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.