Monitoring of viral susceptibility: new challenges with the development ofinfluenza NA inhibitors

With the clinical development of anti-viral agents, monitoring for the cont inued susceptibility of wild-type strains has become important in disease m anagement. Various methods have been used to monitor viral susceptibility; the advantages and disadvantages of which depend on the virus, the target a nd the scale of the research being undertaken. The plaque-reduction assay i s valuable for measuring susceptibility of most viruses but is not ideal fo r large-scale monitoring. Yield-reduction, measuring specific virus antigen s, and dye-uptake assays, measuring virus cytopathic effects, are more suit able for high-throughput requirements, but the IC50 value (the concentratio n that inhibits 50% of virus) varies with the viral inoculum. Surveillance of influenza susceptibility to rimantadine/amantadine in the clinic has pre dominantly used EIA-based assays, since plaquing of influenza clinical isol ates is variable. With development of the influenza NA inhibitors it became apparent that current cell-based assays were unsuitable for monitoring sus ceptibility to this new class of drugs. Variability may result from virus s pread directly from cell to cell in culture by-passing the NA function. Fur thermore, mutations selected in the HA, while not apparently contributing t o phenotypic resistance in vivo, may result in cell-culture based resistanc e, and may mask NA resistance in cell culture by modifying receptor-binding specificity. One important distinction between NA inhibitors and other ant iviral enzyme inhibitors is that both target enzyme and inhibitor work extr acellularly. NA assays are therefore most representative of the in vivo sit uation for monitoring susceptibility, supported by HA sequencing. As the cl inical use of NA inhibitors escalates, a major change will be required in a pproaches used to monitor susceptibility of influenza isolates in virology laboratories world-wide. Copyright (C) 2000 John Wiley & Sons, Ltd.