---
title: "Pharmacokinetic variability of baloxavir marboxil among 14 rare avian species"
id: "plos-one-16-pharmacokinetic-diversity-of-baloxavir-marboxil-across-rare-avian-species"
canonical_url: "https://medichelpline.com/clinical-feed/plos-one-16-pharmacokinetic-diversity-of-baloxavir-marboxil-across-rare-avian-species"
content_type: "clinical_feed_article"
specialty: "Infectious Disease"
source_name: "PLOS ONE (Medicine)"
source_url: "https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0361119"
published_at: "2026-10-23T14:00:00.000Z"
evidence_level: "Journal Feed"
license: "CC-BY-NC-4.0 / Informational Use"
---
# Pharmacokinetic variability of baloxavir marboxil among 14 rare avian species
## Provenance & Clinical Metadata
- **Canonical URL:** https://medichelpline.com/clinical-feed/plos-one-16-pharmacokinetic-diversity-of-baloxavir-marboxil-across-rare-avian-species
- **Specialty:** [Infectious Disease](https://medichelpline.com/clinical-feed/infectious-disease.md)
- **Primary Source:** PLOS ONE (Medicine)
- **Source URL:** [Original Journal Publication](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0361119)
- **Published At:** 2026-10-23T14:00:00.000Z
- **Evidence Rating:** Journal Feed
## Executive GIST (TL;DR)
- Investigators administered **baloxavir marboxil** (BXM) to 14 avian species to define pharmacokinetic behavior and inform species-specific dosing. - The study measured both parent drug and the active metabolite **baloxavir acid** (BXA); systemic exposure varied markedly across species. - Oral dosing used multiple administration methods; the cohort showed that drug–food interactions produced only a limited effect on systemic exposure. - Pharmacokinetic analysis linked higher BXA exposure to indicators of drug absorption, suggesting absorption efficiency drives interspecies differences. - Authors developed optimized dosing regimens tailored to each species based on observed exposure parameters. - The work focused on practical antiviral strategies to protect captive rare birds from **HPAIV** and provide a pharmacokinetic foundation for therapeutic use. - The paper reported limitations related to interspecies extrapolation and the need for efficacy and safety studies in diverse species; the source did not report detailed clinical outcome or long-term safety data. - Data supporting the manuscript are available within the article and investigators noted collaborative support from multiple zoological institutions and industry partners.
## Clinical Analysis & Structured Key Points
Pharmacokinetic diversity of baloxavir marboxil across rare avian species | PLOS One Browse Subject Areas ? Click through the PLOS taxonomy to find articles in your field. For more information about PLOS Subject Areas, click here . Article Authors Metrics Comments Media Coverage Reader Comments Figures Figures Abstract High pathogenicity avian influenza virus (HPAIV) poses a serious threat to captive rare birds. Because effective countermeasures are limited, the use of human antiviral drugs to treat HPAIV infections in captive rare birds has been considered. Although the human antiviral drug, baloxavir marboxil (BXM), is not approved for the treatment of influenza virus infection in birds, previous studies demonstrated the antiviral efficacy of BXM in experimental avian models. Pharmacokinetic studies have also revealed substantial interspecies differences following BXM administration, suggesting the need to establish species-specific dosing regimens. In this study, BXM was administered to 14 avian species to optimize dosing regimens for each species. Oral administration of BXM using different methods revealed that drug–food interactions had a limited impact on systemic drug exposure. Pharmacokinetic analysis of BXM and its active metabolite, baloxavir acid (BXA), demonstrated marked interspecies variability in drug exposure, contributing to the development of optimized dosing regimens for each species. Furthermore, systemic exposure to BXA was strongly associated with drug absorption indicators, suggesting that species-specific differences in absorption efficiency largely determine BXA exposure among avian species. This study provides a pharmacokinetic basis for the rational use of BXM in diverse avian species. It contributes to the development of practical antiviral strategies to protect captive rare birds against HPAIVs. Citation: Shimazu Y, Obara RD, Miki M, Osaki