Patrick An Introduction to Medicinal Chemistry 3/e Chapter 19 CHOLINERGICS, ANTICHOLINERGICS

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Patrick Patrick An Introduction to Medicinal An Introduction to Medicinal

ChemistryChemistry 3/e 3/e

Chapter 19Chapter 19

CHOLINERGICS, CHOLINERGICS, ANTICHOLINERGICSANTICHOLINERGICS

& ANTICHOLINESTERASES& ANTICHOLINESTERASES

Part 1: Cholinergics & anticholinesterasesPart 1: Cholinergics & anticholinesterases

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ContentsContents

Part 1: Cholinergics & anticholinesterases

1. Nerve Transmission (3 slides)2. Neurotransmitter3. Transmission process (10 slides)4. 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 agonist5.2. Nicotine and muscarine as cholinergic agonists5.3. 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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CHOLINERGICCHOLINERGICNERVOUSNERVOUSSYSTEMSYSTEM

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1. Nerve Transmission1. Nerve Transmission

Peripheral nervous systemPeripheral nervous system

Peripheral nerves

Muscle

Gastro-intestinal tract (GIT)

Brain

Spinal cord

CNS

Heart

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Link

Link

Sympathetic nervous system = ‘Fight or Flight Response’

Parasympathetic nervous system = ‘Rest and Digest Response’

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CNS(Somatic)

CNS(Autonomic)

Sympathetic

Parasympathetic

NA

Ach(N)

Synapse

Ach (N)

Ach(N)

Ach(N)

Ach(M)

Adrenalmedulla

Adrenaline

Skeletalmuscle

Synapse

AUTONOMIC

Smooth muscleCardiac muscle

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Dual Innervation

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http://entochem.tamu.edu/neurobiology/index.html

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NerveNerve


SynapsesSynapses

100-500AReceptorsReceptors

Release ofneurotransmitters

Receptor binding and new signal

Nerve impulseNerve impulse New signalNew signal

Vesicles containingneurotransmitters

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2. Neurotransmitter2. Neurotransmitter

Acetylcholine (Ach)Acetylcholine (Ach)

CholineCholineAcetylAcetyl

H3C OC NMe 3

O+

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3. Transmission process3. Transmission process

Signal in nerve 1Signal in nerve 1

Acetylcholinesterase enzymeAcetylcholinesterase enzyme

Cholinergic receptorCholinergic receptor.AcetylcholineAcetylcholine

VesicleVesicle

......

...

Nerve 1Nerve 1Nerve 2Nerve 2

SignalSignal

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Vesicles fuse with membrane and release AchVesicles fuse with membrane and release Ach


SignalSignal

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Nerve 2Nerve 2

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• Receptor binds AchReceptor binds Ach• Induced fit triggers 2Induced fit triggers 2oo message message• Triggers firing of nerve 2Triggers firing of nerve 2• Ach undergoes no reactionAch undergoes no reaction

22oo Message Message

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• Ach departs receptorAch departs receptor• Receptor reverts to resting stateReceptor reverts to resting state• Ach binds to acetylcholinesteraseAch binds to acetylcholinesterase

Nerve 2Nerve 2

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Ach hydrolysedAch hydrolysedby acetylcholinesteraseby acetylcholinesterase

Nerve 2Nerve 2Acetylcholine

H3C

C

O

O

Choline

+ NMe3

C

O

H3C OH

Acetic acid

NMe3HO

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Choline binds to carrier proteinCholine binds to carrier protein

Carrier protein for cholineCarrier protein for choline


CholineCholine

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Choline transported into nerveCholine transported into nerve


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Ach resynthesisedAch resynthesised


E 1

Choline

+ CH2 NMe3CH2HO

C

O

H3C SCoA

Acetylcholine

H3C

C

O

ONMe3

E 1 = Choline acetyltransferase

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Ach repackaged in vesiclesAch repackaged in vesicles


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4. Cholinergic receptors4. Cholinergic receptors

Receptor typesReceptor types• Not all cholinergic receptors are identicalNot all cholinergic receptors are identical• Two types of cholinergic receptor - nicotinic and muscarinicTwo types of cholinergic receptor - nicotinic and muscarinic• Named after natural products showing receptor selectivityNamed after natural products showing receptor selectivity

