Primary Objective: To investigate the safety, tolerability, and pharmacokinetics of treatment with NNZ-2591 Oral Solution, 50mg/mL Investigational Medicinal Product (IMP) in children and adolescents with Pitt-Hopkins Syndrome. Secondary Objective: To investigate measures of efficacy during treatment with NNZ-2591 Oral Solution, 50 mg/mL Investigational Medicinal Product in children and adolescents with Pitt-Hopkins Syndrome.