Since its discovery in the late 1950's, FIP was a dreaded diagnosis -- a death sentence with no effective treatment. In 2018, a study published by Pedersen et al. shone a ray of hope that FIP could successfully be treated, using a 3CL protease inhibitor antiviral known as GC376. The following year The following year a second study from Pederson demonstrated that a nucleoside analogue known as GS-441524 was also effective at treating FIP.
Despite the fact that these drugs were not available through veterinarians, desperate cat owners found ways to source the antiviral compounds, treating their cats with or without veterinary assistance.
Luckily, more recently, new anti-viral options which are effective against FIP have emerged, and approval processes have been expedited due to expedited due to their effectiveness against COVID19 for humans, and compounding pharmacies have made these life-saving drugs available in many countries.
Approval Status
GS-441524
GS-441524 is currently the most commonly used antiviral to treat FIP. Unfortunately, the license holder, Gilead Sciences Inc. has so far declined to license or develop this molecule, limiting the ability of vets to legally prescribe it in many countries. It is however available through compounding pharmacies in many countries, including the US.
Remdesivir/GS-5734
Remdesivir/GS-5734 is a pro-drug of GS-441524. Ongoing research done in Australia and the United Kingdom has shown that it can be an effective treatment for FIP, similar to GS-441524. While Remdesivir has not been specifically approved for veterinary applications, it has been approved or authorized for emergency use to treat COVID‑19 in around 50 countries, including the US, UK, and Australia, thus making it legal for off-label prescription use, although not always easily obtainable.
GC376
GC376 is a 3CL protease inhibitor and was the first antiviral shown to be effective at treating FIP. This drug is being developed for the veterinary market by Anivive Lifesciences, and is currently estimated at being about 2 years away from FDA approval.
Early efforts by cat owners to source antiviral drugs on their own made use of GC376, however GS-441524 quickly supplanted it. Currently it is mostly used as an adjunct to GS-441524 for cats with virus strains that have developed viral resistance to GS-441524.
Molnupiravir/EIDD-2801/EIDD-1931
Molnupiravir is a nucleoside analogue which was developed for treatment of COVID19 in humans. It is approved for conditional or emergency use in over a dozen countries, including the US and UK, thus making it legal for off-label prescription use, although not always easily obtainable.
Compounded Molnupiravir is available in the US through compounding pharmacies.