Development of a sensitive chemiluminescent neuraminidase assay for the determination of influenza virus susceptibility to zanamivir

Determination of the sensitivity of influenza viruses to neuraminidase (NA) inhibitors is presently based on assays of NA function because, unlike ava ilable cell culture methods, the results of such assays are predictive of s usceptibility in vivo. At present the most widely used substrate in assays of NA function is the fluorogenic reagent 2'-O-(4-methylumbelliferyl)-N-ace tylneuraminic acid (MUN). A rapid assay with improved sensitivity is requir ed because a proportion of clinical isolates has insufficient NA to be dete ctable in the current fluorogenic assay, and because some mutations associa ted with resistance to NA inhibitors reduce the activity of the enzyme. A c hemiluminescence-based assay of NA activity has been developed that uses a 1,2-dioxetane derivative of sialic acid (NA-STAR) as the substrate. When co mpared with the fluorogenic assay, use of the NA-STAR substrate results in a 67-fold reduction in the limit of detection of the NA assay, from 200 pM (11 fmol) NA to 3 pM (0.16 fmol) NA. A panel of isolates from phase 2 clini cal studies of zanamivir, which were undetectable in the fluorogenic assay, was tested for activity using the NA-STAR substrate. Of these 12 isolates with undetectable NA activity, 10 (83%) were found to have detectable NA ac tivity using the NA-STAR substrate. A comparison of sensitivity to zanamivi r of a panel of influenza A and B viruses using the two NA assay methods ha s been performed. IC50 values for zanamivir using the NA-STAR were in the r ange 1.0-7.5 nM and those for the fluorogenic assay in the range 1.0-5.7 nM (n = 6). The NA-STAR assay is a highly sensitive, rapid assay of influenza virus NA activity that is applicable to monitoring the susceptibility of i nfluenza virus clinical isolates to NA inhibitors. (C) 2000 Academic Press.