Anticholinesterase

[SH4:p251-p262; CEACCP 2004 Vol 4(5) "Anticholinesterase and anticholinergic drugs"]

Class

Anticholinesterase include:

• Edrophonium
• Neostigmine
• Pyridostigmine
• Physostigmine

NB:

• Physostigmine is a natural alkaloid derived from the Calabar bean

Structures

Acetylcholinesterase (AChE)

• Acetylcholinesterase consists of an anionic and an esteratic site
• Anionic site binds to the quaternary nitrogen in ACh
  --> Orientate the ester link of ACh to the esteratic site
• Esteratic site hydrolyse ACh
• Located at:
  * High concentration at NMJ
  * Lower concentration throughout skeletal muscle fibres
  * Encoded by a single gene on chromosome 7q22
• Acetylcholinesterase (AChE) is one of the most efficient enzymes known
  * Each hydrolyse 300,000 molecules of ACh per minute [SH4:p251]
  * Or 4,000 molecule per second [SH4:p253]
  * 50% of the released ACh during the time of diffusion across the synaptic cleft
  --> Near diffusion-limited
•  May also have other functions
  * Nerve growth-promotion
  * Modulation of nAChRs

Structure-activity relationship

• Linking two quaternary ammonium nuclei by a chain of proper length
  --> Increase in anticholinesterase potency and duration
• Neostigmine, pyridostigmine, and edrophonium are quaternary amine
  --> Does not cross BBB
  --> Effect is mostly peripheral stimulation of nAChRs and mAChRs
• Physostigmine is tertiary amine
  --> Can cross BBB
• Organosphates are very lipid soluble
  --> Can cross BBB

Pharmacodynamics

Mechanism of action

• Enzyme inhibition
• Presynaptic effects
• Direct effects on neuromuscular junction (NMJ)

Overall effect is effectively stimulation of the cholinergic system

Enzyme inhibition

• Inhibition of acetylcholinesterase --> Increased ACh in:
  * Preganglionic sympathetic nerve endings
  * Parasympathetic nerve endings
  * NMJ
• Neostigmine and pyridostigmine are hydrolysed by AChE
  --> AChE is carbamylated
  --> Decreased ability to hydrolyse ACh
• Edrophonium is NOT hydrolysed by AChE
  * It forms reversible electrostatic attachment to AChE

NB:

• The differences in mechanisms have little clinical implication

Different mechanisms of AChE inhibition

• Reversible inhibition
• Formation of carbamyl esters
• Irreversible inactivation by organophosphate

Reversible inhibition

e.g. Edrophonium

• Lacks carbamyl group
• Reversible inhibition of AChE by electrostatic attachment to the anionic site on the AChE enzyme
  --> Edrophonium-AChE complex prevents ACh binding to AChE
• Due to reversibility of the inhibition
  --> ACh competes with edrophonium
  --> Increase in ACh concentration increases access to AChE
  --> Thus duration is short
• Predominantly presynaptic action
  * c.f. Slower-onset anticholinesterases (e.g. neostigmine and pyridostigmine) which are predominantly postsynaptic
• Muscarinic effects are mild
  * c.f. Longer-acting anticholinesterases
• Clinical use:
  * Antagonise effects of non-depolarising NMBDs
  * Diagnosis and assessment of therapy adequacy in myasthenia gravis and cholinergic crisis
  * Evaluation the presence of dual blockade produced by suxamethonium

Formation of carbamyl esters

e.g. Neostigmine, pyridostigmine, physostigmine

• Reversible inhibition of AChE by formation of a carbamyl ester complex at the esteratic site on the AChE enzyme
• The carbamylated AChE cannot hydrolyse ACh until the carbamate-enzyme bond dissociates
• Carbamylated AChE has a half-time of 15-30 minutes

