Alexion Pharmaceuticals Inc. has agreed to buy Synageva BioPharma Corp. in a cash-and-stock deal valued at $8.4 billion, a move that will strengthen Alexion's pipeline of rare-disease treatments. shareholder will receive $115 in cash and 0.6581 Alexion share, for a total consideration of about $225.92 a share based on Alexion's After the deal, Alexion will have eight treatments in clinical trials, including Synageva's experimental enzyme-replacement therapy for patients with mucopolysaccharidosis IIIB, a rare metabolic disease.