🎯 Why is mastering the details of treatment so important?
A cat receiving GS-441524 is not guaranteed to be cured. Incorrect dosing, poor monitoring or failure to adjust the dose as weight increases are among the main causes of treatment failure. This guide brings together the most up-to-date knowledge so that veterinarians and owners can make the best decisions at every stage of treatment.
- 💊 Updated protocol: International Cat Care has revised and updated its doses based on clinical experience
- ⚖️ Mandatory weight-based adjustment: Failing to increase the dose as weight increases is the main cause of failure
- 🔬 Monthly monitoring: Full blood work every month during the 3 months of treatment
- 🔄 Relapses are treatable: With higher doses and an additional 12 weeks, cure is still possible
Table of Contents
1. Dosing: oral vs. injectable and updated protocols
Oral or injectable? The key initial decision
The first field studies with GS-441524 were conducted using subcutaneous injections. Today, available data show that the oral route is equally effective for most stable patients, although it has some characteristics the clinician should be aware of.
✅ Advantages of the oral route
- Greater ease of administration for the owner
- Eliminates pain at the injection site
- No risk of abscesses, seromas or skin wounds
- No risk of sarcoma development at the inoculation site
⚠️ Limitations of the oral route
- Delayed onset of effect
- Highly variable absorption (estimated at ~50% according to International Cat Care)
- Not recommended if pre-existing gastrointestinal disorders are present
- Not indicated in extreme cases where rapid action is required
⚠️ When to ALWAYS start with injectable remdesivir
Current ICatCare guidelines recommend initiating with injectable remdesivir during the first 48 hours to stabilise the patient in these three scenarios, before switching to the oral form:
- Cats with neurological involvement that prevents normal swallowing
- Cats with severe dehydration or very advanced disease
- Cats that, for any other reason, cannot be medicated orally
🔬 What do we know about oral pharmacokinetics?
There is currently no published in vivo pharmacokinetic study specific to the feline species that confirms the exact absorption level of oral formulations. Absorption is estimated at around 50% based on International Cat Care's clinical experience.
In 2023, a study on the pharmacokinetics of oral GS-441524 in laboratory cats was published. The University of Edinburgh conducts Therapeutic Drug Monitoring (TDM) trials open to interested veterinarians. There are also publications on pharmacokinetics in other species that provide useful reference information.
Updated International Cat Care protocol
International Cat Care established its first protocol in 2021. After adjusting doses based on accumulated experience and TDM studies, it has progressively updated its recommendations. It was observed that 85% of cats responded to initial doses, but 15% required adjustments due to variations in absorption. Faced with this challenge, it was decided to increase baseline doses to achieve a higher response rate.
The most significant changes compared to previous protocols are:
- GS dose split into two administrations (every 12 h) to optimise serum levels
- The use of high doses can resolve poor absorption issues and improve penetration of the blood-brain and ocular barriers
- Dose adjustment based on response and TDM when necessary
The current recommendation favours the use of the oral form from the start of treatment in stable cats that do not present any of the three exclusion criteria mentioned above.
⚠️ CRITICAL WARNING: Remdesivir ≠ injectable GS-441524
The injectable doses in the ICatCare protocol are for remdesivir, which is NOT available in many countries (e.g. Spain) and requires very different doses from injectable GS-441524.
It is a very common and potentially serious error to apply the GS-441524 dose as if it were remdesivir. The dose required with injectable GS-441524 is much lower than that of remdesivir.
✖ DO NOT use remdesivir doses for GS-441524
✓ Refer to Table 2 for correct injectable GS-441524 doses
📋 Table 1 — Updated ISFM protocol (International Cat Care)
*Administer oral medication on an empty stomach, waiting at least 30 minutes before offering food. May be mixed with a small amount of food.
