Open AccessInhibitory Effects of Silibinin on Cytochrome P‐450 Enzymes in Human Liver Microsomes
Author(s) -
BeckmannKnopp Svane,
Rietbrock Stephan,
Weyhenmeyer Roland,
Böcker Ronald H.,
Beckurts K. Tobias,
Lang Werner,
Hunz Miriam,
Fuhr Uwe
Publication year - 2000
Publication title -
pharmacology & toxicology
Language(s) - English
Resource type - Journals
eISSN - 1600-0773
pISSN - 0901-9928
DOI - 10.1111/j.0901-9928.2000.860602.x
Subject(s) - silibinin , chemistry , cyp2a6 , chlorzoxazone , dextromethorphan , cyp1a2 , non competitive inhibition , hydroxylation , microsome , cytochrome p450 , cyp3a4 , drug metabolism , biochemistry , metabolism , pharmacology , cyp2e1 , enzyme , biology
Silibinin, the main constituent of silymarin, a flavonoid drug from silybum marianum used in liver disease, was tested for inhibition of human cytochrome P‐450 enzymes. Metabolic activities were determined in liver microsomes from two donors using selective substrates. With each substrate, incubations were carried out with and without silibinin (concentrations 3.7–300 μM) at 37° in 0.1 M KH 2 PO 4 buffer containing up to 3% DMSO. Metabolite concentrations were determined by HPLC or direct spectroscopy. First, silibinin IC 50 values were determined for each substrate at respective K M concentrations. Silibinin had little effect (IC 50 >200 μM) on the metabolism of erythromycin (CYP3A4), chlorzoxazone (CYP2E1), S(+)‐mephenytoin (CYP2C19), caffeine (CYP1A2) or coumarin (CYP2A6). A moderate effect was observed for high affinity dextromethorphan metabolism (CYP2D6) in one of the microsomes samples tested only (IC 50 =173 μM). Clear inhibition was found for denitronifedipine oxidation (CYP3A4; IC 50 =29 μM and 46 μM) and S(−)‐warfarin 7‐hydroxylation (CYP2C9; IC 50 =43 μM and 45 μM). When additional substrate concentrations were tested to assess enzyme kinetics, silibinin was a potent competitive inhibitor of dextromethorphan metabolism at the low affinity site, which is not CYP2D6 (K i,c =2.3 μM and 2.4 μM). Inhibition was competitive for S(−)‐warfarin 7‐hydroxylation (K i,c =18 μM and 19 μM) and mainly non‐competitive for denitronifedipine oxidation (K i,n =9 μM and 12 μM). With therapeutic silibinin peak plasma concentrations of 0.6 μM and biliary concentrations up to 200 μM, metabolic interactions with xenobiotics metabolised by CYP3A4 or CYP2C9 cannot be excluded.