Xenon pauses new enrollment in depression drug trials over side-effect concerns

Meanwhile, Xenon said the pause does not impact its epilepsy program, and that it has submitted an application to the U.S. Food and Drug Administration seeking approval for the drug to treat focal seizures, the most common form of epilepsy.

Sept. 17 (Reuters) - Xenon Pharmaceuticals on Thursday said it has temporarily paused enrolling new patients in clinical studies testing its experimental drug for major and bipolar depression, following reports of mental and nervous system-related side effects.

Shares of the drug developer tumbled more than 25% in after-hours trading.

The voluntary pause was a precaution taken after consultations with its safety monitoring board and analyzing adverse events across the trials, Xenon said. The studies are testing azetukalner, an experimental treatment that is also being developed for epilepsy.

Patients already enrolled in the trials will continue their treatment.

The side effects observed in psychiatry studies included confusion, speech difficulties, motor coordination issues and a "very small number" of cases classified under the broad term of "psychosis," company executives said in a conference call.

Psychosis is a mental health condition in which a person loses contact with reality, leading to symptoms such as hallucinations, delusions and disorganized thinking.

The rate of psychosis was "hovering around 1% or under," said Xenon's Chief Medical Officer Christopher Kenney, adding that the events were short in duration and reversible without any long-term negative effects.

While the severity and frequency of the side effects aligned with how the drug worked, the company noted they had not appeared in an earlier mid-stage study for depression.