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Cholinergic Agonists
‘Cholinomimetic agents’
Lecture 2
Dr. Hiwa K. Saaed
Department of Pharmacology & Toxicology
1
2
Cholinergic Agonists
Classification
A. Pharmacologically: by their spectrum of action.
1. Muscarinic agonists:
 Bethanechol
 Methacholine
 Pilocarpine
2. Nicotinic agonists:
 nicotine
3
B. mode of action
1. Direct-acting: bind directly to and activate
muscarinic or nicotinic receptors,
Also subdivided chemically:
A. Esters of choline (including acetylcholine,
bethanechol, methacholine and carbachol)
B. Alkaloids (such as muscarine, pilocarpine and
nicotine).
4
Classification
2. Indirect-acting:
by inhibiting the hydrolysis of endogenous
acetylcholine. also subdivided to
A. Reversible:
neostigmine, physostigmine, pyridostigmine,
ambenomium, edrophonium, donepezil,
galantamine, rivastigmine, tacrine.
B. Irreversible
echothiophate, isoflurophate
5
Classification
Indirect and direct
Some quaternary cholinesterase inhibitors also
have a modest DIRECT action as well,
example, neostigmine, which activates
neuromuscular nicotinic cholinoceptors directly
in addition to blocking cholinesterase.
6
Basic Pharmacology of Direct-acting
 Muscarinic and Nicotinic agents
Acetylcholine & Carbachol: have effects on both
receptors.
 muscarinic agents;
‘Pilocarpine and Bethanechol’ preferentially bind
to muscarinic receptors.
7
Pharmacokinetics
 All the direct-acting cholinergic drugs have
longer durations of action than Ach.
 Choline esters are poorly absorbed and
poorly distributed into the CNS because they
are hydrophilic.
 The tertiary cholinomimetic alkaloids
(pilocarpine, nicotine, lobeline) are well
absorbed from most sites of administration.
8
Pharmacokinetics
 Although all are hydrolyzed in the
gastrointestinal tract (and less active by the oral
route).
 they differ markedly in their susceptibility to
hydrolysis by cholinesterase.
1. Acetylcholine is very rapidly hydrolyzed.
2. Methacholine 3 times more resistant to
hydrolysis
3. Carbachol & Bethanechol are extremely
resistant to hydrolysis.
9
10
 Pharmacology of Ach-like agonist
 Although, it affects almost every system within
the body. it is therapeutically of no
importance!!!! because of
1. its multiplicity of actions,
2. and its rapid inactivation by the cholinesterases.
A. Acetylcholine (Ach)
11
Acetylcholine (Ach)
 CVS: it ↓s heart rate, contractility, and blood
pressure.
 GIT: it ↑s motility of the GIT and bladder.
 Pulmonary system: it ↑s secretions of the
bronchioles
 Eye: miosis and accommodation.
 PNS: contraction of skeletal muscle
 CNS: it affects neurotransmission
 Endocrine system: it causes release of
epinephrine from the adrenal medulla (via Nn
receptors).
 And it stimulates sweat gland secretions.
12
Response mediated by Muscarinic
receptors
13
Response mediated by Nicotinic
receptors
14
Adverse effects:
excessive generalized cholinergic stimulation
DUMBLES:
• Diarrhea and decreased BP,
• Urination,
• Miosis,
• Bronchoconstriction,
• Lacrimation,
• Excitation of skeletal muscle,
• Salivation and Sweating
15
B. Bethanechol
 It has duration of action of about one hour.
 Its major action on the smooth musculature of the bladder
and GIT, causing increased intestinal motility and tone.
 (BBB- Bethanechol stimulates the Bladder and Bowel)
Therapeutic Uses:
 to stimulate the atonic bladder in postoperative, non-
obstructive urinary retention.
16
C. Carbachol actions:
 Systemically: Carbachol has profound effects on both
the CVS and the GIT
 because of its ganglion (Nn receptor) stimulating
activity, and it may first stimulate and then depress
these systems.
 It can cause release of epinephrine from the adrenal
medulla by its nicotinic action.
 Locally instilled into the eye, it mimics the effects of
Ach, causing miosis and a spasm of accommodation.
 is rarely used therapeutically except in the eye to treat
glaucoma. Because of its high potency, and relatively
long duration of action.
