Patrick
An Introduction to Medicinal Chemistry 3/e
Chapter 19
CHOLINERGICS, ANTICHOLINERGICS
& ANTICHOLINESTERASES
Part 1: Cholinergics & anticholinesterases
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Contents
Part 1: Cholinergics & anticholinesterases
1.
2.
3.
4.
Nerve Transmission (3 slides)
Neurotransmitter
Transmission process (10 slides)
Cholinergic receptors (2 slides)
4.1.
Nicotinic receptor (2 slides)
4.2.
Muscarinic receptor - G Protein coupled receptor (2 slides)
5. Cholinergic agonists
5.1.
Acetylcholine as an agonist
5.2.
Nicotine and muscarine as cholinergic agonists
5.3.
Requirements for cholinergic agonists
6. SAR for acetlcholine (6 slides)
7. Binding site (muscarinic) (3 slides)
8. Active conformation of acetylcholine (2 slides)
9. Instability of acetylcholine
10. Design of cholinergic agonists (7 slides)
11. Uses of cholinergic agonists (2 slides)
[46 slides]
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CHOLINERGIC
NERVOUS
SYSTEM
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1. Nerve Transmission
Peripheral nervous system
CNS
Brain
Peripheral nerves
Muscle
Heart
Gastrointestinal
tract
(GIT)
Spinal cord
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Sympathetic nervous system = ‘Fight or Flight Response’
Parasympathetic nervous system = ‘Rest and Digest Response’
Link
Link
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1. Nerve Transmission
Peripheral nervous system
Skeletal
muscle
CNS
(Somatic)
CNS
(Autonomic)
Sympathetic
Ach
(N)
Synapse
Ach (N)
NA
Adrenaline
Parasympathetic
Ach
(N)
Adrenal
medulla
AUTONOMIC
Synapse
Ach
(N)
Ach
(M)
Smooth muscle
Cardiac muscle
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Dual Innervation
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http://entochem.tamu.edu/neurobiology/index.html
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1. Nerve Transmission
Synapses
100-500A
Receptors
New signal
Nerve impulse
Nerve
Nerve
Vesicles containing
neurotransmitters
Release of
neurotransmitters
Receptor binding
and new signal
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2. Neurotransmitter
Acetylcholine (Ach)
O
+
C
H 3C
Acetyl
NMe 3
O
Choline
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3. Transmission process
Signal in nerve 1
...
Nerve 1
Signal
...
.
Nerve 2
...
Acetylcholinesterase enzyme
Acetylcholine
Vesicle
Cholinergic receptor
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3. Transmission process
Vesicles fuse with membrane and release Ach
Nerve 1
Nerve 2
Signal
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3. Transmission process
Nerve 2
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3. Transmission process
•
•
•
•
Receptor binds Ach
Induced fit triggers 2o message
Triggers firing of nerve 2
Ach undergoes no reaction
2o Message
Nerve 2
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3. Transmission process
•
•
•
Ach departs receptor
Receptor reverts to resting state
Ach binds to acetylcholinesterase
Nerve 2
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3. Transmission process
Ach hydrolysed
by acetylcholinesterase
O
O
C
H 3C
O
Ac e tylcholine
HO
C
NMe3
H 3C
Ac e tic acid
OH
+
NMe3
Choline
Nerve 2
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3. Transmission process
Choline binds to carrier protein
Choline
Nerve 1
Nerve 2
Carrier protein for choline
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3. Transmission process
Choline transported into nerve
Nerve 1
Nerve 2
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3. Transmission process
Ach resynthesised
Nerve 2
Nerve 1
E 1 = Choline acetyltransferase
O
O
C
H 3C
SCoA
+
E1
HO
CH2 CH2 NMe3
Choline
C
H 3C
NMe3
O
Acetylcholine
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3. Transmission process
Ach repackaged in vesicles
Nerve 1
Nerve 2
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4. Cholinergic receptors
Receptor types
• Not all cholinergic receptors are identical
• Two types of cholinergic receptor - nicotinic and muscarinic
• Named after natural products showing receptor selectivity
HO
NMe
Me
N
Nicotine
Activates cholinergic
receptors at nerve synapses
and on skeletal muscle
O
CH2NMe3
L-(+)-Muscarine
Activates cholinergic
receptors on smooth
muscle and cardiac muscle
Acetylcholine is natural messenger for both receptor types
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• Nicotine comprises up the 3% the dry weight of the tobacco leaf
• When inhaled, it rapidly crosses the blood-brain barrier where it
activates (acts as an agonist at) the acetylcholine receptors
• Addiction to nicotine is reported to be one of the hardest
addictions to break.
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• Muscarine is the active poisonous ingredient in several
species of mushrooms
• Ingestion causes severe nausea and diarrhea as the
muscarine acts as an acetylcholine agonist. Also causes
perspiration and lacrimation (tearing).
