# Efficacy of GS-441524 for Feline Infectious Peritonitistis: A Systematic Review (2018-2024)
## 4 Discussion
One recent study suggests that even a 6-week course might suffice for some mild cases, but relapse remains a risk [34]. In general, 12 weeks is still advised, and if there is any residual disease at 12 weeks, extending treatment or increasing the dose for a few extra weeks is recommended [16]. Our review found that some cats required a second round of treatment after a relapse--often at a higher dose or for a longer period--to fully cure the disease. As a safety margin, some practitioners treat for 2 weeks beyond the resolution of clinical signs to ensure clearance of the virus. In our dataset, about 4% of cats relapsed after the first round and were then successfully retreated; thus, monitoring after the initial treatment period is crucial. Relapses and Resistance: A small number of cats did not respond or relapsed repeatedly. Viral RNA from some of these refractory cases has shown mutations in the virus'spolymerase or spike proteins, potentially conferring partial resistance to GS-441524 [9]. Molnupiravir has been used as a \"rescue\" in these cases, as it has a different mechanism (causing lethal mutagenesis of the virus). [PERSON] et al. (2022) reported that 8/11 cats who failed with GS-441524 treatment achieved remission when switched to Molnupiravir [9]. This highlights the importance of having multiple antiviral options available, especially as wider use of GS-441524 could exert selective pressure on the virus. The combination or sequential use of GS-441524 and Molnupiravir is an active area of investigation. Safety and Side Effects: GS-441524 has a wide therapeutic margin in cats; most adverse effects are localized or mild. The occurrence of urolithiasis in two cases is intriguing--it may be that GS-441524 (or a metabolite) can crystallize in urine in some circumstances. Ensuring cats stay well-hydrated during treatment might be a reasonable precaution, especially at high doses. The injection pain issue has largely been mitigated by the move to oral formulations. Many owners now prefer to use oral GS-441524 from day one (to avoid subjecting the cat to daily painful injections) [35]. In situations where oral absorption might be compromised (e.g., a cat with vomiting or inappetence), initial injectable dosing can be performed and then transitioned to oral once the cat is stable [10]. Overall, the tolerability of GS-441524 is excellent compared to many chemotherapeutics or antiviral drugs used in other species. Treating FIP with GS-441524 offers new hope, yet animal welfare considerations must remain central. While many cats experience remission, treatment failures can occur, especially with under-dosing, advanced disease, or drug resistance. Adverse effects, such as injection site pain, vomiting, or liver enzyme elevation, require careful monitoring. Furthermore, reliance on grey-market sources raises ethical and safety concerns due to unregulated drug quality. Ensuring informed consent, adequate veterinary oversight, and access to humane alternatives is crucial. A balanced approach respects both the drive to save lives and the responsibility to avoid undue suffering during experimental or costly interventions. Regulatory Status and Access: Despite the compelling evidence of efficacy, GS-441524 is only approved in certain countries, and supplied as a compounded pharmaceutical. Remdesivir, which is chemically very similar, is now legal for veterinary FIP treatment in Australia, Canada, the UK, and some EU countries. It is being used off-label by veterinarians in the USA as a legal workaround [10]. The FDA's guidance in 2022 opened the door for compounding Remdesivir or similar drugs for FIP under certain conditions [10]. The Federation of Veterinarians of Europe has also advocated for legal pathways to access these life-saving treatments [10]. In Europe, the use of extemporaneous products containing GS-441524 has now been made possible by some manufacturers, giving a legal and safer chance of survival for FIP patients [36].