Less than two months after announcing its U.S. headquarters will be in Massachusetts, Shire Pharmaceuticals is buying a New Jersey-based maker of gastrointestinal drugs, NPS Pharma for $5.2 billion. The purchase price represents a 51 percent premium to the share price of NPS when reports of the pending deal first emerged on Dec. 16, and is 9 percent higher than it's stock as of Friday's close.