BioMarin Pharmaceutical was granted orphan drug designation by the FDA for its drug candidate, amifampridine phosphate (3,4-diaminopyridine or 3,4-DAP) for the treatment of Lambert Eaton Myasthenic Syndrome (LEMS). LEMS is a rare autoimmune disease with the primary symptoms of muscle weakness. Muscle weakness in LEMS is caused by autoantibodies to voltage gated calcium channels leading to a reduction in the amount of acetylcholine released from nerve terminals.  Approximately 50 percent of LEMS patients diagnosed have small cell lung cancer.

Amifampridine phosphate works by blocking potassium channel efflux in nerve terminals so that action potential duration is increased.

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