SANTA CLARA,
Calif. (AP) — XenoPort Inc. will
stop studying the drug candidate arbaclofen placarbil as a potential treatment
for severe heartburn symptoms after it failed to show superiority to a placebo
in a mid-stage clinical trial.

The Santa Clara,
Calif., company said the drug,
known as AP, was studied in patients with gastroesophageal reflux disease who
did not experience complete relief of symptoms while being treated with proton
pump inhibitors. Patients suffering from the condition experience acid backing
up into the esophagus, often causing burning pain in the chest.

XenoPort studied four AP dose regimens, and none showed
statistically significant improvements over a placebo. The company said it will
continue to analyze the data, but it does not believe the results warrant
further development of AP in treating the disease.

Company shares sank 11 percent, or 74 cents, to $5.93 in
morning trading.

XenoPort said it will continue planning a late-stage study
of AP in multiple sclerosis patients with spasticity. A company spokesman said
that study is expected to start by the middle of the year.

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.