Population Pharmacokinetic/Pharmacodynamic Analysis of Intravenous Zanamivir in Healthy Adults and Hospitalized Adult and Pediatric Subjects With Influenza.

Zanamivir is a potent and highly selective inhibitor of influenza neuraminidase in which the inhibition of this enzyme prevents the virus from infecting other cells and specifically prevents release of the new virion from the host cell membrane. It is available as an oral powder for inhalation and intravenous formulations. The current population pharmacokinetic model based on data from eight studies of subjects treated with the intravenous formulation (125 healthy adults and 533 hospitalized adult and pediatric subjects with suspected or confirmed influenza) suggested a decreased zanamivir clearance in pediatric and renal impairment adult subjects. It also indicates that b.i.d. dosing is necessary to keep the exposure in influenza infected subjects above the 90% inhibitory concentration values of recently circulating viruses over the dosing interval. In the exposure-response analysis (phases II and III studies), no apparent relationship was found between zanamivir exposure and clinically relevant pharmacodynamic end points.

Neuraminidase inhibitors as a strategy for influenza treatment: pros, cons and future perspectives.

Introduction: Influenza represents a major public health threat worldwide. Implementation of good personal health and hygiene habits, together with vaccination, is the most effective tools to reduce influenza burden both in community and in healthcare setting. However, achieving adequate vaccination rates is challenging, and vaccination does not always guarantee complete protection. Neuraminidase inhibitors represent an important measure to reduce the risk of influenza-related complications among high-risk patients developing influenza infection. Areas covered: Neuraminidase inhibitors have been proven to be safe and effective in reducing influenza severity, duration of symptoms, hospitalizations, and influenza-related-mortality. Here the authors review the available data on neuraminidase inhibitors, including the mechanism of action, pharmacokinetics, efficacy, safety and current indications for their use in clinical practice. Expert opinion: Although vaccination is the most effective tool to reduce influenza-associated morbidity and mortality, neuraminidase inhibitors represent an important option for the treatment of patients with influenza infection, particularly in high-risk categories. Moreover, antivirals play an important role in influenza prevention and prophylaxis in selected settings.

Intrapulmonary Pharmacokinetics of Laninamivir, a Neuraminidase Inhibitor, after a Single Nebulized Administration of Laninamivir Octanoate in Healthy Japanese Subjects.

A single dose of laninamivir octanoate (LO) inhaled using a dry powder inhaler (DPI) is effective for the treatment and prophylaxis of influenza. Nebulizers are an option for pediatric and elderly patients who may have difficulty in using a DPI. A single-center, open-label study was conducted to evaluate the plasma and intrapulmonary pharmacokinetics (PK) of laninamivir after a single nebulized administration of LO in healthy male Japanese subjects for identifying a safe and effective dosage regimen for a nebulizer. A single dose of LO (40 to 320 mg) was administered using a nebulizer, and plasma concentrations of LO and laninamivir were analyzed up to 168 h after inhalation by validated liquid chromatography-tandem mass spectrometry methods. Subgroups of 6 subjects each underwent bronchoalveolar lavage at specified time intervals over 4 to 168 h following a single nebulized administration of LO (160 mg), and the concentrations in epithelial lining fluid (ELF) were calculated by the urea diffusion method. PK parameters were determined by noncompartment analysis. Inhaled nebulized LO was found to be safe and well tolerated up to the highest dose evaluated (320 mg). Plasma laninamivir concentrations increased almost dose proportionally. Laninamivir concentrations in ELF exceeded the 50% inhibitory concentrations for viral neuraminidase up to 168 h after the nebulized inhalation of 160 mg LO. Thus, similarly to the DPI, ELF concentration profiles of laninamivir after a single nebulized administration support its long-lasting effect against influenza virus infection. This study has been registered at JAPIC Clinical Trials Information (http://www.clinicaltrials.jp/) under registration no. JAPIC CTI-152996.

Preparation, characterization and pulmonary pharmacokinetics of a new inhalable zanamivir dry powder.

