(Reuters) – Acadia Pharmaceuticals Inc said on Monday its add-on treatment for schizophrenia failed to meet the main goal in a late-stage study.

The company’s shares were down 16% at $21.62 in extended trading.

In the study, 396 patients with moderate-to-severe psychotic symptoms were given Acadia’s treatment pimavanserin or placebo as an add-on to their current antipsychotic treatment, the company said.

Adding pimavanserin to existing antipsychotic treatment showed a consistent trend in improvement of psychotic symptoms, however, the results did not show statistical significance in achieving the main goal, Acadia said.

A positive trend was observed in achieving the key secondary goal, Acadia said.

The U.S. Food and Drug Administration in 2016 approved pimavanserin for the treatment of hallucinations and delusions associated with Parkinson’s disease.

Schizophrenia is a chronic, debilitating and often progressive mental illness characterized by disturbances in thinking, emotional reaction, and behavior.

Acadia said about 1% of the U.S. population develops schizophrenia during their lifetime, citing the National Mental Health Institute data.

(Reporting by Aakash Jagadeesh Babu in Bengaluru; Editing by Anil D’Silva and Sriraj Kalluvila)