Cholinergic System Muscarinic Agonists PDF

Cholinergic System Muscarinic Agonists DIRECT AGONISTS FARM-7225 JOSÉ O. COLÓN SÁEZ PHD. Objectives • Learn the steps involved in cholinergic neurotransmission. • Recognize the organ effects of cholinomimetics and the receptors involved. • Understand for both Direct and Indirect cholinomimeti...

Cholinergic System Muscarinic Agonists DIRECT AGONISTS FARM-7225 JOSÉ O. COLÓN SÁEZ PHD. Objectives • Learn the steps involved in cholinergic neurotransmission. • Recognize the organ effects of cholinomimetics and the receptors involved. • Understand for both Direct and Indirect cholinomimetics the: ✓ Mechanism of action ✓ Pharmacodynamics ✓ Pharmacokinetics ✓ Therapeutic ✓ Adverse applications effects Cholinergic agonists Often called parasympathomimetic drugs or cholinomimetics because their action mimics the action of ACh in the ANS. Cholinergic Neurotransmission Neurotransmission in cholinergic neurons  Choline Acetyltransferase involves six sequential steps: ✓ Acetyl CoA ✓ Choline  Acetylcholine(ACh)  Cholinergic Receptors ✓ Muscarinic ✓ Nicotinic  Acetylcholinesterase (AChE) Rate Limiting Step Regulation of Neurotransmitter release • Autoreceptors: located on or close to the axon terminal of a neuron, the neuron's own transmitter can modify transmitter release. ✓ NE released from sympathetic neurons may interact with α2 receptors to inhibit neuronal release of NE. ✓ ACh released from parasympathetic neurons may interact with M4 receptors to inhibit neuronal release of ACh. v Regulation of Neurotransmitter release • Heteroreceptors: are presynaptic receptors that respond to neurotransmitters, neuromodulators, or neuro-hormones released from adjacent cells. ✓ ACh can influence the release of NE from sympathetic neurons by acting on M4 receptors. ✓ NE can influence the release of ACh from parasympathetic neurons by acting on α2 receptors. Acetylcholine ACh: naturally occurring neurotransmitter, with no systemic therapeutic applications (actions are diffuse, and its hydrolysis is rapid).  Cholinergic synapses are found at: ✓ Peripheral Nervous system ❖ Somatic motor nerves ❖ Autonomic Nervous System ✓ CNS  ACh administered systemically, can act on multiple sites; however, its penetration to the CNS is limited, and the amount that reaches target areas is limited due to hydrolysis by plasma butyrylcholinesterase. Muscarinic Agonists Muscarinic, based on the observation that the alkaloid muscarine acts selectively at those sites and produces the same qualitative effects as ACh. Amanita Muscaria  Muscarinic agonists mimic the effects of ACh at most sites. They are longer-acting congeners of ACh or natural alkaloids, some of which stimulate nicotinic as well as muscarinic receptors. The basis for the therapeutic uses of the muscarinic receptor agonists and antagonists can be deduced from the physiological effects of ACh at the parasympathetic nervous system. Cardiovascular System (M2): ✓negative chronotropic effect. ✓negative inotropic effect. ✓Vasodilation (M3). Respiratory Tract (M3): ✓Bronchoconstriction. ✓Secretions. Urinary Tract (M3): ✓Bladder contraction. ✓Urine formation. GI Tract (M3): ✓Secretions. ✓GI motility. Glands (M3): ✓Stimulates secretion (lacrimal, salivary, and sweat glands). Eye (M3): ✓Miosis ✓Increased secretion CNS effects: All five muscarinic receptor subtypes are found in the brain, and recent studies suggest that muscarinic receptor-regulated pathways may have an important role in cognitive function, motor control, appetite regulation, nociception, and other processes. Muscarinic Receptors RECEPTOR M1 M3 M5 Gq INTRACELLULAR TRANSDUCER Phospholipase C Diacyl-glycerol IP3 ELECTRICAL MECHANICAL PHYSIOLOGICAL RESPONSES • Depolarization • Smooth muscle ➢ Contraction ➢ Dilation • Glandular secretion Muscarinic Receptors M3 mediated vasodilation Muscarinic Receptors RECEPTOR M2 Gi INTRACELLULAR TRANSDUCER M4 Go Adenylyl cyclase cAMP ELECTRICAL • Hyperpolarization MECHANICAL (heart) PHYSIOLOGICAL • Cardiac inhibition RESPONSES Muscarinic Agonists Esters Synthetic Synthetic Alkaloids Methacholine  Hydrolyzed by acetylcholinesterases at a relatively slow rate.  Lacks nicotinic actions.  