Metabolism of [C-14]ceftiofur hydrochloride in swine after intramuscular injections

Twelve mix-breed swine (26.5-42.5 kg) received three intramuscular doses of [C-14]ceftiofur hydrochloride at 24 h intervals. Three males and three females received 6.76 +/- 0.83 mg of [C-14]ceftiofur free acid equivalents (CE)/kg body weight (bw)/day, while three males and three females received 4.41 +/- 0.97 mg of CE/kg bw/day. All swine were euthanatized 12 h after the last dose. Most of the C-14-residues (> 80%) in tissues were associated with macromolecules. The rest of the C-14-activity corresponded to desfuroylceftiofur (DFC)-cysteine. The type and percentages of metabolites present in the urine of swine were similar to those previously observed for ceftiofur sodium (Gilbertson et al., J. Agric. Food Chem. 1995, 43, 229-234), with DFC-cysteine and DFC-dimer being the major ones. In plasma, the major portion of the C-14-activity ( > 85%) was associated with macromolecules. DFC-cysteine was the only detectable C-14-component of the macromolecule free plasma fraction. This study indicated that ceftiofur hydrochloride was absorbed in the same ionic form as ceftiofur sodium, acceding to the same primary metabolite DFC. The type of metabolites present in swine after intramuscular injection were the same, regardless of the salt administered.