Sage Therapeutics Inc said on Wednesday its drug along with an antidepressant helped improve depressive symptoms after three days in a late-stage study, paving way for the drug developer to submit data for U.S. approval this year.

The company and partner Biogen Inc said the drug, zuranolone, met the study's secondary goal of showing a statistically significant improvement in symptoms over a two-week period.

However, shares of Sage fell 10% to $39 before the bell as investors focused on the benefit of the drug not lasting significantly beyond two weeks.

"The clinical benefit is modest and the effects are not durable," said Truist Securities analyst Joon Lee, adding that the regulatory process for the drug is likely to be "wrought with noise, controversy" over the next 12-18 months.

In the study among 440 participants with major depressive disorder, zuranolone along with antidepressants led to a greater reduction in symptoms, such as feelings of sadness and anxiety, from baseline compared with a combination of placebo and antidepressants.

Major depressive disorder (MDD) is a common but serious mood disorder in which people experience depressive symptoms and it affects over 16 million American adults every year.

The disorder is mainly treated with antidepressants, which could take up to six weeks to show effect and patients are typically required to continue the treatment for months after.

"What this study has done, we believe, is just substantiated that zuranolone could be a unique treatment option for patients," said Jeff Jonas, chief innovation officer at Sage Therapeutics.

"We believe that if people can get better in days versus weeks... that's an option that patients deserve to have."

Sage plans to start submitting data for U.S. approval of the drug for major depressive disorder on a rolling basis early this year and is aiming to complete the application by the second half of the year.