Cholinomimetics part 2 Flashcards
What are the two classes of cholinomimetic drugs
Directly acting
Indirectly acting
What are the two types of directly acting cholinomimetic drugs
Typical agonists at muscarinic receptors
1) choline esters (bethanechol
(2) alkaloids (pilocarpine)
What are muscarinic receptor agonists usually referred to as
Muscarinic agonists, as a group, are often referred to as parasympathomimetic, because the main effects that they produce in the whole animal resemble those of parasympathetic stimulation. The structures of ACh and related choline esters are given in Table 14.3. They are agonists at both mAChRs and nAChRs, but act more potently on mAChRs (see Fig. 14.1). Bethanechol, pilocarpine and cevimeline are the main ones used clinically.
What are the key features of the ACh molecule
The key features of the ACh molecule that are important for its activity are the quaternary ammonium group, which bears a positive charge, and the ester group, which bears a partial negative charge and is susceptible to rapid hydrolysis by cholinesterase. Variants of the choline ester structure (see Table 14.3) have the effect of reducing the susceptibility of the compound to hydrolysis by cholinesterase, and altering the relative activity on mAChRs and nAChRs.
Summarise the effects of bethanechol and pilocarpine
Bethanechol, which is a hybrid of these two molecules, is stable to hydrolysis and selective for mAChRs, and is occasionally used clinically (see clinical box, p. 184). Pilocarpine is a partial agonist and shows some selectivity in stimulating secretion from sweat, salivary, lacrimal and bronchial glands, and contracting iris smooth muscle (see later), with weak effects on gastrointestinal smooth muscle and the heart.
Why do the directly acting cholinomimetics not have selectivity for nicotinic receptors
Despite having a very similar structure to ACh- which allows them to be recognised by the muscarinic receptors
We know that small changes in structure can effect their activity- and it transpires that these small deviations of structure from ACh means that they cannot be recognised by nicotinic receptors
Where is pilocarpine derived from
Derived from the leaves of a South American shrub Pilocarpus
Summarise the pharmacological properties of pilocarpine
Non-selective muscarinic agonist; good lipid solubility; t1/2 ≈ 3-4h
Non-selective muscarinc agonist means that it can act on all muscarinic receptor sub-types
What is pilocarpine particularly useful to treat and list some of its side effects
Particularly useful in ophthalmology as a local treatment for glaucoma
Side effects: Blurred vision, sweating, gastro-intestinal disturbance and pain, hypotension, respiratory distress
Side effects are due to simulation of muscarinic receptors
How can pilocarpine be given locally to treat glaucoma
Eye drops
What type of cholinomimetic is bethanehcol
Minor modification of acetylcholine, produces an M3 AChR selective agonist- most drugs aren’t great at selecting between different sub-types
Summarise the pharmacological properties of bethanechol
Resistant to degradation, orally active and with limited access to the brain (t1/2 ≈ 3-4h)
Not metabolised by CAT
As it has limited access to the brain- we see less CNS related side effects
What are the main clinical uses of bethanechol and list some of its side effects
Mainly used to assist bladder emptying and to enhance gastric motility- often given post-op to kick start gastric motility again after the anaesthesia
Side effects: sweating, impaired vision, bradycardia, hypotension, respiratory difficulty
Why are the side effects of bethanechol more pronounced than that of pilocarpine
Given systemically (oral) as compared to the topical administration of pilocarpine Therefore it has more access to the muscarinic receptors expressed in the body to elicit side effects.
What is cevimeline
Cevimeline – newer M3-selective cholinomimetic- greater degree of selectivity than bethanechol
Which diseases can bethanechol be used to treat
Treatment of bladder and gastrointestinal hypotoniaa
What is the ultimate effect of indirectly acting cholinomimetic drugs
To raise the endogenous concentration of ACh (less is broken down by CAT)
Summarise the actions of the indirectly acting cholinomimetic drugs
Increase effect of normal parasympathetic nerve stimulation
Reversible anticholinesterases: physostigmine, neostigmine, donepezil (‘Aricept’)
Irreversible anticholinesterases: ecothiopate, dyflos, sarin
How can drugs that enhance cholinergic transmission act
Drugs that enhance cholinergic transmission act either by inhibiting cholinesterase (the main group) or by increasing ACh release.
What is the action of cholinesterase enzymes
Metabolise acetylcholine to choline and acetate
What are the two types of cholinesterase enzymes
Two types which differ in distribution, substrate specificity and function:
Acetylcholinesterase (true or specific cholinesterase)
Butyrylcholinesterase (pseudocholinesterase)
Describe the structure of the cholinesterases
Both consist of globular catalytic subunits, which constitute the soluble forms found in plasma (BuChE) and cerebrospinal fluid (AChE). Elsewhere, the catalytic units are linked to accessory proteins, which tether them like a bunch of balloons to the basement membrane (at the neuromuscular junction) or to the neuronal membrane at neuronal synapses (and also, oddly, the erythrocyte membrane, where the function of the enzyme is unknown).
