Shares of XenoPort Inc. sank Monday after the drug developer said a potential treatment for multiple sclerosis patients failed in late-stage clinical testing, and it will stop developing the drug.

The Santa Clara, Calif., company said the treatment, labeled arbaclofen placarbil, failed to show a statistically significant improvement for patients taking it compared to a fake drug. XenoPort was testing the drug to treat patients with spasticity due to multiple sclerosis.

The study involved 228 patients at 30 U.S. sites.

Spasticity is a condition connected to some neurological disorders and injury to the spinal cord. It can cause decreased range of motion, weakness, and coordination problems. Multiple sclerosis is a disease in which the immune system attacks healthy nerves. It can cause pain, numbness, slurred speech, impaired vision, muscle weakness, and neurological problems.

XenoPort shares fell more than 11 percent, or 78 cents, to $5.97 in late-morning trading Monday, while broader trading indexes climbed slightly. That pushed the stock price down more than 23 percent so far this year.