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Topic 10 Drugs of the NervousSystem
Ch 19,20 PatrickPart VI- Nervous system -Corey
ContentsContents
Part 1: Cholinergics & anticholinesterases
1. Nerve Transmission2. Neurotransmitter3. Transmission4. Cholinergic receptors
4.1. Nicotinic receptor4.2. Muscarinic receptor - G Protein coupled receptor
5. Cholinergic agonists5.1. Acetylcholine as an agonist5.2. Nicotine and muscarine as cholinergic agonists5.3. Requirements for cholinergic agonists
6. SAR for acetylcholine7. Binding site (muscarinic)8. Active conformation of acetylcholine9. Instability of acetylcholine10. Design of cholinergic agonists11. Uses of cholinergic agonists
CHOLINERGICCHOLINERGICNERVOUSNERVOUSSYSTEMSYSTEM
1. Nerve Transmission1. Nerve Transmission
Peripheral nervous systemPeripheral nervous system
Peripheral nerves
Muscle
Gastro-intestinal tract (GIT)
Brain
Spinal cord
CNS
Heart
1. Nerve Transmission1. Nerve Transmission
Peripheral nervous systemPeripheral nervous system
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
NerveNerve
1. Nerve Transmission1. Nerve Transmission
SynapsesSynapses
100-500AReceptorsReceptors
Release ofneurotransmitters
Receptor binding and new signal
Nerve impulseNerve impulse New signalNew signal
Vesicles containingneurotransmitters
2. Neurotransmitter2. Neurotransmitter
Acetylcholine (Ach)Acetylcholine (Ach)
CholineCholineAcetylAcetyl
H3C OC NMe 3
O+
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
3. Transmission process3. Transmission process
Vesicles fuse with membrane and release AchVesicles fuse with membrane and release Ach
Nerve 1Nerve 1Nerve 2Nerve 2
SignalSignal
3. Transmission process3. Transmission process
Nerve 2Nerve 2
Nerve 2Nerve 2
3. Transmission process3. Transmission process
•• 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
3. Transmission process3. Transmission process
•• Ach departs receptorAch departs receptor•• Receptor reverts to resting stateReceptor reverts to resting state•• Ach binds to acetylcholinesteraseAch binds to acetylcholinesterase
Nerve 2Nerve 2
3. Transmission process3. Transmission process
Ach hydrolysedAch hydrolysedby acetylcholinesteraseby acetylcholinesterase
Nerve 2Nerve 2Acetylcholine
H3C
C
O
O
Choline
+ NMe3
C
O
H3C OH
Acetic acid
NMe3
HO
3. Transmission process3. Transmission process
Choline binds to carrier proteinCholine binds to carrier protein
Carrier protein for cholineCarrier protein for choline
Nerve 1Nerve 1Nerve 2Nerve 2
CholineCholine
3. Transmission process3. Transmission process
Choline transported into nerveCholine transported into nerve
Nerve 1Nerve 1Nerve 2Nerve 2
3. Transmission process3. Transmission process
Ach Ach resynthesisedresynthesised
Nerve 1Nerve 1Nerve 2Nerve 2
E 1
Choline
+ CH2 NMe3CH2HO
C
O
H3C SCoA
Acetylcholine
H3C
C
O
O
NMe3
E 1 = Choline acetyltransferase
3. Transmission process3. Transmission process
Ach repackaged in vesiclesAch repackaged in vesicles
Nerve 1Nerve 1Nerve 2Nerve 2
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
Peripheral nervous systemPeripheral nervous system
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
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)
The binding sitesThe binding sites
α
α
γ
δ
β
Two ligand binding sitesTwo ligand binding sitesmainly on mainly on αα-subunits-subunits
Ion channelIon channel
22xxα, β, γ, δα, β, γ, δ subunits subunits
BindingBindingsitessites
4.1 Nicotinic receptor4.1 Nicotinic receptor
CellCellmembranemembrane
α
αδ
β
γ
Activation of a signal proteinActivation of a signal protein•• Receptor binds messenger leading to an induced fitReceptor binds messenger leading to an induced fit•• Opens a binding site for a signal protein (G-protein)Opens a binding site for a signal protein (G-protein)
closed
messenger
inducedfit
open
4.2 Muscarinic receptor - G Protein coupled receptor4.2 Muscarinic receptor - G Protein coupled receptor
G-proteinbound
G-proteinsplit
Activation of membrane bound enzymeActivation of membrane bound enzyme•• G-Protein is split and subunit activates a membrane boundG-Protein is split and subunit activates a membrane bound
enzymeenzyme•• Subunit binds to an allosteric binding site on enzymeSubunit binds to an allosteric binding site on enzyme•• Induced fit results in opening of an active siteInduced fit results in opening of an active site•• 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
5. Cholinergic agonists5. Cholinergic agonists
