Seaside Therapeutics expects to recruit around 150 patients in 25 clinical sites in the US.
In the study patients will be randomized to receive either STX209 or placebo.
The primary efficacy endpoint will evaluate social behavior.
The drug has showed efficacy in preclinical models, suggesting that it may improve function in individuals with FXS and ASD.
Seaside president and CEO Randall Carpenter said in the open-label Phase 2a study of STX209, they observed improvements in social impairment – a core symptom of autism spectrum disorders – including symptoms such as preference to be alone, being withdrawn or isolated, and lack of social reactivity.
"We are spearheading late-stage development of a drug candidate that has the potential to change the treatment paradigm for autism spectrum disorders – addressing core symptoms – and are truly excited about the prospect of helping patients and their families achieve an improved quality of life," Carpenter said.