PHARMACOKINETICS AND BIOEQUIVALENCE OF PARENTERALLY ADMINISTERED DORAMECTIN IN CATTLE

Plasma concentrations of doramectin in 40 cattle dosed by subcutaneous (sc) or intramuscular (i.m.) injection (200 mu g/kg) were compared to assess the bioequivalence of the two routes of administration. Peak concentration (C-max), and areas under the concentration curve (AUC(0-infinity)) were determined from plasma concentrations. Animals treated by the sc route showed a mean AUC(0-infinity) of 457 +/- 66 ng . day/mL (+/- SD) and a mean C-max of 27.8 +/- 7.9 ng/mL. Results from the i.m. treatment group showed a mean AUC(0-infinity) of 475 +/- 82 ng . day/mL and a mean C-max of 33.1 +/- 9.0 ng/mL. Absorption constants (k(a)) determined by modelling were 0.542 +/- 0.336 day(-1) after sc administration and 0.710 +/- 0.357 day(-1) after i.m. administration. The 90% confidence limits on the difference between mean AUC(0-infinity) values for the sc and i.m. groups fell within 20% of the mean value for the subcutaneous group. C-max was somewhat greater for the i.m. route. The 90% confidence limits on the difference in mean In(T-max + 1) also fell within 20% of the mean sc value. Based on this analysis, bioequivalence of the sc and i.m. formulation has been established.