Medical Pharmacology Chapter 6:  Autonomic Pharmacology: Cholinergic Drugs

• Acetylcholinesterase Inhibitors

   

  • "Acetylcholinesterase and Cholinesterase Inhibitors"

    Attribution: Farmer S Acetylcholinesterase and Cholinesterase Inhibitors https://www.youtube.com/watch?v=d2gJ3gHR5Mk

  • Overview

    • Three domains describe the acetylcholinesterase active site: an acyl binding region, the choline binding region, and a peripheral anionic site.

    • There are three classes of anticholinesterase agents

  • Reversible, Short-Acting Anticholinesterases

    • Reversible inhibitors, edrophonium (Tensilon) and tacrine (Cognex), associate with the choline binding domain.

      • The short duration of edrophonium (Tensilon) action is due to its binding reversibility and rapid renal clearance.

      • Tacrine (Cognex), being more lipophillic, has a longer duration.

    • Some reversible inhibitors, propidium and fasiculin, a toxic peptide, bind at the peripheral anionic site.

  • Carbamylating Agents: Intermediate-Duration Acetylcholinesterase Inhibitors

    • Physostigmine and Neostigmine are acetylcholinesterase inhibitors that form a moderately stable carbamyl-enzyme derivative.

    • The carbamyl-ester linkage is hydrolyzed by the esterase, but much more slowly compared to acetylcholine.

    • As a result, enzyme inhibition by these drugs last about 3 - 4 h (t _½ = 15 - 30 min).

    • Neostigmine possesses a quaternary nitrogen and thus has a permanent positive charge.

    • By contrast, physostigmine is a tertiary amine.

  •  Phosphorylating Agents: Long-Duration Acetylcholinesterase Inhibitors

    • Organophosphate acetylcholinesterase inhibitors, such as diisopropyl fluorophosphate (DFP) form stable phosphorylated serine derivatives.

    • For DFP the enzyme effectively does not regenerate following inhibition.

      • Furthermore, in the case of DFP, the loss, termed "aging", of an isopropyl group, further stabilizes the phosphylated enzyme.

      • The application of the terms "reversible" and "irreversible" depends on the duration of enzyme inhibition rather than strictly based on mechanism. (see above)

    • Examples of "reversible" acetylcholinesterase inhibitors that may be used clinically include both carbamylating agents and those that associate only with the choline binding domain.

  • "Reversible" Anticholinesterases Used Clinically

    • Edrophonium

    • Pyridostigmine-Used in treatment of myasthenia gravis

    • Neostigmine

    • Physostigmine

    • Demecarium

    • Ambenonium-Used in treatment of myasthenia gravis.

• Pharmacokinetics of Acetylcholinesterase Inhibitors

  • Duration of Action: Principles

    • Anticholinesterase duration of action: based on rate of disappearance from plasma.

    • Clinical Anesthesia Context:

      • Anticholinesterase drugs are administered when effects of nondepolarizing neuromuscular-blocking agents are diminishing

      • Note: duration of action of edrophonium is approximately the same compared to that of neostigmine in anesthetized patients. (edrophonium had been usually considered a short-acting agent)

  • Renal Clearance:  anticholinesterase drugs

    • Actively secreted into renal tubule lumen

    • Renal clearance:

      •  50% for neostigmine elimination

      •  75% for edrophonium and pyridostigmine elimination

      •  Elimination halftimes -- significantly prolonged in renal failure

        • In renal failure, plasma clearance of anticholinesterase drugs is prolonged more substantially than plasma clearance of neuromuscular-blocking agents-- making recurarization

    • In the absence of renal clearance (renal insufficiency), hepatic metabolism is involved to the following extents:

      • Neostigmine -- 50%

      • Edrophonium -- 30%

      • Pyridostigmine -- 25%

  • Carbamates

    •  Physostigmine, a tertiary amine, is readily absorbed following systemic administration and may be absorbed systemically after conjunctival use.

    •  Neostigmine, also a carbamylating inhibitor, is a quaternary nitrogen compound and, as a result, is poorly absorbed.

