Thanks for the link, golfyeti, which I was familiar with. But the point is that BT status is not a form of approval, it’s a recognition that a drug is looking promising for an important condition on the basis of preliminary data, and merits expedited development with the FDA’s support. See:

https://www.fda.gov/regulatory-information/food-and-drug-administration-safety-and-innovation-act-fdasia/frequently-asked-questions-breakthrough-therapies