Amifampridine Phosphate for the Treatment of Congenital Myasthenic Syndromes

Each patient will participate in an open-label unblinded drug escalation/treatment run-in
phase for up to 4 weeks until stable dose and frequency of amifampridine phosphate is
achieved for 7 days. After this phase, blinded treatment effect will be assessed in a
randomized fashion of continuation or cessation of drug (Placebo) starting with Period I
(duration 7 days). Following experimental Period 1, patients will be returned to the stable
dose administered at the end of the open-label run-in period for approximately 2 weeks,
followed by cross over treatment in Period 2 dosing for 7 days. After completion of Period 2,
patients will be eligible for expanded access with restoration of open-label amifampridine
phosphate at the same dose and frequency as established in the run in phase of the study.

Inclusion Criteria:

Individuals eligible to participate in this study must meet all of the following inclusion
criteria:

1. Patient's or parent 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, or the patient's legal guardian or caregiver with durable power of
attorney can provide written informed consent. An assent form must also be signed if
in the judgement of the IRB the children are capable of providing assent.

2. Male or female age 2 and above.

3. Body weight ≥10 kg.

4. Genetically-confirmed CMS involving acetylcholine receptor defect, Rapsyn deficiency,
MuSK deficiency, Dok-7 deficiency, SYT2 deficiency,SNAP25B deficiency, and fast
channel syndrome.

5. MFM 20 or 32 score equal or less than 48 or 76, respectively, at Screening.

6. In patients naïve to 3,4-DAP or amifampridine phosphate, improvement of >20% in MFM20
or MFM32 scores after open label period of up titration of dose

7. In patients previously stabilized on 3,4-DAP or amifampridine phosphate, history of
meaningful improvement in motor function (in opinion of investigator)

8. Willingness of patients receiving pyridostigmine, prednisone, albuterol, ephedrine, or
fluoxetine to remain on a stable dose of these medications throughout the study
interval.

9. Female patients of childbearing potential must have a negative pregnancy test (serum
human chorionic gonadotropin [HCG] at Screening); and must practice effective,
reliable contraceptive regimen during the study.

10. Ability to participate in the study based on overall health of the patient and disease
prognosis, as applicable, in the opinion of the investigator; and able to comply with
all requirements of the protocol.

Exclusion Criteria:

Individuals who meet any of the following exclusion criteria are not eligible to
participate in the study:

1. CMS subtype diagnosis of acetylcholinesterase deficiency, slow-channel syndrome, LRP4
deficiency, and plectin deficiency.

2. Cardiac conduction defects on Screening ECG.

3. Seizure disorder.

4. Abnormal liver function tests at Screening.

5. Abnormal kidney function tests at Screening.

6. Abnormal electrolyte values at Screening.

7. Pregnancy or breastfeeding at Screening or planning to become pregnant at any time
during the study.

8. Any systemic bacterial or other infection, which is clinically significant in the
opinion of the investigator and has not been treated with appropriate antibiotics.

9. Treatment with an investigational drug (other than amifampridine phosphate), device,
or biological agent within 30 days before Screening or while participating in this
study.

10. Any other medical condition that, in the opinion of the investigator, might interfere
with the patient's participation in the study, poses an added risk for the patient, or
confound the assessment of the patient.

11. History of drug allergy to any pyridine-containing substances or any amifampridine
phosphate excipient(s).