Cholinergic Agents

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Cholinergic Agents
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Cholinergic Agents
Alkaloids Nicotine Lobeline Arecoline Muscarine Pi
locarpine
Synthetic Agents Dimethylphenylpiperazinium-(DMPP)
Oxotremorine Methacholine Bethanechol Carbachol C
evimeline
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Nicotine

• Nicotine mimics the actions of acetylcholine at
  nicotinic sites
• Cell body of the postsynaptic neurons
• sympathetic and parasympathetic divisions
• Chromaffin cells of the adrenal medulla
• End plate of skeletal muscle fiber
• Affinity for NN sites versus NM sites
• Used as an insecticide

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Muscarine

• Muscarine mimics the actions of acetylcholine at
  smooth muscles, cardiac muscles, and glands
• Poisoning by muscarine produces intense effects
  qualitative to those produced by cholinergic
  stimulation of smooth muscles, cardiac muscle,
  and glands
• Muscarine is found in various mushrooms
• Amanita muscaria content of muscarine is very
  low
• Inocybe sp content of muscarine is high
• Clitocybe sp content of muscarine is high

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Pilocarpine

• Has muscarinic actions
• Used for xerostomia
• Used for glaucoma

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Structure of Acetylcholine and its Derivatives
Acetylcholine
Methacholine
Bethanechol
Carbachol
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Therapeutic Uses of Cholinergic Agonists

• Dentistry
• Pilocarpine
• Cevimeline
• Ophthalmology
• Pilocarpine
• Carbachol
• Gastrointestinal tract
• Bethanechol
• Urinary bladder
• Bethanechol

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Contraindications to the Use of Choline Esters

• Hyperthyroidism
• Asthma
• Coronary insufficiency
• Peptic ulcer
• Organic obstruction in bladder or
  gastrointestinal tract

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Toxicity of Choline Esters

• Flushing
• SWEATING (diaphoresis)
• Abdominal cramps
• Spasm of the urinary bladder
• Spasm of accomodation
• Miosis
• Headache
• Salivation
• Bronchospasm
• Lacrimation
• Hypotension
• Bradycardia

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Agents That Inhibit Acetylcholinesterase
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Acetylcholinesterase
(True Cholinesterase)
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Acetylcholinesterase (1)

• Sites of location
• Cholinergic neurons
• Cholinergic synapses
• Neuromuscular junction
• Red blood cells
• Substrates
• Acetylcholine is the best substrate
• Methacholine is a substrate
• Hydrolyzes ACh at greater velocity than choline
  esters with acyl groups larger than acetate or
  proprionate

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Acetylcholinesterase (2)

• Esters that are not substrates
• Bethanechol
• Carbachol
• Succinylcholine
• Its inhibition produces synergistic interaction
  with methacholine and additive actions with
  bethanechol and carbachol
• Drugs that block its hydrolysis of esters are
  called cholinesterase inhibitors

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Drug Interactions of Choline Esters and
Inhibitors of Acetylcholinesterase - Synergism
versus Additivity

• Methacholine
• Carbachol
• Bethanechol

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Butyrylcholinesterase
(Plasma esterase, pseudocholinesterase, serum
esterase, BuChE, PseudoChE)
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Butyrylcholinesterase (1)

• Sites of location
• Plasma, liver, glial cells, other tissues
• Substrates
• Butyrylcholine is the best
• Acetylcholine
• Succinylcholine
• Procaine

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Butyrylcholinesterase (2)

• Esters that are not substrates
• Methacholine, bethanechol, and carbachol
• Is inhibited by carbamyl and organophosphate
  inhibitors of acetylcholinesterase

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Active Site of Acetylcholinesterase
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Interaction of AChE and Acetylcholine
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(No Transcript)
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Acetylation of AChE and Release of Choline
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Hydroxyl Group of Water Attacks the Carbonyl
Group of Acetylated-AChE to Liberate AChE
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Carbamyl Inhibitors of AChE
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Carbamyl Inhibitors of AChE (1)

• Their action promoting accumulation of ACh at
  muscarinic or nicotinic receptors is the basis of
  their pharmacological, therapeutic, and toxic
  actions
• Are derivatives of carbamic acid
• Bind covalently to the esteratic site of AChE,
  resulting in carbamylation of the enzyme

Carbamic acid
Carbamic acid ester
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Carbamyl Inhibitors of AChE (2)

• Quaternary compounds bind to the ionic binding
  site of AChE
• Their induce accumulation of AChE at nicotinic
  and muscarinic sites, producing pharmacological
  responses qualitative to cholinergic stimulation
• Inhibition of AChE is reversible, in the order of
  hours
• Are metabolized in the plasma by plasma esterases

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Carbamyl Inhibitors of AChE (3)

• High doses produce skeletal muscle weakness due
  to depolarizing blockade at the end plate of the
  neuromuscular junction
• High doses produce a profound fall in cardiac
  output and blood pressure
• Their inhibition of AChE is not reversed by
  pralidoxime

