The FDA placed a full clinical hold on Alcobra’s MDX, or metadoxine extended release, currently being studied in attention deficit hyperactivity disorder and fragile X syndrome, a genetic condition that causes developmental problems.

The hold affects Alcobra’s ongoing Phase III study of MDX in adult patients with ADHD, which has enrolled nearly 500 subjects. The FDA indicated that the clinical hold is due to adverse neurological findings in a pre-clinical study, Alcobra said, and is not related to results from the Phase III study.

In 2013, FDA granted orphan drug status to metadoxine for the treatment of fragile X syndrome, and in 2015, the FDA granted a Fast Track designation to MDX for fragile X syndrome.