Tamiflu
The H274Y mutation in the influenza A/H1N1 neuraminidase active site following oseltamivir phosphate treatment leave virus severely compromised both in vitro and in vivo.

Ives JA, Carr JA, Mendel DB, Tai CY, Lambkin R, Kelly L, Oxford JS, Hayden FG, Roberts NA.

Roche Products Ltd., 40 Broadwater Road, Welwyn Garden City, AL7 3AY, Hertfordshire, UK. jane.ives roche.com

Oseltamivir carboxylate is a potent and specific inhibitor of influenza A and B neuraminidase (NA). Oseltamivir phosphate, the ethyl ester prodrug of oseltamivir carboxylate, is the first orally active NA inhibitor available for the prophylaxis and treatment of influenza A and B. It offers an improvement over amantadine and rimantadine which are active only against influenza A and rapidly generate resistant virus. The emergence of virus resistant to oseltamivir carboxylate in the treatment of naturally acquired influenza infection is low (about 1%). The types of NA mutation to arise are sub-type specific and largely predicted from in vitro drug selection studies. A substitution of the conserved histidine at position 274 for tyrosine in the NA active site has been selected via site directed mutagenesis, serial passage in culture under drug pressure in H1N1 and during the treatment of experimental H1N1 infection in man. Virus carrying H274Y NA enzyme selected in vivo has reduced sensitivity to oseltamivir carboxylate. The replicative ability in cell culture was reduced up to 3 logs, as was infectivity in animal models of influenza virus infection. Additionally, pathogenicity of the mutant virus is significantly compromised in ferret, compared to the corresponding wild type virus. Virus carrying a H274Y mutation is unlikely to be of clinical consequence in man. Copyright 2002 Elsevier Science BV.

Online source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12103431&dopt=Abstract oseltamivir Tamiflu



Tamiflu
Mechanism by which mutations at his274 alter sensitivity of influenza a virus n1 neuraminidase to oseltamivir carboxylate and zanamivir.

Wang MZ, Tai CY, Mendel DB.

Gilead Sciences, Inc., Foster City, California 94404, USA. Michael_Wang gilead.com

Oseltamivir carboxylate is a potent and specific inhibitor of influenza neuraminidase (NA). An influenza A/H1N1 variant selected in vitro with reduced susceptibility to oseltamivir carboxylate contains a His274Tyr mutation. To understand the mechanism by which a His274Tyr mutation gives rise to drug resistance, we studied a series of NA variant proteins containing various substitutions at position 274. Replacement of His274 with larger side chain residues (Tyr or Phe) reduced the NA sensitivity to oseltamivir carboxylate. In contrast, replacement of His274 with smaller side chain residues (Gly, Asn, Ser, and Gln) resulted in enhanced or unchanged sensitivity to oseltamivir carboxylate. Previous studies have suggested that the slow-binding inhibition of NA by oseltamivir carboxylate is a result of the reorientation of Glu276. Loss of this slow-binding inhibition in the His274Tyr and His274Phe mutant NA but not in His274Asn, His274Gly, His274Ser, or His274Gln supports the conclusion that the conformational change of Glu276 is restricted in the His274Tyr and His274Phe mutant NA upon oseltamivir carboxylate binding. Interestingly, His274Asn, as well as His274Gly, His274Ser, and His274Gln, also displayed reduced sensitivity to zanamivir and its analogue, 4-amino-Neu5Ac2en. Substitution of His274 with Tyr in influenza A/Tokyo/3/67 (H3N2) recombinant NA did not affect the susceptibility to oseltamivir carboxylate. These data indicate that the volume occupied by the amino acid side chain at position 274 can influence the sensitivities of influenza N1 NA but not of N2 NA to both oseltamivir carboxylate and zanamivir.

Online source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12435681&dopt=Abstract oseltamivir Tamiflu



Tamiflu
[Effectiveness of oseltamivir treatment against influenza type A and type B infection in children]

[Article in Japanese]

Mitamura K, Sugaya N, Nirasawa M, Shinjoh M, Takeuchi Y.

Department of Pediatrics, Nippon Kokan Hospital.

We investigated the effectiveness of oseltamivir treatment against influenza virus infection in children. We treated 131 patients (mean age, 5.8 +/- 3.6 years) with oseltamivir (4 mg/kg/day for 5 days) during the 2001-2002 epidemic. All of the patients had been diagnosed with influenza using a rapid diagnosis kit. When treatment was initiated within 48 hours of the onset of fever, 44% of the patients became afebrile (< 37.5 degrees C) within one day, and 86% of them recovered within two days. The average duration of fever after the initiation of oseltamivir treatment was 1.7 days. Oseltamivir was equally effective against both influenza type A and type B. No differences in the effectiveness of oseltamivir treatment were observed between young children (< 4 years of age) and school-aged children (> 6 years of age). No obvious side effects were observed.

