, memory dysfunction, perception, emotionalbehavior or personality and Cognitive skills (such as calculation,abstract thinking, or judgment). Most types of dementia arenonreversible (degenerative). Nonreversible means the changes inthe brain that are causing the dementia cannot be stopped orturned back [12,13].

Alzheimer’s disease (AD) is the major cause of dementia, and is amultifaceted neurodegenerative disorder characterized at a mo-lecular level by protein misfolding and aggregation, oxidativestress, mitochondrial abnormalities, and neuroinflammatory pro-cesses [14,15]. It is characterized by a gradual onset and progressionof deficits in more than one area of cognition, including episodicmemory, mood and behavior changes, language, praxis and atten-tion, and the most common early symptom is difficulty inremembering newly learned information [16,17]. Brain areainvolved is the basal forebrain, cortex and amygdala, which are theareas involved in learning, memory, attention and emotionalregulation. There are two forms of AD: Sporadic AD and Familial AD(FAD), Sporadic AD is characterized by a severe progressive declinein cognition and increased neuronal cell death, and Familial AD(FAD) develops much faster and is caused by mutations in com-ponents of the amyloid pathway such as Amyloid Precursor Protein(APP), apolipoprotein E4 (ApoE4), presenilin-1 and presenilin-2(PS1 and PS2) and sortilin-related receptor 1 (SORL1) [18,19].

3. History of AD

AD is named after a German physician, Alois Alzheimer, whofirst described it in the early 20th century on November 4, 1906 inTubingen (Wilkins, 1969). In 1901, Alois Alzheimer, a doctor at thestate asylum in Frankfurt, studied a patient Auguste D, 51-year-oldwomanwith symptoms of cognition and language deficits, auditoryhallucinations, delusions, paranoia and aggressive behavior. Afterthe death of the patient in 1906, Alois Alzheimer working with EmilKraepelin carried out the post-mortem of the brain and came toknow that her brain exhibited arteriosclerotic changes, senile pla-ques, and neurofibrillary tangles and he subsequently publishedthe observations in 1907 [20]. In 1910, Kraepelin coined the term‘Alzheimer’s disease’ e a term still used to refer to the most com-mon cause of senile dementia.

In 1950s, increasing interest in theories and general ideas alongwith the development of molecular biology lead to the formation ofgenetic code concept [21]. In 1959, it was widely accepted that AD

was exclusively a rare pre-senile disorder [22]. In 1963, Terrystudied histologic pattern of the tangles and plaque [23]. Plaqueswere interpreted to be: amyloid fibrillar core, surrounded by un-myelinated dystrophic axons and dendrites containing filaments,dense bodies and paired helical filaments (PHF) [24]. In 1976, DrRobert Katzman reviewed the frequency and mortality of AD andhighlighted the need for focused research in this area [25]. As withincreased population age the prevalence of AD increased and wasbound to increase even further. Epidemiologists, clinical neurolo-gists, radiologists, psychiatrists, and psychologists quickly becameactive in developing better diagnostic methods for AD [26].

The prolonged history of scientific efforts to characterize betterthe clinical features of dementia perhaps can be described in thecontext of six arbitrarily defined epochs (Fig. 1). Factors affectingdisease progression are summarized in Fig. 2 [27e29].

4. Pathogenesis of AD

The exact cause of the Alzheimer’s disease is still uncertain, butin general the following hypothesis has been put forward on thebasis of the various causative factors (Fig. 3).

4.1. Cholinergic hypothesis

Degeneration of neurons has been associated with loss ofmemory function. The discovery of a link between the clinicalsymptoms of the disease (memory loss) and specific cholinergicdeficits in the brains of peoplewith AD, by Peter Davies in 1976, wasa landmark because it opened the door for modern neurochemistry[30]. In this hypothesis deficiency of a critical neurotransmitter,acetylcholine, in brain was observed either due to decreased pro-duction of neurotransmitter or amplified acetylcholinesterase ac-tivity [31]. This decreased level of the neurotransmitter causesimpairment of the cholinergic neurotransmission leading to theloss of intellectual abilities. This hypothesis generally implies thatthe cholinergic augmentation will improve the cognition in AD.

4.2. Amyloid hypothesis

Histological studies of the brain of the personwith AD indicatedthe presence of plaque, which lead to exclusive study of these ob-jects. In 1984, building block of amyloidogenic peptidewas found to

Fig. 3. Schematic representation of AD pathogenesis in light of cholinergic, amyloid and tau hypotheses.

M. Singh et al. / European Journal of Medicinal Chemistry 70 (2013) 165e188 167

be amyloid beta protein that forms the amyloid fibrils in theneuritic plaques. The amyloid cascade hypothesis considers that ADis caused by the abnormal processing of b-amyloid [32,33].

In the amyloid hypothesis, a misfolded form of amyloid beta, anoligomeric species, mainly toroidal or star-shaped deposited in thebrain may encourage apoptosis by physically piercing the cellmembrane. Plaque amyloid depositions or partially aggregatedsoluble b-amyloid then start the neurotoxic cascade and causesneurodegeneration that leads to AD [33,34]. Oxidative imbalance,oxidative stress and functional changes in the production of b-amyloid are the early steps of this disease [23]. The abnormalmetabolism of b-amyloids forms neurotoxic species due to mu-tations in some of the components of the amyloid pathway, suchas APP, ApoE4, presenilin-1 and presenilin-2 (PS1 and PS2), andSORL1 are responsible for autosomal-dominant early onset fa-milial Alzheimer’s disease [35,36]. Therefore, inhibitors of b-am-yloid aggregation appear as interesting candidates to treat AD inits earlier phases [37,38]. In 1990s, the amyloid theory became apowerful driving force that has dominated the direction ofresearch.

4.3. Tau hypothesis

In 1985, J.P. Brion and André Delacourte were the first tosuggest that tau might be the main component of neurofibrillarytangles [39]. Soon after, in 1988, Michel Goedert and collabora-tors cloned the cDNA of PHF-tau. Tau proteins, abundantly pre-sent in neurons in the central nervous system, stabilize themicrotubules. In this process, hyperphosphorylated tau (thealtered protein) begins to couple with other threads of tau.Eventually, they form neurofibrillary tangles inside nerve cellbodies [40]. The formation of neurofibrillary tangles results indisintegration of microtubules, collapsing the neuron’s transportsystem [41]. This may lead to malfunctions in biochemicalcommunication between neurons and later results in the deathof the cells [42]. This is one of the expected reasons for thedeposition of the plaques in the brain.

4.4. Calcium hypothesis

The calcium hypothesis of brain aging and dementia began totake shape in 1982. The original highly exploratory hypothesiscame into being in 1984 with little data or circumstantial evidenceto support. The role of activation of the amyloidogenic pathway inremodeling the neuronal Ca2þ signaling pathways responsible forcognitionwas explored by the calcium hypothesis of AD. Hydrolysisof the APP yields two products that can influence Ca2þ signaling.Firstly, the amyloids released to the outside form oligomers thatenhance the entry of Ca2þ that is pumped into the endoplasmicreticulum (ER) further enhancing the sensitivity of the ryanodinereceptors (RYRs) to increase the amount of Ca2þ being releasedfrom the internal stores. Secondly, the APP intracellular domainmay alter the expression of key signaling components such as theRYR. This remodeling of Ca2þ signaling is proposed to result in thelearning and memory deficits that occur early during the onset ofAD. The fact that Ca2þ can either increase or decrease the strengthof central glutamatergic synapses complicates learning mecha-nisms coordinated by neuronal calcium signaling systems. There isa bidirectional relationship between Ca2þ signaling and the amy-loidogenic pathway [43,44]. An increase in Ca2þ can stimulate themetabolism of amyloid [45,46]. The amyloid metabolism results inan upregulation of Ca2þ signaling by enhancing both the entry ofexternal Ca2þ and release of Ca2þ from the internal stores. Thisupregulation of Ca2þ may account for both progressive decline inmemory and increase in neuronal cell apoptosis that occurs duringAD. As a result, the change in Ca2þ signaling in AD may switch thebrain from a system of memory storage to one of memory loss(Fig. 4).

4.5. Isoprenoid change

The isoprenoid changes in Alzheimer’s disease differ from thoseoccurring during normal aging. During normal aging the humanbrain shows a progressive increase in the level of dolichol andreduction in level of ubiquinone but there is no change in

Fig. 4. Schematic representation of calcium hypothesis of AD.


concentration of cholesterol and dolichyl sulfate. In Alzheimer’sdisease, the situation is reversed with decreased levels of dolicholand increased levels of ubiquinone. The concentrations of dolichylphosphate are also increased, while cholesterol remains un-changed. The increase in the sugar carrier dolichyl phosphate mayreflect an increased rate of glycosylation in the diseased brain andthe increase in the endogenous antioxidant ubiquinone an attemptto protect the brain from oxidative stress for instance induced bylipid per oxidation [47].

5. Most promising target for the treatment of AD:acetylcholinesterase

Acetylcholinesterase (EC 3.1.1.7; AChE) belongs to the a/b hy-drolase fold protein super family; a group defined by commonstructural homology and includes the cholinesterases, carbox-ylesterases and lipases. Its principal physiological function is therapid hydrolysis of acetylcholine in the synapse and neuromuscularjunction, resulting in the termination of the nerve impulse [48]. Thethree-dimensional structure of AChE has been first determined onTorpedo californica (Tc) in 1991 via detailed analysis of the various

Fig. 5. Diagrammatic representation of active site of cholinesterase.

structural elements through a combination of structural studiesand site-directed mutagenesis [49,50]. Target enzyme AChE con-sists of a narrow gorge with two separate ligand binding sites. Inmedicine, AChE inhibitors are used mainly in the treatment ofAlzheimer’s disease (AD), glaucoma, neuromuscular blockade insurgical anesthesia and myasthenia gravis [51]. In vertebrates, twotypes of cholinesterase enzymes are present, Acetylcholinesterase(AChE) and Butyrylcholinesterase (BuChE), both efficiently cata-lyzes the acetylcholine hydrolysis. AChE is closely related tobutyrylcholinesterase (EC 3.1.1.8; BuChE). The two enzymes aredistinguished on the basis of substrate specificities, tissue distri-bution and sensitivity to inhibitors [52,53].

5.1. Structure

AChE comprises mainly of two sites: catalytic triad and pe-ripheral anionic site.

5.1.1. Catalytic triadThe AChE contains a catalytic triad (Ser200, His440 and Glu327)

located at the bottom of a deep and narrow gorge (about 20�A longand as narrow as 4.5 �A), lined with 14 aromatic residues (e.g.decamethonium) [54]. The active site also contains a subside (the“anionic subside”), including Trp84 as a key residue for the inter-action with the quaternary ammonium group of the substrateacetylcholine and other ligands via cation-p interaction, locatednear the bottom of the cavity. Another conserved aromatic residue,Phe330, is also involved in the interaction [55]. The cDNA sequenceindicates that there is a continuous stretch of 13 amino acids thatlikely comprise a leader sequence that is missing in the nativeenzyme. This peptide is rich in hydrophobic amino acids, which isconsistent with a putative membrane-spanning function. Thenative enzyme contains a single polypeptide of 575 amino acids.The AChE active site consists of three major domains: (1) anesteratic locus (e.g. nerve agents), comprised of the active siteserine [56,57] (2) an anionic locus (e.g. tacrine) that is 14.7 �A fromthe esteratic serine [58e60] and (3) a hydrophobic region that iscontiguous with or near the esteratic and anionic loci and that isimportant in binding aryl substrates and active site ligands.A fourth domain in the enzyme binds cationic ligands, such asgallamine, D-tubocurarine, and decamethonium. With the help ofsuch developments in the field of structural determination ofacetylcholinesterase and the features of the catalytic triads leads todesign and development of novel AChE inhibitors as potentialtherapeutic agents for the treatment of AD [61] (Fig. 5).

5.1.2. Peripheral anionic site (PAS)The peripheral anionic site also known as b-anionic site (e.g.

aflatoxins, donepezil, huperzine) of AChE is not a well-defined areathat is located at the entrance of the catalytic gorge and isapproximately 14 �A distant from the active centre [62e65]. Ligandoccupation of the peripheral anionic site frequently changes theconformation of the active center [66,67] which contains Tyr 70,Asp 72, Tyr 121, Trp 279 and Tyr 334. Among these amino acidresidues Trp 279 is a key residue, responsible for the adhesionfunction of the enzyme AChE [48]. The aromatic site contains loopsand it has good conformational flexibility. The PAS binds to sub-strate transiently as the first step in the catalytic pathway,enhancing catalytic efficiency by trapping substrate on its way tothe active site. Amyloid b peptide interacts with the peripheralanionic site resulting in the formation of amyloid plaques andconsequent damage to the cholinergic neurons [68]. The peripheralanionic site is a target for a number of toxins and also promisingdrugs [69,70]. The design of new AChEIs able to interact simulta-neously with both the active and the peripheral site of the enzyme

Fig. 6. Structures and names of G-type and V-type agents (in order of discovery).


AChE (dual binding site AChEIs) constitutes the main goal pursuedwith the conjunctive approaches directed to the development ofnew anticholinesterase agents with expanded pharmacologicalprofile, as a consequence of their higher affinity for the enzyme andtheir interference in aggregation of b-amyloid through AChE pe-ripheral site blockade [71]. AChE’s significance is in its being tar-geted by a variety of anti-cholinesterases, ranging from snakevenoms to pesticides and the nerve gases [48].

5.2. Dual role of AChE in Alzheimer

The main stress of the cholinergic hypothesis is on theenhanced activity of the enzyme acetylcholinesterase. The studieshave suggested that AChE is responsible for several non-catalyticactions including the pro-aggregating activity of Ab. In individualshaving Alzheimer, the activity of the acetylcholinesterase in-creases and leads to the augmented breakdown of the neuro-transmitter acetylcholine and causes the decline in theacetylcholine level in the brain. Another relation between theenzyme and AD has been the partial involvement of the enzyme inthe formation of amyloid plaques and neurofibrillary tangles. Ithas been shown that AChE promotes the aggregation of b-amyloidpeptide fragments by forming a complex with the growing fibrils.These complexes have been shown to be more cytotoxic than b-amyloid fibrils alone [72,73]. A structural domain of AChE thatpromotes b-amyloid peptide fibril formation has been identifiedto be the peripheral anionic site of the enzyme [74,75]. The mol-ecules that interact either exclusively with PAS or with both cat-alytic and peripheral binding sites of AChE prevent the pro-aggregating activity of AChE toward Ab. Furthermore, studieshave also revealed that several AChE inhibitors not only facilitatecholinergic transmission, but also interfere with the synthesis,deposition and aggregation of toxic Ab. Thus, AChE inhibition hasbeen documented as a critical strategy for the effective manage-ment of AD. Accordingly, compounds showing dual binding withthe AChE, that is, with catalytic and peripheral sites represent newtherapeutic agents for treatment of AD.