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Carbamyl Inhibitors of AChE (4)

• Quaternary ammonium compounds do not cross the
  blood-brain barrier
• For oral administration, high doses must be given

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Neostigmine Carbamylates Acetylcholinesterase
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Slow Hydrolysis of Carbamylated-AChE and Enzyme
Liberation
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Organophosphate Inhibitors of Acetylcholinesteras
e
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Organophosphate Inhibitors of Acetylcholinesterase
(1)

• Chemical characteristics
• Promote accumulation of ACh at
• NM nicotinic receptor
• NN nicotinic receptor
• Muscarinic receptor

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Organophosphate Inhibitors of AChE (2)

• Their action promoting accumulation of ACh at
  the muscarinic receptor of the ciliary muscle is
  the basis of their therapeutic effectiveness in
  open angle glaucoma
• Only two of these agents are used for
  therapeutics
• Echothiophate for glaucoma
• Diisopropylflurophosphate (DFP) for glaucoma (?)

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Organophosphate Inhibitors of AChE (3)

• Inhibition of AChE by these agents is
  irreversible
• New enzyme synthesis is required for recovery of
  enzyme function
• They also inhibit pseudocholinesterase
• Metabolized by A-esterases (paroxonases) present
  in plasma and microsomes. They are metabolized by
  CYP450.

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Organophosphate Inhibitors of AChE (4)

• Enzyme inhibition by these agents can be reversed
  by cholinesterase reactivators such as
  pralidoxime if administered before aging of
  AChE has occurred. Inhibition by agents that
  undergo rapid aging is not reversed.
• Except for echothiophate, these agents are
  extremely lipid soluble, and some are very
  volatile.

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Diisopropylflurophosphate (DFP) is a Substrate
for AChE
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The Extremely Slow Hydrolysis of
Phosphorylated-AChE
New enzyme synthesis is required for recovery of
enzyme function
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Various States of Acetylcholinesterase
Clockwise free AChE, acetylated AChE,
carbamylated AChE, phosphorylated AChE
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Acetylated-AChE Is Very Rapdily Hydrolyzed
P
AChE Acetylcholine ? AChE-acetylated choline
AChE-acetylated H2O ? AChE acetate
Hydrolysis of AChE-acetylated is rapid, in the
order of microseconds
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Carbamylated-AChE Is Hydrolyzed Slowly
AChE Carbamyl inhibitor ? AChE-carbamylated
noncarbamylated metabolite
AChE-carbamylated H2O ? AChE carbamic acid
derivative
Hydrolysis of the AChE-carbamylated is slow, in
the order of hours. The carbamylated enzyme is
reversibly inhibited, and recovery of function is
in the order of hours
Enzyme after phosphorylation by neostigmine
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Phosphorlylated-AChE Is Hydrolyzed Extremely
Slowly
AChE organophosphate inhibitor ?
AChE-phosphorylated nonphosphorylated
metabolite
AChE-phosphorylated H2O ? AChE phosphorylated
derivative
Hydrolysis of the AChE-phosphorylated is
extremely slow, in the order of days. The
phosphorylated enzyme is considered to be
irreversibly inhibited, and recovery of function
is in the order of days. Pralidoxime, a
reactivating agent, may be adminstered to a
subject before the enzyme has aged.
Enzyme after phosphorylation by DFP
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AGING OF ACETYLCHOLINESTERASE
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Loss of An Alkyl Group From Phosphorylated
AChE Ages the Enzyme
AChE, phosphorylated and inhibited by DFP
Aged AChE
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Aging of Phosphorylated- AChE
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Cholinesterase Reactivation
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Reactivation of Phosphorylated
Acetylcholinesterase

• Oximes are used to reactivate phosphorylated AChE
• The group (NOH) has a high affinity for the
  phosphorus atom
• Pralidoxime has a nucleophilic site that
  interacts with the phosphorylated site on
  phosphorylated-AChE

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Pralidoxime Reacts Chemically with
Phosphorylated-AChE
The oxime group makes a nucleophilic attack upon
the phosphorus atom
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Oxime Phosphonate and Regenerated AChE
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Limitations of Pralidoxime

• Pralidoxime does not interact with
  carbamylated-AChE
• Pralidoxime in high doses can inhibit AChE
• Its quaternary ammonium group does not allow it
  to cross the blood brain barrier
• Aging of phosphorylated-AChE reduces the
  effectiveness of pralidoxime and other oxime
  reactivators

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Other Cholinesterase Reactivators

• Diacetylmonoxime
• Crosses the blood brain barrier and in
  experimental animals, regenerates some of the CNS
  cholinesterase
• HI-6 is used in Europe
• Has two oxime centers in its structure
• More potent than pralidoxime

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Edrophonium
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Edrophonium is a Short Acting Inhibitor that
Binds to the Ionic Site but Not to the Esteratic
Site of AChE
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Pharmacology of Acetylcholinesterase Inhibition
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Inhibition of Acetylcholinesterase Produces
Stimulation of All Cholinergic Sites
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Carbamyl Inhibitors of AChE

