Feline Infectious Peritonitis (FIP) is a severe fatal immune-augmented disease 
in cat population. It is caused by FIP virus (FIPV), a virulent mutant strain of 
Feline Enteric Coronavirus (FECV). Current treatments and prophylactics are not 
effective. The in vitro antiviral properties of five circular Triple-Helix 
Forming Oligonucleotide (TFO) RNAs (TFO1 to TFO5), which target the different 
regions of virulent feline coronavirus (FCoV) strain FIPV WSU 79-1146 genome, 
were tested in FIPV-infected Crandell-Rees Feline Kidney (CRFK) cells. RT-qPCR 
results showed that the circular TFO RNAs, except TFO2, inhibit FIPV 
replication, where the viral genome copy numbers decreased significantly by 
5-fold log10 from 10(14) in the virus-inoculated cells to 10(9) in the circular 
TFO RNAs-transfected cells. Furthermore, the binding of the circular TFO RNA 
with the targeted viral genome segment was also confirmed using electrophoretic 
mobility shift assay. The strength of binding kinetics between the TFO RNAs and 
their target regions was demonstrated by NanoITC assay. In conclusion, the 
circular TFOs have the potential to be further developed as antiviral agents 
against FIPV infection.