Pharmacokinetics and dosage recommendations for an oseltamivir oral suspension for the treatment of influenza in children

Pharmacokinetics of Commonly Used Medications in Children Receiving Continuous Renal Replacement Therapy: A Systematic Review of Current Literature

BACKGROUND AND OBJECTIVE

The use of continuous renal replacement therapy (CRRT) for renal support has increased substantially in critically ill children compared with intermittent modalities owing to its preferential effects on hemodynamic stability. With the expanding role of CRRT, the quantification of extracorporeal clearance and the effect on primary pharmacokinetic parameters is of the utmost importance. Within this review, we aimed to summarize the current state of the literature and compare published pharmacokinetic analyses of commonly used medications in children receiving CRRT to those who are not.

METHODS

A systematic search of the literature within electronic databases PubMed, EMBASE, Cochrane Library, and Web of Science was conducted. Published studies that were included contained relevant information on the use of commonly administered medications to children, from neonates to adolescents, receiving CRRT. Pharmacokinetic parameters that were analyzed included volume of distribution, total clearance, extracorporeal clearance, area under the curve, and elimination half-life. Information regarding CRRT circuit, flow rates, and membrane components was analyzed to investigate differences in pharmacokinetics between each modality.

RESULTS

Forty-five studies met the final inclusion criteria within this systematic review, totaling 833 pediatric patients, with 586 receiving CRRT. Antimicrobials were the most common pharmacological class represented within the literature, representing 81% (35/43) of studies analyzed. Children receiving CRRT largely had similar volume of distribution and total clearance to critically ill children not receiving CRRT, suggesting reno-protective dose adjustments may lead to subtherapeutic dosing regimens in these patients. Overall, there was a tendency for hydrophilic agents, with a low protein binding to undergo elevated total clearance in these children. However, results should be interpreted with caution because of the large variability amongst patient populations and heterogeneity with CRRT modalities, flow rates, and use of extracorporeal membrane oxygenation within studies. This review was able to identify that variation in solute removal, or CRRT modalities, properties (i.e., flow rates), and membrane composition, may have differing effects on the pharmacokinetics of commonly administered medications.

CONCLUSIONS

The current state of the literature regarding medications administered to children receiving CRRT largely focuses on antimicrobials. Significant gaps remain with other commonly used medications such as sedatives and analgesics. Overall reporting of patient clinical characteristics, CRRT settings, and circuit composition was poor, with only 10% of articles including all relevant information to assess the impact of CRRT on total clearance. Changes in pharmacokinetics because of CRRT often required higher than labeled doses, suggesting renally adjusted or reno-protective doses may lead to subtherapeutic dosing regimens. A thorough understanding of the interplay between patient, drug, and CRRT-circuit factors are required to ensure adequate delivery of dosing regimens to this vulnerable population.

Ontogeny of Drug-Metabolizing Enzymes

Almost 50% of prescription drugs lack age-appropriate dosing guidelines and therefore are used "off-label." Only ~10% drugs prescribed to neonates and infants have been studied for safety or efficacy. Immaturity of drug metabolism in children is often associated with drug toxicity. This chapter summarizes data on the ontogeny of major human metabolizing enzymes involved in oxidation, reduction, hydrolysis, and conjugation of drugs. The ontogeny data of individual drug-metabolizing enzymes are important for accurate prediction of drug pharmacokinetics and toxicity in children. This information is critical for designing clinical studies to appropriately test pharmacological hypotheses and develop safer pediatric drugs, and to replace the long-standing practice of body weight- or surface area-normalized drug dosing. The application of ontogeny data in physiologically based pharmacokinetic model and regulatory submission are discussed.

Substitutions at H134 and in the 430-loop region in influenza B neuraminidases can confer reduced susceptibility to multiple neuraminidase inhibitors

With the introduction of the influenza specific neuraminidase inhibitors (NAIs) in 1999, there were concerns about the emergence and spread of resistant viruses in the community setting. Surveillance and testing of community isolates for their susceptibility to the NAIs was initially carried out by the Neuraminidase Inhibitor Susceptibility Network (NISN) and has subsequently been taken on by the global WHO influenza network laboratories. During the NISN surveillance, we identified two Yamagata lineage influenza B viruses with amino acid substitutions of H134Y (B/Auckland/2/2001) or W438R (B/Yokohama/12/2005) which had slightly elevated IC₅₀ values for zanamivir and/or oseltamivir, but not sufficiently to be characterized as mild outliers at the time. As it has now been well demonstrated that mixed populations can mask the true magnitude of resistance of a mutant, we re-examined both of these isolates by plaque purification to see if the true susceptibilities were being masked due to mixed populations. Results confirmed that the B/Auckland isolate contained both wild type and H134Y mutant populations, with mutant IC₅₀ values > 250 nM for both oseltamivir and peramivir in the enzyme inhibition assay. The B/Yokohama isolate also contained both wild type and W438R mutant populations, the latter now demonstrating IC₅₀ values > 400 nM for zanamivir, oseltamivir and peramivir. In addition, plaque purification of the B/Yokohama isolate identified viruses with other single neuraminidase substitutions H134Y, H134R, H431R, or T436P. H134R and H431R viruses had IC₅₀ values > 400 nM and >250 nM respectively against all three NAIs. All changes conferred much greater resistance to peramivir than to zanamivir, and less to oseltamivir, and affected the kinetics of binding and dissociation of the NAIs. Most affected affinity (K_m) for the MUNANA substrate, but some had decreased while others had increased affinity. Despite resistance in the enzyme assay, no reduced susceptibility was seen in plaque reduction assays in MDCK cells for any of the mutant viruses. None of these substitutions was in the active site. Modelling suggests that these substitutions affect the 150 and 430-loop regions described for influenza A NAs, suggesting they may also be important for substrate and inhibitor binding for influenza B NAs.

Mechanistic Population Pharmacokinetic Model of Oseltamivir and Oseltamivir Carboxylate Accounting for Physiological Changes to Predict Exposures in Neonates and Infants

A mechanistic population‐pharmacokinetic model was developed to predict oseltamivir exposures in neonates and infants accounting for physiological changes during the first 2 years of life. The model included data from 13 studies, comprising 436 subjects with normal renal function (317 pediatric subjects (≥ 38 weeks postmenstrual age (PMA), ≥ 13 days old) and 119 adult subjects < 40 years). Concentration–time profiles of oseltamivir and its active metabolite, oseltamivir carboxylate (OC), were characterized by a four‐compartment model, with absorption described by three additional compartments. Renal maturational changes were implemented by description of OC clearance with allometric function of weight and Hill function of PMA. Clearance of OC increased with weight up to 43 kg (allometric coefficient 0.75). Half the adult OC clearance was reached at a PMA of 45.6 weeks (95% confidence interval (CI) 41.6–49.6) with a Hill coefficient of 2.35 (95% CI 1.67–3.04). The model supports the European Union/United States‐approved 3 mg/kg twice‐daily oseltamivir dose for infants < 1 year (PMA ≥ 38 weeks) and allows prediction of exposures in preterm neonates.

