A semiphysiological population pharmacokinetic model of agomelatine and its metabolites in Chinese healthy volunteers

AIMS: Agomelatine is an antidepressant for major depressive disorders. It undergoes extensive first-pass hepatic metabolism, and displays irregular absorption profiles and large interindividual variability (IIV) and interoccasion variability (IOV) of pharmacokinetics. The objective of this study was to characterize the complex pharmacokinetics of agomelatine and its metabolites in healthy subjects. METHODS: Plasma concentration-time data of agomelatine and its metabolites were collected from a four period, cross-over bioequivalence study, in which 44 healthy subjects received 25 mg agomelatine tablets orally. Non-linear mixed effects modelling was used to characterize the pharmacokinetics and variability of agomelatine and its metabolites. Deterministic simulations were carried out to investigate the influence of pathological changes due to liver disease on agomelatine pharmacokinetics. RESULTS: A semiphysiological pharmacokinetic model with parallel first-order absorption and a well-stirred liver compartment adequately described the data. The estimated IIV and IOV of the intrinsic clearance of agomelatine were 130.8% and 28.5%, respectively. The IIV of the intrinsic clearance turned out to be the main cause of the variability of AUC-based agomelatine exposure. Simulations demonstrated that a reduction in intrinsic clearance or liver blood flow, and an increase in free drug fraction had a rather modest influence on agomelatine exposures (range: -50 % to 200 %). Portosystemic shunting, however, substantially elevated agomelatine exposure by 12.6 - 109.1-fold. CONCLUSIONS: A semiphysiological pharmacokinetic model incorporating first-pass hepatic extraction was developed for agomelatine and its main metabolites. The portosystemic shunting associated with liver disease might lead to significant alterations of agomelatine pharmacokinetics, and lead to substantially increased exposure