Donepezil: an Important Prototype to the Design of New Drug Candidates for Alzheimer’S Disease

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Send Orders for Reprints to [email protected] 2 Mini-Reviews in Medicinal Chemistry, 2014, 14, 2-19 Donepezil: An Important Prototype to the Design of New Drug Candidates for Alzheimer’s Disease

Maria Cecília Rodrigues Simões1,2, Flávia Pereira Dias Viegas1,2, Marcella Soares Moreira1, Matheus de Freitas Silva1, Mariana Máximo Riquiel1, Patrícia Mattos da Rosa1, Maísa Rosa Castelli1, Marcelo Henrique dos Santos1,3, Marisi Gomes Soares1 and Claudio Viegas Jr1*

1Laboratório de Fitoquímica e Química Medicinal – LFQM, Instituto de Química, Universidade Federal de Alfenas, 37130-000, Alfenas-MG, Brazil; 2Programa de Pós-Graduação em Química, Universidade Federal de Alfenas, 37130-000, Alfenas-MG, Brazil; 3Faculdade de Farmácia, Universidade Federal de Alfenas, 37130-000, Alfenas-MG, Brazil

Abstract: Alzheimer’s disease (AD) is a progressive and incurable neurodegenerative disorder, with a dramatic socio- economic impact. The progress of AD is characterized by a severe loss in memory and cognition, leading to behavioral changing, depression and death. During the last decades, only a few anticholinergic drugs were launched in the market, mainly acetylcholinesterase inhibitors (AChEIs), with indications for the treatment of initial and moderate stages of AD. The search for new AChEIs, capable to overcome the limitations observed for rivastigmine and tacrine, led Sugimoto and co-workers to the discovery of donepezil. Besides its high potency, donepezil also exhibited high selectivity for AChE and a very low toxicity. In this review, we discuss the main structural and pharmacological attributes that have made donepezil the first choice medicine for AD, and a versatile structural model for the design of novel AChEIs, in spite of multipotent and multitarget-directed ligands. Many recent data from literature transdue great efforts worldwide to produce modifications in the donepezil structure that could result in new bioactive chemical entities with innovative structural pattern. Furthermore, multi-potent ligands have also been designed by molecular hybridization, affording rivastigmine-, tacrine- and huperzine-donepezil potent and selective AChEIs. In a more recent strategy, structural features of donepezil have been used as a model to design multitarget-directed ligands, aiming at the discovery of new effective drug candidates that could exhibit concomitant pharmacological activities as dual or multi- enzymatic inhibitors as genuine innovative therapeutic alternatives for the treatment of AD. Keywords: Acetylcholinesterase inhibitors, Alzheimer’s disease, Donepezil, Molecular hybridization, Neurodegenerative disorders, Rational drug design.

Alzheimer’s Disease (AD) is a neurodegenerative disorder of acetylcholine (ACh), accomplished by aggregation and characterized by an insidious onset and a complex chronic accumulation of extracellular -amyloid (A) peptide as senile multi-factorial progress, affecting hippocampus and frontal neuritic plaques, and intracellular formation of neurofibrillary cortex in brain. This devastating pathology manifests its tangles (NFTs), composed by a hyperphosphorylated form of symptoms as a severe loss in memory, language skills decline the microtubule-associated protein tau, oxidative stress, and and other cognitive impairments, with dramatic behavioral neuronal loss [1, 3, 4]. The Ca2+ ion plays an important role changes that progress to depression and, eventually, death. in the cerebral homeostasis, acting as a second messenger in Recent data points that AD is responsible for ca. 50-60% of the brain. The imbalance of Ca2+ is currently considered one all cases of dementia in people over age 65. It is estimated of the main causes of neurodegeneration due to A effects on that more than 4.5 million people have AD in the US and 18 the capacity of membrane cells to regulate their permeability million worldwide [1-3]. and internal concentration of ions Ca2+. The Ca2+-associated The etiology of AD remains unclear, however many neurodegeneration begins when A causes an increase in the pathophysiologic hallmarks of the disease have been ion influx as a result of the activation of the N-Methyl-D- disclosed and are currently well established. They involve a aspartate receptors by the neurotransmitter glutamate [3b]. complex network of interconnected factors such as a rapid Besides cognitive and motor changes, AD patients also onset of cholinergic dysfunction, with remarkable depletion present diverse behavioral alterations as irritability, anxiety, depression, disorientation and restlessness [4]. To date, AD remains incurable and with few available therapeutic *Address correspondence to this author at the Laboratório de Fitoquímica e alternatives to ameliorate cognition and life quality of the Química Medicinal – LFQM, Instituto de Química, Universidade Federal de patient, arousing special attention and efforts in the search Alfenas, 37130-000, Alfenas-MG, Brazil; Tel: +55-35-32991466; E-mails: [email protected]; and [email protected] for new effective drugs.

CURRENT AVAILABLE TREATMENT FOR AD plays an important role in the process of amyloid aggregation, and thus could be another target for AChE inhibitors that All current drugs available for the treatment of AD are specifically bind to this site or could act as multi-point only symptomatic, acting mainly as acetylcholinesterase ligands, leading to a subside in the senile plaque formation inhibitors (AChEIs) [3b]. Drugs from this therapeutic class [2]. are supported by the “Cholinergic Hypothesis” that points to restore the cholinergic deficit in central nervous system (CNS) THE DISCOVERY OF DONEPEZIL by selective inhibition of AChE enzyme, and thus result in a The search for a new AChE inhibitor capable to delay of the cognitive decline and in the control of AD overcome rivastigmine limitations in oral availability and symptoms [1, 2, 3]. other pharmacokinetic parameters (e.g. short duration) and During the last two decades, only few anticholinergic hepatotoxic effects of tacrine, led Sugimoto and co-workers drugs have been launched in the market, and are mainly from Eisai Co. to start in 1983 a broad research project indicated for the treatment of mild and moderate stages of involving tacrine derivatives. However, they initially failed the disease, such as tacrine (1), donepezil (2), rivastigmine to develop a new non-toxic and effective tacrine derivative (3) and galanthamine (4, Fig. 1). Another drug recently and turned their attention to a N-benzylpiperazine derivative, approved by FDA is memantine (5, Fig. 1), that acts as an that was originally synthesized and tested for arterial antagonist of glutamate receptors, being indicated for the sclerosis. This compound was tested for AChE inhibition treatment of moderate and severe stages of AD [1, 2, 5]. and showed a very poor activity (IC50 = 12600 nM), but its However, due to a number of adverse peripheral effects arising singular structural feature was intrigating and promising. from the excessive activation of cholinergic system, including Using this compound as a model, they synthesized more than confusion, hallucinations, behavioral abnormalities, nausea, 700 derivatives, to discover that replacing N-benzylpiperazine gastric irritation and hepatotoxicity, these drugs have a quite with N-benzylpiperidine moiety, a dramatic increase in limited clinical use [3b]. Besides the treatment with AChEIs potency for AChE inhibition was observed. In order to develop and memantine, many other therapeutic approaches, such new derivatives with better pharmacological profile for as the use of neurotrophic and anti-inflammatory drugs, clinical tests, they worked in structural optimization of antioxidant compounds and formulations, compounds that N-benzylpiperidine derivatives, and discovered compound could interfere in A-aggregation process have been exploited 1-benzyl-4-[2-(N-benzoylamino)ethyl]piperidine (6, Fig. 2) in the search for new effective therapeutic alternatives. In that showed an IC50 = 560 nM in AChE inhibition [6a, b]. As this context, the more recent approach that has emerged to a new strategy in drug design, they decided to explore the support the design of more effective chemical entities have replacement of an amide group in 6 by a ketone moiety, but considered the multifactorial and complex interconnected with no improvement in activity. Furthermore, the replacement and, in some cases parallel or simultaneous, biochemical of the aliphatic amide (6) with cyclic amide subunit led pathways in AD. This strategy is called Multi-target directed to derivative 7. In a preliminary evaluation, compound ligands (MTDLs) or multifunctional ligands, that is based on 1-benzyl-4-(2-isoindolin-2-ylethyl)piperidine (7) was identified the fact that using a one-target-direct drug, it is not always as the most potent of the series (IC50 = 98 nM), and was likely that the therapeutical effect will be effective to block selected as a new prototype for further structure-activity disease evolution [3b]. studies, including variation in many other structural subunits and also structurally constrained derivatives. A comparative Considering that AChE enzyme presents two main analysis of pharmacological data revealed that 2-indolinone binding sites for drug interaction, the catalytic site and the subunit of compound 7 could be replaced with an indanone peripheral anionic site (PAS), another hypothesis to be moiety, without significant loss in AChE inhibitory potency. considered is the effect of AChE enzyme in the A peptide Among several indanone derivatives structurally inspired on aggregation. It is known that AChE anionic binding site compound 7, compounds 8 and 9 (Fig. 2) exhibited the best

