A Brief History of GS

For decades, Feline Infectious Peritonitis (FIP) was one of the most devastating diagnoses for cats and effectively represented a death sentence. Caused by a mutation of the feline coronavirus (FCoV), the disease triggers a fatal immune-mediated inflammatory response throughout the cat’s body. Feline infectious peritonitis (FIP) was first identified in 1963 by veterinarians at the Angell Memorial Animal Hospital in Boston (Holzworth, 1963), and FIPVs were discovered to be mutations of the Feline enteric coronavirus (FECV) that develop within the bodies of cats affected by FIP (Vennema et al., 1995; Poland et al., 1996). However, before 2017, no cure for FIP existed and ‘treatment’ consisted of reducing the severity of the symptoms (Izes et al., 2020) and palliative care. Dr. Niels Pedersen, who had joined the faculty of the School of Veterinary Medicine at UC Davis in 1972, had co-authored his first research paper on FIP as far back as 1964, but it wasn’t until 2016-2019 that the first signs emerged of a breakthrough in the treatment of FIP, with reports of antivirals – specifically protease inhibitors and nucleoside analogs (NUCs) – that were able to target the proteins in the FIPV necessary for viral replication. Dr. Pedersen reached out to Gilead Science, the biopharmaceutical company responsible for Tamiflu and other antivirals, and they provided several molecules for testing.

The first drug to be investigated was GC376, an inhibitor of the main protease of FIPVs (Kim et al., 2016; Pedersen et al., 2018), which was able to cure all experimentally infected cats and one third of  cats with naturally occurring effusive and non-effusive FIP, but proved to be less effective for the ocular or neurological variants of FIP (Pedersen et al., 2018).

The second of these drugs was the adenine nucleotide analog GS-441514, the active metabolite of the prodrug GS-5734 (Remdesivir) – later used to treat people with SARS-CoV-2 infections (Murphy et al., 2018; Pedersen et al., 2019; Li et al., 2022; Cosaro et al., 2023). Small trials conducted in 2018 at the Feline Research Laboratory (FRL) at UC Davis showed that GS-441514 could not only extend a cat’s life but could actually save it. It was also able to cure not only all of the experimentally infected cats (Murphy et al., 2018) but also most of the cats with naturally occurring wet and dry FIP (Pedersen et al., 2019), and at higher dosages. also showed potential in the fight against ocular and neurological FIP (Pedersen et al., 2019) with an overall cure rate of just over 90% (Jones et al., 2021).

Despite this startling breakthrough, however, Gilead refused to license the drug for veterinary use. Although the initial experimental and field studies of GS-441524 had been a collaboration between researchers at Gilead Sciences and UC Davis, with the onset of the COVID-19 pandemic in 2019, Gilead Sciences refused to grant animal rights for GS-441524. Dr. Pedersen (2020) explained that due to the close relationship of Remdesivir (GS-5734) to GS-441524, Gilead was concerned this could interfere with the development of Remdesivir for human use.

The publication of the first field trial results (Pedersen et al., 2019) led almost immediately to pressure from cat owners and rescues. They quickly created a market and a treatment network to avail themselves of the unapproved GS-441524 produced by Chinese pharma manufacturers, who had immediately responded to this new opportunity. These developments for the most part excluded vets, most of whom chose to wait for the drug to be legalized (Jones et al. 2021). The result was the swift transition of GS-441524 from the laboratory to a treatment for FIP, via a rapidly expanding worldwide network of Facebook and other social media-based support groups.

By 2016, the Bova Group (Bova Aus and Bova UK), established in 1968 as a human pharmacy, had transitioned to veterinary compounding and had become the largest veterinary compounder in Australia, before expanding operations to the UK in 2017 (Stokes pharmacy & Bova group partner, n.d.). In 2021, Bova launched its GS-441524 formulations in Australia and the UK and ran multiple clinical trials using their formulation for GS tablets (Stokes pharmacy & Bova group: a partnership transforming the landscape, n.d.). Although GS-441524 had been initially developed as an injectable drug, the transition to an oral formulation represented a crucial advancement in making FIP treatment a more feasible option for most cat owners (FIP treatment GS-441524 – now available in the U.S., 2024).

Canada Takes Center Stage

The next chapter in the FIP story unfolded in Canada. In 2022, Dr. Jeff Aramini designed and conducted a groundbreaking case series study in Canada, using research-quality oral medications, compounded at a pharmacy in Ontario. Overseen by a Licensed Ontario DVM, this study aimed to make FIP treatment less painful, more affordable, and more accessible. By April 2025, Dr. Aramini had treated over 600 FIP cases (Jeff Aramini, 2025).

On February 9, 2024, Dr. Scott Weese, also based in Ontario, Canada, made the exciting announcement that they had been able to get Health Canada’s Veterinary Drugs Directorate (VDD) approval to import legally compounded GS-441524 and remdesivir from the UK
(Weese, 2024). By November 20, 2024, the Canadian Ontario-based Trutina Pharmacy was producing pharmaceutical-grade GS-441524 legally, in collaboration with BOVA UK.
(Weese, 2024).

United States Progress

In May 2024, Stokes Pharmacy in the US announced that, in partnership with Bova UK, compounded GS-441524 would be available June 1, 2024, by veterinary prescription for individual patients and, in certain states, for veterinary office use (Stokes pharmacy & Bova group partner, n.d.). The FDA’s Center for Veterinary Medicine (CVM) notified veterinarians that compounded GS-441524 falls under Guidance for Industry #256 and could be legally compounded and prescribed with a veterinary prescription.