AChE is a sensitive target for both natural and syntheticcholinergic toxins. Among the natural anti-AChEs are plant-derivedcarbamates and glycoalkaloid inhibitors [76]. A natural inhibitor ofAChE was also found in a mollusk [77]. Blue-green algae is

equipped with anatoxins, highly effective toxins that block theactive site [78,79]. Green mamba venom includes the neurotoxicpeptide fasciculin, which blocks the entrance to the active and theperipheral sites of AChE [80]. Use of anti-AChEs as defense andattack weapons in nature, therefore, preceded their use by humans.Synthetic anti-AChEs were first studied andmanufactured as highlypoisonous organophosphate and carbamate nerve gases and in-secticides. In the clinic, controlled use of AChE inhibitors has provedvaluable for the treatment of diseases that involve compromisedacetylcholine-mediated neurotransmission.

5.3. Journey of AChE inhibitors

5.3.1. Pre-war eraNerve agents (NAs) act by interacting with the enzyme acetyl-

cholinesterase via phosphorylation, leading to CVS, CNS, respira-tory failure and seizures [81]. Examples of NAs include tabun, Sarin,Soman, cyclosarin, and VX [82e84]. Peripheral manifestations ofnerve gas includes wheezing, cough, dyspnea, sweating, salivation,nausea, vomiting, diarrhea, hyperglycemia, metabolic acidosis,ketosis, atrioventricular blocks, hypotension and Central NervousSystem Manifestations includes headache, dizziness, impairedmemory, anxiety, tension, emotional instability, lethargy, ataxia,seizures, respiratory depression, severe muscle contractions fol-lowed by paralysis [85]. The mechanism of action, i.e. cholines-terase inhibition, was discovered during World War-II by German,English and US scientists, the datawas published only after theWar[86]. NAs inhibit AChE by making reversible complex which isreactivated by recommended antidotes including atropine,scopolamine, pralidoxime chloride, and anticonvulsant medica-tions [87]. Oximes, such as pralidoxime chloride, act as chemicalreactivators of inhibited AChE and must be administered to allpatients after NA exposure to overcome their effects. Oximes canreactivate bound AChE by removing the OPC from the OPCeAChEcomplex [88].

Historically, the synthesis of the first potentially lethal OPC,tetraethyl pyrophosphate, occurred in 1854 in the laboratory of DeClermont in France. In the 1930s, major research funding wasconcentrated on production and scaling of the nerve gases. Thehighly toxic OPCs Tabun, Sarin, and Soman were developed duringpesticide research by the Germans [89]. In 1934, a project on

Fig. 7. Structures and names of organophosphate-based pesticides.


synthetic insecticides was started at I.G. Farben Industrie (Ger-many) by Otto Bayer who assigned all further research to thechemist Gerhard Schrader. In 1936, the German chemist GerhardSchrader, reasonably working on insecticides, developed anextremely effective organophosphate insecticide called tabun [90].In 1937, a sample of tabun was sent to the Ministry of War andfurther research determined that tabun exerted its effect by inter-fering with nerve transmission. Thus, the G-series NAs were born.Schrader was given a secret lab to continue research and devel-opment. In 1938, Schrader synthesized Sarin. In 1940, secret con-struction began on multiple pilot plants for small-scale tabunproduction. In 1944, German scientist and Nobel Laureate (Chem-istry, 1938) Richard Kuhn discovered the nerve agent Soman[91,92]. NAs were first produced by German scientists duringWWIIand were known by German code names: GA (tabun), GB (Sarin),GD (Soman), GF (cyclosarin), and VX. In 1952, the compound VX(venom compound X) was formulated. VX (V for venomous) wasdiscovered by the British and produced by the Americans afterWorld War II (WWII) [93]. It is the most lethal of all the NAs.Structures and names of G-type and V-type agents has been shownin Fig. 6.

5.3.2. Post war eraAfter the war the twentieth century population saw the scarcity

of food and the funding went into crop growth and crop protection(pest management). The average population age was less andgrowth was the need of the hour, hence this era (the 1940s and the1950s) saw the development of pesticides for crops and was namedas the pesticide era [94].

To dispose of pests (i.e. mainly insects) by chemical means thepreparation of pesticides is entangled with that of the nerve agentssince they possess similar structural features and formulas. Themode of action is same as nerve agents, by inhibition of AChE, theenzyme responsible for breaking down acetylcholine (Ach) whichis a neurotransmitter found at neuromuscular junctions but are lesshazardous [95]. Organophosphates cause a delayed neuropathythat has been termed organophosphate-induced delayed neuro-toxicity (OPIDN). OPIDN is a progressive neurological conditioncharacterized by weakness, ataxia and subsequent paralysis of thelimbs [96,97]. Most AChE-inhibiting pesticides are divided into twodifferent categories, organophosphates and carbamates; they differin the reversibility of their effects. Carbamates like carbofurantemporarily inhibit AChE [98].

Fig. 8. Timeline history of acetylcholinesterase inhibitors.


Before the 1930s, pesticides took the form of chlorinated organiccompounds. As with related chemical warfare developments,chlorine-containing pesticide compounds eventually gave way tophosphorus compounds. The development of organophosphate-based pesticides (insecticides) after World War II (1940se1950s)synthesized second generation of compounds including chlorpyr-ifos, coumaphos, cyanophos, demeton, demeton-S-methyl, diaz-inon, dichlorvos, dioxathion, glyphosate, fonofos, malaoxon,malathion, methamidaphos, mevinphos, oxydemeton-methyl,paraoxon, and parathion. Nerve agents differ from pesticides interms of much greater potency of the nerve agents whereas pes-ticides possess longer duration of the biological effects [99].Structures and names of selected relevant organophosphate-basedpesticides are shown in Fig. 7 [100].

The growth in synthetic pesticides accelerated in the 1940s withthe discovery of the effects of DDT, BHC, aldrin, dieldrin, endrin,chlordane, parathion, captan. In 1946 resistance to DDT by houseflies was reported [101]. Organophosphorous compounds haveimportance to those concerned with military as well as with agri-cultural matters. These are too toxic that they kill the mammalsmuch more readily than they would kill insects. Common symp-toms associated with Carbamate and Organophosphate Pesticideare poisoning fatigue, headache, dizziness, blurred vision, excessivesweating/salivation, nausea/vomiting, stomach, cramps, and diar-rhea, inability to walk, weakness, chest discomfort, constriction ofpupils, unconsciousness, muscle twitching, running nose, drooling,breathing difficulty, coma and death [99].

Under pressure of World War II, the pharmaceutical manufac-turers rapidly adaptedmass productionmethods for the antibiotics.The second quarter of the 20th century marked the flowering of theantibiotic era: a new and dramatic departure in the production ofdisease-fighting drugs [102]. Fleming’s discovery of penicillin in1929 went undeveloped and Florey and Chain studied it in 1940.Antibiotic discoveries came rapidly in the 40’s. In the Pre-AntibioticEra prior to the 1940’s millions of people died from common

bacterial infections. In the last 50 years all the bacterial infectionscan almost always be cured by using antibiotics [7].

When, in 1894, Behring and Roux announced the effectivenessof diphtheria antitoxin, pharmaceutical scientists both in Europeand in the United States rushed to put the new discovery intoproduction [103]. Parke, Davis & Company was among the pioneers.The serum became available in 1895, and lives of thousands ofchildren were saved. Inoculation of horses with diphtheria toxinwas the first step of many in producing antitoxin. In 1903, Parke-Davis received U.S. Biological License No. 1. New improved biolog-ical products have continued to become available, climaxed in 1955by poliomyelitis vaccine [104].

Pharmacological inhibitors of AChE are important in controllingdiseases that involve impaired acetylcholine-mediated neuro-transmission. Use of anti-AChEs as defense and attack weapons innature, therefore, preceded their use by humans. Synthetic anti-AChEs were first studied and manufactured as highly poisonousorganophosphate and carbamate nerve gases and insecticides. Inthe clinic, controlled use of AChE inhibitors has proved valuable forthe treatment of diseases that involve compromised acetylcholine-mediated neurotransmission. For example, Alzheimer’s diseaseinvolves selective loss of cholinergic neurons in the brain [105]. Inmyasthenia gravis, auto-antibodies reduce the number of nicotinicacetylcholine receptors at the neuromuscular junction [106]. AChEinhibition increases the synaptic concentration of acetylcholine andallows a higher occupancy rate and longer duration at its receptor[107]. Nevertheless, anti-AChE therapeutics do not address theetiology of the diseases for which they are used.

The twentieth century was marked by an incredible rise in lifeexpectancy and an equally impressive decline in infant mortality inthe developed world. Prosperous people live longer and old agecarries a high risk of dementia, a condition that is so far neitherpreventable nor curable. Alois Alzheimer described the Alzheimerdisease in 1907, but it was not until 60e70 years later that any newsignificant developments were reported on the pathology of this

Fig. 9. Natural flavones used in AD.

Table 1Structures of acetylcholinesterase inhibitors.

tbl1Compound no. Structure

1

2

3

4

5

6

7

8


disease. The number of laboratories involved and the pace ofresearch on AD remained quite slow till the 1980s. Drug discoveryis a time process involving target site identification, followed byvalidation of target, drug’s ability, designing a molecule, testing in-vitro, in-vivo, toxicity and safety analysis. All this process is alaborious process. In the year 1976, Peter Davies proved that themajor cause of Alzheimer was the loss of Acetylcholine neuroncausing deficiency of ACH in the brain. The link between decreasinglevels of acetylcholine and Alzheimer’s disease has been estab-lished due to the purpose of acetylcholine with regards to memoryand Alzheimer’s being a disease related to loss of memory. Centrallyacting agents were sought after for the treatment to increase thelevel of Ach in the brain. Hence, AChE became a target for AD.Pioneering research in the field of Nerve Agents and Pesticides hadalready been carried out at mass level with maximum fundingconcentrated on research in inhibition of AChE. This backgroundresearch work made it possible to get the first drug for ADbelonging to this class and four approved drugs belong to this classas well (Fig. 8).

In the mid-1980s, office of Alzheimer’s disease, established by,the Director of NIA, T. Franklin Williams, with the support of NIH,Director James Wyngaarden joined NIA in AD research. In 1984,development of national, interdisciplinary research program spe-cifically focused on the causes and the leaders at the NIA decide thecourse of AD and the differences between AD and normal aging. In1986, Summers published the first results obtained with tacrine (1)in the New England Journal of Medicine [108]. It was the first-approved drug among the first generation cholinesterase in-hibitors. New, well-tolerated and more efficient second-generationanticholinesterase inhibitors, such as donepezil (1996), and riva-stigmine (1998), subsequently made their appearance; later in2000, galantamine was brought out. The use of memantine, anNMDA receptor antagonist, to treat patients in moderate to severestages of the disease was approved in 2002 [109]. Various naturalderivatives such as flavones including Apigenin, Oroxylin A,Luteolin, have marked effect on central nervous system to improvememory and cognition, hence beneficial in the treatment of AD[110] (Fig. 9).

9
5.4. Classification of acetylcholinesterase inhibitors
Various natural, semisynthetic, and synthetic derivatives for ADtreatment have been synthesized and can be broadly classifiedbased on chemical structures. The majority of drugs for AD

Table 1 (continued )

Compound no. Structure

10

11

12

13

14

15

16



17

18

19

20

21

22

23

24

25

26

(continued on next page)




27

28

29

30

31

32

33

34

35

36



37

38

39

40

41a

41b

42

43

44

45 N

NCH3

MeOOC

OCH3

OH




46

47

48

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treatment focuses on the cholinergic hypothesis. Various studieshave been conducted on the designing of new highly efficient andactive derivatives of already existing FDA approved drugs.

5.4.1. Tetrahydroacridine derivatives (tacrine (1,2,3,4-tetrahydroacridine)) analogs

Tacrine (1, Table 1) was the first drug approved for treatment ofAD in 1993 [111]. It is a potent inhibitor of both AChE and BuChE. Itsstructure has been modified on the basis of importance of 4-aminopyidine motif in the tetrahydroacridine system.

4-Aminopyridyl moiety in the tetrahydroacridine group wasfound to be important for the potent activity. The modifications,categorized into heterocyclic ring modification and 4-amino sub-stitution, can be targeted keeping 4-aminopyridyl moietyconserved.

5.4.1.1. Five membered heterocyclic ring derivatives. It includes themodification of either ring A or ring C, whereas ring B remainsintact.

5.4.1.1.1. Substituted pyrazole derivatives. These derivatives in-cludes the replacement of the ring A of tacrine nucleus with thesubstituted pyrazole ring, structurally planned on basis of thebioisosteric relationship between the quinoline ring, included inthe 1,2,3,4 tetrahydroacridine system of THA and the azaheter-ocyclic pyrazolo [3,4-b]pyridine system [112].Compound 2, bearinga phenyl group in position 3, showed the best activity (Table 1).

5.4.1.1.2. Furan derivatives. These derivatives include thereplacement of the ring A of doctrine by substituted furan ringsystems. Newly derivedmolecules showed the acetylcholinesteraseinhibitory potential with the high degree of selectivity toward theacetylcholinesterase (Compounds 3 and 4, Table 1) [113].

5.4.1.1.3. Thieno derivative. These derivatives involve therepalcement of ring A with the substituted thiophene ring, butthese compounds were found to possess little inhibitory activityagainst the acetylcholinesterase [114].

5.4.1.2. Six membered substituted heterocyclic ring derivatives.The ring A can also be replaced with the substituted six memberedheterocyclic rings.

5.4.1.2.1. Pyridine substituted derivatives. These series of com-pound comprises of modification in both A and C ring, involves thereplacement of the ring A of tacrine with the substituted sixmembered heterocyclic ring i.e. substituted pyridine, along with achange in the size of the C ring of tacrine. Out of the number ofderivatives synthesized, the compound 5 and 6 (n ¼ 1, 2) (Table 1)possess highest acetylcholinesterase inhibitory potential [115,116].

5.4.1.2.2. Substituted pyran derivatives. These derivatives resultfrom replacement of the ring A of tacrine with the substitutedpyran, a heterocyclic aromatic ring system [116].Out of the varioussynthesized substituted pyran derivatives, derivatives 7 and 8(Table 1) have shown the highest acetylcholinesterase inhibitorypotential with IC50 values of 8.68 � 10�7 M and 1.82 � 10�7 Mrespectively.

5.4.1.3. Fused heterocyclic rings analogs. These derivatives includethe replacement of ring A of tacrine with the fused heterocyclic


rings, one of the ring system used is pyrazolo [3,4-b]pyridine [113].The compound 9 have shown good inhibitory potential againstacetylcholinesterase with IC50 value of 6.4 mM (Table 1).

