GlaxoSmithKline (NYSE: GSK) is wasting no time in trying to seeking additional indications for its new restless leg syndrome drug Horizant.

The British drug maker, which has its U.S. headquarters in Research Triangle Park, has filed a supplemental new drug application with the Food and Drug Administration for Horizant as a treatment for postherpetic neuralgia in adults – the pain patients feel in the area where they had a shingles infection. Such pain can persist for years.

Horizant, developed with drug partner Xenoport (NASDAQ: XNPT), just received FDA approval for the treatment of restless leg syndrome in April. Restless leg syndrome is characterized by a feeling of pulling, itching, tingling, burning or aching in the legs. The symptoms can be relieved by moving the legs.

Horizant was discovered and developed by XenoPort. In 2007, the Santa Clara, California-based company reached a development agreement with GSK that granted XenoPort an up front payment of $75 million and additional milestone payments up to $65 million for work leading to commercialization of the compound as a restless leg syndrome drug.

The agreement also provides for further development of compound. GSK will pay XenoPort up to $210 million in other potential development and regulatory milestone payments if the drug can be developed in other indications. GSK has studied the compound as a migraine headache treatment and for applications in neuorpathic pain. The compound last year failed in a phase 2 clinical trial studying migraine pain.

Horizant is not approved or licensed anywhere in the world for postherpectic neuralgia.

Get the latest news alerts: Follow LTW at Twitter.