Yesterday, Shionogi, a Japanese pharmaceutical company, announced its novel investigational respiratory syncytial virus (RSV) oral antiviral candidate, S-337395, achieved its primary endpoint in a phase 2 clinical trial. In the highest dose group of S-337395, there was an 88.94% reduction in viral load (P<0.0001), and also a statistically significant improvement in clinical symptom scores.1
Trial Specifics
This trial was a randomized, placebo-controlled, double-blind human challenge study conducted in healthy adults. The antiviral efficacy and safety of S-337395 were evaluated when administered orally once daily for 5 days. The S-337395 treatment group showed a statistically significant reduction in viral load compared to the placebo group, achieving the primary endpoint. 1
S-337395 was generally safe and well tolerated, there were no serious or severe adverse events, and no dose-dependent increase in incidence or severity of adverse events. No participants discontinued due to adverse events.1
What You Need to Know
Shionogi’s investigational RSV oral antiviral, S-337395, met its primary endpoint in a phase 2 human challenge trial. The highest dose group achieved an 88.94% reduction in viral load (P<0.0001) and showed a significant improvement in clinical symptoms, demonstrating strong antiviral efficacy.
S-337395 was well tolerated, with no serious or severe adverse events, no dose-dependent increase in adverse events, and no participants discontinuing due to side effects, supporting its potential as a safe RSV treatment.
Unlike F protein inhibitors that block new viral infections, S-337395 inhibits RSV replication inside infected cells by targeting RNA polymerase activity, potentially offering higher efficacy and faster viral load reduction.
As this was a human challenge trial, so healthy subjects were infected with RSV to investigate the onset of disease and the progression of symptoms, including the viral load, after administration of a treatment or placebo.
This trial was a randomized, placebo-controlled, double-blind human challenge study conducted to verify the efficacy and safety of S-337395. It involved 114 healthy adults who were actively inoculated with RSV. Participants were administered S-337395 or placebo for 5 days. The primary endpoint was the area under the curve of the viral load over time.1
About the Antiviral
S-337395 is a novel investigational oral treatment for RSV infection discovered through joint research with UBE. It is a low-molecular-weight compound with a novel mechanism that inhibits the RNA-dependent RNA polymerase activity of the L protein possessed by the RSV , thereby inhibiting the transcription and replication of the viral genome. Unlike F protein inhibitors, which exert their effect by preventing new viral infection of cells extracellularly, S-337395 works by preventing viral proliferation within infected cells, thus potentially offering higher efficacy and a more rapid reduction in viral load.1
Late last year, Shinogi received a Fast Track designation from the FDA for S-337395. The FDA’s Fast Track designation is designed to facilitate the development and expedite the review of potential new therapies that may treat serious conditions and fulfill an unmet medical need. Respiratory infections caused by RSV are diseases that most children contract at least once by the age of two and many children continue to experience recurrent infections throughout their lives.2
The antiviral is being jointly developed with UBE Corporation.
References
1.Shionogi Announces Positive Results from Phase 2 Trial of Respiratory Syncytial Virus Oral Antiviral Candidate S-337395. Shionogi press release. January 30, 2025. Accessed January 31, 2025.
https://www.shionogi.com/global/en/news/2025/01/20250130_2.html
2.Receives U.S. FDA Fast Track Designation for the Novel Anti-Respiratory Syncytial Virus Drug Candidate S-337395. Shionogi press release. October 24, 2024. Accessed January 31, 2025.
