Biopharmaceutical company XenoPort Inc and its partner GlaxoSmithKline Plc said their neuropathic pain drug met the main goal of reducing the intensity of pain in a mid-stage trial.

Shares of XenoPort surged as much as 29 percent, before paring some gains to trade up $5.30 at $25 Thursday morning. The stock was the second biggest percentage gainer on Nasdaq. GlaxoSmithKline was trading flat at $38.93 on the New York Stock Exchange.

The companies said the drug, XP13512 or GSK1838262, reduced the intensity of pain compared with a dummy drug, and was well tolerated at all doses in the study.

The drug is being studied to treat adults with post-herpetic neuralgia, a neuropathic pain syndrome that usually follows an outbreak of shingles -- an infection of a nerve and the area of skin supplied by the nerve -- and mostly affects people over fifty years.

(Reporting by Anuradha Ramanathan in Bangalore; Editing by Deepak Kannan)