5.1 Acetylcholine as an agonist5.1 Acetylcholine as an agonist
AdvantagesAdvantages•• Natural messengerNatural messenger•• Easily synthesizedEasily synthesized
DisadvantagesDisadvantages•• Easily hydrolysed in stomach (acid catalysed hydrolysis)Easily hydrolysed in stomach (acid catalysed hydrolysis)•• Easily hydrolysed in blood (Easily hydrolysed in blood (esterasesesterases))•• 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
5. Cholinergic agonists5. Cholinergic agonists
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
5. Cholinergic agonists5. Cholinergic agonists
5.3 Requirements for cholinergic agonists5.3 Requirements for cholinergic agonists
•• Stability to stomach acids and Stability to stomach acids and esterasesesterases
•• Selectivity for cholinergic receptorsSelectivity for cholinergic receptors
•• Selectivity between Selectivity between muscarinic muscarinic and nicotinic receptorsand nicotinic receptors
•• Knowledge of binding siteKnowledge of binding site
•• SAR for acetylcholineSAR for acetylcholine
AcetoxyAcetoxy
EthyleneEthylenebridgebridge
44 oo Nitrogen Nitrogen
Me O
NMe3
O
AcetoxyAcetoxy
EthyleneEthylenebridgebridge
Nitrogen Nitrogen
Me O
NMe3
O
6. SAR for acetylcholine6. SAR for acetylcholine
Quaternary nitrogen is essentialQuaternary nitrogen is essential
Bad for activityBad for activity
OCMe3
H3C
O
ONMe2
H3C
O
•• 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
Bad for activityBad for activity
O NMe3H3C
O
OH3C
O
NMe3
AcetoxyAcetoxy
EthyleneEthylenebridgebridge
44 oo Nitrogen Nitrogen
Me O
NMe3
O
AcetoxyAcetoxy
EthyleneEthylenebridgebridge
Nitrogen Nitrogen
Me O
NMe3
O
Ester is importantEster is important
6. SAR for acetylcholine6. SAR for acetylcholine
Bad for activityBad for activity
ONMe3
H3CNMe3
H3C
AcetoxyAcetoxy
EthyleneEthylenebridgebridge
44 oo Nitrogen Nitrogen
Me O
NMe3
O
AcetoxyAcetoxy
EthyleneEthylenebridgebridge
Nitrogen Nitrogen
Me O
NMe3
O
Minimum of two methyl groups on quaternary nitrogen Minimum of two methyl groups on quaternary nitrogen
6. SAR for acetylcholine6. SAR for acetylcholine
Bad for activityBad for activity
ON
H3C
OEt
Et
Et
ActiveActive
ON
H3C
OEt
Me
Me
AcetoxyAcetoxy
EthyleneEthylenebridgebridge
44 oo Nitrogen Nitrogen
Me O
NMe3
O
AcetoxyAcetoxy
EthyleneEthylenebridgebridge
44 oo Nitrogen Nitrogen
Me O
NMe3
O
Methyl group of Methyl group of acetoxyacetoxy group cannot be extended group cannot be extended
6. SAR for acetylcholine6. SAR for acetylcholine
Bad for activityBad for activity
ONMe3
O
H3C
AcetoxyAcetoxy
EthyleneEthylenebridgebridge
44 oo Nitrogen Nitrogen
Me O
NMe3
O
AcetoxyAcetoxy
EthyleneEthylenebridgebridge
Nitrogen Nitrogen
Me O
NMe3
O
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
6. SAR for acetylcholine6. SAR for acetylcholine
AcetoxyAcetoxy
EthyleneEthylenebridgebridge
44 oo Nitrogen Nitrogen
Me O
NMe3
O
Trp-307Asp311
Trp-613Trp-616
Asn-617
O N
H
H
CO2
7. Binding site (muscarinic)7. Binding site (muscarinic)
hydrophobic pocket
hydrophobic pocket
hydrophobic pocketsO O
CH3
N
CH3
CH3
CH3
Trp-307Asp311
Trp-613Trp-616
Asn-617
O N
H
H
CO2
7. Binding site (muscarinic)7. Binding site (muscarinic)
H-bonds
Ionic bond
vdw
vdw
vdwO O
CH3
N
CH3
CH3
CH3
•• Possible induced dipole dipole interaction between quaternaryPossible induced dipole dipole interaction between quaternarynitrogen and hydrophobic aromatic rings in binding sitenitrogen and hydrophobic aromatic rings in binding site
•• NN++ induces dipole in aromatic rings induces dipole in aromatic rings
7. Binding site (muscarinic)7. Binding site (muscarinic)
R
NMe3
δ δ --δ δ --
δ δ ++δ δ ++
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 equal the mostActive conformation does not necessarily equal the most
stable conformationstable conformation
8. Active conformation of acetylcholine8. Active conformation of acetylcholine
MUSCARINE
OMe CH2NMe3
HO
CH2NMe3Me
O
O
O
OMe
NMe3
H
H
Rigid Analogues of acetylcholineRigid Analogues of acetylcholine
•• Rotatable bonds Rotatable bonds ‘‘lockedlocked’’ within ring 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
O
N
4.4A
Nicotinic receptor
O
N
5.9A
•• Neighbouring group participationNeighbouring group participation
•• Increases Increases electrophilicityelectrophilicity of carbonyl group of carbonyl group
•• Increases sensitivity to nucleophilesIncreases sensitivity to nucleophiles
9. Instability of acetylcholine9. Instability of acetylcholine
O
C
O
Me3N
