1 Cao Yongxiao 曹永孝 Department of Pharmacology ; Chapter 9 Anticholinesterase Drugs ＆ Cholinesterase Reactivators

2 西安交大医学院药理学系 曹永孝 Anticholinesterase Drugs Cholinesterase (ChE) inhibitors comprise another group drugs mimicing ACh by inhibiting the hydrolysis of endogenous ACh. Cholinoceptor agonists bind to and activate cholinoceptors 西安交大医学院药理学系 曹永孝

3 ． 西安交大医学院药理学系 曹永孝 Anti-ChE agents produce effects by inhibiting AChE, decreasing hydrolysis of ACh, and increasing ［ A Ｃ h ］. ACh evoke an increased responses. Mode of action of Anticholinesterase drugs Drugs act primarily where ACh is physiologically released and are amplifiers of endogenous ACh. Their primary effects are at the active site of this enzyme.

4 Basic Pharmacology of Anticholinesterase Drugs The action of ACh is terminated by destruction of AChE in synapses. Their pharmacodynamic properties are almost identical. The chief differences between members of the group are chemical and pharmacokinetics. 西安交大医学院药理学系 曹永孝

5 Chemistry The ChE inhibitors fall into three chemical groups: (1) simple alcohols bearing a quaternary ammonium group, eg, edrophoniun; (2) carbamic acid esters of alcohols bearing quaternary or tertiary ammonium groups ； (3) organic phosphoric acid.

6 (1) simple alcohol bearing a quaternary (2) carbamic acid esters of alcohols bearing tertiary or quaternary ammonium groups

7 Absorption of the quaternary carbamates from the conjunctiva, skin and lungs is poor, since their charge renders them relatively insoluble in lipids. Larger doses are required for oral administration than for injection.

8 Distribution  Distribution into CNS is negligible.  Physostigmine, in contract, is well absorbed from all sites and can be used topically in the eye. It is distributed into CNS and is more toxic than the more polar quaternary carbamates.  The carbamates are relatively stable in aqueous solution but can be metabolized by ChE.  The duration of effect is determined chiefly by the stability of the inhibitor-enzyme complex. 西安交大医学院药理学系 曹永孝

9 Organophosphate ChE inhibitors  are well absorbed from the skin, lung, gut, and conjunctiva, making them dangerous to humans and highly effects as insecticides.  They are relatively less stable than carbamates.  Echothiophate is the most stable organophosphates. It can be made up in aqueous solution for ophthalmic use and retains activity for weeks. 西安交大医学院药理学系 曹永孝

10 The dash line is the bond that is hydrolyzed in binding to the enzyme The shaded ester bond is the points of detoxification in mammals and birds

11 The thiophosphate insecticides are lipid soluble and rapidly absorbed. They must be activated in the body by conversion to the oxygen analogs

12 Malathion are rapidly metabolized to inactive products in birds and mammals but not in insects; they are safe. Parathion is not detoxified effectively in vertebrate; it is dangerous to man and not available for general public use.

13  Malathion is the least toxic organophosphates due to the presence of ester functions in its molecule. Insects have much lower esterases activity than the mammals. 西安交大医学院药理学系 曹永孝

14 The thiophosphate being quite lipid-soluble is rapidly absorbed and is activated in the body by conversion to the oxygen analogy. Malathion are rapidly metabolized to inactive products in birds and mammals but not in insects; these agents are considered safe enough. Parathion is not detoxified effectively in vertebrate; it is considerably dangerous to humans and is not available for general public use.  The thiophosphate being quite lipid-soluble is rapidly absorbed and is activated in the body by conversion to the oxygen analogy.  Malathion are rapidly metabolized to inactive products in birds and mammals but not in insects; they are considered safe enough.  Parathion is not detoxified effectively in vertebrate; it is considerably dangerous to humans and is not available for general public use. 西安交大医学院药理学系 曹永孝