Summarise the characteristics pop acetylcholinesterase
Found in all cholinergic synapses (peripheral and central)
Very rapid action (hydrolysis; >10 000 reactions per second)
Highly selective for acetylcholine
Describe the actions of acetylcholinesterase
The bound AChE at cholinergic synapses serves to hydrolyse the released transmitter and terminate its action rapidly. Soluble AChE is also present in cholinergic nerve terminals, where it has a role in regulating the free ACh concentration, and from which it may be secreted; the function of the secreted enzyme is so far unclear. AChE is quite specific for ACh and closely related esters such as methacholine. Certain neuropeptides, such as substance P (Ch. 19) are inactivated by AChE, but it is not known whether this is of physiological significance.
Describe the correspondence between the distribution of cholinergic synapses and that of AChE
Overall, there is poor correspondence between the distribution of cholinergic synapses and that of AChE, both in the brain and in the periphery, and AChE most probably has synaptic functions additional to disposal of ACh, although the details remain unclear
What residue is present at the active site of acetylcholinesterase
Serine residue- which has an OH group- which hydrolyses ACh- inactivating it- splitting it into acetate and choline- happens very quickly
Summarise the key characteristics of butyrylcholinesterase
Found in plasma and most tissues but not cholinergic synapses
Broad substrate specificity - hydrolyses other esters e.g. suxamethonium
Is principal reason for low plasma acetylcholine
Shows genetic variation- drugs such as suxamethonoium broken down by different rates by different people- different sensitivities
Describe the activity of butyrylcholinesterase
BuChE has a widespread distribution, being found in tissues such as liver, skin, brain and gastrointestinal smooth muscle, as well as in soluble form in the plasma. It is not particularly associated with cholinergic synapses, and its physiological function is unclear. It has a broader substrate specificity than AChE. It hydrolyses the synthetic substrate butyrylcholine more rapidly than ACh, as well as other esters, such as procaine, suxamethonium and propanidid (a short-acting anaesthetic agent; see Ch. 42). The plasma enzyme is important in relation to the inactivation of the drugs listed previously.
Describe the genetic variation in the activity of butyrylcholinesterase
Genetic variants of BuChE causing significantly reduced enzymic activity occur rarely (see Ch. 12), and these partly account for the variability in the duration of action of these drugs. The short duration of action of ACh given intravenously (see Fig. 14.1) results from its rapid hydrolysis in the plasma
Why is ACh strictly a neurotransmitter and not a hormone
Normally, AChE and BuChE between them keep the plasma ACh at an undetectably low level, so ACh is strictly a neurotransmitter and not a hormone.
Describe the two regions of the active sites of the anticholinesterases
Both AChE and BuChE belong to the class of serine hydrolases, which includes many proteases such as trypsin. The active site of AChE comprises two distinct regions (Fig. 14.8): an anionic site (glutamate residue), which binds the basic (choline) moiety of ACh; and an esteratic (catalytic) site (histidine + serine). As with other serine hydrolases, the acidic (acetyl) group of the substrate is transferred to the serine hydroxyl group, leaving (transiently) an acetylated enzyme molecule and a molecule of free choline.
Describe the hydrolysis of the serine acetyl group
Spontaneous hydrolysis of the serine acetyl group occurs rapidly, and the overall turnover number of AChE is extremely high (over 10,000 molecules of ACh hydrolysed per second by a single active site).
Describe the effect of cholinesterase inhibitors when given at a low dose
Enhanced muscarinic activity- raises endogenous conc of ACh
Describe the effect of cholinesterase inhibitors when given at a moderate dose
Further enhancement of muscarinic activity
Increased transmission at ALL autonomic ganglia (nAChRs)
Need more ACh to stimulate nicotinic receptors
Describe the effect of cholinesterase inhibitors when given at a high dose
Depolarising block at autonomic ganglia & NMJ
Toxic
Receptors switch off and become inactivated- respiratory depression - diaphragm
What are the medium-duration anticholinesterases
These include neostigmine and pyridostigmine, which are quaternary ammonium compounds of clinical importance, and physostigmine (eserine), a tertiary amine, which occurs naturally in the Calabar bean.7
Summarise action of the reversible anticholinesterases
Physostigmine, neostigmine
Compete with acetylcholine for active site on the cholinesterase enzyme
Donate a carbamyl group to the enzyme, blocking the active site and preventing acetylcholine from binding
Carbamyl group removed by slow hydrolysis (mins rather than msecs) Increase duration of acetylcholine activity in the synapse
What is a key property of the reversible cholinesterases
These drugs are all carbamyl, as opposed to acetyl, esters and all possess basic groups that bind to the anionic site. Transfer of the carbamyl group to the serine hydroxyl group of the esteratic site occurs as with ACh, but the carbamylated enzyme is very much slower to hydrolyse (see Fig. 14.8), taking minutes rather than microseconds.
