Approval of Alexion Pharmaceutical Inc.'s Soliris (eculizumab) injection to treat neuromyelitis optica spectrum disorder (NMOSD) in adult patients who are anti-aquaporin-4 (AQP4) antibody positive makes it the first and only FDA-approved treatment. It's also one of the most expensive treatments in the world, making it vulnerable to off-label use and the eventual creep of biosimilars into the market.