Flexion Therapeutics (NSDQ:FLXN) said today that the FDA accepted its New Drug Application for Zilretta – its extended-release steroid injection. The Burlington, Mass.-based company won Fast Track status from the watchdog for its osteoarthritis injection in 2015.

Flexion touts Zilretta as the 1st intra-articular, extended-release therapy for patients with knee osteoarthritis pain. The injection uses microsophere technology to combine a commonly administered, short-acting corticosteroid with a polylactic-co-glycolic acid matrix.

The new drug application is based on data from a pivotal phase III clinical trial evaluating Zilretta in patients with moderate to severe osteoarthritis knee pain. The randomized, double-blind study enrolled 484 patients at 37 centers to compare the injection to a placebo and a commonly injected intra-articular corticosteroid.

In both comparisons, patients who were treated with Zilretta had a statistically significant reduction in average daily pain after 12 weeks. No drug-related serious adverse events were reported.

“The FDA’s acceptance of the NDA for Zilretta is a seminal milestone for Flexion and encouraging news for the many millions of people who are confronting OA of the knee,” president & CEO Dr. Michael Clayman said in prepared remarks. “The pain associated with OA of the knee can be debilitating, and we believe that, if approved, Zilretta could serve as an important new treatment option for patients suffering from this progressive condition.”

The company filed its NDA in mid-December last year.  The federal watchdog said it established a user fee goal date of October 6, 2017, in accordance with the standard 10-month review process.

FLXN shares closed at $19.04 apiece today, down -3.5%.