Development of a surface plasmon resonance assay to measure the binding affinity of wild-type influenza neuraminidase and its H274Y mutant to the antiviral drug zanamivir.

Influenza is one of the most common infections of the upper respiratory tract. Antiviral drugs that are currently used to treat influenza, such as oseltamivir and zanamivir, are neuraminidase (NA) inhibitors. However, the virus may develop resistance through single-point mutations of NA. Antiviral resistance is currently monitored by a labelled enzymatic assay, which can be inconsistent because of the short half-life of the labelled product and variations in the assay conditions. In this paper, we describe a label-free surface plasmon resonance (SPR) assay for measuring the binding affinity of NA-drug interactions. Wild-type (WT) NA and a histidine 274 tyrosine (H274Y) mutant were expressed in High Five™ (Trichoplusia ni) insect cells. A spacer molecule (1,6-hexanediamine) was site-specifically conjugated to the 7-hydroxyl group of zanamivir, which is not involved in binding to NA, and the construct was immobilized onto a SPR sensor Chip to obtain a final immobilization response of 431 response units. Binding responses obtained for WT and H274Y mutant NAs were fitted to a simple Langmuir 1:1 model with drift to obtain the association (ka ) and dissociation (kd ) rate constants. The ratio between the binding affinities for the two isoforms was comparable to literature values obtained using labelled enzyme assays. Significant potential exists for an extension of this approach to test for drug resistance of further NA mutants against zanamivir and other antiviral drugs, perhaps paving the way for a reliable SPR biosensor assay that may replace labelled enzymatic assays.