Astrazeneca plc’s Alexion rare diseases unit has taken an option to buy all remaining equity in Caelum Biosciences Inc., adding a rare disease drug to its pipeline in a deal worth $500 million. At the center of the deal is CAEL-101, a potentially first-in-class monoclonal antibody designed to tackle the toxic amyloid deposits that build up in organs of people with the rare disease light chain amyloidosis.