Arbaclofen vs. Placebo in the Treatment of Children and Adolescents With ASD (ARBA)

Inclusion Criteria: 1. Outpatients 5-17 years of age inclusive. 2. Meet Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5). DSM-5 criteria will be established by a clinician with expertise with individuals with ASD. Diagnosis will be supported by the Autism Diagnostic Observation Schedule, Second Edition (ADOS-2). 3. Complex language to qualify for ADOS-2 modules 3 or 4. 4. If already receiving stable concomitant medications affecting behaviour, have stable regimens with no changes during the preceding 6 weeks prior to Screening, and will not electively initiate new or modify ongoing medications for the duration of the study. 5. If already receiving stable non-pharmacological educational and behavioural interventions, have continuous participation during the preceding 3 months prior to Screening, and will not electively initiate new or modify ongoing interventions for the duration of the study. 6. Have normal physical examination and laboratory test results at Screening. If abnormal, the finding(s) must be deemed clinically insignificant by the Investigator. 7. Ability to obtain written informed consent from the participant, if developmentally appropriate. If a participant does not have the capacity to consent, ability to obtain assent (if developmentally appropriate), as well as written informed consent from their parent(s)/legal guardian(s). Exclusion Criteria: 1. Pregnant females; sexually active females on inadequate birth control. 2. Have a serious medical condition that, based on Investigator judgment, might interfere with the conduct of the study, confound interpretation of the study results, or endanger their own well-being. Have evidence of any significant hematological, endocrine, cardiovascular (including uncorrected symptomatic congenital heart disease), respiratory, renal, hepatic, or gastrointestinal disease, not including mild common pediatric diseases in these areas that are stable (e.g. mild asthma, constipation, etc.). 3. Have unstable epilepsy (i.e. seizures occurring within the last 6 months), or have epilepsy and not on stable doses of antiepileptic medications (i.e. dose changes within the last 3 months). 4. Have a history of drug abuse. 5. Have hypersensitivity to arbaclofen or any components of its formulation. 6. Unable to tolerate venipuncture procedures for blood sampling. 7. Actively enrolled in another intervention study. 8. Taking racemic bacblofen, vigabatrin, tiagapine, riluzole, clobazam or regular benzodiazepine use (prn and hs use is allowed). 9. Unable to take oral medications. 10. Known hypersensitivity to racemic baclofen. 11. Inability to speak and understand English sufficiently enough to allow for the completion of all study assessments (parent/legal guardian; participant).