Acetylcholine is natural messenger for both receptor typesAcetylcholine is natural messenger for both receptor types

Activates cholinergic Activates cholinergic receptors at nerve synapsesreceptors at nerve synapsesand on skeletal muscleand on skeletal muscle

Activates cholinergic Activates cholinergic receptors on smoothreceptors on smoothmuscle and cardiac musclemuscle and cardiac muscle

NicotineNicotine

N

NMe

L-(+)-MuscarineL-(+)-Muscarine

OMe CH2NMe3

HO

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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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CNS(Somatic)

CNS(Autonomic)

Sympathetic

Parasympathetic

NA

Ach(N)

Synapse

Ach (N)

Ach(N)

Ach(N)

Ach(M)

Adrenalmedulla

Adrenaline

SkeletalMuscle

SOMATIC

Synapse

AUTONOMIC

Smooth MuscleCardiac Muscle

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Cellmembrane

Five glycoprotein subunitstraversing cell membrane

ReceptorBindingsite Messenger

Control of Cationic Ion Channel:Control of Cationic Ion Channel:

4.1 Nicotinic receptor4.1 Nicotinic receptor

Cellmembrane

Inducedfit

‘Gating’(ion channel opens)

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The binding sitesThe binding sites

Two ligand binding sitesTwo ligand binding sitesmainly on mainly on -subunits-subunits

Ion channelIon channel

xx subunits subunits

BindingBindingsitessites

4.1 Nicotinic receptor4.1 Nicotinic receptor

CellCellmembranemembrane

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Activation of a signal proteinActivation of a signal protein• Receptor binds messenger leading to an induced Receptor binds messenger leading to an induced

fitfit• Opens a binding site for a signal protein (G-Opens a binding site for a signal protein (G-

protein)protein)

closed

messenger

inducedfit

open

4.2 Muscarinic receptor - G Protein coupled receptor4.2 Muscarinic receptor - G Protein coupled receptor

G-proteinbound

G-proteinsplit

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Activation of membrane bound enzymeActivation of membrane bound enzyme• G-Protein is split and subunit activates a G-Protein is split and subunit activates a

membrane bound enzymemembrane bound enzyme• Subunit binds to an allosteric binding site on Subunit binds to an allosteric binding site on

enzymeenzyme• Induced fit results in opening of an active Induced fit results in opening of an active

sitesite• Intracellular reaction is catalysedIntracellular reaction is catalysed

active site(closed)

active site(open)

Enzyme Enzyme

Intracellular reaction

4.2 Muscarinic receptor - G Protein coupled receptor4.2 Muscarinic receptor - G Protein coupled receptor

subunit

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5. Cholinergic agonists5. Cholinergic agonists

5.1 Acetylcholine as an agonist5.1 Acetylcholine as an agonist

AdvantagesAdvantages• Natural messengerNatural messenger• Easily synthesisedEasily synthesised

DisadvantagesDisadvantages• Easily hydrolysed in stomach (acid catalysed hydrolysis)Easily hydrolysed in stomach (acid catalysed hydrolysis)• Easily hydrolysed in blood (esterases)Easily hydrolysed in blood (esterases)• No selectivity between receptor typesNo selectivity between receptor types• No selectivity between different target organsNo selectivity between different target organs

Ac2ONMe3+

O HO NMe3 NMe3AcO

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5.2 Nicotine and muscarine as cholinergic agonists5.2 Nicotine and muscarine as cholinergic agonists

AdvantagesAdvantages• More stable than AchMore stable than Ach• Selective for main cholinergic receptor typesSelective for main cholinergic receptor types• Selective for different organsSelective for different organs

DisadvantagesDisadvantages• Activate receptors for other chemical messengersActivate receptors for other chemical messengers• Side effects Side effects

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5.3 Requirements for cholinergic agonists5.3 Requirements for cholinergic agonists

• Stability to stomach acids and esterasesStability to stomach acids and esterases

• Selectivity for cholinergic receptorsSelectivity for cholinergic receptors

• Selectivity between muscarinic and Selectivity between muscarinic and nicotinic receptorsnicotinic receptors

