LONDON — The drug maker Shire PLC, seeking to strengthen its portfolio of treatments for rare diseases, said Monday that it would buy ViroPharma Inc. for about $4.2 billion in cash.
The Dublin-based drug maker would pay $50 for each share of ViroPharma, a bio-pharmaceutical company based in Exton, Pa. That represents a 27 percent premium on the company’s closing price Friday, the last trading day before the deal was announced.
Shares of ViroPharma soared 25 percent to close Monday at $49.42.
ViroPharma focuses on serious diseases with few, if any, available therapies.
Its products include Cinryze, which is used to prevent and treat attacks of hereditary angioedema, a rare genetic disorder that can cause dangerous swelling of the throat or larynx.
It also makes the seizure drug Buccolam and the antibiotic Vancocin.
The company generated $428 million in worldwide revenue last year.
ViroPharma said last month that US sales of Cinryze, which account for most of its revenue, climbed 22 percent in the third quarter to $102.2 million. It expects North American sales for the treatment to total $405 million this year.
Shire said Cinryze will act as a complement to one of its drugs, Firazyr, which treats acute hereditary angioedema attacks.
ViroPharma also is developing maribavir, a potential treatment for a virus that can lead to serious disease or death in people with compromised immune systems.
The experimental drug has received an orphan drug designation from both European and US regulators. That could lead to market exclusivity if the drug is approved.
The companies expect the deal to close by the end of this year or soon after.