Peramivir binding affinity with influenza A neuraminidase and research on its mutations using an induced-fit docking approach

Influenza A virus (IAV) has caused epidemic infections worldwide, with many strains resistant to inhibitors of a surface protein, neuraminidase (NA), due to point mutations on its structure. A novel NA inhibitor named peramivir was recently approved, but no exhaustive computational research regarding its binding affinity with wild-type and mutant NA has been conducted. In this study, a thorough investigation of IAV-NA PDB entries of 9 subtypes is described, providing a list of residues constituting the protein-ligand binding sites. The results of induced-fit docking approach point out key residues of wild-type NA participating in hydrogen bonds and/or ionic interactions with peramivir, among which Arg 368 is responsible for a peramivir-NA ionic interaction. Mutations on this residue greatly reduced the binding affinity of peramivir with NA, with 3 mutations R378Q, R378K and R378L (NA6) capable of deteriorating the docking performance of peramivir by over 50%. 200 compounds from 6-scaffolds were docked into these 3 mutant versions, revealing 18 compounds giving the most promising results. Among them, CMC-2012-7-1527-56 (benzoic acid scaffold, IC50 = 32 nM in inhibitory assays with IAV) is deemed the most potential inhibitor of mutant NA resisting both peramivir and zanamivir, and should be further investigated.

Route of Oseltamivir Administration Affects Metabolite Concentrations in Critically Ill Children

We performed a prospective cohort study to investigate oseltamivir administration in critically ill children. We found that enteric tube administration of oseltamivir resulted in lower concentrations of its active metabolite compared with oral delivery. These findings could have significant clinical implications, and more studies are required to better understand the effects of administration route on potential lower systemic metabolite exposure.

Use of antiviral drugs for seasonal influenza: Foundation document for practitioners-Update 2019

This document updates the previous AMMI Canada Foundation Guidance (2013) on the use of antiviral therapy for influenza.

Pharmacokinetics of Oral and Intravenous Oseltamivir Treatment of Severe Influenza B Virus Infection Requiring Organ Replacement Therapy

BACKGROUND AND OBJECTIVES

Patients with severe influenza virus infection, multi-organ failure and organ replacement therapy may absorb and metabolize neuraminidase inhibitors differently. Systematic pharmacokinetic/pharmacodynamic clinical trials are currently lacking in this high-risk group. Inadequate dosing increases the risk of treatment failure and drug resistance, especially in severely ill patients with elevated virus loads. This study aims to explore the impact of organ replacement therapy on oseltamivir drug concentrations.

METHODS

Serial pharmacokinetic/pharmacodynamic measurements and Sieving coefficients were assessed in two patients with severe influenza B infection requiring organ replacement therapy.

RESULTS

Patient #1, a 9-year-old female with severe influenza B virus infection, biventricular assist device, and continuous veno-venous hemodiafiltration, received 75 mg oral oseltamivir twice-daily for 2 days, then intravenous oseltamivir with one-time renoprotective dosing (40 mg), followed by regular intravenous administration of 100 mg twice-daily. Plasma oseltamivir carboxylate concentrations were stable initially, but only regular administration of 100 mg resulted in virus load decline and clinical improvement. Patient #2, a 28-year-old female with influenza B virus infection requiring extracorporeal membrane oxygenation, received 75 mg oral oseltamivir twice-daily, resulting in erratic oseltamivir blood concentrations. In both patients, drug concentrations remained well within safety margins.

CONCLUSIONS

In severe cases with multi-organ failure, administration of 100 mg intravenous oseltamivir twice-daily provided reliable drug concentrations, as opposed to renoprotective and oral dosing, thereby minimizing the risk of treatment failure and drug resistance. Evidence-based pediatric dosing recommendations and effective intravenous antiviral treatment modalities are needed for intensive care patients with life-threatening influenza disease.

Pharmacokinetics of Oseltamivir in Young and Very Elderly Subjects

Background:

Pharmacokinetic studies of oseltamivir in very elderly patients (≥80 y) have not previously been performed.

Objective:

To compare the pharmacokinetics of oseltamivir and the active carboxylate metabolite in healthy young and very elderly Japanese subjects.

Methods:

Young (20–35 y, fasting, n = 7) and very elderly subjects (≥80 y, fed, n = 5) were enrolled in single-center studies and received a single oral dose of oseltamivir 75 mg. Plasma and urine samples were collected (24 h) for pharmacokinetic analysis, and safety was assessed.

Results:

The time to maximum plasma concentration (tmax for oseltamivir was delayed in the very elderly compared with the young subjects (2.30 vs 0.71 h, respectively). Furthermore, oseltamivir maximum plasma concentration (Cmax) and AUCinf were 52% and 80% higher, respectively, in the very elderly compared with the young subjects. Oral clearance was 45% lower in elderly patients, possibly due to the effects of administration of oseltamivir with a meal. For the active metabolite, oseltamivir carboxylate, Cmax and AUCinf values were, respectively, 22% and 91% higher in the very elderly subjects than in the young subjects, while oral clearance was 50% lower in the elderly population. The increased exposure of the active metabolite is likely to correlate with an age-related decline in renal function. For both oseltamivir and the active metabolite, there was large interpatient variability in the Cmax values. The data reported here indicate that oseltamivir would be effective in both of these populations, as trough concentrations for the active metabolite at 12 and 24 hours exceeded the 50% inhibitory concentration against the neuraminidase of influenza A and B isolates by more than 50-fold. Oseltamivir was well tolerated in both groups.

Conclusions:

Exposures (AUCinf) to both the parent drug and active metabolite were increased by more than 80% in the small number of very elderly subjects presented here. However, oseltamivir was well tolerated by these subjects.

Bench to bed evidences for pharmacokinetic and pharmacodynamic interactions involving oseltamivir and chinese medicine

Oseltamivir (OA), an ethyl ester prodrug of oseltamivir carboxylate (OC), is clinically used as a potent and selective inhibitor of neuraminidase. Chinese medicines have been advocated to combine with conventional drug for avian influenza. The current study aims to investigate the potential pharmacokinetic and pharmacodynamic interactions of a Chinese medicine formula, namely, Yin Qiao San and Sang Ju Yin (CMF1), commonly used for anti-influenza in combination with OA in both rat and human, and to reveal the underlined mechanisms. It was found that although C_max, AUC and urinary recovery of OC, as well as metabolic ratio (AUC_OC/AUC_OA), were significantly decreased in a dose-dependent manner following combination use of CMF1 and OA in rat studies (P < 0.01), such coadministration in 14 healthy volunteers only resulted in a trend of minor decrease in the related parameters. Further mechanistic studies found that although CMF1 could reduce absorption and metabolism of OA, it appears to enhance viral inhibition of OA (P < 0.01). In summary, although there was potential interaction between OA and CMF1 found in rat studies, its clinical impact was expected to be minimal. The coadministration of OA and CMF1 at the clinical recommended dosages is, therefore, considered to be safe.

