Anticholinesterase: Basic Pharmacology and Various Uses

Presenter: Dr. Suresh PradhanModerator: Dr. Yogesh Dhakal

Synthesis and Hydrolysis of ACh

Acetylcholinesterase• type B carboxylesterase enzyme• ‘True Cholinesterase’ / ‘Specific Cholinesterase’• one of the most efficient enzymes known

⌐ a single molecule able to hydrolyze estimated 300,000 molecules of Ach every minute

• responsible for the control of neurotransmission at the neuromuscular junction by hydrolyzing acetylcholine• rapid hydrolysis of acetylcholine removes excess

neurotransmitter from the synapse• preventing overstimulation and tetanic excitation of

the postsynaptic muscle• are present along the length of the muscle fiber

• each molecule of acetylcholinesterase has an active surface with two important binding sites• an anionic site and an esteratic site• negatively charged anionic site on acetylcholinesterase

molecule is responsible for electrostatically binding the positively charged quaternary nitrogen group on the acetylcholine molecule• esteratic site forms covalent bonds with the carbamate

group at the opposite end of the acetylcholine molecule and is responsible for the hydrolytic process• a secondary or peripheral anionic site has been

proposed• binding of ligands to the peripheral anionic site results

in inactivation of the enzyme

ClassificationREVERSIBLE IRREVERSIBLE

CARBAMATES

ACRIDINE

PhysostigmineNeostigminePyridostigmineRivastigmine

ORGANOPHOSPHORUS Compounds

Tacrine

PIPERIDINES Donepezil

NON - CARBAMATE QUATERNARY AMMONIUM

Edrophonium

Echothiophateis the only organophosphorus

used clinically (Glaucoma)

Chemical Structure

Quaternary Ammonium group

Carbamate moietyrenders lipid insolubleso cannot cross BBB

covalent bonding with AChE

lacks Carbamate group so binds non-covalently to AChElipid insoluble due to Quaternary ammonium group

structure similar to Neostigmine but quaternary ammonium group incorporated into the phenol ring

has Carbamate group but no quaternary Ammonium; thus lipid soluble : Crosses BBB

Reversal of neuromuscular blockade• theoretically possible by three

principal mechanisms: · a decrease in enzymatic metabolism of acetylcholine by cholinesterase, thereby increasing receptor binding competition

· an increase in presynaptic release of acetylcholine

· a decrease in the concentration of the NMBD, hence, freeing the postsynaptic receptors

Mechanism of Action• enzyme inhibition

inhibit the enzyme acetylcholinesterase reversibly-results in greater availability of ACh at its sites of action,

• presynaptic effects in the absence of nondepolarizing neuromuscular-blocking

drugs, administration of an anticholinesterase drug may produce spontaneous contractions (fasciculations) of skeletal muscles

these presynaptic effects are abolished by a small dose of a nondepolarizing neuromuscular-blocking drug, suggesting that acetylcholine receptors are involved

• direct effects on the neuromuscular junction

• Neostigmine and to some extent pyridostigmine display some limited pseudocholinesterase- inhibiting activity, but their effect on acetylcholinesterase is much greater.

• Edrophonium has little or no effect on pseudocholinesterase

Metabolism and Clearance

• Hepatic metabolism accounts for • 50% elimination of Neostigmine• 30% of Edrophonium• 25% of Pyridostigmine

• metabolites of anticholinesterases do not contribute significantly to the effects of parent drug

• Physostigmine is almost completely metabolized by plasma esterases.

Metabolism and Clearance…

• renal clearance : 50% elimination of Neostigmine 75% elimination of

Edrophonium and Pyridostigmine

• actively secreted into the lumens of renal tubules

• elimination half times greatly prolonged in Renal dysfunction

Antagonism of Nondepolarizing neuromuscular blockade by acetylcholinesterase inhibitors depends primarily on following factors: (a)the depth of the blockade when reversal is attempted(b)the anticholinesterase chosen(c)the dose administered(d)the rate of spontaneous clearance of the

neuromuscular blocker from plasma(e)the choice and depth of anesthetic agents administered

• reversal of ND-NMB requires only the nicotinic cholinergic effects of the anticholinesterase drugs• muscarinic cholinergic effects of Anticholinesterase drugs

are attenuated or prevented by concurrent administration of anticholinergic drugs• when reversing neuromuscular blockade, the primary

goal is to maximize nicotinic transmission with a minimum of muscarinic side effects

