It has to be a substantial improvement over SOC on one or more clinically significant endpoints so I doubt that safety would be a decider, but I could be wrong.

And as to 2MartiniGi's post. You can't get a rolling review for BTD but here is a case of a drug that was denied at first but got BTD a year+ later:

Quote:

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Ariad’s initial BTD application for AP26113 in indication of anaplastic lymphoma
kinase (ALK)-positive NSCLC was denied in August 2013. This initial BTD request was based on an analysis of 24 evaluable patients in the phase I segment of a multi-center Phase I/II trial....

The FDA denied this initial BTD request due to the “relatively short follow-up” and small patient population in its trial. But one year later in October 2014, Ariad submitted additional clinical data from phase II segment of the same phase I/II trial.

The request on BTD status was based on an analysis of 72 evaluable patients.... Based on those additional submitted clinical data the FDA finally granted BTD status for AP26113.

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