March 11 (Reuters) - Acadia Pharmaceuticals ( ACAD )

said on Monday it does not plan to conduct further clinical

trials of its antipsychotic drug, pimavanserin, after it failed

to improve negative symptoms of schizophrenia in a late-stage

study.

Shares of the California-based company fell 16% in extended

trading.

The decision comes after two failed attempts to secure

regulatory approval for the expanded use of pimavanserin in the

treatment of psychosis related to dementia and Alzheimer's

disease in 2021 and 2022, respectively.

In the current study that involved 454 adult patients,

pimavanserin did not outperform a placebo in reducing negative

symptoms of schizophrenia, including poor socialization and lack

of motivation, Acadia said.

Schizophrenia is a chronic mental disorder that causes

distortion in thoughts, hallucinations and feelings of fright

and paranoia, and affects less than 1% of the U.S. population,

according to the American Psychiatric Association.

In 2016, pimavanserin, sold under the brand name Nuplazid,

became the first drug approved in the United States to treat

hallucinations and delusions associated with psychosis

experienced by some people with Parkinson's disease.

Nuplazid contributed $522.7 million in revenue last year,

making up nearly 72% of the company's total sales.

Treatments approved in the U.S. for schizophrenia include

antipsychotic generic drugs such as risperidone and olanzapine.