This work describes a new dry powder for inhalation containing zanamivir, which is less hygroscopic than Relenza®. The powders were prepared via a spray-drying technique using mannitol as the carrier. A 5(3) central composite design was used to optimize the formulations. The final optimized powders, characterized with an angle of repose 37.48°, an aerodynamic diameter of 2.346 µm and in vitro deposition of 58.54%, were obtained by using the predicted variable values. Relenza® absorbed a significant amount of water at 66%, 75% and 85% relative humidity (RH; weight changes of approximately 1.38%, 2.18% and 3.72%, respectively). In contrast, the weight change for the zanamivir dry powder inhalation (DPI) was negligible when the RH was increased to 66%. The in vivo potential for the optimized powders was studied further in rats via the endotracheal administration of an 8.4 mg/kg dose. The bioavailability was 116% relative to Relenza®. Fluorescence imaging monitored the zanamivir dry powder inhalers in rats. The results indicated that the zanamivir DPIs were effectively delivered to the lung. These results indicate that the spray-dried zanamivir DPIs were promising for pulmonary delivery.

Computational assay of Zanamivir binding affinity with original and mutant influenza neuraminidase 9 using molecular docking.

Based upon molecular docking, this study aimed to find notable in silico neuraminidase 9 (NA9) point mutations of the avian influenza A H7N9 virus that possess a Zanamivir resistant property and to determine the lead compound capable of inhibiting these NA9 mutations. Seven amino acids (key residues) at the binding site of neuraminidase 9 responsible for Zanamivir-NA9 direct interactions were identified and 72 commonly occurring mutant NA9 versions were created using the Sybyl-X 2.0 software. The docking scores obtained after Zanamivir was bound to all mutant molecules of NA9 revealed 3 notable mutations R292W, R118P, and R292K that could greatly reduce the binding affinity of the medicine. These 3 mutant NA9 versions were then bound to each of 154 different molecules chosen from 5 groups of compounds to determine which molecule(s) might be capable of inhibiting mutant neuraminidase 9, leading to the discovery of the lead compound of potent mutant NA9 inhibitors. This compound, together with other mutations occurring to NA9 identified in the study, would be used as data for further research regarding neuraminidase inhibitors and synthesizing new viable medications used in the fight against the virus.

Zanamivir Amidoxime- and N-Hydroxyguanidine-Based Prodrug Approaches to Tackle Poor Oral Bioavailability.

The neuraminidase (NA) inhibitor zanamivir (1) is potently active against a broad panel of influenza A and B strains, including mutant viruses, but suffers from pharmacokinetic (PK) shortcomings. Here, distinct prodrug approaches are described that aimed at overcoming zanamivir's lack of oral bioavailability. Lowering the high basicity of the 4-guanidino group in zanamivir and of a bioisosteric 4-acetamidine analog (5) by N-hydroxylation was deemed to be a plausible tactic. The carboxylic acid and glycerol side chain were also masked with different ester groups. The bioisosteric amidine 5 turned out to be potently active against a panel of H1N1 (IC50 = 2-10 nM) and H3N2 (IC50 = 5-10 nM) influenza A viruses (NA inhibition assay). In vitro PK studies showed that all prodrugs were highly soluble, exhibited low protein binding, and were bioactivated by N-reduction to the respective guanidines and amidines. The most promising prodrug candidates, amidoxime ester 7 and N-hydroxyguanidine ester 8, were subjected to in vivo bioavailability studies. Unfortunately, both prodrugs were not orally bioavailable to a convincing degree (F ≤ 3.7%, rats). This finding questions the general feasibility of improving the oral bioavailability of 1 by lipophilicity-increasing prodrug strategies, and suggests that intrinsic structural features represent key hurdles.

Optimizing antiviral therapy for influenza: understanding the evidence.

Influenza is an important cause of annual epidemics of respiratory viral infection associated with significant morbidity and mortality. Three classes of drugs, the M2 ion channel, neuraminidase and RNA-dependent RNA polymerase inhibitors, are approved for the prevention and treatment of influenza. Due to widespread resistance to the class, the M2 ion channel inhibitors are not recommended currently for therapy. The only polymerase inhibitor, favipiravir, is approved only in Japan and its use is highly restricted. Despite significant data to support the early use of the neuraminidase inhibitors, their use in all patient populations is suboptimal. The data to support the early use of neuraminidase inhibitors will be reviewed, as will current data on the utilization rates in ambulatory and hospitalized populations.

Safety and pharmacokinetics of intravenous zanamivir treatment in hospitalized adults with influenza: an open-label, multicenter, single-arm, phase II study.