H, Yoshimoto Y, Hayashi M, et al. (2026) Pharmacokinetic diversity of baloxavir marboxil across rare avian species. PLoS One 21(10): e0361119. https://doi.org/10.1371/journal.pone.0361119 Editor: Naveen Kumar, National Institute of High Security Animal Diseases of Indian Council of Agricultural Research: National Institute of High Security Animal Diseases, INDIA Received: August 17, 2026; Accepted: October 6, 2026; Published: October 23, 2026 Copyright: © 2026 Shimazu et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: All data are available within the manuscript. Funding: This work was mainly funded by JAC Environmental & Animal Protection Foundation, Japan. This research was partially conducted as part of a collaborative research project between Hokkaido University and Shionogi & Co., Ltd. It was supported by the Environment Research and Technology Development Fund [JPMEERF20254004] of the Environmental Restoration and Conservation Agency provided by the Ministry of the Environment of Japan. Additionally, this work was partially supported by the Japan Agency for Medical Research and Development (AMED) [JP223fa627005, JP20wm0125008, JP253m0134005 and JP263f0134005]. Y.Sh. is supported by the World-Leading Innovative and Smart Education Program (1801) from the Ministry of Education, Culture, Sports, Science and Technology, Japan and the Japan Science and Technology Agency SPRING grant [JPMJSP2119]. Shionogi & Co., Ltd. provided support in the form of salaries for authors R.D.O., M.M., H.O., K.N., and T.S. AWS Co., Ltd. provided support in the form of salaries for authors K.Toy., M.O., and I.H. Shikoku Aquarium Development Co., Ltd. provided support in the form of salary for author M.K. These funders did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific roles of these authors are articulated in the Author Contributions section. Competing interests: R.D.O., H.O., and T.S. are employees of Shionogi & Co., Ltd. M.M. and K.N. are former employees of Shionogi & Co., Ltd. K.Toy., M.O., and I.H. are employees of AWS Co., Ltd., and M.K. is an employee of Shikoku Aquarium Development Co., Ltd. These commercial affiliations did not influence the study design, data analysis, interpretation of the results, or the conclusions of this study. The remaining authors declare no competing interests. This does not alter our adherence to PLOS ONE policies on sharing data and materials. 1. Introduction High pathogenicity avian influenza viruses (HPAIVs) cause severe and often fatal disease in a wide range of avian species [ 1 ]. Since 2020, H5 HPAIVs belonging to HA clade 2.3.4.4b have been widely disseminated throughout wild bird populations, causing extensive outbreaks in poultry and substantial mortality in wild birds [ 2 ]. This ongoing panzootic has also affected rare avian species across multiple regions, including Europe, Asia, Africa, the Americas, and Antarctica [ 3 , 4 ]. Despite the ongoing risk of HPAIV infection, birds in zoological institutions are often housed in open enclosures to meet animal welfare requirements, which inherently limits the implementation of strict biosecurity measures. Consequently, zoos have experienced HPAIV outbreaks among captive birds, leading to the loss of rare avian species, such as snowy owls ( Bubo scandiacus ), black swans ( Cygnus atratus ), and Humboldt penguins ( Spheniscus humboldti ) [ 5 , 6 ]. Although some European countries have implemented vaccination programs against HPAIV in captive birds, vaccine efficacy varies among species [ 7 ]. In countries where vaccination is not implemented for poultry, such as Japan, controlling HPAIV infection in both zoos and poultry farms relies primarily on enhanced biosecurity measures [ 8 ]. The standard recommendation when HPAIV infection is confirmed in captive flocks is the culling of infected individuals and contact birds to prevent further spread and eliminate the virus. However, from the standpoint of conservation medicine, the euthanasia of endangered avian species may not be desirable. Accordingly, therapeutic options for HPAIV infections in captive rare birds have been explored. Several anti-influenza drugs, originally developed for human influenza, have been evaluated for the treatment of HPAIV infections in a chicken model. Human anti-influenza drugs are classified into neuraminidase (NA) and polymerase acidic protein (PA) inhibitors [ 9 , 10 ]. NA, a viral surface glycoprotein with sialidase