Mexiletine is a medication used to treat abnormal heart rhythms. It is also being investigated for use as a muscle relaxant in patients with Charcot-Marie-Tooth disease (CMT).
How mexiletine works
For muscle contraction, the brain sends an electric signal to the muscles through so-called motor neurons. The transmission of the electric signals requires the opening and closing of sodium channels in the membrane (the outer layer of the cell) of the motor neuron. When an electric signal stimulates a nerve fiber, sodium channels open, positively charged sodium ions enter the cell, and the inner side of the membrane becomes positive. Once the signal has passed, sodium channels close again.
When the electric signal reaches the point where the motor neuron meets the muscle fiber, a neurotransmitter is released by the nerve cell and binds to receptors on the muscle fiber. This again activates sodium channels in the muscle membrane, and the electric signal stimulates muscle contraction.
A muscle cramp occurs when muscles contract suddenly and involuntarily. CMT patients experience frequent and severe muscle cramps.
Mexiletine blocks sodium channels and consequently reduces the influx of sodium ions into the cell. It may decrease the signal intensity or frequency, promoting muscle relaxation.
Mexiletine in clinical trials
A randomized Phase 2 clinical trial (NCT02561702) assessed the potential of mexiletine as a muscle relaxant in CMT patients. Four patients with CMT received either mexiletine (three oral doses of 150 mg per day) or a placebo for five to seven days. This was followed by a washout period of seven days, during which the participants received no treatment. Then the groups switched. The participants who received mexiletine before were then given a placebo for five to seven days and vice versa.
Two hours after treatment, the clinical evaluator provoked a cramp in the hamstring muscles, which are located in the back of the thigh between the hip and the knee.
After receiving mexiletine, two participants reported a decrease in cramp duration, and one patient reported a lessening in cramp intensity. After receiving a placebo, no patients reported a reduction in cramp duration, and two patients reported a decrease in cramp intensity.
Mexiletine should not be used in patients with serious heart conditions.
The medication may cause liver injury. As a result, in individuals with liver disease, mexiletine should be used with caution.
Common side effects of mexiletine are heartburn, nausea, dizziness, and problems with coordination.
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