BACKGROUND: Intravenous zanamivir is a neuraminidase inhibitor suitable for treatment of hospitalized patients with severe influenza. METHODS: Patients were treated with intravenous zanamivir 600 mg twice daily, adjusted for renal impairment, for up to 10 days. Primary outcomes included adverse events (AEs), and clinical/laboratory parameters. Pharmacokinetics, viral load, and disease course were also assessed. RESULTS: One hundred thirty patients received intravenous zanamivir (median, 5 days; range, 1-11) a median of 4.5 days (range, 1-7) after onset of influenza; 83% required intensive care. The most common influenza type/subtype was A/H1N1pdm09 (71%). AEs and serious AEs were reported in 85% and 34% of patients, respectively; serious AEs included bacterial pulmonary infections (8%), respiratory failure (7%), sepsis or septic shock (5%), and cardiogenic shock (5%). No drug-related trends in safety parameters were identified. Protocol-defined liver events were observed in 13% of patients. The 14- and 28-day all-cause mortality rates were 13% and 17%. No fatalities were considered zanamivir related. Pharmacokinetic data showed dose adjustments for renal impairment yielded similar zanamivir exposures. Ninety-three patients, positive at baseline for influenza by quantitative polymerase chain reaction, showed a median decrease in viral load of 1.42 log10 copies/mL after 2 days of treatment. CONCLUSIONS: Safety, pharmacokinetic and clinical outcome data support further investigation of intravenous zanamivir. CLINICAL TRIALS REGISTRATION: NCT01014988.

Safety, tolerability and pharmacokinetics of orally inhaled zanamivir: a randomized study comparing Rotacap/Rotahaler and Rotadisk/Diskhaler in healthy adults.

BACKGROUND: During a pandemic, the need for available anti-influenza medications increases. There has been extensive use of the approved zanamivir Rotadisk/Diskhaler but no clinical data are available for administration by an alternative Rotacap/Rotahaler presentation. METHODS: In this randomized three-way crossover study, each healthy adult received zanamivir 10 mg every 12 h for 5 days via Rotadisk/Diskhaler, via Rotacap/Rotahaler and placebo via Rotacap/Rotahaler, with a washout period between treatments. Safety assessments were conducted throughout the study and at follow-up. Serial blood samples for pharmacokinetic analysis were collected over a 12-h dose interval on day 5 of each treatment period. Pharmacokinetic parameters were compared using a mixed-effects model. RESULTS: A total of 18 healthy adults were recruited and 17 subjects completed the study. A total of 20 adverse events (AEs) were reported (all grade 1) by nine subjects, with no AE reported ≥1× in any treatment group. Nasal congestion, reported by one subject in the zanamivir Rotadisk/Diskhaler group, was the only drug-related AE. No serious AEs or withdrawals due to AEs occurred during the study. There were no significant changes in clinical laboratory values, vital signs or spirometry. Serum zanamivir exposures were similar after administration via Rotacap/Rotahaler and Rotadisk/Diskhaler. Both oral inhalation presentations are likely to deliver similar zanamivir concentrations to sites of influenza infection in the respiratory tract. CONCLUSIONS: The safety and pharmacokinetic results from this study support the use of the Rotacap/Rotahaler presentation, potentially allowing an increased number of zanamivir treatment courses to be supplied in the event of an influenza pandemic.

Pharmacokinetics of zanamivir following intravenous administration to subjects with and without renal impairment.

Intravenous zanamivir is in clinical development for the treatment of influenza in hospitalized patients, many of whom have renal impairment. This open-label study evaluated zanamivir pharmacokinetics and clinical safety following a single 100-mg intravenous infusion dose in subjects with impaired renal function compared with normal renal function. Male and female subjects between 18 and 79 years of age were recruited, four subjects to each renal function group (normal function and mild, moderate, and severe impairment). Serial blood samples were collected up to 24 h after dose administration (48 h for the severe renal impairment group) to estimate zanamivir serum pharmacokinetic parameters. Urine was collected over the same 24-h (or 48-h) period for estimation of renal clearance (CLR). Zanamivir pharmacokinetics were assessed by regression analysis of systemic clearance (CL) and CLR as a function of creatinine clearance (CLCR). Safety evaluations included adverse-event monitoring, vital signs, electrocardiogram, and clinical laboratory assessments. Zanamivir clearance (total and renal) significantly decreased with decreasing renal function, with corresponding increases in area under the concentration-time curve and elimination half-life. Renal impairment had no apparent effects on peak concentration or volume of distribution. Regression analysis indicated that zanamivir clearance was highly correlated (r(2) = 0.89) with creatinine clearance: CL ≅ 7.08 + 0.826 · CLCR. There were no patterns or trends in adverse events, and no new safety concerns were identified following administration of intravenous zanamivir. Results from this study support the inclusion of subjects with renal impairment, with appropriate dose adjustment, in studies to evaluate intravenous zanamivir in the treatment of influenza.

