Analysts predict top annual revenue for GlaxoSmithKline's Requip could hit $500 million, quite a change for a drug that delivers global revenues around the $200 million range. But GSK's ability to leverage its exclusive hold on the restless legs syndrome market may depend on its ability to obtain additional patents, such as a sustained-release version. The immediate-release formula goes off patent in 2008, noted Stewart Adkins, a Lehman Brothers analyst. At least two other firms are developing RLS treatments, Schwarz Pharma and XenoPort. Requip gained marketing approval for RLS earlier this month, although Rx data suggest a significant amount of offlabel prescribing. A stronger version has been available for Parkinson's disease.