Muscarinic Side Effects

NEOSTIGMINE• carbamate with Quaternary Ammonium Group• lipid Insoluble ; doesn’t cross BBB• recommended dosage- 0.04 – 0.08 mg/kg (maximum of 5

mg in adults)• repeating the dose has no benefit as AChE are already maximally inhibited• onset : 5 to 10 minutes ( peak at 10 min)• duration : lasts for more than 1 hour• paediatric and geriatric patients:

⌐ onset is more rapid⌐ requires smaller dosing⌐ duration of action is prolonged in elderly

• Glycopyrrolate is the preferred anticholinergic to be used along with Neostigmine• 0.2 mg Glycopyrrolate per mg of Neostigmine• If Atropine is used, 0.4 mg per mg of Neostigmine• Neostigmine crosses placenta and can result in Fetal

Bradycardia• Atropine preferred in Pregnant patients receiving

Neostigmine• With higher doses of Neostigmine

⌐Paradoxical potentiation of NM Blockade⌐may cause ACh channel blockade⌐can cause a weak Depolarizing neuromuscular Blockade

PYRIDOSTIGMINE• carbamate with Quaternary Ammonium Group• only as 20% as potent as Neostigmine• dose : 0.1 to 0.4 mg/kg (Max of 20 mg in adults)• onset : Slower – 10 to 15 minutes• duration : Longer - > 2 hours• preferred Anticholinergic :• Glycopyrrolate• 0.05 mg per mg of Pyridostigmine

• If Atropine used; 0.1 mg per mg of Pyridostigmine

EDROPHONIUM• Quaternary Ammonium Group• lacks carbamate group• unlike other agents, it forms reversible

electrostatic attachment to the enzyme• Edrophonium appears to have presynaptic

effects : Enhancement in the release of Ach• less than 10% as potent as Neostigmine• dose : 0.5 – 1 mg/kg• most Rapid Onset : 1-2 min• shortest duration / but higher doses

prolong the duration to >1 hour

• Preferred Anticholinergic : Atropine• Rapid onset is well matched to that of

Atropine• 0.014 mg Atropine per mg Edrophonium• If Glycopyrrolate used; it must be given

several minutes prior to avoid Bradycardia (0.007mg/mg)

• muscarinic effects of Edrophonium are less pronounced• pediatric and geriatric patients are not

more sensitive to it• not as effective as Neostigmine in

reversing intense NM blockade• more effective in reversing Mivacurium

blockade

PHYSOSTIGMINE• natural alkaloid derived from Calabar bean

(Physostigma venenosum)• tertiary amine• lacks quaternary ammonium : lipid soluble• penetrates CNS: limits use as reversal agent• effective in reversing:• central anticholinergic actions due to Atropine

or Scopolamine overdosages• benzodiazepine and volatile anaesthetic

induced CNS depression and delirium

• effective in preventing post-operative shivering (0.04 mg/kg)• may partially antagonize the respiratory depression

caused by Morphine.• glycopyrrolate will not reverse the CNS effects of

Physostigmine.• almost completely metabolized by plasma esterases--

renal excretion not important

Uses of Anticholinersterases

• primary use : to reverse the Nondepolarizing neuro-muscular blockade

• reversal agents are routinely given to patients who have received non-depolarizing muscle relaxants

OTHER USES OF ANTICHOLINESTERASES• Edrophonium- Tensilon test - diagnosis of Myasthenia gravis

TENSILON TEST

2 mg IV Edrophonium

Myasthenia gravis : Improvement of the

symptoms

LEMS : No improvement

Cholinergic crises : Worsening of

symptoms

• Neostigmine, Pyridostigmine and Ambenonium – Standard Anticholinesterases used in Myasthenia gravis.• Oral dose is much higher as- Neostigmine and

Pyridostigmine are lipid insoluble.• Muscarinic side effects need to be controlled with

anticholinergic agents• Bromide toxicity – with Pyridostigmine Bromide

• Decrease intraocular pressure in Narrow angle and wide angle Glaucoma

• Physostigmine eye drops

• Ecothiophate (Organophosphate with quaternary structure)

• Demecarium

GLAUCOMA

• Neurodegenerative : Cholinergic neurons also affected• Mild to moderate Alzheimer

disease.• Cerebroselective AChEi :• donepezil• rivastigmine• tacrine (significant liver toxicity)• galantamine