O NH2 CH3 CH3

N O CH3 N O N CH O CH H C 3 3 N 3 2 H C O 3 1 3

NH2 H3C O N CH3 O CH3

H3C 5 OH 4

Fig. (1). Current commercial available drugs for the treatment of AD. 4 Mini-Reviews in Medicinal Chemistry, 2014, Vol. 14, No. 1 Simões et al.

N O N O . HCl . HCl N N H 6 7

O N . HCl O . HCl N

8 9

Fig. (2). Compound 1-benzyl-4-[2-(N-benzoylamino)ethyl]piperidine (6) and its derivatives 7-9 with improved AChE inhibitory properties.

results, with IC50 values of 230 and 150 nM, respectively. aromatic residues Trp84 and Trp279 (Fig. 3). On the other Further modifications in the structure of compound 9 led hand, the replacement of the piperidine ring with the bioisoster to the dimethoxy-derivative 1-benzil-4-[5,6-dimethoxy-(1- piperazine ring provokes a dramatic 19-fold decrease in indanone)-2-yl]-methylpiperidine hydrochloride (2, Fig. 1) affinity by AChE [10]. that was shown to be the most potent inhibitor with an IC50 = 5.7 nM and was selected as the lead-molecule of that series [6a, b]. This new compound, also known as E2020, has shown to be an AChE inhibitor (IC50 = 5.7 nM) 1250-fold more selective than for butyrylcholinesterase (BuChE, IC50 = 7138 nM), which is more selective than physostigmine (IC50 AChE/ IC50 BuChE = 11.9) and tacrine (IC50 AChE/ IC50 BuChE = 0.9). In vivo assays showed that E2020 has a longer duration effect than physostigmine at a dose of 5 mg/Kg (v.o) and causes a remarkable increase in ACh levels in the cerebral cortex of rats, without significant toxicity in short term studies [7]. The efficacy of donepezil was also evaluated in double- blind, placebo controlled, phase II and III trials with more than 1000 volunteers, revealing that its use resulted in important improvements in memory, concentration, language skills and reasoning, without signs of hepatotoxicity [8]. Face to these results, donepezil was approved by FDA in 1996, with indications for the treatment of mild and moderate AD. STRUCTURAL FEATURES AND MECHANISM OF ACTION OF DONEPEZIL In the structure of donepezil, N-benzylpiperidine and indanone moieties were identified as important interaction binding sites with AChE, and are responsible for inhibitory selectivity. In spite of donepezil had been developed as a racemic mixture, with both enantiomers exhibiting the same activity [9], the eutomer is the R isomer, which exhibited 5-fold more affinity for AChE (Ki = 3.35 nM) than the Fig. (3). Tridimensional structure of E2020 and its interaction S isomer (Ki = 17.5 nM) [10]. Donepezil is recognized by binding sites with AChE. AChE by interactions in the middle gorge of the active site of the enzyme, mainly by three subunits: the benzyl moiety, In the pharmacodynamics, donepezil acts as an anti- the nitrogen atom at the piperidine ring, and the dimethoxy- cholinesterase drug, increasing the intra-synaptic acetylcholine indanone portion (Fig. 3). These interactions involve direct (ACh) levels and, as a consequence, alleviates the cholinergic contacts mediated by water molecules that seem to be crucial symptoms of AD. The adverse cholinergic effects of donepezil for binding and specificity. Some studies with other donepezil are insignificant, and other transient effects include tremor, analogues evidenced the importance of the two aromatic nausea and vomit. Donepezil has a linear absorption, reaching functional groups in the structure of the inhibitor, that probably maximum plasmatic concentration within 3-5 hours after play an essential role in - stacking interactions with the administration, without influence of feeding. It also exhibits Donepezil: An Important Prototype to the Design of New Drug Candidates Mini-Reviews in Medicinal Chemistry, 2014, Vol. 14, No. 1 5 a high capacity in the transposition of blood-brain barrier O (BBB), reaching a cerebral concentration of 7-fold higher than in plasma, being therefore considered an inhibitor of N central action. Donepezil is mainly metabolized in the liver, and excreted by renal via, as well as its metabolites, requiring prudence in administration to patients with renal and hepatic insufficiency [8, 11]. NH THE STRUCTURE OF DONEPEZIL AS A MODEL FOR RATIONAL DESIGN OF NEW DRUG CANDIDATES 10 The Design of New Acetylcholinesterase Inhibitors by Structural Modifications in Donepezil Structure Fig. (4). Structure of the new structure-based donepezil analogue [(1-benzyl-4-oxopiperidin-3-ylidene)methyl]-1H-indol (10). In 2001, Andreani and co-workers synthesized a new family of donepezil analogues, planned by molecular modeling, In another work, Contreras and co-workers investigated aiming to obtain new AChEIs structurally more compact and minaprine (11) derivatives (Fig. 5) with the goal of identify a less flexible than the original prototype, but keeping its more potent and selective AChE inhibitors, drawn from donepezil major spatial features and physic-chemical properties. Among all tested compounds, 3-[(1-benzyl-4-oxopiperidin- molecule as a model. They prepared four new compounds 3-ylidene)-methyl]-1H-indol (10, Fig. 4) showed the best (12-15), with a phenylpiridazine or a benzylpiperidine subunits in order to investigate their pharmacophoric contribution in the activity with an IC50= 6.0 ± 2 M, but less potent than the improvement of AChE inhibitory activity in comparison to reference drugs tacrine (IC50 = 0.18 ± 0.05 M) and compound 11 (IC50 = 85 M) [13]. donepezil (IC50 = 0.04 ± 0.01 M). Further computational studies revealed that probably this low inhibitory potency Among all four compounds tested, compound 12 showed could be attributed to a significant loss in the interaction of the weaker inhibitory activity with an IC50= 5.4 M. The the ligand and the Trp279 residue, which could be decisive insertion of an electronegative chlorine atom in the benzene for molecular recognition. Moreover, rigidity of [(4- ring of 13 resulted in a significant increase in inhibitory activity oxopiperidin-3-ylidene)-methyl]-indole moiety could make (IC50= 1.3 M). However, the most active compounds were more difficult the passage of the ligand through the gorge 14 and 15, containing a central piridazine subunit and a that conducts to the enzyme active site, blocking the access spacer-carbon side-chain between the piridazine ring and the of the ligand to the binding site, although the new ligand was tertiary amine. Compound 14 exhibited an IC = 0.74 M, capable to reach the enzymatic target with adequate lipophilicity 50 but compound 15, the most active of that series, disclosed (log P = 3.35) [12]. the pharmacophoric importance of the benzylpiperidine