Insoluble to lipid cell membranes, poor absorption from the GI tract and does not cross the BBB. Mechanism of Action: Profound cardiovascular effect limit the therapeutic use. ✓ Causes both bradycardia and hypotension. Methacholine Therapeutic applications: diagnosis of bronchial hyperreactivity (asthma and COPD). ✓ Methacholine challenge test: a subject inhales aerosolized methacholine → bronchoconstriction (asthmatics react to lower doses of drug). Adverse effects: Other therapeutic uses are limited by its adverse cardiovascular effects. ✓ Bradycardia and hypotension, which arise from its function as a cholinomimetic. Carbachol Structurally related to acetylcholine, in which the acetate is replaced by carbamate.  Not hydrolyzed by acetylcholinesterases.  Has both muscarinic as well as nicotinic actions. Mechanism of Action: has profound effects on both the cardiovascular system and the gastrointestinal system because of its ganglion-stimulating activity. ✓ Can cause release of epinephrine from adrenal medulla. Carbachol Therapeutic applications: Because of its high potency, receptor non-selectivity, and relatively long duration of action, carbachol is rarely used therapeutically. ✓ Eye drops are used to ↓ eye pressure (glaucoma) and constrict pupils for cataract surgery. Adverse effects: At doses used ophthalmologically, little or no side effects occur due to lack of systemic penetration. • Not administered systemically because of its relatively larger component of nicotinic action at autonomic ganglia. Bethanechol Structurally related to acetylcholine, in which the acetate is replaced by carbamate and the choline is methylated.  Not hydrolyzed by acetylcholinesterases.  Lacks nicotinic actions. Mechanism of Action: Directly stimulates muscarinic receptors in smooth musculature of the bladder and gastrointestinal tract. Bethanechol Therapeutic applications: urologic treatment. ✓ Non-obstructive ❖ Neurogenic urinary retention. atony (poor muscular condition). ❖ Postpartum. ❖ Postoperative. ✓ Paralytic ileus ❖ Megacolon (dilation of the colon associated with prolonged constipation). Adverse effects: effects of generalized cholinergic stimulation (sweating, salivation, flushing, ↓HR/BP, nausea, abdominal pain, diarrhea, and bronchospasm). ADME Absorption: polarity dependent (poor for ACh, quaternary ammonium), intravenous, subcutaneous and intramuscular for local effects (ACh). Metabolism: Highly dependent on the susceptibility to acetylcholinesterase (AChE). Basic and Clinical Pharmacology, 13e> Cholinoceptor-Activation Contraindications for choline esters Contraindications: asthma, hyperthyroidism, coronary insufficiency, and acid-peptic disease. ✓ Bronchoconstrictor action could precipitate an asthmatic attack. ✓ Hyperthyroid patients may develop atrial fibrillation. ✓ Hypotension induced by these agents can severely reduce coronary blood flow, especially if it is already compromised. ✓ The gastric acid secretion produced by the choline esters can aggravate the symptoms of acid-peptic disease. Causes Cholinergic Side effects (DUMBELLS), depending on administration route. Pilocarpine A naturally occurring alkaloid, obtained from the leaves of tropical South American shrubs from the genus Pilocarpus. ADME: lipid soluble, penetrates the CNS at therapeutic doses. Stable to hydrolysis by Acetylcholinesterases.  It has Muscarinic actions only.  Less potent than acetylcholine and its derivatives. Mechanism of Action: When applied locally to cornea produces rapid miosis and contraction of ciliary muscle. ✓ Vision is fixed making cannot focus for far situated objects. Pilocarpine  Most potent stimulators of secretions (secretagogue): sweat, tears, and saliva. Therapeutic applications: used to promote glandular secretions. ✓ Salivation in xerostomia from irradiation of the head and neck. ✓↑ Salivation and ↑ tears in patients with Sjgoren's syndrome. ✓ Sweat chloride test: cystic fibrosis diagnosis. https://www.cff.org/intro-cf/sweat-test#:~:text=If%20you%20show%20symptoms%20of,reliable%20way%20to%20diagnose%20CF. Pilocarpine Drug of choice in emergency ↓ of intraocular pressure. ✓ Both narrow-angle (also called closedangle) and wide-angle (also called openangle) glaucoma. Adverse effects: can enter the brain and cause CNS disturbances. ✓ Stimulates salivation. profuse sweating and Glaucoma  Progressive eye disease classified as open angle glaucoma (most common) and close angle glaucoma. ✓ Both types of glaucoma cause an elevated eye pressures that may result in optic nerve damage and permanent blindness if not treated.  