There are so many interesting, off-label novel uses for drugs that have already been developed. Recently my wife & I adopted a kitten that developed FIP, a disease caused by a feline coronavirus that was nearly 100% fatal up until just a few years ago.

Turns out, a drug developed by Gilead Sciences (GS-441524) which is the active metabolite of remdesivir, is remarkably effective at treating the disease. It's a nucleoside analog, so it essentially is a slightly modified adenosine molecule which disrupts the virus RNA replication process.

Within 12 hours of the first dose our cat went from dying to making a full turnaround and complete recovery following treatment. For us it was truly a miracle drug, and it only gained awareness recently due to the COVID-19 pandemic [1]. It still isn't technically "approved" for this use case, but is prescribed off-label by vets as of 2024 once the FDA relaxed its position [2]. Folks needed to seek it out on the black market (and still do in some areas).

[1] https://www.theatlantic.com/science/archive/2020/05/remdesiv...

[2] https://www.fda.gov/animal-veterinary/cvm-updates/fda-announ...