Risk factors for need of mechanical ventilation in children with influenza A(H1N1)pdm09

OBJECTIVE

The pandemic caused by influenza A(H1N1)pdm09 virus peaked between July and August of 2009 in southern Brazil, with the highest incidence in children and young adults. In the post-pandemic period, there was an increase in the incidence of cases during the winter months of 2011 and 2012 in Brazil, similar to seasonal influenza virus. Since infections due to pandemic influenza are still occurring, the present study aimed to investigate the risk factors for worse outcome in children.

METHODS

A retrospective cohort study was performed by reviewing the charts of hospitalized patients younger than 14 years with reverse transcription-polymerase chain reaction (RT-PCR) positive for influenza A(H1N1)pdm09 during the first pandemic wave in six Brazilian tertiary centers. Need for mechanical ventilation was defined as the severity of outcome; age, chronic diseases, bacterial and viral co-detection, chest radiograph findings, and use of oseltamivir were possible predictors.

RESULTS

In the present study, 120 patients were included. In a multivariate analysis, chronic diseases (prevalence ratio: 2.613, 95% CI: 1.267-5.386) and viral co-detection (prevalence ratio: 2.43, 95% CI: 1.203-4.905) were statistically associated with worse outcome (p<0.05).

CONCLUSIONS

The presence of chronic diseases as predictors reinforces previous finding. Furthermore, viral co-detection was found to be a risk factor. Further studies are necessary to confirm this association.

Influenza neuraminidase inhibitors: antiviral action and mechanisms of resistance

There are two major classes of antivirals available for the treatment and prevention of influenza, the M2 inhibitors and the neuraminidase inhibitors (NAIs). The M2 inhibitors are cheap, but they are only effective against influenza A viruses, and resistance arises rapidly. The current influenza A H3N2 and pandemic A(H1N1)pdm09 viruses are already resistant to the M2 inhibitors as are many H5N1 viruses. There are four NAIs licensed in some parts of the world, zanamivir, oseltamivir, peramivir, and a long-acting NAI, laninamivir. This review focuses on resistance to the NAIs. Because of differences in their chemistry and subtle differences in NA structures, resistance can be both NAI- and subtype specific. This results in different drug resistance profiles, for example, the H274Y mutation confers resistance to oseltamivir and peramivir, but not to zanamivir, and only in N1 NAs. Mutations at E119, D198, I222, R292, and N294 can also reduce NAI sensitivity. In the winter of 2007-2008, an oseltamivir-resistant seasonal influenza A(H1N1) strain with an H274Y mutation emerged in the northern hemisphere and spread rapidly around the world. In contrast to earlier evidence of such resistant viruses being unfit, this mutant virus remained fully transmissible and pathogenic and became the major seasonal A(H1N1) virus globally within a year. This resistant A(H1N1) virus was displaced by the sensitive A(H1N1)pdm09 virus. Approximately 0.5-1.0% of community A(H1N1)pdm09 isolates are currently resistant to oseltamivir. It is now apparent that variation in non-active site amino acids can affect the fitness of the enzyme and compensate for mutations that confer high-level oseltamivir resistance resulting in minimal impact on enzyme function.

I222 Neuraminidase mutations further reduce oseltamivir susceptibility of Indonesian Clade 2.1 highly pathogenic Avian Influenza A(H5N1) viruses

We have tested the susceptibility to neuraminidase inhibitors of 155 clade 2.1 H5N1 viruses from Indonesia, isolated between 2006-2008 as well as 12 clade 1 isolates from Thailand and Cambodia from 2004-2007 using a fluorometric MUNANA-based enzyme inhibition assay. The Thailand and Cambodian clade 1 isolates tested here were all susceptible to oseltamivir and zanamivir, and sequence comparison indicated that reduced oseltamivir susceptibility we observed previously with clade 1 Cambodian isolates correlated with an S246G neuraminidase mutation. Eight Indonesian viruses (5%), all bearing I222 neuraminidase mutations, were identified as mild to extreme outliers for oseltamivir based on statistical analysis by box plots. IC50s were from 50 to 500-fold higher than the reference clade 1 virus from Viet Nam, ranging from 43-75 nM for I222T/V mutants and from 268-349 nM for I222M mutants. All eight viruses were from different geographic locales; all I222M variants were from central Sumatra. None of the H5N1 isolates tested demonstrated reduced susceptibility to zanamivir (IC50s all <5 nM). All I222 mutants showed loss of slow binding specifically for oseltamivir in an IC50 kinetics assay. We identified four other Indonesian isolates with higher IC50s which also demonstrated loss of slow binding, including one virus with an I117V mutation. There was a minimal effect on the binding of zanamivir and peramivir for all isolates tested. As H5N1 remains a potential pandemic threat, the incidence of mutations conferring reduced oseltamivir susceptibility is concerning and emphasizes the need for greater surveillance of drug susceptibility.

Similarity in Pharmacokinetics of Oseltamivir and Oseltamivir Carboxylate in Japanese and Caucasian Subjects

The pharmacokinetics of oseltamivir and oseltamivir carboxylate in healthy Japanese (n = 14) and Caucasian (n = 14) males were compared. Subjects in each ethnic group were randomized to twice-daily oral oseltamivir 75 mg, 150 mg, or placebo for 13 doses. Oseltamivir was well tolerated across doses and ethnic groups. Oseltamivir was rapidly absorbed and hydrolyzed to oseltamivir carboxylate in all subjects. The mean plasma concentration-time profiles for oseltamivir and oseltamivir carboxylate were similar in Japanese and Caucasian subjects. At steady state, there was no evidence of any ethnic difference in the individual AUC(0-12) values for oseltamivir or oseltamivir carboxylate. Despite a significant difference in group mean body weight (approximately 20 kg) between the Japanese and Caucasian subjects, there was no evidence that dose-adjusted AUC(0-12) and C(max) for oseltamivir carboxylate were affected by body weight or ethnicity. Day 7 trough concentrations (C(min)) for oseltamivir carboxylate markedly exceeded the IC(50) (50% inhibitory concentration) against influenza A and B isolates. In conclusion, the results of this study support the use of the same dose regimens of oseltamivir in both Caucasian and Japanese subjects because of similarity in pharmacokinetics.

Pharmacokinetic properties of anti-influenza neuraminidase inhibitors

Neuraminidase inhibitors are the mainstay of anti-influenza treatment. Oseltamivir is the most widely used drug but is currently available only as an oral formulation. Resistance spreads rapidly in seasonal H1N1 influenza A viruses, which were universally resistant in 2008, because of the H275Y mutation in the neuraminidase (NA) gene. Oseltamivir is a prodrug for the active carboxylate metabolite. Ex vivo conversion in blood samples may have confounded early pharmacokinetic studies. Oseltamivir shows dose linear kinetics, and oseltamivir carboxylate has an elimination half-life (t(1/2) β) after oral administration in healthy individuals of approximately 7.7 hours. Oseltamivir carboxylate is eliminated primarily by tubular secretion, and both clearance and tissue distribution are reduced by probenecid. The H275Y mutation in NA confers high-level oseltamivir resistance and intermediate peramivir resistance but does not alter zanamivir susceptibility. Zanamivir is available as a powder for inhalation, and a parenteral form is under development. Zanamivir distributes in an apparent volume of distribution approximating that of extracellular water and is rapidly eliminated (t(1/2) β of approximately 3.0 hours). Peramivir is slowly eliminated (t(1/2) β of 7.7-20.8 hours) and is prescribed as either a once-daily injection or as a single infusion. Laninamivir is a recently developed slowly eliminated compound for administration by inhalation.

