Alexion's ($ALXN) Soliris is arguably the most expensive drug in the world. The rare-disease therapy fights two life-threatening diseases--and it costs up to $400,000 per year, depending upon the patient. And if the company has its way, the drug may be targeted at another group of patients. In a Phase II study, Soliris reduced the frequency of attacks in patients with relapsing neuromyelitis optica, a serious neurological disorder that worsens with each relapse. The 14-patient trial met that primary goal "with high degrees of clinical and statistical significance," Alexion said in a statement. Story