What is the recommended GS-441524 dosing protocol and duration for confirmed or suspected FIP in cats?

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GS-441524 (GS) dosing for feline infectious peritonitis (FIP) is form-dependent, with higher doses required for neurological and ocular disease due to limited blood-brain and blood-ocular barrier penetration. The standard oral treatment duration is 84 days (12 weeks), followed by an 84-day observation period.4 For effusive FIP, oral GS-441524 is dosed at 12.5–25 mg/kg/day, with the lower end of this range used for uncomplicated effusive disease.1 Subcutaneous GS-441524 at 4.0 mg/kg SC q24h for at least 12 weeks was established as the effective dose in the foundational field trial, where 2.0 mg/kg SC q24h produced an unacceptable relapse rate.6 Oral dosing of 10–15 mg/kg q24h has been used successfully in effusive and non-effusive disease, with remission achieved by day 84 in 56% of cats on this regimen; cats requiring retreatment responded to escalation to 15–20 mg/kg.10 Across 84-day oral treatment protocols, success rates are 82.2% for effusive FIP, 85.1% for mixed effusive/non-effusive FIP, and 93.9% for non-effusive FIP.12 Neurological FIP requires dose escalation to 5–10 mg/kg SC q24h for at least 84 days. At 5 mg/kg SC q24h, two less severely affected neurological cases resolved, but the most severely affected cat failed to respond until stepwise escalation to 10 mg/kg.7 This pharmacokinetic rationale is supported by cerebrospinal fluid concentrations of GS-441524 measuring approximately 20% of plasma concentration, with a 10 mg/kg dose achieving CSF concentrations of 0.8–2.7 μM.7 Oral dosing of 10–20 mg/kg/day has been used with successful results in neurological cases.3 Overall survival for neurological FIP is 60%.2 Ocular FIP is treated at the same higher dose range of up to 20 mg/kg/day, reflecting similarly limited drug penetration through the blood-ocular barrier, with 22–33% of plasma concentration detected in the aqueous humor.3 Ocular FIP carries the highest survival rate at 100% in one cohort.2 Of cats presenting with both ocular and neurological dis