Norfloxacin nicotinate pharmacokinetics in unweaned and weaned calves

The pharmacokinetics of norfloxacin nicotinate were investigated in unweaned and weaned calves. Following intravenous administration of 7.5 and 15 mg/kg (calculated as norfloxacin base) the clearance values were 8.5 +/- 2.0 or 7.7 +/- 1.2 mL/min.kg (unweaned calves) and 11.7 +/- 3.2 or 16.1 +/- 3.3 mL/min.kg (weaned calves). Norfloxacin mean residence time and volume of distribution values were 211 +/- 33 or 227 +/- 41 min (unweaned calves) and 185 +/- 79 or 128 +/- 18 minutes (weaned calves), and 1.8 +/- 0.3 or 1.7 +/- 0.1 L/kg (unweaned calves) and 2.0 +/- 0.7 or 2.1 +/- 0.7 L/kg (weaned calves) following administration of the lower and higher dose, respectively. These results indicated that norfloxacin pharmacokinetics were similar at a dose range of 7.5-15 mg/kg. However, a significant difference was observed in clearance, mean residence time and the half-life values between the unweaned and weaned calves. The only major pharmacokinetic parameter which did not show a significant difference between the investigated groups was the volume of distribution. The pharmacokinetic differences between the non-ruminating (unweaned) and ruminating (weaned) animals seemed to result from changes in drug clearance. The absorption rate after intramuscular administration appeared to change as a result of dose increase. Norfloxacin bioavailability following intramuscular administration ranged from 73 to 106%. The results suggested that larger injection volumes may reduce the extent of absorption.