Saxena AK and Saini, J Alzheimers Neurodegener Dis 2018, 4: 015 DOI: 10.24966/AND-9608/100015 HSOA Journal of Alzheimer’s and Neurodegenerative Diseases Review Article Abbreviation The Structural Hybrids of Ace- AD- Alzheimer’s Disease tylcholinesterase Inhibitors in ACh- Acetylcholine AChE- Acetylcholinesterase the Treatment of Alzheimer’s BChE- Butyrylcholinesterase Aβ- Amyloid Beta Disease: A Review CAS- Catalytic Anionic Site Ratni Saini and Anil K Saxena* PAS- Peripheral Anionic Site Division of Medicinal and Process Chemistry, CSIR Central Drug Research Institute, Lucknow, India CNS- Central Nervous System THA- Tetrahydroacridine DPPH- 1,1-diphenyl-2-picrylhydrazyl ABTS - (2,20-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) IC50- The half maximal Inhibitory Concentration APP- Amyloid Precursor Protein Abstract Introduction Alzheimer’s Disease (AD) is characterized by the loss of mem- ory and learning ability in elderly patients affecting large population Alzheimer’s Disease (AD) is the most common form of irrevers- worldwide. The enzyme Acetylcholinesterase (AChE), (E.C.3.1.1.7) ible neurological disorder of elderly patients that has affected large plays a major role in the hydrolysis of the released neurotransmit- population worldwide [1]. It is clinically characterized by progressive ter acetylcholine. Most of the clinically used drugs to treat AD are cognitive impairments or defined by a loss of memory and learning Acetylcholinesterase Inhibitors (AChEIs). These drugs can provide ability, together with a reduced ability to perform daily routine activ- symptomatic benefits only and suffer with loss of therapeutic poten- ities and adverse array of neuropsychiatric symptoms such as apathy, tial with time. Therefore, there is an urgent need of novel cholinester- verbal and physical agitation, irritability, anxiety, depression, delu- ase inhibitors with wider therapeutic window for the treatment of AD. sions and hallucinations [2]. Characteristic neuropathologic findings The strategies targeting the AChE enzyme along with other target(s) include selective neuronal and synaptic losses, extracellular neuritic like Butyrylcholinesterase (BChE), amyloid-β (Aβ), β-secretase-1 plaques containing the β-amyloid peptide and Neurofibrillary Tangles (BACE), metals (Cu2+, Zn2+, or Fe2+), antioxidant properties and free (NFTs) composed of hyperphosphorylated forms of the tau (τ) protein radical scavenging capacity have been mainly focused in the last [3]. Still existing treatments for mild to moderate AD include the use five years. A number of hybrid molecules incorporating sub-struc- of Acetylcholinesterase (AChE) inhibitors such as tacrine, donepezil tures with the desired well-established pharmacological profile into ivastigmine, galanthamine, (Figure1) and the use of N-methyl-D-as- a single scaffold have been investigated. The main sub structures partate (NMDA) antagonist memantine [4]. However, these drugs used in developing these molecules are derived from diverse chem- are unable to slow or prevent AD progression, rather can provide ical classes such as acridine, quinoline, carbamates, huperzine and symptomatic benefits only and suffer from major drawback of loss of other heterocyclic analogs. It has been followed by optimization of therapeutic potential with time. Thus, increasing daily doses in such activity through structural modifications of the prototype molecules circumstances increases the side effects until the pause of the treat- for developing the Structure Activity Relationship (SAR). This has led ment. The major side effects are specifically caused by the peripheral to the development of novel molecules with desired AChE inhibitory activity of these drugs on cholinesterase enzyme. As the average age activity along with other desirable pharmacological properties. This is increasing all over the world, and so the AD (66% in the developing review summarizes the current therapeutic strategies for the devel- countries), there is an urgent need for novel therapeutics, which could opment of these AChEI in the last seven years. act as anti-Alzheimer agents with less side effects than the known Keywords: Acetylcholinesterase; Alzheimer’s disease; Amyloid-β commercial drugs for the treatment of the AD. The clinical trial re- peptides; Catalytic anionic site, Dual binding site inhibitors; Medic- sults from current developmental pipeline [5] involving molecules inal chemical hybridization; Molecular hybridization; Multi-target-di- as anti-amyloid, anti-tau, neuroprotective (calpain inhibitor, estrogen rected ligands; Neurodegenerative disorder; Peripheral anionic site receptor beta agonist, Peroxisome Proliferator-Activated Receptor (PPAR)-gamma agonist, Gamma-Aminobutyric Acid (GABA)-recep- *Corresponding