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What GS-441524 is and why it is studied for FIP
FIP is a viral disease of cats that has historically been fatal once it develops. It is often described as effusive (“wet”) or non-effusive (“dry”), and the form of the disease matters both for treatment decisions and for how far study results can be applied. GS-441524 is the compound that has been tested against FIP in cats in the studies below. It does not have FDA approval for this use.
What the clinical studies show
Three reports are most relevant. They differ in population, schedule, and design, so their results should not be combined into a single success rate.
| Study | Cats | Route and schedule | Duration | Reported result | Main limit |
|---|---|---|---|---|---|
| Pedersen et al., 2019 (naturally occurring FIP) | 31 enrolled | Subcutaneous, every 24 hours; 2 mg/kg initially, increased to 4 mg/kg when indicated | At least 12 weeks | Of 26 cats completing the planned course, 18 remained healthy after one treatment round at publication; 8 relapsed within 3–84 days | Outcomes of one cohort and study period, not a population cure rate |
| 2024 study comparing GS-441524 with molnupiravir | Number not stated in the cited listing | Not stated | Not stated | Reported as similarly effective to molnupiravir; laboratory parameters normalized within 6–7 weeks (study authors, 2024) | Limited to that study’s population and conditions; not evidence that the drugs are interchangeable |
| 2026 randomized, double-blind non-inferiority trial (trial listing) | 20 cats with non-effusive FIP: 10 given oral remdesivir, 10 given oral GS-441524 | Oral, every 24 hours | 84 days | Results not stated in the trial listing | Small; restricted to non-effusive disease |
The 2019 naturally occurring FIP study
Pedersen and colleagues treated cats with FIP that had arisen naturally rather than through experimental infection, which is closer to the situation most owners face. Four severely ill cats died or were euthanized within two to five days of starting treatment, and a fifth died or was euthanized after 26 days. The eight relapses shown in the table are the reason follow-up matters: a cat that appears to respond still needs monitoring after the course is complete. The authors also described the study as one of safety and efficacy, which is why its regimen is informative even though it cannot be copied directly.
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The 2024 and 2026 comparisons
The 2024 study placed GS-441524 alongside molnupiravir, another antiviral. Its laboratory normalization timeline describes recovery in that group of cats, not how long any individual cat should be treated. The 2026 trial used a non-inferiority design, which asks whether one drug is not meaningfully worse than another. With 20 cats, all with non-effusive disease, it cannot show that the two drugs work the same way in a particular cat, especially one with effusive FIP.
Published doses: historical protocols, not prescriptions
Dose questions are the most common reason people search this topic, so it helps to see exactly what the published protocols were and what they were not.
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- Initial protocol in the 2019 study (Pedersen et al.): 2 mg/kg subcutaneously every 24 hours for at least 12 weeks.
- Escalation in the same study: increase to 4 mg/kg subcutaneously every 24 hours when indicated.
- The authors’ conclusion in the full text: they considered the 4 mg/kg regimen more effective in their observations and suggested it as a basis for future treatment. This is a historical conclusion from one study.
- 2024 and 2026 studies: the dose is not stated in the summaries available. The 2026 trial used oral dosing every 24 hours for 84 days.
Do not combine these protocols, calculate a dose from your cat’s body weight, or assume that a route or duration from one study applies to another form of disease.
Why your veterinarian sets the dose
- Disease form. Effusive and non-effusive FIP present differently, and the 2026 trial covered only non-effusive cats.
- Clinical response. The iCatCare update states that dosage should be adjusted according to clinical response.
- Setting. The iCatCare update notes that intensive hospital care during the first week of a study may have contributed to that study’s success. Trial conditions are not automatically reproduced at home.
- Guideline context. The 2025 ABCD (Advisory Board on Cat Diseases) guideline frames dosing within the clinical context of each cat rather than as a fixed number.
Side effects and monitoring
This article does not list specific side effects. The 2019 study was designed to assess safety as well as efficacy, but the summaries available for this article do not include a full adverse-event breakdown, and a complete, current list of adverse effects for GS-441524 in cats has not been confirmed here. A list compiled from memory could be wrong, so the practical step is to ask your veterinarian what to watch for in your cat specifically. Treatment should be prescribed and monitored by a veterinarian who can assess response and any adverse signs in light of the cat’s disease and the formulation used.
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Useful questions to ask at the start of treatment:
- Which signs of an adverse reaction should prompt a call, and how quickly?
- Which examinations and laboratory tests will be repeated, and how often?
- Which signs of relapse should you watch for after treatment ends?
- Who covers urgent problems outside clinic hours?
US status: compounded, not FDA-approved
The FDA Center for Veterinary Medicine issued the following statement on May 10, 2024:
“Because no FDA approved drug is available to treat Feline Infectious Peritonitis in cats, the U.S. Food and Drug Administration 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 Feline Infectious Peritonitis under the conditions listed in Guidance for Industry (GFI) #256 Compounding Animal Drugs from Bulk Drug Substances.”
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What enforcement discretion does and does not mean
The statement describes a policy about which compounded products the agency will not pursue under approval requirements, and only when every condition is met: a veterinarian’s prescription, for a specific cat, for FIP, under the conditions in GFI #256. It does not make the product FDA-approved, and compounded GS-441524 should not be described that way. The notice also states that compounded products made from bulk drug substances are unapproved drugs. Quality, labeling, and dosing of any compounded product depend on the pharmacy that makes it, so ask for that information directly.
Outside the United States
The FDA statement covers US compounding only. The legal status of GS-441524 in other countries is not established here. Check with your national veterinary regulator and a local veterinarian.
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Access: compounded oral products and what to verify
The Cornell Feline Health Center reported that Stokes Pharmacy, working with the Bova Group, announced a compounded oral GS-441524 formulation for FIP treatment in the US in 2024. That is a dated announcement. It does not confirm current stock, current prescribing steps, or any ongoing partnership between the pharmacy and another company.
Quick Recap
- Ask your veterinarian whether a compounded GS-441524 product is appropriate for your cat’s form of FIP.
- Ask which pharmacy they would use and whether that pharmacy is currently making the oral formulation.
- Confirm that the prescription is written for your cat and that the pharmacy can supply your location.
- Do not buy from unverified online drug vendors.
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