Shanghai, China, August 14, 2026
Shanghai Ark Biopharmaceutical Co., Ltd. (ArkBio) announced that the U.S. Food and Drug Administration (FDA) has approved a Phase II clinical trial of ziresovir (AK0529) in severely ill infants infected with respiratory syncytial virus (RSV), including patients requiring treatment in pediatric intensive care units (PICUs). The regulatory milestone expands the clinical development program for ziresovir into one of the most critically ill pediatric populations affected by RSV. The randomized, double-blind, placebo-controlled multicenter study is designed to evaluate the clinical efficacy of the direct-acting antiviral, with particular attention to oxygen treatment duration and sustained recovery.
Ziresovir Targets Severe RSV in Infants
RSV is a major cause of lower respiratory tract infections in infants and young children, creating a substantial global burden of hospitalization and severe disease. ArkBio cited estimates of approximately 33 million RSV-related lower respiratory tract infection cases, 3.6 million hospitalizations, and more than 100,000 deaths annually. Severely infected infants may experience rapid disease progression and require hospitalization and intensive respiratory support, particularly in PICUs. According to ArkBio, treatment for hospitalized patients with severe RSV remains largely focused on supportive care, highlighting the need for effective antiviral therapies targeting the virus itself. Ziresovir is being developed as a direct-acting RSV antiviral designed to address this unmet clinical need. ArkBio said the candidate successfully completed a Phase III clinical trial with positive results in patients aged 1 to 24 months in China. The company reported that treatment significantly reduced viral load, accelerated resolution of respiratory symptoms, and was associated with a lower risk of recurrent wheezing and asthma during a 24-month follow-up period. The company also reported a favorable safety profile in the study.
FDA-Approved Phase II Study Focuses on PICU Patients
The newly authorized Phase II clinical trial will evaluate ziresovir in severely ill RSV-infected infants requiring hospitalization in a PICU. The study will use a multicenter, randomized, double-blind, placebo-controlled design, providing a structured assessment of the candidate’s efficacy and safety in a high-risk population. One of the key clinical measures will be the duration of oxygen treatment, alongside sustained recovery, with the results intended to provide clinical evidence for potential subsequent registrational development. The trial represents an important expansion of ziresovir’s clinical development because it moves the program into patients with some of the most serious manifestations of RSV infection. ArkBio said previous clinical findings have been published in The New England Journal of Medicine and The Lancet Child & Adolescent Health. The company also stated that ziresovir was included in the World Health Organization’s 2025 Paediatric Drug Optimization publication for RSV as a therapeutic drug.
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ArkBio Expands Respiratory Drug Development
The FDA’s Phase II authorization strengthens ArkBio’s global clinical development strategy for ziresovir and broadens the company’s focus on respiratory and pediatric diseases. Founded in 2014, ArkBio is a commercial-stage biotechnology company developing therapeutics for respiratory, lung, and pediatric conditions through proprietary technology platforms and strategic collaborations. Beyond ziresovir, the company’s pipeline includes AK3280, an anti-fibrotic candidate being evaluated in Phase III registrational trials for idiopathic pulmonary fibrosis, and AK0901, an ADHD treatment approved and commercialized in China. ArkBio has also established collaborations with organizations including Roche, Genentech, The Scripps Research Institute, the Institute of Microbiology of the Chinese Academy of Sciences, and Qilu Pharmaceutical.
The advancement of ziresovir into an FDA-authorized Phase II study represents a significant step in the global development of an RSV-specific antiviral for severely ill infants. By evaluating the candidate in PICU patients and measuring clinically meaningful outcomes such as oxygen-treatment duration and sustained recovery, the study could generate important evidence for the next stage of development. The program also highlights continuing efforts in pediatric infectious disease drug development to address serious RSV disease with targeted antiviral treatment rather than relying solely on supportive care.
Source: ArkBio press release
