# Efficacy of GS-441524 for Feline Infectious Peritonitistis: A Systematic Review (2018-2024)
## 4 Discussion  
In this systematic review, we compiled and analyzed the published evidence on treating FIP with GS-441524. The results indicate that GS-441524 is efficacious, transforming FIP from a fatal disease into a treatable condition for the majority of affected cats. Across 650 reported cases, approximately 85% survived and achieved remission with GS-441524 therapy, which is a striking outcome considering the historical near-100% mortality of FIP. Our analysis confirms several clinical impressions and provides some new insights:  
High Overall Efficacy: GS-441524, as a sole therapy, yielded about an 83% success rate in pooled data. When including combination treatments, success exceeded 90% in some cohorts. These findings concur with earlier reports of \(\sim\)80-90% cure rates in field use of GS-441524 or its prodrugs [32; 33]. By comparison, historically, no treatment reliably induced remission in FIP, so this represents a significant improvement. Effusive vs. Non-Effusive FIP: Cats with non-effusive (dry) FIP responded somewhat better than those with effusive FIP, which aligns with clinical experience. Effusive FIP is the more acute fulminant form with a shorter clinical course, and extensive immune-complex deposition, which may be harder to fully reverse, even with antivirals. Additionally, large-volume effusions can be caused by a high viral load and severe inflammation. Still, it is notable that even in effusive FIP, roughly 78% of cats survived--again, a vast improvement on 0%. Neurological/Ocular FIP: Cats with CNS involvement remain the most challenging to cure. The blood-brain barrier limits the penetration of GS-441524; even at higher doses, drug levels in cerebrospinal fluid are much lower than in plasma. This likely explains why standard dosing is often insufficient for neuro-FIP. Clinicians now often use \(\sim\)10 mg/kg for neuro cases (and some advocate even higher or twice-daily dosing) [16]. Alternative strategies, such as using more lipophilic analogues or combination therapy (e.g., adding drugs that cross into the CNS), might further improve neuro-FIP outcomes. The success seen with Molnupiravir in rescue cases is encouraging--Molnupiravir is orally administered and can penetrate the CNS. Combining GS-441524 with Molnupiravir or other CNS-penetrant antivirals may be a fruitful approach in the future. Combination Therapy: While our analysis did not prove a significant advantage to combining GS-441524 with other antivirals, the numerically higher survival in those cohorts and the particular success in some tough cases (e.g., all neuro cases on GS + Remdesivir survived) identify this strategy as promising in certain scenarios. It is logical that two antivirals targeting different viral proteins (polymerase and protease, for instance) could have additive or synergistic effects. Moreover, Remdesivir essentially delivers the same active molecule but via a different route--some protocols start with Remdesivir injections to rapidly achieve high blood levels and then switch to oral GS-441524 for convenience [8]. Australian veterinarians have adopted Remdesivir (since it is legally obtainable) followed by oral GS-441524, with reported outcomes similar to those in this review (an approximately 90% cure rate) [10]. Interferon omega's role is less clear; it has immunomodulatory effects and might help in controlling secondary infections or modulating the immune response. In a couple of small studies, adjunct interferon seemed to help, but those were not controlled comparisons. As a minimum, we can say that interferon was not detrimental, and it could be considered as an adjunct, especially in non-effusive FIP cases [21]. Treatment Duration: Most studies treated for at least 12 weeks, which has become the standard recommendation (this duration was originally extrapolated from the first experimental trial of GS-441524) [24]. A few reports experimented with shorter courses (e.g., 8 weeks) or longer ones (beyond 12 weeks). Anecdotally, shorter courses have higher relapse rates [11].