AstraZeneca is attempting to fend off impending generic competition to its best-selling drug, the anti-cholesterol drug rosuvastatin calcium (Crestor), by getting it approved to treat a rare disease characterized by ultrahigh levels of bad cholesterol. In an unusual legal argument, the company says the drug is entitled to 7 years of additional market exclusivity under the Orphan Drug Act, a 3-decade-old law that encourages pharmaceutical companies to develop treatments for rare diseases. Critics say AstraZeneca is trying to abuse the law, since the overwhelming use of rosuvastatin calcium is for treating adults with high cholesterol, not children with the rare disease. The drug, a cholesterol-lowering statin, was prescribed 20.3 million times in the United States last year, the second-most of any brand-name drug, according to the prescription tracker IMS Health. It is scheduled to lose patent protection on July 8, potentially exposing it to an onslaught of generic competition. Michele Meixell, a spokeswoman for AstraZeneca, says the company pursued the new use of the drug as "part of our standard practice to address unmet needs." She says the Orphan Drug Act "provides exclusivity as an incentive for pharmaceutical companies to develop products to help address the needs of an underserved patient population."