(Adds details on the study, shares)
June 15 (Reuters) - Sage Therapeutics said on Tuesday its experimental depression drug showed significant improvement in symptoms compared to placebo in a late-stage study.
The 543-participant study assessed Sage's once-daily oral drug, zuranolone, as a two-week course for the mood disorder that impedes patients' ability to carry out daily activities such as work, school or social interactions.
Depression affects over 16 million American adults every year and is mainly treated with antidepressants such as Eli Lilly and Co's Prozac and Pfizer Inc's Zoloft, which could take up to six weeks to show effect and patients are typically required to continue treatment for months after.
Zuranolone was generally well-tolerated in the study and showed a safety profile consistent with its previous clinical studies, Sage and partner Biogen said in a joint statement.
However, shares of Cambridge, Massachusetts-based Sage were down nearly 20% before the bell, as investors were concerned about how long the symptom improvement will last, some analysts said.
Those who responded to zuranolone at day 15 maintained about 86% of the improvement on average at day 42 or four weeks after the therapy ended, Sage and Biogen said.
"The stock reaction may be mixed as investor expectations on key Day 15/Day 42 endpoints were a little higher, but we'd reiterate the results are fundamentally positive," Jefferies analyst Andrew Tsai said.
Biogen tied up with Sage in a deal worth over $1.5 billion plus milestone payments in November to co-develop drugs for depression and other neurological disorders.
(Reporting by Manojna Maddipatla in Bengaluru; Editing by Arun Koyyur and Shinjini Ganguli)