17
 Act on muscarinic receptors
 Used in diagnosis of asthma and
bronchial hyperreactivity
D. Methacholine
18
E. Pilocarpine (an alkaloid)
 is a tertiary amine, and is stable not hydrolyzed
by AchE.
 It is far less potent compared with Ach and its
derivatives.
 Pilocarpine exhibits muscarinic activity
 is used primarily in ophthalmology, topically
produces a rapid miosis and contraction of the
ciliary muscle; a spasm of accommodation; the
vision is fixed at some particular distance, making
it impossible to focus.
19
20
E. Pilocarpine action
 Pilocarpine is one of the most potent
stimulators of secretions such as sweat, tears,
and saliva,
 but its lack of selectivity (adverse effects)
limited its use:
1. can enter the brain and causes CNS
disturbances.
2. It stimulates profuse sweating and salivation.
Pilocarpine actions:
21
Sjogren syndrome
 Sjogren syndrome, immunologic disorder with
destruction of the exocrine glands leading to
the mucosal dryness (dry mouth and lack of
tears)
 is usually treated with cevimeline, a cholinergic
drug that also has the drawback of being
nonspecific.
 Recent studies have shown that mouth sprays
of Pilocarpine are beneficial in promoting
salivation in patients with Xerostomia (dryness
of the mouth).
22
is the DOC in the emergency lowering of IOP of both narrow-angle
and a wide-angle glaucoma.
Therapeutic use in glaucoma:
23
is extremely effective in
opening the trabecular
meshwork around
Schlemm canal result in
an increased drainage of
aqueous humor
Organ system effects of cholinergic agonists
24
INDIRECT-ACTING
CHOLINOMIMETICS
(ANTICHOLINESTERASE)
Dr. Hiwa K. Saaed
Department of Pharmacology & Toxicology
Basic Pharmacology of the Indirect-Acting
Cholinomimetics
Q. How do they work?
 By inhibiting ChE, protect Ach from hydrolysis.
 Their pharmacodynamic properties are almost identical.
 The chief differences between members of the group are
chemical and pharmacokinetic.
26
Chemistry of the Indirect-Acting
Cholinomimetics
1. Simple alcohols bearing a quaternary ammonium
group, e.g., edrophonium
2. Carbamic acid esters of alcohols bearing
 Quaternary ammonium groups (e.g., neostigmine)
 Tertiary ammonium groups (e.g., physostigmine).
3. Organic derivatives of phosphoric acid
(organophosphates, e.g., echothiophate).
Physostigma venenosum27
Reversible Irreversible bind covalently to ChE.
Carbamates Acridine organophosphates
Physostigmine
Neostigmine
Pyridostigmine
Edrophonium
Rivastigmie
Donepezil
tacrine Dyflos (DFP) , Echothiophate Drug
Parathion, Malathion (insecticide)
Diazinon, Tabun, sarin, soman (nerve
gases for chemical warfare)
28
Cholinesterase inhibitors
Irreversible cholinesterase inhibitors
 Irreversible: Only the organophosphate inhibitors,
 because they bind covalently to ChE, and can
permanently inactivate the enzyme.
 The effects of organophosphates can last as long as a
week, which is approximately the time, needed to
synthesize a new molecule of ChE.
Q. Is it at all possible to reverse the effects of organophosphates? In
most cases, no.
However, if Pralidoxime “2-PAM” (a cholinesterase reactivator) is
given before a process called aging.
29
 Aging: the
organophosphate binds to
ChE and loses one of its
alkyl groups, then it may
be possible to remove the
organophosphate from
ChE.
30
Aging:
 Edrophonium, Neostigmine,
Pyridostigmine:
 Absorption from the conjunctiva,
skin, and lungs is predictably
poor.
 Distribution into the CNS is
negligible.
 Physostigmine (lipid
soluble):
 is well absorbed from all
sites and can be used
topically in the eye.
 It is distributed into the
CNS and is more toxic
Synthetic quaternary ammonium
agents
Naturally occurring tertiary
amine:
31
Absorption, Distribution, and Metabolism
All are well absorbed from the skin, lung, gut, and conjunctiva except for
echothiophate; Therefore, dangerous to humans and highly effective as
insecticides.