5.4.1.4. Amino substituted derivative5.4.1.4.1. Imidazole substituted derivatives. These derivatives

include substitution of substituted imidazole ring at the aminogroup of the 4-aminopyridine ring of tacrine. This kind of modifi-cation possess additional abilities such as inhibition of histaminemetabolism, N-methyltransferase (NMT) and histamine H3 recep-tor antagonism which can improve reduced cognitive functionsalong with acetylcholinesterase inhibition [117,118].One of thecompound (10) among these designed molecules possesses goodAChE inhibitory potential (Table 1).

5.4.1.4.2. Piperidine substituted derivatives. The amino group oftacrine has also been substituted with the piperidine, both the ar-omatic nucleus are separated by the alkyl or an oxyphenyl group inthe tether and led to the design of the most effective compound 11(Table 1) possessing good acetylcholinesterase inhibition [117,118].

5.4.1.4.3. 1,2-Dithiolane derivative. In these derivatives, aminogroup has been substituted with 1,2-dithiolane, five memberedheterocyclic ring. The most common derivative is the tacrine-lipoicacid dimer. Lipoic acid (universal antioxidant) has been combinedwith a tacrine moiety by a carbon chain of varied length (2e7methylene groups). The most potent derivative with three methy-lene group linker, compound 12, showed IC50 of 6.96 nM (Table 1).Along with acetylcholinesterase inhibition, it also shows an addi-tional antioxidant property [119].

5.4.1.4.4. Heteroarylpiperazine substitution. Besides acetylcho-linesterase inhibition, these moieties also exhibit antiemetic ac-tivity via producing 5-HT-3 agonism. The 5HT-3 organism can bepossibly due to the 2-piperazinylquinoline-4-carboxamideattached to the tacrine ring. Compound 13 (Table 1) has beenfound to possess the optimum activity with IC50 ¼ 4.1 nM againsthuman AChE [119].

5.4.1.4.5. Tacrine dimers. Homodimers of two tacrine moietiesconnected by oligomethylene chains of different lengths have alsobeen designed [120]. These compounds are more hydrophobic thanTHA because of the introduced alkylene chain that interactssimultaneously with the catalytic site and the peripheral anionicsite. Among them, a compound containing two tacrine subunitswhose amino groups are connected by an heptamethylene chain,14, which was designed taking into account the existence of twobinding sites for tacrine in AChE, is about 1000 times more potentthan tacrine, although its toxicity is not known yet (Table 1).

5.4.1.4.6. Melatoninetacrine hybrids. Amino group has also beensubstitutedwith themelatonin and the hybrid so formed have beenreported to possess AChE inhibitory potential, strong antioxidantactions and is able to directly scavenge a variety of reactive oxygenspecies [120].Compound 15 possess the optimum activity with IC50value of 2 nM (Table 1).

5.4.1.4.7. Tacrine-drugs dimers. The substitution at amino groupof tacrine with other drugs of AD like Donepezil (18) producesTacrine and Donepezil related Hybrid (Table 1). Out of the varioussynthesized hybrid compounds two compounds 16, 17 possess theoptimumAChE inhibitory potential [121]. All of the newcompoundsdemonstrated significant inhibition of beta-amyloid aggregationand were shown to be more potent than parent compounds.

5.4.2. N-Benzylpiperidine derivativesDonepezil (18) was approved in 1996 for the treatment of mild-

to-moderate AD. It has been designed as AChE bivalent inhibitor.The basic structure of donepezil has been modified and the effecton the inhibitory potential of the compounds was observed. Ring ofdonepezil has been kept intact as such while indanone ring of the

donepezil has been substituted with the other heterocyclic ringsystems [122].

5.4.2.1. Benzisoxazole substituted derivatives. These derivativesworked on the replacement of indanone ring of Donepezil withbenzisoxazole ring system. Among the synthesized N-benzylpi-peridine-benzisoxazole derivatives, compound 19 (Table 1)exhibited the potent AChE inhibitory activity with IC50 value of0.8e14 nM [123,124].

5.4.2.2. 1,2,4-Thiadiazolidinone substituted derivatives. These de-rivatives involves replacement of indanone system of donepezilwith 1,2,4-thiadiazolidinone ring [124].The compound 20 pos-sesses activity profile comparable with tacrine instead of donepezil(Table 1). It also displayed more selectivity toward the AChE(IC50 ¼ 0.14 mM).

5.4.2.3. Benzylpiperidinone substituted derivatives. These de-rivatives involve the replacement of indanone system of donepezilwith substituted indole or pyrrole ring [125]. The compound 21(Table 1) possess the AChE inhibitory potential (IC50 ¼ 8 mM).

5.4.2.4. Aroyl(thio)urea substitution. These compounds involve thereplacement of indanone ring with aroyl (Thio) urea system whilethe N-benzylpiperidine ring systemwas kept intact. The compound22 (Table 1) exhibited an in vitro nanomolar AChE inhibitory activityand in vivo it reversed scopolamine-induced amnesia in the passiveavoidance paradigm in ratswith dose 1000-fold lower than its LD50.

5.4.2.5. Other N-benzylpiperidine derivatives. TAK-147 (23) andTAK-802 (24) have been developed by the Takeda Chemical In-dustries (Table 1). Preliminary SAR studies concluded that theintroduction of an additional ring in the structure of TAK-147 (23)caused an increase in the biological activity whereas furtherintroduction of the hydroxy and fluoro groups results in decreasedactivity [126].

5.4.3. Benzophenone derivativesThese compounds encompass an aromatic function and a ter-

tiary amino moiety connected by a suitable spacer. In particular, thebenzophenone nucleus as aromatic function and the N,N-benzylmethylamine as tertiary amino function has been selected. Com-pound 25 and 26 possess optimum AChE inhibitory potential(Table 1) [127].

5.4.4. Di-benzofuran analogsRivastigmine is a small molecule that easily crosses the blood

brain barrier and possesses BuChE and AChE inhibitory properties.It (27) was approved in 2000 for the treatment of mild-to-moderateAD (Table 1). In 2007, it was reformulated for delivery through atransdermal patch. This has resulted in significantly lower GI sideeffects compared to the oral capsule.

Various derivatives of rivastigmine have been synthesized butthe compound 28, i.e. the 5-thia-1-azacyclopenta[a]naphthalenederivative, exhibited the highest activity (Table 1). Replacement ofthe methyl chain with an ethyl chain results in 14e23 folddecreased AChE inhibitory activity. The alkyl substituent on thecarbamoyl nitrogen determined the profile of activity. Replacementof the methyl group in the carbamoyl moiety by a 1-phenylethylsubstituent caused significant decrease in inhibitory activity to-wards both AChE and BuChE [128].

5.4.5. Galantamine analogsGalantamine was approved for the treatment of mild-to-

moderate AD in February 2001. Galantamine (29), an alkaloid


isolated from the Caucasian snow-drop (Galanthus woronowii) andfrom the bulbs of different species of the Amaryllidaceae family, is acentrally acting, selective, reversible, and competitive acetylcho-linesterase (AChE) inhibitor, as well as an allosteric modulator ofthe neuronal nicotinic receptor for acetylcholine [129e131]. Gal-antamine has also been shown to enhance g-aminobutyric acid(GABA) and glutamate release in hippocampus slices (Table 1)[132].

5.4.5.1. Open D-ring galantamine analogs. New optically pure openD-ring galantamine analogs have been synthesized [133] and themost active inhibitor was found to be compound 30 (Table 1).

5.4.5.2. N-Hexyl-benzyl piperidine substituted derivatives.Structure activity relationship (SAR) studies reveal that substitutionon the nitrogen atom of galantamine has been favorable for AChEinhibitory activity, may be the substituent display interaction withthe peripheral anionic site (PAS). N-substituted galantamine de-rivatives designed by selecting benzyl amino groups and modifiedbenzyl amino moieties (such as amide group) as pharmacophoricunits and incorporating them into the galantamine molecule forbetter AChE inhibitory activity. Besides, different lengths of thealkyl chain between galantamine and benzyl amino moieties hasbeen explored, the compound 31 (Table 1) was observed to be themost active [134].

5.5. Natural compounds exhibiting acetylcholinesterase inhibitorypotential

5.5.1. Alkaloids and related compounds5.5.1.1. Physostigmine and its analogs. Physostigmine, an alkaloidisolated from Physostigma venenosum, possesses AChE inhibitoryactivity. Physostigmine may penetrate the CNS, so the developmentof physostigmine derivatives has been undertaken. This resulted inobtaining additional lipophilic analogs of physostigmine such asphenserine and geneserine [135e137]. The first medical treatmentof glaucoma was introduced by Ludwig Laqueur, Professor ofOphthalmology at Strassburg, who found that physostigmine(eserine) would lower tension in glaucomatous eyes. He began hisstudies on the drug in 1875. In 1876, he personally learned aboutthe hypotensive effect of physostigmine [138]. It was the first AChEinhibitor investigated for the treatment of AD. It is isolated from theseeds of P. venenosum.

Some geneserine derivatives with a substituted phenyl ringattached to the carbamoyl nitrogen atom have been synthesized[139]. The most active compounds of the series were 32 and 33(Table 1).

Several potent cholinesterase inhibitors in the series of phen-serine and geneserine analogs have been identified. Phenserinederivatives containing one, two or three methyl groups at differentpositions of the phenyl ring have been synthesized. In the group ofmono-substituted derivatives, the compound with a 20-methylsubstituent (34) has been the most active and selective inhibitor ofAChE (Table 1). Compounds with a disubstituted phenyl ringexhibited similar activity with selectivity for AChE but tri-substituted derivatives were found to be inactive [140]. Phenser-ine has a dual effect: decreasing beta-amyloid precursor proteinand has a reversible AChE inhibition and has reached Phase IIIclinical trials (2003e2004).

Tolserine another analog of physostigmine differs from phen-serine at the 2-methyl substitution on its phenylcarbamoyl moiety.Preclinical studies were initiated in 2000, and it was shown to be200-fold more selective against hAChE versus BuChE [141]. Tol-serine proved to be a highly potent inhibitor of human AChEcompared to its structural analogs physostigmine and phenserine.

In the structure of physostimine, replacement of the nitrogens inthe pyrrolodine ring with oxygen led to new derivatives [142]. TheAChE inhibitory activity of enantiomers of tetrahydrofur-obenzofuran andmethanobenzodioxepines has also been reviewed[143]. It has been revealed that (3aS) tetrahydrofurobenzofurans(35) and (5S)-methanodioxepines (36) analogs were potent AChEinhibitors (Table 1).

5.5.1.2. Piperidine alkaloids. New piperidine alkaloids: (�)-3-O-acetyl-spectaline and (�)-spectaline have been obtained from theflowers of Senna spectabilis sin. Cassia spectabilis, Leguminoseae[144e147]. An acetylcholine fragment, which could be recognizedin the structure of these alkaloids, prompted to design and syn-thesize new semi-synthetic piperidine alkaloids with expectedAChE inhibitory activity. Compounds 37 and 38 have found to beeffective inhibitors of rat brain tissue AChE and weak inhibitors ofBuChE (Table 1). These compounds displayed selectivity towardsbrain AChE [144] and showed mean IC50 value of 7.32 and 15.1 mM,respectively.

5.5.1.2.1. Huperzine derivatives. Huperzine A and B are alkaloidsisolated from the Chinese herb Huperzia serrata, which is a memberof the Lycopodium species, used in traditional herbal medicine. Itinduces significant improvement of memory in aged subjects andpatients with Alzheimer’s disease. Both alkaloids are potentreversible inhibitors of AChE [148].

Huperzine A (39) has been found more potent than huperzine B(40) in AChE inhibition; however, HupB exhibited a higher thera-peutic index in comparisonwith HupA. HupA has been approved inChina as a drug for the treatment of AD (Table 1).

A series of tacrine-huperzine A hybrids also have been synthe-sized [149]. Huprines Z and Y, (41) with combined carbobicyclicsubstructure of (�)-huperzine A and the 4-aminoquinoline sub-structure of tacrine. Huprine Z and huprine Y found to be moreactive than both tacrine and (�)-huperzine A as inhibitors of bothhuman and bovine AChE; however they showed intermediateBuChE inhibitory activity (Table 1) [150,151].

The highest activity was exhibited by compound tacrine andhuprine Y joined with a tether with an N-methylamine groupinserted in the middle of the methylene linker i.e. compound 42(Table 1).

5.5.1.3. Indole alkaloids. The species Tabernaemontana australis(Müell. Arg) Miers (sin. Peschiera australis), which flourishes inSouth America, has been a source of iboga alkaloids that inhibitAChE. Four indole alkaloids: coronaridine (43), voacangine (44),voacangine hydroxyin-dolenine (45) and rupicoline (46), showedanticholinesteric activity at the same concentration as the referencecompounds physostigmine and galantamine (Table 1) [152].

5.5.1.4. Lycorine alkaloids. Lycorine (47) was the first Amaryllida-ceous alkaloid with a tetracyclic pyrrole [d,e] phenanthridine(galanthan) skeleton that demonstrated weak inhibitory activityagainst AChE (Table 1). Among more than 20 lycorine related al-kaloids assoanine (48) displayed the highest AChE activity (Table 1).

Secolycorines prepared from lycorine through chemical modi-fications also showed inhibitory activity against AChE. Compound49withN-methyl and 50withN-butyl substituentwere found to bemore potent than N-ethyl or N-propyl derivatives (Table 1) [153].

5.5.1.5. Isoquinoline alkaloids. Isoquinoline alkaloids represent asuccessful template for the identification of potent AChE inhibitors[154]. One type of structure has been represented by bisbenzyli-soquinoline (BBIQ) derivatives. Acetylcholinesterase inhibitorsamong new synthesized series of isoquinoline and dihy-droisoquinoline derivatives have been discovered. These


compounds have been designed as monomeric analogs of BBIQ. 1-Benzyl-3,4-dihydroisoquinoline and 1-benzyl-isoquinoline struc-ture displayed AChE inhibitory activity in the micromolar range[155]. The most active compounds of the series were 51 and 52(Table 1).

A number of bisbenzylisoquinoline (BBIQ) (53) alkaloids; suchas fangchinoline, atherospermoline, and fenfangjine E, isolatedfrom root of Stephania tetrandra S. Moore, Menispermaceae family,were found to inhibit acetylcholinesterase enzyme in the micro-molar range (Table 1) [156].

Stephaoxocanes belonging to a small family of isoquinolinealkaloids with a tetracyclic skeleton have been known in folkmedicine for a long time. Several tricyclic analogs of these ste-phaoxocanidine alkaloids also exhibited AChE inhibitory activity.Modifications of alkaloid structure included replacement of theoxocane ring with functionalized alkyl chains. One of the mostpotent compounds in this series was found to be the analog ofstephaoxocanidine (54) with a lactone ring i.e. (5,6-dimethoxy-7H-8-oxa-1-aza-phenalen-9-one), i.e. (55) which exhibited similaractivity to that of Narcissus extracts enriched in galantamine(Table 1) [157].