Irreversible inactivation

• Organophosphate combines with AChE at the esteratic site
  --> Stable inactive complex (i.e. does not hydrolyse)
• Echothiophate interacts with BOTH the esteratic site and anionic subsites
  --> Extreme potency
  * Only one used clinically
• Other organophosphate (e.g. parathion, malathion) are used as insecticides

Malathion

• Malathion is a selective insecticide
  * Enzyme necessary for its metabolism is absent in insects
• Malathion is hydrolysed by phosphorylphosphatases --> Excreted in urine

Others

• Nerve gases (tabum, saran, soman) are extremely lipid-soluble
  --> May be absorbed through intact skin

Presynaptic effects

• An anticholinesterase may produce spontaneous contractions (fasciculations) of skeletal muscles
  * Only in the absence of non-depolarising NMBDs
• Fasciculations may be due to direct stimulation of presynaptic nAChRs
  --> Increased availability of ACh

Direct effects on NMJ

• Anticholinesterase may produce some form of NMJ blockade
  * But only at doses far greater than administered clinically
• Possible mechanism
  = Excess of ACh at NMJ --> Desensitisation block

Dose-response curve

• Dose-response curve are parallel between neostigmine and pyridostigmine
  --> Similar mechanism
• Dose-response curve for edrophonium is NOT parallel with that of neostigmine or pyridostigmine
  --> Different mechanism of action for edrophonium
  * Dose-response curve for edrophonium is flatter
• When the NMJ blockade is still intense at the time of the reversal being initiated
  --> Dose response curve is shifted to the right
  * The shift is more important for edrophonium and pyridostigmine (than for neostigmine)
  * Neostigmine is preferable when antagonising >90% twitch depression [SH4:p258]

NB:

• Dose-response curve for neostigmine is to the left of pyridostigmine
  --> At a given response, less neostigmine is required
  * i.e. Neostigmine is more potent
• But when combined, the effects of edrophonium and other anticholinesterases are only additive
  --> Suggesting a similar mechanism [SH4:p258]
• Potency ratio between anticholinesterase drugs depends on:
  * The non-depolarising NMBD being antagonised (and its inherent speed of spontaneous recovery)
  * Depth of NMJ blockade when reversal is initiated
  * The selected end-point

Ceiling effect

• Once acetylcholinesterase is maximally inhibited
  --> Additional anticholinesterase will not further antagonise nondepolarising NMJ blockade
• Persistent NMJ blockade despite large doses neostigmine (70mcg/kg)
  --> Indication for further mechanical ventilation

Effects by systems

• The effects are due to accumulation of ACh at muscarininc and nicotinic cholinergic receptors
• Muscarinic cholinergic effects are evoked by lower concentration of ACh than are required for nicotinic effects at autonomic ganglia and NMJ
• Co-administration of an anticholinergic drug to prevent adverse muscarininc cholinergic effects
  * Anticholinergic drugs selectively block muscarinic cholinergic receptors

NMJ

• Anticholinesterases
  --> Increase the amount of ACh available at NMJ
  --> NMJ are more likely to bind with ACh than with non-depolarising NMBDs
  --> Reversal of NMJ blockade
• In overdose, anticholinesterases
  --> Excessive amount of ACh at NMJ
  --> Depolarisation NMJ blockade

CVS

• Bradycardia
  * Most likely due to slowing of the conduction at AV node (and slowed rate at SA node)
  * No change in ventricular conduction, and contractility [SH4:p262]
  * Denervated heart (e.g. in heart transplant) are exquisitely sensitive to the bradycardic effect of neostigmine
• Possible decrease in BP
  * Due to decrease in systemic vascular resistance

Respiratory

• Smooth muscle fibres of bronchioles and ureters are contracted
  --> Can potentially produce bronchoconstriction
• Increased tracheobronchial secretions

 

GIT

• Increased gastric secretion (by parietal cells)
• Increased motility (especially in large bowel)
  * Due to accumulated ACh acting on ganglion cells of Auerbach's plexus and on smooth muscle fibres
• Increased incidence of post-operative nausea and vomiting (PONV)
  * Especially neostigmine [CEACCP 2004 Vol 4(5):p166]
• Sometimes used to treat achalasia
• At high doses
  --> Vomiting, diarrhoea, and incontinence