The organophosphate cholinesterase inhibitors
are also rapidly metabolized (detoxify)
by other pathways to inactive products
in birds and mammals but not in insects
and Unfortunately, fish ; these agents
are therefore considered safe enough for
sale to the general public.
is not detoxified effectively in
vertebrates; thus, it is
considerably more dangerous
than malathion to humans and
livestock and is not available
for general public use.
Malathion and a few other
organophosphate insecticides
Parathion
32
Thiohosphate (parathion, malathion, and related compounds)
• Are quite lipid-soluble rapidly; absorbed by all routes.
• They must be activated in the body by conversion to the oxygen
analogs, a process that occurs rapidly in both insects and vertebrates.
Treatment of AChE Poisoning
AChE Adverse effects:
 Excessive cholinergic stimulation
1. Atropine: Reverses muscarinic but not nicotinic
2. Pralidoxime (2-PAM):
33
Actions of AChE Inhibitors
1. CNS:
• Low doses: CNS activation
• High: coma and respiratory arrest
2. Eye, respiratory tract, GI & urinary tract: The same as
muscarinic agonists
3. Cardiovascular:
• Heart: Bradycardia, ↓contraction, ↓COP
• Blood vessels? No effect
4. Neuromuscular junction:
• low dose ↑ force of contraction
• high dose Muscle fasciculation and depolarizing blockade
34
The major therapeutic uses of the
cholinomimetics:
1. Eye:
 glaucoma,
 accommodative esotropia
2. Gastrointestinal & Urinary tracts:
 postoperative atony,
 neurogenic bladder
3. Neuromuscular junction:
 myasthenia gravis,
 curare-induced neuromuscular paralysis
NB. Cholinesterase inhibitors, but not direct-acting acetylcholine
receptor agonists; are extremely valuable as therapy for
myasthenia.??
35
Drugs Used in Myasthenia Gravis
Drug Duration of Action
Diagnosis:
Edrophonium I.V (improvement) 5-15 min
Treatment:
Neostigmine (do not cross BBB) 0.5-2 hours
Pyridostigmine 3-6 hours
Ambenonium 4-8 hours
36

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L2

  • 1. Cholinergic Agonists ‘Cholinomimetic agents’ Lecture 2 Dr. Hiwa K. Saaed Department of Pharmacology & Toxicology 1
  • 3. Classification A. Pharmacologically: by their spectrum of action. 1. Muscarinic agonists:  Bethanechol  Methacholine  Pilocarpine 2. Nicotinic agonists:  nicotine 3
  • 4. B. mode of action 1. Direct-acting: bind directly to and activate muscarinic or nicotinic receptors, Also subdivided chemically: A. Esters of choline (including acetylcholine, bethanechol, methacholine and carbachol) B. Alkaloids (such as muscarine, pilocarpine and nicotine). 4 Classification
  • 5. 2. Indirect-acting: by inhibiting the hydrolysis of endogenous acetylcholine. also subdivided to A. Reversible: neostigmine, physostigmine, pyridostigmine, ambenomium, edrophonium, donepezil, galantamine, rivastigmine, tacrine. B. Irreversible echothiophate, isoflurophate 5 Classification
  • 6. Indirect and direct Some quaternary cholinesterase inhibitors also have a modest DIRECT action as well, example, neostigmine, which activates neuromuscular nicotinic cholinoceptors directly in addition to blocking cholinesterase. 6
  • 7. Basic Pharmacology of Direct-acting  Muscarinic and Nicotinic agents Acetylcholine & Carbachol: have effects on both receptors.  muscarinic agents; ‘Pilocarpine and Bethanechol’ preferentially bind to muscarinic receptors. 7
  • 8. Pharmacokinetics  All the direct-acting cholinergic drugs have longer durations of action than Ach.  Choline esters are poorly absorbed and poorly distributed into the CNS because they are hydrophilic.  The tertiary cholinomimetic alkaloids (pilocarpine, nicotine, lobeline) are well absorbed from most sites of administration. 8
  • 9. Pharmacokinetics  Although all are hydrolyzed in the gastrointestinal tract (and less active by the oral route).  they differ markedly in their susceptibility to hydrolysis by cholinesterase. 1. Acetylcholine is very rapidly hydrolyzed. 2. Methacholine 3 times more resistant to hydrolysis 3. Carbachol & Bethanechol are extremely resistant to hydrolysis. 9
