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.