Primary screening data.

Dengue virus (DENV) is a mosquito-transmitted flavivirus that circulates globally as four distinct serotypes and poses a substantial threat to public health. There are an estimated ~96 million symptomatic infections yearly, including severe cases of dengue fever, underscoring the urgency of identifying effective therapeutics targeting all four serotypes. Nucleoside analogs, which mimic endogenous nucleosides to inhibit viral RNA replication, offer a promising strategy for broad-spectrum antiviral development. Here, we conducted a high-throughput screen of 1,101 nucleoside analogs against DENV serotype 2 (DENV2) in a panel of human cell models, including human epithelial cells, hepatocytes, and fibroblasts. Candidates that were active against DENV2 were screened against all four serotypes. Since flaviviruses including West Nile virus and Zika virus are also important human pathogens, we screened these compounds for activity and identified compounds that were broadly active in these cellular and viral models. We further evaluated antivirals in primary human keratinocytes and fibroblasts, which are early targets of mosquito-transmitted DENV infection. From this screen, we identified 23 nucleoside analogs with broad antiviral activity against DENV and focused on two purine analogs UPGNUC255 and UPGNUC558, that demonstrated potent pan-flaviviral activity achieving >10-fold viral load reduction across all four DENV serotypes and other flaviviruses across cell models. Mechanistic studies revealed that both compounds target the viral RNA-dependent RNA polymerase (RdRp) domain of NS5. Resistance to UPGNUC558 was associated with a conserved S604T substitution, conferring cross-resistance to other 2′C-substituted nucleoside analogs. Resistance to UPGNUC255 was linked to a previously unknown R355Q mutation, located near the catalytic GDD motif of RdRp. These findings highlight UPGNUC255 and UPGNUC558 as promising leads for the development of broad-spectrum antiviral agents against flaviviruses.