Combination of ganciclovir and trifluridine prevents drug-resistance emergence in HSV-1

Ocular herpes simplex virus 1 (HSV-1) infections can lead to visual impairment. Long-term acyclovir (ACV) prophylaxis reduces the frequency of recurrences but is associated with drug resistance. Novel therapies are needed to treat drug-resistant HSV-1 infections. Here, we describe the effects of trifluridine (TFT) in combination with ACV or ganciclovir (GCV) on HSV-1 replication and drug-resistance emergence. Wild-type HSV-1 was grown under increasing doses of one antiviral (ACV, GCV, or TFT) or combinations thereof (ACV + TFT or GCV + TFT). Virus cultures were analyzed by Sanger sequencing and deep sequencing of the UL23 [thymidine kinase (TK)] and UL30 [DNA polymerase (DP)] genes. The phenotypes of novel mutations were determined by cytopathic effect reduction assays. TFT showed overall additive anti-HSV-1 activity with ACV and GCV. Five passages under ACV, GCV, or TFT drug pressure gave rise to resistance mutations, primarily in the TK. ACV + TFT and GCV + TFT combinatory pressure induced mutations in the TK and DP. The DP mutations were mainly located in terminal regions, outside segments that typically carry resistance mutations. TK mutations (R163H, A167T, and M231I) conferring resistance to all three nucleoside analogs (ACV, TFT, and GCV) emerged under ACV, TFT, ACV + TFT pressure and under GCV + TFT pressure initiated from suboptimal drug concentrations. However, higher doses of GCV and TFT prevented drug resistance in the resistance selection experiments. In summary, we identified novel mutations conferring resistance to nucleoside analogs, including TFT, and proposed that GCV + TFT combination therapy may be an effective strategy to prevent the development of drug resistance.