Cholinesterase Inhibitors

Mechanism of Action

Cholinesterase inhibitors function by inhibiting cholinesterase from hydrolyzing ACh into acetate and choline, thereby increasing both the availability and duration of ACh action at neuromuscular junctions. The cholinesterase enzyme has 2 active sites: an anionic site (formed by tryptophan) and an esteractic site (formed by serine). Cholinesterase inhibitors, such as organophosphates, inhibit cholinesterase from cleaving ACh by interacting with the serine esteractic site. This leads to the accumulation of ACh and prolonged activation of its associated receptors.[19]

Cholinesterase inhibitors are classified as reversible, irreversible, or pseudo-reversible. Reversible cholinesterase inhibitors are generally utilized for therapeutic purposes. In contrast, irreversible and pseudo-reversible inhibitors are often used in pesticides and biowarfare (nerve agents).[20][21]

Pharmacokinetics

The general pharmacokinetic properties are summarized in the table below for better comprehension. Detailed pharmacokinetic information for each drug should be obtained from its product labeling.

Table 2. Pharmacokinetic Properties of Selected Cholinesterase Inhibitors

Abbreviations: BBB, blood-brain barrier; CNS, central nervous system; CYP, cytochrome P450; IV, intravenous.

Administration

Available Dosage Forms and Strengths

Cholinesterase inhibitors are available in multiple dosage forms and can be administered via intramuscular (IM), intravenous (IV), or oral routes, depending on the specific agent and clinical indication. The specific formulation and route of administration vary depending on the individual agent and its clinical use. For example, neostigmine is available as an injectable solution to counteract the effects of muscle relaxants at the end of surgery. In contrast, an oral form of neostigmine is available for the treatment of myasthenia gravis. Rivastigmine, commonly prescribed for patients with dementia, is available as a transdermal patch, which is also frequently used in clinical practice.[22][23]

Cholinesterase inhibitors are available in several formulations for both central nervous system and peripheral nervous system indications. Donepezil is available as film-coated tablets in strengths of 5 mg, 10 mg, and 23 mg. Galantamine is formulated as immediate-release tablets and extended-release capsules in strengths ranging from 4 mg to 24 mg. Benzgalantamin is available as delayed-release tablets in 5 mg, 10 mg, and 15 mg strengths. Rivastigmine is available as oral capsules (1.5 mg, 3 mg, 4.5 mg, and 6 mg) and as transdermal patch systems delivering 4.6 or 9.5 mg per 24 hours.[24] Pyridostigmine is available as 60 mg immediate-release tablets, 180 mg extended-release tablets, an oral solution (60 mg/5 mL), and parenteral formulations. Neostigmine is available as a sterile injectable solution in concentrations such as 0.5 mg/mL and 1 mg/mL. Physostigmine is available as an injectable solution, typically at 1 mg/mL, for IV or IM use, and is used exclusively for the management of anticholinergic toxicity.  

Adult Dosage

Adult dosing varies according to indication, disease severity, hepatic and renal impairment, and concomitant medications. 

• Donepezil is typically initiated at 5 mg once daily and titrated to 10 mg after several weeks, as tolerated. The 23 mg formulation is reserved for patients with moderate-to-severe Alzheimer disease who have demonstrated tolerance to 10 mg daily.

• Galantamine is initiated at the lowest tolerated dose and titrated every 4 weeks to achieve therapeutic targets. Extended-release formulations may allow once-daily administration up to 24 mg.

• Benzgalantamine follows a similar titration schedule as galantamine, with a starting dose of 10 mg daily (divided into 5-mg doses) and gradual escalation based on tolerability.

• Rivastigmine also requires slow titration, beginning with low doses of oral capsules administered twice daily or a 4.6 mg per 24 hours transdermal patch. The dose is increased after several weeks, as tolerated, to reach the target maintenance level. The maximum dose of the rivastigmine transdermal system is 13.3 mg per 24 hours once daily, and is used in patients with severe Alzheimer dementia.

