Clinical Trial of Sodium Phenylbutyrate in Children With Spinal Muscular Atrophy Types II or III

Inclusion Criteria: Subjects must meet all of the following inclusion criteria within 14 days prior to receiving the first dose of study drug. * Weakness and hypotonia consistent with a clinical diagnosis of spinal muscular atrophy (SMA) type II or III * Laboratory documentation of homozygous absence of SMN1 exon 7 * Type III subjects who are unable to get up from the floor unaided. "Unaided" is defined as not using a device or obtaining assistance from another person. * 2 years of age or older, but younger than 12 years of age, at the time of enrollment * Written informed consent of parents/guardian * Weight greater than or equal to 10 kilograms * Laboratory results drawn within 14 days prior to start of study drug demonstrating: Hemoglobin within normal range at the clinical site; White Blood Cell Count ≥ 3000/mm³; Platelet Count ≥ 75,000/mm³; Lipase and Amylase ≤ 1.5 x upper limit of normal (ULN) in the absence of associated clinical symptoms; AST and ALT ≤ 2.5 x ULN; Bilirubin ≤ 1.5x ULN in the absence of associated clinical symptoms; Sodium ≥ 130 and ≤ 150 mmol/L; Potassium ≥3.0 and ≤ 5.5 mmol/L; Chloride ≤ 110 mmol/L; Calcium ≥ 8.0 mg/dL; Bicarbonate ≥ 16 mmol/L; Glucose ≥ 55 and ≤ 160 mg/dL; BUN ≤ 39 mg/dL; Creatinine ≤ 1.5 x ULN * Subject is expected to survive for at least 6 months following study entry. Exclusion Criteria: Subjects who meet any of the following criteria will be excluded from participating in the study: * Evidence of renal dysfunction, blood dyscrasia, hepatic insufficiency, symptomatic pancreatitis, cardiac arrhythmia, congenital heart defect, known history of metabolic acidosis, hypertension, significant central nervous system impairment, or neurodegenerative or neuromuscular disease other than SMA. * Any adverse event ≥ Grade 3 at the time of screening based on the protocol toxicity grading table * Any acute co-morbid condition interfering with the well-being of the subject within 7 days of enrollment including bacterial infection, viral infectious process, food poisoning, temperature \> 99.0ºF, need for acute treatment or observation due to any other reason, as judged by the investigator * ≥ Grade 2 vomiting; * ≥ Grade 2 liver dysfunction/failure (clinical); * Any abnormality noted on EKG except for asymptomatic sinus arrhythmia * History of allergy/sensitivity to sodium phenylbutyrate * Use of sodium phenylbutyrate within 30 days of study entry * Serious illness requiring systemic treatment and/or hospitalization within 14 days prior to study entry (Subjects are not eligible following serious illness until therapy is complete and the subject is stable, or until the subject is on therapy and stable for at least 14 days.) * Use of medications intended for the treatment of SMA including riluzole, valproic acid, hydroxyurea, oral use of albuterol, sodium phenylbutyrate, butyrate derivatives, creatine, carnitine, growth hormone, anabolic steroids, probenecid, oral or parenteral use of corticosteroids at entry, haloperidol, agents anticipated to increase or decrease muscle strength or agents with known or presumed histone deacetylase (HDAC) inhibition within 30 days prior to study entry Notes: Subjects who use a nebulizer or require an inhaler to receive albuterol will be allowed in the study; however oral use of albuterol is prohibited. Topical use of steroids will be allowed. Oral use of steroids is not allowed at entry, but these may be used as clinically indicated while on study. Event grading will be based on the toxicity-grading table in the protocol.