Inclusion Criteria:  Clinical diagnosis of PMS with a documented disease-causing genetic abnormality of SHANK3. Males or females aged 3-12 years. Subjects with a Clinical Global Impression - Severity (CGI-S) score of 4 or greater at the Screening visit. Each subject must be able to swallow the study medication provided as a liquid solution. Caregiver(s) must have sufficient English language skills.  Exclusion Criteria:  Clinically significant abnormalities in safety laboratory tests at Screening. Abnormal QTcF interval or prolongation at Screening. Any other clinically significant finding (as determined by the Investigator) on ECG at the Screening visit. Excluded concomitant treatments. Actively undergoing regression or loss of skills. Unstable seizure profile. Current clinically significant (as determined by the Investigator) cardiovascular, renal, hepatic, gastrointestinal, respiratory, endocrine disease, or clinically significant organ impairment. Current clinically significant (as determined by the Investigator) hypo or hyperthyroidism Type 1 or Type 2 diabetes mellitus requiring insulin (whether well controlled or uncontrolled), or uncontrolled Type 1 or Type 2 diabetes. Has planned surgery during the study. History of, or current, cerebrovascular disease or brain trauma. History of, or current catatonia or catatonia-like symptoms. History of, or current, malignancy. Current major or persistent depressive disorder (including bipolar depression). Significant, uncorrected visual or uncorrected hearing impairment. Allergy to strawberry.