A GlaxoSmithKline logo is seen outside one of its buildings in west London
The U.S. Supreme Court Monday took on a case that will decide if GlaxoSmithKline's sales reps are exempt from overtime pay laws, an issue that could cost the pharmaceutical industry billions of dollars. Reuters

XenoPort (NASDAQ: XNPT) and GlaxoSmithKline Plc (LON: GSK) said the U.S. Food and Drug Administration (FDA) has approved Horizant, a treatment for restless legs syndrome (RLS) that the companies developed together.

The drug is an extended-release formulation of gabapentin enacarbil, a once-daily treatment for moderate-to-severe RLS.

About a year ago, the FDA requested for more information about a link between treatment with the drug and tumors in rats.

The effectiveness of Horizant was studied in two 12-week clinical trials in adults. The trials showed that people taking the medication had an improvement in their RLS symptoms, compared with people taking an inactive pill (placebo), the companies said.

RLS is a disorder that causes a strong urge to move the legs. This urge often occurs with unpleasant feelings in the legs. People who have RLS describe feeling pulling, itching, tingling, burning, or aching in their legs, and moving the legs temporarily relieves these feelings. The urge to move often happens when a person is inactive, and the symptoms typically are worse in the evening and early morning

Shares of California-based XenoPort soared 57 percent to $10 on Thursday following the news of its first U.S. drug approval.

Shares of GlaxoSmithKline are trading 0.74 percent higher at 1,226.18 pence at 11:40 am BST Friday on the London Stock Exchange.

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