A Phase 3, Double-blind, Placebo-controlled, Randomized Discontinuation Study Followed by Open-label Extension Evaluating Efficacy and Safety 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.

- 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.