'How can we make sense of the fact that animal studies and early clinical trials seem to show promise, yet there is very limited official approval for these therapies? There are 2 possible explanations: One scenario is that the strict requirements of RCTs and regulatory approval are causing many potentially valuable treatments to be left behind. The other scenario is that both animal studies and early clinical trials may have limitations in their design, such as a lack of proper randomization and blinding, which affects their internal validity'

https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3002667#sec027