5.5.1.6. Xanthostigmine alkaloids and its derivatives5.5.1.6.1. 2-Arylidenebenzocycloalkanone derivative. The key

feature of these derivatives is a 2-arylidenebenzocycloalkanonemoiety that provides the ability to bind at the AChE peripheral siteresponsible for promoting the beta amyloid aggregation. 2-arylidenebenzocycloalkanone analogs prepared with the aim totarget simultaneously both the catalytic and the peripheral AChEbinding sites. The insertion of an alkoxy spacer chain of suitablelength (three or seven methylene units) and introduction of a ste-rically encumbering ð-electron-rich arylidene moiety into thearylidene aryl ring resulted in increased contact area with the PAS.The selected benzocycloalkanone moieties are benzofuran-3-one,3,4-dihydro-2H-naphthalen-1-one, chroman-4-one and indan-1-one [158]. The most active one of the series has been compound56 (Table 1).

5.5.1.6.2. 3-[X-(Benzylmethylamino) alkoxy] xanthen-9-ones an-alogs. These new derivatives devoid of the carbamoyl substituentin the phenyl ring have been synthesized. The effect of the linkerlength and the presence of different substituents in the phenyl ringwere both tested [159]. Best results were obtained for the com-pounds having three (57) and seven (58) methylene units respec-tively (Table 1).

5.5.1.7. Carbazole alkaloids. Carbazole alkaloids also possess theacetylcholinesterase inhibitory potential. Mahanimbine (59) analkaloid has been isolated from the Murraya koenigii leaves bysolvent extraction, via petroleum ether. Mahanimbine [3, 5-dimethyl-3-(4-methylpent-3-enyl)-11H-pyrano [5,6-a] carbazole]contain a carbazole nucleus which is responsible for its activity. Theauthors reported the acetylcholinesterase inhibitory activity ofcarbazole alkaloids by most widely used method i.e. Ellman’smethod [160].

The various structural feature of carvedilol (60) as inhibitors ofAlzheimer beta-amyloid fibril formation have also been identified.Based on their work they also studied the beta amyloid inhibitorypotential of various derivatives of carbazole like SB-211475 (61), SB-209995 (62), 9-acetylcarbazole (63) and hydroxy carbazole (64). Allof the compounds when tested for their beta amyloid fibril for-mation inhibitory potential have shown the good results [161](Table 1).

The berberine derivatives have been synthesized and evaluatedas the potent acetylcholinesterase inhibitors. A simple structureeactivity relationship analysis showed that the AChE inhibitory

potency has closely related to the length of the alkylene chain.Compounds with four methylene groups between the berberineand aromatic ring units, i.e. with n ¼ 4 being the best inhibitors inthe series. Various derivatives (65, (Table 1)) with good inhibitorypotential have been synthesized with n varying from 2 to 6 [162].

New class of multi target directed ligand for inhibition ofacetylcholinesterase and beta amyloids aggregation in AD has alsobeen synthesized. Since substituted carbazoles are efficient in-hibitors of beta amyloids fibril formation, the carbazole moiety ofcarvedilol was selected to design a new pharmacophore and havebeen joined with the tacrine derivative like 6-cholrotacrine andsynthesized the carbacrine. All the designed compounds werefound to be effective AChEIs in the nanomolar range and morepotent than tacrine and its 6-chloro derivative. In particular, car-bacrine was able to inhibit AChE activity in the nanomolar range,block in vitro beta amyloids self aggregation and aggregationmediated by AChE, antagonize NMDARs and reduce oxidativestress, and the most active compound of the carbacrine series wascompound (66) (Table 1) [163].

5.5.2. TerpenoidsNovel meroterpenoid AChE inhibitors have been isolated from

microbial metabolites. Solid state fermentation of Aspergillus ter-reus led to isolation of terreulactones AeD and another polarmetabolite isoterreulactone-A (67). Terreulactone A (68) and Ter-reulactone D (69) are meroterpenoid type compounds; with mixedpolyketide terpenoid structures that showed AChE inhibition(Table 1). Isoterreulactone A is also a meroterpenoid but contains aseven-membered lactone skeleton in its structure and it is 10 timesweaker AChEI than terreulactones [164,165].

These new inhibitors (70, (Table 1)) contain a modified het-erocycle which is supposed to interact with the catalytic site andnew substituents connected to the nitrogen atom N 20, which issupposed to be responsible for the binding to the peripheral site ofthe enzyme [166].

5.5.3. SteroidsNatural cholinesterase-inhibiting steroids isolated from Sarco-

cocca saligna have been reported. These pregnane-type compoundswith steroidal skeleton and monomethylamino or dimethylaminosubstituents, either at C-3 and/or at C-20 position of the basicsteroidal skeleton have been found to inhibit both AChE and BuChEin micromolar ranges. Among over twenty new steroids, the mostactive ones were salignenamide-E (71), salignenamide-F (72) andaxillaridine-A (73, (Table 1)). All investigated steroids containamino nitrogen atoms at positions C-3 and/or C-20. These play animportant role in the inhibitory activity of these compounds[167,168].

5.6. Miscellaneous AChE inhibitors

5.6.1. Aminobenzoic acid derivativesThe arylamides and arylimides structurally related with ACh,

derived from the p-aminobenzoic acid (PABA) have been designedand developed [169]. Various meta and para aminobenzoic acidderivatives act as the potential AChE inhibitors [170,171]. Amongthe tested PABA derivatives, the most active have been compound74 and 75 (Table 1) [170,172].

5.6.2. 1-[Bis(4-fluorophenyl)-methyl] piperazine derivativeSeveral derivatives of 1-[bis(4-fluorophenyl)-methyl] pipera-

zine with various heterocyclic rings have been synthesized thatexhibited AChE inhibitory activity [171].The highest potencyagainst AChE was exhibited by compound (76, (Table 1)) withpyrrolidine-substituted piperazine.


5.6.3. Obidoxime and structurally related oximesSeveral analogs of obidoxime, derivatives of pyridinium oxime

ether have been developed. It has been found that the monopyr-idinium compound and the bispyridinum compound displayedinhibitory activity towards AChE in the low micromolar range. Thegreatest inhibitory activity observed for compound 77 with monochloro and 78 with 2,6-dichloro substituents in the phenyl ring(Table 1). These dimeric compounds may interact with both activeand peripheral binding sites of the enzyme [173e175].

5.6.4. Thienoxazinones derivativesThere have been several 2-secondary-amino-substituted thie-

noxazinones derivatives compound 80, with inhibitory activityagainst AChE (Table 1). It contains a bulky benzyl residue attachedto the basic nitrogen atom. Moreover, 2-secondary-amino-substituted thienoxazinones led to discovery of newtetrahydropyrido-anellated thieno [1,3]-oxazinones as potent in-hibitors of AChE. Among the tricyclic 1,3-oxazin-4-ones, com-pounds 79, 80, 81 and 82 showed inhibitory activity in the submicromolar range (Table 1) [176].Thus, these compounds may bindto the active site gorge of AChE in a manner similar to donepezil.

5.6.5. Phenothiazine derivativesVarious derivatives of phenothiazine (83, (Table 1)) have been

synthesized. Some of the synthesized derivatives inhibited bothBuChE and AChE [177].

5.6.6. QuinazoliniminesThere have been potent inhibitors of cholinesterases in the se-

ries of Quinazolinimines [178].Among new synthesized molecules,compounds 84e87 showed moderate inhibition activity againstcholinesterase (Table 1). Novel tricyclic Quinazolinimines, tetracy-clic dibenzodiazocines and related analogs have also been synthe-sized and tested [179].

5.6.7. Bis-(�)-nor-meptazinol derivativeA bis-(�)-nor-meptazinol derivative (88), in which the two

meptazinol rings being linked by a nonamethylene spacer, a novelacetylcholinesterase inhibitor inhibits both catalytic activity andbeta amyloid peptide aggregation (Table 1) [180].

5.6.8. Cis-2,6-dimethyl piperidine sulphonamides derivativeDonepezil is a widely prescribed AChE inhibitor, which displays

a piperidine ring in its structure. These piperidine sulfonamideshave been subjected to in vitro AChE enzyme inhibition studies[181].The most active compound of series showing the AChEinhibitory potential were 89 and 90 (Table 1).

5.6.9. Carbamate derivative5.6.9.1. Substituted phenyl-N-butyl carbamates. These compounds(91, (Table 1)) have been found to possess potent, irreversible, pe-ripheral anionic site-directed inhibition of AChE [182]. The X can besubstituted with H to OMe, NO2.

5.6.9.2. Alkane-1-N-butylcarbamate-n-ols and 1,n-alkane-di-N-butylcarbamates. These compounds are identified as “pseudo-irreversible” (“pseudo-substrate”) inhibitors of AChE. The mostactive one has been the hexadecane derivative 92 with one car-bamoyl group (Table 1). The authors also suggested that com-pounds, which contained two carbamate moieties, interacted withboth peripheral and catalytic active sites [183].

5.6.9.3. Phenylcarbamates. Phenylcarbamates structurally relatedto Rivastigmine were evaluated, in vitro and in vivo for biologicalactivity. Among these compounds that showed the highest activity

is 1-[1-(3-dimethylcarbamoyloxyphenyl) ethyl] piperidine] (93,(Table 1)). Meta-substituted derivatives inhibited cholinesterasesmore strongly than ortho-substituted compounds [184].

5.6.10. 2-Phenoxy-indan-1-one derivativesThese derivatives comprise a new group of cholinesterase in-

hibitors with a dimeric structure i.e. two pharmacophoric moietiesi.e. 5, 6-dimethoxy-indan-1-one derived from Donepezil anddialkyl-benzylamine derived from Rivastigmine. These compoundsare able to interact with central and peripheral binding site of AChEand prevented catalytic and noncatalytic actions of the enzyme[185].The compounds 94 and 95 exhibited the highest activityamong all of the designed compounds (Table 1).

5.6.11. Nelumbo nuciferaN. nucifera is an aquatic plant with numerous medicinal prop-

erties. The primary effect of this plant is as an AChE inhibitor ratherthan as BACE1 inhibitors [186].There are no reports of humanstudies. Preclinical and clinical safety and toxicity data is notreported.

5.6.12. Himatanthus lancifoliusH. lancifolius is a shrub. Seidl et al. conducted a study to deter-

mine if there were any AChE inhibiting properties from the H.lancifolius extract (Seidl et al., 2010). The dichloromethane (DCM),and ethyl acetate (EtOAc), fractions showed significant AChEinhibitory effects and Uliene was the significant compound presentin both fractions [187].

5.6.13. GalanginGuo et al. studied 21 different flavonoids for potential AChE

inhibition properties in the brain in vitro. Flavonoids have been ofgreat interest in AD research and treatment because of their freeradical scavenging properties. A flavonol isolated from RhizomaAlpiniae officinarum called galangin demonstrated the highestinhibitory effects on AChE activity [188].Clinical and preclinicaltoxicities have not been established yet.

5.6.14. Cardanol derivativesDe Paula et al. designed new AChEI from nonisoprenoid

phenolic lipids (NIPLs) of Anacardium occidentale. The studyconcluded that the most promising candidates to the developmentof AchEI for AD treatment were derived from cardanol. Clinical andpreclinical toxicities have not been established yet [189].

5.6.15. MetrifonateMetrifonate is a long-acting irreversible ChEI that was originally

used to treat schistosomiasis. The clinical trials were discontinuedduring phase III. Metrifonate is not an option for AD treatment atthis time. Metrifonate is not an approved AD treatment but showedefficacy that was outweighed by safety risks [190].

5.6.16. Coumarin derivativesCoumarins are naturally occurring phytochemicals in many

plant species. Coumarins primarily interact with PAS of AChE, soscientists have put their efforts in synthesizing dual inhibitors ofAChE by incorporating a catalytic site interacting moiety withcoumarin through an appropriate spacer. Coumarin derivatives arerecently reviewed by Anand et al. [191].

5.6.17. Oxoisoaporphine-based inhibitorsA series of novel oxoisoaporphine-based inhibitors (10-

aminoalkylamino-1-azabenzanthrone AreNH(CH2)nNR1R2) ofacetylcholinesterase (AChE) has been designed, synthesized, andtested for their ability to inhibit AChE, butyrylcholinesterase (BChE)


and AChE-induced b-amyloid (Ab) aggregation. Molecular dockingsimulations on the oxoisoaporphine derivatives with AChE from T.californica have demonstrated that 1-azabenzanthrone moiety ofthe ligands could interact with peripheral anionic site (PAS) ofacetylcholinesterase, especially with Trp 279 of PAS. A series ofoxoisoaporphine derivatives 96 with different basic side chain(n ¼ 2 and 3) at 10-position of 1-azabenzanthrone were designedand synthesized, and their anti-AChE, BChE and AChE-inducedAb140 aggregation activities were tested [192].

The structure of terminal groups of side chain has effect on theirinhibitory activities. High inhibitory potency was found to beassociated with diethylamine at the end of side chain whereasdimethylamine, pyrrolidine and piperidine derivatives showed lesspotency. Compounds which possessed hydroxyl group at the end ofside chain exhibited much weaker anti-AChE potency, whichcaused approximately 16- to 340-fold decrease compared withdiethylamine derivatives in activity. Compound 102 (IC50 of0.72 � 0.03 uM) showed the highest inhibitory activity againstAChE and BChE (13.4 mM) (Table 1). The anti-Ab aggregating effectseems to be dependent on the length of the side chain. Since theinhibitory potency was increased by increasing the length betweenthe 1-azabenzanthrone and terminal nitrogen atoms.

5.6.18. Tricyclic analogs of stephaoxocanidineThe synthesis of simplified analogs of the novel isoquinoline

alkaloid stephaoxocanidine, carrying the oxazaphenalene ABC-ringsystem of the natural product, and their activity as inhibitors of theenzyme acetylcholinesterase, is reported [157]. 5,6-Dimethoxy-7H-8-oxa-1-aza-phenalen-9-one (97, (Table 1)) was as active as aNarcissus extract enriched in galantamine. The stephaoxocanes area small family of isoquinoline alkaloids recently uncovered byJapanese, Chinese and Brazilian scientists, which shared the tetra-cyclic skeleton 98 (Table 1). Till date, only five members are knowni.e. stephaoxocanidine (99) and stephaoxocanine (100, (Table 1))isolated from Stephania cepharantha Hayata, excentricine (101,(Table 1)) and N-methylexcentricine (102), from Stephaniaexcentrica and eletefine (103, (Table 1)) isolated from Cissampelospareira [193e195].

Interestingly, besides galantamine other alkaloids such asisoquinoline derivatives from Amarillidaceae as well as

Fig. 10. Tacrine-4-oxo-4H-chromene hybrids.

protoberberines, and quaternary benzophenanthridine and iso-quinoline alkaloids including sanguinarine and N-alkyl carnegi-nium salts, have been shown to display acetylcholinesteraseinhibitory activity [196e198]. It was observed that lactone exhibi-ted an IC50 of 19.6 mM, a remarkable activity.

