Firdapse and Ruzurgi to Treat LEMS

Lambert-Eaton myasthenic syndrome (LEMS) is a rare autoimmune disease that affects the neuromuscular junction — the point at which nerve and muscle cells meet. The disease leads to muscle weakness, pain, and fatigue, among other symptoms.

With no current cure, LEMS can be treated to alleviate some symptoms, and slow disease progression. The U.S. Food and Drug Administration approved Firdapse (amifampridine phosphate), developed by Catalyst Pharmaceuticals, in 2018 — the first oral medication for adults with LEMS. The agency then approved the Jacobus Pharmaceuticals-developed therapy Ruzurgi (amifampridine) the following year as the first treatment for younger patients, ages 6 to 16. Doctors can prescribe Ruzurgi off-label for adult patients.

How should the medications be stored?

Firdapse contains a modified form of amifampridine that is more stable so the medication can be stored at room temperature. Ruzurgi tablets can be stored at room temperature for up to three months, but the oral suspension formulation, between doses, should be stored in the refrigerator for up to 24 hours.

How do the medications treat LEMS?

LEMS develops as a result of the body mistakenly making autoantibodies against a protein called voltage-gated calcium channels (VGCCs) found at the neuromuscular junction. These autoantibodies cause the nerve cells not to be able to take up calcium. Calcium is necessary to convert the electrical signal sent from the brain into a chemical signal by driving the release of a neurotransmitter, acetylcholine, at the neuromuscular junction.

Firdapse and Rugurzi contain a small molecule that binds to another channel on the nerve cells, called the voltage-gated potassium channel. Blocking this channel slows or prevents the nerve cells from “resetting” after sending a signal to the muscles. This means that nerve cells can send a longer-lasting nerve impulse, which causes the remaining VGCC to open and stay open for longer. This increases the signal that nerve cells send to the muscles and can help compensate for the lower number of VGCCs in LEMS. Researchers believe the mechanisms of the two medications to be similar.

What are the side effects?

Both medications can cause side effects such as stomach pain, indigestion, dizziness, and nausea. They can also cause seizures, even in patients who have not previously had seizures, and therefore should not be given to patients with a history of seizures.

Last updated: Aug. 17, 2020

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