Carbachol

Carbachol (Carbastat, Carboptic, Isopto Carbachol, Miostat), also known as carbamylcholine, is a drug that binds and activates the acetylcholine receptor. Thus it is classified as a cholinergic agonist. It is primarily used for various ophthalmic purposes, such as for treating glaucoma, or for use during ophthalmic surgery. It is generally administered as an ophthalmic solution (i.e. eyedrops).

Chemistry and pharmacology
Carbachol is a choline ester and a positively charged quaternary ammonium compound. It is not well absorbed in the gastro-intestinal tract and does not cross the blood-brain barrier. It is usually administered topical ocular or through intraocular injection. Carbachol is not easily metabolized by cholinesterase, it has a two to 5 minute onset of action and its duration of action is 4 to 8 hours with topical administration and 24 hours for intraocular administration. Since carbachol is poorly absorbed through topical administration, benzalkonium chloride is mixed in to promote absorption.

Carbachol is a parasympathomimetic that stimulates both muscarinic and nicotinic receptors. In topical ocular and intraocular administration its principal effects are miosis and increased aqueous humour outflow.

In the cat and rat, carbachol is well-known for its ability to induce rapid eye movement (REM) sleep when microinjected into the pontine reticular formation. Carbachol elicits this REM sleep-like state via activation of postsynaptic muscarinic cholinergic receptors (mAChRs).

Indications
Carbachol is primarily used in the treatment of glaucoma, but it is also used during ophthalmic surgery. Carbachol eyedrops are used to decrease the pressure in the eye for people with glaucoma. It is sometimes used to constrict the pupils during cataract surgery.

Topical ocular administration is used to decrease intraocular pressure in people with primary open-angle glaucoma. Intracellular administration is used to produce miosis after lens implantation during cataract surgery. Carbachol can also be used to stimulate bladder emptying if the normal emptying mechanism is not working properly.

In most countries carbachol is only available by prescription.

Contraindications
Use of carbachol, as well as all other muscarinic receptor agonists, is contraindicated in patients with asthma, coronary insufficiency, gastroduodenal ulcers, and incontinence. The parasympathomimetic action of this drug will exacerbate the symptoms of these disorders.

Overdose
The effects of a systemic overdose will probably be similar to the effects of a nerve agent (they both act on the cholinergic system, increasing cholinergic transmission), but its toxicity is much weaker and it is easier to antagonize in overdose. When administered ocularly there is little risk of such effects, since the doses are much smaller (see topical vs systemic administration).