Cholinergic drugs 2

anticholinesterase agents: introduction

the function of acetylcholinesterase (ache) in terminating the action of acetylcholine (ach) at the junctions of the various cholinergic nerve endings with their effector organs or postsynaptic sites is considered in chapter 6. drugs that inhibit ache are called anticholinesterase (anti-che) agents. they cause ach to accumulate in the vicinity of cholinergic nerve terminals and thus are potentially capable of producing effects equivalent to excessive stimulation of cholinergic receptors throughout the central and peripheral nervous systems. in view of the widespread distribution of cholinergic neurons across animal species, it is not surprising that the anti-che agents have received extensive application as toxic agents, in the form of agricultural insecticides, pesticides, and potential chemical warfare "nerve gases." nevertheless, several compounds of this class are widely used therapeutically others that cross the blood–brain barrier have been approved or are in clinical trials for the treatment of alzheimer s disease.

prior to world war ii, only the "reversible" anti-che agents were generally known, of which physostigmine is the prototype. shortly before and during world war ii, a new class of highly toxic chemicals, the organophosphates, was developed chiefly by schrader at i.g. farbenindustrie, first as agricultural insecticides and later as potential chemical warfare agents. the extreme toxicity of these compounds was found to be due to their "irreversible" inactivation of ache, which resulted in prolonged enzyme inhibition
mechanism of action of ache inhibitors

the mechanisms of the action of compounds that typify the three classes of anti-che agents are also shown in figure 8–2.

three distinct domains on ache constitute binding sites for inhibitory ligands and form the basis for specificity differences between ache and butyrylcholinesterase: the acyl pocket of the active center, the choline subsite of the active center, and the peripheral anionic site (taylor and radic , 1994 reiner and radic , 2000). reversible inhibitors, such as edropinghonium and tacrine, bind to the choline subsite in the vicinity of tryptophan 86 and glutamate 202 (silman and sussman, 2000) (figure 8–2b). edropinghonium has a brief duration of action because its quaternary structure facilitates renal elimination and it binds reversibly to the ache active center. additional reversible inhibitors, such as donepezil, bind with higher affinity to the active center.

other reversible inhibitors, such as propidium and the snake peptidic toxin fasciculin, bind to the peripheral anionic site on ache. this site resides at the rim of the gorge and is defined by tryptophan 286 and tyrosines 72 and 124 (figure 8–1).

drugs that have a carbamoyl ester linkage, such as physostigmine and neostigmine, are hydrolyzed by ache, but much more slowly than is ach. the quaternary amine neostigmine and the tertiary amine physostigmine exist as cations at physiological ph. by serving as alternate substrates to ach (figure 8–2c), attack by the active center serine generates the carbamoylated enzyme. the carbamoyl moiety resides in the acyl pocket outlined by phenylalanines 295 and 297. in contrast to the acetyl enzyme, methylcarbamoyl ache and dimethylcarbamoyl ache are far more stable (the half-life for hydrolysis of the dimethylcarbamoyl enzyme is 15 to 30 minutes). sequestration of the enzyme in its carbamoylated form thus precludes the enzyme-catalyzed hydrolysis of ach for extended periods of time. in vivo, the duration of inhibition by the carbamoylating agents is 3 to 4 hours.

the organophosphorus inhibitors, such as diisopropyl fluorophosphate (dfp), serve as true hemisubstrates, since the resultant conjugate with the active center serine phosphorylated or phosphonylated is extremely stable (figure 8–2d). the organophosphorus inhibitors are tetrahedral in configuration, a configuration that resembles the transition state formed in carboxyl ester hydrolysis. similar to the carboxyl esters, the phosphoryl oxygen binds within the oxyanion hole of the active center. if the alkyl groups in the phosphorylated enzyme are ethyl or methyl, spontaneous regeneration of active enzyme requires several hours. secondary (as in dfp) or tertiary alkyl groups further enhance the stability of the phosphorylated enzyme, and significant regeneration of active enzyme usually is not observed. hence, the return of ache activity depends on synthesis of a new enzyme. the stability of the phosphorylated enzyme is enhanced through "aging," which results from the loss of one of the alkyl groups.