• The antidote is atropine, an acetycholine antagonist at the
muscarinic receptor.
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Peripheral nervous system
Skeletal
Muscle
CNS
(Somatic)
CNS
(Autonomic)
Sympathetic
SOMATIC
Ach
(N)
Synapse
Ach (N)
NA
Adrenaline
Parasympathetic
Ach
(N)
Adrenal
medulla
AUTONOMIC
Synapse
Ach
(N)
Ach
(M)
Smooth Muscle
Cardiac Muscle
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4.1 Nicotinic receptor
Control of Cationic Ion Channel:
Receptor
Binding
site
Cell
membrane
Five glycoprotein subunits
traversing cell membrane
Messenger
Induced
fit
Cell
membrane
‘Gating’
(ion channel
opens)
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4.1 Nicotinic receptor
The binding sites
Binding
sites
Ion channel
a
b
Cell
membrane
d
a
a
g
g
a
b
d
2xa, b, g, d subunits
Two ligand binding sites
mainly on a-subunits
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4.2 Muscarinic receptor - G Protein coupled receptor
Activation of a signal protein
• Receptor binds messenger leading to an induced fit
• Opens a binding site for a signal protein (G-protein)
messenger
induced
fit
closed
open
G-protein
bound
G-protein
split
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4.2 Muscarinic receptor - G Protein coupled receptor
Activation of membrane bound enzyme
• G-Protein is split and subunit activates a membrane bound
enzyme
• Subunit binds to an allosteric binding site on enzyme
• Induced fit results in opening of an active site
• Intracellular reaction is catalysed
Enzyme
subunit
active site
(closed)
Enzyme
active site
(open)
Intracellular
reaction
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5. Cholinergic agonists
5.1 Acetylcholine as an agonist
Advantages
• Natural messenger
• Easily synthesised
O
+
HO
NM e3
AcO
NMe3
NMe3
Ac2O
Disadvantages
• Easily hydrolysed in stomach (acid catalysed hydrolysis)
• Easily hydrolysed in blood (esterases)
• No selectivity between receptor types
• No selectivity between different target organs
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5. Cholinergic agonists
5.2 Nicotine and muscarine as cholinergic agonists
Advantages
• More stable than Ach
• Selective for main cholinergic receptor types
• Selective for different organs
Disadvantages
• Activate receptors for other chemical messengers
• Side effects
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5. Cholinergic agonists
5.3 Requirements for cholinergic agonists
•
Stability to stomach acids and esterases
•
Selectivity for cholinergic receptors
•
Selectivity between muscarinic and nicotinic receptors
•
Knowledge of binding site
•
SAR for acetylcholine
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6. SAR for acetlcholine
Ethylene
bridge
O
Me
O
Acetoxy
NMe3
4 o Nitrogen
Quaternary nitrogen is essential
O
H3C
O
O
CMe3
Bad for activity
H3C
O
NMe2
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6. SAR for acetylcholine
Ethylene
bridge
O
Me
O
Acetoxy
•
•
NMe3
4 o Nitrogen
Distance from quaternary nitrogen to ester is important
Ethylene bridge must be retained
O
O
H 3C
O
NMe3
H 3C
Bad for activity
O
NMe3
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6. SAR for acetylcholine
Ethylene
bridge
O
Me
O
NMe3
4 o Nitrogen
Acetoxy
Ester is important
H 3C
O
NMe3
H 3C
NMe3
Bad for activity
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6. SAR for acetylcholine
Ethylene
bridge
O
Me
O
NMe3
4 o Nitrogen
Acetoxy
Minimum of two methyl groups on quaternary nitrogen
O
O
H 3C
Et
Et
O
N
Et
H 3C
O
Me
Me
Et
Bad for activity
N
Active
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6. SAR for acetylcholine
Ethylene
bridge
O
Me
NMe3
O
4 o Nitrogen
Acetoxy
Methyl group of acetoxy group cannot be extended
O
H 3C
O
NMe3
Bad for activity
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6. SAR for acetylcholine
Ethylene
bridge
O
Me
Acetoxy
O
NMe3
4 o Nitrogen
Conclusions:
• Tight fit between Ach and binding site
• Methyl groups fit into small hydrophobic pockets
• Ester interacting by H-bonding
• Quaternary nitrogen interacting by ionic bonding
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7. Binding site (muscarinic)
hydrophobic
pocket
CH3
Trp-307
Asp311
CH3
CO 2
N
O
O
Trp-616 Trp-613
H
O
N
H
CH3
hydrophobic
pockets
CH3
hydrophobic
pocket
Asn-617
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7. Binding site (muscarinic)
Trp-307
vdw
CH3
Ionic bond
Asp311
CO 2
N
O
CH3
CH3
vdw
O
H-bonds
H
O
N
Trp-616 Trp-613
CH3
vdw
H
Asn-617
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7. Binding site (muscarinic)
•
•
Possible induced dipole dipole interaction between quaternary
nitrogen and hydrophobic aromatic rings in binding site
