The endpoint was met at week 12 against placebo achieved p value of <0.0001. Zilretta also demonstrated significant analgesia against placebo at weeks 1 through 16.
Flexion said that in pre-specified analyses, Zilretta achieved statistical significance against placebo in validated OA and quality of life secondary measures through week 12.
Zilretta also demonstrated statistical significance on WOMAC A (pain), WOMAC B (stiffness) and WOMAC C (function) through week 12 against both placebo and immediate-release triamcinolone acetonide.
Flexion Therapeutics president and CEO Michael Clayman said: "We believe that Zilretta has the potential to become an important new non-opioid treatment in a therapeutic area that hasn’t seen meaningful innovation in many years.
"We look forward to working closely with the U.S. Food and Drug Administration (FDA) as we prepare to submit our New Drug Application (NDA) for Zilretta."
Zilretta, which was designed using microsphere technology, is aimed to offer localized pain relief over a period of months and minimize systemic exposure as well as prevent severe side effects common to oral therapies prescribed for OA pain.
The progressive breakdown and eventual loss of cartilage in one or more joints causes OA.
Over 12 million people in the US currently suffer from the painful and debilitating effects of knee OA. The existing oral treatment options for OA knee pain include non-steroidal anti-inflammatory drugs.