GlaxoSmithKline ($GSK) is one step closer to further bolstering its respiratory lineup. The British pharma giant says it's nailed the primary endpoint in a Phase III study of its injectable candidate mepolizumab, which will help set the severe eosinophilic asthma treatment up for its first regulatory filings. Last year, GSK nabbed the FDA's blessing for two potential respiratory blockbusters, Anoro and Breo. But sales of the pair won't touch Advair's $8.25 billion-a-year haul, and the off-patent behemoth has recently seen its market share challenged by both branded and generic rivals. More