The pharmacokinetics of oseltamivir and oseltamivir carboxylate in a critically ill pediatric patient receiving extracorporeal membrane oxygenation and continuous venovenous hemodialysis

This report details the pharmacokinetics of oseltamivir and oseltamivir carboxylate following administration of high-dose oseltamivir in a critically ill child receiving extracorporeal membrane oxygenation (ECMO) and continuous venovenous hemodialysis (CVVHD). A 6-year-old critically ill male patient suffering from a presumed viral illness was transferred to our institution's pediatric intensive care unit from an outside hospital after developing respiratory failure and cardiomegaly. ECMO and oseltamivir therapy were initiated upon admission, and CVVHD was started on hospital day 3. Pharmacokinetic sampling occurred at an oseltamivir dose of approximately 4 mg/kg on hospital day 6. The patient's oseltamivir and oseltamivir carboxylate area under the plasma concentration time curves for the 12-hour dosing interval (AUC(0-12)) were 30.5 and 905 ng/mLhr, respectively. Drug clearance by CVVHD was 31.6 mL/min for oseltamivir and 26.9 mL/min for oseltamivir carboxylate. Pre- and postoxygenator oseltamivir and oseltamivir carboxylate plasma concentrations did not differ substantially. The patient's oseltamivir carboxylate plasma concentrations remained well above the reported mean 50% inhibitory concentration for 2009 pandemic H1N1 virus. However, despite receiving twice the standard dose of oseltamivir, the oseltamivir carboxylate AUC(0-12) in our patient was less than that reported in noncritically ill pediatric subjects. The reduced oseltamivir carboxylate AUC(0-12) found in our patient was most likely due to decreased drug absorption.

Post-marketing assessment of neuropsychiatric adverse events in influenza patients treated with oseltamivir: an updated review

A 2008 review by our group concluded that the risk of neuropsychiatric adverse events (NPAEs) in influenza patients was not increased by oseltamivir exposure, and did not identify any mechanism by which oseltamivir or its metabolites could cause or worsen such events. The current article reviews new information on this topic. Between September 16, 2007 and May 15, 2010, 1,805 spontaneously-reported NPAEs were identified in 1,330 patients receiving oseltamivir: 767 (42.5%) from Japan, 296 (16.4%) from the USA, and 742 (41.1%) from other countries. NPAEs were more common in children: 1,072 (59.4%) events were in those aged ≤16 years. NPAEs often occurred within 48 h of treatment initiation (953 events; 52.8%). Nearly half of the events were serious in nature (838; 46.4%). The three largest categories of events were abnormal behavior (457 events, 25.3%), miscellaneous psychiatric events (370; 20.5%), and delusions/perceptual disturbances (316 events, 17.5%). A total of 1,545 events (85.6%) in eight different categories were considered to be delirium or delirium-like. Twenty-eight suicide-related events were reported. A US healthcare claims database analysis showed that the risk of NPAEs in 7,798 oseltamivir-treated patients was no higher than that in 10,411 patients not on antivirals, but a study on oseltamivir and abnormal behavior in Japan was less conclusive. NPAE frequency in oseltamivir-exposed Japanese and Taiwanese children with influenza was the same as in unexposed children. New analysis of the UK General Practice Research Database showed that the relative adjusted risk of NPAEs in influenza patients was 2.18-times higher than in the general population. Other epidemiology studies report frequent occurrence of encephalitis and similar disorders in influenza patients independently of oseltamivir exposure. The new data support the findings of the original assessment. Evidence suggests that influenza-related encephalopathies are caused by influenza-induced inflammatory responses, but more work is needed to confirm the underlying mechanisms.

Characteristics of a widespread community cluster of H275Y oseltamivir-resistant A(H1N1)pdm09 influenza in Australia

Background. Oseltamivir resistance in A(H1N1)pdm09 influenza is rare, particularly in untreated community cases. Sustained community transmission has not previously been reported.

Methods. Influenza specimens from the Asia–Pacific region were collected through sentinel surveillance, hospital, and general practitioner networks. Clinical and epidemiological information was collected on patients infected with oseltamivir-resistant viruses.

Results. Twenty-nine (15%) of 191 A(H1N1)pdm09 viruses collected between May and September 2011 from Hunter New England (HNE), Australia, contained the H275Y neuraminidase substitution responsible for oseltamivir resistance. Only 1 patient had received oseltamivir before specimen collection. The resistant strains were genetically very closely related, suggesting the spread of a single variant. Ninety percent of cases lived within 50 kilometers. Three genetically similar oseltamivir-resistant variants were detected outside of HNE, including 1 strain from Perth, approximately 4000 kilometers away. Computational analysis predicted that neuraminidase substitutions V241I, N369K, and N386S in these viruses may offset the destabilizing effect of the H275Y substitution.

Conclusions This cluster represents the first widespread community transmission of H275Y oseltamivir-resistant A(H1N1)pdm09 influenza. These cases and data on potential permissive mutations suggest that currently circulating A(H1N1)pdm09 viruses retain viral fitness in the presence of the H275Y mutation and that widespread emergence of oseltamivir-resistant strains may now be more likely.

Quantitative determination of oseltamivir and oseltamivir carboxylate in human fluoride EDTA plasma including the ex vivo stability using high-performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry

Oseltamivir, the ethyl ester prodrug of the neuramidase inhibitor oseltamivir carboxylate, is licensed for the treatment of patients with influenza virus infection. Here we describe the development and validation of an assay for the simultaneous quantification of oseltamivir and oseltamivir carboxylate in human fluoride EDTA plasma including the ex vivo stability using liquid chromatography coupled to tandem mass spectrometry. Sample pretreatment consisted of protein precipitation with 8% (v/v) trichloroacetic acid in water using only 50 μL plasma. Chromatographic separation was performed on a reversed phase C18 column (150 mm × 2.0 mm ID, particle size 4 μm) with a stepwise gradient using 0.1% formic acid and methanol at a flow rate of 250 μL/min. A triple quadrupole mass spectrometer operating in the positive ionization mode was used for detection and drug quantification. The method was validated over a range of 3-300 ng/mL for oseltamivir and 10-10,000 ng/mL for oseltamivir carboxylate. Deuterated oseltamivir and oseltamivir carboxylate were used as internal standards. The intra-assay accuracies and precisions for oseltamivir were between -8.8 and 16.3% at the LLOQ level, whereas for all other concentration levels this was -8.6 and 14.5%. For oseltamivir carboxylate the intra-assay accuracies and precisions were between -10.9 and 10.7% at all levels. Furthermore, oseltamivir was stable in plasma and whole blood ex vivo in commercially available fluoride EDTA tubes for at least 24h at 2-8 °C. This method is now applied for the determination of both compounds in specific patient populations to evaluate current dosing guidelines.

