SAN FRANCISCO (AP) — Nektar Therapeutics shares sank in extended trading Thursday after the pharmaceutical maker said that a recent mid-stage study of its pain drug failed to meet its goal.

The San Francisco company’s drug, known as NKTR-181, is designed with a slow rate of entry into the brain, which the company says could make it less likely to be abused and could reduce other common side effects for similar drugs, such as sedation and respiratory depression.

Nektar said the study failed to meet its end goal because patients who made it to the randomized portion of the trial, in which they randomly received a placebo or the drug, did not show the expected increase in pain scores that was anticipated when they were put on the placebo.

The company said it remained committed to bringing the drug to patients suffering from chronic pain.

Nektar develops drugs for oncology, pain and other treatment areas.

Shares of the company lost 26 percent to $10.30 in after-hours trading. Its stock added 44 cents to close regular trading at $13.85, up 87 percent for the year.