The Food and Drug Admistration approved CSL Behring's hemophilia B gene therapy, a one-off infusion that frees patients from regular treatments but costs $3.5 million a dose, making it the most expensive medicine in the world.

CSL Behring's Hemgenix, administered just once, cut the number of bleeding events expected over the course of a year by 54%, a key study of the therapy found. It also freed 94% of patients from time-consuming and costly infusions of Factor IX, which is currently used to control the potentially deadly condition.

"While the price is a little higher than expected, I do think it has a chance of being successful because 1) existing drugs are also very expensive and 2) hemophilia patients constantly live in fear of bleeds," said Brad Loncar, a biotechnology investor and chief executive officer of Loncar Investments. "A gene therapy product will be appealing to some."