O NH N NN 11

Lead optimization Strucutre-activity relationship

CH3 CH3

NH N Cl NH N NN NN 12 13

NH NH NN NN

14 N H 15 N

Fig. (5). Chemical structure of minaprine (11) and its derivatives 12-15 with AChE inhibitory activity. 6 Mini-Reviews in Medicinal Chemistry, 2014, Vol. 14, No. 1 Simões et al. group for AChE inhibition, showing to be the most promise stacking interaction with Trp84 residue more suitable. This inhibitor of the series with an IC50= 0.12 M [13]. observation was confirmed in vitro in which compound 18 showed the best inhibitory profile in the series with an IC50 = Omran and co-workers synthesized other 64 new AChE 0.06 M [14]. inhibitors, also using the donepezil structure as a scaffold model. They modified the structure of donepezil by a central In order to explore other suitable modifications on the amide-piperazine subunit and the nature of the aromatic pharmacophoric dimethoxy-indanone and benzylpiperidine moiety in the indanone portion. In this series, 11 new subunits in the structure of donepezil, Belluti and co-workers compounds showed to be potent AChE inhibitors in a proposed a novel series of benzophenone analogues with an nanomolar range. Derivative 18 (Fig. 6) exhibited an inhibitory insertion of a tertiary amine in the side chain. Among the activity (IC50 = 0.06 M) comparable to donepezil (IC50 = sixteen new donepezil-based analogues of this series, 0.02 M), used as a standard drug. This compound was planned compounds 19 and 20 demonstrated the best activity profile, from derivative 16, that has showed a weak inhibitory inhibiting AChE with IC50 values of 0.46 and 0.57 M, activity (IC50= 4 M), being 200-fold less potent than respectively, but less potent than the prototype donepezil. donepezil. In the first efforts to get better inhibitory profile for compound 16, indanone subunit was replaced with an Nevertheless, it was observed that the presence of a hidroxyimino-di-bromo-cyclopentatiophene system, leading benzyl group linked to an amine nitrogen atom is essential to compound 17, that showed a significant improve in for AChE inhibitory action. Probably, this effect is due to a more suitable fit in the enzyme cavity, once 6 of the 16 inhibitory potency (IC50= 0.56 M). Further studies by molecular modeling were indicative that the presence of compounds tested exhibited weak inhibitory activity, when halogen substituents like F or Cl atoms in the orto position benzylamine moiety was modified by other functional of the benzyl system could change the spatial orientation of groups or substituents [15]. the benzylpiperazine subunit, and then makes the - –

O H3CO

H3CO O HN

N N Molecular modification 16 Lead optimization F HON Br O N Molecular modification Br S Activity optimization N Structure-activity relationship N S H Br Br 18 17

Fig. (6). Chemical structures of the donepezil-based amide-piperazine derivatives 16-18 with remarkable acetylcholinesterase inhibitory activities.

O O H3C CH3

H3C N O 19 O O H3C Conformational restriction annelation-aromatization of the side chain H3C N ring oppening O 2 O CH3 O H3C CH2

H3C N O 20 Fig. (7). Some donepezil-based benzophenone analogues with AChE inhibitory properties. Donepezil: An Important Prototype to the Design of New Drug Candidates Mini-Reviews in Medicinal Chemistry, 2014, Vol. 14, No. 1 7

A new series of N-benzylpiperidine-benzoxazol derivatives keeping the dimethoxy-indanone moiety of 2 and inserting was synthesized by Villalobos and co-workers, with the goal N-dissubstituted O-phenoxy-benzylamine subunit with of evaluate new bioisosteric analogues of donepezil, maintaining different substituents at nitrogen atom (23, Fig. 9). inhibitory properties, but without a stereogenic center in the Pharmacological evaluation of this series revealed that original structure. The benzoxazol system was elected as a possible non-classic bioisoster of the original dimethoxy- the nature and the position of the substituents bound to the benzylamine nitrogen atom are relevant for AChE inhibitory indanone system (2, Fig. 8), leading to compounds 21 e 22 as activity. Compounds with substituents in the para-position potential AChE inhibitors. Both compounds had shown to be were the most active, as well as those with the presence of a the strongest AChE inhibitors in the series, with IC values 50 pyrrolidine group that showed the best pharmacophoric of 2.8 and 0.80 nM, respectively [16]. These results proved contribution. Compound 26 (Fig. 9) was the most active to be an important contribution to the rational design of new donepezil-based derivatives, where the N-benzylpiperidine- (IC50= 0.050 M), followed by compound 24 with an IC50= 0.10 M. The other compounds of the series were weaker benzoxazol scaffold could be exploited as a novel simplified AChE inhibitors, with compounds 23 (IC = 0.15 M) and framework suitable in the design of AChE inhibitors. 50 25 (IC50= 0.13 M) being almost equipotent to compound Design and Synthesis of Novel Donepezil-rivastigmine 24. Molecular docking data showed that compound 26 was Hybrid Compounds capable to interact with AChE very similarly to donepezil, which corroborate the observed in vitro results [17]. As mentioned above, the main pharmacophoric groups in donepezil, responsible for its adequate recognition by AChE, Shen and co-workers reported in 2008 the synthesis and are the 5,6-dimethoxy-indan-1-one and N-benzylpiperidine pharmacologic evaluation of another series of donepezil- subunits. Otherwise, in rivastigmine, the carbamoyl group rivastigmine hybrid derivatives (29). They planned a new and dialkylbenzylamine subunits play important roles in the structural pattern for AChE inhibitors based on changes in interaction with the enzyme, including in the active site. nature of the spacer between the benzene ring and the tertiary Thus, Sheng and co-workers proposed a new series of hybrid amine group of donepezil, aiming to evaluate the influence derivatives of donepezil (2) and rivastigmine (3, Fig. 9), on the enzymatic affinity. It was proposed a 2 to 4-carbon

O R1 N O H CO 3 Bioisoterism

H3CO N Strcuture-activity optimization N 2 21: R1=NHCOMe

22: R1= N O

Fig. (8). Structure of new N-benzyl piperidine-benzoxazol derivatives (21 and 22) with improved AChE inhibitory activity.