Seasonal and pandemic influenza: an overview with pediatric focus

Influenza is an important cause of respiratory illness in children, who have the highest attack rates during the annual influenza outbreaks [60]. Clinical infection ranges from subclinical illness to complicated disease that affects multiple organs. Annual vaccination remains the most effective strategy for the prevention and control of influenza [2]. Recently developed antiviral drugs offer new approaches to the prevention and treatment of influenza.

Oseltamivir in seasonal, pandemic, and avian influenza: a comprehensive review of 10-years clinical experience

Oseltamivir (Tamiflu®; F. Hoffmann-La Roche Ltd, Basel, Switzerland) is an orally administered antiviral for the treatment and prevention of influenza A and B infections that is registered in more than 100 countries worldwide. More than 83 million patients have been exposed to the product since its introduction. Oseltamivir is recommended by the World Health Organization (WHO) for use in the clinical management of pandemic and seasonal influenza of varying severity, and as the primary antiviral agent for treatment of avian H5N1 influenza infection in humans. This article is a nonsystematic review of the experience gained from the first 10 years of using oseltamivir for influenza infections since its launch in early 2000, emphasizing recent advances in our understanding of the product and its clinical utility in five main areas. The article reviews the pharmacokinetics of oseltamivir and its active metabolite, oseltamivir carboxylate, including information on special populations such as children and elderly adults, and the co-administration of oseltamivir with other agents. This is followed by a summary of data on the effectiveness of oseltamivir treatment and prophylaxis in patients with all types of influenza, including pandemic (H1N1) 2009 and avian H5N1 influenza. The implications of changes in susceptibility of circulating influenza viruses to oseltamivir and other antiviral agents are also described, as is the emergence of antiviral resistance during and after the 2009 pandemic. The fourth main section deals with the safety profile of oseltamivir in standard and special patient populations, and reviews spontaneously reported adverse event data from the pandemic and pre-pandemic periods and the topical issue of neuropsychiatric adverse events. Finally, the article considers the pharmacoeconomics of oseltamivir in comparison with vaccination and usual care regimens, and as a component of pandemic influenza mitigation strategies.

Oseltamivir in seasonal, avian H5N1 and pandemic 2009 A/H1N1 influenza: pharmacokinetic and pharmacodynamic characteristics

Oseltamivir is the ester-type prodrug of the neuraminidase inhibitor oseltamivir carboxylate. It has been shown to be an effective treatment for both seasonal influenza and the recent pandemic 2009 A/H1N1 influenza, reducing both the duration and severity of the illness. It is also effective when used preventively. This review aims to describe the current knowledge of the pharmacokinetic and pharmacodynamic characteristics of this agent, and to address the issue of possible therapeutic drug monitoring. According to the currently available literature, the pharmacokinetics of oseltamivir carboxylate after oral administration of oseltamivir are characterized by mean ± SD bioavailability of 79 ± 12%, apparent clearance of 25.3 ± 7.0 L/h, an elimination half-life of 7.4 ± 2.5 hours and an apparent terminal volume of distribution of 267 ± 122 L. A maximum plasma concentration of 342 ± 83 μg/L, a time to reach the maximum plasma concentration of 4.2 ± 1.1 hours, a trough plasma concentration of 168 ± 32 μg/L and an area under the plasma concentration-time curve from 0 to 24 hours of 6110 ± 1330 μg · h/L for a 75 mg twice-daily regimen were derived from literature data. The apparent clearance is highly correlated with renal function, hence the dosage needs to be adjusted in proportion to the glomerular filtration rate. Interpatient variability is moderate (28% in apparent clearance and 46% in the apparent central volume of distribution); there is no indication of significant erratic or limited absorption in given patient subgroups. The in vitro pharmacodynamics of oseltamivir carboxylate reveal wide variation in the concentration producing 50% inhibition of influenza A and B strains (range 0.17-44 μg/L). A formal correlation between systemic exposure to oseltamivir carboxylate and clinical antiviral activity or tolerance in influenza patients has not yet been demonstrated; thus no formal therapeutic or toxic range can be proposed. The pharmacokinetic parameters of oseltamivir carboxylate after oseltamivir administration (bioavailability, apparent clearance and the volume of distribution) are fairly predictable in healthy subjects, with little interpatient variability outside the effect of renal function in all patients and bodyweight in children. Thus oseltamivir carboxylate exposure can probably be controlled with sufficient accuracy by thorough dosage adjustment according to patient characteristics. However, there is a lack of clinical study data on naturally infected patients. In addition, the therapeutic margin of oseltamivir carboxylate is poorly defined. The usefulness of systematic therapeutic drug monitoring in patients therefore appears to be questionable; however, studies are still needed to extend the knowledge to particular subgroups of patients or dosage regimens.

Pharmacologic considerations for oseltamivir disposition: focus on the neonate and young infant

Across much of the world, pandemic H1N1 infection has produced a significant healthcare crisis, reflected in significant morbidity and mortality. Statistics reveal that infection-associated deaths among individuals without pre-existing conditions (e.g. immunosuppression) are clustered in pregnant women and young infants. In developing countries where the availability of influenzae vaccine is limited, the only currently available pharmacologic counter-measure for H1N1 disease is oseltamivir, a neuraminidase inhibitor with excellent in vitro activity against the virus. This drug is available in oral solid and liquid formulations, has excellent peroral bioavailability in adults, and generally has a very favorable safety profile. Many observational studies indicate that oseltamivir treatment is associated with symptomatic improvement in pediatric patients with H1N1 infection and, therefore, is considered to represent a viable therapeutic option for use in children. However, the disposition of the ethyl ester prodrug and its active metabolite has not been well characterized in infants and children. Presently, data are available from only two published investigations and preliminary summary information from a recent presentation of an ongoing study. Given that recent in vitro data support the importance of a target exposure-response profile for the active metabolite of oseltamivir and that many processes known to modulate drug disposition have a developmental basis, understanding the potential impact of age on oseltamivir disposition becomes crucial in the development of age-appropriate dosing regimens for the drug. In this review, the impact of ontogeny on processes that are important in regulating the absorption, distribution, metabolism, and excretion of oseltamivir and its active metabolite are considered. Data from both animal and human investigations are presented in the context of defining how development might influence the dose-exposure relationship and, most importantly, the significant variability associated with it. In addition, the available pediatric pharmacokinetic data for oseltamivir and its active metabolite are summarized and current 'information gaps' deserving of future study are presented.

