British drugmaker GlaxoSmithKline PLC and XenoPort Inc. said Thursday they will partner to develop and market a XenoPort drug candidate to treat nerve pain and Restless Legs Syndrome (RLS).

XenoPort shares hit a 52-week high of $30.37, and were up $4.71, or 18.7 percent, to $29.95 in morning trading on the Nasdaq at more than four times their average trading volume. Shares have traded between $15.50 and $27.75 the past 52 weeks.

Under the exclusive agreement which is subject to U.S. antitrust review, GlaxoSmithKline will pay XenoPort $75 million up front for worldwide rights, with the exception of certain countries in Asia, to XP13512, an improved form of the nerve drug gabapentin.

XenoPort is also entitled to up to $65 million in development milestone payments, $210 million in other potential development and regulatory milestone payments and up to $290 million in potential sales milestone payments. Maximum milestone payments depend on getting drug market approval to treat neuropathic pain and RLS.

If XP13512 is approved for sale, XenoPort is also entitled to royalties.

The drug candidate is currently in late-stage clinical trials for RLS and in mid-stage clinical trials for neuropathic pain. RLS is a nerve disorder in which the limbs -- most often the legs -- tremble or twitch uncontrollably.

U.S. shares of GlaxoSmithKline rose 54 cents to $55.89 on the New York Stock Exchange.