The study objective is to explore the safety and tolerability of STX209 in subjects with Autism Spectrum Disorders and to obtain preliminary data on several measures of efficacy in treating irritability. We hypothesize that STX209 will be safe and well-tolerated.

Inclusion Criteria:

• Male or female subjects 6 to 17 years of age, inclusive.
• Diagnosis of Autistic spectrum disorders
• Clinical Global Impression - Severity (CGI-S) rating for aberrant behavior of moderate or higher at screening and at Visit 1 (Day 1).
• An Aberrant Behavior Checklist (ABC-C) Irritability Subscale score ≥16 at screening and at Visit 1 (Day 1).
• If the subject is already receiving stable non-pharmacologic educational, behavioral, and/or dietary interventions, participation in these programs must have been continuous during the 3 months prior to Screening (Visit 1) and subjects and their parent/caregiver/legally authorized representative (LAR) will not electively initiate new or modify ongoing interventions for the duration of the study.
• Exclusion Criteria:
• Subjects with known genetic disorders associated with PDD such as fragile X syndrome.
• Subjects with a history of a seizure disorder who are not currently receiving treatment with antiepileptic medication.
• Subjects with any medical condition, including alcohol and drug abuse that might interfere with the conduct of the study, confound interpretation of the study results, or endanger their own well-being.
• Subjects who plan to initiate or change pharmacologic or non-pharmacologic interventions during the course of the study.
• Subjects currently treated or have been treated in the last 2 weeks with any psychotropic medication except anti-epileptics or who have been treated with fluoxetine in the last 4 weeks.
• Subjects currently treated with vigabatrin or tiagabine.
• Subjects taking another investigational drug currently or within the last 30 days.
• Subjects who have a history of hypersensitivity to racemic baclofen.