Evaluation of Baloxavir Marboxil Susceptibility in Pediatric Patients with Influenza: A Phase IIIb Multicenter, Single-Arm, Open-Label Study

The primary objective of this study is to evaluate the **prevalence** of pre-dose and treatment-emergent polymerase acidic protein (PA) I38X substitutions and other resistance-associated PA substitutions by type and subtype of **influenza** virus in patients treated with baloxavir marboxil. This is clinically relevant as it aims to understand the resistance patterns that may emerge during treatment, which is crucial for optimizing therapeutic strategies and improving patient outcomes in managing influenza.

Secondary objectives include:

• Evaluating the virologic characteristics of circulating and post-treatment viruses in patients treated with baloxavir marboxil. This will provide insights into the viral dynamics and potential changes in virus behavior post-treatment.
• Further evaluating the safety of a single dose of baloxavir marboxil by assessing the frequency, severity, and timing of adverse events. This is important for ensuring the safety profile of the medication in pediatric patients.

The clinical trial involves a total of **300 participants** diagnosed with **influenza**. The study population includes both male and female subjects, encompassing a broad age range from 2 years and older. Participants were selected based on the presence of symptoms suggestive of influenza, confirmed by a positive local influenza test within 24 hours prior to full study screening. Additionally, participants were required to have a negative test for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) within 48 hours before screening. The trial includes a vulnerable population, indicating that special considerations are in place to ensure their safety and compliance with study procedures. Participants or their caregivers must be able to understand the study requirements and provide informed consent. The study does not specify particular lifestyle considerations such as diet or physical activity. The selection criteria ensure that the study population is representative of individuals affected by influenza, allowing for a comprehensive evaluation of the trial's objectives.

The clinical trial is designed as a **Phase IIIb**, multicenter, single-arm, open-label surveillance study to evaluate the susceptibility to **baloxavir marboxil** in pediatric patients diagnosed with **influenza**. The primary objective is to assess the prevalence of pre-dose and treatment-emergent polymerase acidic protein (PA) I38X substitutions and other resistance-associated PA substitutions by type and subtype of the influenza virus. The trial is expected to commence recruitment on December 31, 2023, and conclude by May 1, 2027.

Participants will undergo a series of study visits, beginning with an inclusion (screening) visit. During this visit, eligibility will be confirmed through a positive local influenza test and a negative severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) test. The screening must occur within 24 hours of a positive influenza test and 48 hours of a negative SARS-CoV-2 test. The time interval between the onset of influenza symptoms and the start of pre-dose examinations should not exceed 48 hours. Participants or their guardians must provide written informed consent prior to the pre-dose examinations.

Following the screening, participants will receive the investigational product, **Xofluza 2 mg/mL granules for oral suspension**, administered orally. The primary endpoints include the incidence of resistance-associated pre-treatment substitutions determined at baseline (Day 1) and treatment-emergent substitutions based on sampling on Days 4, 6, and 10. Secondary endpoints will assess the incidence of treatment-emergent substitutions by age group, vaccination status, and influenza virus type and subtype, as well as viral titers and susceptibility to baloxavir marboxil.

The expected duration of participant involvement is approximately 10 days, with the possibility of early termination if participants fail to comply with study procedures or if adverse events necessitate withdrawal. The study will monitor the incidence and severity of adverse events, with severity determined according to the National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE) version 5.0. The trial is not classified as a low-intervention study, and it is part of an ongoing development program for the molecule.

The clinical trial involves the administration of **Xofluza 2 mg/mL granules for oral suspension**, which contains the active substance **baloxavir marboxil**. This pharmaceutical form is specifically designed as an oral suspension, facilitating ease of administration in pediatric patients. The medication is administered orally, with a maximum daily dose of 80 mg, and the treatment period is limited to a single day. The formulation is a pediatric-specific preparation, ensuring appropriate dosing and safety for the target population. The active substance, baloxavir marboxil, is a chemical compound known for its antiviral properties, particularly against influenza viruses. The chemical structure of baloxavir marboxil is denoted by its synonym, S-033188, and it is classified under the ATC code J05AX25.

In this study, no non-experimental treatments such as standard-of-care therapy, placebo, or comparator treatments are utilized. The focus is solely on the administration of the experimental medication, Xofluza, to evaluate its efficacy and resistance profile in pediatric patients with influenza. Compliance with the dosing schedule is monitored to ensure accurate assessment of the treatment's effects. The study is designed as a Phase IIIb multicenter, single-arm, open-label surveillance study, aiming to evaluate the prevalence of pre-dose and treatment-emergent polymerase acidic protein (PA) I38X substitutions and other resistance-associated PA substitutions by type and subtype of influenza virus in patients treated with baloxavir marboxil.

Efficacy in this clinical trial will be assessed through a series of primary and secondary endpoints. The primary endpoints include the incidence of resistance-associated pre-treatment substitutions determined at baseline (Day 1) and the incidence of resistance-associated treatment-emergent substitutions based on sampling on Days 4, 6, and 10. These endpoints are designed to evaluate the prevalence of pre-dose and treatment-emergent polymerase acidic protein (PA) I38X substitutions and other resistance-associated PA substitutions by type and subtype of the influenza virus in patients treated with **baloxavir marboxil**.

Secondary endpoints will further explore the incidence of resistance-associated treatment-emergent substitutions by age groups (< 5 years vs. ≥ 5 years), the incidence of novel treatment-emergent mutations in PA, and the incidence of resistance-associated treatment-emergent substitutions by baseline vaccination status (vaccinated vs. not vaccinated in the last 6 months). Additionally, the study will assess the incidence of influenza virus type (A or B) and subtype (A/H1 or A/H3) in participants by study period, viral titers by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) at baseline and post-baseline timepoints, and the susceptibility to **baloxavir marboxil** by phenotyping of post-baseline samples with novel genotypic PA substitutions. The incidence and severity of adverse events, including serious adverse events, will be determined according to the National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE) version 5.0.