activity, plays a role in facilitating the release of progeny virions from infected host cells. Evaluation of NA inhibitors, such as oseltamivir, zanamivir, and peramivir, in HPAIV-infected chickens revealed that none of them completely averted disease onset or mortality [ 11 , 12 ]. This limited efficacy may reflect that NA inhibitors primarily suppress viral release without inhibiting viral replication. In contrast, PA inhibitors act during viral RNA transcription by inhibiting the PA subunit of the influenza virus RNA-dependent RNA polymerase complex, which possesses the endonuclease activity responsible for cap-snatching, a process essentially required for viral mRNA transcription [ 13 ]. Baloxavir marboxil (BXM) is a PA inhibitor approved for clinical use in humans and is an orally administered prodrug that is converted to its pharmacologically active form, baloxavir acid (BXA) [ 14 ]. BXA treatment inhibited the replication of H5 HPAIVs in vitro, while oral administration of BXM suppressed viral replication and improved survival in an H5N1-infected mouse model [ 15 ]. In chickens that exhibit extremely rapid disease progression and typically die within 2–3 days after HPAIV infection, prophylactic administration of BXM at 2.5 mg/kg at the time of HPAIV inoculation was sufficient to prevent mortality [ 12 ]. However, in chickens, administration initiated one day after infection failed to confer therapeutic benefits, even at substantially higher doses. This limited efficacy is likely attributable to the narrow therapeutic window imposed by the rapid disease progression in this species [ 16 ]. In contrast to chickens, many avian species, including ducks, sparrows, crows, herons, white-tailed eagles, and wild waterfowl, exhibit a prolonged disease course following HPAIV infection, with clinical progression occurring over several days [ 17 – 20 ]. In ducks, which typically die 4–7 days after HPAIV infection, BXM treatment was initiated 48 hours (h) post-infection and continued at 2.5 mg/kg for 5 consecutive days, effectively reducing both mortality and virus shedding [ 21 ]. These findings suggest that therapeutic administration of BXM after infection may be feasible in avian species with slower disease progression. In that study, systemic exposure to BXA, as measured by the area under the concentration–time curve (AUC), correlated with these therapeutic effects, suggesting that maintaining BXA exposure above a certain threshold is critical for antiviral efficacy. These findings provide a basis for considering the application of BXM treatment to rare avian species affected by HPAIV. Despite these findings, the safety and pharmacokinetics of BXM in avian species have not yet been fully characterized. Although no clinical signs or organ toxicity were observed in chickens, even at doses up to 62.5 mg/kg administered for 28 consecutive days, corresponding to a 71-fold safety margin, the species-specific drug toxicity of BXM cannot be excluded, as exemplified by the severe toxicity of diclofenac, a nonsteroidal anti-inflammatory drug, in vulture species [ 22 , 23 ]. Moreover, the pharmacokinetic profiles of drugs vary considerably among the different avian species. For example, the elimination half-life of carboplatin, a platinum-based antineoplastic agent, is correlated with body weight across avian species, whereas that of amikacin, an aminoglycoside antibiotic, shows little species variation [ 24 , 25 ]. In contrast, the pharmacokinetic properties of opioids are significantly influenced by the species [ 26 ]. Therefore, a direct evaluation of the pharmacokinetic profile of each target species is essential to establish appropriate dosing regimens. In addition, BXM absorption is reduced by food intake in humans, raising the possibility that species-specific feeding habits and administration conditions influence BXA exposure in birds [ 27 ]. Furthermore, as clinical use in birds would likely require repeated administration over several days, it is necessary to evaluate the potential safety and drug accumulation under repeated-dose regimens. In the present study, the effects of different oral administration methods on the pharmacokinetics of BXA were evaluated in white-tailed eagles ( Haliaeetus albicilla ) to assess the influence of feeding conditions on systemic BXA exposure. Subsequently, a comprehensive pharmacokinetic evaluation following BXM administration was conducted in a diverse range of rare avian species in Japan. In addition, a repeated-dose study was conducted in a Eurasian eagle-owl ( Bubo bubo ) to evaluate the potential drug accumulation associated with consecutive dosing. Based on the pharmacokinetic data obtained for these species, practical and species-optimized dosing regimens using commercially available BXM formulations were proposed for therapeutic use in infected individuals and potential metaphylactic use in exposed birds following detection of HPAIV infection within a flock or facility. 