The anticholinesterase drug is therefore hydrolysed, but at a negligible rate compared with ACh, and the slow recovery of the carbamylated enzyme means that the action of these drugs is quite long-lasting.
Summarise physostigmine
Naturally occurring tertiary amine from Calabar beans
Primarily acts at the postganglionic parasympathetic synapse (t1/2 ≈ 30 mins)
Used in the treatment of glaucoma, aiding intraocular fluid drainage
Also used to treat atropine poisoning, particularly in children
Lipid soluble
What is atropine
Muscarinic receptor antagonist- competitive antagonist- thus surmountable with physositgmine.
The atropine is then metabolised quickly
Summarise the irreversible anti cholinesterase enzymes
Organophosphate compounds:
ecothiopate, dyflos, parathion and sarin
Rapidly react with the enzyme
active site, leaving a large blocking
group
This is stable and resistant to hydrolysis - recovery may require the production of new enzymes (days/weeks)
Only ecothiopate in clinical use, but the others are commonly used as insecticides (and as nerve gas!)
Describe the structural properties of the irreversible anti cholinesterase enzymes
Irreversible anticholinesterases (see Table 14.8) are pentavalent phosphorus compounds containing a labile group such as fluoride (in dyflos) or an organic group (in parathion and ecothiophate). This group is released, leaving the serine hydroxyl group of the enzyme phosphorylated (see Fig. 14.8).
What are the main uses of the irreversible anticholinesterases
Most of these organophosphate compounds, of which there are many, were developed as weapons, such as sarin, and the more potent VX (10 mg of which through skin contact is said to be fatal) which acquired notoriety as an agent of state-sponsored assassination.8 Some are used as pesticides as well as for clinical use; they interact only with the esteratic site of the enzyme and have no cationic group. Ecothiophate is an exception in having a quaternary nitrogen group designed to bind also to the anionic site.
Describe the activity of the inactivated, phosphorylated enzyme
The inactive phosphorylated enzyme is usually very stable. With drugs such as dyflos, no appreciable hydrolysis occurs, and recovery of enzymic activity depends on the synthesis of new enzyme molecules, a process that may take weeks. With other drugs, such as ecothiophate, hydrolysis occurs over the course of a few days, so that their action is not strictly irreversible.
Which property does the use of the irreversible anti-cholinesterases depend on
Dyflos and parathion are volatile non-polar substances of very high lipid solubility, and are rapidly absorbed through mucous membranes and even through unbroken skin and insect cuticles; the use of these agents as war gases or insecticides relies on this property.
Describe the other actions of the irreversible anticholinesterases
The lack of a specificity-conferring quaternary group means that most of these drugs block other serine hydrolases (e.g. trypsin, thrombin), although their pharmacological effects result mainly from cholinesterase inhibition.
Summarise ecothiopiate
Potent inhibitor of acetylcholinesterase
Slow reactivation of the enzyme by hydrolysis takes several days
Used as eye drops in treatment of glaucoma, acting to increase intraocular fluid drainage with a prolonged duration of action
Systemic side effects: sweating,
blurred vision, GI pain, bradycardia,
hypotension, respiratory difficulty
Summarise the effects of the anti cholinesterase drugs on the CNS
Non-polar anticholinesterases (e.g. physostigmine; nerve agents) can cross the blood brain barrier
Low doses: Excitation with possibility of convulsions
High doses: Unconsciousness, respiratory depression, death- due to cessation of respiration
Describe the effects of the anti cholinesterase drugs on the CNS
Tertiary compounds, such as physostigmine, and the non-polar organophosphates penetrate the blood–brain barrier freely and affect the brain. The result is an initial excitation, which can result in convulsions, followed by depression, which can cause unconsciousness and respiratory failure. These central effects result mainly from the activation of mAChRs, and are antagonised by atropine. The use of anticholinesterases to treat senile dementia is discussed in
Describe organophosphate poisoning
Accidental exposure to organophosphates used in insecticides, or deliberate use as nerve agents can cause severe toxicity ( increased muscarinic activity; CNS excitation; depolarising NM block)
Summarise the treatment of organophosphate posinoning
Treatment: atropine (iv); artificial respiration; pralidoxime (iv)- only drug which can reverse the block- but needs to be given within first couple of hours on being intoxicated
NB: Phosphorylated enzyme ‘ages’ within few hours
What does the ‘ageing’ of the phosphorylated enzyme mean
Its inactivation becomes irrversible after a couple of hours
Distinguish between the actions of the pralidoxine and atropine
Pralidoxine works on the inactivated enzyme- so reduces cholinergic transmission at both the nicotinic and muscarinic synapses
Atropine- will block muscarinic receptors
Summarise the effects of the cholinomimetics
Cholinomimetics decrease heart rate and cardiac output, increase exocrine gland activity, increase non-vascular smooth muscle contractility and cause miosis