Secretion

• Increase production of secretions innervated by postganglionic cholinergic fibres
  * e.g. bronchial, lacrimal, sweat, salivery, gastric, intestinal, and acini pancreatic gland

NB:

• Sweat glands are cholinergic, even though they are innervated by sympathetic nervous system

 

Eyes

When topically applied to the eye, anticholinesterase can cause:

• Constriction of iris sphincter
  --> Miosis
• Constriction of ciliary muscle
  --> Inability to focus for near vision
• Intraocular pressure (IOP) declines because outflow of aqueous humour is facilitated

NB:

• Interference with accomodation is usually shorter in duration than miosis
• Usually topically applied

Others

Myasthenia gravis

• In patients with myasthenia gravis, who received non-depolarising NMBDs
  * Giving anticholinesterase drugs (to reverse the NMJ blockade) is associated with a risk of cholinergic crisis
  * [SH4:p257]
  * ??? why

Plasma cholinesterase activity

• Neostigmine and pyridostigmine produce prolonged and marked inhibition of plasma cholinesterase
  * Edrophonium does not inhibit plasma cholinesterase

Pharmacokinetics

• Normal renal and hepatic function
  --> No significant different between anticholinesterase drugs
• Similar pharmacokinetics
  --> Differences in potency are due to pharmacokinetic reasons

Absorption

Lipid solubility

• Anticholinesterase containing quaternary ammonium group are poorly lipid soluble
  * i.e. edrophonium, neostigmine, pyridostigmine
  --> Poor GIT absorption and unpredictable CNS effect
• Lipid soluble anticholinesterase
  * i.e. tertiary amine (physostigmine) and organophosphates
  --> Good GIT absorption and predictable CNS effects

Distribution

Volume of distribution (Vd)

• Quaternary ammonium anticholinesterases have a large Vd (0.7-1.4 L/kg)
  * Compared to nondepolarising NMBDs and  considering their low lipid solubility
  * May be due to extensive tissue storage in liver and kidney

Metabolism

• In the absence of renal function, hepatic metabolism account for
  * 50% of the clearance of neostigmine
  * 30% of the clearance of edrophonium
  * 25% of the clearance of pyridostigmine
• Physostigmine --> Hydrolysed at ester bond
  * By plasma esterase [CEACCP 2004 Vol 4(5):p166]

Neostigmine

• Metabolised by plasma esterase to a quaternary alcohol [CEACCP 2004 Vol 4(5):p166]
• Rest (which is 50% [SH4] to 60% [CEACCP]) is excreted unchanged in urine

Metabolites

• None of the metabolites contribute significantly to activity
• Principle metabolite for neostigmine = 3-hydroxyphenyltrimethylammonium
  --> About 1/10 the activity of the parent compound
• Principle metabolite for pyridostigmine = 3-hydroxy-N-methylpyridinium
  --> Inactive
• Principle metabolite for edrophonium = edrophonium glucuronide (conjugation)
  --> Inactive

Elimination

Renal clearance

• Anticholinesterase drugs are actively secreted into the renal tubular lumen
  --> Thus clearance is higher than GFR
• Neostigmine
  --> Renal clearance accounts for 50% of elimination
• Edrophonium and pyridostigmine
  --> Renal clearance accounts for 75% of elimination
• In renal failure, elimination of anticholinesterase is more prolonged than nondepolarising NMBDs are
  --> Recurarisation unlikely
• Physostigmine does NOT depend on renal function for elimination
  * Unlike other anticholinesterases [CEACCP 2004 Vol 4(5):p166]

Elimination half-lifes

[CEACCP 2004 Vol 4(5):p166]

• Edrophonium = 110 min
• Neostigmine = 77 min
• Pyridostigmine = 113 min

[SH4:p254]

• Edrophonium = 110 min
• Neostigmine = 77 min
• Pyridostigmine = 112 min

NB:

• ??? Not sure why edrophonium has longer half-life, yet duration of action is shorter than, or about the same as neostigmine. I wonder if it has anything to do with the competitive nature of the antagonism by edrophonium .