from the foregoing account, it is apparent that the terms reversible and irreversible as applied to the carbamoyl ester and organophosphorate anti-che agents, respectively, reflect only quantitative differences in rates of decarbamoylation or dephosphorylation of the conjugated enzyme. both chemical classes react covalently with the enzyme serine in essentially the same manner as does ach
available therapeutic agents

the compounds described here are those commonly used as anti-che drugs and che reactivators in the united states. preparations used solely for ophthalmic purposes are described in chapter 63. conventional dosages and routes of administration are given in the discussion of therapeutic applications (see below).

physostigmine salicylate (antilirium) is available for injection. physostigmine sulfate ophthalmic ointment and physostigmine salicylate ophthalmic solution also are available. pyridostigmine bromide is available for oral (mestinon) or parenteral (regonol, mestinon) use. neostigmine bromide (prostigmin) is available for oral use. neostigmine methylsulfate (prostigmin) is marketed for parenteral injection. ambenonium chloride (mytelase) is available for oral use. tacrine (cognex), donepezil (aricept), rivastigmine (exelon), and galantamine (reminyl) have been approved for the treatment of alzheimer s disease.

pralidoxime chloride (protopam chloride) is the only ache reactivator currently available in the united states and can be obtained in a parenteral formulation. hi-6 is available in several european and near eastern countries.

paralytic ileus and atony of the urinary bladder

in the treatment of both these conditions, neostigmine generally is preferred among the anti-che agents. the direct parasympathomimetic agents (chapter 7) are employed for the same purposes.

neostigmine is used for the relief of abdominal distension and acute colonic pseudo-obstruction from a variety of medical and surgical causes (ponec et al., 1999). the usual subcutaneous dose of neostigmine methylsulfate for postoperative paralytic ileus is 0.5 mg, given as needed. peristaltic activity commences 10 to 30 minutes after parenteral administration, whereas 2 to 4 hours are required after oral administration of neostigmine bromide (15 to 30 mg). it may be necessary to assist evacuation with a small low enema or gas with a rectal tube.

when neostigmine is used for the treatment of atony of the detrusor muscle of the urinary bladder, postoperative dysuria is relieved, and the time interval between operation and spontaneous urination is shortened. the drug is used in a similar dose and manner as in the management of paralytic ileus. neostigmine should not be used when the intestine or urinary bladder is obstructed, when peritonitis is present, when the viability of the bowel is doubtful, or when bowel dysfunction results from inflammatory bowel disease.

glaucoma and other ophthalmologic indications

glaucoma is a complex disease characterized by an increase in intraocular pressure that, if sufficiently high and persistent, leads to damage to the optic disc at the juncture of the optic nerve and the retina irreversible blindness can result. of the three types of glaucoma—primary, secondary, and congenital—anti-ache agents are of value in the management of the primary as well as of certain categories of the secondary type (e.g., aphakic glaucoma, following cataract extraction) congenital glaucoma rarely responds to any therapy other than surgery. primary glaucoma is subdivided into narrow-angle (acute congestive) and wide-angle (chronic simple) types, based on the configuration of the angle of the anterior chamber where the aqueous humor is reabsorbed.

narrow-angle glaucoma is nearly always a medical emergency in which drugs are essential in controlling the acute attack, but the long-range management is often surgical (e.g., peripheral or complete iridectomy). wide-angle glaucoma, on the other hand, has a gradual, insidious onset and is not generally amenable to surgical improvement in this type, control of intraocular pressure usually is dependent upon continuous drug therapy.

