Rapastinel, the depression drug that Allergan picked up in its $560 million acquisition of Naurex, missed its primary endpoint in a trio of pivotal trials. What’s more, an interim analysis from a fourth study “suggests the primary and key secondary endpoints will not be met.”

Allergan was testing the NMDA receptor modulator as an adjunct therapy in patients with major depressive disorder (MDD). In all, the three phase 3 studies enrolled 1,510 patients with MDD who had a partial response to antidepressant therapy. In two of the trials, patients received weekly injections of placebo or rapastinel on top of their oral antidepressants, the company said in a statement Wednesday afternoon. The third study was similar in design, except that it also evaluated a second, lower dose of rapastinel. The drug was well tolerated in all three of the studies but failed to differentiate from placebo on the primary and key secondary endpoints.

"We are deeply disappointed with these results, and they are a vivid reminder that drug development is extremely challenging, especially in mental health,” said David Nicholson, Allergan’s R&D chief, in the statement.

Whitepaper Download Reducing the Complexity and Costs of Channel Planning and Logistics How can you make the process of bringing your product to market less complex while also reducing costs? This Whitepaper identifies opportunities to simplify channel strategies for biopharma companies, their customers and patients. Discover how you can deliver savings and innovation to your business. Download for Free

While these data are grim, the drug isn’t quite dead in the water. Allergan is also testing it as a monotherapy for MDD. It is in a phase 3 trial for MDD patients and a phase 2 study in patients with MDD who are at imminent risk of suicide. The Big Pharma will look into how these data might affect its other rapastinel programs and make a decision “over the course of 2019,” Nicholson said.

Allergan’s stock dipped about 5% in after-hours trading.

The company forked over $560 million up front to acquire Naurex back in 2015, picking up a pair of NMDA receptor modulators, rapastinel and NRX-1074, also called apimostinel, that would “strengthen Allergan’s long-term growth profile.”

RELATED: Aptinyx bags $70M to fund multidrug CNS trial strategy

At the time, Naurex spun out its development platform into a new company, Aptinyx, which then entered a research collaboration with Allergan to develop NMDA drugs for central nervous system disorders. Under the deal, Allergan has first dibs on in-licensing candidates that come out of the partnership. Last May, Allergan pulled that trigger on the oral, small-molecule drug AGN-241751, which it will advance for MDD.