Irish pharmaceutical company Shire said Monday it would buy U.S. drug maker ViroPharma in an effort to strengthen its rare disease treatment portfolio. Shire said it would pay $4.2 billion for outstanding shares of ViroPharma, which comes out to $50 per share, a 27 percent premium above Friday's closing price, The New York Times reported. ViroPharma took in $428 million in net revenue in 2012, most of that from Cinryze, which is made to treat angioedema, a rare blood disease. "The acquisition of ViroPharma will immediately benefit Shire and is entirely consistent with our clear strategic objective of strengthening our rare disease portfolio. It brings us a new growth driving product which augments our already strong growth prospects," Shire Chief Executive Officer Flemming Ornskov said. ViroPharma Chairman and Chief Executive Officer Vincent Milano said the Pennsylvania firm would "benefit from Shire's innovation, scale and global reach." Shire already produces Firazyr, a treatment for angioedema. It also produces attention deficit hyperactivity disorder treatments Vyvanse and Adderall XR.