Alexion Pharmaceuticals Inc. has demonstrated the commercial potential for rare disease drugs with its complement inhibitor Soliris (eculizumab) and long-lasting follow-up Ultomiris (ravulizumab) driving blockbuster sales. A host of other companies are hoping to compete with Ultomiris, which is U.S. FDA-approved for paroxysmal nocturnal hemoglobinuria and atypical hemolytic uremic syndrome.