Antivirals for influenza: strategies for use in pediatrics

Influenza infection is annually responsible for significant morbidity and mortality, particularly among the very young and old. Recently updated guidelines recommend influenza vaccination of all children aged 6 months to 18 years; however, childhood vaccination remains underutilized. Furthermore, concerns over the reduced efficacy of vaccination in children have further heightened the need for effective treatment schemes. Antiviral therapies have emerged as attractive options in the battle against influenza infection. These agents include the adamantanes (amantadine and rimantadine) and neuraminidase inhibitors (zanamivir, oseltamivir, and peramivir). Broad-scale use of adamantane antivirals has been severely limited in recent years because of high resistance rates and their inability to cover influenza type B. Neuraminidase inhibitors cover influenza types A and B, and have been promulgated to first-line therapy because of historically low resistance rates and relatively infrequent side effects. Moreover, these agents are effective options in combating non-seasonal influenza strains, including H5N1 and pandemic 2009 H1N1. Oseltamivir may be particularly appealing for treating children since it is available in multiple oral dosage formulations, whereas commercially available zanamivir use is limited in young children because it requires inhalation. However, the emergence of resistance to oseltamivir among influenza A strains may limit its usefulness. Additional concerns with neuraminidase inhibitor use in pediatrics center around emerging reports, primarily from Japan, that have temporally linked oseltamivir to significant neuropsychiatric events in children of varying ages. Numerous novel antiviral agents are under development, but most are far from market approval. In addition to treating and preventing the initial burden of pediatric influenza infection, antiviral therapies may significantly reduce secondary bacterial infections (including pneumonia and otitis media), unnecessary antibiotic prescribing, and healthcare-associated costs.

Structural and functional basis of resistance to neuraminidase inhibitors of influenza B viruses

We have identified a virus, B/Perth/211/2001, with a spontaneous mutation, D197E in the neuraminidase (NA), which confers cross-resistance to all NA inhibitors. We analyzed enzyme properties of the D197 and E197 NAs and compared these to a D197N NA, known to arise after oseltamivir treatment. Zanamivir and peramivir bound slowly to the wild type NA, but binding of oseltamivir was more rapid. The D197E/N mutations resulted in faster binding of all three inhibitors. Analysis of the crystal structures of D197 and E197 NAs with and without inhibitors showed that the D197E mutation compromised the interaction of neighboring R150 with the N-acetyl group, common to the substrate sialic acid and all NA inhibitors. Although rotation of the E275 in the NA active site occurs upon binding peramivir in both the D197 and E197 NAs, this does not occur upon binding oseltamivir in the E197 NA. Lack of the E275 rotation would also account for the loss of slow binding and the partial resistance of influenza B wild type NAs to oseltamivir.

Plasma concentrations of oseltamivir and oseltamivir carboxylate in critically ill children on extracorporeal membrane oxygenation support

INTRODUCTION

To evaluate the effect of extracorporeal membrane oxygenation (ECMO) support on pharmacokinetics of oseltamivir and oseltamivir carboxylate (OC) in children.

METHODOLOGY

Steady state 0-12 hour pharmacokinetic sampling was performed in new influenza A (H1N1) infected children treated with oseltamivir while on ECMO support. Cmax, Cmin and AUC(0-12 h) were calculated. The age-specific oseltamivir dosage was doubled to counter expected decreased plasma drug concentrations due to increased volume of distribution on ECMO support.

PRINCIPAL FINDINGS

Three patients were enrolled aged 15, 6 and 14 years in this pharmacokinetic case series. For two children the OC plasma concentrations were higher than those found in children and adults not on ECMO. These increased plasma concentrations related to the increased oseltamivir dosage and decreased kidney function. In one patient suboptimal plasma concentrations coincided with a decreased gastric motility.

CONCLUSION

Oseltamivir pharmacokinetics do not appear to be significantly influenced by ECMO support. Caution is required in case of nasogastric administration and decreased gastric motility. Due to the limited number of (paediatric) patients available further multicenter studies are warranted.

Pharmacokinetics of oseltamivir: an oral antiviral for the treatment and prophylaxis of influenza in diverse populations

Influenza is a transmissible viral pathogen that continues to cause substantial morbidity and mortality. Oseltamivir is an orally administered antiviral medication that selectively inhibits the influenza neuraminidase enzymes that are essential for viral replication. Treatment of infected children ≥1 year and adults of all ages may decrease the severity and duration of the symptoms of infection, while prophylactic dosing can prevent their onset. Oseltamivir is ingested in the form of a prodrug (oseltamivir phosphate) that is rapidly converted by hepatic esterases into the active metabolite, oseltamivir carboxylate. Oseltamivir carboxylate has high bioavailability and penetrates sites of infection at concentrations that are sufficient to inhibit viral replication. The pharmacokinetics of oseltamivir and oseltamivir carboxylate are dose proportional after repeated doses of up to 500 mg twice daily. This predictable profile means that oseltamivir is suitable for use in diverse patient populations, which may include young children and elderly patients, various ethnic groups and those with renal or hepatic impairment. As the potential for drug interactions is low, oseltamivir is also suitable for use in patients with co-morbid conditions who are likely to be receiving concomitant medications.

Clinical effectiveness of oseltamivir for influenza A(H1N1) virus with H274Y neuraminidase mutation

OBJECTIVE

To evaluate the clinical effectiveness of oseltamivir therapy started within 48h of the onset for influenza A(H1N1) virus with H274Y neuraminidase (NA) mutation.

METHODS

Virus was isolated before and four to six days after starting oseltamivir treatment from 73 outpatients with influenza A(H1N1) virus in the 2007-2008 and 2008-2009 seasons. NA inhibition assays (IC(50)) and sequence analyses were done using influenza viruses isolated from these patients. Body temperature was evaluated before and on the second, third, and fourth days after starting treatment.

RESULTS

H274Y mutation was not shown in the 2007-2008 season (44 patients) and shown in all 29 patients in the 2008-2009 season by NA sequence analyses. The mean IC(50) before oseltamivir treatment was significantly higher in 2008-2009 (319.3+/-185.4 nM) than in 2007-2008 (1.5+/-0.8 nM; p<.001). Patients < or =15 years with oseltamivir-resistant virus infection had a higher ratio of patients persisted virus after oseltamivir treatment than patients >15 years (50% and 11.8%, respectively, p=0.038), and a significant higher body temperature during oseltamivir treatment, compared to patients < or =15 years treated for oseltamivir-sensitive virus infection.

CONCLUSION

The clinical effectiveness of oseltamivir for the A(H1N1) virus was reduced in the 2008-2009 season compared with the previous season, especially in children, probably due to the H274Y mutation. Oseltamivir seems to be not recommended for children and patients with high-risk underlying diseases infected with H274Y mutated A(H1N1) virus.

Antivirals for the treatment and prevention of epidemic and pandemic influenza

Influenza is a highly contagious and debilitating disease that imposes an excess burden of complications and mortality. Antiviral therapy is the primary intervention for treatment and post‐exposure prophylaxis (PEP) of influenza. Amantadine and rimantadine are members of the M2 class of antiviral agents and are moderately effective in influenza management. However, their utility is compromised by high levels of resistance, tolerability concerns and a lack of efficacy against influenza B. An alternative class of agents, the neuraminidase inhibitors (NIs), represent the most advanced form of antiviral therapy available, and act by specifically inhibiting the neuraminidase enzymes that are present on all influenza subtypes. Two NIs, oseltamivir and zanamivir, are currently available for clinical use. Oseltamivir, the most widely used NI, is administered orally as a prodrug (oseltamivir carboxylate) and systemically distributed to all potential infection sites. Zanamivir, a second NI, is administered by inhalation via a disk inhaler and deposited primarily in the respiratory tract. When administered within 48 hours of symptom onset, both agents significantly reduce illness duration and symptom severity, and decrease the rate of influenza‐associated complications. With oseltamivir, greater benefits are detected with earlier treatment initiation (<12 hours). In PEP, both NIs effectively protect the close contacts of index cases from symptomatic influenza. Oseltamivir and zanamivir are generally well tolerated and associated with a low level of resistance. Emerging evidence supports the activity of both NIs against the H5N1avian influenza infection, which is a pandemic candidate. However, the WHO currently recommends the use of oseltamivir for the management of suspected cases, given the systemic nature of the H5N1 challenge. Ongoing studies are exploring the effectiveness of oseltamivir, zanamivir and other NIs for pandemic management.

