Other muscarinic receptor antagonists
It is exceptional that all properties of atropine are simultaneously useful in the same patient and effort has been made to obtain compounds having a greater specificity of action and particular pharmacokinetic features. Their adverse effects and their contraindications are however quite similar to those of atropine.
Mydriatic agents
Tropicamide is a muscarinic receptor antagonist used as a mydriatic.
Tropicamide is different from atropine by its shorter duration of action which is approximately 1 hour and half. The effect appears in 10 minutes, is maximum in 15 or 20 minutes. The pupil finds its normal diameter in approximately 6 hours.
Cyclopentolate is presented in the form of an ophthalmic solution with mydriatic effect of shorter duration than atropine.
Gastric acid secretion inhibitors
Numerous muscarinic receptor antagonists, including pirenzepine which has a quite specific action on the stomach, have been used in the management of peptic ulcer disease. Pirenzepine was withdrawn from the market after the introduction in therapeutics of H2antihistamines and proton pump inhibitors which are more effective and better tolerated.
Bronchodilatators
The two muscarinic receptor antagonists used as bronchodilatators are oxitropium and ipratropium.
They are given by pulmonary route, in the form of aerosol, in the preventive and curative treatment of asthma. Their efficacy is however lower than that of beta-mimetics.
The advantage of these products compared to atropine result from a pharmacokinetic particularity: as they involve a quaternary ammonium in their chemical formula, they are only little or not absorbed by bronchi and thus have a predominantly local effect.
There is also a preparation for nasal use for rhinorrhea treatment.
Tiotropium, anticholinergic of long duration of action, used in inhalation, is released on the market in Switzerland and Belgium but not in France.
With vesical indications
Tolterodine is a cholinergic antagonist whose effect on the bladder prevails. It is used in the treatment of vesical instability with symptoms of pressing micturition or incontinence.
Oxybutynine, which is a muscarinic receptor antagonist and has direct effects on smooth muscles, is proposed for the treatment of urinary incontinence of adults and could be used in the treatment of infantile enuresis.
Trospium, an anticholinergic known for a long time, involving a quaternary ammonium group in its structure, was the subject of recent studies in urinary disorders.
Used as antispasmodics
Muscarinic receptor antagonists have antispasmodic properties, but all the antispasmodic agents are not necessarily muscarinic receptor antagonists. The antispasmodic effect of some of them results from a direct effect on smooth muscle, often by calcium-channel antagonism.
Muscarinic receptor antagonists used as antispasmodics are, in addition to atropine itself, dihexyverine, prifinium and propantheline.
Among antispasmodic drugs having at the same time a muscarinic receptor antagonist effect and a direct effect on smooth muscles one can quote tiemonium., although its effect on smooth muscle activity predominates over its muscarinic receptor antagonist activity.
Note:
The antispasmodics with direct smooth muscle effect such as phloroglucinol, pinaverium, mebeverine and trimébutine are also used in the treatment of hepatic and renal colics, colopathies and dysmenorrhea.
Used as antiparkinsonians
It is admitted that Parkinson disease is the consequence of the destruction of the dopaminergic neurons, what induce an imbalance between dopaminergic and cholinergic effects, with cholinergic excess. Muscarinic receptor antagonists used as antiparkinsonians are trihexyphenidyl also called benzhexol, tropatepine and biperidene.
These drugs, in particular trihexyphenidyl, are used frequently for prevention and treatment of dyskinesia elicited by neuroleptic agents.
Other drugs
It is necessary to insist on the fact that numerous drugs whose principal property is not to be muscarinic receptor antagonists have antimuscainic effects and the corresponding undesirable effects. It is the case of many neuroleptic agents and antidepressants.
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