15 All thiophosphate-type (P  S) AChE inhibitors need to be bioactivated by the cytochrome P-450 enzymes to the corresponding phosphate esters (P  O) (i.e., the P  S is not as electrophilic as P  O to allow nucleophilic attack by the Ser-OH). Since insects appear to have more reactive oxidative enzymes 西安交大医学院药理学系 曹永孝

16 Phosphoramide-type AChE inhibitors are inactive. The selective toxicity is due to the differences in the biotransformations between insects and mammals 西安交大医学院药理学系 曹永孝

17 Pharmacodynamics Mechanism of Action 西安交大医学院药理学系 曹永孝  Then, the covalent acetylenzyme bond is split. It takes about 150 ms. First, ACh binds to the enzyme’s active site and is hydrolyzed, yielding free choline and the acetylated enzyme.

18  The figure shows how the positively charged nitrogen in the acetylcholine molecule is attracted to the ionic site on acetylcholinesterase, and hydrolysis is catalyzed at the esteric site to form choline and acetic acid.

19 The figure shows how a cholinesterase inhibitor (in this case, a nerve agent) attaches to the serine hydroxyl group on acetylcholinesterase. This prevents acetylcholine from interacting with the cholinesterase enzyme and being broken down.

20 AChE inhibitors bind to AChE active site and are hydrolyzed Ch Mechanism of Action of AChE Inhibitors Electrostatically Hydrogen bond Covalent bond Irreversible bond Carbamate esters agents undergo a two-step hydrolysis sequence. However, the covalent bond of the carbamoylated enzyme is more resistant to the second process. Edrophonium consisting of quaternary alcohols bind electrostatically and by hydrogen bonds to the active site, preventing access of ACh. The enzyme-inhibitor complex is short-lived.

21 AChE inhibitors bind to AChE active site and are hydrolyzed Ch Mechanism of Action of AChE Inhibitors Electrostatically Hydrogen bond Covalent bond Irreversible bond Organo-phosphates also undergo initial binding and hydrolysis by the enzyme, resulting in a phosphorylated active site. The covalent phosphorus- enzyme bond is extremely stable and hydrolyzed in a very slow rate.

22  After the initial binding-hydrolysis step, the phosphorylated enzyme complex may undergo a process called aging. This process involves the breaking of one of the oxygen-phosphorus bonds of the inhibitor and further strengthens the phosphorus enzyme bond.

23 Organ System Effects  Central Nervous System —  In low concentrations, the lipid- soluble cholinesterase inhibitors cause diffuse activation on the electroencephalogram and a subjective alerting response.  In higher concentrations, they cause generalized convulsions, which may be followed by coma and respiratory arrest.

24  The effects of the cholinesterase inhibitors on these organ system, all of which are well innervated by the parasympathetic nervous system, are quite similar to the effects of the direct-acting cholinomimetics. Eye, Respiratory Tract, Gastrointestinal Tract, Urinary Tract — 西安交大医学院药理学系 曹永孝

25 Cardiovascular System-  The ChE inhibitors can increase activation in ganglia supplying the heart.  In the heart, the effects on the parasympathetic predominate. Thus, ChE inhibitors mimic the effects of vagal nerve activation.  Negative chronotropic, dromotropic, and inotropic effects are produced, and cardiac output falls. 西安交大医学院药理学系 曹永孝

26  At low concentrations, the ChE inhibitors moderately prolong and intensify the action of ACh, increase strength of contraction.  At higher concentration, the accumulation of ACh may result in fibrillation of muscle fibers.  Neostigmine, a quaternary carbamate cholinesterase inhibitors, agonize N 2 receptor.  This contributes to the effectiveness at therapy for myasthenia. Neuromuscular Junction— 西安交大医学院药理学系 曹永孝

27 Clinical Uses  A. The Eye  Glaucoma is with increasing intraocular pressure. Cholinesterase inhibitors reduce intraocular pressure by increasing ACh concentration, contracting the ciliary body and facilitating outflow of aqueous humor. 西安交大医学院药理学系 曹永孝

28 Gastrointestinal and Urinary Tracts  Postoperative ileus and congenital megacolon involve depression of smooth muscle activity without obstruction.  Urinary retention may occur postoperative or postpartum or secondary to spinal cord injury.  Of the cholineaterase inhibitions, neostigmine is the most widely used for these applications. 西安交大医学院药理学系 曹永孝