• Knowledge of binding siteKnowledge of binding site

• SAR for acetylcholineSAR for acetylcholine

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AcetoxyAcetoxy

EthyleneEthylenebridgebridge

44oo NitrogenNitrogen

Me ONMe3

O

AcetoxyAcetoxy


NitrogenNitrogen

Me ONMe3

O

6. SAR for acetlcholine6. SAR for acetlcholine

Quaternary nitrogen is essentialQuaternary nitrogen is essential

Bad for activityBad for activity

OCMe3H3C

O

ONMe2H3C

O

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• Distance from quaternary nitrogen to ester is importantDistance from quaternary nitrogen to ester is important• Ethylene bridge must be retainedEthylene bridge must be retained

6. SAR for acetylcholine6. SAR for acetylcholine


O NMe3H3C

O

OH3C

O

NMe3

AcetoxyAcetoxy



Me ONMe3

O

AcetoxyAcetoxy


NitrogenNitrogen

Me ONMe3

O

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Ester is importantEster is important



ONMe3H3C

NMe3H3C

AcetoxyAcetoxy



Me ONMe3

O

AcetoxyAcetoxy


NitrogenNitrogen

Me ONMe3

O

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Minimum of two methyl groups on quaternary nitrogen Minimum of two methyl groups on quaternary nitrogen



ON

H3C

O Et

Et

Et

ActiveActive

ON

H3C

O Et

Me

Me

AcetoxyAcetoxy



Me ONMe3

O

AcetoxyAcetoxy



Me ONMe3

O

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Methyl group of acetoxy group cannot be extended Methyl group of acetoxy group cannot be extended



ONMe3

O

H3C

AcetoxyAcetoxy



Me ONMe3

O

AcetoxyAcetoxy


NitrogenNitrogen

Me ONMe3

O

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Conclusions:Conclusions: • Tight fit between Ach and binding site Tight fit between Ach and binding site • Methyl groups fit into small hydrophobic pocketsMethyl groups fit into small hydrophobic pockets• Ester interacting by H-bondingEster interacting by H-bonding• Quaternary nitrogen interacting by ionic bondingQuaternary nitrogen interacting by ionic bonding


AcetoxyAcetoxy



Me ONMe3

O

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Trp-307Asp311

Trp-613Trp-616

Asn-617

O N

HH

CO2

7. Binding site (muscarinic)7. Binding site (muscarinic)

hydrophobic pocket

hydrophobic pocket

hydrophobic pockets

O O

CH3

N

CH3

CH3

CH3

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Trp-307Asp311

Trp-613Trp-616

Asn-617

O N

HH

CO2


H-bonds

Ionic bond

vdw

vdw

vdwO O

CH3

N

CH3

CH3

CH3

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• Possible induced dipole dipole interaction Possible induced dipole dipole interaction between quaternary nitrogen and hydrophobic between quaternary nitrogen and hydrophobic aromatic rings in binding sitearomatic rings in binding site

• NN++ induces dipole in aromatic rings induces dipole in aromatic rings


R

NMe3----

++++

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O

C

H

H

OMe

H

H

Me

N

Me

Me

• Several freely rotatable single bondsSeveral freely rotatable single bonds

• Large number of possible conformationsLarge number of possible conformations

• Active conformation does not necessarily Active conformation does not necessarily equal the most stable conformationequal the most stable conformation

8. Active conformation of acetylcholine8. Active conformation of acetylcholine

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MUSCARINE

OMe CH2NMe3

HO

CH2NMe3Me

O

O

O

OMe

NMe3

H

H

Rigid Analogues of acetylcholineRigid Analogues of acetylcholine

• Rotatable bonds ‘locked’ within ringRotatable bonds ‘locked’ within ring• Restricts number of possible conformationsRestricts number of possible conformations• Defines separation of ester and N Defines separation of ester and N

8. Active conformation of acetylcholine8. Active conformation of acetylcholine

Muscarinic receptor

ON

4.4A

Nicotinic receptor

ON

5.9A

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• Neighbouring group participationNeighbouring group participation

• Increases electrophilicity of carbonyl groupIncreases electrophilicity of carbonyl group