N+ induces dipole in aromatic rings
d+
d-
R
NMe3
d+
d-
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8. Active conformation of acetylcholine
O
H
H
Me
C
Me
Me
N
O
Me
H
H
•
Several freely rotatable single bonds
•
Large number of possible conformations
•
Active conformation does not necessarily equal the most
stable conformation
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8. Active conformation of acetylcholine
Rigid Analogues of acetylcholine
HO
Me
O
CH2NMe3
Me
H
O
O
NMe3
O
CH2NMe3
Me
O
H
MUSCARINE
•
•
•
Rotatable bonds ‘locked’ within ring
Restricts number of possible conformations
Defines separation of ester and N
O
4.4A
O
5.9A
N
N
Muscarinic
receptor
Nicotinic
receptor
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9. Instability of acetylcholine
Me 3N
d O
d C
H 3C
O
•
Neighbouring group participation
•
Increases electrophilicity of carbonyl group
•
Increases sensitivity to nucleophiles
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10. Design of cholinergic agonists
Requirements
•
Correct size
•
Correct pharmacophore - ester and quaternary nitrogen
•
Increased stability to acid and esterases
•
Increased selectivity
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10. Design of cholinergic agonists
Use of steric shields
Rationale
•
Shields protect ester from nucleophiles and enzymes
•
Shield size is important
•
Must be large enough to hinder hydrolysis
•
Must be small enough to fit binding site
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10. Design of cholinergic agonists
Me
O
Methacholine
Me
hinders binding to esterases
and provides a shield to
nucleophilic attack
O
NMe3
*
asymmetric centre
Properties
• Three times more stable than acetylcholine
• Increasing the shield size increases stability but decreases
activity
• Selective for muscarinic receptors over nicotinic receptors
• S-enantiomer is more active than the R-enantiomer
• Stereochemistry matches muscarine
• Not used clinically
HO
Me
O
O
MUSCARINE
CH2NMe3
Me
O
Me
O
(S)
CH2NMe3
Me
H
O
H
(R)
CH2NMe3
Me
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• The primary clinical use of methacholine is as an
acetylcholine agonist at the muscarinic receptor
• As such, it is utilized in a test for asthma, called the
‘bronchial challenge test’
• The methacholine provokes bronchoconstriction
• Asthmatic patients, which already have airway hyperactivity,
are more sensitive to the effect of methacholine, and this
reaction can be quantified using a breathing test called
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spirometry..
10. Design of cholinergic agonists
Use of electronic factors
•
•
Replace ester with urethane
Stabilises the carbonyl group
H 2N
O
O
C
C
H 2N
O
=
d
H 2N
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C
d
10. Design of cholinergic agonists
O
C
H2N
NMe3
O
Carbachol
Properties
• Resistant to hydrolysis
• Long lasting
• NH2 and CH3 are equal sizes. Both fit the hydrophobic pocket
• NH2 = bio-isostere
• Muscarinic activity = nicotinic activity
• Used topically for glaucoma
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• Glaucoma is an eye disease, characterized by increased
intraocular pressure. It leads to irreversible loss of vision and is
the second leading cause of blindness.
• Treatments for glaucoma focus on relieving the pressure.
• Carbachol causes miosis (constriction of the pupil), by
contracting the ciliary muscle, tightening the trabecular meshwork
and allowing increased outflow of the aqueous humour
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10. Design of cholinergic agonists
Steric + Electronic factors
O
Me
C
H2N
O
*
NMe3
Bethanechol
Properties
•
Very stable
•
Orally active
•
Selective for the muscarinic receptor
•
Used to stimulate GI tract and urinary bladder after surgery
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10. Design of cholinergic agonists
Nicotinic selective agonist
O
C
Me
O
*
NMe3
* asymmetric centre
Me
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11. Uses of cholinergic agonists
Nicotinic selective agonists
Treatment of myasthenia gravis
- lack of acetylcholine at skeletal muscle causing weakness
Muscarinic selective agonists
•
Treatment of glaucoma
•
Switching on GIT and urinary tract after surgery
•
Treatment of certain heart defects. Decreases heart muscle
activity and decreases heart rate
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Peripheral nervous system
Skeletal
Muscle
CNS
(Somatic)
CNS
(Autonomic)
Sympathetic
SOMATIC
Ach
(N)
Synapse
Ach (N)
NA
Adrenaline
Parasympathetic
Ach
(N)
Adrenal
medulla
AUTONOMIC
Synapse
Ach
(N)
Ach
(M)
Smooth Muscle
Cardiac Muscle
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