    •  Neostigmine is hydrolyzed by plasma esterases with metabolites excreted in the urine.

  •  Organophosphates

    • Most organophosphorous acetylcholinesterase inhibitors are well- absorbed lipophillic agents.

    • Organophosphates are generally hydrolyzed by serum and tissue esterases and hydrolytic products are renally excreted.

    • Some organophosphorus chemicals are substrates for mixed-function oxidases that convert phosphorothioates (P=S) to phorphorates (P=O).

    • Organophosphate anticholinesterases are themselves hydrolyzed by liver esterases also called A-esterases or paraoxonases. The extent of paraoxon toxicity in humans is dependent on an A-esterase genetic polymorphism.

    • Lipophillicity and intrinsic reactivity (phosophorylation rate constant) are two important factors in determining the lethality of human exposure to organophosphate inhibitors.

Stoelting, R.K., "Anticholinesterase Drugs and Cholinergic Agonists", in Pharmacology and Physiology in Anesthetic Practice, Lippincott-Raven Publishers, 1999, 224-237;  Taylor, P. Anticholinesterase Agents, In, Goodman and Gillman's The Pharmacologial Basis of Therapeutics,(Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) TheMcGraw-Hill Companies, Inc.,1996, pp.161-174.

• Differences between Parathion and Malathion

  • Parathion

    • Parathion, a low volatility and aqueous-stable, organophosphate is used as an agriculural insecticide.

    • Parathion is converted to paraoxon by mixed function oxidases. Both the parent compound and its metabolite are effective acetylcholinesterase inhibitors (P=S to P=O).

    • Parathion probably is the most common cause of accidental organophosphate poisoning and death.

    • The phosphothioate structure is present in other common insecticides: dimpylate, fenthion, and chlorpyrifos.

  • Malathion

    • Malathion is converted to the oxygen form (P=S to P=O).

    • Inactivation rates (hydrolysis) vary between species.

    • Inactivation rates are much higher in mammals and birds than insects.

    • Accidental poisoning and death is not observed with malathion with acute toxicity seen in suicide or deliberate poisoning. (lethal dose in man is about 1g/kg)

    • Spraying over populated areas with malathion has been used in control of Medierranean fruit flies and mosquitoes.

    • Malathion is used in treatment of lice infestations.

Taylor, P. Anticholinesterase Agents, In Goodman and Gillman's The Pharmacologial Basis of Therapeutics, (Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) The McGraw-Hill Companies, Inc.,1996, p. 167.

• Inhibition Mechanisms

  • Overview: 

    • Inactivation of acetylcholinesterase by organophosphates or carbamates requires either carbamylation or phosphorylation of an active-site reactive serine.

    • Phosphorylation of this serine leads to a very stable acyl-enzyme complex.

    • Deacylation (dephosphorylations) reactions may occur very slowly, thus effective inhibiting enzyme activity for long periods.

    • Reactivation of phosphorylated enzyme may be possible using a nucleophile such as pyridine-2-aldoxime (2-PAM).

      •  2-PAM reactivation may be not be possible, depending on the stability of the phosphoryl enzyme derivative.

• Acetylcholinesterase Locations

  • Acetylcholinesterase: post-synaptic cholinergic membranes.

  • Inhibition of acetylcholinesterase with subsequent acetylcholine accumulation causes:

    1. Enhanced muscarinic responses at parasympathetic effector sites.

    2. Nicotinic receptor stimulation and then paralysis (depolarization block) at autonomic ganglia and skeletal muscle.

    3. CNS cholinergic neuronal stimulation

  • Effects of increased acetylcholine levels can be blocked or reduced by atropine (muscarinic antagonist) at:

    • Parasympathetic effector sites.

    • Autonomic ganglia (muscarinic receptor population).

    • Subcortical CNS sites.

