Oseltamivir (Tamiflu) Dosing Strategies for Use During Influenza Prophylaxis (VA01)

In vitro studies have determined that the 50 % inhibitory concentrations (IC50) of Ro 64-0802 against influenza neuraminidases ranged from 0.3 to 22 nmol/L (0.08 - 0.28 g/L or 0.08 -0.28 ng/mL). IC50 values against influenza strains in cell culture were somewhat higher and more variable ranging from 0.6 to 155 nmol/L (0.17 - 32.8 g/L or 0.17 - 32.8 ng/mL). Tamiflu has also been shown to have in vitro and mouse challenge activity against the H5N1 virus. EC50 values against H5N1 strain replication in Madin Darby canine kidney (MDCK) cells ranged from 7.5-12 M and neuraminidase activity from 7.0-15 nM (IC50 values).

The current U.S. FDA approved recommendations for influenza A and B treatment using oseltamivir suggest a dose of 75mg taken orally twice daily for 5 days at the onset of symptoms or laboratory confirmation of infection. Prophylaxis against influenza A infection for those people exposed to or at high risk for exposure suggests a dose of 75 mg orally taken once daily for up to 6 weeks. No clinical trials in humans infected with or requiring prophylaxis for the H5N1 variant have been performed. Oseltamivir treatment in a human pediatric case of H5N1 influenza pneumonia has been published. Oseltamivir was given late in the course of illness and the child subsequently expired. Several people were given a prophylactic course of oseltamivir after avian influenza (H7N7, H7N3) outbreaks in the Netherlands and in British Columbia, which appeared to be effective in preventing additional human cases.

Clinical trials using oseltamivir for influenza treatment have recently been reviewed. Two phase III placebo controlled, blinded studies were performed in adults, three in geriatric populations, and one in pediatric patients. In general, flu symptoms were decreased by one day or greater in those patients who received Tamiflu compared to placebo. Several prophylaxis studies using oseltamivir have been conducted in healthy unvaccinated adults and have also been recently reviewed. In several trials involving either prophylaxis after household exposure or after exposure in the community have demonstrated that the incidence of influenza was significantly reduced (range 70-90%) in those patients receiving Tamiflu, 75mg orally once daily for 42 days compared to placebo. Side effect profiles in the prophylaxis studies indicate that nausea and vomiting were more commonly found in the Tamiflu arm when compared to placebo. There was no difference in side effect incidence in younger compared to older adult (> 65 years) populations.