O O H3C

H3C O N 2 O O O Molecular O Hybridization N O O R2 N R N 3 R1

3 23: R1=CH3, R2= CH3, R3= CH3 ( para) 24: R2= H, R2=CH2-CH3, R3= CH2-CH3 ( para) 25: R1=CH3, R2,R3=piperidine (para) 26: R1=H, R2, R3= pyrrolidine (para) 27: R1=H, R2,R3=N-methyl piperazyne (para) 28: R1=H, R2,R3= morpholine ( para)

Fig. (9). New series of 2-phenoxyindan-1-one derivatives (23-28) designed by molecular hybridization between donepezil (2) and rivastigmine (3). 8 Mini-Reviews in Medicinal Chemistry, 2014, Vol. 14, No. 1 Simões et al.

O O O Molecular modification O R1 O O N X O Structure-activity O R2 optimization

29 N 30: X=(CH2)2, R1,R2= piperidine H 31: X= (CH2)2, R1= CH2-CH3, R3=CH2-CH3 32: X= CO(CH2)2, R1=CH2-CH3,R3= CH2-CH3 33: X=CO(CH2)2,R1=CH3,R2=CH2-CH3 Fig. (10). Chemical structures of compounds 29-33 planned as new AChE inhibitors by molecular hybridization of donepezil and rivastigmine. spacer in the side chain connecting the aromatic ring and the phtalimide bioisoster to be recognized by the PAS of the amino group, was proposed in different positions (compounds enzyme, connected by spacer subunits with different lengths. 30 and 31), besides the insertion of a carbonyl group in A series of 22 new tacrine-donepezil hybrid compounds was compounds 32 and 33 (Fig. 10). synthesized and assayed against AChE (from bovine erythrocytes) and human BuChE. The selective inhibitory Among all the 27 new substances synthesized, compounds potency was evaluated by the Ellman’s method [19], revealing 30 (IC = 37.3 nM) and 31 (IC = 42.1 nM), with two 50 50 phtalimide derivatives 34 (IC = 2.8 nM; IC = 75 methylene groups in the side chain, showed the higher AChE 50 AChE 50 BuChE nM) and 35 (IC = 2.4 nM; IC = 90 nM) (Fig. 11), inhibitory activities, being more potent than all other 50 AChE 50 BuChE as the most active and selective from the series, showing derivatives with three or four-carbon as spacer subunits between 26.8-fold and 37.5-fold higher selectivity for human AChE, the aromatic ring and the amino group. These results confirmed respectively. that the size of the spacer subunit is decisive for a favorable interaction between the ligand and the enzyme. On the other Based on these results, compound 35 was selected as a hand, compounds that possess a carbonyl group between the lead-molecule for further studies by molecular modeling. aromatic ring and the tertiary amine showed inhibitory Computational results suggested that this compound could properties similar to donepezil, with compounds 32 and 33 be, as expected, capable to interact with both binding sites of showing the highest AChE inhibitory activity with IC50 = AChE. Thus, compound 35 is a genuine example of a novel 0.78 nM and 1.85 nM, respectively. Taking into account both promising drug candidate for AD treatment exhibiting a dual molecular modifications, compound 32, with an ethyl ketone mechanism of action [20]. subunit as a spacer moiety, exhibited the best anticholinesterase More recently, Camps and co-workers used the same profile and revealed a new scaffold to be exploited in the approach to draw other donepezil-tacrine hybrid compounds search for novel effective and multipotent AChE inhibitors (36-39,Fig. 12) as dual inhibitor candidates capable to interact [18]. simultaneously with PAS and with some amino acid residues New Hybrid Donepezil-Tacrine Prototypes at the midgorge of AChE. The structural feature of these new hybrid molecules contemplate the basic structure of donepezil, The search for AChE inhibitor drug candidates with an with an insertion of a tacrine subunit as a substituent on the innovative mechanism of action, led Alonso and co-workers piperidine nitrogen atom, and insertion of a variable spacer [20] to design a series of new donepezil-tacrine hybrid unit and the substituent groups on acridine moiety. All compounds that could inhibit AChE by simultaneous compounds of this series were able to inhibit both human interaction with two different binding sites, the PAS and AChE and bovine BuChE in a nanomolar or subnanomolar catalytic site. The molecular architecture of these ligands range. Some of these compounds, especially compounds 36 combine the 6-chlorotacrine subunit for interaction with the and 37 that possess a donepezil-indanone subunit, also showed active site, and the indanone moiety of donepezil or its significant A anti-aggregation properties, which make these

O O N N N O N H N NH N N O O H 35 CH3 34

Fig. (11). Chemical structure of phtalimide derivatives 34 and 35, designed as new donepezil-tacrine hybrid compounds. Donepezil: An Important Prototype to the Design of New Drug Candidates Mini-Reviews in Medicinal Chemistry, 2014, Vol. 14, No. 1 9 compounds promise and innovative anti-Alzheimer drug activity of BuChE (IC50= 3500 nM), but exhibited a higher candidates [21]. potency in inhibition of AChE from Electrophorus electricus (Ee) (IC50= 53 nM). Its apparent ability for dual interaction X with AChE was confirmed by molecular modeling studies MeO that revealed favorable and effective interactions at the catalytic site and PAS, concomitantly. Optimization of compound 41 led to the homologue series 42, and the MeO 36, X=O; R=H; n=2 discovery of the more conformationally restricted compound 37, X=O; R=Cl; n=2 N 43 that showed a significant selective AChE inhibitory activity 38, X=H,H; R=H; n=2 (IC50= 37 nM) and also a moderate and selective inhibitory n activity of MAO-A (IC = 41 M). Face to these results, HN 39, X=H,H; R=Cl; n=2 50 compound 43 was selected as the lead-molecule in the series, with a new structural pattern for optimization and development of multi-target directed drug candidates for the treatment of N R AD [22a-d]. Fig. (12). Chemical structure of tacrine-donepezil hybrid compounds In another approach for drug design, Shao Dong and co- 36-39 with selective and multipotent properties proposed by Camps workers also used tacrine and donepezil as models for and co-workers. planning a new series of hybrid inhibitors of AChE. Among five new molecules, compound 44 (Fig. 17) was the most In another work, Samadi and co-workers also proposed a potent and selective for AChE inhibition, with as IC50 = 6.0 new class of tacrine-donepezil hybrid analogues (40, Fig. 13), nM, being 37-fold more potent and 31-fold more selective aiming for the discovery of multipotent inhibitors of than tacrine (IC50= 223 nM). Molecular docking studies monoamine oxidase A and B (MAO-A and MAO-B), AChE confirmed that the high affinity for AChE of compound 44 is and BuChE. The first selected active derivative from probably due to multiple interactions involving the acrydine this series, was compound 40, with the pharmacophore subunit, the benzyl system and the nitrogen atom of the N-benzylpiperidine of donepezil, which was additionally piperidine nucleus. On the other hand, compound 45 (Fig. 17) modified to furnish compound 41, that showed poor inhibitory had shown to be the second best AChE inhibitor with an IC50

