ViroPharma is attracting preliminary interest from several pharmaceutical companies about buying the orphan drugmaker, according to three people familiar with the matter. While ViroPharma is not pursuing a sale, potential buyers are looking at the 9-year-old company because of its pipeline of drugs for rare diseases, said the sources, who requested anonymity because they are not permitted to speak to the media.

Two of the sources said a deal is not imminent for ViroPharma, whose market capitalization is $1.8 billion and which makes a drug to treat swelling caused by a genetic disorder. A ViroPharma spokeswoman declined to comment.

Pharmaceutical companies are increasingly interested in developing or buying orphan drugs that treat rare diseases, as their more traditional products have lost market share to generic competitors. Big Pharma's interest has increased since 2011, when French drugmaker Sanofi bought Genzyme, the first company to show that it could make money from treatments of diseases with small patient populations, for $20.1 billion, one of the sources said.

ViroPharma is one of a handful of orphan drug makers that pharmaceutical companies are interested in acquiring, said the people familiar with the matter. Another orphan drug maker, BioMarin Pharmaceuticals, has also been viewed as an attractive target, but since its shares have surged almost 60% over the past twelve months to a market value of more than $8 billion, it is too expensive for most potential buyers, sources said. A spokeswoman for BioMarin was not immediately available for comment.

ViroPharma would not be a cheap acquisition either. The company's stock is up more than 20% over the past twelve months, trading around $27 per share. It was not clear what price ViroPharma would want.

Last week, ViroPharma said its maribavir drug, which is designed to treat patients who develop a type of herpes virus and have impaired cell mediated immunity, received orphan drug designation in Europe. With the orphan drug designation, ViroPharma has gained ten years of potential market exclusivity if the product is approved for marketing in the European Union and the orphan designation is maintained. The drug already has orphan designation status in the United States.