Clomipramine hydrochloride restricts feline infectious peritonitis virus replication in immortalized and primary feline cells

Feline infectious peritonitis (FIP), caused by feline coronavirus (FCoV), is a severe and often fatal disease in cats. Although recent advances in antiviral therapy have improved clinical outcomes, limitations related to cost, accessibility, regulatory approval, and antiviral resistance highlight the need for additional therapeutic strategies. Host-directed antiviral approaches targeting cellular processes required for viral replication may represent an alternative strategy. In this study, we investigated whether clomipramine hydrochloride, a veterinary drug with lysosomotropic properties and previously reported antiviral activity against several enveloped viruses, could inhibit FIPV replication. Clomipramine pretreatment reduced FIPV RNA and protein expression and decreased infectious virus production in Crandell-Rees Feline Kidney (CRFK) cells in a concentration-dependent manner, with an EC₅₀ of 10.8 μM. The calculated CC₅₀ was 60.5 μM, corresponding to a selectivity index of 5.6. Clomipramine also retained antiviral activity when administered immediately after infection or at 4 h postinfection, although the effect was reduced with delayed treatment. Antiviral activity was further observed in primary feline cells derived from abdominal and pleural effusion samples. Clomipramine treatment reduced FIPV-induced expression of the interferon-stimulated genes MX1 and CXCL10, while clomipramine alone did not induce their expression in uninfected cells. The reduction in MX1 and CXCL10 expression closely paralleled the decrease in viral RNA levels, suggesting that these changes were likely associated with reduced viral stimulation rather than direct induction or suppression of these interferon-stimulated genes. LysoTracker staining showed reduced fluorescence in FIPV-infected cells, whereas staining was retained in clomipramine-treated infected cells. These findings may reflect reduced virus-associated perturbation of lysosomal compartments following inhibition of FIPV replication. Together, these findings identify clomipramine as a potential host-directed antiviral candidate against FIPV and support further investigation of its antiviral activity and therapeutic potential.