Amifampridine Phosphate for the Treatment of Congenital Myasthenic Syndromes

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).