  • 10. 10  Pharmacology of Ach-like agonist
  • 11.  Although, it affects almost every system within the body. it is therapeutically of no importance!!!! because of 1. its multiplicity of actions, 2. and its rapid inactivation by the cholinesterases. A. Acetylcholine (Ach) 11
  • 12. Acetylcholine (Ach)  CVS: it ↓s heart rate, contractility, and blood pressure.  GIT: it ↑s motility of the GIT and bladder.  Pulmonary system: it ↑s secretions of the bronchioles  Eye: miosis and accommodation.  PNS: contraction of skeletal muscle  CNS: it affects neurotransmission  Endocrine system: it causes release of epinephrine from the adrenal medulla (via Nn receptors).  And it stimulates sweat gland secretions. 12
  • 13. Response mediated by Muscarinic receptors 13
  • 14. Response mediated by Nicotinic receptors 14
  • 15. Adverse effects: excessive generalized cholinergic stimulation DUMBLES: • Diarrhea and decreased BP, • Urination, • Miosis, • Bronchoconstriction, • Lacrimation, • Excitation of skeletal muscle, • Salivation and Sweating 15
  • 16. B. Bethanechol  It has duration of action of about one hour.  Its major action on the smooth musculature of the bladder and GIT, causing increased intestinal motility and tone.  (BBB- Bethanechol stimulates the Bladder and Bowel) Therapeutic Uses:  to stimulate the atonic bladder in postoperative, non- obstructive urinary retention. 16
  • 17. C. Carbachol actions:  Systemically: Carbachol has profound effects on both the CVS and the GIT  because of its ganglion (Nn receptor) stimulating activity, and it may first stimulate and then depress these systems.  It can cause release of epinephrine from the adrenal medulla by its nicotinic action.  Locally instilled into the eye, it mimics the effects of Ach, causing miosis and a spasm of accommodation.  is rarely used therapeutically except in the eye to treat glaucoma. Because of its high potency, and relatively long duration of action. 17
  • 18.  Act on muscarinic receptors  Used in diagnosis of asthma and bronchial hyperreactivity D. Methacholine 18
  • 19. E. Pilocarpine (an alkaloid)  is a tertiary amine, and is stable not hydrolyzed by AchE.  It is far less potent compared with Ach and its derivatives.  Pilocarpine exhibits muscarinic activity  is used primarily in ophthalmology, topically produces a rapid miosis and contraction of the ciliary muscle; a spasm of accommodation; the vision is fixed at some particular distance, making it impossible to focus. 19
  • 21.  Pilocarpine is one of the most potent stimulators of secretions such as sweat, tears, and saliva,  but its lack of selectivity (adverse effects) limited its use: 1. can enter the brain and causes CNS disturbances. 2. It stimulates profuse sweating and salivation. Pilocarpine actions: 21
  • 22. Sjogren syndrome  Sjogren syndrome, immunologic disorder with destruction of the exocrine glands leading to the mucosal dryness (dry mouth and lack of tears)  is usually treated with cevimeline, a cholinergic drug that also has the drawback of being nonspecific.  Recent studies have shown that mouth sprays of Pilocarpine are beneficial in promoting salivation in patients with Xerostomia (dryness of the mouth). 22
  • 23. is the DOC in the emergency lowering of IOP of both narrow-angle and a wide-angle glaucoma. Therapeutic use in glaucoma: 23 is extremely effective in opening the trabecular meshwork around Schlemm canal result in an increased drainage of aqueous humor
  • 24. Organ system effects of cholinergic agonists 24
  • 25. INDIRECT-ACTING CHOLINOMIMETICS (ANTICHOLINESTERASE) Dr. Hiwa K. Saaed Department of Pharmacology & Toxicology
  • 26. Basic Pharmacology of the Indirect-Acting Cholinomimetics Q. How do they work?  By inhibiting ChE, protect Ach from hydrolysis.  Their pharmacodynamic properties are almost identical.  The chief differences between members of the group are chemical and pharmacokinetic. 26