ALZHEIMER’S DISEASE

• Overdosage of Atropine or Scopolamine

• Restlessness and Confusion

• Physostigmine used – Lipid soluble

Central Anticholinergic Syndrome

• Physostigmine abolishes the somnolent effects of Opioids and Volatile anaesthetics

• Physostigmine may also be used to reverse the ventilatory depression caused by Morphine without decreasing its analgesic effects

• Physostigmine (40ugm/kg IV) following anaesthesia decreases the incidence of postoperative shivering

• Diagnosis and management of Cardiac Dysrhythmias• Edrophonium – Paroxysmal SVTs• 5-10mg slows heart rate with no effect on ventricular

contractility, conduction or peripheral vascular tone

• Post-operative Ileus / Urinary Retention• Neostigmine 0.5 to 1 mg SC• Organic Obstruction has to be ruled out first

• Belladonna Poisoning• Anticholinergic excess• Physostigmine preferred

• Neostigmine as neuraxial anaesthesia adjunct• Intrathecal (50 – 100 mcg)• Epidural (1 – 4 mcg/kg)

• Analgesia produced is probably by inhibiting the breakdown of spinal released Ach which is increased in presence of pain

• Disadvantages – High incidence of nausea and vomiting, pruritis, prolongation of sensory and motor block

Sugammadex• Sugammadex is in the family of cyclic dextrose units

used as solubilizing agents since 1953• Modfied gamma-cyclodextrine-comprised of 8 sugar

molecules that form a rigid ring with a central lipophilic cavity• Initially discovered when a compound was needed

to increase the solubility of rocuronium in a specific media• Observed permanent binding of the rocuronium

molecule to the center of the sugammadex molecule in a 1:1 ratio

Mechanism of action• Hydrophobic interactions trap the drug in the

cyclodextrine cavity forming a tight water-soluble complex in a 1:1 ratio (encapsulation)• A concentration gradient is created with no free

unbound NDMR in the plasma as compared to the extravascular compartment• Favors movement of NDMR (rocuronium or

veuronium) into the plasma where they are rapidly encapsulated• Terminates the NDMR’s action and restrains the

drug in extracellular fluid where it cannot interact with nicotinic acetylcholine receptors

Pharmacodyamics• Sugammadex is biologically inactive and does not

bind to plasma proteins• Metabolism of sugammadex very limited, and the

drug is predominantly eliminated unchanged by the kidneys- approximately 75%• sugammadex should be avoided in patients with a

creatinine clearance of <30 mL per minute

Pharmacokinetics• Sugammadex, used in appropriate doses, is capable of

reversing any depth of neuromuscular blockade (profound or shallow) induced by rocuronium or vecuronium• During rocuronium or vecuronium-induced

neuromuscular blockade, intravenous administration of sugammadex results in rapid removal of free rocuronium or vecuronium molecules from the plasma• Sugammadex is ineffective against succinylcholine and

benzylisoquinolinium neuromuscular blockers such as mivacurium, atracurium, and cisatracurium because it cannot form inclusion complexes with these drugs

Dosing Available: 2mL or 5mL vials (100mg/mL)

Dose of Sugammadex Indication Mean Recovery time

to TOF 0.9

16mg/kg Immediate Reversal after 1.2 mg/kg rocuronium

1.5 minutes

4mg/kg Routine reversal of deep neuromuscular block

3 minutes

2mg/kg Routine reversal of moderate block

2 minutes

Safety and Tolerability• The U.S. Food and Drug Administration has expressed

concerns about the safety of sugammadex, citing its possible association with allergic reactions, and bleeding

• sugammadex has not been approved for clinical use in the United States• more than 5 million doses of sugammadex have been

administered worldwide

THANK YOU!!!

• SUCCINYLCHOLINE and Depolarizing Blockade• Neostigmine and other AChEi prolong the

depolarizing blockade produced by Succinylcholine• increased Ach concentration will increase the motor

end plate depolarization• neostigmine and pyridostigmine has some

Psuedocholinesterase inhibiting action

General Findings• Extensive research and numerous clinical trials have

evaluated the safety, efficacy, and usefulness of sugammadex• A metaanalysis including 18 randomized controlled

trials demonstrated that sugammadex can reverse rocuronium induced blockade faster than neostigmine at all levels of blockade in a dose dependent manner• In trials of immediate reversal sugammadex

(16mg/kg) was administered 3 minutes after profound blockade by rocuronium and showed faster recovery than patients who underwent spontaneous recovery from succinylcholine