• Organ Systems Affected by Anticholinesterase Agents

  • Opthalmological Uses of Anticholinesterase Drugs

    • When applied to the conjunctiva, acetylcholinesterase inhibitors produce:

      • Constriction of the pupillary sphincter muscle (miosis)

      • Contraction of the ciliary muscle (paralysis of accommodation or loss of far vision).

    • Loss of accommodation disappears first, while the miotic effect is longer lasting.

    • During miosis, elevated intraocular pressure (glaucoma) declines due to enhanced flow of aqueous humor.

    •  In glaucoma, elevation of intraocular pressure can cause damage to the optic disc and blindness.

    • There are three types of glaucoma:

      • Primary

      • Secondary (aphakic (no lens) glaucoma, following cataract removal)

      • Congenital.

        • Of the three, primary glaucoma responds to anticholinesterase treatment.

    • Primary glaucoma may either be narrow angle (acute, congestive) or wide-angle (chronic, simple) depending on the angle configuration of the anterior chamber.

    • Narrow angle glaucoma, a medical emergency, may rely on drug treatment to control the attack, although surgery may be required for long-term management (iridectomy, peripheral or complete).

    • Anticholinesterase used for management of glaucoma or accommodative esotropia (esotropia (eso- (inward) + Gr. trepein to turn) [Deviation of visual axis toward that of the other other when fusion is possible]

    • Anticholinesterases Used in Treating Glaucoma

      1. Physostigmine (eserine)

      2. Demecarium (Humorsol)

      3. Echothiophate (Phospholine)

      4. Isoflurophate (Floropryl)

  • Gastrointestinal and Urinary Bladder

    • Neostigmine is the anticholinesterase agent of choice for treatment of paralytic ileus or urinary bladder atony.

    • Direct acting cholinomimetic drugs are also useful.

  • Myasthenia Gravis

    • "Acetylcholinesterase Inhibitors for Myasthenia Gravis"

      Attribution: Drugs in Motion (https://www.youtube.com/channel/UChQt4-Yl0nWy4bq1gKwh-6A) https://www.youtube.com/watch?v=-TzclKd2kL0

    • Myasthenia Gravis appears to be caused by the binding of anti-nicotinic receptor antibodies to the nicotinic cholinergic receptor.

    • Binding studies using snake alpha-neurotoxins determined a 70% to 90% reduction of nicotinic receptors per motor endplate in myasthenic patients. Receptor number is reduced by:

      • Increased receptor turnover (rapid endocytosis)

      • Blockade of the receptor binding domain

      • Antibody damage of postsynaptic muscle membrane

    • A related disease, Lambert-Eaton syndrome, is a presynaptic disorder in which acetylcholine release is impaired due to autoantibodies against P/Q type calcium channels.

      • Most patients with this syndrome have a malignancy, usually small cell lung carcinoma.

      • Treatment includes immunosuppression and plasmapheresis.

    • Anticholinesterase, edrophonium (Tensilon), is useful in differential diagnosis for myasthenia gravis.

      • In this use, edrophonium (Tensilon) with its rapid onset (30 s) and short duration (5 min) may cause an increase in muscle strength.

      • This change is due to the transient increase in acetylcholine concentration at the end plate.

      • Edrophonium (Tensilon) may also be used to differentiate between muscle weakness due to excessive acetylcholine (cholinergic crisis) and inadequate drug dosing.

    • Anticholinesterase drugs provide partial improvement in myasthenia gravis by increasing the amount of acetylcholine available at neuromuscular junctions.

    • Of the anticholinesterases listed below, pyridostigmine (oral) is the one most widely used in the U.S.

      • Anticholinesterases Used in Treating Myasthenia Gravis

        • Neostigmine (Prostigmin)

        • Pyridostigmine (Mestinon)

        • Ambenonium

    • Conditions that resemble myasthenia gravis include:

      • Lambert-Eaton myasthenic syndrome, a presynaptic disorder. (NEJM 332: 1467, 1995 review)

        • Patients with Lambert-Eaton disorder have depressed or absent reflexes, show autonomic changes such as xerostomia and impotence and incremetal responses to repetitive nerve stimulation. Treatment includes plamapheresis and immunosupression.

