An Open-label Study of NNZ-2591 in Pediatric Participants With Phelan-McDermid Syndrome

Inclusion Criteria: 1. Male or female pediatric participants with Phelan-McDermid syndrome ages 3 to 12 years (inclusive) at the time of signing the informed consent for the antecedent study. 2. Participant must have completed all applicable study visits for the antecedent study in which they participated. 3. Body weight ≥ 10 kg at Screening/Baseline. 4. Participants with a PMSA-S overall score ≥ 3 at the Screening and Baseline visits. 5. Not actively undergoing regression or loss of skills. Exclusion Criteria: 1. Use of exclusionary medication or unstable treatment regimens of acceptable concomitant medications as required by the protocol. 2. Participants with seizures must be controlled on no more than 2 anticonvulsant medications (not counting rescue medications). 3. Psychotropic medications or any other medication used for a chronic illness (not including antibiotics, pain relievers, anti-diarrheals, and laxatives) with doses and dosing regimen that have not been stable for at least 4 weeks before Screening. If the treatment was discontinued, the discontinuation must have occurred no fewer than 2 weeks before the start of Screening. 4. Any intercurrent seizures in the past 6 months and /or more than 1 seizure in the past 12 months. •A single febrile seizure in the 6 months prior to screening is allowable if no rescue medication was required. 5. Abnormal liver function laboratory results during the Screening period, as defined by the protocol 6. Abnormal QT interval on Screening ECG as defined by the protocol.