ArkBio’s Ziresovir Demonstrates Clinical Efficacy in Hospitalized Infants with Respiratory Syncytial Virus

Image Credit: Adobe Stock Images/udomsak

Results from a Phase III clinical trial, including two-year follow-up data, show that ArkBio’s ziresovir (AK0529), a novel antiviral for respiratory syncytial virus (RSV), is the first therapy in its class to demonstrate both significant clinical efficacy and long-term respiratory benefits in hospitalized infants under six months of age—a population with the highest risk for severe RSV infection. Full results from the trial were published in The Lancet Child & Adolescent Health.¹

"This pivotal Phase III trial provides the first solid evidence of an effective antiviral treatment for RSV in high-risk infants,” said Xin Ni, principal investigator, Airflo study, professor, Beijing Children's Hospital, Capital Medical University, in a press release. “Following our initial publication in The New England Journal of Medicine, this deeper analysis in infants under six months published in The Lancet Child & Adolescent Health further validates ziresovir's safety, efficacy, and long-term benefit. We are proud of our team's commitment and look forward to bringing this drug to patients in China and worldwide."

The multicenter, randomized, double-blind, placebo-controlled trial evaluated infants aged one month to two years with virologically confirmed RSV infection, including a subgroup of 188 infants aged six months or younger. Participants were randomly assigned in a 2:1 ratio to receive either ziresovir or placebo orally for five days, followed by a two-year observational period. The study’s primary endpoint was the change in Wang Bronchiolitis Clinical Score (WBCS) from baseline to day three in the intention-to-treat infected population.^1,2

In infants under six months of age, ziresovir demonstrated a 3.5-point reduction in WBCS, compared to a 2.2-point reduction in the placebo group, which translates to a 54.5% relative difference. Patients administered ziresovir showed statistically significant improvements in respiratory rate, wheezing, and chest retractions.

By day five, the ziresovir group also showed a greater reduction in viral load (2.51 log₁₀ copies/mL vs. 1.87 log₁₀ in the placebo group; p=0.024), reinforcing its antiviral activity, according to the investigators. Additionally, the time to symptom resolution was faster for ziresovir-treated patients, with a hazard ratio of 1.53 for wheezing resolution (p=0.021) and 1.49 for retraction relief (p=0.018). While trends toward shorter hospital stays and reduced oxygen use were observed, these differences were not deemed statistically significant.

Long-term respiratory outcomes further supported ziresovir’s benefit. Infants in the treatment group experienced a 3.6-fold lower incidence of recurrent wheezing (0.18% vs. 0.65%; p=0.0048), with 2.6 times fewer wheezing episodes overall (1.2 vs. 3.1). Asthma incidence was also lower in the ziresovir group (3% vs. 5%).

In terms of safety, treatment-emergent adverse events (AEs) occurred in 18% of the treatment group compared to 11% in the placebo group, with no reported drug-related serious AEs or deaths.¹

Annually, RSV results in approximately 2.1 million outpatient visits in pediatric patients under five years of age, with an additional 600,000 hospitalizations. According to Contagion Live, this makes it the most common cause of hospital admission in children under one year of age. RSV can often lead to other severe conditions, such as bronchiolitis, particularly in vulnerable groups.³ According to the Centers for Disease Control and Prevention, RSV causes anywhere from 100,000 to 150,000 hospitalizations in patients over 60 years of age annually.⁴

References

1. The Lancet Child & Adolescent Health Publishes Phase III Trial and 24-Month Follow-Up Results of Ziresovir for RSV in Infants Under 6 Months. PR Newswire. April 15, 2025. Accessed April 16, 2025. https://prnmedia.prnewswire.com/news-releases/the-lancet-child--adolescent-health-publishes-phase-iii-trial-and-24-month-follow-up-results-of-ziresovir-for-rsv-in-infants-under-6-months-302428382.html

2. Efficacy and safety of ziresovir in hospitalised infants aged 6 months or younger with respiratory syncytial virus infection in China: findings from a phase 3 randomised trial with 24-month follow-up. ScienceDirect. Accessed April 16, 2025. https://www.sciencedirect.com/science/article/abs/pii/S2352464225000677

3. Respiratory Syncytial Viral Infection: Treatment With Ziresovir in Infants. ContagionLive. January 12, 2025. Accessed April 16, 2025. https://www.contagionlive.com/view/respiratory-syncytial-viral-infection-treatment-with-ziresovir-in-infants

4. Surveillance of RSV. CDC. Accessed April 16, 2025. https://www.cdc.gov/rsv/php/surveillance/index.html