  • 27. Chemistry of the Indirect-Acting Cholinomimetics 1. Simple alcohols bearing a quaternary ammonium group, e.g., edrophonium 2. Carbamic acid esters of alcohols bearing  Quaternary ammonium groups (e.g., neostigmine)  Tertiary ammonium groups (e.g., physostigmine). 3. Organic derivatives of phosphoric acid (organophosphates, e.g., echothiophate). Physostigma venenosum27
  • 28. Reversible Irreversible bind covalently to ChE. Carbamates Acridine organophosphates Physostigmine Neostigmine Pyridostigmine Edrophonium Rivastigmie Donepezil tacrine Dyflos (DFP) , Echothiophate Drug Parathion, Malathion (insecticide) Diazinon, Tabun, sarin, soman (nerve gases for chemical warfare) 28 Cholinesterase inhibitors
  • 29. Irreversible cholinesterase inhibitors  Irreversible: Only the organophosphate inhibitors,  because they bind covalently to ChE, and can permanently inactivate the enzyme.  The effects of organophosphates can last as long as a week, which is approximately the time, needed to synthesize a new molecule of ChE. Q. Is it at all possible to reverse the effects of organophosphates? In most cases, no. However, if Pralidoxime “2-PAM” (a cholinesterase reactivator) is given before a process called aging. 29
  • 30.  Aging: the organophosphate binds to ChE and loses one of its alkyl groups, then it may be possible to remove the organophosphate from ChE. 30 Aging:
  • 31.  Edrophonium, Neostigmine, Pyridostigmine:  Absorption from the conjunctiva, skin, and lungs is predictably poor.  Distribution into the CNS is negligible.  Physostigmine (lipid soluble):  is well absorbed from all sites and can be used topically in the eye.  It is distributed into the CNS and is more toxic Synthetic quaternary ammonium agents Naturally occurring tertiary amine: 31 Absorption, Distribution, and Metabolism All are well absorbed from the skin, lung, gut, and conjunctiva except for echothiophate; Therefore, dangerous to humans and highly effective as insecticides. The organophosphate cholinesterase inhibitors
  • 32. are also rapidly metabolized (detoxify) by other pathways to inactive products in birds and mammals but not in insects and Unfortunately, fish ; these agents are therefore considered safe enough for sale to the general public. is not detoxified effectively in vertebrates; thus, it is considerably more dangerous than malathion to humans and livestock and is not available for general public use. Malathion and a few other organophosphate insecticides Parathion 32 Thiohosphate (parathion, malathion, and related compounds) • Are quite lipid-soluble rapidly; absorbed by all routes. • They must be activated in the body by conversion to the oxygen analogs, a process that occurs rapidly in both insects and vertebrates.
  • 33. Treatment of AChE Poisoning AChE Adverse effects:  Excessive cholinergic stimulation 1. Atropine: Reverses muscarinic but not nicotinic 2. Pralidoxime (2-PAM): 33
  • 34. Actions of AChE Inhibitors 1. CNS: • Low doses: CNS activation • High: coma and respiratory arrest 2. Eye, respiratory tract, GI & urinary tract: The same as muscarinic agonists 3. Cardiovascular: • Heart: Bradycardia, ↓contraction, ↓COP • Blood vessels? No effect 4. Neuromuscular junction: • low dose ↑ force of contraction • high dose Muscle fasciculation and depolarizing blockade 34
  • 35. The major therapeutic uses of the cholinomimetics: 1. Eye:  glaucoma,  accommodative esotropia 2. Gastrointestinal & Urinary tracts:  postoperative atony,  neurogenic bladder 3. Neuromuscular junction:  myasthenia gravis,  curare-induced neuromuscular paralysis NB. Cholinesterase inhibitors, but not direct-acting acetylcholine receptor agonists; are extremely valuable as therapy for myasthenia.?? 35
  • 36. Drugs Used in Myasthenia Gravis Drug Duration of Action Diagnosis: Edrophonium I.V (improvement) 5-15 min Treatment: Neostigmine (do not cross BBB) 0.5-2 hours Pyridostigmine 3-6 hours Ambenonium 4-8 hours 36