H3C
! "
! +
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 Increased stability to acid and esterasesesterases
•• Increased selectivityIncreased selectivity
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
10. Design of cholinergic agonists10. Design of cholinergic agonists
10. Design of cholinergic agonists10. Design of cholinergic agonists
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--enantiomerenantiomer is more active than the is more active than the RR--enantiomerenantiomer•• 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 O
NMe3
O Me
*
10. Design of cholinergic agonists10. Design of cholinergic agonists
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! "
=
10. Design of cholinergic agonists10. Design of cholinergic agonists
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- = bio-isostereisostere •• Muscarinic activity = nicotinic activityMuscarinic activity = nicotinic activity•• Used topically for glaucomaUsed topically for glaucoma
CarbacholCarbacholO
C
O
H2N
NMe3
10. Design of cholinergic agonists10. Design of cholinergic agonists
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
10. Design of cholinergic agonists10. Design of cholinergic agonists
Nicotinic selective agonistNicotinic selective agonist
Me
C
O
O * asymmetric centreNMe3
Me
*
11. Uses of cholinergic agonists11. Uses of cholinergic agonists
Nicotinic selective agonistsNicotinic selective agonists
Treatment of myasthenia gravisTreatment of myasthenia gravis
- lack of acetylcholine at skeletal muscle causing weakness - lack of acetylcholine at skeletal muscle causing weakness
Muscarinic selective agonistsMuscarinic selective agonists
•• Treatment of glaucomaTreatment of glaucoma
•• Switching on GIT and urinary tract after surgerySwitching on GIT and urinary tract after surgery
•• Treatment of certain heart defects. Decreases heart muscleTreatment of certain heart defects. Decreases heart muscleactivity and decreases heart rateactivity and decreases heart rate
Peripheral nervous systemPeripheral nervous system
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
ContentsContents
Part 1: Cholinergics & anticholinesterases
1. Nerve Transmission2. Neurotransmitter3. Transmission process4. Cholinergic receptors
4.1. Nicotinic receptor4.2. Muscarinic receptor - G Protein coupled receptor
5. Cholinergic agonists5.1. Acetylcholine as an agonist5.2. Nicotine and muscarine as cholinergic agonists5.3. Requirements for cholinergic agonists
6. SAR for acetlcholine7. Binding site (muscarinic)8. Active conformation of acetylcholine9. Instability of acetylcholine10. Design of cholinergic agonists11. Uses of cholinergic agonists
CHOLINERGICCHOLINERGICNERVOUSNERVOUSSYSTEMSYSTEM
1. Nerve Transmission1. Nerve Transmission
Peripheral nervous systemPeripheral nervous system
Peripheral nerves
Muscle
Gastro-intestinal tract (GIT)
Brain
Spinal cord
CNS
Heart
1. Nerve Transmission1. Nerve Transmission
Peripheral nervous systemPeripheral nervous system
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
NerveNerve
1. Nerve Transmission1. Nerve Transmission
SynapsesSynapses
100-500AReceptorsReceptors
Release ofneurotransmitters
Receptor binding and new signal
Nerve impulseNerve impulse New signalNew signal
Vesicles containingneurotransmitters
ContentsContents
Part 2: Cholinergics & anticholinesterases
12. Cholinergic Antagonists (Muscarinic receptor)12.1. Atropine12.2. Hyoscine (scopolamine)12.3. Comparison of atropine with acetylcholine12.4. Analogues of atropine12.5. Simplified Analogues12.6. SAR for Antagonists12.7. Binding Site for Antagonists
13. Cholinergic Antagonists (Nicotinic receptor)13.1. Curare13.2. Binding13.3. Analogues of tubocurarine
12. Cholinergic Antagonists (Muscarinic receptor)12. Cholinergic Antagonists (Muscarinic receptor)
•• Drugs which bind to cholinergic receptor but do not activate itDrugs which bind to cholinergic receptor but do not activate it•• Prevent acetylcholine from bindingPrevent acetylcholine from binding•• Opposite clinical effect to agonists - lower activity of Opposite clinical effect to agonists - lower activity of
acetylcholineacetylcholine
Postsynapticnerve
Ach
Antagonist
Ach
Postsynapticnerve
Ach
12. Cholinergic Antagonists (Muscarinic receptor)12. Cholinergic Antagonists (Muscarinic receptor)
