EFFECTS OF AGE ON THE PHARMACOKINETICS OF SINGLE-DOSE CEFTIOFUR SODIUM ADMINISTERED INTRAMUSCULARLY OR INTRAVENOUSLY TO CATTLE

The effects of maturation on the intravenous (IV) and intramuscular (I M) pharmacokinetics of ceftiofur sodium following a dose of 2.2 mg cef tiofur equivalents/kg body weight were evaluated in 16 one-day-old Hol stein bull calves (33-53 kg body weight initially; Group 1) and 14 six -month-old Holstein steers (217-276 kg body weight initially; Group 2) . Group 1 calves were fed unmedicated milk replacer until 30 days of a ge and were then converted to the same roughage/concentrate diet as Gr oup 2, Groups 1-IV and 2-IV received ceftiofur sodium TV, and Groups 1 -IM and 2-IM received ceftiofur sodium IM, Group 1 calves were dosed a t 7 days of age and at 1 and 3 months of age; group 2 calves were dose d at 6 and 9 months of age, Blood samples were obtained serially from each calf, and plasma samples were analysed using an HPLC assay that c onverts ceftiofur and all desfuroylceftiofur metabolites to desfuroylc eftiofur acetamide. C-max values were similar in all calves, and were no higher in younger calves than in older calves. Plasma concentration s remained above 0.150 mu g ceftiofur free acid equivalents/mL for 72 h in 7-day-old calves, but were less than 0.150 mu g/mL within 48 h fo llowing IV or IM injection for 6- and 9-month-old calves. Intramuscula r bioavailability, assessed by comparing the model-derived area under the curve (AUC(mod)) from IM and IV injection at each age, appeared to be complete. After IV administration, the AUC(mod) in 7-day-old and 1 -month-old calves (126.92+/-21.1 mu g . h/mL and 135.0+/-21.6 mu g . h /mL, respectively) was significantly larger than in 3-, 6- and 9-month -old calves (74.0+/-10.7 mu g . h/mL, 61.0+/-17.7 mu g . h/mL and 68.5 +/-12.8 mu g . h/mL, respectively; P < 0.0001), The Vd((ss)) decreased linearly within the first 3 months of life in cattle (0.345+/-0.0616 L/kg, 0.335+/-0.919 L/kg and 0.284+/-0.0490 L/kg, respectively; P=0.03 1), indicative of the decreasing extracellular fluid volume in maturin g cattle. The Cl-B was significantly smaller in 7-day-old and 1-month- old calves (0.0178+/-0.00325 L/h . kg and 0.0167+/-0.00310 L/h . kg, r espectively) than in 3-, 6- and 9-month-old calves (0.0303+/-0.0046 L/ h . kg, 0.0398+/-0.0149 L/h . kg and 0.0330+/-0.00552 L/h . kg, respec tively; P less than or equal to 0.001). This observation may be indica tive of maturation of the metabolism and/or excretion processes for ce ftiofur and desfuroylceftiofur metabolites. The approved dosage regime ns for ceftiofur sodium of 1.1-2.2 mg/kg administered once daily for u p to 5 consecutive days will provide plasma concentrations above the M IC for bovine respiratory disease pathogens for a longer period of tim e in neonatal calves than in older calves. Peak plasma concentrations of ceftiofur and desfuroylceftiofur metabolites were no higher in neon atal calves than in more mature cattle, highly suggestive that peak ti ssue concentrations would be no higher in neonatal calves than in more mature cattle.