benzydamine noxygenation as a measure of flavincontaining monooxygenase activity benzydamine is a nonsteroidal antiinflammatory drug that undergoes flavincontaining monooxygenase fmodependent metabolism to a stable noxide this metabolite can be quantified with high specificity and sensitivity by using a simple reversephase highperformance liquid chromatography hplc assay with fluorescence detection studies with recombinant fmo enzymes demonstrate that fmoi and fmo3 are the primary catalysts of benzydamine noxygenation with minimal contributions from cytochrome p450 enzymes investigations conducted with human liver microsomes confirm that fmo3 in large part is responsible for benzydamine noxide formation in this tissue these features render benzydamine a useful in vitro probe for fmo activity in a wide range of tissues and cell types in addition benzydamine appears to be a suitable in vivo probe for human liver fmo3 this chapter provides a detailed account of the experimental protocol for determining rates of formation of benzydamine noxide by fmocontaining enzyme fractions