Assessment of the sensitivity of Cambodian H5N1 strains towards Oseltamivir and Zanamivir

Title of Contribution: Assessment of the sensitivity of Cambodian H5N1 strains towards Oseltamivir and Zanamivir

Author(s): Monica J. Naughtin, Philippe Buchy

Affiliation(s): Virology Unit, Institut Pasteur du Cambodge, 5 Monivong Blvd, Phnom Penh

Abstract:

Since 2003, currently circulating H5N1 avian influenza strains have caused outbreaks in 12 countries, including most of South East Asia, and have caused over 300 human fatalities, including 7 in Cambodia.  We have isolated and characterized the molecular determinants of 33 among 144 H5N1 viruses isolated from poultry and human species between 2003-2007.  Several strains have molecular alterations which are predicted to affect sensitivity to NA inhibitors.  The aim of this study is to assess the susceptibility of a panel of Cambodian H5N1 isolates to Oseltamivir (TamifluTM) and Zanamivir (RelenzaTM).  A chemoluminescence-based in vitro assay of NA activity, which utilizes the artificial NA substrate 1,2-dioxetane derivative of sialic acid (NA-STAR), was used to determine the concentration of drug required to inhibit 50% of NA enzyme activity (IC50).  These results demonstrate that Cambodian H5N1 strains from 2003-2007 from human and avian species all exhibit IC50 values in the highly sensitive range (0.1-1 nM) for both Oseltamivir and Zanamivir.  Additionally, an in vivo bioassay was developed to measure NA inhibitor sensitivity of H5N1 strains on MDCK-SIAT1 cells.  These results suggest that EC50 values obtained from the bioassay compliment the IC50 values obtained from the NA-STAR assay, and confirm that the two Cambodian strains predicted to have altered responses to these drugs are in fact highly sensitive to Oseltamivir. Development of resistance to NA inhibitors is a concern in the wake of an influenza pandemic, as these drugs will be one of the first lines of defense to protect the community against the pandemic strain until a suitable vaccine can be developed.  This study contributes to the WHO global surveillance program for monitoring NA inhibitor resistance amongst Influenza strains.