Horizon Pharma (HZNP - Get Report) shares plunged Thursday after the biopharmaceutical company reported that its neuromuscular disorder treatment failed late-stage trials. The drug has already been approved by the U.S. Food and Drug Administration for use in two other rare genetic conditions. The company says today's announcement does not affect its full year 2016 adjusted net sales, though the company has previously said that it expects its rare disease treatments to account for about 60% of its net sales by 2020.