Permeability enhancers dramatically increase zanamivir absolute bioavailability in rats: implications for an orally bioavailable influenza treatment.

We have demonstrated that simple formulations composed of the parent drug in combination with generally regarded as safe (GRAS) permeability enhancers are capable of dramatically increasing the absolute bioavailability of zanamivir. This has the advantage of not requiring modification of the drug structure to promote absorption, thus reducing the regulatory challenges involved in conversion of an inhaled to oral route of administration of an approved drug. Absolute bioavailability increases of up to 24-fold were observed when Capmul MCM L8 (composed of mono- and diglycerides of caprylic/capric acids in glycerol) was mixed with 1.5 mg of zanamivir and administered intraduodenally to rats. Rapid uptake (t(max) of 5 min) and a C(max) of over 7200 ng/mL was achieved. Variation of the drug load or amount of enhancer demonstrated a generally linear variation in absorption, indicating an ability to optimize a formulation for a desired outcome such as a targeted C(max) for enzyme saturation. No absorption enhancement was observed when the enhancer was given 2 hr prior to drug administration, indicating, in combination with the observed tmax, that absorption enhancement is temporary. This property is significant and aligns well with therapeutic applications to limit undesirable drug-drug interactions, potentially due to the presence of other poorly absorbed polar drugs. These results suggest that optimal human oral dosage forms of zanamivir should be enteric-coated gelcaps or softgels for intraduodenal release. There continues to be a strong need and market for multiple neuraminidase inhibitors for influenza treatment. Creation of orally available formulations of inhibitor drugs that are currently administered intravenously or by inhalation would provide a significant improvement in treatment of influenza. The very simple GRAS formulation components and anticipated dosage forms would require low manufacturing costs and yield enhanced convenience. These results are being utilized to design prototype dosage forms for initial human pharmacokinetic studies.

Effect of intravenous zanamivir on cardiac repolarization.

STUDY OBJECTIVE: To assess the effect of a therapeutic and supratherapeutic intravenous dose of the neuraminidase inhibitor zanamivir on QT and rate-corrected QT intervals. DESIGN: Randomized, placebo-controlled, single-dose, four-period, balanced crossover study. SETTING: Clinical research unit. SUBJECTS: Forty healthy adults were randomized to receive intravenous zanamivir at two dose levels, oral moxifloxacin, and placebo; 38 subjects completed all four study treatments. INTERVENTION: Subjects were randomized to receive a single intravenous dose of zanamivir 600 mg (therapeutic dose) with oral moxifloxacin placebo, a single intravenous dose of zanamivir 1200 mg (supratherapeutic dose) with oral moxifloxacin placebo, oral moxifloxacin 400 mg (positive control) with intravenous zanamivir placebo, or intravenous zanamivir placebo with oral moxifloxacin placebo. Subjects crossed over to all other treatments, with each treatment separated by a 7-day washout period. MEASUREMENTS AND MAIN RESULTS: Zanamivir pharmacokinetics were dose proportional; the pharmacokinetic exposure from zanamivir 1200 mg was 2 times higher than that from 600 mg, the maximum dose under clinical evaluation. For both 600-mg and 1200-mg doses of intravenous zanamivir, the upper limit of the 90% confidence interval (CI) for the placebo-adjusted mean change from baseline of the QT interval corrected for heart rate using Fridericia's formula (ΔΔQTcF) was less than 10 msec at all time points. The sensitivity of the study to detect modest increases in QT interval was established with the positive control, moxifloxacin. The maximum ΔΔQTcF value for zanamivir 1200 mg was 1.73 msec (90% CI -0.40 to 3.87 msec), which was observed within 30 minutes after dosing, and 11.21 msec (90% CI 8.81-13.60) for moxifloxacin, observed at 4 hours after dosing. No relationship was observed between zanamivir serum concentration and ΔΔQTcF. Zanamivir was generally well tolerated, with very few adverse events; none were serious or severe. CONCLUSION: Intravenous zanamivir does not affect cardiac repolarization. Accordingly, treatment with intravenous zanamivir does not require additional cardiac monitoring beyond the standard of care.