| Clinical presentation | Oral GS dose* | Remdesivir IV/SC dose |
|---|---|---|
| Effusion without ocular or neurological signs | 15 mg/kg every 24 h or divided into 2 doses | 10 mg/kg every 24 h |
| No effusion without ocular or neurological signs | 15 mg/kg every 24 h or divided into 2 doses | 12 mg/kg every 24 h |
| Ocular signs ± effusion | 20 mg/kg every 24 h or divided into 2 doses | 15 mg/kg every 24 h |
| Neurological signs ± effusion | 10 mg/kg every 12 h | 20 mg/kg every 24 h |
📋 Table 2 — Doses from social media working groups (PIF Warriors)
*Same oral administration instructions as in Table 1. In severe cases, doses may be higher than those indicated and administered every 12 h.
| Clinical presentation | Oral GS dose* | Injectable GS dose |
|---|---|---|
| Effusion without ocular or neurological signs | 12–18 mg/kg/24 h | 6–8 mg/kg/24 h |
| No effusion without ocular or neurological signs | 12–18 mg/kg/24 h | 6–8 mg/kg/24 h |
| Ocular signs ± effusion | 16 mg/kg/24 h | 8 mg/kg/24 h |
| Neurological signs ± effusion | Minimum 20 mg/kg/24 h | Minimum 10 mg/kg/24 h |
Treatment duration: 84 days as standard and the evidence for 42 days
📅 Why is a minimum of 84 days of treatment recommended?
Regardless of the protocol used, current recommendations are to treat for a minimum of 84 days, for two fundamental reasons:
- The experience gained with the drug GC-376 against FIP in cats with natural infection from Pedersen's 2018 study
- The half-life of macrophages within tissues is approximately 84 days. To eliminate the virus at the intracellular level, treatment must be extended beyond this half-life
🌟 Could 42 days be sufficient?
In June 2024, a study was presented in which 20 cats affected by the wet form of FIP were treated with oral GS-441524 for 42 days at a dose of 15 mg/kg/24 h, with a success rate of 95%. A subsequent study including 64 cats treated for 42 days at the same dose achieved a success rate of 91%.
These results open the possibility that shorter treatments may be effective in selected cases (effusive FIP, adequate dose). The total duration of treatment always depends on the normalisation of haematological and biochemical parameters and the clinical progression of the patient.
⚠️ Fundamental rule for the owner: complete consistency
The veterinarian must emphasise to the owner two non-negotiable aspects:
- Always administer at the same time
- Never miss a dose under any circumstances
- Weigh the cat weekly and adjust the dose with any weight gain
One of the main causes of treatment failure is not increasing the dose as the cat gains weight.
2. Monitoring during treatment
Monitoring protocol: what to do and when
Monitoring is based on N. Pedersen's studies. A complete monthly blood panel is recommended during the 3 months of treatment, plus check-ups during the observation period (3 months following completion). Severe cases require more frequent monitoring. At each visit, a full clinical examination is also performed. Abdominal ultrasound or radiography will be indicated based on the symptoms present.
What normalises first and what takes longer?
✅ Table 3 — Normalisation of haematological and biochemical parameters
| Parameter | Estimated time to normalisation |
|---|---|
| Leukocytosis | 2–4 weeks |
| Lymphocytes (in case of lymphopenia) | 1 week |
| Bilirubin | 2–4 weeks |
| Globulins | Increase in wet forms at 3–4 weeks; then normalise |
| Haematocrit | 6–8 weeks |
| Albumin | 8 weeks |
| A/G ratio | ≥0.6 at end of treatment |
| AGP (Alpha-1 acid glycoprotein) | Progressive normalisation during treatment |
✅ Table 4 — Normalisation of clinical signs and symptoms
| Symptom / Sign | Estimated time to resolution |
|---|---|
| Energy and appetite | Improvement within 1–2 days; normal by 2 weeks |
| Resolution of pyrexia (fever) | 12–48 hours |
| Pleural effusion | Gone by day 7 |
| Abdominal effusion | Improvement at 10–14 days; gone by 2 weeks |
| Ocular symptoms | Not apparent at 7–14 days |
| Weight gain | 1–1.5 kg during treatment |
| Neurological symptoms | Variable |
🔍 Table 5 — Organ normalisation and persistence of fluid
- Reduction in lymph node size: progressive during treatment
- A study published in 2022 on the follow-up of 18 cases in complete remission over one year confirmed the persistence of abdominal lymphadenopathy without clinical relevance. This finding should not cause alarm once treatment has ended
- There may be persistence of residual fluid without this necessarily implying therapeutic failure
3. Signs of lack of response to treatment
When to suspect treatment is not working
When the patient's response is inadequate, symptoms persist or follow-up blood work yields unfavourable results, the following factors must be systematically evaluated:
⚠️ Possible causes of treatment failure
- Reconsider the diagnosis: is it really FIP?
