(Reuters) - An advisory panel to the U.S. Food and Drug Administration on Thursday voted against approving Alkermes Plc’s treatment for depression in patients with an inadequate response to standard antidepressant therapies.

The decision comes two days after FDA staff reviewers bit.ly/2QegxPF flagged abuse potential for the opioid-based depression treatment and raised questions on its efficacy.

“I don’t feel the evidence that was presented was substantial to show that there was efficacy ... I’m not convinced there is a big enough treatment effect to make a clinical difference,” said Terri Warholak, a panel member.

The panel voted 21-2 against the drug, ALKS 5461, a once-daily pill combining samidorphan and buprenorphine developed as an add-on treatment for major depressive disorder (MDD), saying the data did not support a favorable benefit-risk profile.

ALKS 5461 is designed to rebalance brain function that becomes dysregulated in the state of depression and was seen as a driver of future revenue for the Dublin-based drugmaker.

FDA staffers in the review on Tuesday also questioned the non-availability of trial data on the effects of the treatment on unborn infants when the treatment was administered to pregnant women.

“We don’t know what the mechanism of action is ... and the likelihood of dependence and withdrawal,” said Jane Acri, also a panel member.

“I think it’s a very vulnerable population at risk for drug dependence in the first place.”

The FDA, which is slated to announce its final decision on Alkermes’ drug by Jan. 31, 2019, is not mandated to follow the recommendation of the panel, but generally does.

MDD affects about 16.2 million people in the United States, of which nearly two-thirds do not respond to currently approved therapies, highlighting the need for new therapies with novel mechanisms of action to treat the disorder.

To date, only three antipsychotic drugs have been approved as add-on treatments for MDD. Alkermes’ treatment would be the first opioid-based product for depression, if approved.

There are currently very few treatments for depression.