The 5'-Isobutyryl Ester Prodrug of 4'-Thiouridine Inhibits SARS-CoV‑2 Replication in Culture and in a Syrian Hamster...

Although the SARS-CoV-2 pandemic is now largely controlled through widespread vaccination and available therapeutics, emerging variants could undermine their overall effectiveness. 4'-Thiouridine (TU) and its 5'-isobutyryl prodrug (iBu TU) were identified as potent inhibitors of SARS-CoV-2 replication across various cell models, including advanced primary human respiratory models. Both compounds exhibited no significant toxicity across a range of cell culture systems, with no signs of mitochondrial damage or elevated lactic acid production. Furthermore, TU and iBu TU tested negative for mutagenicity, aneugenic and clastogenic effects, alleviating concerns about genotoxicity. They also showed no interaction with a broad spectrum of ion channels, GPCRs, and enzymes, further confirming their favorable safety profile. TU is metabolized intracellularly to its active triphosphate form (TU-TP), which effectively disrupts viral RNA replication by acting as a delayed chain terminator. iBu TU is rapidly converted to TU in plasma and displays superior oral bioavailability compared to TU (F = 90% vs 42% in monkeys). In a Syrian hamster model infected with SARS-CoV-2, oral iBu TU significantly reduced viral loads (1.7 to 4.5 log₁₀) and improved lung health without adverse effects. These findings highlight iBu TU as a promising oral antiviral candidate for treating SARS-CoV-2 infection.