Shares of rare-disease specialist United Therapeutics (NASDAQ: UTHR) dropped nearly 11% today after the company announced that a phase 3 trial evaluating Orenitram in patients with pulmonary arterial hypertension, or PAH, will continue through completion after an independent interim analysis was conducted. That may sound like an odd reason to send shares lower, but investors were hoping that the trial would be stopped early had the drug shown overwhelming efficacy.