Biotransformation of benzydamine by microsomes and precision-cut slices prepared from cattle liver

1. Benzydamine (BZ), a non-steroidal anti-inflammatory drug used in human and veterinary medicine, is not licensed for use in food-producing species. Biotransformation of BZ in cattle has not been reported previously and is investigated here using liver microsomes and precision-cut liver slices. 2. BZ was metabolized by cattle liver microsomes to benzydamine N-oxide (BZ-NO) and monodesmethyl-BZ (Nor-BZ). Both reactions followed Michaelis-Menten kinetics (Km = 76.4 +/- 16.0 and 58.9 +/- 0.4 microM Vmax = 6.5 +/- 0.8 and 7.4 +/- 0.5 nmolmg(-1) min(-1) respectively); sensitivity to heat and pH suggested that the N-oxidation is catalysed by the flavin-containing monooxygenases. 3. BZ-NO and Nor-BZ were the most abundant products derived from liver slice incubations, and nine other BZ metabolites were found and tentatively identified by LC-MS. Desbenzylated and hydroxylated BZ-NO analogues and a hydroxylated product of BZ were detected, which have been reported in other species. Product ion mass spectra of other metabolites, which are described here for the first time, indicated the formation of a BZ N- -glucuronide and five hydroxylated and N+-glucuronidated derivatives of BZ, BZ-NO and Nor-BZ. 4. The results indicate that BZ is extensively metabolized in cattle. Clearly, differences in metabolism compared with, for example, rat and human, will need to be considered in the event of submission for marketing authorization for use in food animals.