As of January 2017, Catalyst and another US company, Jacobus Pharmaceutical, which had been manufacturing and giving it away for free since the 1990s, were both seeking FDA approval for their iterations and marketing rights.

In May 2019, the US Food and Drug Administration (FDA) approved amifampridine tablets under the brand name Ruzurgi for the treatment of Lambert-Eaton myasthenic syndrome (LEMS) in people 6 to less than 17 years of age.

[5] The dose-limiting side effects include tingling or numbness, difficulty sleeping, fatigue, and loss of muscle strength.

[5] In Lambert–Eaton myasthenic syndrome, acetylcholine release is inhibited as antibodies involved in the host response against certain cancers cross-react with Ca2+ channels on the prejunctional membrane.

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

Biological half-life (2.5±0.7 hrs) and the area under the curve (AUC = 117±77 ng∙h/ml) also vary widely between subjects, but are nearly independent of food intake.

[23][24][25] Doctors at the Assistance Publique – Hôpitaux de Paris had created a phosphate salt of 3,4-DAP (3,4-DAPP), and obtained an orphan designation for it in Europe in 2002.

[26] In 2008 EUSA submitted an application for approval to market the phosphate form to the European Medicines Agency under the brand name Zenas.

[28] EUSA, through a vehicle called Huxley Pharmaceuticals, sold the rights to 3,4-DAPP to BioMarin in 2009,[29] the same year that 3,4-DAPP was approved in Europe under the new name Firdapse.

[30] A group of UK neurologists and pediatricians petitioned to prime minister David Cameron in an open letter to review the situation.

[9][18] Meanwhile, in Europe, a task force of neurologists had recommended 3,4-DAP as the firstline treatment for LEMS symptoms in 2006, even though there was no approved form for marketing; it was being supplied ad hoc.

[24] In December 2015, a group of 106 neuromuscular doctors who had worked with both Jacobus and BioMarin/Catalyst published an editorial in the journal, Muscle & Nerve, expressing concern about the potential for the price of the drug to be dramatically increased should Catalyst obtain FDA approval, and stating that 3,4-DAPP represented no real innovation and didn't deserve exclusivity under the Orphan Drug Act, which was meant to spur innovation to meet unmet needs.

[22][42] Catalyst responded to this editorial with a response in 2016 that explained that Catalyst was conducting a full range of clinical and non-clinical studies necessary to obtain approval in order to specifically address the unmet need among the estimated 1500-3000 LEMs patients since about 200 were receiving the product through compassionate use – and that this is exactly what the Orphan Drug Act was intended to do: deliver approved products to orphan drug populations so that all patients have full access.

[45][19] Catalyst cut 30% of its workforce, mainly from the commercial team it was building to support an approved product, to save money to conduct the trials.

[49][50] In May 2019, the privately held US company Jacobus Pharmaceutical, Princeton, New Jersey gained approval by the FDA for amifampridine tablets (Ruzurgi) for the treatment of LEMS in patients 6 to less than 17 years of age.

With this decision, the Eleventh Circuit rejected the FDA's interpretation of orphan exclusivity and concluded that the agency had improperly approved a competitor product from Jacobus Pharmaceutical.