AIMS To determine the FMO and P450 isoform selectivity for metabolism of benzydamine and caffeine, two potential in vivo probes for human FMO. METHODS Metabolic incubations were conducted at physiological pH using substrate concentrations of 0.01-10 mM with either recombinant human FMOs, P450s or human liver microsomes serving as the enzyme source. Products of caffeine and benzydamine metabolism were analysed by reversed-phase h.p.l.c. with u.v. and fluorescence detection. RESULTS CYP1A2, but none of the human FMOs, catalysed metabolism of caffeine. In contrast, benzydamine was a substrate for human FMO1, FMO3, FMO4 and FMO5. Apparent Km values for benzydamine N-oxygenation were 60 +/- 8 microM, 80 +/- 8 microM, > 3 mM and > 2 mM, for FMO1, FMO3, FMO4 and FMO5, respectively. The corresponding Vmax values were 46 +/- 2 min-1, 36 +/- 2 min-1, < 75 min-1 and < 1 min-1. Small quantities of benzydamine N-oxide were also formed by CYPs 1A1, 1A2, 2C19, 2D6 and 3A4.
Conclusions: FMO1 and FMO3 catalyse benzydamine N-oxygenation with the highest efficiency. However, it is likely that the metabolic capacity of hepatic FMO3 is a much greater contributor to plasma levels of the N-oxide metabolite in vivo than is extrahepatic FMO1. Therefore, benzydamine, but not caffeine, is a potential in vivo probe for human FMO3.