Pharmacokinetic and pharmacodynamic modelling of metoprolol in rabbits with liver failure

The pharmacokinetic and pharmacodynamic profiles of metoprolol were studied in adult male rabbits given 3.2 mg/kg i.v. before and during liver failure. The partition of metoprolol between blood cells and plasma averaged 1.14 in both conditions. Plasma protein binding, concentration-independent, was 32% and 17% in normal and pathological status, respectively. With normal liver function the terminal elimination half-life for the drug was 0.54-0.96 h, rising to 1.0-2.1 h in liver failure. Differences of the same order were observed for total plasma drug clearance (average 3.7 vs 1.5 1/h/kg), MRT (0.77 vs 1.92 h), AUC (0.9 vs 2.2 mg h/l) and k10 (3.17 vs 1.80 h-1). Liver impairment did not affect the volume of distribution of the central compartment, the steady-state volume of distribution and the other intercompartmental rate constants. Although metoprolol was eliminated in the urine, the amount excreted was low (1.5% of the administered dose) in both conditions. The pharmacokinetic model was extended by an 'effect compartment', which has no influence on the predetermined mass of drug in the body, to analyse the relationship between heart rate fall and changes in metoprolol plasma concentrations. After drug administration, heart rate fell rapidly about 90 beats in both states. The mean unbound plasma concentration producing 50% of this reduction was double during liver failure compared to normal condition (0.03 vs 0.07 mg/l), but the temporal aspects of drug equilibration with site of action were similar.