Clinical EffectsClinical Effects•• Decrease of saliva and gastric secretionsDecrease of saliva and gastric secretions•• Relaxation of smooth muscle Relaxation of smooth muscle •• Decrease in motility of GIT and urinary tractDecrease in motility of GIT and urinary tract•• Dilation of pupilsDilation of pupils
UsesUses•• Shutting down digestion for surgeryShutting down digestion for surgery•• Ophthalmic examinationsOphthalmic examinations•• Relief of peptic ulcersRelief of peptic ulcers•• Treatment of ParkinsonTreatment of Parkinson’’s Diseases Disease•• AnticholinesteraseAnticholinesterase poisoning poisoning•• Motion sicknessMotion sickness
12.1 Atropine12.1 Atropine
•• Racemic Racemic form of form of hyoscyaminehyoscyamine•• Source - roots of belladonna (1831) (deadly nightshade)Source - roots of belladonna (1831) (deadly nightshade)•• Used as a poisonUsed as a poison•• Used as a medicine Used as a medicine
decreases GIT motility decreases GIT motility antidote for antidote for anticholinesterase anticholinesterase poisoningpoisoningdilation of eye pupilsdilation of eye pupils
•• CNS side effects - hallucinationsCNS side effects - hallucinations
*
N
H
O
C
O
Me
CH
CH2OH
easily racemised
12. Cholinergic Antagonists (Muscarinic receptor)12. Cholinergic Antagonists (Muscarinic receptor)
12.2 12.2 Hyoscine Hyoscine (scopolamine)(scopolamine)
•• Source - thorn Source - thorn apple-apple-Datura-Datura-jimsonweedjimsonweed•• Medical use - treatment of motion sicknessMedical use - treatment of motion sickness•• CNS effects, hallucinationsCNS effects, hallucinations
*
N
H
O
C
O
Me
CH
CH2OHO
H
H
12. Cholinergic Antagonists (12. Cholinergic Antagonists (Muscarinic Muscarinic receptor)receptor)
12.3 Comparison of atropine with acetylcholine12.3 Comparison of atropine with acetylcholine
•• Relative positions of ester and nitrogen similar in both moleculesRelative positions of ester and nitrogen similar in both molecules•• Nitrogen in atropine is ionisedNitrogen in atropine is ionised•• Amine and ester are important binding groups (ionic + H-bonds)Amine and ester are important binding groups (ionic + H-bonds)•• Aromatic ring of atropine is an extra binding group (Aromatic ring of atropine is an extra binding group (vdWvdW) ) •• Atropine binds with a different induced fit - no activationAtropine binds with a different induced fit - no activation•• Atropine binds more strongly than acetylcholineAtropine binds more strongly than acetylcholine
N
O
C
O
Me
H
CH
CH2OH
C
O
O CH3
NMe3
CH2CH2
12. Cholinergic Antagonists (Muscarinic receptor)12. Cholinergic Antagonists (Muscarinic receptor)
12.4 Analogues of atropine12.4 Analogues of atropine
•• Analogues are fully ionized Analogues are fully ionized •• Analogues unable to cross the blood brain barrierAnalogues unable to cross the blood brain barrier•• No CNS side effectsNo CNS side effects
Atropine methonitrate(lowers GIT motility)
N
H
O
C
O
CH3
CH
CH2OH
H3CNO3
Ipratropium(bronchodilator & anti-asthmatic)
N
H
OC
O
CH(CH3)2
CH
CH2OH
H3C
Br
12. Cholinergic Antagonists (Muscarinic receptor)12. Cholinergic Antagonists (Muscarinic receptor)
12.5 Simplified Analogues12.5 Simplified Analogues
Pharmacophore = ester + basic amine + aromatic ringPharmacophore = ester + basic amine + aromatic ring
Amprotropine
N
CH2
CH2
CH2
O
C
O
Et
CH
CH2OH
Et
Tridihexethyl bromide
HO C CH2CH2N(Et)3 Br
Propantheline chloride
Cl
O C
O
O CH2CH2 N
CH
Me
CH
Me Me
Me
Me
12. Cholinergic Antagonists (Muscarinic receptor)12. Cholinergic Antagonists (Muscarinic receptor)
12.5 Simplified Analogues12.5 Simplified Analogues
Tropicamide(opthalmics)
Cyclopentolate(opthalmics)
Benztropine(Parkinsons disease)
Benzhexol(Parkinsons disease)
Pirenzepine(anti-ulcer)
CH
CH2OH
O
N
CH2CH3
N
N
H
OCH
Me
N
CH
NN
CHN
C O
CH2
N
N
O
Me
12. Cholinergic Antagonists (Muscarinic receptor)12. Cholinergic Antagonists (Muscarinic receptor)
CH
O
O
OH
Me2N
12.6 SAR for Antagonists12.6 SAR for Antagonists
Important featuresImportant features•• Tertiary amine (ionised) or a quaternary nitrogenTertiary amine (ionised) or a quaternary nitrogen•• Aromatic ringAromatic ring•• EsterEster•• NN-Alkyl groups (R) can be larger than methyl (unlike agonists)-Alkyl groups (R) can be larger than methyl (unlike agonists)•• Large branched acyl groupLarge branched acyl group•• RR’’ = aromatic or heteroaromatic ring = aromatic or heteroaromatic ring•• Branching of aromatic/heteroaromatic rings is importantBranching of aromatic/heteroaromatic rings is important
R2N
CH2
CH2
O
C
O
CH
R'
R' R' = Aromatic or Heteroaromatic
