Phosphate-modified PROTECs as potent antivirals against respiratory syncytial virus by targeting the fusion protein

Respiratory syncytial virus (RSV) remains a major cause of severe respiratory infections, highlighting the need for antiviral strategies with novel mechanisms. Here, we extended our protein-radical-oxidation targeting enediyne chimera (PROTEC) strategy to the RSV fusion (F) protein. BMS-433771 was employed as the protein-targeting group and conjugated to a maleimide-based acyclic enediyne bearing phosphate hydrophilicity-adjusting groups, which simplified synthesis compared with our previous sulfate-containing PROTECs. Three analogues with different linker lengths (n = 3, 5, and 7) were synthesized and evaluated. All compounds showed low cytotoxicity (CC₅₀ > 100 µM) and potent anti-RSV activity. PROTEC-R7 (n = 7) exhibited the highest potency, with an IC₅₀ of 110.3 nM and a selectivity index > 906. In contrast, the non-targeted control EDY-R showed substantially weaker activity (IC₅₀ = 16.3 µM). These results demonstrate the importance of targeted delivery and linker engineering and support PROTEC as a promising strategy for targeting viral surface proteins.