DRUGS MODULATING CHOLINESTERASE ENZYME

ORGANOPHOSPHOROUS POISONING

Dr. POOJA. M

DRUGS MODULATING CHOLINESTERASE ENZYME

ACETYLCHOLINE

Acetic acid ester of cholineCH3-CO-CH2-CH2N+(CH3)3Cl-

O

ACh is synthesized in the axon terminal.

Acetylation of choline with acetyl coenzyme A catalysed by enzyme choline acetyl transferase.

Choline + acetyl coA AcetylcholineChAT

Newly synthesized ACh is transported into the vesicle mediated by H+ - ACh antiporter.

Presynaptic action potential rise in intracellular Ca2+ release of ACh into the synaptic cleft.

ACh in the synaptic cleft diffuses and binds to the muscarinic and nicotinic receptors producing the respective effects.

Degradation of ACh is by the enzyme Acetylcholinesterase.

Acetylcholine Choline+ Acetic acid. Acetylcholinesterase

ANTICHOLINESTERASES

Anticholinesterases are the agents that bind to and inhibit AChE elevating the concentration of endogenously released ACh in the synaptic cleft.

Accumulated ACh activates nearby cholinergic receptors.

Also known as Indirectly acting ACh receptor agonists.

Classification

REVERSIBLE1. Carbamates-

PhysostigmineNeostigminePyridostigmineEdrophoniumRivastigmineDonepezilGalantamine

2. Acridine-Tacrine

IRREVERSIBLE1. Organophosphates-

Dyflos (DFP)EcothiophateMalathionDiazinon

2. Carbamates-CarbarylPropoxur

CHEMISTRY• Anti- ChEs are either esters of carbamic acid or derivatives

of phosphoric acid.

• Carbamates may have a non polar tertiary amino N+ (Physostigmine) or a quaternary N+ (Neostigmine).

• Carbamates with tertiary N+ Lipid soluble

• Carbamates with quaternary N+ Lipid insoluble.

• All organophosphates are lipid soluble except echothiophate which is water soluble.

MECHANISM OF ACTION• AChE is an extremely active enzyme.

• ACh binds to the enzyme’s active site, undergoes hydrolysis choline + acetylated enzyme.

• Splitting of covalent acetylated enzyme bond with addition of water (Hydration).

• Cholinesterase inhibitors Inhibit AChE Increase in concentration of endogenous ACh at cholinoceptors.

• Carbamate esters (Neostigmine and Physostigmine) 2 step hydrolysis similar to ACh.

• The covalent bond more resistant to hydration prolonged action (t1/2- 15-30 min)

• Edrophonium(quaternary alcohol) binds electrostatically and by hydrogen bonds to the active site.

• Covalent bond not involved (t1/2- 2-10 min)

• Organophosphates binding and hydrolysis phosphorylated active site.

• Covalent phosphorous enzyme bond- extremely stable, hydrolyses in water at a slow rate.

• Aging breaking of one oxygen-phosphorous bonds of the inhibitor and strengthening the phosphorous enzyme bond.

• Once aging has occurred enzyme inhibitor complex is more stable difficult to break.

• Pralidoxime (Cholinesterase reactivator) breaks the phosphorous enzyme bond if given before aging.

PHARMACOLOGICAL ACTIONS• Anti-AChE’s act on muscarinic, ganglionic, skeletal muscles

and CNS sites.

• Lipid soluble agents more marked muscarinic and CNS effects.

Stimulates ganglia but action on skeletal muscles is less prominent.

• Lipid insoluble agents marked effect on skeletal muscles, stimulates ganglia.

Muscarinic effect less prominent. No CNS effects.

• CVS- Muscarinic action bradycardia, hypotensionGanglionic stimulation increases heart rate and BP.

• Eye Contraction of the circular muscle of iris MiosisContraction of ciliary muscle spasm of accomodation, reduction in the intraocular tension.

• Skeletal musclesAccumulated ACh rebinds with the cholinoceptors twitching and fasciculations.Force of contraction is increased in myasthenic muscles.

At higher doses, persistent depolarisation of endplates blockade of neuromuscular transmission weakness and paralysis.

• GIT Tone and peristalsis in the GIT is increased, sphincters relax abdominal cramps, bowel evacuation.

• BladderContraction of detruser muscle, relaxation of bladder trigone and sphincter voiding

PHARMACOKINETICS

• Physostigmine-

Rapidly absorbed from GIT and parenteral sites.

Penetrates cornea freely.

Crosses blood brain barrier.

Hydrolysed by ChE.

PHARMACOKINETICS• Neostigmine- Poorly absorbed orally. Does not penetrate cornea or BBB. Partially hydrolysed and partially excreted unchanged in the

urine.

• Organophosphates- Absorbed from all sites including intact skin and lungs. Undergo hydrolysis in the body.

INDIVIDUAL COMPOUNDS• PHYSOSTIGMINE Natural alkaloid from Physostigma venenosum. Tertiary amine derivative. Important use- Miotic Dose- 0.5- 1mg oral/ parenteral

0.1- 1% eye drops Duration of action- Systemic- 4- hrs

In eye- 6-24hrs Adverse effect- convulsions, bradycardia, fall in cardiac

output, paralysis of skeletal muscles.

• NEOSTIGMINE Synthetic quaternary ammonium compound.

