A Phase 3 Study of Amifampridine Phosphate in Patients With Lambert Eaton Myasthenic Syndrome (LEMS)

Inclusion Criteria: Individuals eligible to participate in this study must meet all of the following inclusion criteria: * ≥18 years of age * Confirmed diagnosis of LEMS * Normal respiratory function * Normal swallowing function * If receiving peripherally acting cholinesterase inhibitors a stable dose is required for at least 7 days prior to Screening. * If receiving oral immunosuppressants a stable dose is required for at least 90 days prior to Screening. * Negative pregnancy test for females of childbearing potential * If sexually active, willing to use 2 acceptable methods of contraception * Willing to perform all study procedures as physically possible. * Willing and able to provide written informed consent after the nature of the study has been explained and prior to the start of any research-related procedures. Exclusion Criteria: Individuals who meet any of the following exclusion criteria are not eligible to participate in the study: * History of epilepsy or seizure. * Known active brain metastasis. * Use of Fampridine (4-aminopyridine), and any form of 3,4-diaminopyridine other than the IP provided, such as amifampridine base or Firdapse, during the study. * Use of medications known to lower the epileptic threshold within 7 days or 5 half-lives. * Use of medications which inhibit neuromuscular junction function within 7 days or 5 half-lives. * Use of IVIG, plasmapheresis (plasma exchange), or immunoadsorption within 90 days * Use of guanidine hydrochloride within 7 days * Use of rituximab within 12 months * History of drug allergy to any pyridine-containing substances or any amifampridine phosphate excipient(s). * Use of any other investigational productwithin 30 days * Treatment with a concomitant medication that prolongs the QT/QTc interval within 7 days or 5 half-lives. * Treatment with sultopride (4-amino-N-\[(1-ethylpyrrolidin-2-yl)methyl\]-5-ethylsulfonyl-2-methoxybenzamide) within 7 days. * An abnormal electrocardiogram (ECG). * Documented history of arrhythmias. * History of additional risk factors for torsade de pointes. * Breastfeeding or pregnant or planning to become pregnant (self or partner) at any time during the study. * Likely or expected to require treatment for cancer within 3 months (90 days) after entering. * History of severe renal impairment or evidence of severe renal impairment * Any condition that places the patient at high risk of poor treatment compliance or of not completing the study. * History of uncontrolled asthma.