- Concomitant diseases: other infections, neoplasms
- Inadequate dose: failure to increase the dose with weight gain
- Incorrect administration by the caregiver
- Poor-quality drugs
- Development of resistance
📋 Table 6 — Warning signs: parameters indicating lack of response
| Parameter / Sign | Alarm threshold |
|---|---|
| Persistent pyrexia | More than 7 days |
| Persistent effusions | More than 3–4 weeks |
| Hyperglobulinaemia | More than 6–8 weeks |
| Albumin/globulin ratio <0.6 | More than 8–10 weeks |
| Persistent hyperbilirubinaemia | More than 3 weeks |
| White blood cell abnormalities | More than 3 weeks |
| Appearance of new FIP signs | At any point during treatment |
Recommended action when there is lack of response: Increase the dose by 2.5–5 mg/kg for the injectable form and 5–10 mg/kg for the oral form, and continue treatment until the next review. If near the end of treatment, maintain until parameters have been normal for at least 2 weeks.
4. Side effects and adverse reactions
What can happen and how often?
The safety profile of GS-441524 is generally favourable. Most adverse effects are mild and transient. Details by route of administration are provided below.
🔸 General adverse effects (oral and injectable)
- Increase in ALT enzyme activity: it is not entirely clear whether this is a consequence of the disease or a drug side effect
- Mild peripheral eosinophilia
- GS uroliths: documented in 2 cats undergoing FIP treatment
💉 Additional adverse effects with injectable (SC) administration
- Pain at the injection site
- Development of abscesses, seromas and skin wounds
- Pruritic reactions with self-inflicted lesions
- Vagal responses following parenteral administration
- Anaphylactic reaction (very rare)
These cutaneous reactions may be visible and distressing for owners; it is important to inform them in advance of their possible occurrence and that the oral route avoids them entirely.
⚠️ Specific adverse effects of injectable remdesivir
- Development or worsening of pleural effusion within the first 48 h of treatment, which in some cases requires drainage. This effusion does not always have a high protein content
- Cats may appear depressed or show signs of nausea a few hours after IV administration
5. Drug interactions to consider
Drugs that may complicate clinical interpretation
🧠 Drugs that cross the blood-brain barrier
Certain drugs that penetrate the central nervous system, such as fluoroquinolones and some external and internal antiparasitic agents, can cause neurological side effects. This may create confusion as to whether the patient is progressing towards a neurological form of FIP. The clinician must weigh the pros and cons in each individual case.
💊 Corticosteroids: limited and targeted use only
Corticosteroids may be used during the first days of treatment in very specific cases:
- Haemolytic anaemia
- Neurological signs
- Inflammatory bowel disease
- Dermatitis
They should be discontinued as soon as possible. Depot or long-acting formulations are absolutely contraindicated, as it is impossible to know the drug concentration at any given time or predict when its action will end. Numerous cases have been reported of cats on antiviral treatment that worsened 5–6 weeks after the administration of depot formulations (personal observation by the authors).
⚠️ L-Lysine: discontinue during treatment
It is recommended to discontinue the use of L-Lysine during treatment with GS-441524. It is postulated that it could lead to depletion of arginine levels in kittens and result in growth delays, although further studies are needed to confirm this.
6. Special precautions: persistent effusions and pregnancy
Effusions refractory to treatment: what is happening?
Some cats may present pleural or abdominal effusion that does not resolve with treatment. There are three distinct reasons why effusions may persist:
a) Persistent infection
Persistence of the infection with resulting inflammation due to inadequate treatment, use of poor-quality drug or development of antiviral resistance.
b) Circulatory damage
Chronic damage to the circulatory system with increased capillary pressure preventing reabsorption of the effusion. Usually due to low-grade infection or residual fibrosis.
c) Another effusion-generating pathology
Heart disease, liver disease, hypoproteinaemia or neoplasms that do not correspond to a diagnosis of FIP.