• Pyridostigmine dosing for myasthenia gravis is individualized, typically using immediate-release tablets administered multiple times daily, with dose adjustments based on clinical response. The extended-release 180 mg formulation is administered once or twice daily, depending on clinical needs.

• Neostigmine is dosed by body weight to reverse nondepolarizing neuromuscular blockade. The drug is typically administered via the IV route at 0.03 to 0.07 mg/kg, slowly, with concurrent anticholinergic therapy to prevent muscarinic adverse effects.

• Physostigmine is used only for severe anticholinergic toxicity and is administered via the IV route with cautious, titrated dosing based on clinical response, due to its short half-life and risk of bradyarrhythmia and seizures.

Specific Patient Populations

The table below summarizes key information for specific patient populations. Clinical decisions should always be guided by the individual patient’s condition, the most recent product labeling, and institutional protocols.

Table 3. Dosage Adjustments of Cholinesterase Inhibitors in Specific Patient Populations 

Adverse Effects

Cholinesterase inhibitors increase the overall availability of ACh. Thus, symptoms of parasympathetic nervous system overstimulation, such as increased hypermotility, hypersecretion, bradycardia, miosis, diarrhea, and hypotension, may occur.

A major concern when prescribing cholinesterase inhibitors or exposure to organophosphates is the potential for developing a cholinergic crisis, also known as SLUDGE syndrome.[31]

SLUDGE is a mnemonic that stands for:

• S: Salivation
• L: Lacrimation
• U: Urination
• D: Diaphoresis
• G: Gastrointestinal upset
• E: Emesis

Another commonly used mnemonic is DUMBBELLS, which also includes miosis and bronchospasm:

• D: Diaphoresis, diarrhea
• U: Urination (incontinence)
• M: Miosis (pinpoint pupils)
• B: Bradycardia
• B: Bronchospasm
• E: Emesis
• L: Lacrimation
• L: Lethargy and low blood pressure
• S: Salivation [32]

Temporary adverse effects when initiating patients on cholinesterase inhibitors include headaches, insomnia, and minor gastrointestinal issues. More concerning effects include lightheadedness, weakness, and weight loss. In some cases, prolonged muscle contraction may also be a presenting feature in patients exposed to cholinesterase inhibitors.[33][34]

Drug-Drug Interactions

Donepezil: Donepezil is highly protein-bound and exhibits weak in vitro inhibition of CYP3A4 and CYP2D6. Strong CYP3A4 and CYP2D6 inhibitors, such as ketoconazole and quinidine, can inhibit donepezil metabolism, with ketoconazole increasing its exposure. CYP inducers, such as phenytoin, carbamazepine, dexamethasone, rifampin, and phenobarbital, may increase donepezil clearance and reduce its efficacy.

Galantamine: Galantamine is metabolized via several pathways, with CYP2D6 and CYP3A4 being the principal contributors. Strong CYP3A4 inhibitors (eg, ketoconazole) increase systemic exposure. CYP2D6 inhibitors, such as fluoxetine, quinidine, and paroxetine, can reduce clearance and increase bioavailability. Cimetidine increases galantamine exposure, whereas ranitidine shows no significant effect. Coadministration with digoxin may increase the risk of bradyarrhythmia.

Benzgalantamine: Benzgalantamine shares the same interaction profile as galantamine, its active metabolite. The drug antagonizes anticholinergic drugs, potentially reducing their clinical effect. Additive cholinergic effects may occur when combined with cholinomimetics or other cholinesterase inhibitors. Synergistic effects are also expected if benzgalantamine is used concomitantly with succinylcholine and cholinergic agonists such as bethanechol.

Rivastigmine: Concomitant use with metoclopramide increases the risk of extrapyramidal reactions and should be avoided. Rivastigmine may enhance the effects of cholinomimetic drugs and may diminish the therapeutic response to anticholinergic medications, such as oxybutynin or tolterodine. Rivastigmine can also cause additive bradycardia when combined with beta-blockers; therefore, concurrent use with beta-blockers should be avoided in patients with bradycardia or a history of syncope.

