The Exton-based drug ViroPharma has been sold to Shire Pharmaceuticals for $4.2 billion. The sale is part of a larger trend in the Philadelphia region.

Shire, an Irish company, already has a big presence in Chester County. It's known for producing a medication for attention deficit and hyperactivity disorder, but its European sales have dropped.

That's spurring Shire to acquire new drugs by purchasing smaller companies, according to Alfred Wertheimer, a Temple University professor of pharmacy economics.

"I think it's part of a trend. The pharma companies are feeling pressure -- as you know the pipelines haven't been as robust lately as they have sometime in the past," Wertheimer said. "And even when somebody has a new drug candidate, it still takes some years to get it through the FDA.

"In order to boost sales and earnings, you buy somebody smaller. (Large) companies have cash and borrowing ability," he said. "I see this as part of a national and continuing trend of modest-sized companies being swallowed up."

ViroPharma's drug Cinryze treats the rare immune disorder hereditary angioedema, which involves sudden inflammation. It also has the antibiotic Vancocin that is now hitting the generic market.