What is Firdapse (amifampridine) used to treat?

Firdapse is used to treat Lambert-Eaton Myasthenic Syndrome (LEMS), a rare autoimmune disorder.

The disease is caused by autoantibodies against the voltage-gated calcium channels located in the neuromuscular junction, and results in progressive limb muscle weakness. The disease generally affects the extremities, especially the legs, but occasionally affects the muscles of the mouth, throat, and eyes. The autonomic nervous system can also be affected, with symptoms of dry mouth, dry skin, constipation, blurred vision, impaired sweating, and blood pressure variations. The disease can be life threatening when the weakness involves respiratory muscles.

Firdapse is a broad spectrum potassium channel blocker that works by causing depolarization of the presynaptic membrane. This action results in the opening of slow voltage-dependent calcium (Ca2+) channels and a subsequent influx of Ca2+ which induces the exocytosis of synaptic vesicles containing Acetylcholine (ACh) to release more ACh into the synaptic cleft. Neuromuscular transmission is enhanced and results in improved muscle function.