Action profile

• Edrophonium, neostigmine, and pyridostigmine
  * Reach peak and decrease rapidly within 5-10 minute
  * Vd = 0.7 - 1.4 L/kg
  * Elimination half-time = 60 - 120 minutes
  * Clearance = 8 - 16 mL/kg/min (much greater than GFR)

Onset of action

[SH4:p254]

• Onset of action for edrophonium = 1 - 2 minutes
• Onset of action for neostigmine = 7 - 11 minutes
  * Initial onset is about 1 min, but peak action is in around 10 minute [CEACCP]
• Onset of action for pyridostigmine = 16 minutes

NB:

• Faster onset of action for edrophonium may be due to its predominately presynaptic action
  * c.f. neostigmine and pyridostigmine exert predominantly postsynaptic action
  * i.e. action on ACh release instead of acetylcholinesterase inhibition
• Slower onset for neostigmine and pyridostigmine are NOT related to the need to form active metabolites

Duration of action

• Duration of action of anticholinesterase are related to its plasma clearance
• e.g. Half-time of carbamylated enzymes are 15-30 minutes
  --> Shorter than the elimination half-time of anticholinesterase (60-120 min)
• Edrophonium, neostigmine, and pyridostigmine have similar duration of action [SH4:p258]
  * [SH4:p255, fig 9-9] definitely showed difference in duration of action (Pyridostigmine > Neostigmine > Edrophonium)
  * [SH4:p254, tab 9-1] also showed duration Pyridostigmine > Edrophonium > Neostigmine
• [SH4:p254]
  * Edrophonium duration of action = 60 min
  * Neostigmine duration of action = 54 min
  * Pyridostigmine duration of action = 76 min
• [CEACCP 2004 Vol 4(5) "Anticholinesterase and anticholinergic drugs"]
  * Edrophonium duration of action = 10 minutes
  * Neostigmine duration of action = 20-30 minutes
  * Pyridostigmine duration of action = 360 minutes

NB:

• In the past edrophonium was considered a short-acting drug
  --> Apparently controlled studies showed not much difference in duration with neostigmine
  * [SH4:p255]

Clinical

Usage

1. Reversal of NMJ blockade produced by nondepolarising NMBDs
2. Treatment of certain drug-induced CNS effects
3. Treatment of myasthenia gravis
4. Treatment of glaucoma
5. Treatment of mild-to-moderate Alzheimer's disease
6. Other uses
   * Treatment of paralytic ileus and atony of urinary bladder
   * Analgesia (used intrathecally or epidurally)
   * Diagnosis of cardiac dysrhymthia (paroxysmal supraventricular tachycardia)
   * Treatment of post-operative shivering

Reversal of nondepolarising NMJ blocade

• Edrophonium, neostigmine, or pyridostigmine are used to increase availability of acetylcholine at NMJ
• Physostigmine is not used for this purpose due to the excessive dosage required
• Neostigmine is preferable when antagonising >90% twitch depression [SH4:p258]
• Effect is dose-related
  * With a ceiling effect

NB:

• Due to the predominantly presynaptic action of edrophonium
  --> Train-of-four ratio is higher after edrophonium than after neostigmine or pyridostigmine

Co-administration of anticholinergic drugs

• Usually given with anticholinergic drugs (atropine or glycopyrrolate)
  --> Attenuation of the unwanted muscarinic effects
• Usually the anticholinergic drugs chosen need to have faster onset to minimise risk of bradycardia

