GlaxoSmithKline (NYSE: GSK) and XenoPort, Inc. (Nasdaq: XNPT) Wednesday received a Complete Response letter from the U.S. Food and Drug Administration (FDA) regarding the New Drug Application (NDA) for Horizant™(gabapentin enacarbil) Extended-Release Tablets, an investigational non-dopaminergic treatment for moderate-to-severe primary Restless Legs Syndrome (RLS)...