Omeros shares dropped 9.2 percent in light after-hours trading Thursday after the Seattle biotechnology company reported that a Phase 3 clinical trial yielded mixed results and fell short of its primary benchmark.

The company said the testing with 344 patients treated with Omeros’s OMS103HP drug to reduce pain and inflammation following arthroscopic surgery found a statistically significant reduction in pain and need for postoperative narcotics.

But the agreed-upon primary criterion for the drug’s effectiveness was a patient-reported assessment of knee swelling, stiffness and similar symptoms, and by that measure the trial failed to achieve its goal.

Citing “the strength and consistency of the data” from the testing, Omeros said it’s on track to begin another Phase 3 trial in the first half of 2013.

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Its shares ended regular trading at $5.84, down 16 cents, and lost an additional 54 cents in the after-hours session.