Thus,

• Edrophonium (fastest in onset) is usually given with atropine (faster onset than glycopyrrolate)
• Neostigmine + atropine
  --> Early tachycardia and late bradycardia is more likely
  * Due to faster onset time and shorter duration of action of atropine
• Neostigmine + glycopyrrolate
  --> HR more stable, and late bradycardia is less likely
  * Glycopyrrolate action profile more closely matches neostigmine

Factors influencing reversal of NMJ blockade

• Intensity of NMJ blockade at the time of reversal
• The nondepolarising NMBD used
• End-point selected
• Other factors [SH4:p259]
  * Certain antibiotics
  * Hypothermia
  * Respiratory acidosis (pCO2 > 50 mmHg)
  * Hypokalaemia and metabolic acidosis

NB:

• Edrophonium is less effective than neostigmine in reversing deep NMJ blockade
• Edrophonium is more effective against atracurium
• Neostigmine is more effective against vecuronium

Treatment of certain drug-induced CNS effects

Central cholinergic syndrome

• Physostigmine is used to treat central cholinergic syndrome due to atropine or scopolamine
  * Physostigmine 15-60 mcg/kg IV
• Duration of action for physostigmine is shorter than anticholinergic drugs
  --> Repeated dosing may be necessary

Others

• Physostigmine may reduce postoperative somnolence after anaesthesia with a volatile anaesthetic agent
• Physostigmine may reduce the somnolent effect of opioids

Treatment of myasthenia gravis

• Neostigmine, pyridostigmine, and ambenonium
  --> Standard anticholinesterase drugs used in symptomatic treatment of myasthenia gravis
  * Presumably improve muscle response by increasing ACh availability
• Due to quaternary ammonium structure in neostigmine and pyridostigmine
  --> Poor oral absorption
  --> Oral dose of neostigmine is 30 times the IV dose
• Pyridostigmine is longer duration of action
  --> Used in treatment of myasthenia gravis

Testing the adequacy of anticholinergic drug therapy

• Edrophonium is used to test the adequacy anticholinergic drug therapy
  --> Edrophonium 1mg IV every 1-2 minutes
• If symptom improves
  --> Anticholinergic therapy has been inadequate
• If increased muscle weakness (due to cholinergic crisis)
  --> Therapy adquate
• When used to diagnose myasthenia gravis
  --> Edrophonium 2mg followed by 8 mg IV 30 seconds later [CEACCP 2004 Vol 4(5):p166]

Treatment of glaucoma

• Anticholinesterase drugs decreases intraocular pressure in narrow-angle and wide-angle glaucoma
  * Due to a decrease in resistance to outflow of aqueous humour
• Longer term treatment (>6 month) with topical long-acting anticholinesterase (e.g. echothiophate, demecarium, isoflurophate)
  --> Risk of cataracts
• Prolonged use of ecothiophate and physostigmine eye drops
  --> Risk of acquired cholinesterase deficiency
  --> Risk of prolonged NMJ blockade by NMBDs metabolised by this enzyme [CEACCP 2004 Vol 4(5):p167]
• No risk of cataracts with shorter-acting anticholinesterases
• In contrast, topical beta-adrenergic antagonists (e.g. timolol)
  * No miosis
  * Decrease IOP by decreases secretion of aqueous humour

Treatment of Alzheimer's Disease

• Anticholinesterases are recommended for treatment of mild-to-moderate Alzheimer disease
• Four centrally acting drugs are availabe:
  * Tacrine
  * Donepezil
  * Rivastigmine
  * Galantamine
• Tacrine is rarely used due to hepatotoxic effects in nearly 40% of patients
• Donepezil is effective once daily dosing, without hepatotoxicity

Treatment of post-operative shivering

• Physostigmine 40mcg/kg IV
• Similar efficacy to pethidine and clonidine

Postoperative analgesia

[SH4:p262]

• Neostigmine 50-100mcg intrathecally or 1-4mcg/kg epidurally
• High incidence of N&V, pruritus, and prolonged block
• No respiratory depression
• No neurotoxicity with intrathecal administration of neostigmine with paraben preservative