author: Anil K Saxena, Division of Medicinal and Process tor modulator), Phosphodiesterase (PDE)-9A inhibitor, Butyrylcho- Chemistry, CSIR Central Drug Research Institute, Lucknow, India, Tel: +91 9839012951; E-mail: [email protected] linesterase (BChE) inhibitor, 5-Hydroxytryptamine (5-HT)-6 receptor antagonist etc., but the success rate is very poor (0.4%) due to high Citation: Saxena AK, Saini R (2018) The Structural Hybrids of Acetylcholines- attrition rate, 99.6% indicate that the most of the candidate drugs are terase Inhibitors in the Treatment of Alzheimer’s Disease: A Review. J Alzhei- failing to meet the primary cognitive and functional end points [5-7]. mers Neurodegener 4: 015. Hence extensive efforts are made for safe and effective anti-Alzhei- Received: August 26, 2017; Accepted: February 14, 2018; Published: Feb- mer’s agents [8]. Among several targets explored, the AChE is the ruary 28, 2018 major target in providing clinically effective anti-Alzheimer’s agents. Citation: Saxena AK, Saini R (2018) The Structural Hybrids of Acetylcholinesterase Inhibitors in the Treatment of Alzheimer’s Disease: A Review. J Alzheimers Neurodegener 4: 015. • Page 2 of 25 • located at the bottom of the gorge and the second is Peripheral Anion- ic Site (PAS) which is about 15 Å above the catalytic active site and located at the entrance of the gorge [10]. According to the recently reported the crystal structures of human AChE (hAChE) in complex with Fasciculin 2 (FAS-2) and huprine W (PDB code 4BDT) the catalytic active site is lined by aromatic res- idues and itself can be divided into two sub-unites namely, catalytic triad (also known as esteratic sub-site) and anionic sub-site. The cat- alytic triad sub-site consists glutamic acid (E202), serine (S203) and histidine (H447), while anionic sub-site consists tryptophan (W86). Figure 1: Structures of tacrine and it analogs, donepezil, rivastigmine and The catalytic triad is involved in the hydrolysis of the neurotransmit- galanthamine. ter acetylcholine, while the quaternary group of the choline moiety interacts with of tryptophan (W86) of the anionic site of the CAS. The adjacent area of catalytic triad is known as “oxyanion hole” which is Cholinergic Hypothesis formed by glycine and alanine residues and interacts with carbonyl In search for novel therapeutic approaches towards AD, the mech- oxygen of ACh. Whereas the PAS consisting of tryptophan (W286), anism-based therapeutic approaches targeting β-amyloid, tau pathol- tyrosine (Y337) and phenylalanine (F338) is believed to guide the ogies and cholinergic hypothesis have been mainly addressed in last entrance of suitable choline esters into the catalytic domain of AChE several years [3]. The cholinergic hypothesis [9] has been the most [11] (Figure 2). The binding of any ligand at PAS blocks the entry of widely used in search of drug(s) for treatment of AD. According to substrates and the exit of the products from the base of the active site. this hypothesis, enzyme AChE hydrolyzes the neurotransmitter ACh Some ligands such as bulky bis quaternary compounds bind with PAS and breakdown it into acetic acid and choline (Figure 2) and creates and hence block the entrance of acetylcholine in the catalytic gorge deficiency of the ACh at the synaptic gap which leads to the termina- therefore partially inhibit activity of AChE. tion of synaptic transmission in brain. The AD is caused by reduced level of the neurotransmitter Acetylcholine (ACh) at the synaptic gap Selectivity for AChE over BChE in central nerve system in mammals. There are two cholinesterase en- It is evident that AChE promotes amyloid fibril formation, more zymes that metabolize acetylcholine, namely, a) AChE and b) BChE. specifically; the peripheral site of enzyme AChE actively involved in The AChE is mainly found in neuromuscular junctions and chemical aggregation of amyloid-beta (Aβ) peptides [12]. The AChE interacts synapses of the cholinergic type and plays a major role in the hydro- with growing amyloid fibrils and form AChE-Amyloid complex that lysis of the released neurotransmitter ACh in central nervous system. accelerates assembly of Aβ peptides which leads to Aβ deposits such as mature senile plaque present in the AD brain [13,14]. The AChEIs bind only with CAS prevents the hydrolysis of ACh thus rectify the deficit of ACh in brain. Whereas the AChEI which blocks the entire active gorge including catalytic active site and peripheral site of en- zyme of AChE, known as dual binding site inhibitors not only prevent the hydrolysis of ACh but also inhibit Aβ aggregation. So these dual AChEIs are considered more effective anti-Alzheimer’s agents than the single biding site inhibitors. The BChE is produced in liver and is mainly found in blood plas- ma and