ABSTRACT
Objective: To investigate the inhibition of tetrahydropalmatine (THP) enantiomers on main cytochrome P450 (CYP450) in human liver microsomes and the mechanism. Methods: The effects of (-)-THP and (+)-THP on the activies of main phase I metabolic enzymes in human liver microsomes, CYP1A2, CYP2C9, CYP2C19, CYP3A1, CYP2E1, and CYP2D6 were investigated using Cocktail probe drugs method; The effect of preincubation with (-)-THP on the CYP2D6 substrate dextromethorphan demethylation activity in human liver microsomes were investigated using a two-step incubation scheme and study its inhibitory mechanism was studied; The enzyme inhibitory kinetic parameters of CYP2D6 by (-)-THP were investigated using time-dependent incubation with human liver microsomes. Results: The inhibitory effect of (+)-THP on CYP450 subtype was not significant, while.CYP2D6 activity was significantly inhibited by (-)-THP (IC50 = 0.46 μmol/L). The value of IC50 preincubation with or without NADPH were 2.40 and 0.46 μmol/L, respectively, IC50 (-) NADPH/IC50 (+) NADPH = 5.22. And the enzyme inhibitory kinetic parameters of Ki and Kinact were 0.690 μmol/L and 0.0846 min-1, respectively. Conclusion: (-)-THP has a strong inhibition on CYP2D6, and the type of inhibition is mechanism-based inhibiton (MBI), which should indicate a potential metabolic drug-drug interaction mediated by (-)-THP in clinical application.
ABSTRACT
Objective To investigate the effects ofamiodarone (AMD) on simvastatin (SV) in human liver microsomes and the possible underlying mechanisms. Methods Time-, NADPH- and concentration-dependent inhibitions were tested in HLM. The logarithm of relative inhibition values was plotted versus preincubation time (0, 5, 10, 15, 20min) for a series concentration of AMD used (0, 2, 5,25, 50 μ mol/L), and the slopes determined by linear regression. These slope values represente the observed inactivation rate constants (kobs). A double-reciprocal plot was then constructed using the reciprocal of the ko～ (y-axis) and the reciprocal of the associated inhibitor concentration (x-axis) to estimate the values ofkinact and K, which were two principal kinetic constants that were specific for mechanism-based inhibition (MBI).drug-drug interactions (DDI) potential was predicted based on in vitro data and by using the in vitro-in vivo extrapolation. Results The time-, concentration- and NADPH-dependent charactga'istics confirmed that when SV was the substrate of CYP3A4, the inhibition of AMD to CYP3A4 is MBI. Kj and kinact value were calculated to be 5.1 μ mol/L and 0.018min-1 The Clint of SV was reduced 2.96-5.63 fold when it was administrated with AMD. Conclusion Based on the results, AMD would inhibit SV metabolism via the mechanism-based manner, which would lead to DDI when they are taken together. Careful clinical observation is recommended when AMD and SV have to be simultaneously prescribed.