12. Cholinergic Antagonists (Muscarinic receptor)12. Cholinergic Antagonists (Muscarinic receptor)
InactiveInactiveActiveActive
Cl
O C
O
O CH2CH2 N
CH
Me
CH
Me Me
Me
Me CH2
C
O
O CH2CH2NR2
12.6 SAR for Antagonists12.6 SAR for Antagonists
12. Cholinergic Antagonists (Muscarinic receptor)12. Cholinergic Antagonists (Muscarinic receptor)
Tertiary amine (ionized) Tertiary amine (ionized) or quaternary nitrogenor quaternary nitrogen
Aromatic ringAromatic ringEsterEsterNN-Alkyl groups (R) can be -Alkyl groups (R) can be larger than methyllarger than methylRR’’ = aromatic or = aromatic or heteroaromaticheteroaromaticBranching of Branching of Ar Ar rings importantrings important
Quaternary nitrogenQuaternary nitrogen
Aromatic ringAromatic ringEsterEsterNN-Alkyl groups = methyl-Alkyl groups = methyl
RR’’ = H = H
SAR for AntagonistsSAR for Antagonists SAR for AgonistsSAR for Agonists
12.6 SAR for Antagonists vs. Agonists12.6 SAR for Antagonists vs. Agonists
12. Cholinergic Antagonists (Muscarinic receptor)12. Cholinergic Antagonists (Muscarinic receptor)
RECEPTOR SURFACE
Acetylcholinebinding site
12.7 Binding Site for Antagonists12.7 Binding Site for Antagonists
van der Waalsbinding regionsfor antagonists
12. Cholinergic Antagonists (Muscarinic receptor)12. Cholinergic Antagonists (Muscarinic receptor)
12.7 Binding Site for Antagonists12.7 Binding Site for Antagonists
CH
Me
MeCH
MeMe
N
Me
CH2CH2OC
O
O
Cl
H2N Asn
CO2
CH2
CH2
O
O
C
NMeR2
O
12. Cholinergic Antagonists (Muscarinic receptor)12. Cholinergic Antagonists (Muscarinic receptor)
13. Cholinergic Antagonists (Nicotinic receptor)13. Cholinergic Antagonists (Nicotinic receptor)
13.1 Curare13.1 Curare•• Extract from curare plant- Extract from curare plant- Strychnos toxifera •• Used for poison arrowsUsed for poison arrows•• Causes paralysis (blocks acetylcholine signals to muscles)Causes paralysis (blocks acetylcholine signals to muscles)•• Active principle = Active principle = tubocurarinetubocurarine
TubocurarineTubocurarine
N
HO
MeO
O
CH2
OMe
N
Me
Me
H
O
Me
CH2
OH
H
H
13. Cholinergic Antagonists (Nicotinic receptor)13. Cholinergic Antagonists (Nicotinic receptor)
PharmacophorePharmacophore •• Two quaternary centres at specific separation (1.15nm)Two quaternary centres at specific separation (1.15nm)•• Different mechanism of action from atropine based antagonistsDifferent mechanism of action from atropine based antagonists•• Different binding interactionsDifferent binding interactions
Clinical usesClinical uses •• Neuromuscular blocker for surgical operationsNeuromuscular blocker for surgical operations•• Permits lower and safer levels of general anaestheticPermits lower and safer levels of general anaesthetic•• Tubocurarine Tubocurarine used (previously) as neuromuscular blockerused (previously) as neuromuscular blocker but side effects but side effects
13.2 Binding13.2 Binding
a) Receptor dimer
S
b) Interaction with tubocurarine
protein complex(5 subunits)diameter=8nm
8nm
9-10nm
N N
N N TubocurarineAcetylcholine binding site
13. Cholinergic Antagonists (Nicotinic receptor)13. Cholinergic Antagonists (Nicotinic receptor)
Probably not bridging Ach sites
13.3 Analogues of tubocurarine13.3 Analogues of tubocurarine
•• Long lastingLong lasting•• Long recovery timesLong recovery times•• Side effects on heartSide effects on heart
•• Esters incorporatedEsters incorporated•• Shorter lifetime (5 min)Shorter lifetime (5 min)•• Fast onset and short durationFast onset and short duration•• Side effects at autonomic gangliaSide effects at autonomic ganglia
DecamethoniumDecamethonium
Me3N(CH2)10NMe3
SuxamethoniumSuxamethonium
Me3NCH2CH2 O
C
O
CH2
C
O
O CH2CH2NMe3CH2
13. Cholinergic Antagonists (Nicotinic receptor)13. Cholinergic Antagonists (Nicotinic receptor)
13.3 Analogues of tubocurarine13.3 Analogues of tubocurarine
•• Steroid acts as a spacer for the quaternary centres (1.09nm)Steroid acts as a spacer for the quaternary centres (1.09nm)•• Acyl groups are added to introduce the Ach skeletonAcyl groups are added to introduce the Ach skeleton•• Faster onset then tubocurarine but slower than suxamethoniumFaster onset then tubocurarine but slower than suxamethonium•• Longer duration of action than suxamethonium (45 min)Longer duration of action than suxamethonium (45 min)•• No effect on blood pressure and fewer side effectsNo effect on blood pressure and fewer side effects
13. Cholinergic Antagonists (Nicotinic receptor)13. Cholinergic Antagonists (Nicotinic receptor)
