EXPRESS: Large cell lymphoma in four cats after successful treatment of feline infectious peritonitis with oral GS-441524 - a novel clinical observation.

Case Series SummaryFatal feline infectious peritonitis (FIP), caused by feline coronavirus (FCoV), can now be cured with GS-441524. Only a few unexpected clinical and laboratory observations have been reported with treatment, including lymphocytosis, eosinophilia and long-term persistence of abdominal lymphadenomegaly. Yet immune overstimulation associated with FIP might have negative long-term consequences. This report describes four cases of large cell lymphoma (LCL) arising within 2 years after FIP diagnosis and successful treatment with legally sourced oral GS-441524 (15 mg/kg once daily), representing an incidence of 2.0% (4/202) in a large treatment cohort. At LCL diagnosis, two cats were <2 years old, one was 9 years old and one was 13 years old. All cats showed weight loss, three had hyporexia and two had chronic vomiting; all were feline leukemia virus and feline immunodeficiency virus negative. LCL was diagnosed by histology (n = 3) or cytology (n = 1) at 81, 365 (n = 2) and 595 days after FIP diagnosis/treatment start. The cats died a median of 15.5 days after LCL diagnosis. Neither a high FCoV viral load nor FCoV antigen, as determined by semiquantitative reverse transcriptase polymerase chain reaction (RT-qPCR) and immunohistochemistry, respectively, was detected in any of the available samples. PCR for antigen receptor rearrangements revealed a monoclonal B cell population in two cats, supporting a diagnosis of LBCL.Relevance and Novel InformationThe incidence of LCL reported here among cats in remission from FIP after legally sourced oral GS-441524 treatment is remarkably high compared to the general feline population. LCL should be considered a potential "long-FIP syndrome" and/or a long-term complication after GS-441524 treatment. The pathogenesis of LCL in this context requires further clarification.