The proposed $4.2 billion buyout of ViroPharma Inc. fits Shire plc's long-standing growth through acquisition model, but the new CEO also wants to focus on internal R&D in rare diseases. This deal is an incremental step to boost rare disease revenues while the internal pipeline matures.

On Nov. 11, Shire said it would buy ViroPharma for $50 per share primarily to gain Cinryze, a complement 1 (C1) esterase inhibitor that is marketed in the U.S. and six European countries to prevent hereditary angioedema (HAE) attacks.