Pharmacokinetic interactions between Flunixin and Sulphadimidine in horses

The pharmacokinetic aspects of sulphadimidine were studied in clinically he althy (control) and Flunixin-medicated horses after a single intravenous an d oral administration of 100 mg/kg body weight. Plasma sulphadimidine conce ntration were determined by high-performance liquid chromatography (HPLC). Following the intravenous injection, all plasma sulphadimidine data were be st approximated by a two-compartment open model using sequential, weight no n-linear regression. Flunixin induced a 67 % increase in the rate of sulpha dimidine return to the central compartment from peripheral tissues (K-21) a nd there were a trend to a 30 % increase in K-12. The sulphadimidine elimin ation half-life was decreased 21 %, the Vdss was reduced by 18 % and MRT wa s decreased by 20 %. Following the oral administration, sulphadimidine was rapidly absorbed in control and Flunixin-medicated horses with absorption h alf-lives (t(1/2 ab)) of 0.5 and 0.43 hours respectively. The peak plasma concentration (C-max) were 93.7 and 109 mu g/ml attained at (t(max)) 2.36 and 1.9 hours respectively. The elimination half-life after oral administration (t(1/2 ab)) was shorter in flunixin pre-medicated horse s than in control ones. The systemic bioavalability percentages (F %) of sulphadimidine after oral administration of 100 mg/kg body weight was 79.3 and 71.2 % in control and flunixin medicated horses, respectively. Therefore care should;be exercised in the use of sulphadimidine in equine patients concurrently treated with flunixin.