Seven ABC-ring analogs of stephaoxocanidine have been syn-thesized and their activity as inhibitors of acetylcholinesterase wastested. Lactone 100 (Table 1), was found to be the most potentcompound of this series. Transformation of the lactone moietyfurnished less active compounds but did not abolish the acetyl-cholinesterase inhibiting activity. Unexpectedly, however, intro-duction of a functionalized side chain partially resembling ring D ofthe tetracyclic natural products, did not improve the activity.

5.6.19. Oxoisoaporphine and oxoaporphine derivativesOxoaporphine (104, (Table 1)) alkaloids were designed and

synthesized as acetylcholinesterase (AChE) and/or butyr-ylcholinesterase (BuChE) inhibitors [192]. The AChE inhibitory po-tential of synthetic oxoaporphine derivatives was decreased about2e3 orders of magnitude as compared with that of oxoisoapor-phine derivatives. The synthetic oxoisoaporphine derivativesexhibited high AChE inhibitory activity with IC50 values in thenanomolar range and high selectivity for AChE over BuChE (45- to1980-fold).

Newly synthesized oxoisoaporphine derivatives, 9-(3-piperidinopropionamido)-1-azabenzanthrone methiodide salt,prospectively showed themost powerful inhibitory potency towardAChE with IC50 value in sub-nanomolar level. The only differencebetween oxoaporphine (105, (Table 1)) and oxoisoaporphine (106,(Table 1)) alkaloids is the position of nitrogen atom in thepharmacophore.

5.6.19.1. Hybrids of oxoisoaporphine-tacrine congeners. The newhybrids consist of a unit of 1-azabenzanthrone and a tacrine (107,(Table 1)) or its congener, connected through an oligomethylenelinker containing an amine group at variable positions [199]. Thesehybrids exhibit high AChE inhibitory activity with IC50 values in thenanomolar range inmost cases. Those, bearing a tetrahydroacridinemoiety, exhibit a significant in vitro inhibitory activity toward theAChE-induced and self-induced Ab aggregation, whichmakes thempromising anti-Alzheimer drug candidates.

It was predicted that hybrids of oxoisoaporphine-tacrine con-geners in which the two pharmacophores were separated by alinker of a suitable length would have both greater inhibitory po-tency and selectivity than tacrine or oxoisoaporphine itself andshould be involved in neurotrophic activity. These compoundsconsist of a unit of tacrine or its congeners, which occupies thesame position as tacrine at the AChE active site, and the 1-azabenzanthrone moiety whose position along the enzyme gorgeand the peripheral site can be modulated by a suitable tether thatconnects tacrine and 1-azabenzanthrone. The proper tether lengthfor the linker between the two anchoring groups, 9-aminoacridineand 9-amino-1-azabenzanthrone, seemed to be six. Various fla-vones were used as adjuvant with some active moieties due to theirantioxidant activity. In that context, a new family of tacrine-4-oxo-4H-chromene hybrids has been designed, synthesized, and evalu-ated biologically for Alzheimer’s disease (AD) [200] (Fig. 10).

5.6.20. Recent synthetic analogsThe first AChEI developed was THA, but using the drug caused

dose-dependent but reversible liver toxicity. The 7-methoxytacrine(7-MEOTA) is an analog of THA that has far less toxicity and ispharmacologically equivalent to THA [201]. In a study by Kor-abecny, fourteen 7-MEOTA analogs were synthesized. Models ofhuman recombinant AChE (hAChE) and human plasmatic BuChE


(hBuChE) were used to evaluate these new analogs in vitro andwere compared to THA and 7-MEOTA. Ladostigil is a novel anti-Alzheimer’s disease drug with neuroprotective, multimodal brain-selective monoamine oxidase, and cholinesterase inhibitor prop-erties [202]. Ladostigil (108, (Table 1)) also prevented the age-related reduction in cortical AChE activity and the increase inbutyrylcholinesterase activity in the hippocampus presently in aPhase II clinical trial and intended for the treatment of Alzheimer’sdisease and dementia co morbid with extrapyramidal disordersand depression.

AP2238 (109, (Table 1)) is the first published compound to bindboth anionic sites of AChE. The potency against AChE is comparableto donepezil, while its ability to contrast b-amyloid aggregation ishigher. There are reports on a series of hybrids developed fromdonepezil and AP2238 in which the indanone core from donepezilis linked to the phenyl-N-methylbenzylaminomoiety fromAP2238.There are no reports of human studies. Preclinical and clinicalsafety and toxicity have not been established yet [203].

A great deal has been learned about the pathogenesis of neu-rodegeneration, after less than three decades. Novel interventionstrategies are being developed to improve the neuro-toxicitycaused by abnormal metabolic products and prevent processesthat lead to cell death. A large number of clinical trials are under-way, both industry and government sponsored studies with widelyused drugs (e.g. antioxidants, anti-inflammatory agents, statins andvitamins) that might reduce the risk of Alzheimer’s disease.Intensive studies are underway on multiple fronts, from basic sci-ence to genetics to drug therapy to care giving. Some of the keyfactors that influenced the pace of progress and helped to changethe ‘status’ of dementia research were: a) increases in researchfunding, b) recruitment of new scientific talent; convergence ofknow-how and technologies, c) several crucial discoveries in mo-lecular neurobiology.

6. Conclusion

Research is always driven by the funding, which in turn gener-ates facilities. Drug discovery is a tedious process and involves a lotof time as well as money. Research funding has always been toachieve a myopic goal. The path of research on AD started in 1906,but the research was hampered by the war times. In the post wartime, as the population grew older the research was reorientedtowards the study of Alzheimer. The path of research of AChE in-hibitors started in 1876, with use of these inhibitors in glaucoma.Then the toxicity of these compounds was misused for pest man-agement and pesticides came into existence. With the eruption ofwar, toxic compounds of this category became more toxic nerveagent. In this era all, the funding was going in the development ofnerve gases. Their mechanistic studies were studied in detail tocounter adverse actions and antidotes came into the picture. Again,after the war, funding went for wellbeing of younger population fordevelopment of antibiotics and immunization. As the populationgrew older, AD again came to front row of funding. These two pathsmerged with discovery of the fact that brain Acetylcholine levelsare depleted in AD. Normally, identification and validation of targetis a time consuming process. The whole research time was reducedwith development of AChE inhibitors as drug for AD due to highlydeveloped research facilities in the field of AChE during the worldwar era. Hence, the first four drugs that were approved for ADbelonged to this class. As it is a past therapy of AD, numerous newapproaches such as Tau-based therapies, dealing with oxidativestress, targeting cellular Ca2þ handling, anti-inflammatory therapy,amyloid targeted strategies (b-Secretase inhibitors and g-Secrea-tase modulators) are currently being employed for the treatment ofAD. Among them, b-Secretase (BACE) inhibitors and g-Secreatase

modulators (GSMs) are emerging as promising AD therapy. Inrecent times, many companies and academic groups have ongoingb-Secretase inhibitors and GSMs development programs and alsosome of them have entered early phase clinical trials but are stilllagging behind in the studies. On the other hand, the potential ofAChE inhibitors is well explored and hence it won in the race ofdrug development in AD treatment.

References

[1] C. Snyder, M.D. Lusdwig Laqueur, I saw for the first time in front of both myeyes the ominous colored halos, Arch. Ophthalmol. 72 (1964) 111e113.

[2] M. Eto, Organophosphorus Pesticides: Organic and Biological Chemistry, CRCPress Inc., Cleveland, Ohio, 1974, pp. 57e121.

[3] A.T. Tu, Basic information on nerve gas and the use of sarin by Aum Shin-rikyo, J. Mass Spectrom. Soc. Jpn. 44 (1996) 293e320.

[4] T.C. Marrs, R.L. Maynard, F.R. Sidell, Organophosphates nerve agents, in:Chemical Warfare Agents: Toxicology and Treatment, John Wiley & Sons,New York, 1996, pp. 83e100.

[5] J. Kassa, Review of oximes in the antidotal treatment of poisoning byorganophosphorus nerve agents, J. Toxicol. Clin. Toxicol. 40 (2002) 803e816.

[6] H. Landsberg, Prelude to the discovery of penicillin, Isis 40 (1949) 225e227.[7] F. Gaertner, L. Kim, Current applied recombinant DNA projects, Trends Ecol.

Evol. 3 (1988) S4eS7.[8] M.B. Graeber, S. Kosel, E. Grasbon-Frodl, H. Moller, P. Mehraein, Histopa-

thology and APOE genotype of the first Alzheimer’s disease patient AugusteD, Neurogenetics 1 (1998) 223e228.

[9] A.K. Ghosh, M. Brindisi, J. Tang, Developing b-secretase inhibitors for treat-ment of Alzheimer’s disease, J. Neurochem. 1 (2012) 71e83.

[10] M. Pettersson, A.F. Stepan, G.W. Kauffman, D.S. Johnson, Novel g-secretasemodulators for the treatment of Alzheimer’s disease: a review focusing onpatents from 2010 to 2012, Neurosci. Med. Chem. 23 (2013) 1e18.

[11] W.K. Summers, Administration of monoamine acridines in cholinergicneuronal deficit states, US 4816456, 1989.

[12] M. Pievani, W. de Haan, T. Wu, W.W. Seeley, G.B. Frisoni, Functional networkdisruption in the degenerative dementias, Lancet Neurol. 10 (2011) 829e843.

[13] R.J. Caselli, B.F. Boeve, The degenerative dementias, in: C.G. Goetz (Ed.),Textbook of Clinical Neurology, third ed., Saunders Elsevier, Philadelphia,2007, pp. 699e702.

[14] M. Goedert, M.G. Spillantini, A century of Alzheimer’s disease, Science 314(2006) 777e781.

[15] L.M. Bolognesi, A. Cavalli, M.V.L. Bartolini, M. Rosini, V. Andrisano,M. Recanatini, C. Melchiorre, Multi-target-directed drug design strategy:from a dual binding site acetylcholinesterase inhibitor to a trifunctionalcompound against Alzheimer’s disease, J. Med. Chem. 50 (2007) 6446e6449.

[16] M. Storandt, Cognitive deficits in the early stages of Alzheimer’s disease,Assoc. Psychol. Sci. 17 (2008) 198e202.

[17] L.R. Squire, Memory systems of the brain: a brief history and currentperspective, Neurobiol. Learn. Mem. 82 (2004) 171e177.

[18] M.J. Berridge, Calcium hypothesis of Alzheimer’s disease, Pflugers Arch. 459(2010) 441e449.

[19] R.H. Wilkins, I.A. Brody, Alzheimer’s disease, Arch. Neurol. 21 (1969) 109e110.

[20] A. Alzheimer, Over a eigenartigo disease of the cerebral cortex, Allg. Z.Psychiat. 64 (1907) 146e148.

[21] F.H. Crick, L. Barnett, S. Brenner, R.J. Watts-Tobin, General nature of thegenetic code for proteins, Nature 192 (1961) 1227e1232.

[22] R.D. Terry, N.K. Gonatas, M. Weiss, Ultrastructural studies in Alzheimer’spresenile dementia, Am. J. Pathol. 44 (1964) 269e297.

[23] R.D. Terry, The fine structure of neurofibrillary tangles in Alzheimer’s dis-ease, J. Neuropathol. Exp. Neurol. 22 (1963) 629e642.

[24] H.M. Wi�sniewski, H.K. Narang, R.D. Terry, Neurofibrillary tangles of pairedhelical filaments, J. Neurol. Sci. 27 (1976) 173e181.

[25] R. Katzman, The prevalence and malignancy of Alzheimer’s disease, Arch.Neurol. 33 (1976) 299e308.

[26] R. Katzman, K. Bick, The rediscovery of Alzheimer disease during the 1960sand 1970s, in: P.J. Whitehouse, k. Maurer, M.A. Ballenger (Eds.), Concepts ofAlzheimer’s Disease, Biological, Clinical, and Cultural Perspectives, JohnsHopkins University Press, Baltimore, 2000, pp. 104e114.

[27] J. Gehrmann, Y. Matsumoto, G.W. Kreutzberg, Microglia: intrinsic immu-neffector cell of the brain, Brain Res. Rev. 20 (1995) 269e287.

[28] A. Ott, R.P. Stolk, F. van Harskamp, H.A. Pols, A. Hofman, M.M. Breteler,Diabetes mellitus and the risk of dementia: the Rotterdam study, Neurology53 (1999) 1937e1942.

[29] M. Prince, M. Cullen, A. Mann, Risk factors for Alzheimer’s disease and de-mentia: a case-control study based on the MRC elderly hypertension trial,Neurology 44 (1994) 97e104.

[30] R. Katzman, K. Bick, The cholinergic story: hope for the patient and family,in: Alzheimer Disease, the Changing View, first ed., Academic Press, London,2000, p. 182 (Chapter 5).

http://refhub.elsevier.com/S0223-5234(13)00631-4/sref1

































































































[31] P.M. Arce, R.I.M. Franco, G.C.G. Munoz, C. Perez, B. Lopez, M. Villaroya,G.M. Lopez, G.A. Garcia, Neuroprotective and cholinergic properties ofmultifunctional glutamic acid derivatives for the treatment of Alzheimer’sdisease, J. Med. Chem. 22 (2009) 7249e7257.

[32] G.G. Glenner, C.W. Wong, Alzheimer’s disease: initial report of the purifi-cation and characterization of a novel cerebrovascular amyloid protein,Biochem. Biophys. Res. Commun. 120 (1984) 885e890.

[33] J. Hardy, D.J. Selkoe, The amyloid hypothesis of Alzheimer’s disease: progressand problems on the road to therapeutics, Science 297 (2002) 353e356.

[34] D.J. Selkoe, D. Schenk, Alzheimer’s disease: molecular understanding pre-dicts amyloid-based therapeutics, Annu. Rev. Pharmacol. Toxicol. 43 (2003)545e584.

[35] L.M. Bekris, C. Yu, T.D. Bird, D.W. Tsuang, Genetics of Alzheimer disease,J. Geriatr. Psychiatry Neurol. 23 (2010) 213e227.

[36] F. Liu, M.A. Ikram, A.C. Janssens, M. Schuur, I. de Koning, A. Isaacs,M. Struchalin, A.G. Uitterlinden, J.T. den Dunnen, K. Sleegers, K. Bettens,C. Van Broeckhoven, J. van Swieten, A. Hofman, B.A. Oostra, Y.S. Aulchenko,M.M. Breteler, C.M. van Duijn, A study of the SORL1 gene in Alzheimer’sdisease and cognitive function, J. Alzheimer’s Dis. 18 (2009) 51e64.

[37] B.J. Tabner, O.M. El-Agnaf, S. Turnbull, M.J. German, K.E. Paleologou,Y. Hayashi, L.J. Cooper, N.J. Fullwood, D. Allsop, Hydrogen peroxide isgenerated during the very early stages of aggregation of the amyloid pep-tides implicated in Alzheimer disease and familial British dementia, J. Biol.Chem. 280 (2005) 35789e35792.

