Plasma and pulmonary pharmacokinetics of desfuroylceftiofur acetamide after weekly administration of ceftiofur crystalline free acid to adult horses

Summary Reasons for performing study Current labelling for the use of ceftiofur crystalline free acid (CCFA) in horses states that 2 i.m. doses must be administered 4 days apart to provide 10 days of therapeutic coverage. A 10 day treatment regimen is not sufficient for the long‐term treatment of horses with severe lung consolidation or pleuropneumonia. There are currently no data to guide an appropriate dosing interval when a longer treatment regimen is warranted. Objectives To determine steady‐state plasma and pulmonary epithelial lining fluid (PELF) concentrations of desfuroylceftiofur acetamide (DCA) after weekly i.m. administration of CCFA to adult horses. Study design Experimental study. Methods Seven adult horses received i.m. CCFA at a dose of 6.6 mg/kg bwt on Day 0, Day 4 and every 7 days thereafter for 3 additional doses. Concentrations of DCA in plasma and PELF were measured at various time intervals. Results After weekly i.m. administration, the mean (± s.d.) steady‐state peak DCA concentration in plasma (2.87 ± 1.50 μg/ml) was significantly higher than that in PELF (0.84 ± 0.53 μg/ml). Mean terminal half‐lives in plasma (77.5 ± 17.5 h) and PELF (92.8 ± 59.0 h) were not significantly different. Concentrations of DCA in plasma and PELF remained in the therapeutic range for the entire dosing interval. Conclusions After the initial 2‐dose regimen 4 days apart, weekly i.m. administration of CCFA was well tolerated and resulted in plasma and PELF DCA concentrations above the minimal inhibitory concentration that inhibits growth of at least 90% of common lower respiratory tract pathogens of horses. Potential relevance Weekly administration of CCFA would appear appropriate when a treatment regimen longer than 10 days is warranted based on clinical signs and disease severity.