Stopping the Breakdown: How Acetylcholinesterase Inhibitors Help Fight Alzheimer's Disease

Authors: Abdullah Choudhary (1) and Ronald Ganzorig (1)

Editor: Swabir Silayi, Ph.D.

Author Affiliatons: (1) Thomas Jefferson High School for Science and Technology 

Introduction

Acetylcholine (ACh) is a neurotransmitter that is essential for memory, learning, executive function, and various other cognitive processes [1]. In the healthy brain, cholinergic neurons, which release ACh, modulate cortical and hippocampal activity in order to support memory formation and recall. Alzheimer’s Disease (AD) is characterized in part by degeneration of these cholinergic neurons and ultimately leads to a deficiency of ACh in many key brain regions [2,10]. This “cholinergic hypothesis” of AD was first proposed in the 1980s and suggests that loss of ACh directly contributes to cognitive decline [4]. Indeed, postmortem and imaging studies confirm that the severe loss of ACh-producing neurons typically associated with AD correlates with impaired cognition [10]. Thus, to counteract this deficit, enhancing cholinergic signaling has been a major therapeutic strategy in AD.

Acetylcholinesterase Function in the Brain

Acetylcholinesterase (AChE) is a membrane-bound enzyme that is responsible for terminating cholinergic signals in the brain. It is located within the synaptic cleft (the gap between neurons) and hydrolyzes ACh after it is released. AChE breaks ACh down into choline and acetate, and essentially “turns off” the neurotransmitter signal so that new neural messages can be easily sent. Remarkably, AChE is one of the fastest enzymes and can split an ACh molecule into its constituent molecules within mere microseconds [3]. By doing so, AChE prevents the overstimulation of neurons and ensures that each nerve signal is relatively brief. Clearly, this high-efficiency enzyme is indispensable for normal neuromuscular function and cognitive processing. However, in AD, the already-present cholinergic deficit means that there is too little ACh available in the first place. Therefore, excess AChE activity in this context can exacerbate the shortage of ACh, further impairing neurotransmission and cognitive function [2,10].

Acetylcholinesterase Inhibitors and Their Mechanisms

In Alzheimer’s disease, the loss of cholinergic neurons leads to a reduction of synaptic acetylcholine (ACh), impairing overall memory and cognition. Acetylcholinesterase inhibitors (AChEIs) address this deficit by occupying the enzyme’s active-site gorge and slowing the breakdown of ACh into its constituents (hydrolysis)  in the synaptic cleft, which enhances cholinergic transmission [2]. Donepezil is the most widely prescribed AChEI in the world. It fits into the gorge and interacts both with the catalytic serine at the base and a nearby pocket through aromatic stacking interactions. These interactions confer high specificity for acetylcholinesterase and allow a long half-life of approximately 70 hours, so patients usually take it once a day [5,7]. Another AChEI, rivastigmine, works by attaching a carbamate functional group to the enzyme’s active serine, temporarily inactivating both acetylcholinesterase and butyrylcholinesterase. This dual enzyme coverage can sustain ACh levels longer, but it requires twice-daily dosing and gradual/careful dose increases to manage certain side effects such as nausea and slow heartbeat [7]. Furthermore, Galantamine has a mechanism that reversibly inhibits AChE and enhances nicotinic acetylcholine receptor responsiveness at the same time, which substantially boosts presynaptic ACh release in surviving neurons [8]. A generalized mechanism diagram for these AChEI’s are presented in Figure 1 below. Moreover, despite their vastly different mechanisms and dosing schedules, studies show that donepezil, rivastigmine, and galantamine provide very similar cognitive benefits, so doctors typically choose one based on side-effect profiles, dosing preferences, and cost rather than potential efficacy differences [2].

Figure 1: Cholinergic Synapse and site of action of AChE inhibitors (Adapted from S Mogul & D Wilkinson https://doi.org/10.1586/14737175.1.1.61)

