Florfenicol in non-lactating dairy cows: pharmacokinetics, binding to plasma proteins, and effects on phagocytosis by blood neutrophils.

Serial blood samples were collected and plasma concentrations of florfenicol (FLO) were measured following the administration of an intravenous bolus of 50 mg/kg FLO to five healthy non-lactating dairy cows. A triexponential equation provided the best fit of the data for four of the five cows. The mean value for beta corresponded to a half-life of 3.2 h. The mean apparent volume of distribution was 0.67 l/kg, and the mean body clearance was 0.15 l/kg/h. The extent of binding of FLO to bovine plasma proteins was determined in vitro at concentrations of 5 micrograms/ml and 50 micrograms/ml by equilibrium dialysis and ultrafiltration. The drug was 18% and 19% bound by equilibrium dialysis, and 23% and 19% bound by ultrafiltration, at 5 micrograms/ml and 50 micrograms/ml, respectively. Phagocytosis of 32phosphorus-labelled Staphylococcus aureus by bovine blood neutrophils was compared in vitro between neutrophils incubated in phosphate-buffered saline alone or in combination with 5, 125, or 1000 micrograms/ml chloramphenicol or FLO. There was no significant effect of chloramphenicol at any concentration. Florfenicol significantly inhibited phagocytosis at all concentrations, but the percentage inhibition was small. The clinical significance, if any, of this effect of FLO remains to be demonstrated.