[38] K. Leuner, S. Hauptmann, R. Abdel-Kader, I. Scherping, U. Keil,J.B. Strosznajder, A. Eckert, W.E. Muller, Mitochondrial dysfunction: the firstdomino in brain aging and Alzheimer’s disease, Antioxid. Redox Signal. 9(2007) 1659e1675.

[39] N. Crespo-Biel, C. Theunis, F.V. Leuven, Protein tau: prime cause of synapticand neuronal degeneration in Alzheimer’s disease, Int. J. Alzheimer’s Dis.2012 (2012) 1e13.

[40] M. Goedert, M.G. Spillantini, R.A. Crowther, Tau proteins and neurofibrillarydegeneration, Brain Pathol. 1 (1991) 279e286.

[41] K. Iqbal, A.C. Alonso, S. Chen, M.O. Chohan, E. El-Akkad, C.X. Gong, S. Khatoon,B. Li, F. Liu, A. Rahman, H. Tanimukai, I. Grundke-Iqbal, Tau pathology inAlzheimer disease and other tauopathies, Biochim. Biophys. Acta 1739(2005) 198e210.

[42] W. Chun, G.V. Johnson, The role of tau phosphorylation and cleavage inneuronal cell death, Front. Biosci. 12 (2007) 733e756.

[43] L. Bojarski, J. Herms, J. Kuznicki, Calcium dysregulation in Alzheimer’s dis-ease, Neurochem. Int. 52 (2008) 621e633.

[44] K.N. Green, F.M. LaFerla, Linking calcium to Ab and Alzheimer’s disease,Neuron 59 (2008) 190e194.

[45] N. Pierrot, S.F. Santos, C. Feyt, M. Morel, J.P. Brion, J.N. Octave, Calcium-mediated transient phosphorylation of tau and amyloid precursor proteinfollowed by intraneuronal amyloid-beta accumulation, J. Biol. Chem. 281(2006) 39907e39914.

[46] H.W. Querfurth, D.J. Selkoe, Calcium ionophore increases amyloid betapeptide production by cultured cells, Biochemistry 33 (1994) 4550e4561.

[47] C. Edlund, M. Söderberg, K. Kristensson, Isoprenoids in aging and neurodegeneration, Neurochem. Int. 25 (1994) 35e38.

[48] G. Johnson, W.S. Moore, The peripheral anionic site of acetylcholinesterase:structure, functions and potential role in rational drug design, Curr. Pharm.Des. 12 (2006) 217e225.

[49] A. Shafferman, C. Kronman, A. Flashner, M. Leitner, H. Grosfed, A. Ordentlich,Y. Gozes, S. Cohen, N. Ariel, D. Barak, Mutagenesis of human acetyl cholin-esterase. Identification of residue involved in the catalytic activity and inpolypeptide folding, J. Biol. Chem. 267 (1992) 17640e17648.

[50] A. Shafferman, D. Barak, D. Kaplan, A. Ordentlich, C. Kronman, B. Velan,Functional requirements for the optimal catalytic configuration of the AChEactive center, Chem. Biol. Interact. 157 (2005) 123e131.

[51] L. Blichfeldt-Lauridsen, B.D. Hansen, Anesthesia and myasthenia gravis, ActaAnaesthesiol. Scand. 56 (2012) 17e22.

[52] R.M. Lane, S.G. Potkin, A. Enz, Targetting acetylcholinesterase and butyr-ylcholinesterase in dementia, Int. J. Neuropsychopharmacol. 9 (2006) 101e124.

[53] G. Zimmerman, H. Soreq, Termination and beyond: acetylcholinesterase as amodulator of synaptic transmission, Cell Tissue Res. 326 (2006) 655e669.

[54] D.M. Quinn, Acetylcholinesterase: enzyme structure, reaction dynamics, andvirtual transition states, Chem. Rev. 87 (1987) 955e979.

[55] J. Massoulié, S. Bon, The molecular forms of cholinesterase and acetylcho-linesterase in vertebrates, Annu. Rev. Neurosci. 5 (1982) 57e106.

[56] K. MacPhee-Quigley, P. Taylor, S. Taylor, Primary structures of the catalyticsubunits from two molecular forms of acetylcholinesterase. A comparison ofNH2-terminal and active center sequences, J. Biol. Chem. 260 (1985) 12185e12189.

[57] N.K. Schaffer, H.O. Michel, A.F. Bridges, Amino acid sequence in the region ofthe reactive serine residue of eel acetylcholinesterase, Biochemistry 12(1973) 2946e2950.

[58] T.L. Rosenberry, Acetylcholinesterase, Adv. Enzymol. Relat. Areas Mol. Biol.43 (1975) 103e218.

[59] M. Harel, I. Schalk, L. Ehret-Sabatier, F. Bouet, M. Goeldner, C. Hirth,P.H. Axelsen, I. Silman, J.L. Sussman, Quaternary ligand binding to aromaticresidues in the active-site gorge of acetylcholinesterase, Proc. Natl. Acad. Sci.90 (1993) 9031e9035.

[60] J. Massoulie, A. Anselmet, S. Bon, E. Krejci, C. Legay, N. Morel, S. Simon,Acetylcholinesterase: C-terminal domains, molecular forms and functionallocalization, J. Physiol. (Paris) 92 (1998) 183e190.

[61] T.L. Rosenberry, P. Barnett, C. Mays, Acetylcholinesterase, Meth. Enzymol. 82(1982) 325e339.

[62] M. Pohanka, Cholinesterases, a target of pharmacology and toxicology, Bio-med. Pap. Med. Fac. Univ. Palacky Olomouc Czech Repub. 155 (2011) 219e230.

[63] H.A. Berman, J. Yguerabide, P. Taylor, Fluorescence energy transfer onacetylcholinesterase: spatial relationship between peripheral site and activecenter, Biochemistry 19 (1980) 2226e2235.

[64] G. Mooser, D.S. Sigman, Ligand binding properties of acetylcholinesterasedetermined with fluorescent probes, Biochemistry 13 (1974) 2299e2307.

[65] P. Taylor, S. Lappi, Interaction of fluorescence probes with acetylcholines-terase. The site and specificity of propidium binding, Biochemistry 14 (1975)1989e1997.

[66] H.A. Berman, P. Taylor, Fluorescent phosphonate label for serine hydrolases,pyrenebutyl methylphosphonofluoridate: reaction with acetylcholines-terase, Biochemistry 17 (1978) 1704e1713.

[67] H.A. Berman, W. Becktel, P. Taylor, Spectroscopic studies on acetylcholines-trase: influence of peripheral site occupation on active-centre conformation,Biochemistry 20 (1981) 4803e4810.

[68] N.C. Inestrosa, M.C. Dinamarca, A. Alvarez, Amyloidecholinesterase in-teractions. Implications for Alzheimer’s disease, FEBS J. 275 (2008) 625e632.

[69] H. Haviv, D.M. Wong, I. Silman, J.L. Sussman, Bivalent ligands derived fromHuperzine A as acetylcholinesterase inhibitors, Curr. Top. Med. Chem. 7(2007) 375e387.

[70] J. Eichler, A. Anselment, J.L. Sussman, J. Massoulie, I. Silman, Differential ef-fects of peripheral site ligands on Torpedo and chicken acetylcholinesterase,Mol. Pharmacol. 45 (1994) 335e340.

[71] A. Castro, A. Martinez, Peripheral and dual binding site acetylcholinesteraseinhibitors: implications in treatment Alzheimer’s disease, Mini-Rev. Med.Chem. 1 (2001) 267e272.

[72] A. Alvarez, C. Opazo, R. Alarcon, J. Garrido, N.C. Inestrosa, Acetylcholin-esterase promotes the aggregation of amyloid-beta-peptide fragments byforming a complex with the growing fibrils, J. Mol. Biol. 272 (1997) 348e361.

[73] A. Alvarez, R. Alarcon, C. Opazo, E.O. Campos, F.J. Munoz, F.H. Calderon,F. Dajas, M.K. Gentry, B.P. Doctor, F.G. De Mello, N.C. Inestrosa, Stable com-plexes involving acetylcholinesterase and amyloid-beta peptide change thebiochemical properties of the enzyme and increase the neurotoxicity ofAlzheimer’s fibrils, J. Neurosci. 18 (1998) 3213e3223.

[74] G.V. De Ferrari, M.A. Canales, I. Shin, L.M. Weiner, I. Silman, N.C. Inestrosa,A structural motif of acetylcholinesterase that promotes amyloid beta-peptide fibril formation, Biochemistry 40 (2001) 10447e10457.

[75] N.C. Inestrosa, A. Alvarez, C.A. Perez, R.D. Moreno, M. Vicente, C. Linker,Acetyl cholinesterase accelerates assembly of amyloid-beta-peptides intoAlzheimer’s fibrils: possible role of the peripheral site of the enzyme, Neuron16 (1996) 881e891.

[76] S.N. Abramson, Z. Radic, D. Manker, D.J. Faulkner, P. Taylor, Onchidal: anaturally occurring irreversible inhibitor of acetylcholinesterase with a novelmechanism of action, Mol. Pharmacol. 36 (1989) 349e354.

[77] W. Carmichael, The toxins of cyanobacteria, Sci. Am. 270 (1994) 78e86.[78] S. Matsunaga, R.E. Moore, W.P. Niemczura, W.W. Carmichael, Anatoxin-a(s),

a potent anticholinesterase from Anabaena flos-aquae, J. Am. Chem. Soc. 111(1989) 8021e8023.

[79] M. Harel, G.J. Kleywegt, R.B.G. Ravelli, I. Silman, J.L. Sussman, Crystal struc-ture of an acetylcholinesteraseefasciculin complex: interaction of a three-fingered toxin from snake venom with its target, Structure 3 (1995) 1355e1366.

[80] B. Goozner, L.I. Lutwick, E. Bourke, Chemical terrorism: a primer for 2002,J. Assoc. Acad. Minor. Phys. 13 (2002) 14e18.

[81] Y. Solberg, M. Belkin, The role of excitotoxicity in organophosphorous nerveagents central poisoning, Trends Pharmacol. Sci. 18 (1997) 183e185.

[82] J. Bajgar, Organophosphates/nerve agent poisoning: mechanism of action,diagnosis, prophylaxis, and treatment, Adv. Clin. Chem. 38 (2004) 151e216.

[83] T. Namba, C.T. Nolte, J. Jackrel, D. Grob, Poisoning due to organophosphateinsecticides, Am. J. Med. 50 (1971) 475e492.

[84] F.R. Sidell, J. Borak, Chemical warfare agents: II. Nerve agents, Ann. Emerg.Med. 21 (1992) 865e871.

[85] C.P. Holstege, M. Kirk, F.R. Sidell, Chemical warfare: nerve agent poisoning,Crit. Care Clin. 4 (1997) 923e942.

[86] G.B. Koelle, R.L. Volle, B. Holmstedt, A.G. Karczmar, R.D. O’brien, Anticho-linesterase agents, Science 141 (1963) 63e65.

[87] W.P. Bozeman, D. Dilberio, J.L. Schauben, Biologic and chemical weapons ofmass destruction, Emerg. Med. Clin. North Am. 20 (2002) 975e993.

[88] E.E. Rosenbaum, R.J. Hersechler, S.W. Jacob, An antidote to parathionpoisoning: pralidoxime chloride (Protopam chloride), J. Am. Med. Assoc. 192(1965) 314e315.

[89] R.F. Clark, Insecticides: organic phosphorus compounds and carbamates, in:L.R. Goldfrank, N.E. Flomenbaum, N.A. Lewin, M.A. Howland, R.S. Hoffman,L.S. Nelson (Eds.), Goldfrank’s Toxicologic Emergencie, seventh ed., McGraw-Hill, New York, 2002, pp. 1346e1360.

[90] R.T. Delfino, T.S. Ribeiro, J.D. Figueroa-Villar, Organophosphorus compoundsas chemical warfare agents: a review, J. Braz. Chem. Soc. 20 (2009) 407e428.






































































































































































































































[91] J. Paxman, R.A. Harris, Higher Form of Killing: the Secret Story of Chemicaland Biological Warfare, Hill and Wang, New York, 1982, pp. 138e139.

[92] NE Museum, R. Kuhn, Biography, http://www.nobel.se/chemistry/laureates/1938/kuhn-bio.html.

[93] J.K. Smart, History of chemical and biological warfare: an Americanperspective, in: F.R. Sidell, E.T. Takafuji, D.R. Franz (Eds.), Medical Aspects ofChemical and Biological Warfare, Textbook of Military Medicine, Part I:Warfare Weaponry, and the Casualty, Office of the Surgeon General, Wash-ington, DC, 1997, pp. 9e86.

[94] H. Daly, J.T. Doyen, A.H. Purcell, Introduction to Insect Biology and Diversity(Chapter 13), second ed., Oxford University Press, New York, 1998, pp. 279e300.

[95] T.R. Fukuto, Mechanism of action of organophosphorus and carbamate in-secticides, Environ. Health Perspect. 87 (1990) 245e254.

[96] M.B. Abou-Donia, Organophosphorus ester-induced delayed neurotoxicity,Annu. Rev. Pharmacol. Toxicol. 21 (1981) 511e548.

[97] S.Y. Wu, J.E. Casida, Subacute neurotoxicity induced in mice by potentorganophosphorus neuropathy target esterase inhibitors, Toxicol. Appl.Pharmacol. 139 (1996) 195e202.

[98] C.C. Yu, R.L. Metcalf, G.M. Booth, Inhibition of acetylcholinesterase frommammals and insects by carbofuran and its related compounds and theirtoxicities toward these animals, J. Agric. Food Chem. 20 (1972) 923e926.

[99] F.R. Sidell, Clinical effects of organophosphorus cholinesterase inhibitors,J. Appl. Toxicol. 14 (1994) 111e113.

[100] K. Hayden, M. Norton, D. Darcey, T. Ostbye, P. Zandi, J. Breitner, K. Welsh-Bohmer, Occupational exposure to pesticides increases the risk of incidentAD: the Cache County study, Neurology 19 (2010) 1524e1530.

[101] S. Barnes, The influence of certain biological factors on the resistance of bed-bugs (Cimex lectularius, L.) to DDT, Bull. Entomol. Res. 36 (1946) 419e422.

[102] S.C. Arya, N. Agarwal, A risk for returned travellers: the “post-antibiotic era”,Med. J. Aust. 194 (2011) 55e56.

[103] F.J. Grundbacher, Behring’s discovery of diphtheria and tetanus antitoxins,Immunol. Today 13 (1992) 188e190.

[104] A.M. Bain, Historical note on poliomyelitis in Uganda, East Afr. Med. J. 43(1966) 62e64.

[105] J. Patrick, J. Lindstrom, Autoimmune response to acetylcholine receptor,Science 180 (1973) 871e872.

[106] J.C. Keesey, Contemporary opinions about Mary Walker: a shy pioneer oftherapeutic neurology, Neurology 51 (1998) 1433e1439.