Clinical Effects in Alzheimer’s Patients

Placebo-controlled trials demonstrated that AChEIs gave modest but consistent effects in mild-to-moderate levels of Alzheimer's Disease. Taking the 70-point Alzheimer’s Disease Assessment Scale-Cognitive Subscale (ADAS-Cog), patients after taking 6 months of donepezil, rivastigmine, or galantamine scored 2 to 3 more points than those on the placebo, slowing the expected decline rate by 30 to 40 percent [9]. Additionally, similar effects were displayed in similar tests, such as the Clinician’s Interview-Based Impression of Change, as well as in daily activities, demonstrating that small improvements in memory and attention can lead to maintaining independence [9]. Follow-up studies describe these benefits persist between nine to twelve months before the drug's efficacy gets reduced due to continued neuron loss. Observational data suggest that long-term AChEI treatment can delay nursing home placement by some months and reduce caregiver burden; however, there is no conclusive evidence of increased survival [6]. Moreover, nausea, vomiting, diarrhea, dizziness or bradycardia are side effects that are seen in as many as 30 percent of these effects, clinicians start with low doses and progress upwards slowly, advise taking with meals, and prescribe evening dosing and, in the case of rivastigmine, utilize a transdermal patch that provides steady medication over 24 hours [7]. In spite of these difficulties, cost-effectiveness analyses by the Alzheimer's Association determine AChEIs to be a worthwhile symptomatic therapy in early Alzheimer’s in comparison to more expensive cases of disease-modifying options [6].

Structural Insights and Drug Design

High-resolution X-ray crystallography has been crucial in terms of mapping AchE’s 3-dimensional structure and guiding new inhibitor design. In 1991, the enzyme from the Torpedo electric ray was shown to have a 20-Å-deep, narrow gorge lined with aromatic residues that guide ACh to three amino acids in the active site where it is broken down [3]. This discovery pinpointed this specific gorge as the prime target for inhibition. Years later, in 1999, the human AChE-donepezil complex (PDB ID 1EVE)  showed how donepezil spans the entire gorge, blocking ACh access and stabilizing π-π interactions with tryptophan and phenylalanine residues [5]. Medicinal chemists used this information to optimize rivastigmine’s reactive carbamate group and explore galantamine’s dual effects. Later studies combining crystallography and computer modeling helped discover additional sub-pockets and peripheral binding sites, leading to dual-site inhibitors with stronger binding and longer duration. Comparative structures of AChE and butyrylcholinesterase enabled the design of BuChE-selective inhibitors to lessen peripheral side effects. Today, there is an ongoing focus on improving blood-brain barrier penetration, fine-tuning pharmacokinetics and minimizing off-target interactions in the next-generation AChEIs [3,5].

Therapeutic Challenges, Limitations and Future Directions

Acetylcholinesterase inhibitors remain symptomatic treatments; they do not remove amyloid plaques nor tau tangles and cannot stop neuronal loss [2]. As Alzheimer’s progresses and cholinergic neurons are lost, the maximal benefit of AChEIs falls, and side-effect burdens cause up to one-third of patients to discontinue therapy even with patches and prodrug strategies [7]. Newer strategies target irreversible or long-acting inhibitors with selective central nervous system targeting to maintain enzyme inhibition with minimal peripheral activity [10]. Nanocarriers and ligand-directed delivery systems are being investigated to target drugs in brain tissue without exposing other organs. Combination therapies, such as fixed-dose donepezil and memantine, target both cholinergic and glutamatergic pathways for more global cognitive support [6]. At the same time, monoclonal antibodies against amyloid and tau do hold true disease-modifying potential, particularly in early Alzheimer’s treatment. Continued drug purification. Innovations in delivery and combination with new treatments will be essential to expanding and improving the quality of life for patients with Alzheimer’s disease.

Glossary

Neurotransmitter: A chemical messenger that is released by neurons to transmit signals across a synapse to another neuron or target cell.

Cholinergic Neuron: A nerve cell that uses acetylcholine as its primary neurotransmitter.Acetylcholine (ACh): A neurotransmitter that is involved in memory, learning, executive functioning, movement, etc.

Acetylcholinesterase (AChE): An enzyme that is located in the synaptic cleft that terminates cholinergic signaling by hydrolyzing acetylcholine into choline and acetate.

Synaptic Cleft: The small gap between the presynaptic and postsynaptic neurons where neurotransmitters are released in vesicles.

Hydrolysis: A chemical reaction in which a molecule is split into two parts by the addition of water (for ACh, this yields choline and acetate).

AChE Inhibitor: A drug that binds to acetylcholinesterase and slows the breakdown of acetylcholine, essentially prolonging its action.

Carbamate Functional Group: A chemical functional group represented by (–O–C(=O)–NH–)Nicotinic Acetylcholine Receptor: A type of ion channel on neurons that opens in response to acetylcholine binding, allowing ions to flow and generating an electrical signal.

Gorge (Active-Site Gorge): The narrow tunnel within acetylcholinesterase that guides acetylcholine from the synaptic cleft to the enzyme’s catalytic center.

References

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