✅ Approach to persistent effusion
- Complete fluid study. Note: performing a coronavirus PCR on fluid during treatment may yield false negatives
- If the study suggests active FIP: increase the dose and extend treatment
- If the result is negative: consider whether another pathology may be involved
- Consider performing exploratory laparotomy with biopsy sampling if in doubt
- Residual effusions due to fibrosis usually resolve over time, except in cases of major circulatory damage
Pregnancy: can a pregnant queen be treated?
🐾 The recommendation is to treat
Cases of pregnant queens with FIP treated with GS-441524 have been documented with favourable outcomes for both the disease and the kittens. Pregnant queens are usually in subclinical stages when pregnancy occurs; the immunosuppression inherent to gestation favours disease progression, generally in the last trimester.
- Placental transmission has been described but is an uncommon phenomenon
- Kittens of untreated sick mothers typically die in the foetal stage or early postnatal period
- Queens treated with GS-441524 can give birth to healthy kittens
- In rat studies, the metabolite GS-441524 crossed the placental barrier following remdesivir administration, but there are no studies yet determining the concentration that reaches the foetus in queens
Current recommendation: treat pregnant females, as they respond rapidly to treatment and are capable of providing the necessary care for their kittens.
7. Comorbidities: FIP alongside other diseases
Antiviral treatment does not interfere with other conditions
Based on the authors' experience, FIP may be accompanied by other pathologies. It is important to highlight that antiviral treatment does not interfere with the treatment of these concurrent conditions.
🦠 Infectious diseases
- Leukaemia (FeLV) and Feline Immunodeficiency Virus (FIV): These complicate follow-up due to overlapping symptoms and laboratory abnormalities, but cure is possible. Bear in mind that 90–95% of cats with progressive FeLV do not survive beyond 3 years of age
- Feline panleukopenia: If the cat survives panleukopenia, the prognosis for FIP is favourable
- Feline rhinotracheitis: Specific medication can be started without issue. Corticosteroids may worsen this condition
- Haemoplasmosis: Worsens anaemia and requires specific treatment (antibiotics ± corticosteroids)
🛡️ Immune-mediated diseases
- Pemphigus foliaceus
- Atopic dermatitis
- Lymphoplasmacytic gastritis/enteritis
- Lymphoplasmacytic pododermatitis
- Immune-mediated haemolytic anaemia (may be primary or secondary to FIP): may require corticosteroids at immunosuppressive doses or even alkylating agents
🔬 Other documented comorbidities with successful treatment
Neoplastic conditions
- Lymphoma (simultaneous chemotherapy or after FIP treatment)
- Chronic lymphocytic leukaemia
Other conditions
- Dermatophytosis (simultaneous antifungals)
- Cardiac diseases (the FIP–feline myocarditis relationship has recently been described)
- Polycystic kidney disease
- Feline gingivostomatitis complex
- Feline asthma
- Tetralogy of Fallot
8. Treatment of relapse
What is a relapse and when does it occur?
📖 Clinical definition of relapse
Relapses can occur in two ways:
- During treatment: with worsening of symptoms
- During the observation period: during the 12 weeks following the end of treatment
The most plausible explanation is that the virus has taken refuge in the central nervous system following treatment of dry or effusive FIP without neurological/ocular involvement but with insufficient doses, unable to effectively penetrate the blood-brain or ocular barriers. Relapses may present differently from the initial clinical picture.
Retreatment protocol for relapse
✅ Relapse is not the end: it responds to retreatment
Retreatment in case of relapse should ideally be instituted for another 12 weeks, with doses always higher than those used at the end of the previous treatment:
💉 Minimum injectable dose
10 mg/kg/day
Always higher than the dose at the end of the first treatment
💊 Minimum oral dose
20 mg/kg divided into two doses
Always higher than the dose at the end of the first treatment
Unfortunately, we currently have no test that can predict which patients will relapse. This remains one of the most important challenges in FIP treatment.
