Investigation of morantel metabolism and its application in veterinary drug residue screening

Food Safety authorities routinely test food of animal origin to verify that veterinary drug residues (VDRs) are within maximum residue limits. For morantel, a gas chromatography - mass spectrometry (GC-MS) confirmatory method is effective, but it involves an extensive sample preparation that is not suitable for multiple analytes. Furthermore, the resulting marker residue is not specific to morantel. Alternatively, using major morantel metabolites as specific VDRs in liquid chromatography - mass spectrometry (LC-MS) screening methods would increase productivity, especially when implemented in multi-residue screening methods.

In this study, our workflow used in vitro incubations of morantel and stable isotope labeled morantel with liver S9 fractions. LC combined with high resolution mass spectrometry (HRMS) and Compound Discoverer software was used to identify major morantel metabolites. Detailed tandem MS (MS/MS and MS³) experiments were used to determine the structures of metabolites. Multiple phase I and phase II metabolites of morantel were identified, including hydroxylated and cysteine-conjugated metabolites. Five major metabolites, identified from in vitro metabolism studies using porcine or bovine liver S9 fractions, were thoroughly investigated. The presence of these metabolites was confirmed using pseudo-incurred bovine liver tissue. Metabolic reaction sites were proposed for these metabolites, including a hydroxylation site for one metabolite, that contradicts previous findings.

These five major metabolites reported are specific to morantel and can be readily implemented in a multi-residue screening method. This workflow is adaptable for other veterinary drugs and the major advantages of using this approach include time efficiency in sample preparation and the ability to incorporate the VDRs into multiple-residue screening methods.