H R1 NC CN NC CN

SAR studies 2 N N n N N N H N H H N R2 40 41 (R1 = H or Ph; R2 = H or Me; n = 0, 1, 2, 3)

Lead compound Optimization Annelation

R1 CN

n NH N N N N

42 Me (R1 = H or Ph; n = 0, 1, 2, 3)

Pharmacological optimizartion New lead- compund discovery

H CN

N N N N Me 43

Fig. (13). New tacrine-donepezil hybrid compounds 40-43 with multipotent activity as inhibitors of AChE and MAO-A and B. 10 Mini-Reviews in Medicinal Chemistry, 2014, Vol. 14, No. 1 Simões et al.

N O N H N N N H H N NH 2 45 O 44

O N O H H N MeO N N N HN H H O N MeO 46 47

Fig. (14). New tacrine-donepezil-based AChE inhibitors 44-47 proposed by Shao Dong and co-workers.

value of 10.2 nM. Otherwise compounds 46 and 47 (Fig. 17) showing a very higher activity with an IC50 value of 17.2 were the most selective, but weaker inhibitors with IC50 M. Its apparent ability to interact with both the active site values of 230 and 869 nM, respectively [23]. and PAS of AChE could indicate compound 50 as a potential and efficient new donepezil-huperzine-A active hybrid Donepezil and Huperzine-A as Model Prototypes for New Drug Candidates inhibitor, with an innovative and very singular molecular framework to be further exploited in the development of new Huperzine-A (48), is an alkaloid isolated from Huperzia drugs for AD [25]. serrata with a reversible and high potent AChE inhibitory New Multi-target Directed Ligands Designed from the activity, exhibiting lower toxicity than tacrine and donepezil Structure of Donepezil [24]. Due to these quite particular relevant properties, Zeng and co-workers elected this molecule as a model for the In 2003, Piazzi and co-workers used the MTDLs design of a novel molecular pattern of donepezil-huperzine- approach for the design of a new class of AChE inhibitors, A hybrid ligands. Molecular docking studies and capable to partially block the enzyme activity on -amyloid conformational analysis revealed that compound 49 (Fig. 15) residues. Initially, they evaluated in silico a series of inhibitors could probably be a weak cholinesterase inhibitor in based on computational sequential docking (DOCK 4.1) over comparison to E2020, showing an IC50= 190 M. Structural human AChE, leading to the selection of ligand AP2238 optimization led to compound 50, that presents two methoxy (51, Fig. 16), with a similar structure to donepezil, and substituents at carbonyl and hydroxyl positions related to 49, exhibiting a bioisosteric dimethoxy-coumarin subunit and a

O H3CO H H N O CH3 H3CO N O N H3C 2 O Strcuture-activity CO HO optimization H3C NH H3C HN Molecular H hybridization N N N O 50 49 H2N 48

Fig. (15). New donepezil-huperzine-A hybrid derivatives proposed by Zeng and co-workers.

O O O H3C

H3C O CH3 N

Fig. (16). Compound AP2238 (51) designed by Piazzi and co-workers as the first drug candidate prototype with dual anti-amyloid and AChE inhibitory properties. Donepezil: An Important Prototype to the Design of New Drug Candidates Mini-Reviews in Medicinal Chemistry, 2014, Vol. 14, No. 1 11

O O O

H3CO S N

F H3CO OH H H N N N

2 O 52 O Molecular Hibridization

New molecular scaffold for dual AChE-BACE-1 inhibitor drug prototypes

R2 n

H H N N N N R1

O O

O Molecular optimization O SAR studies S N

F OH N

NH NH NH

O O O

Molecular optimization O O SAR studies S N

F OH H NH NH N

O O O N

O O S N

OH H NH NH N

O O N

Fig. (17). Design of a new series of hybrid drug candidates synthesized by Zhu et al. with dual and multi-target directed properties. 12 Mini-Reviews in Medicinal Chemistry, 2014, Vol. 14, No. 1 Simões et al.

N-benzylamino group. This compound was synthetized and respectively. Among the more active molecules, compound tested for inhibition of both AChE and A-aggregation, and 55 exhibited the best multiple mode of action, showing a the results disclosed this molecule as a novel innovative drug high potency in inhibition of both AChE and BACE-1, with candidate, showing an IC50= 44.5 nM for AChE inhibition. IC50 values of 1.83 and 0.567 M, respectively. This compound The structure-activity analysis pointed out a discrete relationship also showed a significant antioxidant effect in a H2O2-induced between the interactions of compound 51 with peripheral cytotoxicity assay. In addition, in vivo studies demonstrated sites of AChE and its dual inhibitory activity of both the that an intraventricular administration of compound 54 was enzyme and A-aggregation. Compound 51 showed a higher capable to diminish the A1-40 production in transgenic rats. ability to inhibit A-aggregation than donepezil, with also a These data evidenced the multiple-target properties for high selectivity for AChE [26]. This was the first compound compound 54 and that this molecule could be an interesting in the literature that was planned as a dual ligand for the PAS and promising drug candidate for AD [27a]. of AChE and inhibitor of A-aggregation. Further computational studies evidenced that one of the main structural subunits Recently, the coumarin scaffold also inspired Catto and responsible for binding of compound 51 at the AChE anionic co-workers in the design of a new series of substituted site was the benzyl group that makes strong - stacking coumarins, and in particular 6,7-dimethoxy-coumarine- interactions with the indole ring of Trp86 residue [26]. derivatives tethered to a benzylamine moiety as potential and selective dual binding site inhibitors of acetylcholinesterase In another approach, Zhu and co-workers also searching [27b]. Among the eighteen coumarin derivatives, varying for dual drug candidates of therapeutic use for AD planned a substituents on the 6,7-positions of the coumarin subunit and series of dual inhibitors of AChE and -secretase (BACE-1). the nature of the substituent on the -carbonyl position, the The structural design was based on the combination of the most potent amide derivative 56 (Fig. 18) exhibited AChE pharmacophoric N-benzylpiperidine subunit from donepezil inhibitory potency (IC = 0.0076 M, from bovine erythrocytes) (2) and the diaminobenzyl moiety present in the structure of 50 comparable to the reference drug donepezil (IC50= 0.0042 M). BACE-1 inhibitor compound 52 linked by a substituted Further kinetic studies revealed that compound 56 acts as a tertiary amine spacer (Fig. 17). reversible, mixed-type inhibitor, in accordance to a dual In order to evaluate the contribution of the central spacer binding site mode of interactions with AChE, with a Ki= subunit in biological activity, the authors also synthesized a 8.6 nM. In addition, derivative 56 showed a good selectivity variety of other analogues based on the original scaffold. All for AChE (IC50 BuChE= 33.0 M) and an outstanding compounds were tested for AChE and BACE-1 inhibitory inhibitory activity against human AChE with an IC50 of 43 activity, revealing compounds 53-55 (Fig. 17) as the most nM. These biological results clearly indicate compound 56 active of the series. Compound 53 showed an IC50 values of as a multipotent drug candidate prototype that is still being 2.5 X 10-5 and 0.616 M for inhibition of AChE and BACE- carried on investigation for its anti-amyloid aggregation and 1, respectively, while compound 54 showed an IC50 values of enantioselective interactions on the AChE-binding sites 2.5 and 1.39 M for inhibition of BACE-1 and AChE [27b].

