Pharmacokinetics, intramuscular bio-availability, and bioequivalence of amoxycillin in donkeys

The disposition kinetics of intravenously (i.v.) administered amoxycillin sodium (10 mg/kg) was determined in four adult donkeys. The elimination half-life (t1/2 beta) of 47.3 +/- 5.1 min, the apparent volume of distribution (Varea) of 0.325 +/- 0.093 l/kg and the total body clearance (ClB) of 4.76 +/- 1.33 ml/min.kg were very close to the corresponding values reported for the horse injected i.v. with amoxycillin sodium. It was evident that because of the rapid rate of drug elimination, limited distribution volume, and high clearance values, the i.v. injected amoxycillin sodium (10-20 mg/kg) was likely to result in potentially therapeutic tissue concentrations for a short time (2-3 h) only. Three formulations of 15% amoxycillin trihydrate suspension in oil were injected intramuscularly (i.m.) to each donkey at 10 mg/kg. Drug absorption was rapid and the elimination half-life (t1/2el) ranged between 6 and 10 h. Mean peak serum amoxycillin concentrations (Cmax) were, however, rather low (0.81-1.68 micrograms/ml) and the i.m. bioavailability was 25.8-45.1% for the different formulations. Although the three formulations were considered to be intramuscularly bio-equivalent and appeared to be well tolerated, it was estimated that multiple SID (once daily) treatments at 10-20 mg/kg could result in tissue concentrations of potential therapeutic value for the treatment of infections caused by very susceptible pathogens only.