Identification of cephapirin metabolites and degradants in bovine milk by electrospray ionization-ion trap tandem mass spectrometry

Liquid chromatography-ion trap tandem mass spectrometry (LC-MS/MS) with ele ctrospray ionization was used to identify cephapirin metabolites and degrad ants in milk from cows dosed with cephapirin. The milk was extracted accord ing to a previously published procedure. Structures for various components were tentatively identified by their molecular weight, product ion mass spe ctra, and/or correspondence to standard mass spectra. These components may have occurred as metabolites or as degradants that occurred on storage or d uring extraction. Compounds identified in the milk included cephapirin, des acetylcephapirin, cephapirin lactone, hydrolyzed cephapirin, and a reduced cephapirin lactone that has not previously been reported. Methylcephapirin was also identified, possibly as a trace contaminant in the formulation. An alysis of incurred milk extracts showed that cephapirin and desacetylcephap irin are the major residues in milk. Desacetylcephapirin residues persisted about as long as the parent drug. The detection limit for both residues by LC-MS/MS was similar to1 ng/mL in milk. These results have implications fo r microbiological methods or rapid test kits, if such methods or kits respo nd to cephapirin metabolites and degradants present in the milk.