LONDON (AFP) - British drugs group Shire has agreed to buy United States-based rare disease specialist ViroPharma for about US$4.2 billion (S$5.2 billion) in cash.

The board of directors of the two firms "unanimously" approved the tie-up for a "total consideration of approximately US$4.2 billion". the pair said in a statement. The cash deal was pitched at US$50 per ViroPharma share, a premium of 27 per cent compared with the closing level on Friday.

Pennsylvania-based ViroPharma, which is listed in New York, is an antiviral pharmaceutical group whose leading product is Cinryze. The drug is used in the treatment of hereditary angioedema (HAE), a rare genetic inflammatory disorder.

Shire, which is listed on the London stock market but based in Ireland, already produces Firazyr, a drug used to treat acute attacks of hereditary angioedema.

"The acquisition of ViroPharma will immediately benefit Shire and is entirely consistent with our clear strategic objective of strengthening our rare disease portfolio," said Shire chief executive Flemming Ornskov. "It brings us a new growth-driving product which augments our already strong growth prospects."

He added: "Shire is also excited by the prospect of being able to offer two complementary treatments, Firazyr for the treatment of acute HAE attacks and Cinryze for prophylactic treatment of patients suffering from HAE."

Shire predicted the deal would generate annual cost savings of US$150 million by 2015.

ViroPharma had total worldwide net revenues of US$428 million last year. The group forecasts that revenues will climb to between US$445 million and US$465 million this year.