• Physostigmine
• Neostigmine (N)
• Pyridostigmine (N)
• Ambenonium (N)
• Demecarium (N)
• Carbaryl

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Pharmacology of Carbamyl Inhibitors of
Acetylcholinesterase

• Eye
• Exocrine glands
• Cardiac muscle
• Smooth muscles
• Skeletal muscle
• Toxicity

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Therapeutic Uses of Inhibitors of
Acetylcholinesterase

• Glaucoma (wide angle)
• Atony of the bladder
• Atony of the gastrointestinal tract
• Intoxication by antimuscarinic agents (use
  physostigmine)
• Intoxication by tricyclic antidepressants (TCAs)
  or phenothiazines (use physostigmine)
• Recovery of neuromuscular function after
  competitive blockade of NN receptor of skeletal
  muscle fibers
• Myasthenia gravis

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Therapeutic Uses of Edrophonium

• Diagnosis of myasthenia gravis
• In conjunction with chosen therapeutic agent to
  determine proper dose of agent

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Determining Proper Dose of AChE Inhibitor
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Inhibitors of AChE Are Used for Therapy of
Alzheimers Disease

• Tacrine
• Donepezil
• Rivastigmine
• Galantamine

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Organophosphate Inhibitors of AChE
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Some Organophosphate Inhibitors of
Acetylcholinesterase

• Tetraethylpyrophosphate
• Echothiophate (N)
• Diisopropylflurophosphate (DFP)
• Sarin
• Soman
• Tabun
• Malathion
• Parathion
• Diazinon
• Chlorpyrifos
• Many others

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Organophosphate Inhibitors - 2
Diisopropylfluorophosphate (DFP)
Soman
Tabun
Sarin
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Echothiophate
Therapeutic use - local application to the eye
for wide angle glaucoma
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Conversion of Parathion to Paraoxon
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Conversion of Malathion to Malaoxon
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Malathion Is Hydrolyzed by Plasma Carboxylases in
Birds and Mammals but Not Insects
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Carboxyl Esterases

• Preferentially hydrolyzes aliphatic esters
• Malathion is a substrate
• Are inhibited by organophosphates
• May also be called aliesterases

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Uses of Malathion

• Insecticide
• Therapeutics
• Used as a lotion for Pediculus humanus capitis
  associated with pediculosis
• 0.5 solution in 78 isopropranolol is
  pediculicidal and ovicidal
• Ovide is the brand name
• Primoderm was the former brand name

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Malathion Metabolism

• Rapidly metabolized by birds and mammals
• Plasma carboxylases are involved
• Insects do not possess the enzyme
• Organophosphates inhibit malathion metabolism
• Malathion is toxic to fish

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Aryl Esterases

• Are found in the plasma and liver
• Hydrolyzes organophosphates at the
• P-F bond
• P-CN bond
• Phosphoester bond
• Anhydride bond

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EPA And Organophosphates

• Diazinon
• No longer allowed to be manufactured for indoor
  use in as of March 1, 2001 or for garden use as
  of June 3, 2001
• Found in Real Kill, Ortho, Spectracide
• Limited agricultural use is allowed
• Chlorpyrifos (Dursban) has been phased out
• Parathion has been phased out for agricultural
  use in the United States

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NERVE AGENT VX
NERVE AGENT VX
Chemical name O-ETHYL-S-(2-DIISOPROPYLAMINOMETHYL
)METHYL-PHOSHONOTHIOLATE Trade name
PHOSPHONOTHIOIC ACID
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NERVE AGENT VX
Chemical name O-ETHYL-S-(2-DIISOPROPYLAMINOMETHYL
)METHYL-PHOSHONOTHIOLATE Trade name
PHOSPHONOTHIOIC ACID
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Organophosphates as Nerve Gas Agents in Chemical
Warfare (1)

• Extremely volatile agents such as sarin, tabun,
  soman, and agent VX may be used as nerve agents
  in chemical warfare.
• Accumulation of ACh at cholinergic receptors
  produces effects reflecting stimulation of
  cardiac muscle, smooth muscles and glands. Such
  effects would be identical to those caused by
  muscarine poisoning.
• Bradycardia and hypotension occur. However, in
  some cases, tachycardia may be observed, due to
  intense sympathetic discharge in response severe
  hypoxemia.

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Organophosphates as Nerve Gas Agents in Chemical
Warfare (2)

• Irreversible inhibition of acetylcholinesterase
  by these agents produces accumulation of ACh at
  the end plate of skeletal muscle fibers. This in
  turn leads to depolarizing blockade of the NM
  nicotinic receptor. Skeletal muscle paralysis
  occurs. Movement is impossible. The diaphragm is
  also paralyzed. The individual eventually dies
  due to respiratory paralysis.
• Pralidoxime, atropine, and removal of the person
  from the source of exposure are all to be
  employed in cases of posioning.

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Use of Pyridostigmine During the Gulf War