Mestinon (pyridostigmine bromide) is a drug commonly used for treating myasthenia gravis. The drug is currently under investigation for spinal muscular atrophy (SMA).

Mode of action

Mestinon is a cholinesterase inhibitor that reversibly inhibits the esterase enzyme that is responsible for breaking down acetylcholine, a neurotransmitter that transmits nerve signals to the muscles, resulting in increased levels as well as duration of the acetylcholine activity.

SMA is a genetic neurodegenerative disease that involves impairment of the neuromuscular transmission. So Mestinon, which facilitates the nerve signal transmission at the neuromuscular junction (NMJ) by inhibiting acetylcholine breakdown, could be a potential therapy for SMA.

It is an analog of Prostigmin (neostigmine), but Mestinon is effective for a longer period of time and has fewer side effects on the digestive system.

Dosage

The drug is available as a regular tablet (60 mg), an extended-release tablet (180 mg), and a syrup (60 mg per teaspoon). The dose to be taken depends on the patient’s response. Its overdose may cause severe muscle weakness.

A doctor’s advice is recommended if the patient has allergies, asthma, seizures, heart disease, intestinal or urinary obstruction, or is taking other medications.

Safety information for its use in pregnant or breastfeeding women is inadequate. The benefits and risks of the drug should be carefully considered before it is used among these women.

Side effects include drowsiness, nausea, diarrhea, and pain in the abdomen.

Unused medication should be disposed of following a pharmacist’s instructions.

Clinical trials

The drug is currently being studied in an open-label Phase 2 clinical study (NCT02227823), at the Centre Hospitalier Régional de la Citadelle in Belgium to evaluate its safety and effectiveness on motor function in SMA type 3 patients who are able to walk. Its primary aim is to study the increase in motor performance and strength by electromyogram and the six-minute walk test after six months of treatment. The study is expected to be completed by July 2017.

Another Phase 2 clinical trial (NCT02941328), called the SPACE trial, is investigating the effect of Mestinon compared to placebo in patients with SMA types 2, 3 and 4 ages 12 and above. It is a double-blind (where neither the participants nor the investigators know who is getting the drug and who is getting placebo), cross-over (both Mestinon and placebo are given to participants in different study periods) trial that is currently recruiting participants in the Netherlands.

It aims to study the effect of oral Mestinon on the fatigability of patients (a measure of how fast they get tired) after an eight-week treatment period, with the dose slowly increasing from 2 mg/kg per day to 6 mg/kg per day (four daily doses) to manage any side effects. The study is estimated to be completed by December 2017.

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