29  Myasthenia gravis is a disease affecting skeletal muscle neuromuscular junction. An autoimmune process produces antibodies that decrease the number of N 2 receptors. Frequent findings are ptosis, difficulty in speaking and swallowing, and extremity weakness. Even respiratory muscles. 西安交大医学院药理学系 曹永孝

30  Myasthenia gravis is a disease affecting skeletal muscle neuromuscular junction. ChE inhibitors can increase ACh concentration, excite N 2 receptors, which strengthen contraction. Patients are also treated with immunosuppressant drugs and some with thymectomy. 西安交大医学院药理学系 曹永孝

31 Heart  The short-acting cholinesterase inhibitor edrophonium had been used to treat supraventricular tachycardia. 西安交大医学院药理学系 曹永孝

32 Antimuscarinic Drug in Toxication  Atropine intoxication is potentially lethal in children and may cause prolonged severe behavioral disturbances in adults. The tricyclicantidepressants in overdosage, also cause severe muscarinic blockade.  Cholinesterase inhibitors can antagonize the blockage of M receptors (atropine overdosage). 西安交大医学院药理学系 曹永孝

33 CNS: Alzheimer’s disease  Tacrine is a drug with anticholinesterase and other cholinomimetic action that has been used for the treatment Alzheimer’s disease.  Donepezil, galantamine, and rivastigmine are more selective AChE inhibitors that appear to have the same modest clinical benefic as tacrine in treatment of cognitive dysfunction in Alzheimer’s patient.  Donepezil may be given once daily because of its long half-life. 西安交大医学院药理学系 曹永孝

34 Toxicity of Cholinesterase Inhibitors  The acute toxicity: The initial signs are those of M symptoms: miosis, salivation, sweating, vomiting, diarrhea, and bronchial constriction. CNS involvement usually follow rapidly, accompanied by peripheral N effects, especially depolarizing N 2 blockade. Chronic exposure to organophosphate ChE inhibitor cause neuropathy with demyelination of axons

35 ChE Reactivators  Cholinesterase reactivation, capable of regenerating active enzyme from the organophosphorus-cholinesterase complex, is available to treat organophosphorus poisoning.  These oxime agents include pralidoxime (PAM), diacteylmonoxime (DAM), and others. 西安交大医学院药理学系 曹永孝

36 The figure shows how a cholinesterase inhibitor (in this case, a nerve agent) attaches to the serine hydroxyl group on acetylcholinesterase. This prevents acetylcholine from interacting with the cholinesterase enzyme and being broken down.

37 Partially electropositive nitrogen on 2-PAM is attracted to electronegative anionic site on cholinesterase. Then, cholinesterase is regenerated

38 Mode and Use of Action of the Cholinesterase Reactivators  The oxime group (NOH) has a very high affinity for the phosphorus atom, and are able to hydrolyze the phosphorylated enzyme.  Pralidoxime is the most effective in regenerating ChE. Because of its positive charge, it does not enter CNS and is ineffective in CNS poisoning.  Diacetylmonoxime can cross the blood brain barrier and can regenerate cholinesterase in CNS. 西安交大医学院药理学系 曹永孝

39 PAM reverses the effects of the organophosphate anticholinesterase agents. 1. PAM combines with and splits off the phosphorus from the ester site on cholinesterases in such a way that the enzyme is restored. 2.With reactivation of the enzyme, the effects of ACh begin to disappear. 3. The treatment must be within hours, because the phosphorylated enzyme slowly changes to a form that cannot be reversed. 西安交大医学院药理学系 曹永孝

40 PAM and atropine have synergism in treating organophosphate ChE inhibitor poisoning. Organophosphate ChE inhibitors result in: M effects N effects CNS effects PAM Atropine 西安交大医学院药理学系 曹永孝

41 glad to share my knowledge with you, and want you to become a master of the subject, pharmacology 西安交大医学院药理学系 曹永孝