Raised intraocular pressure (above 21 mmHg or 2.8 kPa) is the most important and only modifiable risk factor for glaucoma. Cholinergic Crisis 33 Atropine competitively antagonizes ACh at muscarinic receptors to reverse excessive secretions, miosis, bronchospasm, vomiting, diarrhea, diaphoresis, and urination. Cholinergic System Muscarinic Agonists INDIRECT AGONISTS FARM-7225 JOSÉ O. COLÓN SÁEZ PHD. Cholinergic agonists Often called parasympathomimetic drugs or cholinomimetics because their action mimics the action of ACh in the ANS. Acetylcholinesterase (AChE) The actions of acetylcholine in the cholinergic synapse are terminated by enzymatic hydrolysis, by the action of AChE, which is present in high concentrations in cholinergic synapses.  Indirect-acting cholinomimetics have their primary effect at the active site of this enzyme, some also have direct actions at nicotinic receptors. The ACh released is rapidly hydrolyzed by AChE, such that the lifetime of free ACh within the nerve-muscle synapse (∼200 μsec). Types of Cholinesterases 1. Acetylcholinesterase (AChE), present at synapses. ✓ Bound to the basement membrane in the synaptic cleft. ✓ Present as a soluble form in cholinergic nerve terminals. 2. Butyrylcholinesterase (pseudocholinesterase, BChE), produced in the liver and present in soluble form in the plasma. ✓ Broader substrate specificity than AChE; hydrolyses many different choline esters. ✓ Together with AChE, keeps acetylcholine concentration in plasma nearly undetectable. AChE Inhibitors  Anti-ChE agents have received extensive application as toxic agents (agricultural insecticides, pesticides), and potential chemical warfare "nerve gases”.  Several compounds are widely used therapeutically; some that cross the blood-brain barrier have been approved for the treatment of Alzheimer's disease. AChE Inhibitors  Reversible: ✓ Edrophonium ✓ Physostigmine ✓ Neostigmine ✓ Pyridogstimine  Irreversible: ✓ Echothiophate ✓ Sarin ✓ Parathion ✓ Soman ✓ Malathion ✓ Tabun AChE Inhibitors  Reversible: hydrolyzed by AChE, but much more slowly than ACh. 1. Quaternary alcohols: Edrophonium ✓ The enzyme-inhibitor complex does not involve a covalent bond (short-lived ≈ 2–10 minutes)(Competitive inhibitor of AChE). 2. Carbamate esters: Pyridostigmine ✓ The Physostigmine, Neostigmine and covalent bond of the carbamoylated enzyme is more resistant to the second (hydration) process (prolonged action ≈ of 30 minutes - 6 hours). AChE Inhibitors • Irreversible: phosphorylated conjugate with AChE. 1. Organophosphates: ✓ The covalent phosphorus-enzyme bond is extremely stable and hydrolyzes in water at a very slow rate (hundreds of hours). ❖ Reactivation time is more than regeneration. • The stability of the phosphorylated enzyme is enhanced through "aging" which results from the loss of one of the alkyl groups. Hence, the return of AChE activity depends on synthesis of a new enzyme. “Aging” of Organophosphates Aging: loss of one of the alkyl groups cause irreversibility. ✓ Reactivation time of carbamylated enzyme <6 h vs phosphorylated >100 h (>regeneration). ✓ Rate of aging varies between organophosphate compound (2 min for soman, as much as 48 hours for other organophosphates). • If given before aging, pralidoxime (strong nucleophile) can break the phosphorus-enzyme bond. ✓ “cholinesterase regenerator”. AChE Inhibitors Pharmacology  The anti-AChE agents potentially can produce all the following effects: 1. Stimulation of muscarinic receptor responses at autonomic effector organs (Parasympathetic effect). 2. Stimulation, followed by depression (paralysis), of all autonomic ganglia and skeletal muscle. 