[107] H.C. Hartzell, S.W. Kuffler, D. Yoshikami, Post-synaptic potentiation: inter-action between quanta of acetylcholine at the skeletal neuromuscular syn-apse, J. Physiol. 25 (1975) 1427e1463.

[108] W.K. Summers, L.V. Majovski, G.M. Marsh, K. Tachiki, A. Kling, Oral tetra-hydroaminoacridine in long-term treatment of senile dementia, Alzheimertype, N. Engl. J. Med. 315 (1986) 1241e1245.

[109] B. Reisberg, R. Doody, A. Stoffler, F. Schmitt, S. Ferris, H.J. Mobius, MemantineStudy Group, Memantine in moderate-to-severe Alzheimer’s disease,N. Engl. J. Med. 348 (2003) 1333e1341.

[110] T.A. Pham, H. Che, P.T. Phan, J.W. Lee, S.S. Kim, H. Park, Oroxylin A analogsexhibited strong inhibitory activities against iNOS-mediated nitric oxide(NO) production, Bioorg. Med. Chem. Lett. 22 (2012) 2534e2535.

[111] E.J. Barreiro, C.A. Camara, H. Verli, L. Brazil-Mas, N.G. Castro, W.M. Cintra,Y. Aracava, C.R. Rodrigues, C.A.M. Fraga, Design, synthesis, and pharmaco-logical profile of novel fused pyrazolo[4,3-d]pyridine and pyrazolo[3,4-b][1,8]naphthyridine isosteres: a new class of potent and selective acetyl-cholinesterase inhibitors, J. Med. Chem. 46 (2003) 1144e1152.

[112] J.L. Marco, C. De los Rios, M.C. Carreiras, J.E. Banos, A. Badia, N.M. Vivas, Syn-thesis and acetylcholinesterase/butyrylcholinesterase inhibition activity of 4-amino-2,3-diaryl-5,6,7,8-tetrahydrofuro(and thieno)[2,3-b]-quinolines, and4-amino-5,6,7,8,9-pentahydro-2,3-diphenylcyclohepta[e]furo(and thieno)-[2,3-b]pyridines, Arch. Pharm. 7 (2002) 347e353.

[113] J.L. Marco, C. De los Rios, M.C. Carreiras, J.E. Banos, A. Badia, N.M. Vivas,Synthesis and acetylcholinesterase/butylcholinesterase inhibition activity oftacrine like analogs, Bioorg. Med. Chem. 9 (2001) 727e732.

[114] J.L. Marco, C. De los Rios, A.G. Garcia, M. Villarroya, M.C. Carreiras, C. Martins,A. Eleuterio, A. Morreale, M. Orozco, F. Luque, Synthesis, biological evalua-tion and molecular modelling of diversely functionalized heterocyclic de-rivatives as inhibitors of acetylcholinesterase/butyrylcholinesterase andmodulators of Ca2þ channels and nicotinic receptors, Bioorg. Med. Chem. 12(2004) 2199e2218.

[115] G. Petroianu, K. Arafat, C. Sasse, H. Stark, Multiple enzyme inhibitions byhistamine H3 receptor antagonists as potential procognitive agents, Phar-mazie 61 (2003) 179e182.

[116] S. Morisset, E. Traiffort, J.C. Schwartz, Inhibition of histamine versus acetyl-choline metabolism as a mechanism of tacrine activity, Eur. J. Pharmacol. 315(1996) R1eR2.

[117] M. Rosini, V. Andrisano, M. Bartolini, M.L. Bolognesi, P. Helia, A. Minarini,A. Tarozzi, C. Melchiorre, Rational approach to discover multipotent anti-Alzheimer drugs, J. Med. Chem. 48 (2005) 360e363.

[118] A. Cappelli, A. Gallelli, M. Manini, M. Anzini, L. Mennuni, F. Makovec,M.C. Menziani, S. Alcaro, F. Ortuso, S. Homer, Further studies on the inter-action of the 5-hydroxytryptamine3 (5-HT3) receptor with arylpiperazineligands development of a new 5-HT3 receptor ligand showing potentacetylcholinesterase inhibitory properties, J. Med. Chem. 48 (2005) 3564e3575.

[119] Y. Pang, P. Quiram, T. Jelacic, F. Hong, S. Brimijon, Highly potent, selective,and low cost bis-tetrahydroaminacrine inhibitors of acetylcholinesterase,J. Biol. Chem. 271 (1996) 23646e23649.

[120] M.I. Rodriguez-Franco, M.I. Fernandez-Bachiller, C. Perez, B. Hernandez-Ledesma, B. Bartolome, Novel tacrineemelatonin hybrids as dual-actingdrugs for Alzheimer disease, with improved acetylcholinesterase inhibitoryand antioxidant properties, J. Med. Chem. 49 (2006) 459e462.

[121] D. Alonso, I. Dorronsoro, L. Rubio, P. Munoz, Donepeziletacrine hybridrelated derivatives as new dual binding site inhibitors of AChE, Bioorg. Med.Chem. 13 (2005) 6588e6597.

[122] P. Camps, X. Formosa, C. Galdeano, T. Gomez, D. Munoz-Torrero,M. Scarpellini, E. Viayna, A. Badia, M.V. Clos, A. Camins, M. Pallas,M. Bartolini, F. Mancini, V. Andrisano, J. Estelrich, M. Lizondo, A. Bidon-Chanal, F.J. Luque, Novel donepezil-based inhibitors of acetyl-and butyr-ylcholinesterase and acetylcholinesterase-induced b-amyloid aggregation,J. Med. Chem. 51 (2008) 3588e3598.

[123] A. Villalobos, J.F. Blake, C.K. Biggers, T.W. Butler, D.S. Chapin, Y.L. Chen,J.L. Ives, S.B. Jones, D.R. Liston, A.A. Nagel, Novel benzisoxazole derivatives aspotent and selective inhibitors of acetylcholinesterase, J. Med. Chem. 37(1994) 2721e2734.

[124] A. Martinez, E. Fernandez, A. Castro, S. Conde, M.I. Rodriguez-Franco,J. Banos, A. Badia, N-Benzylpiperidine derivatives of 1,2,4-thiadiazolidinoneas new acetylcholinesterase inhibitors, Eur. J. Med. Chem. 35 (2000) 913e922.

[125] A. Andreani, A. Cavalli, M. Granaiola, M. Guardigli, A. Leoni, A. Locatelli,R. Morigi, M. Rambaldi, M. Recanatini, A. Roda, Synthesis and screening forantiacetylcholinesterase activity of (1-benzyl-4-oxopiperidin-3-ylidene)methylindoles and pyrroles related to donepezil, J. Med. Chem. 44 (2001)4011e4014.

[126] Y. Ishihara, K. Hirai, M. Miyamoto, G. Goto, Central cholinergic agents: syn-thesis and evaluation of 3-[1-(phenylmethyl)-4-piperidinyl]-1-(2,3,4,5-tetrahydro-1H-1-benzazepin-8-yl)-1-propanones and their analogs as cen-tral selective acetylcholinesterase inhibitors, J. Med. Chem. 37 (1994) 2292e2299.

[127] F. Belluti, L. Piazzi, A. Bisi, S. Gobbi, M. Bartolini, A. Cavalli, P. Valenti,A. Rampa, Design, synthesis, and evaluation of benzophenone derivatives asnovel acetylcholinesterase inhibitors, Eur. J. Med. Chem. 44 (2009) 1341e1348.

[128] M.L. Bolonesi, M. Bartolini, A. Cavalli, V. Andrisano, M. Rosini, A. Minarini,C. Melchiorre, Design, synthesis and biological evaluation of conforma-tionally restricted rivastigmine analogues, J. Med. Chem. 47 (2004) 5945e5952.

[129] N.F. Proskurnina, A.P. Yakoleva, Oral solution containing galanthamine and asweetening agent, J. Gen. Chem. 22 (1957) 1899e1902.

[130] G.M. Bores, R.W. Kosley, Galanthamine derivatives for the treatment ofAlzheimer’s disease, Drugs Future 21 (1996) 621e635.

[131] S. Lilienfeld, Galantamine: a novel cholinergic drug with a unique dual modeof action for the treatment of patients with Alzheimer’s disease, CNS DrugRev. 8 (2002) 159e176.

[132] M.D. Santos, M. Alkondon, E.F. Pereira, Y. Aracava, H.M. Eisenberg,A. Maelicke, E.X. Albuquerque, The nicotinic allosteric potentiating ligandgalantamine facilitates synaptic transmission in the mammalian centralnervous system, Mol. Pharmacol. 61 (2002) 1222e1234.

[133] D. Herlem, M.T. Martin, C. Thal, C. Guillou, Synthesis and structure activityrelationships of open d-ring galanthamine analogues, Bioorg. Med. Chem.Lett. 13 (2003) 2389e2391.

[134] P. Jia, R. Sheng, J. Zhang, L. Fang, Q. He, B. Yang, Y. Hu, Design, synthesis andevaluation of galanthamine derivatives as acetylcholinesterase inhibitors,Eur. J. Med. Chem. 44 (2009) 772e784.

[135] D.J. Triggle, J.M. Mitchell, R. Filler, The pharmacology of physostigmine, CNSDrug Rev. 4 (1998) 87e136.

[136] Z.J. Zhan, H.L. Bian, W.J. Wang, G.W. Shan, Synthesis of physostigmine ana-logues and evaluation of their anticholinesterase activities, Bioorg. Med.Chem. Lett. 20 (2010) 1532e1534.

[137] S. Iijima, N.H. Greig, P. Garofalo, E.L. Spangler, B. Heller, A. Brossi, D.K. Ingram,Phenserine: a physostigmine derivative that is a long-acting inhibitor ofcholinesterase and demonstrates a wide dose range for attenuating ascopolamine-induced learning impairment of rats in a 14-unit T-maze,Psychopharmacology (Berlin, Germany) 112 (1993) 415e420.

[138] C. Linden, A. Alm, Latanoprost and physostigmine have mostly additiveocular hypotensive effects in human eyes, Arch. Ophthalmol. 115 (1997)857e861.

[139] C. Bartolucci, M. Siotto, E. Ghidini, G. Amari, P.T. Bolzoni, M. Racchi, G. Villetti,M. Delcanale, D. Lamba, Structural determinants of Torpedo californicaacetylcholinesterase inhibition by the novel and orally active carbamatebased anti-Alzheimer drug ganstigmine (CHF-2819), J. Med. Chem. 49 (2006)5051e5058.

[140] Q.S. Yu, H.W. Holloway, J.L. Flippen-Anderson, B. Hoffman, A. Brossi,N.H. Grieg, Methyl analogues of the experimental Alzheimer drug phenser-ine: synthesis and structure/activity relationships for acetyl and butyr-ylcholinesterase inhibitory action, J. Med. Chem. 44 (2001) 4062e4071.

[141] G. Orhan, I. Orhan, N. Oztekin-Subutay, F. Ak, B. Sener, Contemporary anti-cholinesterase pharmaceuticals of natural origin and their synthetic ana-logues for the treatment of Alzheimer’s disease, Recent Pat. CNS DrugDiscovery 4 (2009) 43e51.




http://www.nobel.se/chemistry/laureates/1938/kuhn-bio.html

http://www.nobel.se/chemistry/laureates/1938/kuhn-bio.html






















































































































































































































[142] Z.J. Zhan, H.L. Bian, J.W. Wang, W.G. Shan, Synthesis of physostigmine ana-logues and evaluation of their anticholinesterase activities, Bioorg. Med.Chem. Lett. 20 (2011) 1532e1534.

[143] W. Luo, Q.S. Yu, M. Zhan, D. Parrish, J.R. Deschamps, S.S. Kulkarni,H.W. Holloway, G.M. Alley, D.K. Lahiri, A. Brossi, N.H. Greig, Novel anticho-linesterases based on the molecular skeletons of furobenzofuran andmethanobenzodioxepine, J. Med. Chem. 48 (2005) 986e994.

[144] C. Viegas Jr., V.S. Bolzani, L.S. Pimentel, N.G. Castro, R.F. Cabral, R.S. Costa,C. Floyd, M.S. Rocha, M.C. Young, E.J. Barreiro, C.A. Fraga, New selectiveacetylcholinesterase inhibitors designed from natural piperidine alkaloids,Bioorg. Med. Chem. 13 (2005) 4184e4190.

[145] V.S. Bolzani, A.A.L. Gunatilaka, D.G.I. Kingston, Bioactive and other piperidinealkaloids from Cassia leptophylla, Tetrahedron 51 (1995) 5929e5934.

[146] C. Viegas Jr., V.S. Bolzani, E.J. Barreiro, M.C. Young, M. Furlan, D. Tomazela,M.N. Eberlin, Further bioactive piperidine alkaloids from the flowers andgreen fruits of Cassia spectabilis, J. Nat. Prod. 67 (2004) 908e910.

[147] M.S. Alexandre-Moreira, C. Viegas Jr., A.L.P. Miranda, V.S. Bolzani,E.J. Barreiro, Antinociceptive profile of (�)-spectaline: a piperidine alkaloidfrom Cassia leptophylla, Planta Med. 69 (2003) 795e799.

[148] H. Jiang, X. Luo, D. Bai, Progress in clinical, pharmacological, chemical andstructural biological studies of huperzine A: a drug of traditional Chinesemedicine origin for the treatment of Alzheimer’s disease, Curr. Med. Chem.10 (2003) 2231e2252.

[149] A. Badia, J.E. Banos, P. Camps, J. Contreras, D.M. Gorbig, D.M. Diego Munoz-Torrero, M. Simon, N.M. Vivas, Synthesis and evaluation of tacrineehuper-zine a hybrids as acetylcholinesterase inhibitors of potential interest for thetreatment of Alzheimer’s disease, Bioorg. Med. Chem. 6 (1998) 427e440.

[150] M. Mar Alcala, N.M. Vivas, S. Hospital, P. Camps, T.D. Munoz, A. Badia,Characterisation of the anticholinesterase activity of two new tacrineehuperzine A hybrids, Neuropharmacology 44 (2003) 749e755.

[151] P. Camps, X. Formosa, D. Munoz-Torrero, J. Petrignet, A. Badia, V. Clos,Synthesis and pharmacological evaluation of huprine-tacrine heterodimers:subnanomolar dual binding site acetylcholinesterase inhibitors, J. Med.Chem. 48 (2005) 1701e1704.

[152] M.T. Andrade, J.A. Lima, A.C. Pinto, C.M. Rezende, M.P. Carvalho, R.A. Epifanio,Indole alkaloids from Tabernaemontana australis (Muell. Arg) Miers thatinhibit acetylcholinesterase enzyme, Bioorg. Med. Chem. 13 (2005) 4092e4095.

[153] S.S. Lee, U. Venkatesham, C.P. Rao, S.H. Lam, J.H. Lin, Preparation of secoly-corines against acetylcholinesterase, Bioorg. Med. Chem. 15 (2007) 1034e1043.