The Evolving Landscape

In the meantime, the FIP treatment environment continues to evolve rapidly. Ever more pharmacies are entering the market for compounded FIP treatment, driving down costs, increasing availability, and expanding treatment options – in particular for GS-441524 in the United States. Canada also has seen significant progress since March 2025, with the arrival of Clearpoint Pharmacy, followed shortly thereafter by Summit and Create Pharmacies, alongside rapidly expanding educational opportunities for veterinary treatment providers.

As we are now nearing the end of 2025, we look forward to more research and even greater awareness, access and affordability of FIP treatment In Canada and worldwide! However, much still remains to be done before we can truly state with confidence that “FIP is no longer a death sentence!” 

References:

Cosaro, E., Pires, J., Castillo, D., Murphy, B. G., & Reagan, K. L. (2023). Efficacy of Oral Remdesivir Compared to GS-441524 for Treatment of Cats with Naturally Occurring Effusive Feline Infectious Peritonitis: A Blinded, Non-Inferiority Study. Viruses, 15(8), 1680. https://doi.org/10.3390/v15081680

FIP treatment GS-441524 – now available in the U.S. (2024). https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/fip-treatment-gs-441524-now-available-us 

Holzworth, J. (1963). Some important disorders of cats. Cornell Veterinarian 53, 157–160. 

Izes, A.M., Yu, J., Norris, J.M., Govendir, M. (2020). Current status on treatment options for feline infectious peritonitis and SARS-CoV-2 positive cats. Veterinary Quarterly 40, 322–330.

Jeff Aramini – Pet Nation (2025). https://petnation.care/team/jeff-aramini

Jones, S., Novicoff, W., Nadeau, J., Evans, S. (2021). Unlicensed GS-441524-like antiviral therapy can be effective for at-home treatment of feline infectious peritonitis. Animals 11, 2257. 

Kim, Y., Liu, H., Galasiti Kankanamalage, A.C., Weerasekara, S., Hua, D.H., Groutas, W.C., Chang, K.O., Pedersen, N.C. (2016). Reversal of the progression of fatal coronavirus Infection in cats by a broad-spectrum coronavirus protease inhibitor. PLoS Pathogens 12:e1005531. 

Li, Y., Cao, L., Li, G., Cong, F., Li, Y., Sun, J., Luo, Y., Chen, G., Li, G., Wang, P., Xing, F., Ji, Y., Zhao, J., Zhang, Y., Guo, D., & Zhang, X. (2022). Remdesivir Metabolite GS-441524 Effectively Inhibits SARS-CoV-2 Infection in Mouse Models. Journal of medicinal chemistry, 65(4), 2785–2793. https://doi.org/10.1021/acs.jmedchem.0c01929

Murphy, B.G., Perron, M., Murakami, E., Bauer, K., Park, Y., Eckstrand, C., Liepnieks, M., Pedersen, N.C. (2018). The nucleoside analog GS-441524 strongly inhibits feline infectious peritonitis (FIP) virus in tissue culture and experimental cat infection studies. Veterinary Microbiology 219, 226-233.

Pedersen, N.C., Kim, Y., Liu, H., Galasiti Kankanamalage, A.C., Eckstrand, C., Groutas, W.C., Bannasch, M., Meadows, J.M., Chang, K.O. (2018). Efficacy of a 3C-like protease inhibitor in treating various forms of acquired feline infectious peritonitis. Journal of Feline Medicine and Surgery 20, 378-392. 

Pedersen, N.C., Perron, M., Bannasch, M., Montgomery, E., Murakami, E., Liepnieks, M., Liu, H. (2019). Efficacy and safety of the nucleoside analog GS-441524 for treatment of cats with naturally occurring feline infectious peritonitis. Journal of Feline Medicine and Surgery 21, 271-281. 

Pedersen, N. (2020). Dr. Pedersen GS-441524 update. https://www.sockfip.org/2020-dr-pedersen-gs-441524-update/ 

Poland, A.M., Vennema, H., Foley, J.E., Pedersen, N.C. (1996). Two related strains of feline infectious peritonitis virus isolated from immunocompromised cats infected with the feline enteric coronavirus. Journal of Clinical Microbiology 34, 3180-3184.

Stokes pharmacy & Bova group: a partnership transforming the landscape of veterinary medicine (n.d.). https://www.stokespharmacy.com/stokes-pharmacy-bova-group-partnership-in-veterinary-medicine/#:~:text=How%20did%20the%20partnership%20with,it%20meets%20the%20Bova%20specifications

Stokes pharmacy & Bova group partner to bring oral treatment for Feline Infectious Peritonitis to U.S. (n.d.). https://www.stokespharmacy.com/stokes-bova-partner-for-feline-infectious-peritonitis-treatment/#:~:text=More%20About%20Bova:,vet 

Vennema, H., Poland, A., Foley. J/, Pedersen. N.C. (1995). Feline infectious peritonitis viruses arise by mutation from endemic feline enteric coronaviruses. Virology 243, 150-157.

Weese, S. (2024, February 9). Antiviral resistance in cats, Part 2: GS-441524. Worms & germs blog. https://wormsandgermsblog.com/2025/04/articles/animals/cats/antiviral-stewardship-in-cats-pt2-gs-441524/

Weese, S. (2024, November 20). Antiviral resistance in cats, Part 2: GS-441524. Worms & germs blog. https://wormsandgermsblog.com/2025/04/articles/animals/cats/antiviral-stewardship-in-cats-pt2-gs-441524/