Investigators administered baloxavir marboxil (BXM) orally to 14 distinct avian species to characterize pharmacokinetics of the prodrug and its active metabolite, baloxavir acid (BXA). The cohort showed substantial interspecies variability in systemic drug exposure. Measured plasma concentrations of BXM and BXA differed markedly among species, and those differences formed the empirical basis for later dose adjustments described in this report. The study team observed that variability was not confined to a single pharmacokinetic parameter; rather, Cmax, AUC, and absorption-related metrics all contributed to the heterogeneity in exposure.
The authors framed this heterogeneity in the context of zoonotic risk and captive-animal conservation. High pathogenicity avian influenza viruses (HPAIV) threaten zoo collections and rare species, and the inability to apply uniform dosing across taxa complicates the off-label use of human antivirals. Data indicated that a one-size-fits-all approach would not reliably achieve comparable systemic BXA exposure across species, an observation that motivated species-tailored regimens.
The study evaluated oral administration using different practical methods appropriate for captive birds. Investigators tested variations in how BXM was given to accommodate feeding behavior and animal handling constraints. Data indicated that drug–food interactions had a limited impact on systemic exposure in the tested conditions. In other words, although the route and feeding context were adjusted for animal welfare and feasibility, those variables did not meaningfully explain the wide interspecies differences in BXA exposure.
The manuscript did not provide a detailed mechanistic account of gastrointestinal factors for each species. The authors noted, however, that administration practicality in zoological settings was an important consideration when constructing dosing regimens. The study therefore prioritized methods that are feasible for routine veterinary use while reporting exposure outcomes tied to those methods.
Pharmacokinetic analysis revealed a strong association between systemic BXA exposure and indicators of drug absorption. Investigators reported that species with markers consistent with higher absorption efficiency tended to show greater plasma BXA levels. This pattern suggested that interspecies differences in absorption largely determine BXA exposure among the birds studied, rather than differences in metabolism or clearance alone.
Authors emphasized that absorption efficiency may reflect anatomical and physiological variation—such as differences in gastrointestinal transit time, mucosal surface, or feeding behavior—although the study did not experimentally dissect those mechanisms. The source did not report controlled mechanistic experiments to isolate absorption versus metabolism factors; it reported correlative pharmacokinetic relationships derived from observed concentration–time data.
Using the pharmacokinetic data, the study team developed optimized dosing regimens for each species included in the investigation. Those regimens were based on observed BXA exposure and the goal of achieving therapeutic systemic concentrations that have shown antiviral efficacy in prior avian models. Investigators described dosing adjustments tailored to species-specific exposure profiles rather than recommending a single universal dose.
The manuscript positioned these dosing recommendations as a pharmacokinetic foundation for clinical use in captive rare birds. The authors noted that previous experimental models have shown antiviral efficacy of BXM in birds, and that matching exposure across species is a prerequisite for translating that efficacy into practical treatment. The study did not include direct efficacy or long-term safety outcomes for the newly proposed regimens within the rare species cohort.
The authors reported several constraints. First, the pharmacokinetic dataset reflects single-dose or limited administration conditions in a controlled research setting; the source did not present long-term safety, efficacy, or multi-dose pharmacodynamics for the species tested. Second, although absorption indicators correlated with BXA exposure, the paper did not experimentally separate absorption from metabolic clearance mechanisms for each species. Third, regulatory approval for avian use of baloxavir marboxil does not exist; the study provided a pharmacokinetic rationale but did not address regulatory pathways or label changes.
Investigators called for follow-up work: controlled efficacy trials in target species, safety and tolerability assessments under repeated dosing, and mechanistic studies to pinpoint the physiologic determinants of absorption. Those specific unanswered questions remain critical before routine clinical deployment in zoological practice, according to the authors.