Clinical setbacks and a rapidly evolving HCV landscape have left AbbVie Inc. less insulated from potential biosimilar competition for blockbuster Humira adalimumab. Now that Shire plc has decided to accept AbbVie's takeout bid, the specialty pharma's rare disease portfolio and pipeline, along with its ADHD franchise, could help fill a potential revenue gap.

AbbVie's fifth offer since May, when the pharma started to privately court the Irish company, values Shire at £31.4 billion ($54.5 billion) and is a 35% premium to the biotech's first bid of £23.6 billion ($39.9 billion). The deal is 46% cash and 54% stock.