Pyridostigmine: Atropine antagonizes the muscarinic effects of pyridostigmine toxicity or overdose and is used to counteract muscarinic symptoms such as salivation, sweating, and bradycardia. Drugs that interfere with neuromuscular transmission, including aminoglycosides, anesthetics, and antiarrhythmics, should be used cautiously or avoided, as they may worsen muscle weakness in patients with myasthenia gravis. Beta-blockers should be used cautiously in patients with hypertension or glaucoma, as pyridostigmine may worsen bradycardia. Concurrent use of anticholinesterases for glaucoma may produce additive effects, leading to vision problems due to increased cholinergic activity in the eye.[35]

Neostigmine: Neostigmine is primarily metabolized by hepatic microsomal enzymes; caution is advised when used with strong enzyme inducers or inhibitors. When administered postoperatively to reverse neuromuscular blockade, neostigmine may cause residual neuromuscular block.[36] 

Physostigmine: Physostigmine should be used with extreme caution with other cholinesterase inhibitors, including those prescribed for dementia, as concurrent therapy may increase the risk of cholinergic adverse effects. Physostigmine is contraindicated in cases of intoxication with depolarizing neuromuscular blockers, such as succinylcholine (suxamethonium), as it can increase the depolarizing block and worsen neuromuscular paralysis.

Contraindications

As cholinesterase inhibitors can enhance vagal tone by activating the parasympathetic nervous system, they should be used with caution in patients with bradycardia or cardiac conduction disorders, such as sick sinus syndrome, who are at increased risk of syncope and falls. Caution is also warranted in patients receiving antihypertensive therapy due to the potential for severe hypotension.[37]

Moreover, cholinesterase inhibitors are also contraindicated in patients with gastric ulcers due to the increased risk of gastrointestinal bleeding. Peritonitis or mechanical obstruction of the urinary or intestinal tracts typically contraindicates the use of cholinesterase inhibitors.

Administration is likewise contraindicated in patients with a history of allergy or hypersensitivity to cholinesterase inhibitors or their derivatives.[38]

Monitoring

The therapeutic index varies among different classes of cholinesterase inhibitors. Physostigmine, which has a short half-life and a narrow therapeutic index, is associated with adverse effects such as nausea, vomiting, stomach cramps, and diarrhea; therefore, it is not currently recommended for the treatment of dementia. Donepezil, which is used in treating Alzheimer disease, is well absorbed and relatively well tolerated, although higher dosages can produce adverse effects. The oral capsule formulation of rivastigmine can cause gastrointestinal upset; however, the development of a transdermal version improved tolerability in many studies. Both donepezil and rivastigmine are FDA-approved and associated with fewer adverse effects than older cholinesterase inhibitors. The safety and efficacy of newer generations of cholinesterase inhibitors, such as metrifonate, are currently under investigation. In cases where confirming the diagnosis is challenging, red blood cell cholinesterase activity can be measured from a blood sample.[39][40] 

Galantamine is indicated for the treatment of cognitive decline in mild-to-moderate Alzheimer disease. Galantamine is a potent allosteric potentiating ligand of human nicotinic ACh receptors, and it also acts as a weak, competitive, and reversible cholinesterase inhibitor throughout the body.

The American Society of Anesthesiologists recommends using quantitative neuromuscular monitoring in conjunction with clinical assessment to prevent residual neuromuscular blockade. The train-of-four ratio is used, and a value greater than 0.9 is advised before extubation. The adductor pollicis muscle is suggested for neuromuscular monitoring.[41] A recent study found no significant association between cholinesterase inhibitor use and QTc interval prolongation. However, additional studies are required, especially in high-risk patient populations.[42] 

Toxicity

Signs and Symptoms of Overdose

The potential toxicity of cholinesterase inhibitors stems from their mechanism of action, leading to excessive accumulation of acetylcholine. The clinical spectrum of toxicity varies among patients, which is also complicated by the type of cholinesterase inhibitor a patient is exposed to. SLUDGE syndrome, as described above, is the most recognized form of toxicity for cholinesterase inhibitors. Severe respiratory depression can also occur.