Coumarin scaffold N O R R 1 3 H3CO

R O O 2 H3CO 2 R1,R2= H, OCH3 R3= OH, NH2, CO2H

Molecular Hybridization

H3CO N N O H H3CO O O 56

Fig. (18). Design of a new series of coumarin-donepezil hybrid drug candidates, and discovery of the lead-compound coumarin-amide derivative 56 with dual and multi-target directed properties. Donepezil: An Important Prototype to the Design of New Drug Candidates Mini-Reviews in Medicinal Chemistry, 2014, Vol. 14, No. 1 13

In 2011, another recent publication by Bolea and residue. Moreover, a structure-activity relationship study was collaborators reported the synthesis and pharmacological conducted to study the influence of distinct substituents at evaluation of a new family of multi-enzymatic inhibitors positions 5 or 6 in the indanone system. Among 22 molecules capable to interact simultaneously with the enzymes AChE, in the target-series, compounds 60 and 61 (Fig. 20) were the BuChE, MAO-A and MAO-B. The structural architecture of most active, exhibiting inhibitory activity of human AChE the series 58 (Fig. 19) was drawn by molecular hybridization with IC50= 0.056 M and IC50= 0.052 M, respectively, in of the benzylpiperidine moiety of donepezil and the indolyl- comparison to donepezil (IC50= 0.02 M) and AP2238 (IC50 propargyl amine subunit present in the structure of the MAO = 0.044 M) [29]. inhibitor 57, connected by a methylene chain with variable The search for new AChE inhibitors capable to interact length. Compound 59 was identified from the series 58 as with multiple binding sites of the enzyme led Nadri and co- the most promising ligand, showing a good selectivity and workers to explore a series of flavonoid derivatives, based on high potency in the inhibition of MAO-A with an IC = 5.2 nM 50 the structure of aurones (62), that were documented as AChE (IC = 43.0 nM), but less potent and with poor selectivity 50 MAO-B inhibitors, and de donepezil hydrochloride 63 (Fig. 21). The for the AChE (IC = 0.35 M) and BuChE (IC = 0.46 M). 50 50 mechanism of action of this class of natural products seems Further molecular modeling and enzymatic kinetics studies to be related to multiple - stacking interactions between clarified that these inhibitors act in two binding sites of the planar aromatic systems of aurone molecule with diverse AChE, which could explain the inhibitory effect exerted over aromatic amino acid residues along the midgorge of AChE. A aggregation [28]. Compound 64 (Fig. 21) showed a very significant inhibitory Rizzo and collaborators reported a series of donepezil- activity with an IC50= 41 ± 7.94 nM, compared to donepezil based hybrid compounds, containing an additional (IC 50= 28 ± 6.62 nM). The introduction of a fluorine atom at pharmacophoric groupment from AP2238 (51), discovered the positions 2 or 4 of the benzyl subunit resulted in a some years earlier by Piazzi and co-workers [26, 29]. In the remarkable increase in anticholinesterase activity, as could molecular design, the authors modified the original structure be observed for compounds 65 (IC50= 10 ± 6.87 nM), that of 51 by the connection of an indanone nucleus from donepezil was 2.8-fold more potent than donepezil (IC50= 28 ± 6.62) and a phenyl-N-methylbenzylamine of AP2238, aiming to [30]. maintain -interactions with hydroxyl group of Tyr124

H3CO H3C O N H3CO N N 57 CH 2 3

Molecular Hibridization

H3C O Y N n X N

58 CH3

Structural and Pharamcologiucal Optimization

N H3C O N

59 N CH3

Fig. (19). Design of a new class of hybrid inhibitors of MAO and AChE (58) and compound 59 identified as a multipotent inhibitor of MAO and AChE.

14 Mini-Reviews in Medicinal Chemistry, 2014, Vol. 14, No. 1 Simões et al.

O H3CO H3CO O O

H3CO

H3CO CH3 N 2 N 51 Molecular Hibridization

O O R (H C)n 1 2 CH3 N R2

60 - n=5, R1=NEt2, R2=OCH3 61 - n=5, R1=C5H10N, R2=OCH3

Fig. (20). Chemical structure of compounds 60 and 61 with remarkable inhibitory active for human AChE discovered by Rizzo and co- workers.

O O Cl- O O

O O O R N 1 63 62 NH R2 Molecular Hibridization

O H

RO O .X N

64 - R=CH3, X=Br, Y=H Y 65 - R=CH3, X=Cl, Y=F

Fig. (21). Chemical structure of aurone-donepezil hybrid compounds 64 and 65 designed as multi-potent AChE inhibitors.