2. Materials and methods 2.1. Animals and drugs 2.1.1. Animals. Fourteen avian species housed in zoos, aquariums, and wildlife rehabilitation centers in Japan were used in this study. The study species were selected based on their conservation status or on reported cases of HPAIV infection. Among the study species, HPAIV infections have been reported in wild birds in Japan, including several species of conservation concern according to the Red List of the Ministry of the Environment, Japan: common murres ( Uria aalge ; Critically Endangered [CR]), Oriental storks ( Ciconia boyciana ; Endangered [EN]), black-faced spoonbills ( Platalea minor ; Vulnerable [VU]), white-tailed eagles (Vulnerable [VU]), and peregrine falcons ( Falco peregrinus ; Near Threatened [NT]) [ 28 ]. Puna ibis ( Plegadis ridgwayi ) was included as a species closely related to the crested ibis ( Nipponia nippon ), which is classified as Endangered (EN), to provide comparative pharmacokinetic information within the same taxonomic group. The rock ptarmigan included in this study was the Svalbard rock ptarmigan ( Lagopus muta hyperborea Sundevall), a subspecies of the same species as the Japanese rock ptarmigan ( Lagopus muta japonica Clark), which is classified as Endangered (EN). The Eurasian eagle-owl was included as a species closely related to Blakiston’s fish owl ( Bubo blakistoni ), an Endangered (EN) species in which HPAIV infection has been reported, providing comparative pharmacokinetic information for a taxonomically related species of conservation concern. The endemic Amami jay ( Garrulus lidthi ) was included as a species of conservation importance in Japan, although it is not currently classified as threatened. To reflect the conditions commonly encountered in zoological facilities, several penguin species, including Humboldt penguins (Vulnerable), African penguins ( Spheniscus demersus ; Critically Endangered), and southern rockhopper penguins ( Eudyptes chrysocome ; Vulnerable), according to the IUCN Red List, were included. Humboldt penguins have experienced HPAIV infections in both wild and captive settings, African penguins have been infected with HPAIV in the wild, and southern rockhopper penguins are frequently housed in zoos despite the absence of confirmed HPAIV infections [ 6 , 29 , 30 ]. Furthermore, the great white pelicans ( Pelecanus onocrotalus ) and American flamingos ( Phoenicopterus ruber ), both of which have experienced HPAIV outbreaks in zoological collections, were included in this study. The number of animals, age, body weight, and sex of each species are summarized in Table 1 and S1 Table . All individuals were confirmed healthy based on clinical observations. Preliminary hematological examinations were also conducted for certain species when samples were available ( S2 Table ). Throughout the study period, all birds were monitored at least twice daily for clinical signs, including changes in activity, posture, feather condition, respiratory status, and fecal consistency, and abnormal findings were recorded. Particular attention has been paid to signs of diarrhea and anaphylaxis, which have been reported in human clinical trials. Download: PNG larger image TIFF original image Table 1. Species, number of animals, body weight ranges, doses, and administration routes of baloxavir marboxil in the birds included in this study. https://doi.org/10.1371/journal.pone.0361119.t001 2.1.2. Drugs. BXM (20 mg tablets, Shionogi & Co., Ltd., Osaka, Japan) was crushed and weighed based on the body weight of each individual. 