Population pharmacokinetics of laninamivir and its prodrug laninamivir octanoate in healthy subjects and in adult and pediatric patients with influenza virus infection.

Laninamivir octanoate (LO) is a new neuraminidase inhibitor for inhalation. The objectives of this study were to model the population pharmacokinetics of LO and its active metabolite laninamivir after inhaled administration of LO using a pooled population of healthy subjects, and adult and pediatric patients with influenza virus infection from 8 clinical studies, and to evaluate covariate effects on pharmacokinetics. The pharmacokinetics of LO and laninamivir in plasma and urine are well-described by structural models that consist of a 2-compartment model for LO with instantaneous bolus input and first-order elimination; and a 1-compartment model for laninamivir with formation of laninamivir via the metabolic pathway from LO in systemic circulation, entry of laninamivir from the respiratory tract compartment, and linear elimination. Creatinine clearance was identified as a covariate of apparent total clearance for LO and renal clearances for LO and laninamivir, with the largest effect on laninamivir exposure. Body weight was identified to affect distribution volumes of LO and laninamivir and the metabolic clearance of LO; however there was no notable effect on exposures across the wide body weight range evaluated. The population pharmacokinetic model also provides insight into the likely kinetics of drug disposition in the respiratory tract following inhaled administration.

The pharmacokinetic and safety profiles of zanamivir after single and repeat intravenous administration in healthy Japanese males.

WHAT IS KNOWN AND OBJECTIVE: Neuraminidase inhibitors are important options for the treatment of infection by the influenza virus. For the treatment of severe influenza, parenteral administration of a neuraminidase inhibitor may be desirable. This study was conducted to evaluate the pharmacokinetic and safety profiles of intravenous zanamivir, an influenza viral neuraminidase inhibitor, in Japanese subjects to further characterize these profiles particularly following relatively high-doses when compared with inhalation doses and to provide reassurance that there are no marked differences with profiles reported for other ethnically different populations. METHODS: Single doses of 100, 300, 600 mg zanamivir were administered to healthy Japanese men in a randomized, double-blind, ascending dose, placebo-controlled, incomplete three-period cross-over study. In period 3, subjects were given 600 mg of zanamivir on day 1, followed by a 60 h washout period and then a 5-day course of 600 mg zanamivir twice daily. Each subjects received two of three active dosages of zanamivir from 100, 300 and 600 mg, and placebo. RESULTS: Adverse events reported in the study were all mild in intensity and resolved without any treatment. The mean AUC0-∞ values after single intravenous administration of 100, 300 and 600 mg were 16768, 53462 and 100400 ng·h/mL, respectively, demonstrating dose proportionality. No accumulation or time variance was observed after 5 days of twice-daily administration of 600 mg zanamivir. Urinary concentrations of zanamivir after single doses ranging from 100 to 600 mg indicated that over 94% of the zanamivir administered was excreted in urine within 24 h. WHAT IS NEW AND CONCLUSION: Single and 5-day BID repeat dosing of 600 mg were safely administered in Japanese healthy subjects. The pharmacokinetic profile of zanamivir after intravenous administration was consistent with previously reported findings in non-Japanese subjects.

Structure-based design and synthesis of C-1- and C-4-modified analogs of zanamivir as neuraminidase inhibitors.

In order to exploit the 430-cavity in the active sites of neuraminidases, 22 zanamivir analogs with C-1 and C-4 modification were synthesized, and their inhibitory activities against both group-1 (H5N1, H1N1) and group-2 neuraminidases (H3N2) were determined. Compound 9f exerts the most potency, with IC(50) value of 0.013, 0.001, and 0.09 µM against H3N2, H5N1, and H1N1, which is similar to that of zanamivir (H3N2 IC(50) = 0.0014 µM, H5N1 IC(50) = 0.012 µM, H1N1 IC(50) = 0.001 µM). Pharmacokinetic studies of compound 9f in rats showed a much longer plasma half-life (t(1/2)) than that of zanamivir following administration (po dose). Molecular modeling provided information about the binding model between the new inhibitors and neuraminidase, with the elongated groups at the C-1-position being projected toward the 430-loop region. This study may represent a novel starting point for the future development of improved antiflu agents.

Pharmacokinetic mechanism involved in the prolonged high retention of laninamivir in mouse respiratory tissues after intranasal administration of its prodrug laninamivir octanoate.

