FIP was an untreatable, fatal disease until the effects of GS-441524, a novel antiviral compound, were discovered.1 GS-441524 is the metabolite of remdesivir, an injectable antiviral medication originally developed for use in humans. Feline licensing was not pursued by the manufacturer, and the company elected not to sell the product to clinicians, resulting in some cat owners making purchases, typically via the internet, of unlicensed oral and injectable supplements containing remdesivir or GS-441524.2 Weighing a potential cure for a serious disease against the illegality of procuring, dispensing, or administering unregistered drugs posed an ethical quandary.
Is remdesivir or GS-441524 now an option?
In the United Kingdom and Australia, compounded remdesivir and GS-441524 have been legally available for treatment of FIP.3 In the United States, remdesivir received conditional approval for use in humans with coronavirus disease (COVID-19) during the pandemic, but usage was not extended to cats. Clinicians may now prescribe extra-label remdesivir for cats, but it can be difficult to obtain.4 Compounded GS-441524 became available for use in cats in June 2024. Although drugs compounded from bulk substances are unapproved and not strictly legal,5 the FDA states, “The FDA does not intend to enforce new animal drug approval requirements for products compounded from GS-441524, when prescribed by a veterinarian for a specific cat patient for the treatment of FIP for the conditions listed in Guidance for Industry (GFI #256) regarding animal drug compounding.”
Unlicensed remdesivir and GS-441524 products are likely to remain available via the internet; these products may vary in cost, safety, efficacy, and quality.
Ask the Expert: When can remdesivir and GS-441524 be used in cats with FIP, and what should pet owners understand?
What is the evidence?
With various unlicensed forms of GS-441524, most cats exhibit a rapid response to treatment, with few significant adverse effects and overall survival rates of 77% to 100%.2,6,7 Content and quality of GS-441524, method of FIP confirmation, cat populations, length of follow-up, and definition of treatment success have varied in reports, making evaluation and comparison of responses difficult. In one study, 14 cats given an oral form of GS-441524 (and available for follow-up) did not relapse in the year following treatment.8 Two cats developed transient hyperesthesia, and a majority had abdominal lymphadenomegaly following treatment, but it was unclear whether this was related to FIP or therapy. Further study is needed to determine whether transient hyperesthesia and abdominal lymphadenomegaly may be a form of a long FIP syndrome or an adverse drug effect.
A retrospective report of 307 cats given remdesivir alone, oral GS-441524 alone, or remdesivir followed by oral GS-441524 showed an 88.6% survival rate, with 88.4% (median, 180 days; range 0-730 days) overall survival.9 The initial treatment period was 1 to 330 days (median, 84 days). Cats that achieved complete response within 30 days were more likely to survive until the conclusion of the initial treatment period. Variable protocols made it difficult to compare treatment regimens.
Is there a recommended treatment protocol?
Treatment typically consists of oral or injectable administration (or a combination thereof) daily for 12 weeks. In a large retrospective study, signs relapsed in ≈10% of cats during or following treatment (most within 60 days following treatment).9 Some of these cats responded to repeated treatment with the same or an increased dose. Cats with neurologic and ocular forms of FIP appear to require larger doses,10 as vasculitis associated with FIP results in permeability of the blood–brain and blood–ocular barriers, and the medication must penetrate these organs as treatment progresses and barriers heal. A recent report suggested a 42-day protocol may be as effective as the 84-day treatment protocol.11 Adverse effects are mostly benign, with injection site pain being most common10; however, GS-441524 uroliths have been recovered from 3 cats treated for FIP.12
Supportive care is important for cats with FIP, especially in the early phase of treatment. These cats may benefit from hydration, appetite support, and pain control. In some cases, abdominocentesis may be performed to relieve pressure; monitoring of fluid and protein parameters is recommended. In addition, monitoring blood, urine, and imaging parameters can inform care, especially if treated cats may be prone to urolith formation. Dose adjustments are often necessary as cats recover and gain weight.
How should pet owners be educated?
Owners of cats with FIP should be educated about the diagnosis and available treatment options, including treatment duration, cost, and potential adverse effects. Veterinary compounded products are preferable to unlicensed products due to superior quality control, consistency, and documentation. Unlicensed products may vary significantly from amounts specified on product labels.13
Conclusion
FIP treatment should continue to rapidly evolve as novel antiviral drugs become more widely available. Treatment protocols may develop and change, and adverse effects may become clearer. Long-term effects of treatment and disease will need to be studied.