Expanded Access Study Amifampridine Phosphate in Lambert-Eaton Myasthenic Syndrome (LEMS), Congenital Myasthenic Syndrome (CMS), or Downbeat Nystagmus Patients

Inclusion Criteria:

- Male or female:

- 2 years of age at 5 pediatric CMS study sites

- 10 years of age at other study sites.

- Confirmed physician diagnosis of LEMS, CMS or downbeat nystagmus.

- Completion of anti-cancer treatment at least 3 months (90 days) before treatment.

- Negative urine pregnancy test for females of childbearing potential at Screening.

- If sexually active and of childbearing potential, willing to use 2 acceptable methods
of contraception from screening visit until 3 months after the last dose of
investigational product. No adequate clinical data on exposed pregnancies are
available for amifampridine. No nonclinical safety data are available regarding the
effects of amifampridine on reproductive function. Amifampridine phosphate should not
be used during pregnancy. It is unknown whether amifampridine is excreted in human
breast milk. The excretion of amifampridine in milk has not been studied in animals.
Amifampridine phosphate should not be used during breastfeeding.

- Any subject currently participating in studies LMS-002, LMS-002EXT, or CMS 001 is
immediately eligible for enrollment into study EAP-001, as long as inclusion/exclusion
criteria are still met.

- Willing and able to provide written informed consent after the nature of the study has
been explained and before the start of any research-related procedures.

Exclusion Criteria:

- History of epilepsy.

- CMS subtypes including slow-channel syndrome, LRP4 deficiency, and
acetylcholinesterase deficiency.

- Known active brain metastasis. Patients with treated brain metastasis (radiotherapy
and/or surgery) who have completed treatment for their brain metastasis >90 days
before Screening, are neurologically stable (neurological symptoms grade <1), are on a
stable dose of corticosteroids and have no evidence of new disease on magnetic
resonance imaging (MRI) are eligible, provided they meet the other inclusion/exclusion
criteria.

- Current use of dalfampridine (Ampyra®; 4-aminopyridine), and any form of 3,4 DAP other
than the investigational product provided, such as amifampridine base and does not
agree to discontinue use for the duration of the study.

- Use of guanidine hydrochloride within 7 days of starting amifampridine phosphate
treatment.

- History of drug allergy to any pyridine-containing substances or any amifampridine
phosphate excipients (i.e. microcrystalline cellulose, colloidal silicon dioxide or
calcium stearate).

- Use of any other investigational product (other than 3,4 DAP or amifampridine
phosphate) or investigational medical device within 30 days before starting treatment
or requirement for any investigational agent before completion of all scheduled study
assessments.

- An electrocardiogram (ECG) within 6 months before starting treatment that shows
clinically significant abnormality(ies), in the opinion of the patient's personal
physician.

- History of additional risk factors for torsade de pointes (e.g. history of surviving a
near drowning due to loss of consciousness, family history of congenital QT syndrome,
long QT syndrome, family history of unexplained early sudden death, or heart failure).

- Breastfeeding or pregnant or planning to become pregnant (self or partner). Male
patients with breastfeeding partners are not excluded from the study.

- History of severe renal impairment or evidence of severe renal impairment at time of
Screening on laboratory tests, specifically a creatinine clearance <30 mL/min (within
30 days) as calculated using the Cockcroft Gault formula.

- History at time of Screening of laboratory tests (within 30 days) indicating hepatic
impairment:

o In patients without liver metastases from cancer, alanine aminotransferase (ALT),
aspartate aminotransferase (AST), and/or total bilirubin >1.5 × upper limit of normal
(ULN).

- Any condition that, in the view of the Principal Investigator, places the patient at
high risk of poor treatment compliance or of not completing the study.