Currently, it is well known that indanone donepezil a consequence of the presence of a conjugated double bond. moiety is responsible for aromatic - stacking interactions This strategy led to a new series of seventeen conjugated- with PAS of AChE, which in turn contributes greatly for indanone derivatives 66 with various amine groups as molecular recognition, high affinity and selectivity of substituents on the 6-position of the indanone system linked donepezil (2) for AChE. Considering these findings, Meng by methylene spacers of different lengths and different and collaborators [31] proposed the combination of this aromatic rings conjugated to the carbonyl indanone group pharmacophoric subunit of 2 with aromatic and (Fig. 22). All compounds were evaluated for their AChE and heteroaromatic systems, linked by aldol reactions with the BuChE inhibitory properties. The most active compound goal to obtain new multipotent ligands with enhanced was the piperidinemethylene-pyridynil-indanone derivative 67 affinity for the PAS and potential metal-chelating activity as with an IC50= 0.0018 M for AChE, being 47-fold and 14-fold Donepezil: An Important Prototype to the Design of New Drug Candidates Mini-Reviews in Medicinal Chemistry, 2014, Vol. 14, No. 1 15

N O H3CO

H CO 3 2

Molecular Derivatization Biological Optimization

New indanone derivatives

H3CO H3CO

n N R1 O O O O X 67 N 66 Lead-Molecule Optimization

H3CO H3CO N N O N O O 68 69 O

Fig. (22). Structural scaffold of new indanone derivatives 66 and the most active AChE inhibitors and metal-chelating compounds 67-69 designed by structural optimization of donepezil. more potent than tacrine and donepezil, respectively. The search for new multifunctional ligands is capable to Compound 67 showed an IC50 value of 9.5 M for BuChE exert effective disease-modifying effects to reverse AD inhibition, and therefore the best selectivity for AChE (SI= manifestations, Mohamed and co-workers have proposed a 5248.6). Structure-activity comparative analysis revealed new group of heterocyclic, non-fused cholinesterase inhibitors that the best biological profile was achieved for those exploring a pyrimidine-2,4-diamine scaffold. Among these, derivatives with a two-methylene-piperidine substituent at 6- compounds 70 and 71 (Fig. 23) were identified as lead position of indanone subunit [31]. molecules due to their effects on AChE, BuChE and A- aggregation inhibition [32, 33]. More recently, the same Further structural modifications on the structure of the authors decided to explore the pyrimidine-2,4-diamine scaffold lead compound 67 led to the identification of the very potent in the development of new small molecules designed as non- 2-pyrydyl (68) and phenyl (69) derivatives with IC50 values fused heterocyclic-donepezil hybrid compounds (72, Fig. 23) of 0.0044 and 0.0091 M, respectively, for AChE inhibition. possessing multifunctional properties against cholinesterase Both compounds exhibited also a good selectivity for AChE, enzymes, amyloid aggregation (self and AChE-induced) and showing IC50 values of 7.48 and 2.24 M, respectively for BACE-1 [34]. A series of ten compounds were synthesized BuChE with good selectivity (SI=1703 (68) and 246 (69)). and evaluated in vitro for their ability to inhibit both human All three derivatives showed higher potencies in AChE AChE (hAChE) and equine BuChE, hAChE-induced and inhibition than the standard drugs tacrine, donepezil and self-induced A1-40 aggregation, as well as BACE-1. All galanthamine. The study of the mechanism of action of compounds exhibited a broad range of inhibition with compound 67 was indicative of a mixed-type inhibition for IC50 hAChE = 7.7-12.9 M; IC50 eqBuChE= 2.2-11.4 M; hAChE- this class of compounds, which consists of a both non- induced A1-40 aggregation = inactive to 59.3%; self-induced competitive and uncompetitive inhibition. Metal chelating A aggregation = inactive to 48.4% and IC = 0.6 2+ 3+ 2+ 1-40 50 BACE-1 abilities related to Cu , Fe and Zn for compounds 67-69 to >50 M. Structure-activity studies, in comparison with were also evaluated and all compounds showed ability to donepezil and galanthamine, disclosed derivative 73 (Fig. 23)as 2+ 3+ interact with Cu and Fe ions. Due to the best AChE the most promise multifunctional profile, with the best inhibitory activity, cholinesterase selectivity and metal-chelating selective inhibitory activities for hAChE (IC50= 9.90 M; properties, compound 66 (Fig. 22), with a piperidine group IC50 eqBuChE= 11.40 M), AChE-induced aggregation (59.3% linked to the indanone subunit by a 2-carbon spacer and a 4- inhibition, 1.3-fold more potent than galanthamine), as well pyridynil-aromatic system conjugated to the carbonyl indanone as good BACE-1 inhibition (34% at 10 M). The lead functionality, was considered as a potential multifunctional compound 73 also showed a good cell viability of 81% on drug candidate for the treatment of AD [31]. the MTT assay, suggesting that a pyrimidine-2,4-diamine 16 Mini-Reviews in Medicinal Chemistry, 2014, Vol. 14, No. 1 Simões et al.

NH N O NH H3CO N N

N N H3CO 2 N N N 71 N 70 CH3 Dual ChEI and dual mode A-aggregation inhibitor Selective AChEI and AChE-induced A-aggregation inhibitor Molecular Hibridization

OCH3

R1 OCH3

NH N N

N N N N H

72 N N H

Fig. (23). Structural design of a new series of pyrimidine-2,4-diamine-donepezil hybrid compounds 72 and the most active multifunctional derivative 73. scaffold serves as a suitable template for the design and AChE among all other indanone derivatives. In contrast, development of innovative multi-targeting small molecules compound 76 showed to be the highest BuChE inhibitor with -4 as drug candidates for AD treatment [34]. an IC50 value of 8.981 x 10 M. In another recent work, hydrazine-nicotinamide-donepezil CONCLUSIONS hybrid derivatives (74, Fig. 24) were planned by Zurek and During the last two decades, Alzheimer’s disease has co-workers as novel acetyl and butyrylcholinesterase inhibitors been the focus of enormous scientific and medical efforts [35]. In order to evaluate pharmacophoric contributions in and investments around the world, in a run against the time the 4-position of the piperidine ring, the effects of the for more effective and secure medicines and, the so waited, absence of the 3,4-dimethoxy pattern and the carbon length discovery for cure. This neuropathology has great social and in the spacer subunit between the indanone moiety and the nicotinamide group, six derivatives were prepared. All the economic impacts, due to the disastrous functional and behavioral impairments caused in patients, that usually have tested compounds showed higher inhibitory activities for -5 -4 death as the end point, after 8-10 years after the first AChE (IC50= 1.087 x 10 to 1.879 x 10 M) than for BuChE -4 -3 symptoms of AD are recognized. Since donepezil has been (IC = 8.981 x 10 to 2.659 x 10 M), but showed lower 50 launched in the Market in 1996s, this drug has attracted potency in AChE inhibition than donepezil (IC50= 0.601 x -5 special attention due to its AChE inhibitory potency, high 10 M). SAR studies revealed that N-benzylpiperidine moiety is essential for high potency and affinity for AChE, selectivity, low toxicity and good bioavailability. Thus, donepezil has also been exploited as molecular scaffold for as well as the alkyl linker length between the two subunits, design and development of new AChE inhibitor drug indanone and hydrazinonicotinamide. The penthylene-linked candidates. Moreover, molecular hybridization has been the derivatives revealed best activities than their propylene and main approach for rational drug design of new ligands, ethylene linked counterparts. The most potent inhibitor with capable to act as dual, multipotent and/or multi-target directed regards to AChE activity was compound 75 (IC50= 1.087 x 10-5), with the greatest affinity and higher selectivity for mechanisms of action. Donepezil: An Important Prototype to the Design of New Drug Candidates Mini-Reviews in Medicinal Chemistry, 2014, Vol. 14, No. 1 17

N O

H3CO

H3CO 2 pharmacophoric group modification SAR Studies Basic-Structure functionalization Side-chain homologation Prototype-Structure Modification

O n N O N H

74 NHNH2 . HCl R= H, CH2-C6H5 R n= 2, 3 or 5

O n N O N H

75-R= CH2-C6H5, n= 5 NHNH2 . HCl 76- R= H, n= 3 R

Fig. (24). Structural design of a new series of hydrazine-nicotinamide-donepezil hybrid derivatives (74) and compounds 75 and 76, the most active AChE and BuChE inhibitors, respectively.

