Treatment of Feline Infectious Peritonitis in Cats with Molnupiravir: Clinical Observations and Outcomes For 54 Cases

Authors: T. M. Clark, S. J. Coggins, R. Malik, J. King, R. Korman

Abstract:

Objectives: This observational study investigated the use of molnupiravir for treatment of cats with naturally occurring feline infectious peritonitis. Methods: From September 2022 to February 2024, 66 cats diagnosed with FIP across 32 veterinary practices, mainly in Australia, were enrolled. Of these, 54 cats met the inclusion criteria. Complete remission was defined by the resolution of clinical signs and normalisation of A:G ratio (to ≥0.6). Presumptive remission was defined as sustained resolution of FIP-related clinical signs for at least 100 days post cessation of antiviral therapy. Results: In Cohort 1, 18 cats were treated with molnupiravir monotherapy. Thirteen achieved complete remission and three attained presumptive remission, resulting in an overall remission rate of 89% and a provisional cure rate of 72%, with three relapses. Cohort 2 included 29 cats treated with a short induction course of GS-441524 and/or remdesivir before switching to molnupiravir; 23 attained complete remission, and two achieved presumptive remission. The overall cure rate was 86% with no relapses. Seven cats in cohort 3 were initially treated with extended courses of GS-441524, remdesivir, and/or mefloquine and experienced treatment failure or relapse. Molnupiravir was introduced as a rescue therapy; 6 achieved complete remission and 1 achieved presumed remission, resulting in a 100% cure rate with no relapses. Few adverse effects were reported, with the most notable including neutropenia, transient elevations in hepatic enzymes, and polydipsia/polyuria. Conclusion and Relevance: Molnupiravir as a monotherapy, or in combination with other antivirals, represents an accessible, effective treatment for FIP when given at a dosage of 10-15 mg/kg BID. Success occurred across various presentations of FIP, including cases with ocular and neurological involvement.

Keywords: feline infectious peritonitis, FIP, molnupiravir, nucleoside analogue, antiviral