Investigational RSV Antiviral Treatment Reduces Viral Load in Phase 2 Human Challenge Trial
The study of S-337395 showed an 88.9% reduction in RSV viral load and symptom improvement in the highest dose group, with no severe adverse events reported.
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A phase 2 clinical trial evaluating the investigational oral antiviral candidate S-337395 for respiratory syncytial virus (RSV) has demonstrated a statistically significant reduction in viral load among treated participants. The human challenge study demonstrated that the highest dose group exhibited a near 89% reduction in viral load.1
The randomized, placebo-controlled, double-blind study enrolled 114 healthy adults who were actively inoculated with RSV and received once-daily oral administration of S-337395 for 5 days. Individuals in the highest treatment dose group exhibited an 88.94% reduction in viral load (P<0.0001) and a statistically significant improvement in clinical symptom scores compared to those in the placebo group.1
Also, the treatment was generally well tolerated, with no serious or severe adverse events reported and no dose-dependent increase in the incidence or severity of adverse events. No participants discontinued treatment due to adverse events, according to a January 30, 2025, press release from Shionogi.1
S-337395 is being jointly developed by Shionogi and UBE and it functions as an inhibitor of the RSV L protein, a key component in viral replication. “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,” the pharmaceutical company stated in the release.1
In October 2024, the investigational antiviral was granted Fast Track designation by the US FDA, reflecting its potential to address an unmet medical need.2
RSV is a widespread respiratory virus that can cause severe complications, particularly in infants and older adults. In the US, RSV is associated with over 3 million cases annually, with significant hospitalization and mortality rates among individuals aged 60 years and older. Currently, effective antiviral treatment options remain limited, highlighting the need for novel therapeutic approaches.1
References:
1. Shionogi announces positive results from phase 2 trial of respiratory syncytial virus oral antiviral candidate S-337395. News release. Shionogi & Co., Ltd. January 30, 2025. Accessed January 30, 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. News release. Shionogi & Co., Ltd. October 24, 2024. Accessed January 30, 2025. https://www.shionogi.com/us/en/news/2024/10/receives-us-fda-fast-track-designation-for-the-novel-anti-respiratory-syncytial-virus-drug-candidate-s-337395.html
