LONDON (AP) – Drug maker Shire says it is buying the rare disease biopharmaceutical company ViroPharma in a deal worth $4.2 billion.

Shire PLC will pay $50 a share, a 27 percent premium on last week’s closing price. ViroPharma makes CINRYZE, which treats hereditary angioedema, a disease characterized by recurrent attacks of swelling of the skin or mucous membranes. It can be painful, disfiguring and potentially life-threatening.

Dublin-based Shire says it believes that of the approximately 8,000 patients in the United States with the disease, it is actively managed in only about 3,500 patients.

Shire’s CEO, Dr. Flemming Ornskov, said Monday that the acquisition of ViroPharma, which is based in Exton, Pennsylvania, is consistent with the objective of strengthening the company’s rare disease portfolio and brings a growth driving product to the company.

Investors in Shire welcomed the deal, bidding up its stock 4.2 percent to 117 pence on the London Stock Exchange.