3. Stimulation, with occasional subsequent depression, of cholinergic receptor sites in the CNS. AChE Inhibitors Pharmacology At ganglionic level → ↑ ACh: initially excitatory (NnAChR). At higher concentrations, ganglionic blockade (depolarization). • Primary action is to amplify the actions of endogenous ACh. ✓ Effects are similar (but not always identical) to the effects of the directacting cholinomimetic agonists. • Most prominent effects on the cardiovascular & gastrointestinal systems, the eye, and the skeletal muscle. AChE Inhibitors Pharmacology  CNS: convulsions, coma and respiratory arrest.  Eye, respiratory tract, gastrointestinal tract, urinary tract: effects are qualitatively like the effects of the direct-acting cholinomimetics.  Cardiovascular system: ↑ activity sympathetic and parasympathetic ganglia. ✓ The in both parasympathetic limb predominate.  Neuromuscular junction: ✓ Low (therapeutic) concentrations ↑ strength of contraction. ✓ Higher concentrations cause accumulation of ACh (fibrillation of muscle fibers followed by paralysis). Myasthenia Gravis  Autoimmune disease affecting neuromuscular junctions. ✓ Antibodies are produced against the muscle nicotinic receptor channel. ❖ Detected in 85% of myasthenic patients.  Antibodies ↓ muscle nicotinic receptor function by: ✓ Stimulating receptor internalization and degradation. ✓ Binding to the nicotinic receptor and inhibiting function. skeletal muscle Myasthenia Gravis  Symptoms includes ptosis, diplopia, difficulty speaking and swallowing, and extremity weakness. ✓ Severe disease may affect all the muscles, including those necessary for respiration.  Special blood tests reveal ACh autoantibodies in the circulating blood.  Edrophonium test: IV injection of edrophonium chloride briefly corrects muscle weakness (diagnostic).  Anti-AChE agents: pyridostigmine, physostigmine and neostigmine are prescribed to ↑ muscle strengths. Edrophonium  Short-acting AChE inhibitor. ✓ Duration of action of 2 to 10 minutes.  Actions are limited to the periphery.  Diagnosis of myasthenia gravis (Tensilon test). ✓ IV edrophonium → rapid ↑ in muscle strength. ✓ Care must be taken, because excess drug may provoke a cholinergic crisis (antidote???).  Also used to assess cholinesterase inhibitor therapy. ✓ Differentiates cholinergic and myasthenic crises, and for reversing the effects of nondepolarizing neuromuscular blockers after surgery. Neostigmine Synthetic anti-AChE based on Physostigmine.  Cannot cross the BBB. Therapeutic uses: ✓ Atony (GI and Urinary retention) produced by muscarinic receptor antagonists used prior to surgical intervention. ✓ Myasthenia gravis.  Has an additional direct nicotinic agonist effect at the neuromuscular junction. Pyridostigmine: Like neostigmine, but longer-acting (4 vs 6 hrs); chronic management of Myasthenia Gravis. Acetylcholine Release Modulators  Amifampridine and 4-aminopyridine: K+ channel antagonists. ✓ Prolong excitability; slows down repolarization → ↑VCa2+ activation.  Both are >90% bioavailable and are metabolized mainly by acetylation and excreted via the kidneys  Due to effects on CNS and cardiac potassium channels → ↑ seizure activity and prolong QT interval. https://www.fda.gov/news-events/press-announcements/fda-approves-first-treatment-children-lambert-eatonmyasthenic-syndrome-rare-autoimmune-disorder Physostigmine  Found in seed (Calabar bean) of African plant Physostigma venenosum. ✓ Ordeal Poison: accusation of demonic possession. ❖ Winner: vomited (or didn’t chew beans). ❖ Loser: miosis, excessive salivation, decreased bowel/bladder control, seizures, asphyxiation!  Intermediate-acting AChE inhibitor. ✓ Duration of action 30 min - 2 hrs. Physostigmine The first therapeutic use of the drug was in 1877, in the treatment of glaucoma, one of its clinical uses today. Mechanism: Actions at muscarinic and nicotinic sites of the autonomic nervous system, and nicotinic receptors of the neuromuscular junction. • Physostigmine enter and stimulate the cholinergic sites in the CNS. Physostigmine D  An overdose can cause cholinergic U rination syndrome. “DUMBELS” M iosis  The metabolite formed from B physostigmine, eseroline. mesis E Acts as an opioid agonist. L acrimation alivation S weating efecation ✓ ronchospam ronchorea radycardia ✓ ❖ Death can result from failure of the respiratory system. Physostigmine Therapeutic uses: ↑ intestinal and bladder motility, which serve as its therapeutic action in atony of either organ.  