Pharmacokinetics of sodium and trihydrate amoxicillin after intravenous and intramuscular administration in llamas (Lama glama)

The pharmacokinetics of amoxicillin following a single intravenous (i.v.) and intramuscular (i.m.) administration of a conventional sodium formulation and following a single i.m. administration of a trihydrate long acting suspension was investigated in adult llamas. On phase 1, in a cross-over design, six llamas received sodium amoxicillin (20 mg/kg) by the i.v. and i.m. routes. On phase 2, six llamas received i.m. trihydrate amoxicillin suspension (15 mg/kg). Amoxicillin plasma concentrations were determined using a microbiological assay. Significant differences were found for mean peak plasma concentration (40.4 ± 12.1 versus 3.04 ± 1.02 μg/ml), mean elimination half-life (0.86 ± 0.3 versus 9.96 ± 3.1 h) and mean residence time (1.37 ± 0.5 versus 15.1 ± 4.1 h) following sodium and trihydrate i.m. amoxicillin, respectively. Bioavailability was similar for both sodium (1.44 ± 0.2) and trihydrate (1.14 ± 0.4) formulations. The results of the pharmacokinetic/pharmacodynamic indices of time above the minimal inhibitory concentrations (T > MIC) suggest that 20 mg/kg sodium amoxicillin would produce a good therapeutic outcome in infections due to susceptible bacteria administered every 8-12 h, while for 15 mg/kg trihydrate amoxicillin, higher doses or more frequent intervals may be needed for treating low susceptibility bacteria in llamas.