PancuroniumPancuronium (R=Me) (R=Me)
VecuroniumVecuronium (R=H) (R=H)
MeO
NMe
N
O
Me
Me
Me
O
O
H
H H
H
13.3 Analogues of tubocurarine13.3 Analogues of tubocurarine
•• Design based on tubocurarine and suxamethoniumDesign based on tubocurarine and suxamethonium•• Lacks cardiac side effectsLacks cardiac side effects•• Rapidly broken down in blood both chemically and metabolicallyRapidly broken down in blood both chemically and metabolically•• Avoids patient variation in metabolic enzymesAvoids patient variation in metabolic enzymes•• Lifetime is 30 minutesLifetime is 30 minutes•• Administered as an i.v. dripAdministered as an i.v. drip•• Self destruct system limits lifetimeSelf destruct system limits lifetime
AtracuriumAtracurium
NCH2 CH2
C
O
O
MeO
OMe
HN
(CH2)5MeO O
C
OMe
O
CH2 CH2
Me
MeO
OMe
OMe
OMe
13. Cholinergic Antagonists (Nicotinic receptor)13. Cholinergic Antagonists (Nicotinic receptor)
13.3 Analogues of tubocurarine13.3 Analogues of tubocurarine
AtracuriumAtracurium stable at acid pH stable at acid pHHofmann elimination at blood pH (7.4)Hofmann elimination at blood pH (7.4)
N
Me
CH2
Ph
CH C
HO
R
ACTIVE
-H
CHH2C C
O
Ph
R
N
Me
INACTIVE
13. Cholinergic Antagonists (Nicotinic receptor)13. Cholinergic Antagonists (Nicotinic receptor)
13.3 Analogues of 13.3 Analogues of tubocurarinetubocurarine
MivacuriumMivacurium
•• Faster onset (2 min)Faster onset (2 min)•• Shorter duration (15 min)Shorter duration (15 min)
13. Cholinergic Antagonists (Nicotinic receptor)13. Cholinergic Antagonists (Nicotinic receptor)
N
MeO
OMe
H3C
NMeO
OMe
Me
MeO
OMe
OMe
OMe
OO
O
O
ContentsContents
Part 3: Cholinergics & anticholinesterases
14. Acetylcholinesterase14.1. Role14.2. Hydrolysis reaction catalysed14.3. Effect of inhibition14.4. Structure of enzyme complex14.5. Active site - binding interactions14.6. Active site - Mechanism of catalysis
15. Anticholinesterases15.1. Physostigmine15.2. Mechanism of action15.3. Physostigmine analogues15.4. Organophosphates15.5. Anticholinesterases as ‘Smart Drugs’
14. Acetylcholinesterase14. Acetylcholinesterase
Acetylcholinesterase enzymeAcetylcholinesterase enzyme
......
...
Nerve 1Nerve 1Nerve 2Nerve 2
SignalSignal
14.1 Role14.1 Role •• Hydrolysis and deactivation of acetylcholineHydrolysis and deactivation of acetylcholine•• Prevents acetylcholine reactivating receptorPrevents acetylcholine reactivating receptor
14. Acetylcholinesterase14. Acetylcholinesterase
activeactive inactiveinactive
14.2 Hydrolysis reaction catalysed14.2 Hydrolysis reaction catalysed
Acetylcholine
CH3
C
O
O
Choline
+ NMe3
C
O
CH3 OH
Acetic acid
NMe3
HO
14. Acetylcholinesterase14. Acetylcholinesterase
•• Inhibitor blocks acetylcholinesteraseInhibitor blocks acetylcholinesterase•• Ach is unable to bindAch is unable to bind•• Ach returns to receptor and reactivatesAch returns to receptor and reactivates
itit•• Enzyme inhibitor has the same effect asEnzyme inhibitor has the same effect as
a cholinergic agonista cholinergic agonist
22oo Message Message
Nerve 2Nerve 2
Enzyme inhibitorEnzyme inhibitor((AnticholinesteraseAnticholinesterase))
14.3 Effect of inhibition14.3 Effect of inhibition
AchAch
14. 14. AcetylcholinesteraseAcetylcholinesterase14.4 Structure of enzyme complex14.4 Structure of enzyme complex
S
S
SS
S
S
S
S
S S
S
S
S
S
S S
S
S
Enzyme
EnzymeEnzyme
collagen
:
::OO
Serine
Histidine
Tyrosine
OH
N
N
HO
Aspartate
14. 14. AcetylcholinesteraseAcetylcholinesterase14.5 Active site - binding interactions14.5 Active site - binding interactions
•• Anionic binding region similar to cholinergic receptor siteAnionic binding region similar to cholinergic receptor site•• Binding and induced fit strains Ach and weakens bondsBinding and induced fit strains Ach and weakens bonds•• Molecule positioned for reaction with His and SerMolecule positioned for reaction with His and Ser
vdwvdw
vdwvdwhydrophobic pockets
Anionic binding regionEster binding region
Me
N
CH2 CH2
O
C
CH3
O
Me
Me
H-bondIonic
14. Acetylcholinesterase14. Acetylcholinesterase14.6 Active site - Mechanism of catalysis14.6 Active site - Mechanism of catalysis
O HN
NH
:
Serine Histidine(Nucleophile) (Base)
:
:
CH3 C
O
O CH2CH2NMe3
N
NH
O
O
R
C
O
CH3
H
:
:Histidine(Base catalyst)
::
N
NH
O
O
R
C
O
CH3
H
:
Histidine
:: :
:
C
OR
O
CH3
O N
NH
H
:
:
Histidine
Acid catalyst
14. Acetylcholinesterase14. Acetylcholinesterase14.6 Active site - Mechanism of catalysis14.6 Active site - Mechanism of catalysis