Administration

[SH4:p254]

• Edrophonium = 0.5 mg/kg IV
  + Atropine 7 mcg/kg IV
• Neostigmine = 0.043 mg/kg IV
  + Atropine 20 mcg/kg IV OR Glycopyrrolate 10 mcg/kg IV
• Pyridostigmine = 0.35 mg/kg IV
  + Atropine 20 mcg/kg IV OR Glycopyrrolate 10 mcg/kg IV

[CEACCP 2004 Vol 4(5):p166]

• Edrophonium = 1 mg/kg IV
• Neostigmine = 0.05-0.07 mg/kg IV
• Pyridostigmine = 0.1 mg/kg IV (for myasthenia gravis)

Overdose

Symptoms

Overdose produces two types of symptoms: muscarinic and nicotinic

• Muscarinic symptoms include:
  * Miosis
  * Difficulty focusing (eye)
  * Salivation
  * Bronchoconstriction
  * Bradycardia
  * Abdominal cramps
  * Loss of bladder and rectal control
• Nicotinic symptoms include:
  * Skeletal muscle weakness (may progress to paralysis)
  * CNS actions (confusion, ataxia, seizures, coma, respiratory depression)

Treatment of anticholinesterase overdose (including organophosphate)

• Atropine
  * Antagonise muscarinic effects, but not nicotinic effects
  * Does not reactivate AChE
• Pralidoxime (an AChE reactivator)
  * Occasionally used to supplement atropine

Pralidoxime

• 15 mg/kg IV over 2 minutes, repeat again after 20 minutes
• More useful in countering the NMJ effect (nicotinic effect)
• More useful in countering drugs that phosphorylate AChE (e.g. organophosphate)
• Not as useful in countering drugs that carbamylate AChE (e.g. neostigmine)
• Only effective if administered with minutes after exposure
  * Before formation of irreversible bonds
  * The inactivated phosphorylated AChE is stable, and stability is enhanced by a process called "ageing"
  * [CEACCP 2004 Vol 4(5):p167]
• Not useful against CNS effects

Special considerations

Patient age

• Neostigmine in infants and children
  --> Faster onset and greater effect
  * Difference is due to pharmacodynamic reasons
• Edrophonium
  --> No difference for infant, children, and adults
  * Supports the idea that different mechanisms are involved
• In elderly
  --> Duration of action by neostigmine and pyridostigmine are prolonged
  * Due to smaller ECF and slower plasma clearance
  * i.e. due to pharmacokinetic changes
  * Pharmacodynamics are not changed
• In elderly
  * Duration of action is not changed
  * But a higher plasma concentration is required to produce the same effect (compared to in younger patients)

Other related drugs

Organophosphate compounds

Also see "Irreversible inactivation" under "Metabolism"

[CEACCP 2004 Vol 4(5):p167]

• Parathion and malathion are used as insecticides
• Malathion is the main ingredient in dermatological preparation used in pediculosis treatment

Nerve gas

• Tabun, sarin, VX, and soman are highly potent anticholinesterase
  --> Irreversible inactivation of AChE by alkylphosphorylation

Triphasic clinical syndrome

• Initial cholinergic phase (24-48 hours)
• Intermediate phase (4-18 days)
• Third phase of delayed polyneuropathy (7-14 days after exposure)

Tacrine

[CEACCP 2004 Vol 4(5):p166]

• Another short-acting anticholinesterase
  * Like edrophonium
• Crosses BBB
• Used in management of Alzheimer's disease in USA

Donepezil

[CEACCP 2004 Vol 4(5):p167]

• Medium-duration anticholinesterase
• Reversible anticholinesterase
• Used to treat Alzheimer's disease
  * Once daily

Rivastigmine

[CEACCP 2004 Vol 4(5):p167]

• Medium-duration anticholinesterase
• Non-competitive reversible anticholinesterase
• Used to treat Alzheimer's disease
  * Twice daily