Pharmacokinetics of high-dose oseltamivir in healthy volunteers

The effects of loading doses and probenecid coadministration on oseltamivir pharmacokinetics at four increasing dose levels in groups of eight healthy adult Thai volunteers (125 individual series) were evaluated. Doses of up to 675 mg were well-tolerated. The pharmacokinetics were dose linear. Oseltamivir phosphate (OS) was rapidly and completely absorbed and converted (median conversion level, 93%) to the active carboxylate metabolite. Median elimination half-lives (and 95% confidence intervals [CI]) were 1.0 h (0.9 to 1.1 h) for OS and 5.1 h (4.7 to 5.7 h) for oseltamivir carboxylate (OC). One subject repeatedly showed markedly reduced OS-to-OC conversion, indicating constitutionally impaired carboxylesterase activity. The coadministration of probenecid resulted in a mean contraction in the apparent volume of distribution of OC of 40% (95% CI, 37 to 44%) and a reduction in the renal elimination of OC of 61% (95% CI, 58 to 62%), thereby increasing the median area under the concentration-time curve (AUC) for OC by 154% (range, 71 to 278%). The AUC increase for OC in saliva was approximately three times less than the AUC increase for OC in plasma. A loading dose 1.25 times the maintenance dose should be given for severe influenza pneumonia. Probenecid coadministration may allow considerable dose saving for oseltamivir, but more information on OC penetration into respiratory secretions is needed to devise appropriate dose regimens.

Human carboxylesterases HCE1 and HCE2: ontogenic expression, inter-individual variability and differential hydrolysis of oseltamivir, aspirin, deltamethrin and permethrin

Carboxylesterases hydrolyze chemicals containing such functional groups as a carboxylic acid ester, amide and thioester. The liver contains the highest carboxylesterase activity and expresses two major carboxylesterases: HCE1 and HCE2. In this study, we analyzed 104 individual liver samples for the expression patterns of both carboxylesterases. These samples were divided into three age groups: adults (>or= 18 years of age), children (0 days-10 years) and fetuses (82-224 gestation days). In general, the adult group expressed significantly higher HCE1 and HCE2 than the child group, which expressed significantly higher than the fetal group. The age-related expression was confirmed by RT-qPCR and Western immunoblotting. To determine whether the expression patterns reflected the hydrolytic activity, liver microsomes were pooled from each group and tested for the hydrolysis of drugs such as oseltamivir and insecticides such as deltamethrin. Consistent with the expression patterns, adult microsomes were approximately 4 times as active as child microsomes and 10 times as active as fetal microsomes in hydrolyzing these chemicals. Within the same age group, particularly in the fetal and child groups, a large inter-individual variability was detected in mRNA (430-fold), protein (100-fold) and hydrolytic activity (127-fold). Carboxylesterases are recognized to play critical roles in drug metabolism and insecticide detoxication. The findings on the large variability among different age groups or even within the same age group have important pharmacological and toxicological implications, particularly in relation to pharmacokinetic alterations of ester drugs in children and vulnerability of fetuses and children to pyrethroid insecticides.

Pharmacokinetics and tolerability of oseltamivir combined with probenecid

Oseltamivir is an inhibitor of influenza virus neuraminidase, which is approved for use for the treatment and prophylaxis of influenza A and B virus infections. In the event of an influenza pandemic, oseltamivir supplies may be limited; thus, alternative dosing strategies for oseltamivir prophylaxis should be explored. Healthy volunteers were randomized to a three-arm, open-label study and given 75 mg oral oseltamivir every 24 h (group 1), 75 mg oseltamivir every 48 h (q48h) combined with 500 mg probenecid four times a day (group 2), or 75 mg oseltamivir q48h combined with 500 mg probenecid twice a day (group 3) for 15 days. Pharmacokinetic data, obtained by noncompartmental methods, and safety data are reported. Forty-eight subjects completed the pharmacokinetic analysis. The study drugs were generally well tolerated, except for one case of reversible grade 4 thrombocytopenia in a subject in group 2. The calculated 90% confidence intervals (CIs) for the geometric mean ratios between groups 2 and 3 and group 1 were outside the bioequivalence criteria boundary (0.80 to 1.25) at 0.63 to 0.89 for group 2 versus group 1 and 0.57 to 0.90 for group 3 versus group 1. The steady-state apparent oral clearance of oseltamivir carboxylate was significantly less in groups 2 (7.4 liters/h; 90% CI, 6.08 to 8.71) and 3 (7.19 liters/h; 90% CI, 6.41 to 7.98) than in group 1 (9.75 liters/h; 90% CI, 6.91 to 12.60) (P < 0.05 for both comparisons by analysis of variance). The (arithmetic) mean concentration at 48 h for group 2 was not significantly different from the mean concentration at 24 h for group 1 (42 +/- 76 and 81 +/- 54 ng/ml, respectively; P = 0.194), but the mean concentration at 48 h for group 3 was significantly less than the mean concentration at 24 h for group 1 (23 +/- 26 and 81 +/- 54 ng/ml, respectively; P = 0.012). Alternate-day dosing of oseltamivir plus dosing with probenecid four times daily achieved trough oseltamivir carboxylate concentrations adequate for neuraminidase inhibition in vitro, and this combination should be studied further.

The role of oseltamivir in the treatment and prevention of influenza in children

The burden of seasonal influenza in children is poorly recognized, in spite of the potential for severe and even life-threatening illness and common secondary complications. Children are a primary reservoir for the spread of influenza to both family members and the community, which imposes a sizeable social and economic strain. Although vaccination is the primary intervention against childhood influenza, the antiviral neuraminidase inhibitors, oseltamivir and zanamivir, provide treatment options. Oseltamivir is administered orally to children aged > 1 year and has been shown to cost-effectively reduce the influenza disease burden and duration of viral shedding. Additionally, oseltamivir postexposure prophylaxis provides protective efficacy for children and families. Oseltamivir has shown excellent tolerability and a low potential for viral resistance in pediatric studies. In the event of an influenza pandemic, oseltamivir is expected to be at the forefront of containment strategies. This article reviews the pharmacology, efficacy and tolerability of oseltamivir as treatment and prophylaxis in children.