:
:
C
OR
O
CH3
O N
NH
H
:
:
Histidine
H2O
N
NH
OO
H
CCH3
O
H::
:
Histidine
ROH
CO
CH3
O NNH
Histidine
C
O
OHCH3
ON
NH
: _
H :
::
HistidineBasic catalyst
14. Acetylcholinesterase14. Acetylcholinesterase14.6 Active site - Mechanism of catalysis14.6 Active site - Mechanism of catalysis
C
O
OHCH3
O NNH
::
:
: H:
Histidine(Acid catalyst)
C
O
OHCH3
ON
NH
: _
H
:
Histidine
::
C
O
OHCH3
O NNH
: _
H :
::
HistidineBasic catalyst
OHC
O
CH3
OH N
NH
:
14. Acetylcholinesterase14. Acetylcholinesterase
•• Serine and water are poor nucleophilesSerine and water are poor nucleophiles
•• Mechanism is aided by histidine acting as a basic catalystMechanism is aided by histidine acting as a basic catalyst
•• Choline and serine are poor leaving groupsCholine and serine are poor leaving groups
•• Leaving groups are aided by histidine acting as an acid catalystLeaving groups are aided by histidine acting as an acid catalyst
•• Very efficient - 100 x 10Very efficient - 100 x 1066 faster than faster than uncatalyseduncatalysed hydrolysis hydrolysis
•• Acetylcholine hydrolysed within 100 Acetylcholine hydrolysed within 100 µµsecssecs of reaching active of reaching activesitesite
•• An aspartate residue is also involved in the mechanismAn aspartate residue is also involved in the mechanism
14. Acetylcholinesterase14. AcetylcholinesteraseThe catalytic triadThe catalytic triad
•• An An aspartate aspartate residue interacts with the residue interacts with the imidazole imidazole ring ofring ofhistidine histidine to orient and activate itto orient and activate it
Serine Histidine
(Nucleophile) (Base)
O H
N
N
H
O
O
Aspartate
:
:::
::
:
15. 15. AnticholinesterasesAnticholinesterases
•• Inhibitors of acetylcholinesterase enzymeInhibitors of acetylcholinesterase enzyme
•• Block hydrolysis of acetylcholineBlock hydrolysis of acetylcholine
•• Acetylcholine is able to reactivate cholinergic receptorAcetylcholine is able to reactivate cholinergic receptor
•• Same effect as a cholinergic agonistSame effect as a cholinergic agonist
15. 15. AnticholinesterasesAnticholinesterases15.1 Physostigmine15.1 Physostigmine
•• Natural product from the African Natural product from the African calabar calabar (ordeal) bean-(ordeal) bean-PhysostigmaPhysostigma•• Carbamate Carbamate is essential (equivalent to ester of Ach)is essential (equivalent to ester of Ach)•• Aromatic ring is important Aromatic ring is important •• Pyrrolidine Pyrrolidine N is important (ionized at blood pH)N is important (ionized at blood pH)•• Pyrrolidine Pyrrolidine N is equivalent to the quaternary nitrogen of AchN is equivalent to the quaternary nitrogen of Ach
N N
MeOC
O
NMe
H
Me MeUrethane
or
Carbamate
Pyrrolidine N
http://www.floradelaterre.com/index.php?id=19
Physostigmine
O:N
NH
H
MeNH
C
O
O Ar
::
15.2 Mechanism of action15.2 Mechanism of action
O ArC
O
MeNH
O
NHN:
:
H
:
:
O ArC
O
MeNH
O
NHNH
:
:
:
::
:
::
O ArC
O
MeNH
O
NHN
:H
:
:
:
O ArC
O
MeNH
O
NHN
:H
15.2 Mechanism of action15.2 Mechanism of action
Rate of hydrolysis slower by 40 x 10Rate of hydrolysis slower by 40 x 1066
Stable carbamoylintermediate
O H
NH:N
Hydrolysisvery slow
OC
O:N
NHMeNH
-ArOH
15.2 Mechanism of action15.2 Mechanism of action
Carbonyl group'deactivated'
:
O
CON
Me
H: ::N
H
Me
CO
O
Stable carbamoylintermediate
15.3 15.3 Physostigmine Physostigmine analoguesanalogues
•• Simplified analogueSimplified analogue•• Susceptible to hydrolysisSusceptible to hydrolysis•• Crosses BBB as free baseCrosses BBB as free base•• CNS side effectsCNS side effects
•• Fully ionizedFully ionized•• Cannot cross BBBCannot cross BBB•• No CNS side effectsNo CNS side effects•• More stable toMore stable to
hydrolysishydrolysis•• Extra Extra NN-methyl group-methyl group
increases stabilityincreases stability
MiotineMiotine
C
N
O
O
H
Me
Me
CH
NMe2
(ionised at blood pH)
NeostigmineNeostigmine
NMe3OMe
N
Me
C
O
15.4 Organophosphates15.4 Organophosphates
a) Nerve gasesa) Nerve gases
•• Agents developed in World War 2Agents developed in World War 2•• Agents irreversibly inhibit Agents irreversibly inhibit acetylcholinesteraseacetylcholinesterase•• Permanent activation of cholinergic receptors by AchPermanent activation of cholinergic receptors by Ach•• Results in deathResults in death
DyflosDyflos((DiisopropylDiisopropyl fluorophosphonatefluorophosphonate))
iPrO
P
O
FiPrO
SarinSarin
P
Me F
OiPrO
15.4 Organophosphates15.4 Organophosphates
b) Mechanism of actionb) Mechanism of action