      • Neurasthenia

        • Muscle testing indicates a nonorganic disorder, characterized by feelings of fatigue rather than by a loss of muscle power.

      • Thyroid abnormalities (either hyper or hypo- thyroidism) can increase myasthenic weakness.

        • Thyroid testing is definitive.

      • Associated Disorders:

        •  Myasthenia gravis patients often have associated disorders including:

          •  Thymic abnormalities: >70%

          •  Hyperthyroidism 3% - 8%

          •  Other autoimmune disorders--test for rheumatoid factor and antinuclear antibody

          •  Ventilatory dysfunction

        •  Thymic abnormalities: about 35% of patients with epithelial thymoma have myasthenia gravis; furthermore, acetylcholine receptor autoantibody secretion by thymocytes have been reported {Yoshikawa, H and Lennon, V.A. Acetylcholine receptor autoantibody secretion by thymocytes: Relationship to myasthenia gravis, Neurology, 49:562-567,1997}

        •  Of the patients who do not have thymomas, most of the rest have thymic hyperplasia (germinal follicles in the thymus)

          • Most thymomas express acetylcholine receptor epitopes on the surfaces of the neoplastic cells (Lancet 339: 707, 1992; Am. J. Path. 148: 1359 and 1839, 1996).

            • This expression may trigger the disease.

          • Seronegative myasthenia gravis patients do not have thymoma: Neurology 42: 586, 1992.

          • Extended thymectomy is the procedure of choice (Ann. Thoracic Surg. 62: 853, 1996).

Sites of Drug Intervention in Management of Myasthenia Gravis

Drachman, D.B. Myasthenia Gravis and Other Diseases of the Neuromuscular Junction , In Harrison's Principles of Internal Medicine 14th edition, (Isselbacher, K.J., Braunwald, E., Wilson, J.D., Martin, J.B., Fauci, A.S. and Kasper, D.L., eds) McGraw-Hill, Inc (Health Professions Division), 1998, p. 2469-2472.: Figure adapted from Figure 382-2, p. 2471

• Alzheimer's Disease

  • Tacrine (Cognex) or other cholinesterase inhibitiors are useful in treating mild to moderate Alzheimer's dementias.

1. Taylor, P. Anticholinesterase Agents, In, Goodman and Gillman's The Pharmacologial Basis of Therapeutics,(Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) TheMcGraw-Hill Companies, Inc.,1996, pp. 172-173.;

2. Moroi, S.E. and Lichter, P.R. Ocular Pharmacology In, Goodman and Gillman's The Pharmacologial Basis of Therapeutics,(Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) The McGraw-Hill Companies, Inc.,1996, p. 1634;

3. Drachman, D.B. Myasthenia Gravis and Other Diseases of the Neuromuscular Junction , In Harrison's Principles of Internal Medicine 14th edition, (Isselbacher, K.J., Braunwald, E., Wilson, J.D., Martin, J.B., Fauci, A.S. and Kasper, D.L., eds) McGraw-Hill, Inc (Health Professions Division), 1998, p. 2469-2472

• Adverse Effects: Overstimulation of Muscarinic and Nicotinic Receptors

  • Miosis

  • Salivation

  • Sweating

  • Bronchial constriction

  • Vomiting and diarrhea

  • Myasthenia gravis

  • Neuromuscular blockade (nicotinic effect)

  • CNS effects: high doses

Indirect Cholinomimetic Agents

Acetylcholinesterase Inhibitors ("Reversible")	Acetylcholinesterase Inhibitors ("Irreversible")
Neostigmine (Prostigmin) Physostigmine (Antilirium) Edrophonium (Tensilon)	Soman Parathion Malathion Isoflurophate Diisopropylflurorphosphate, DFP Echothiophate

 

Brown, J.H. and Taylor, P. Muscarinic Receptor Agonists and Antagonists, In, Goodman and Gillman's The Pharmacological Basis of Therapeutics, (Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) The McGraw-Hill Companies, Inc.,1996, pp.149-150.