A Randomized, Double-Blind, Placebo-Controlled Study of the Efficacy, Safety, and Tolerability of STX209 (Arbaclofen) Administered for the Treatment of Social Withdrawal in Adolescents and Adults with Fragile X Syndrome

The objective of this study is to explore the efficacy, safety, and tolerability of STX209 (arbaclofen) administered for the treatment of social withdrawal in subjects with Fragile X Syndrome (FXS).

The effects of STX209 in subjects with FXS will be evaluated in a randomized, placebo-controlled, parallel-group, fixed-dose study. The treatment period will begin with all subjects being up-titrated every seven days (as necessary) to their randomized, assigned dose. Once subjects reach the assigned dose, that dose will be maintained for the duration of the treatment period. Up-titration is consistent with real-world use of virtually all medications with indications for neurobehavioral symptoms to minimize side effects associated with abrupt introduction of pharmacotherapy, and also is consistent with FDA-approved label for racemic baclofen.

Subjects who meet protocol criteria will be randomly allocated to receive either STX209 (5 mg BID, 10 mg BID, or 10 mg TID) or placebo in a 1:1:1:1 ratio in the treatment period. Blinding will be maintained by the random allocation to the four treatment arms, through the use of identical tablets containing either active medication or placebo, and through an identical dosing schedule for each treatment arm.

Clinicians will use the Lethargy/Social Withdrawal subscale of the ABC-C to assess efficacy. The CGI-S and CGI-I will also be used to assess the subject's overall behavior. Other efficacy endpoints will include assessments of social withdrawal and other behavioral symptoms of FXS by the parents/caregivers/LARs. An additional rating scale will assess the impact of problem behaviors on parent/caregiver/LAR stress levels.

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