Determination of a dosage regimen of colistin by pharmacokinetic/pharmacodynamic integration and modeling for treatment of G.I.T. disease in pigs.

Colistin is an antimicrobial drug of the polymyxin group and COLIVET SOLUTION is an aqueous solution containing colistin sulphate (2 x 10(6) IU/mL), formulated for oral administration. The target species is the pig, particularly the suckling and post weaning animal. This investigation was undertaken to provide pharmacokinetic and pharmacodynamic data on which to base the selection of dosage rate and interval of the solution for the treatment of porcine colibacillosis. Colistin absorption from the gastrointestinal tract of young pigs, when administered at dosage rates of 25,000, 50,000 and 100,000 IU/kg, was slight or absent. The drug was therefore restricted almost entirely to the required site of action. The colistin concentration-time profile within the jejunum and ileum was established, and this enabled determination of the pharmacokinetic variables, maximum concentration (C(max)) and area under curve (AUC) and derivation of the surrogate indices of antibacterial activity, C(max)/minimum inhibitory concentration (MIC) and AUC/MIC through integration of in vivo data with the results of in vitro potency studies for four strains of Escherichia coli. In the in vitro bacterial growth inhibition studies colistin acted by a concentration-dependent killing mechanism. Numerical values for the surrogate parameter AUC/MIC producing bactericidal and eradication effects of colistin against four strains of E. coli were established by PK-PD modeling based on the sigmoidal E(max) equation. These data were used to predict a daily dosage regimen for colistin.