2.2. Administration experiment 2.2.1. Evaluation of oral administration routes for baloxavir marboxil in white-tailed eagles. Four white-tailed eagles were administered BXM at 2.5 mg/kg. BXM was administered three times at intervals of at least one week, using a different administration method for each trial. For the first administration, crushed BXM was encapsulated and inserted into the oral cavity of rainbow trout, which were then force-fed to the birds. For the second administration, the encapsulated BXM was manually inserted into the oral cavity, followed immediately by forced feeding of the rainbow trout. For the third administration, the crushed BXM was suspended in water and administered via a gastric tube. After each administration, all birds were monitored for clinical abnormalities potentially attributable to the BXM administration. In all trials, 230 μL of blood was collected at 2, 8, 24, and 48 h post-administration (hpa). The collected blood samples were thoroughly mixed with 13.8 μL of an enzyme inhibitor–anticoagulant mixture (60 mmol/L dichlorvos [FUJIFILM Wako Pure Chemical Corp., Osaka, Japan] and sodium heparin [Nipro Corp., Osaka, Japan]) and centrifuged at 1900 × g for 15 min to obtain plasma. Plasma samples were stored at –80°C until further analysis. 2.2.2. Single oral administration of baloxavir marboxil in rare avian species. BXM was orally administered to Humboldt penguins, African penguins, southern rockhopper penguins, puna ibises, common murres, great white pelicans, American flamingos, peregrine falcons, Oriental storks, and black-faced spoonbills. The routes of administration for each species are summarized in Table 1 . These routes were chosen for their practical applicability in animal facilities to ensure safe and effective use even in HPAIV-infected individuals. An initial dose of 2.5 mg/kg was administered to all animals. A second dose of 12.5 mg/kg was administered after an interval of at least two weeks to all species, except black-faced spoonbills. For all species except great white pelicans, 230 μL of blood was collected at 2, 8, 24, and 48 hpa, followed by plasma separation and storage as described above. In great white pelicans, blood sampling was performed at 6, 8, 24, and 48 hpa to minimize the risk of regurgitation during restraint. Additionally, to reduce the physiological burden associated with blood sampling, rock ptarmigans and Amami jays received a single oral dose of BXM at 12.5 mg/kg, with blood samples collected at 2, 8, and 24 hpa under thiopental anesthesia. After each administration, all birds were monitored for clinical abnormalities potentially attributable to the BXM administration. 2.2.3. Repeated administration of baloxavir marboxil in Eurasian eagle-owl. Powdered BXM at 2.5 mg/kg was mixed with horse meat and orally administered to a Eurasian eagle-owl once daily for 7 consecutive days. Blood samples were collected 2, 8, and 24 h after the initial dose, immediately before the seventh dose, and 2, 8, and 24 h after the seventh dose. After an interval of at least one week between dosing periods, a 5.0 mg/kg regimen was administered, followed by a 10 mg/kg regimen. In the 5.0 mg/kg and 10 mg/kg regimens, blood samples were collected immediately before the initial and seventh doses, as well as at 2, 8, and 24 h after each dose. Plasma separation and storage were performed as previously described. After each administration, the bird was monitored for clinical abnormalities potentially attributable to the BXM administration. 2.3. LC–MS/MS analysis of baloxavir marboxil and baloxavir acid Plasma concentrations of BXM and BXA were determined by liquid chromatography–tandem mass spectrometry (LC–MS/MS) using an Agilent 6495B system (Agilent Technologies, Inc., Santa Clara, CA, USA). For BXM analysis, 10 μL of plasma was mixed with 100 μL of methanol and 100 μL of acetonitrile containing the internal standard. For BXA analysis, 3 μL of plasma was mixed with 200 μL of methanol and 200 μL of internal standard solution. The mixture was vortexed and incubated at –80°C for 30 min, followed by centrifugation at –10°C and 10,000 × g for 10 min. The resulting supernatant was mixed with an equal volume of 0.1% EDTA-2K solution (Dojindo Laboratories, Kumamoto, Japan) and analyzed using LC–MS/MS. Stable isotope-labeled internal standards were used for quantification. BXM-d 3 18 O and BXA-racemate-d 4 18 O were used as the internal standards for BXM and BXA analyses, respectively. Chromatographic separation was performed using a 2.1 mm × 100 mm L-column3 C18 metal-free column with a particle size of 2.0 μm (Chemicals Evaluation and Research Institute, Tokyo, Japan) maintained at 45°C. Mobile phase A consisted
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