•• Irreversible Irreversible phosphorylationphosphorylation•• P-O bond very stableP-O bond very stable
P
iPrO F
OiPrO
STABLE
-H
O
P O
iPrO
iPrO
Serine
O
H
Serine
F-
c) Medicinal organophosphatec) Medicinal organophosphate
•• Used to treat glaucomaUsed to treat glaucoma•• Topical applicationTopical application•• Quaternary N is added to improve binding interactionsQuaternary N is added to improve binding interactions•• Results in better selectivity and lower, safer dosesResults in better selectivity and lower, safer doses
EcothiopateEcothiopateMe3N CH2 CH2 S P
O
OEt
OEt
15.4 Organophosphates15.4 Organophosphates
d) Organophosphates as insecticidesd) Organophosphates as insecticides
•• Relatively harmless to mammalsRelatively harmless to mammals•• Agents act as prodrugs in insectsAgents act as prodrugs in insects•• Metabolised by insects to produce a toxic metaboliteMetabolised by insects to produce a toxic metabolite
ParathionParathion
EtO
P
S
EtO O NO2
MalathionMalathion
CH
CH2CO2Et
CO2Et
S
P
MeO
SMeO
15.4 Organophosphates15.4 Organophosphates
d) Organophosphates as insecticidesd) Organophosphates as insecticides
MAMMALS INSECTS
NO2O
P
EtO
SEtO
PARATHION(Inactive Prodrug)
Active drug
Phosphorylates enzyme
DEATHDEATH
Mammalian Metabolism
EtO
P
S
EtO OH
INACTIVE& excreted
EtO
P
O
EtO O NO2Insect
Oxidativedesulphurisation
15.4 Organophosphates15.4 Organophosphates
e) Design of Organophosphate Antidotese) Design of Organophosphate Antidotes
StrategyStrategy•• Strong Strong nucleophile nucleophile required to cleave strong P-O bondrequired to cleave strong P-O bond•• Find suitable Find suitable nucleophile nucleophile capable of cleaving phosphate esterscapable of cleaving phosphate esters•• Water is too weak as a Water is too weak as a nucleophilenucleophile•• Hydoxylamine Hydoxylamine is a stronger is a stronger nucleophilenucleophile
•• Hydroxylamine is too toxic for clinical useHydroxylamine is too toxic for clinical use•• Increase selectivity by increasing binding interactions with activeIncrease selectivity by increasing binding interactions with active
sitesite
Hydroxylamine
+ +RO P
O
OR
ORH2N
O P
O
OR
OR
NH2OHROH
15.4 Organophosphates15.4 Organophosphates
e) Design of Organophosphate Antidotese) Design of Organophosphate Antidotes
•• Quaternary N is added to bind to the anionic regionQuaternary N is added to bind to the anionic region•• Side chain is designed to place the hydroxylamine moiety inSide chain is designed to place the hydroxylamine moiety in
the correct position relative to phosphorylated serinethe correct position relative to phosphorylated serine•• PralidoximePralidoxime 1 million times more effective than 1 million times more effective than
hydroxylaminehydroxylamine•• Cannot act in CNS due to charge - cannot cross Cannot act in CNS due to charge - cannot cross bbbbbb
PralidoximePralidoximeN
CH3
CH N
OH
15.4 Organophosphates15.4 Organophosphates
e) Design of Organophosphate Antidotese) Design of Organophosphate Antidotes
H
N CH N
O
H
MeMe
N
OPNCH
O
OR
OR
CO2 OH
SER
Active Site (Free)
CO2 O
P
O
OR
OR
SER
Active Site (Blocked)
15.4 Organophosphates15.4 Organophosphates
e) Design of Organophosphate Antidotese) Design of Organophosphate Antidotes
•• ProdrugProdrug for for pralidoximepralidoxime•• Passes through BBB as free basePasses through BBB as free base•• Oxidised in CNS to Oxidised in CNS to pralidoximepralidoxime
ProPAMProPAM
N
HH
CH3
NOH
15.4 Organophosphates15.4 Organophosphates
15.5 15.5 AnticholinesterasesAnticholinesterases as as ‘‘Smart DrugsSmart Drugs’’
•• Act in CNSAct in CNS
•• Must cross blood brain barrierMust cross blood brain barrier
•• Used to treat memory loss in Used to treat memory loss in AlzheimersAlzheimers disease disease
•• AlzheimersAlzheimers causes deterioration of cholinergic receptors in causes deterioration of cholinergic receptors inbrainbrain
•• Smart drugs inhibit Ach hydrolysis to increase activity atSmart drugs inhibit Ach hydrolysis to increase activity atremaining receptorsremaining receptors
15.5 15.5 AnticholinesterasesAnticholinesterases as as ‘‘Smart DrugsSmart Drugs’’
Donepezil
Tacrine (Cognex)Toxic side effects
Rivastigmine (Exelon)(analogue of physostigmine)
Galanthamine(daffodil and snowdrop bulbs
Metrifonate(organophosphate)
Xanomeline
Anabaseine(ants and marine worms)
N
NH2
MeO
MeO
O
CH2 N
H
Cl
CH O
C
N
O
Me
Et
Me
NMe2
MeO
P CCl3
O
MeO
OH
HN N
NMe
N
S
N
O Me
N
O
MeO
Me
HO