Prevention and treatment of influenza in high-risk groups: children, pregnant women, immunocompromised hosts, and nursing home residents

The pediatric population experiences preventable hospitalizations and serves as a reservoir for influenza and its transmission to other children as well as adults. As a consequence, the Advisory Committee on Immunization Practices has recommended initiating influenza immunization of children as young as 6 months of age through 23 months of age and, recently, up to 5 years of age. However, immunization of older children has not yet become a priority of the US Public Health Service. As a consequence, the importance of antiviral agents, particularly neuraminidase (NA) inhibitors, cannot be overemphasized. From an epidemiological perspective, influenza resulted in higher childhood mortality than did Bordetella pertussis infection in 2003-2004. During that season, 153 children died of influenza, and two-thirds were <5 years of age. Importantly, nearly 50% of these children were previously healthy, with no underlying illness. Currently, 2 NA inhibitors are approved for the treatment of influenza in children. Zanamivir is approved for children >7 years of age, and oseltamivir is approved for children >1 year of age. Arguably, the younger children are at particular risk for influenza complications and hospitalization. In placebo-controlled studies in children >1 year of age, oseltamivir therapy accelerated resolution of clinical illness and defervescence and decreased both the incidence of otitis media and the concomitant use of antibiotics. However, oseltamivir is not currently approved for children <1 year of age. Three clinical toxicology studies identified neurotoxicity in newborn rats administered this medication. In these preclinical toxicology studies, the dose of oseltamivir exceeded that which would be used in humans. In addition, the metabolism of oseltamivir is different in rats than in humans. A key component of influenza therapy is the possibility for development of resistance. Although in studies performed in North America, resistance was not a frequent event, it has been documented in Japanese children treated with this medication; the adequacy of the dose used has been questioned. Children represent only one unique study population among others. Individuals who are at increased risk for influenza infection include the elderly, the immunocompromised, and pregnant women. Collectively, antiviral medications must be evaluated in populations in which they have not yet been assessed. The development of additional antiviral drugs is an important recommendation for the future, so that antiviral resistance can be circumvented. Similarly, availability of drugs for children <1 year of age is mandatory.

Importance of collection tube during clinical studies of oseltamivir

Ex vivo conversion of the anti-influenza drug oseltamivir to its active metabolite can be inhibited by the esterase inhibitor dichlorvos or by using commercial fluoride-oxalate tubes. Oseltamivir and its active metabolite remain intact in plasma samples during a proposed virus heat inactivation step: incubation at 60 degrees C for 45 min.

Neuraminidase inhibitors for preventing and treating influenza in children

BACKGROUND

During epidemic years, influenza attack rates in children exceed 40%. Options for prevention and treatment include the neuraminidase inhibitors: zanamivir and oseltamivir.

OBJECTIVES

To assess the efficacy, safety and tolerability of neuraminidase inhibitors in the treatment and prevention of influenza infection in children.

SEARCH STRATEGY

We searched the Cochrane Central Register of Controlled Trials (The Cochrane Library Issue 1, 2005); MEDLINE (1966 to April 2005); EMBASE (January 1980 to December 2004); the on-line GlaxoSmithKline Clinical Trials Register; the on-line Roche Clinical Trial Protocol Registry and Clinical Trial Results Database (August 2005); and reference lists of articles. We also scrutinised web sites of European and US regulatory bodies and contacted manufacturers and authors.

SELECTION CRITERIA

Double-blind, randomised, controlled trials comparing neuraminidase inhibitors with placebo or other antiviral drugs in children less than 12 years of age. Additional safety and tolerability data from other sources were also included.

DATA COLLECTION AND ANALYSIS

Four authors applied the inclusion criteria to the retrieved studies, assessed trial quality and extracted data. Data were analysed separately for oseltamivir and zanamivir.

MAIN RESULTS

Three trials involving 1500 children with a clinical case definition of influenza were included, of whom 977 had laboratory-confirmed influenza. Overall, trial quality was good. Oseltamivir reduced the median duration of illness by 26% (36 hours) in healthy children with laboratory-confirmed influenza (P value less than 0.0001). The reduction was only 7.7% (10 hours) in 'at risk' (asthmatic) children, and this did not reach statistical significance (P value = 0.54). Zanamivir reduced the median duration of illness by 24% (1.25 days) in healthy children with laboratory-confirmed influenza (P value less than 0.001). No data in 'at risk' children were available. Only oseltamivir produced a significant reduction in the complications of influenza (particularly otitis media), although there was a trend to benefit for zanamivir. We identified one randomised, controlled trial of oseltamivir for the prevention of influenza transmission in households, reporting data from 222 paediatric contacts. Where index cases had laboratory-confirmed influenza, a protective efficacy of 55% was observed, but this did not reach statistical significance (P value = 0.089). The adverse events profile of zanamivir was no worse than placebo, but vomiting was more common in children treated with oseltamivir.

AUTHORS' CONCLUSIONS

Neuraminidase inhibitors are effective in shortening illness duration in healthy children with influenza, but efficacy in 'at risk' children remains to be proven. Oseltamivir is also effective in reducing the incidence of secondary complications, and may be effective for influenza prophylaxis.

Avian influenza: A review

PURPOSE

A review of the avian influenza A/H5N1 virus, including human cases, viral transmission, clinical features, vaccines and antivirals, surveillance plans, infection control, and emergency response plans, is presented.

SUMMARY

The World Health Organization (WHO) considers the avian influenza A/H5N1 virus a public health risk with pandemic potential. The next human influenza pandemic, if caused by the avian influenza A/H5N1 virus, is estimated to have a potential mortality rate of more than a hundred million. Outbreaks in poultry have been associated with human transmission. WHO has documented 258 confirmed human infections with a mortality rate greater than 50%. Bird-to-human transmission of the avian influenza virus is likely by the oral-fecal route. The most effective defense against an influenza pandemic would be a directed vaccine to elicit a specific immune response toward the strain or strains of the influenza virus. However, until there is an influenza pandemic, there is no evidence that vaccines or antivirals used in the treatment or prevention of such an outbreak would decrease morbidity or mortality. Surveillance of the bird and human populations for the highly pathogenic H5N1 is being conducted. Infection-control measures and an emergency response plan are discussed.

CONCLUSION

Avian influenza virus A/H5N1 is a public health threat that has the potential to cause serious illness and death in humans. Understanding its pathology, transmission, clinical features, and pharmacologic treatments and preparing for the prevention and management of its outbreak will help avoid its potentially devastating consequences.

Rapid degradation of oseltamivir phosphate in clinical samples by plasma esterases

The anti-influenza drug oseltamivir is an ester prodrug activated by hepatic carboxylesterases. Plasma esterases also convert up to 31.8% of the parent compound to the active metabolite after 4 h ex vivo, with wide interindividual variation. This source of error is removed by adding the esterase inhibitor dichlorvos to blood collection tubes.

The use of antiviral drugs for influenza: recommended guidelines for practitioners

The present document outlines current guidelines and supporting literature relating to the use of antiviral drugs for chemoprophylaxis and influenza illness therapy in paediatric and adult settings. The focus is on the management of influenza in interpandemic periods. Where appropriate, the areas in need of additional research are identified. It will be necessary to update aspects of these guidelines as new information emerges. The recommendations that follow represent the results of a joint effort supported by the Canadian Paediatric Society and the Association of Medical Microbiology and Infectious Disease Canada.