Increased ACh at neuromuscular junctions can also be a sign of toxicity and may result in involuntary movements such as muscle fibrillation, fasciculations, and paralysis. These findings should raise a strong suspicion of cholinesterase inhibitor overdose.[43]

Miosis is a common sign of cholinergic toxicity. Excess ACh causes contraction of the sphincter pupillae muscle that encompasses the iris. Miosis is considered one of the most sensitive indicators of exposure to cholinesterase-inhibiting aerosols, including organophosphates and pesticides.[44]

Management of Overdose

First-line treatments for suspected cholinesterase inhibitor toxicity include atropine, pralidoxime (2-PAM), and diazepam. Atropine competitively blocks muscarinic receptor sites, therefore reducing the binding of ACh. However, it does not counteract nicotinic effects such as muscle fasciculations and weakness; thus, ventilation or respiratory support may still be necessary.[45] 

2-PAM functions by reversing the binding of cholinesterase inhibitors to AChE. When administered concurrently, 2-PAM and atropine produce a synergistic effect.[46]

Seizures due to cholinesterase inhibitor toxicity are more apparent in pediatric patients and in adults exposed to nerve agents; therefore, requiring immediate management with diazepam. The American College of Medical Toxicology supports the use of atropine autoinjectors, atropine 1% sublingual, and glycopyrrolate administered IM, IV, or intraosseously as alternative countermeasures. Similarly, IM lorazepam may also be used as an alternative to diazepam.[47] 

Atropine may also be used for donepezil overdose, along with supportive measures. Because overdose management strategies continue to evolve, it is recommended to contact a Poison Control Center to obtain the most up-to-date guidance for managing drug overdose or toxicity.

Enhancing Healthcare Team Outcomes

Effective treatment of patients treated with cholinesterase inhibitors, particularly in cases of toxicity, requires a coordinated interprofessional approach. Because many patients with cholinesterase inhibitor toxicity initially present to the emergency department, triage nurses must be familiar with the associated symptoms, and affected patients often require prompt admission to a monitored setting. Early recognition and timely intervention are essential to improving clinical outcomes.

In cases of cholinesterase inhibitor toxicity, optimal care requires collaboration among nurses, laboratory technicians, pharmacists, physicians, and other healthcare professionals. Patients admitted under these circumstances require close monitoring by nursing staff and clinicians, along with timely laboratory evaluation, accurate assessment of symptoms, and anticipation of appropriate treatment steps. Pharmacists should be consulted regarding the use of atropine, pralidoxime, and benzodiazepines when cholinesterase inhibitor toxicity is suspected. Consultation with a toxicologist or intensivist is often necessary to achieve optimal clinical results, as many cases require advanced or interventional management during hospitalization.

For toxicity related to neostigmine, the anesthesiology team should be promptly notified. Respiratory therapists also play a vital role in treating patients who require mechanical ventilation or noninvasive respiratory support. Through comprehensive interprofessional coordination, patients experiencing cholinergic toxicity are more likely to achieve optimal clinical outcomes.

In addition to managing toxicity, cholinesterase inhibitor therapy for conditions such as dementia, Alzheimer disease, and myasthenia gravis often requires collaboration with a neurologist.[48] Given the clinical complexity of these patients, particularly those with advanced disease who may be unable to participate fully in their care, close communication among prescribing clinicians, nursing staff, and pharmacists is essential. This coordination supports appropriate drug selection, dosing, administration, and monitoring for adverse effects.

Interprofessional collaboration is also critical in cases of poisoning, as patients who receive prompt treatment generally have favorable outcomes, whereas delays in care are associated with increased morbidity. A retrospective cohort study using electronic health records and claims data evaluated prescribing patterns of cholinesterase inhibitors, including donepezil, rivastigmine, and galantamine. Researchers found that donepezil remained the most frequently prescribed agent across dementia subtypes, while galantamine use was comparatively limited, warranting further investigation.[49] Overall, an interprofessional team approach involving physicians, advanced practice providers, pharmacists, and nurses is essential for minimizing adverse effects and optimizing outcomes in patients receiving cholinesterase inhibitor therapy.