Time-dependent kinetics of lignocaine in the isolated perfused rat liver

The kinetics of lignocaine have been investigated in the isolated perfused rat liver preparation. After a low dose (0.3 mg) the drug was eliminated according to first-order kinetics, but after higher doses (7.5, 15.0 mg) and multiple doses (3 X 1.5 mg at 15 min intervals), nonlinear kinetics were observed, which appeared to show time dependence. This was not due to deterioration of the preparation nor was there any evidence of a hepatotoxic effect of lignocaine. The kinetics of lignocaine were also found to be sex-dependent since it was eliminated at a faster rate by livers from male rats compared to those from female rats. Exogenous MEGX (7.5 mg), the mono-N-deethylated metabolite of lignocaine, inhibited the elimination of parent drug (1.5 mg dose). However, evidence was obtained suggesting that a direct effect of this and other end-product metabolites may not be responsible for the observed changes in lignocaine kinetics with time when the compounds are produced endogenously. Studies of the hepatic binding of lignocaine in the preparation showed the presence of high affinity-low capacity and low affinity-high capacity binding sites, which may be the enzymes responsible for aromatic hydroxylation and N-deethylation of the drug, respectively. Further experiments supported the view that an intermediate product of lignocaine, related to the N-deethylation pathway, might be inhibiting its further metabolism.