My small sample size of experience with the FDA: they seem to be very receptive to novel therapies for terminal illness and it's surprisingly straight-forward to get a Phase I trial approved.

There's no static rule uniformly enforcing the initiation of trials, rather your IND (investigational new drug) application is reviewed by experts in the relevant field to determine whether (1) is there a plausible reason to believe your new therapy helps and (2) is it unlikely to kill or severely injure patients.

I can't think of a better way for things to work for late-stage cancer or other untreatable terminal illnesses.