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With the International Nonproprietary Name amifampridine, it is used as a drug, predominantly in the treatment of a number of rare muscle diseases. In Europe, the phosphate salt of amifampridine has been licenced as Firdapse (BioMarin Pharmaceutical) in 2010 as an orphan drug. In the United States, Firdapse is under investigation for the treatment of Lambert-Eaton myasthenic syndrome by Catalyst Pharmaceuticals.
Although not approved for pharmaceutical use in the United States, 3,4-diaminopyridine is available under compassionate use regulations for the treatment of Lambert-Eaton myasthenic syndrome (LEMS). In Lambert-Eaton syndrome, acetylcholine release is inhibited as antibodies meant to target characteristic cancers target Ca2+ channels on the prejunctional membrane instead. 3,4-Diaminopyridine works by blocking potassium channel efflux in nerve terminals so that action potential duration is increased. Ca2+ channels can then be open for a longer time and allow greater acetylcholine release to stimulate muscle at the end plate. A 2005 systematic review from the Cochrane Collaboration found some data favouring its use in LEMS.
It is also used to treat many of the congenital myasthenic syndromes, particularly those with defects in choline acetyltransferase, downstream kinase 7, and those where any kind of defect causes "fast channel" behaviour of the acetylcholine receptor.
The licensing of 3,4-diaminopyridine in 2010 led to a sharp increase in price for the drug. In some cases, this has led to hospitals using an unlicenced form rather than the licensed agent, as the price difference proved prohibitive. BioMarin has been criticised for licencing the drug on the basis of previously conducted research, and yet charging exorbitantly for it. A group of UK neurologists and pediatricians have petitioned to prime minister David Cameron in an open letter to review the situation. The company has responded that it submitted the licensing request at the suggestion of the French government, and points out that the increased cost of a licensed drug also means that it is monitored by regulatory authorities (e.g. for uncommon side-effects), a process that was previously not present. In 2010, company scientists published evidence that amifampridine should be the first-line treatment for LEMS.
In the United States, Catalyst Pharmaceuticals has made 3,4-diaminopyridine phosphate available to patients at no cost under an expanded access program. Patients must be diagnosed with Lambert-Eaton myasthenic syndrome and meet appropriate inclusion criteria.
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