In this concise review, we aimed to report the more of its ability to interact both with the active site and peripheral recent literature examples of the importance of donepezil in anionic site of AChE. the design, molecular optimization and computational studies In a more recent approach, hybrid donepezil-derivatives for the discovery of AChE inhibitors, that could exhibit have been exploited as multi-target directed ligands, in novelty in terms of structural pattern, pharmacodynamics and accordance to a new paradigm of “one drug, multiple targets” pharmacokinetic parameters. As a consequence, modifications that aims to control and treat multifactorial diseases by the on the basic donepezil structure led to the discovery of new classes of donepezil-based AChE inhibitors, as exemplified effect of a single drug with multiple targets involved in the installation, progress and severity of such pathologies. by benzyl-oxo-piperidine-indole analogue (10), phenyl- Considering the multi-factorial pathophysiological aspects pyridazine minaprine analogues (12-15), benzophenone of AD, novel donepezil-based hybrid compounds with high derivatives (19, 20) and N-benzyl piperidine-benzoxazol structural diversity, such as compounds 50-55 were derivatives (21, 22). Molecular hybridization between donepezil discovered as potential AChE and BACE-1 inhibitors. Other and commercial drugs rivastigmine and tacrine also led to the discovery of high potent and selective AChE inhibitors, indolyl-propargyl amine (e.g. 56, 58 and 59), phenyl-N- methylbenzylamine analogues of donepezil (e.g. 60-61) and and some of them with an additional action profile as anti- aurone-analogues (e.g. 64 and 65) were also identified as amyloid agents (compounds 36-39) and multipotent inhibitors potential multi-inhibitors, capable to interact with one or of monoamine oxidase A and B (MAO-A and MAO-B), more the enzymes related to AD, such as AChE, BuChE, AChE and BuChE as observed for compounds 40-47. MAO-A and MAO-B. Based on the same MTDLs approach, Huperzine-A, an abundant natural alkaloid obtained from Huperzia species with remarkable anti-cholinesterase properties other multifunctional compounds have been designed as cholinesterase inhibitors with additional metal-quelating and a currently drug approved for the treatment of AD in properties (e.g. 68, 69) and inhibitors of A-aggregation and China, have also led to the discovery of the promising BACE-1 (e.g. 73). donepezil-hybrid derivative 51, with dual property as a result 18 Mini-Reviews in Medicinal Chemistry, 2014, Vol. 14, No. 1 Simões et al.

These findings are representative of the more recent [4] Sugimoto, H.; Yamanishi, Y.; Limura, Y.; Kawakami, Y. efforts of many research groups around the world and the Donepezil hydrochloride (E2020) and others acetylcholinesterase inhibitors. Curr. Med. Chem., 2000, 7, 303-339. important role of donepezil in different design strategies and [5] Tumiatti, V.; Andrisano, V.; Banzi, R.; Bartolini, M.; Minarini, A.; hypothesis to discover innovative drug candidates for a more Rosini, M.; Melchiorre, C. Structure-activity relationships of effective treatment for AD. acetylcholinesterase noncovalent inhibitors based on a polyamine backbone. 3. Effect of replacing the inner polymethylene chain CONFLICT OF INTEREST with cyclic moieties. J. Med. Chem., 2004, 47(26), 6490-6498. [6] Sugimoto, H.;Tsuchiya, Y.; Sugumi, H.;Higurashi, K.; Karibe, N.; The authors confirm that this article content has no Limura, Y.; Sasaki, A.; Kawakami, Y.; Nakamura, T.; Araki, S.; conflicts of interest. Yamanishi, Y.; Yamatsu,K. Novel Piperidine Derivatives. Synthesis and Anti-Acetylcholinesterase Activity of 1-Benzyl-4-[2- ACKNOWLEDGEMENTS (N-benzoy1amino)et hyllpiperidine Derivatives. J. Med. Chem., 1990, 33(7), 1880-1881 b) Sugimoto, H. Pure Appl. Chem. 1999, This work was supported by grants from INCT de Fármacos 71, 2031-2037. e Medicamentos INOFAR, CNPq, Brazil, #573.564/2008-6) [7] Sugimoto, H.; Limura, Y.; Yamanishi, Y.; Yamatsu, K. Synthesis and and Universal Project (FAPEMIG, Brazil, #APQ-01299-10). Structure-Activity Relationships of Acetylcholinesterase Inhibitors: The authors are also grateful for the fellowships granted by 1-Benzyl-4-[(5,6-dimethoxy-l-oxoindan-2-yl)methyl]piperidine Hydrochloride and Related Compounds. J. Med. Chem., 1995, CAPES, CNPq, and FAPEMIG. 38(24), 4821-4829. [8] Viegas, F.P.D.; Simões, M.C.R.; Rocha, M.D.; Castelli, M.R.; Moreira, ABBREVIATIONS M.S.; Viegas Junior,C. Doença de Alzheimer: Caracterização, ACh = Acetylcholine Evolução e Implicação do Processo Neuroinflamatório. Rev. Virtual Quim., 2011, 4(3), 286-306. AChE = Acetylcholinesterase [9] Kawakami, Y.; Inoue, A.; Kawai, T.; Wakita, M.; Sugimoto, H.; Hopfinger, A.J. The Rationale for E2020 as a Potent AChEIs = Acetylcholinesterase inhibitors Acetylcholinesterase Inhibitor. Bioorg. Med. Chem., 1996, 4(9), 1429-1446. AD = Alzheimer Disease [10] Kryger, G.; Silman, I.; Sussman, J.L. Structure of acetylcholinesterase complexed with E2020 (Aricept®): implications for the design of new A = -amyloid protein anti-Alzheimer drugs. Structure, 1999, 7(3), 297-307. [11] Rogers, S. L.; Friedhoff, L. T. Pharmacokinetic and pharmacodynamic BACE 1 = -secretase profile of donepezil HCl following single oral doses. Br. J. Clin. Pharmacol., 1998, 46(suppl 1), 1-6. 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Received: August 10, 2013 Revised: November 05, 2013 Accepted: December 13, 2013

DISCLAIMER: The above article has been published in Epub (ahead of print) on the basis of the materials provided by the author. The Editorial Department reserves the right to make minor modifications for further improvement of the manuscript.

PMID: 24251806