Placed topically in the eye, it produces miosis and spasm of accommodation, ↓ intraocular pressure. It is used to treat glaucoma, but pilocarpine is more effective.  Used in the treatment of overdoses of drugs with anticholinergic actions, such as atropine, antihistamines, antipsychotics and tricyclic antidepressants. Physostigmine Adverse effects: effects on the CNS may lead to convulsions when high doses are used.  Bradycardia and a fall in cardiac output may occur.  Inhibition of AChE at the skeletal neuromuscular junction causes the accumulation of ACh, ultimately, results in paralysis of skeletal muscle. An overdose can cause cholinergic syndrome. ✓ These effects are rarely seen with therapeutic doses. Tacrine, donepezil, rivastigmine, and galantamine: Treatment to enhance the cognitive function found in Alzheimer's Disease Patients. Indirect-Acting Cholinergic Agonists Anticholinesterases (Irreversible) Phosphorylate the AChE → permanent enzyme inactivation.  Many of the organophosphates (echothiophate is an exception) are highly lipid-soluble liquids.  “nerve gas”: synthesis of several compounds of much greater toxicity was kept secret by the Germans. ✓ Sarin ✓ Soman ✓ Tabun Echothiophate Following covalent modification of acetylcholinesterase, the phosphorylated enzyme slowly releases one of its ethyl groups. Actions: include generalized cholinergic stimulation, paralysis of motor function (causing breathing difficulties). Therapeutic uses: An ophthalmic solution of the drug is used directly in the eye for the chronic treatment of open-angle glaucoma. ✓ Echothiophate produces intense miosis. Echothiophate  Highly polar and more stable than most other organophosphates. When prepared in aqueous solution for ophthalmic use, it retains activity for weeks. ✓ Echothiophate is not a first-line agent in the treatment of glaucoma. In addition to its other side effects, the potential risk for causing cataracts limits its use.  Atropine in high dosage can reverse many of the muscarinic and some of the central effects of echothiophate. Other Organophosphates All organophosphates except echothiophate are distributed to all parts of the body, including the CNS. CNS toxicity is an important component of poisoning.  Parathion and malathion are pro-drugs; converted to the phosphate derivatives in animals and plants and are used as insecticides. ✓ Parathion is not detoxified effectively in vertebrates; it is considerably more dangerous than malathion to humans and livestock and is not available for general public use in the USA. Other Organophosphates  Sarin: lethal at very low doses, death following 1-10 minutes after inhalation (suffocation from lung muscle paralysis). ✓ Ghouta chemical attack, Syrian Civil War in August 2013. ❖ The suburbs around Damascus, were struck by rockets containing sarin. Estimates of the death toll range from at least 281-1,729 people. • Antidotes: ✓ Atropine and pralidoxime. Organophosphates Antidotes • Atropine: mAChR antagonist. • Reactivators: attach to inhibitor & separate inhibitor from acetylcholinesterase (dephosphorylate). ✓ Examples: ✓ Concern: pralidoxime (PAM). not one reactivator for all AChE-Inhibitors. Reactivation of acetylcholinesterase Pralidoxime  Can reactivate inhibited acetylcholinesterase. ✓ Unable to penetrate the CNS.  The presence of a charged group allows it to approach an anionic site on the enzyme, where it displaces the phosphate group of the organophosphate and regenerates the enzyme.  If given before aging of the enzyme has occurred, it can reverse the effects (except for those in the CNS). Reactivation of acetylcholinesterase  With the newer nerve agents, which produce fast aging of the enzyme, pralidoxime is less effective.  Pralidoxime is a weak acetylcholinesterase inhibitor and, at higher doses, may cause side effects like other acetylcholinesterase inhibitors. Limitation: For PAM to work, it must be used immediately following exposure to insecticides or sarin gas or no more than 36 hrs of exposure or else aging will occur and it won’t be able to dissociate the phosphate from the receptor.