9. Frequently asked questions
10. Scientific references
📚 Bibliography from the original article
References correspond to the original numbering from the article published in Centro Veterinario.
- Krentz D, Zenger K, Alberer M, et al. Curing Cats with Feline Infectious Peritonitis with an Oral Multi-Component Drug Containing GS-441524. Viruses. 2021;13(11):2228.
- Krentz D, Zwicklbauer K, Felten S, et al. Clinical Follow-Up and Postmortem Findings in a Cat Cured of FIP with an Oral Antiviral Drug Containing GS-441524. Viruses. 2022;14(9):2040.
- Zwicklbauer K, Krentz D, Bergmann M, et al. Long-term follow-up of cats in complete remission after treatment of FIP with oral GS-441524. J Feline Med Surg. 2023;25(8).
- Pedersen NC, Jacque N. Use of oral GS-441524 for FIP treatment v10. UC Davis, 2021.
- Hartmann K, Day MJ, Thiry E, et al. Feline injection-site sarcoma: ABCD guidelines. J Feline Med Surg. 2015;17(7):606–613.
- Update on Feline Injection-Site Sarcomas. AAHA Guidelines, 2020.
- Yan Y, Li J, Jiao Z, et al. Better therapeutic effect of oral GS441524 vs GC376. Vet Microbiol. 2023;283:109781.
- Pedersen NC, et al. Efficacy and safety of GS-441524 for naturally occurring FIP. J Feline Med Surg. 2019;21(4):271–281.
- Zuzzi-Krebitz AM, et al. Short Treatment of 42 Days with Oral GS-441524 Results in Equal Efficacy as 84-Day Treatment. Viruses. 2024;16(7):1144.
- Addie DD, et al. Alpha-1 Acid Glycoprotein Reduction in Cats Treated for FIP. Viruses. 2022;14(4):744.
- Zwicklbauer K, et al. Long-term follow-up after oral GS-441524 in FIP. J Feline Med Surg. 2023. doi:10.1177/1098612X23118.
- FIP-Update-2024-Vet-Vault-clinical-podcast-Sally-Coggins. Stokes Pharmacy, 2024.
- Protocolo10.pdf. AVEPA–GEMFE, 2024.
- Allinder M, et al. Uroliths composed of GS-441524 in 2 cats with FIP. J Vet Intern Med. 2024;38(1):370–374.
- Bol S, Bunnik EM. Lysine supplementation is not effective for FHV-1 in cats. BMC Vet Res. 2015;11:284.
- Court MH. Feline Drug Metabolism and Disposition. Vet Clin North Am Small Anim Pract. 2013;43(5):1039–1054.
- Pedersen NC. Feline Infectious Peritonitis in Pregnant Cats. DVM. PhD June 24, 2021.
- Yang L, et al. Biotransformation and transplacental transfer of remdesivir and GS-441524 in pregnant rats. eBioMedicine. 2022;81:104095.
- Taylor SS, Coggins S, Barker EN, et al. Retrospective study of 307 cats with FIP treated with remdesivir and GS-441524. J Feline Med Surg. 2023.
- BVetMed ST, Tasker S, Gunn-Moore D, et al. An update on treatment of FIP using antiviral drugs in 2024.
- Pedersen NC. Summary of GS-441524 treatment for FIP. UC Davis.
- Buchta K, et al. Myocarditis in Cats with FIP Cured with GS-441524 and Cardiovascular Treatment. Animals. 2025;15(11):1660.
- Černá P, et al. Immune-Mediated Hemolytic Anemia in Cats with FIP. Pathogens. 2025;14(7):660.
⚠️ Veterinary medical disclaimer
This article is a summary of the clinical article published by Vicky Vives Moya and Esther Garrido Cervera in the journal Centro Veterinario. It does not replace professional veterinary consultation, diagnosis or prescription.
FIP treatment must always be guided by a veterinarian with experience in feline medicine. Each patient must be evaluated individually. The dosing protocols indicated correspond to the recommendations in effect at the time of publication of the original article and may be updated with new scientific evidence.