Online source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12508478&dopt=Abstract oseltamivir Tamiflu



Tamiflu
Oseltamivir for treatment of influenza in healthy adults: pooled trial evidence and cost-effectiveness model for Canada.

O'Brien BJ, Goeree R, Blackhouse G, Smieja M, Loeb M.

Department of Clinical Epidemiology and Biostatistics, McMaster University, Hamilton, Ontario, Canada. obrienb mcmaster.ca

BACKGROUND: Influenza is a common viral respiratory infection that is associated with significant morbidity. Oseltamivir (Tamiflu) is a neuraminidase inhibitor-a new class of antiviral treatment for influenza where efficacy and safety has been established but cost-effectiveness is unknown. METHODS: A decision analytic model was used to estimate the costs and effectiveness of two treatment scenarios for empiric management of otherwise healthy nonelderly patients, presenting with influenza-like illness (ILI) to primary care physicians in Canada: 1) where oseltamivir is reimbursed and on formulary for prescription; and 2) where oseltamivir is not on formulary. Outcomes are influenza-days averted and quality-adjusted life-years (QALYs) gained. Effectiveness, utility, and pneumonia complication risk estimates are by pooled analysis of patient-level data from four clinical trials. Unit cost information (Canadian dollars) was obtained from published sources in Ontario. Probabilistic sensitivity analysis was conducted using Monte Carlo simulation. RESULTS: Of 2288 patients randomized, influenza was confirmed in 1575 (69%) and oseltamivir treatment reduced the mean time to symptom alleviation by 1.08 days (95% confidence interval CI] 0.58-1.59). Infected patients treated with oseltamivir had higher utility scores (quality of life) than placebo patients over the 7 days of follow-up (P <.05). Cost per influenza-day averted with oseltamivir on formulary is 49 US dollars (95% CI 31-107) and the cost per QALY is 57,863 US dollars (95% CI 48,919- 70,149 US dollars). Results are sensitive to the percentage of patients presenting to their physician beyond 48 hours from symptom onset who get oseltamivir and the prevalence of influenza among patients presenting with ILI. CONCLUSIONS: Oseltamivir for treatment of patients with ILI is potentially cost-effective if clinical diagnostic specificity for influenza observed in clinical trials is applicable to routine practice. More population-based information on the prevalence of influenza among early (<48 hours) presenters with ILI would be valuable.

Online source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12641862&dopt=Abstract oseltamivir Tamiflu



Tamiflu
Neuraminidase sequence analysis and susceptibilities of influenza virus clinical isolates to zanamivir and oseltamivir.

McKimm-Breschkin J, Trivedi T, Hampson A, Hay A, Klimov A, Tashiro M, Hayden F, Zambon M.

Division of Health Sciences and Nutrition, Commonwealth Scientific and Industrial Research Organisation, Parkville, Australia. jennifer.mckimm-breschkin csiro.au

The influenza virus neuraminidase (NA) inhibitors zanamivir and oseltamivir were introduced into clinical practice in various parts of the world between 1999 and 2002. In order to monitor the potential development of resistance, the Neuraminidase Inhibitor Susceptibility Network was established to coordinate testing of clinical isolates collected through the World Health Organization influenza surveillance network from different regions of the world (M. Zambon and F. G. Hayden, Antivir. Res. 49:147-156, 2001). The present study establishes the baseline susceptibilities prior to and shortly after the introduction of the NA inhibitors. Over 1000 clinical influenza isolates recovered from 1996 to 1999 were tested. Susceptibilities were determined by enzyme inhibition assays with chemiluminescent or fluorescent substrates with known NA inhibitor-resistant viruses as controls. The 50% inhibitory concentrations (IC(50)s) depended upon the assay method, the drug tested, and the influenza virus subtype. By both assays, the mean zanamivir IC(50)s were 0.76, 1.82, and 2.28 nM for the subtype H1N1 (N1), H3N2 (N2), and B NAs, respectively, and the oseltamivir IC(50)s were 1.2, 0.5, and 8.8 nM for the N1, N2, and B NAs, respectively. The drug susceptibilities of known zanamivir- and oseltamivir-resistant viruses with the NA mutations E119V, R292K, H274Y, and R152K fell well outside the 95% confidence limits of the IC(50)s for all natural isolates. Sequence analysis of the NAs of viruses for which the IC(50)s were above the 95% confidence limits and several control isolates for which the IC(50)s were in the normal range revealed variations in some previously conserved residues, including D151, A203, T225, and E375 (N2 numbering). Known resistance mutations are both influenza virus subtype and drug specific, but there was no evidence of naturally occurring resistance to either drug in any of the isolates.

Online source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12821478&dopt=Abstract oseltamivir Tamiflu