since the cholinergic agonists and che inhibitors also block accommodation and induce myopia, these agents produce transient blurring of far vision, limited visual acuity in low light, and loss of vision at the margin when instilled in the eye. with long-term administration of the cholinergic agonists and anti-che agents, the compromise of vision diminishes. nevertheless, other agents without these side effects, such as adrenergic receptor antagonists, prostaglandin analogs, or carbonic anhydrase inhibitors, have become the primary topical therapies for open-angle glaucoma (alward, 1998) (see chapter 63), with ache inhibitors held in reserve for the chronic conditions when patients become refractory to the above agents. topical treatment with long-acting che inhibitors such as echothiophate gives rise to symptoms characteristic of systemic che inhibition. echothiophate treatment in advanced glaucoma may be associated with the production of cataracts (alward, 1998).

anti-che agents have been employed locally in the treatment of a variety of other less common ophthalmologic conditions, including accommodative esotropia and myasthenia gravis confined to the extraocular and eyelid muscles. adie (or tonic pupil) syndrome results from dysfunction of the ciliary body, perhaps because of local nerve degeneration. low concentrations of physostigmine are reported to decrease the blurred vision and pain associated with this condition. in alternation with a mydriatic drug such as atropine, short-acting anti-che agents have proven useful for breaking adhesions between the iris and the lens or cornea. (for a complete account of the use of anti-che agents in ocular therapy, see chapter 63.)

myasthenia gravis

myasthenia gravis is a neuromuscular disease characterized by weakness and marked fatigability of skeletal muscle (drachman, 1994) exacerbations and partial remissions occur frequently. jolly noted the similarity between the symptoms of myasthenia gravis and curare poisoning in animals and suggested that physostigmine, an agent then known to antagonize curare, might be of therapeutic value. forty years elapsed before his suggestion was given systematic trial.

the defect in myasthenia gravis is in synaptic transmission at the neuromuscular junction. when a motor nerve of a normal subject is stimulated at 25 hz, electrical and mechanical responses are well sustained. a suitable margin of safety exists for maintenance of neuromuscular transmission. initial responses in the myasthenic patient may be normal, but they diminish rapidly, which explains the difficulty in maintaining voluntary muscle activity for more than brief periods.

the relative importance of prejunctional and postjunctional defects in myasthenia gravis was a matter of considerable debate until patrick and lindstrom found that rabbits immunized with the nicotinic receptor purified from electric eels slowly developed muscular weakness and respiratory difficulties that resembled the symptoms of myasthenia gravis. the rabbits also exhibited decremental responses following repetitive nerve stimulation, enhanced sensitivity to curare, and following the administration of anti-ache agents, symptomatic and electrophysiological improvement of neuromuscular transmission. although this experimental allergic myasthenia gravis and the naturally occurring disease differ somewhat, this animal model prompted intense investigation into whether the natural disease represented an autoimmune response directed toward the ach receptor. antireceptor antibodies are detectable in sera of 90% of patients with the disease, although the clinical status of the patient does not correlate precisely with the antibody titer (drachman et al., 1982 drachman, 1994 lindstrom, 2000).

the picture that emerges is that myasthenia gravis is caused by an autoimmune response primarily to the ach receptor at the postjunctional endplate. these antibodies reduce the number of receptors detectable either by snake -neurotoxin-binding assays (fambrough et al., 1973) or by electrophysiological measurements of ach sensitivity (drachman, 1994). the autoimmune reaction enhances receptor degradation (drachman et al., 1982). immune complexes along with marked ultrastructural abnormalities appear in the synaptic cleft and enhance receptor degradation. these events appear to be a consequence of complement-mediated lysis of junctional folds in the endplate. a related disease that also compromises neuromuscular transmission is lambert-eaton syndrome. here, antibodies are directed against ca2+ channels that are necessary for presynaptic release of ach (lang et al., 1998).

in a subset of approximately 10% of patients presenting with a myasthenic syndrome, muscle weakness has a congenital rather than an autoimmune basis. characterization of biochemical and genetic bases of the congenital condition has shown mutations to occur in the acetylcholine receptor which affect ligand-binding and channel-opening kinetics (engel et al., 2003). other mutations occur as a deficiency in the form of ache that contains the collagen-like tail unit (ohno et al., 2000). as expected, following administration of anti-che agents (see below), subjective improvement is not seen in most congenital myasthenic patients