The agency’s letter stated that the company’s application was not sufficiently complete and it is seeking additional supporting information.
There was no comment on the acceptability of the submitted clinical data in the letter and the FDA did not judge the efficacy or safety of Firdapse.
Catalyst Pharmaceuticals chairman and CEO Patrick McEnany said: "We expect to work closely with the FDA over the coming weeks in an effort to resolve the open issues and to define a path forward for a successful resubmission of our application at the earliest point in time.
"We remain focused on delivering on our promise to transform the way people living with LEMS and CMS are provided access to a safe and effective, FDA approved therapy."
The company said it will continue to enroll new patients in its expanded access program and provide Firdapse at no cost for those who meet the enrollment criteria.
In October 2012, Catalyst collaborated with BioMarin Pharmaceuticals, which included the in-licensing of the North American rights to Firdapse and responsibility for its clinical development program.
Firdapse had already secured orphan medicinal product designation in the European Union (EU) and has been marketed in the EU since 2010 to treat LEMS.
The European Academy of Neurology has also recommended Firdapse as first-line symptomatic treatment for LEMS.