London-listed Shire is buying ViroPharma for $4.2 billion its biggest deal yet to strengthen its portfolio of lucrative drugs to treat rare diseases, which are attracting increasing attention from drugs companies as patents expire on their older treatments.

Several companies, including France's Sanofi according to reports, were interested in ViroPharma, which makes Cinryze for the treatment of immune disorder hereditary angioedema.

Treatments for such rare disorders can command huge prices, running into hundreds of thousands of dollars a year per patient.

Shire, which latched onto the rare disease drugs trend years before bigger rivals, said yesterday it had agreed to pay $50 a share in cash for ViroPharma, a 27 per cent premium on Friday's closing price. – (Reuters)