[154] S. Markmee, S. Ruchirawat, V. Prachyawarakorn, K. Ingkaninan, N. Khorana,Isoquinoline derivatives as potential acetylcholinesterase inhibitors, Bioorg.Med. Chem. Lett. 16 (2006) 2170e2172.

[155] N. Khorana, S. Markmee, K. Ingkaninan, S. Ruchirawat, R. Kitbunnadaj,M.R. Pullagurla, Evaluation of a new lead for acetylcholinesterase inhibition,Med. Chem. Res. 18 (2009) 231e241.

[156] T. Ogino, T. Yamaguchi, T. Sato, H. Sasaki, K. Sugama, M. Okada, M. Maruno,Studies on inhibitory activity against acetylcholinesterase of new bisben-zylisoquinoline alkaloid and its related compounds, Heterocycles 45 (1997)2253e2560.

[157] D.A. Bianchi, G.S. Hirschmann, C. Theoduloz, A.B.J. Bracca, T.S. Kaufman,Synthesis of tricyclic analogs of stephaoxocanidine and their evaluation asacetylcholinesterase inhibitors, Bioorg. Med. Chem. Lett. 15 (2005) 2711e2715.

[158] F. Belluti, A. Rampa, L. Piazzi, A. Bisi, S. Gobbi, M. Bartolili, V. Andrisano,A. Cavalli, M. Recanatini, P. Valenti, Cholinesterase inhibitors: xanthos-tigmine derivatives blocking the acetylcholinesterase-induced beta-amyloidaggregation, J. Med. Chem. 48 (2005) 4444e4456.

[159] L. Piazzi, F. Belluti, A. Bisi, S. Gobbi, S. Rizzo, M. Bartolini, V. Andrisano,M. Recanatini, Acetylcholinesterase inhibitors: SAR and enzyme inhibitoryactivity of 3-[u-(benzylmethylamino)alkoxy]xanthen-9-ones, Bioorg. Med.Chem. 15 (2007) 575e583.

[160] S.N. Kumar, P.K. Mukherjee, S. Bhadra, B.P. Saha, B.C. Pal, Acetyl cholines-terase inhibitory potential of a carbazole alkaloid, mahanimbine, fromMurraya koenigii, Phytother. Res. 24 (2010) 629e631.

[161] R.D. Howlett, R.A. George, E.D. Owen, V.R. Ward, E.R. Markwell, Commonstructural features determine the effectiveness of carvedilol, daunomycinand rolitetracycline as inhibitors of Alzheimer b-amyloid fibril formation,Biochem. J. 343 (1999) 419e423.

[162] L. Huang, A. Shi, F. He, X. Li, Synthesis, biological evaluation, and molecularmodeling of berberine derivatives as potent acetylcholinesterase inhibitors,Bioorg. Med. Chem. 18 (2010) 1244e1251.

[163] M. Rosini, E. Simoni, M. Bartolini, A. Cavalli, L. Ceccarini, N. Pascu,D.W. McClymont, A. Tarozzi, M.L. Bolognesi, A. Minarini, V. Tumiatti,V. Andrisano, I.R. Mellor, C. Melchiorre, Inhibition of acetylcholinesterase, b-amyloid aggregation, and NMDA receptors in Alzheimer’s disease: a prom-ising direction for the multi-target-directed ligands gold rush, J. Med. Chem.51 (2008) 4381e4384.

[164] W.G. Kim, K.M. Cho, C.K. Lee, I.D. Yoo, Terrelactone-A, a novel mero terpe-noid with anti-acetyl cholinesterase activity from Aspergillus terreus, Tetra-hedron Lett. 43 (2002) 3197e3198.

[165] I.D. Yoo, K.M. Cho, C.K. Lee, W.G. Kim, Isoterreulactone A, a novel mer-oterpenoid with anti-acetylcholinesterase activity produced by Aspergillusterreus, Bioorg. Med. Chem. Lett. 15 (2005) 353e356.

[166] J. Rouleau, B.I. Iorga, C. Guillou, New potent human acetylcholinesteraseinhibitors in the tetracyclic triterpene series with inhibitory potency onamyloid b-aggregation, Eur. J. Med. Chem. 46 (2011) 2193e2205.

[167] A. Khalid, Zaheer-ul-Haq, S. Anjum, M.R. Khan, Atta-ur-Rahman,M.I. Choudhary, Kinetics and structureeactivity relationship studies onpregnane-type steroidal alkaloids that inhibit cholinesterases, Bioorg. Med.Chem. 12 (2004) 1995e2003.

[168] Zaheer-ul-Haq, B. Wellenzohn, K.R. Liedl, B.M. Rode, Molecular dockingstudies of natural cholinesterase-inhibiting steroidal alkaloids from Sarco-cocca saligna, J. Med. Chem. 46 (2003) 5087e5090.

[169] J. Correa-Basurto, I.V. Alcantara, L. Michel, J. Espinoza-Fonseca, G. Trujillo-Ferrara, p-Aminobenzoic acid derivatives as acetylcholinesterase inhibitors,Eur. J. Med. Chem. 40 (2005) 732e735.

[170] J. Trujillo-Ferrara, L.M. Cano, M. Espinoza-Fonseca, Synthesis, anticholines-terase activity and structureeactivity relationships of m-aminobenzoic acidderivatives, Bioorg. Med. Chem. Lett. 13 (2003) 1825e1827.

[171] C.T. Sadashiva, J.N. Narendra Sharath Chandra, K.C. Ponnappa, T. Veera-basappa Gowda, K.S. Rangappa, Synthesis and efficacy of 1-[bis(4-fluorophenyl)-methyl]piperazine derivatives for acetylcholinesterase inhi-bition, as a stimulant of central cholinergic neurotransmission in Alzheimer’sdisease, Bioorg. Med. Chem. Lett. 16 (2006) 3932e3936.

[172] J. Trujillo-Ferrara, I. Vazquez, J. Espinosa, R. Santillan, N. Farfan, H. Hopfl,Reversible and irreversible inhibitory activity of succinic and maleic acidderivatives on acetylcholinesterase, Eur. J. Pharm. Sci. 18 (2003) 313e322.

[173] V. Alptüzün, P. Kapková, K. Baumann, E. Erciyas, U. Holzgrabe, Synthesis andbiological activity of pyridinium-type acetylcholinesterase inhibitors,J. Pharm. Pharmacol. 55 (2003) 1397e1404.

[174] P. Kapkova, V. Alptüzün, P. Frey, E. Erciyas, U. Holzgrabe, Search for dualfunction inhibitors for Alzheimer’s disease: synthesis and biological activityof acetylcholinesterase inhibitors of pyridinium-type and their Ab-fibrilformation inhibition capacity, Bioorg. Med. Chem. 14 (2006) 472e478.

[175] P. Kapkova, N. Stiefl, U. Sürig, B. Engels, K. Baumann, U. Holzgrabe, Synthesis,biological activity and docking studies of new acetylcholinesterase inhibitorsof the bispyridinium type, Arch. Pharm. (Weinheim) 336 (2003) 523e540.

[176] M. Pietsch, M. Gütschow, Synthesis of tricyclic 1,3-oxazin-4-ones and kineticanalysis of cholesterol esterase and acetylcholinesterase inhibition, J. Med.Chem. 48 (2005) 8270e8288.

[177] S. Darvesh, R.S. McDonald, A. Penwell, S. Conrad, K.V. Darvesh, D. Mataija,G. Gomez, A. Caines, R. Walsh, E. Martin, Structure-activity relationships forinhibition of human cholinesterases by alkyl amide phenothiazine de-rivatives, Bioorg. Med. Chem. 13 (2005) 211e222.

[178] M. Decker, Homobivalent quinazolinimines as novel nanomolar inhibitors ofcholinesterase with dirigible selectivity toward butyrylcholinesterase,J. Med. Chem. 49 (2006) 5411e5413.

[179] M. Decker, F. Krauth, J. Lehmann, Novel tricyclic quinazolinimines andrelated tetracyclic nitrogen bridgehead compounds as cholinesterase in-hibitors with selectivity towards butyrylcholinesterase, Bioorg. Med. Chem.14 (2006) 1966e1977.

[180] A. Paz, Q. Xie, H.M. Greenblatt, W. Fu, Y. Tang, Z. Qiu, J.L. Sussman, The crystalstructure of a complex of acetylcholinesterase with bis-(�)-nor-meptazinolderivative reveals disruption of the catalytic triad, J. Med. Chem. 52 (2009)2543e2549.

[181] H.R. Girisha, N.S. Chandra, S. Boppana, M. Malviya, C.T. Sadashiva,S.R. Kanchugarakoppal, Active site directed docking studies: synthesis andpharmacological evaluation of cis-2,6-dimethyl piperidine sulfonamides asinhibitors of acetylcholinesterase, Eur. J. Med. Chem. 44 (2009) 4057e4062.

[182] G. Lin, C.Y. Lai, W.C. Liao, Molecular recognition by acetylcholinesterase atthe peripheral anionic site: structureeactivity relationships for inhibitionsby aryl carbamates, Bioorg. Med. Chem. 7 (1999) 2683e2689.

[183] G. Lin, H.C. Tseng, A.C. Chio, T.M. Tseng, B.Y. Tsai, A rate determining stepchange in the pre-steady state of acetylcholinesterase inhibitions by 1,n-alkane-di-N-butylcarbamates, Bioorg. Med. Chem. Lett. 15 (2005) 951e955.

[184] C. Mustazza, A. Borioni, M.R. Del Giudice, F. Gatta, R. Ferreti, A. Meneguz,M.T. Volpe, P. Lorenzini, Synthesis and cholinesterase activity of phenyl-carbamates related to Rivastigmine, a therapeutic agent for Alzheimer’sdisease, Eur. J. Med. Chem. 37 (2002) 91e109.

[185] R. Sheng, X. Lin, J. Li, Y. Jiang, Z. Shang, Y. Hu, Design, synthesis, and evalu-ation of 2-phenoxy-indan-1-one derivatives as acetylcholinesterase in-hibitors, Bioorg. Med. Chem. Lett. 15 (2005) 3834e3837.

[186] H.A. Jung, Y.J. Jung, S.K. Hyun, Selective cholinesterase inhibitory activities ofa new monoterpene diglycoside and other constituents from Nelumbonucifera stamens, Biol. Pharm. Bull. 33 (2010) 267e272.

[187] C. Seidl, B.L. Correia, A.E.M. Stinghen, C.A.M. Santos, Acetylcholinesteraseinhibitory activity of uleine from Himatanthus lancifolius, Z. Naturforsch. C 65(2010) 440e444.

[188] A.J. Guo, H.Q. Xie, R.C. Choi, K.Y. Zheng, C.W. Bi, S.L. Xu, T.T. Dong, K.W. Tsim,Galangin, a flavonol derived from Rhizoma Alpiniae Officinarum, inhibitsacetylcholinesterase activity in vitro, Chem. Biol. Interact. 187 (2010) 246e248.

[189] A.A.N. De Paula, J.B.L. Martins, M.L. Dos Santos, New potential AChE inhibitorcandidates, Eur. J. Med. Chem. 44 (2009) 3754e3759.

[190] J.M. López-Arrieta, L. Schneider, Metrifonate for Alzheimer’s disease,Cochrane Database Syst. Rev. 2 (2006) 1e88.

[191] P. Anand, B. Singh, N. Singh, A review on coumarins as acetylcholinesteraseinhibitors for Alzheimer’s disease, Bioorg. Med. Chem. 20 (2012) 1175e1180.
































































































































































































































[192] H. Tang, Y.B. Wei, C. Zhang, F.X. Ning, W. Qiao, S.L. Huang, L. Ma, Z.S. Huang,L.Q. Gu, Synthesis, biological evaluation and molecular modeling of oxoi-soaporphine and oxoaporphine derivatives as new dual inhibitors ofacetylcholinesterase/butyrylcholinesterase, Eur. J. Med. Chem. 44 (2009)2523e2532.

[193] E.V.L. Da-Cunha, M.L. Cornelio, J.M. Barbosa-Filho, R. Braz-Filho, A.I. Gray,Eletefine, a stephaoxocane alkaloid from Cissampelos glaberrima, J. Nat. Prod.61 (1998) 1140e1142.

[194] J.Z. Deng, S.X. Zhao, 2-N-Methylexcentricine, a new alkaloid from roots ofStephania excentrica, J. Nat. Prod. 60 (1997) 294e295.

[195] N. Kashiwaba, S. Morooka, M. Kimura, M. Ono, Stephaoxocanine, a noveldihydroisoquinoline alkaloid from Stephania cepharantha, J. Nat. Prod. 59(1996) 803e805.

[196] T.S. Kaufman, A.B.J. Bracca, Synthetic approaches to carnigine, a simple tet-rahydroisoquinoline alkaloid, Tetrahedron 60 (2004) 10575e10610.

[197] S. Lopez, J. Bastida, F. Viladomat, C. Codina, Acetylcholinesterase inhibitoryactivity of some Amaryllidaceae alkaloids and Narcissus extracts, Life Sci. 71(2002) 2521e2529.

[198] J. Ulrichova, D. Walterova, V. Preininger, V. Simanek, Inhibition of acetyl-cholinesterase activity by some isoquinoline alkaloids, Planta Med. 48 (1983)174e177.

[199] H. Tang, L. Zhao, H. Zhao, S. Huang, S. Zhong, J. Qin, Z. Chen, Z. Huang,H. Liang, Hybrids of oxoisoaporphineetacrine congeners: novel acetylcho-linesterase and acetylcholinesterase-induced b-amyloid aggregation in-hibitors, Eur. J. Med. Chem. 46 (2011) 4970e4979.

[200] M.I. Fernández-Bachiller, C. Pérez, L. Monjas, J. Rademann, M.I. Rodríguez-Franco, New tacrine-4-oxo-4H-chromene hybrids as multifunctionalagents for the treatment of Alzheimer’s disease, with cholinergic, anti-oxidant, and b-amyloid-reducing properties, J. Med. Chem. 55 (2012)1303e1317.

[201] J. Korabecny, K.Musilek, O. Holas, Synthesis and in vitro evaluation ofN-alkyl-7-methoxytacrine hydrochlorides as potential cholinesterase inhibitors inAlzheimer disease, Bioorg. Med. Chem. Lett. 20 (2010) 6093e6095.

[202] O. Weinreb, T. Amit, O. Bar-Am, M.B.H. Youdim, A novel anti-Alzheimer’sdisease drug, ladostigil neuroprotective, multimodal brain-selective mono-amine oxidase and cholinesterase inhibitor, Int. Rev. Neurobiol. 100 (2011)191e215.

[203] S. Rizzo, M. Bartolini, L. Ceccarini, L. Piazzi, S. Gobbi, A. Cavalli, M. Recanatini,V. Andrisano, A. Rampa, Targeting Alzheimer’s disease: novel indanone hy-brids bearing a pharmacophoric fragment of AP2238, Bioorg. Med. Chem. 18(2010) 1749e1760.























































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