Ultra-rarified heir

Alexion Pharmaceuticals Inc. will have to wait three years to find out whether its purchase of Enobia Pharma Inc. for $610 million up front will provide a Soliris-like winner in ultra-Orphan metabolic bone diseases. Alexion hopes to launch Enobia's asfotase alfa to treat hypophosphatasia in late 2014.

Since early 2009, Alexion's market cap has tripled from just over $4 billion to more than $13 billion. The driver has been Soliris, a humanized mAb targeting complement 5 (C5) that is approved to treat a pair of ultra-Orphan indications -paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS) - where there are no therapeutic alternatives...