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. Anecdotally, thousands of cats have been treated successfully using antiviral compounds, and "citizen scientists" have assisted in collecting data to help researchers evaluate the efficacy of the treatment.
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.
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. Two exceptions are the United Kingdom and Australia, where, a veterinary specials manufacturer has made it available in tablet form as an unlicensed special preparation.
Although GS-441524 cannot be legally obtained through veterinary channels in most countries, an activist community of cat owners has emerged to help cat owners acquire the antiviral on their own.
Per a VIN article from 2019, the FDA's enforcement is primarily interested in parties that sell unapproved drugs, not end users.
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 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 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.
Although no formal study of its efficacy has been published, several trials are underway, and it has begun circulating amongst cat owner groups for treatment of cats with virus strains that have developed viral resistance to GS-441524.
Unlike GS and GC, it is teratogenic and should not be used in pregnant or breeding queens.
Veterinary Involvement and Treatment with Unapproved Compounds
Veterinary supportive care and monitoring can be critical to the success of antiviral treatment, and it is in the best interest of the patient that they have an established relationship with a veterinarian who is familiar with their case and with the treatment protocol. The non-approved status of several of the anti-viral compounds that are effective to treat FIP as well as the fact that some owners source the drugs themselves does create some difficulties for vets.
Legally, veterinarians cannot prescribe or dispense non-approved medications, however they can support and advise the owner in order to help them act in the best interests of their cat. For example, veterinarians can:
provide supportive care
monitor and assess the cat during treatment
demonstrate proper injection or pill administration technique
counsel the owner and recommend reliable sources of information
Every veterinarian must determine their own comfort level as to how to engage with owners who are pursuing un-approved treatment.