S-337395

S-337395 is a novel investigational oral antiviral drug developed for the treatment of respiratory syncytial virus (RSV) infection. It is a low-molecular-weight small molecule that inhibits the RNA-dependent RNA polymerase activity of the L protein in RSV, thereby blocking transcription and replication of the viral genome. Unlike F protein inhibitors, which prevent new viral infections extracellularly, S-337395 acts within infected cells to halt viral proliferation, potentially offering higher efficacy and more rapid reduction in viral load. The drug has demonstrated significant reductions in RSV viral load and improvement in clinical symptoms during phase 2 human challenge trials. S-337395 has received Fast Track designation from the U.S. FDA due to its potential to address an unmet medical need for effective RSV treatments. It is under joint development by Shionogi and UBE Corporation; Shionogi leads global clinical development while UBE handles active pharmaceutical ingredient manufacturing.