Remdesivir is active in vitro against tick-borne encephalitis virus and selects for resistance mutations in the viral RNA-dependent RNA polymerase.

Background: Tick-borne encephalitis (TBE) is a neurological disease caused by the tick-borne encephalitis virus (TBEV). Despite available vaccines, breakthrough infections occur, some fatal.

Objectives: As no antiviral therapy for TBE is currently approved, this study evaluated the in vitro activity of already licenced remdesivir (RDV) and sofosbuvir (SOF) for possible drug repurposing against TBEV.

Methods: TBEV was cultured in A549 cells, and the inhibitory effects of RDV (GS-5734), its parent nucleotide GS-441524, and SOF (GS-7977) were assessed.

Results: After 78 h, RDV demonstrated significantly lower EC₅₀ values than SOF (0.14 vs. 11 µM) based on TBEV RNA levels measured by RT-qPCR. RDV also had a lower mean EC₅₀ (0.55 µM) compared to GS-441524 and SOF (>8.9 and 13.1 µM, respectively) using crystal violet staining after 5 days. After 11 passages of TBEV in the presence of RDV, emergence of virus with a higher EC₅₀ (1.32 vs. 0.55 µM) was detected with two mutations (L3122F and Y3278F) in NS5, the viral RNA-dependent RNA polymerase (RdRp), and one substitution in envelope (E) protein (E402G). Similarly, SOF resistance appeared after 20 passages, increasing EC₅₀ values (35.5 vs. 10 µM).

Conclusion: RDV exhibits potent in vitro antiviral activity against TBEV via specific targeting of the viral RdRp as confirmed by the emergence of resistance-associated double NS5 substitutions in vitro in the presence of RDV. While the potential in vivo implications of the observed RDV resistance remain to be determined, these in vitro data support further assessment of RDV for the treatment of TBEV infection.