Laninamivir octanoate (LO) (Inavir; Daiichi Sankyo, Japan) is an ester prodrug of the neuraminidase inhibitor laninamivir. We previously reported that a prolonged high retention of laninamivir in mouse respiratory tissues was achieved by intranasal administration of LO. In this study, we evaluated intrapulmonary pharmacokinetics both in vivo and in vitro to investigate the potential mechanism involved in such a preferable retention. After intranasal administration of LO to mice (0.5 µmol/kg), the drug was distributed from the airway space into the lungs, and laninamivir remained in the lung at 24 hours postdose (2680 pmol/g), with a higher concentration than that in the epithelial lining fluid. The laninamivir was localized mainly on the epithelial cells of airway tracts, determined by microautoradiography using (14)C-labeled LO. In mouse airway epithelial cells, the cellular uptake and hydrolysis of LO were observed over incubation time without any apparent saturation at the highest concentration tested (1000 µM). Furthermore, after additional incubation in drug-free medium, the intracellular laninamivir was released very slowly into the medium with an estimate rate constant of 0.0707 h(-1), which was regarded as a rate-limiting step in the cellular retention. These results demonstrated that the prolonged high retention of laninamivir in the respiratory tissues was attributed to a consecutive series of three steps: uptake of LO into the airway epithelial cells, hydrolysis of LO into laninamivir by intracellular esterase(s), and limited efflux of the generated laninamivir due to its poor membrane permeability. This prodrug approach could be useful for lung-targeting drug delivery.

A chemically programmed antibody is a long-lasting and potent inhibitor of influenza neuraminidase.

Programming an anti-flu strategy: A new and potent neuraminidase inhibitor that maintains long-term systemic exposure of an antibody and the therapeutic activity of the neuraminadase inhibitor zanamivir has been created. This strategy could provide a promising new class of influenza A drugs for therapy and prophylaxis, and validates enzyme inhibitors as programming agents in synthetic immunology.

Synthesis of C-4-modified zanamivir analogs as neuraminidase inhibitors and their anti-AIV activities.

With the introduction of bioisosteres of the guanidinium group together with scaffold hopping, 35 zanamivir analogs with C-4-modification were synthesized, and their inhibitory activities against both group-1 and group-2 neuraminidase (H5N1 and H3N2) were determined. Compound D26 exerts the most potency, with IC(50) values of 0.58 and 2.72 µM against N2 and N1, respectively. Further preliminary anti-avian influenza virus (AIV, H5N1) activities against infected MDCK cells were evaluated, and D5 exerts ∼58% protective against AIV infection, which was comparable to zanamivir (∼67%). In a rat pharmacokinetic study, compound D5 showed an increased plasma half-life (t(1/2)) compared to zanamivir following either intravenous or oral administration. This study may represent a new start point for the future development of improved anti-AIV agents.

Intrapulmonary distribution and pharmacokinetics of laninamivir, a neuraminidase inhibitor, after a single inhaled administration of its prodrug, laninamivir octanoate, in healthy volunteers.

A single inhaled dose of laninamivir octanoate (LO), a long-acting neuraminidase inhibitor, exhibits efficacy in treating both adult and pediatric patients with influenza virus infection. The intrapulmonary pharmacokinetics (PK) of LO and laninamivir, a pharmacologically active metabolite, were investigated by a single-center, open-label study of healthy adult volunteers. Subgroups of five subjects each underwent bronchoalveolar lavage (BAL) 4, 8, 24, 48, 72, 168, and 240 h following a single inhaled administration of LO (40 mg). Plasma, BAL fluid, and alveolar macrophages (AM) were analyzed to determine LO and laninamivir concentrations, using validated liquid chromatography-tandem mass spectrometry methods. The concentrations in epithelial lining fluid (ELF) and AM from the first and subsequent BAL fluid samples were determined separately to explore the drug distribution in airways. Mean laninamivir concentrations in ELF, calculated using the first BAL fluids and BAL fluids collected 4 h after inhaled administration, were 8.57 and 2.40 µg/ml, respectively. The laninamivir concentration in ELF decreased with a longer half-life than that in plasma, and it exceeded the 50% inhibitory concentrations for viral neuraminidases at all time points examined for 240 h after the inhalation. Laninamivir exposure in ELF from the first BAL samples was 3.2 times higher than that in ELF from the subsequent BAL fluid samples. ELF concentration profiles of laninamivir support its long-lasting effect for treatment of patients with influenza virus infection by a single inhaled administration.