Important use- Myasthenia gravis

Dose- 0.5- 2.5 mg IM/SC 15-30 mg orally

Duration of action- 3-4 hrs

Adverse effects- Generalised cholinergic stimulation salivation, flushing, fall in BP, nausea, pain abdomen, diarrhea, bronchospasm.

• PYRIDOSTIGMINE Resembles neostigmine Less potent and long acting Used in Myasthenia gravis Duration of action- 3-6 hrs.

• EDROPHONIUM Similar to neostigmine; more rapidly absorbed. Short duration of action- 10-20 min Used in the diagnosis of Myasthenia gravis Dose- 2-10mg IV

• RIVASTIGMINE Cerebroselective ChE inhibitor Highly lipid soluble Indicated in mild- moderate AD

• DONEPEZIL Cerebroselective and reversible Can be used in severe cases of AD

• GALANTAMINE

Natural alkaloid, cerebroselective

Has direct agonistic action on nicotinic receptors

Produces cognitive and behavioural benefits in AD.

• ECHOTHIOPHATE Organophosphates with quaternary structure

Potent, long acting

Water soluble

Uses- Treatment of open angle galucoma

USES

• Ophthalmic uses- Physostigmine is used-

for the treatment of open angle glaucoma

for reversal of mydriasis

to prevent adhesions between pupils and lens

• Myasthenia gravis- Neostigmine 15mg orally 6 hourly.

Pyridostigmine is an alternative, needs less frequent dosing.

Intolerable muscarinic effects can be blocked by Atropine.

Corticosteroids- immunosuppresant activity.- inhibit production of antibodies increasing

synthesis of nicotine receptors.- Initally 30-60 mg/day followed by 10 mg daily or alternate days as maintainance therapy

Azathioprine and cyclosporine inhibit NR-antibody synthesis by affecting T-cells.

Plasmapheresis

Thymectomy

Myasthenic crisis??

• Overtreatment with anti-ChEs- Overdose persistent depolarisation of muscle

endplate weakness Cholinergic crisis.

Edrophonium test- to differentiate cholinergic crisis and myasthenic crisis.

Edrophonium 2mg IV--

Improvement- Myasthenic crisis

No improvement/ worsening- Cholinergic crisis

• Alzheimers disease-

• Post operative paralytic ileus/ urinary retention- 0.5-1 mg SC neostigmine.

• Cobra bite- Neostigmine+ atropine prevents respiratory paralysis

• Belladona poisoning- 0.5-2 mg IV Physostigmine is a specific antidote for poisoning with belladona or other anticholinergics.

Case Scenario• A 50-year-old farmer is brought to the emergency department

in a semiconscious state and having garlic like smell. A bottle of

pesticide containing Organophosphorous compound was found

beside him says the attender. O/E, the patient has pinpoint

pupils, large amounts of secretions pouring from his mouth. His

heart rate is 120 bpm, BP 90/60 mmHg, and oxygen saturation

65%. His chest has widespread crackles and rhonchi. Fine

fasciculations are apparent in his peri-orbital, chest, and leg

muscles. He also has incontinence of urine and faeces.

ORGANOPHOSPHOROUS POISONING

• Easily available, extensively used as agricultural and household insecticides.

• Accidental as well as suicidal and homicidal poisoning is common.

• OP inhibits acetylcholinesterases rise in ACh activity at nicotinic and muscarinic receptors in CNS.

Clinical Features• GI symptoms- Abdominal cramps, diarrhea, vomiting.

• Excessive salivation, sweating, lacrimation, miosis.

• Involuntary defecation and urination.

• Initial tachycardia followed by bradycardia, fall in BP

• Irritability, disorientation, unsteadiness, tremor, ataxia, convulsions, coma

• Skeletal muscle weakness aggravated by excessive bronchial secretions respiratory arrest and death.

• Diagnosis should be suspected in patients presenting with miosis, sweating, hyperperistalsis and a characteristic garlic like odour.

• Serum and red blood cell cholinesterase activity is usually depressed at least 50% below baseline in those who have severe intoxication.

• Treatment Termination of further exposure to poison

Maintain patent airway, positive pressure respiration

Supportive measures- maintain BP, hydration, control of convulsions with use of diazepam

Specific antidotes- Atropine, Pralidoxime

• ATROPINE Highly effective in counteracting muscarinic symptoms.

High doses required to antagonise central effects.

Atropine 2mg IV every 10 min till signs of atropinisation occur

Continued treatment with maintainance dose may be required for 1-2 weeks.

• CHOLINESTERASE REACTIVATORS Pralidoxime, contains a quaternary ammonium group

attaches to the anionic site of the enzyme.

Oxime end reacts with the phosphorous atom attached to the esteric site.

Oxime-phosphonate formed diffuses leaving the reactivated ChE.

Treatment should be started early before the phosphorylated enzyme has undergone ‘aging’ and become resistant to hydrolysis

• Pralidoxime 1-2g slow IV; Children- 20-40mg/kg

• 30 mg/kg IV loading dose followed by 8-10 mg/kg/hour continuous infusion till recovery.

REFERENCES Basic and clinical pharmacology – Katzung.

Lippincott’s illustrated reviews.

David E Golan’s Principles of pharmacology.

K D Tripathi

H L Sharma

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