As revenue at Pfizer (PFE 1.63%) continues to decline, some relief may come in the form of a new indication for Lyrica, one of the company's top-selling drugs, to treat restless legs syndrome, or RLS.

Lyrica was a top seller during 2013
Lyrica was a top contributor for Pfizer (PFE 1.63%) during 2013. Despite having lost exclusivity in the Canadian market in 2013, Lyrica had sales of $4.6 billion, an increase of 11% for the year. Amid a decreasing revenue environment, Lyrica's sales made up 9% of the top line of $51.6 billion. The losses of branded cholesterol drug Lipitor to generic competition and lower emerging market purchases of arthritis drug Enbrel were part of the headwinds.

During the year, Pfizer (PFE 1.63%) disposed of its animal health division Zoetis and used the proceeds to repurchase common shares worth $16.3 billion. The share repurchase and dividend payments of $6.6 billion helped return $23 billion to shareholders. Pfizer (PFE 1.63%) shares rose 1.4% last week to end at $31.94.

Trial results are encouraging
RLS is a neurological disorder characterized by various unpleasant sensations in the legs that usually cause an uncontrollable urge to move them. The condition worsens when a person is lying down, which disrupts sleeping patterns. It impacts as many as 10% of the U.S. population and more than 5 million adults have moderate to severe symptoms. And it isn't just adults -- an estimated 1 million school-aged kids have the condition.

In the year-long clinical trial, patients suffering from RLS and taking Lyrica (pregabalin) saw improvement in their symptoms comparable to patients taking a higher dose of Mirapex (pramipexole), manufactured by Boehringer Ingelheim. The use of Lyrica and the higher dose of Mirapex showed more significant improvements than a placebo in all measures, except for overall sleep quality and number of nighttime awakenings.

Patients also reported lower rates of symptoms worsening when using Lyrica; after 12 weeks, only 2.1% of patients receiving Lyrica showed worsening symptoms. In comparison, 7.7% of patients taking a high dose of Mirapex reported worsening symptoms. Worsening of symptoms, also known as augmentation, is a key element used to identify suitable therapies to treat RLS. Researchers in the Lyrica study noted limitations with the trial and called for "more head-to-head comparison of treatments..." with study participants that "...include children and elderly patients, those with secondary RLS, and those with mild-to-moderate RLS."

Other competing drugs on the market
One of those treatments is Horizant, manufactured by XenoPort (NASDAQ: XNPT) which works in a similar manner as Lyrica. Horizant is also known as gabapentin enacarbil, and it received FDA approval in 2011 to treat RLS. GlaxoSmithKline's Requip is also a competing treatment.

Both Mirapex and Requip are associated with various, and at times debilitating, side effects. Mirapex, part of Boehringer Ingelheim's Prescription Medicines business segment, which reported sales growth of 13% in 2012, has been used to treat RLS since 2006. Mirapex has had to compete with generics since 2010. The drug had a drop in sales during 2013 of 18%, reflecting generic competition in the U.S. and Europe. Requip is part of the company's central nervous system division, and its weak sales contributed to the division's 8% decline for 2013. The drug had constant currency growth of 15% in smaller markets.

During the first quarter of 2014, XenoPort (NASDAQ: XNPT) will announce results of an FDA required, post-marketing study on the use of low doses of Horizant for treatment of moderate to severe RLS in adults. For now, Horizant is losing money; according to CEO Ronald W. Barrett, PhD, the drug is expected to break even and become profitable by the end of 2015. Last year, the company began promoting the drug in 40 geographical areas in the U.S. In these areas, total prescriptions increased 36% and the number of prescribed pills rose 40% for the 13 weeks ending on December 20, 2013. Horizant is XenoPort's (NASDAQ: XNPT) main product, and the company is also working on another drug to treat multiple sclerosis.

My Foolish conclusion
Industry analysis firm GlobalData estimates that the market for RLS drugs could grow 11.5% annually to reach a value of $770 million by 2017. Generic competition and a small number of people seeking treatment could stall the demand of branded RLS drugs.

However, sales of Lyrica may rise if the drug is regarded as a more effective treatment for RLS with more manageable side effects than Mirapex or Requip. This can help boost results at Pfizer (NYSE: PFE), while the company waits to announce 2014 clinical results for several of its mid-stage and late-stage products currently under development.

It's also important to watch how well Horizant is selling and if future trials will test it and other drugs on the market against Lyrica. The drug was omitted from the Pfizer (NYSE: PFE) trial, so that could be a sign that it might be more effective than Lyrica and its competitors at treating RLS. Investors interested in XenoPort (NASDAQ: XNPT) should take note.