• Increases sensitivity to nucleophilesIncreases sensitivity to nucleophiles

9. Instability of acetylcholine9. Instability of acetylcholine

O

C

O

Me3N

H3C

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10. Design of cholinergic agonists10. Design of cholinergic agonists

RequirementsRequirements

• Correct sizeCorrect size

• Correct pharmacophore - ester and quaternary nitrogenCorrect pharmacophore - ester and quaternary nitrogen

• Increased stability to acid and esterasesIncreased stability to acid and esterases

• Increased selectivityIncreased selectivity

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Use of steric shieldsUse of steric shields

RationaleRationale

• Shields protect ester from nucleophiles and enzymesShields protect ester from nucleophiles and enzymes

• Shield size is importantShield size is important

• Must be large enough to hinder hydrolysisMust be large enough to hinder hydrolysis

• Must be small enough to fit binding siteMust be small enough to fit binding site


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MethacholineMethacholine

PropertiesProperties• Three times more stable than acetylcholineThree times more stable than acetylcholine• Increasing the shield size increases stability but decreases Increasing the shield size increases stability but decreases

activityactivity• Selective for muscarinic receptors over nicotinic receptorsSelective for muscarinic receptors over nicotinic receptors• SS-enantiomer is more active than the -enantiomer is more active than the RR-enantiomer-enantiomer• Stereochemistry matches muscarineStereochemistry matches muscarine• Not used clinicallyNot used clinically

asymmetric centre

hinders binding to esterasesand provides a shield to nucleophilic attack

MUSCARINE

OMe CH2NMe3

HO

H

CH2NMe3Me

Me

O

O

(S) (R)

Me

O

O

H

Me CH2NMe3

Me ONMe3

O 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 spirometry..

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Use of electronic factorsUse of electronic factors

• Replace ester with urethaneReplace ester with urethane• Stabilises the carbonyl groupStabilises the carbonyl group

H2N

C

O

C

O

H2N

H2N

C

O

=

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PropertiesProperties• Resistant to hydrolysisResistant to hydrolysis• Long lastingLong lasting• NHNH22 and CH and CH33 are equal sizes. Both fit the hydrophobic pocket are equal sizes. Both fit the hydrophobic pocket• NHNH22 = bio-isostere = bio-isostere • Muscarinic activity = nicotinic activityMuscarinic activity = nicotinic activity• Used topically for glaucomaUsed topically for glaucoma

CarbacholCarbacholOC

O

H2NNMe3

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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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Steric + Electronic factorsSteric + Electronic factors

PropertiesProperties

• Very stableVery stable

• Orally activeOrally active

• Selective for the muscarinic receptorSelective for the muscarinic receptor

• Used to stimulate GI tract and urinary bladder after surgeryUsed to stimulate GI tract and urinary bladder after surgery

BethanecholBethanechol*H2N

C

O

ONMe3

Me

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Nicotinic selective agonistNicotinic selective agonist

Me

C

O

O * asymmetric centreNMe3

Me

*

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11. Uses of cholinergic agonists11. Uses of cholinergic agonists

Nicotinic selective agonistsNicotinic selective agonists

Treatment of myasthenia gravis Treatment of myasthenia gravis

- lack of acetylcholine at skeletal - lack of acetylcholine at skeletal muscle causing weaknessmuscle causing weakness

Muscarinic selective agonistsMuscarinic selective agonists

• Treatment of glaucomaTreatment of glaucoma

• Switching on GIT and urinary tract after Switching on GIT and urinary tract after surgerysurgery

• Treatment of certain heart defects. Treatment of certain heart defects. Decreases heart muscle activity and Decreases heart muscle activity and decreases heart ratedecreases heart rate

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CNS(Somatic)

CNS(Autonomic)

Sympathetic

Parasympathetic

NA

Ach(N)

Synapse

Ach (N)

Ach(N)

Ach(N)

Ach(M)

Adrenalmedulla

Adrenaline

SkeletalMuscle

SOMATIC

Synapse

AUTONOMIC

Smooth MuscleCardiac Muscle

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Patrick An Introduction to Medicinal Chemistry 3/e Chapter 19 CHOLINERGICS, ANTICHOLINERGICS