Shares of Catalyst Pharmaceuticals Inc. tumbled 40% Tuesday on word that the FDA had cleared Ruzurgi (amifampridine) for treating pediatric patients with Lambert-Eaton myasthenic syndrome (LEMS). Catalyst, of Coral Gables, Fla., currently markets its LEMS treatment, Firdapse (amifampridine phosphate), for adult patients but has come under congressional scrutiny for its hefty $375,000 annual price tag, particularly since the active drug was once available to patients for free.