ABSTRACT:Magnolol is a food additive that is often found in mints and gums. Human exposure to this compound can reach a high dose; thus, characterization of magnolol disposition in humans is very important. Previous studies indicated that magnolol can undergo extensive glucuronidation in humans in vivo. In this study, in vitro assays were used to characterize the glucuronidation pathway in human liver and intestine. Assays with recombinant human UDP-glucuronosyltransferase enzymes (UGTs) revealed that multiple UGT isoforms were involved in magnolol glucuronidation, including UGT1A1, -1A3, -1A7, -1A8, -1A9, -1A10, and -2B7. Magnolol glucuronidation by human liver microsomes (HLM), human intestine microsomes (HIM), and most recombinant UGTs exhibited strong substrate inhibition kinetics. The degree of substrate inhibition was relatively low in the case of UGT1A10, whereas the reaction catalyzed by UGT1A9 followed biphasic kinetics. Chemical inhibition studies and the relative activity factor (RAF) approach were used to identify the individual UGTs that played important roles in magnolol glucuronidation in HLM and HIM. The results indicate that UGT2B7 is mainly responsible for the reaction in HLM, whereas UGT2B7 and UGT1A10 are significant contributors in HIM. In summary, the current study clarifies the glucuronidation pathway of magnolol and demonstrates that the RAF approach can be used as an efficient method for deciphering the roles of individual UGTs in a given glucuronidation pathway in the native tissue that is catalyzed by multiple isoforms with variable and atypical kinetics.

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Abstract1. Human exposure to magnolol can reach a high dose in daily life. Our previous studies indicated that magnolol showed high affinities to several UDP-glucuronosyltransferases (UGTs) This study was designed to examine the in vitro inhibitory effects of magnolol on UGTs, and further to evaluate the possibility of the in vivo inhibition that might happen. 2. Assays with recombinant UGTs and human liver microsomes (HLM) indicated that magnolol (10 µM) can selectively inhibit activities of UGT1A9 and extra-hepatic UGT1A7. Inhibition of magnolol on UGT1A7 followed competitive inhibition mechanism, while the inhibition on UGT1A9 obeyed either competitive or mixed inhibition mechanism, depending on substrates. The K i values for UGT1A7 and 1A9 are all in nanomolar ranges, lower than possible magnolol concentrations in human gut lumen and blood, indicating the in vivo inhibition on these two enzymes would likely occur. 3. In conclusion, UGT1A7 and 1A9 can be strongly inhibited by magnolol, raising the alarm for safe application of magnolol and traditional Chinese medicines containing magnolol. Additionally, given that UGT1A7 is an extrahepatic enzyme, magnolol can serve as a selective UGT1A9 inhibitor that will act as a new useful tool in future hepatic glucuronidation phenotyping.

Aim: To ascertain the effects of erlotinib on CYP3A, to investigate the amplitude and kinetics of erlotinib-mediated inhibition of seven major CYP isoforms in human liver microsomes (HLMs) for evaluating the magnitude of erlotinib in drug-drug interaction in vivo. Methods: The activities of 7 major CYP isoforms (CYP1A2, CYP2A6, CYP3A, CYP2C9, CYP2D6, CYP2C8, and CYP2E1) were assessed in HLMs using HPLC or UFLC analysis. A two-step incubation method was used to examine the time-dependent inhibition of erlotinib on CYP3A.Results: The activity of CYP2C8 was inhibited with an IC 50 value of 6.17±2.0 μmol/L. Erlotinib stimulated the midazolam 1′-hydroxy reaction, but inhibited the formation of 6β-hydroxytestosterone and oxidized nifedipine. Inhibition of CYP3A by erlotinib was substratedependent: the IC 50 values for inhibiting testosterone 6β-hydroxylation and nifedipine metabolism were 31.3±8.0 and 20.5±5.3 μmol/L, respectively. Erlotinib also exhibited the time-dependent inhibition on CYP3A, regardless of the probe substrate used: the value of K I and k inact were 6.3 μmol/L and 0.035 min -1 for midazolam; 9.0 μmol/L and 0.045 min -1 for testosterone; and 10.1 μmol/L and 0.058 min -1 for nifedipine. Conclusion: The inhibition of CYP3A by erlotinib was substrate-dependent, while its time-dependent inhibition on CYP3A was substrateindependent. The time-dependent inhibition of CYP3A may be a possible cause of drug-drug interaction, suggesting that attention should be paid to the evaluation of erlotinib's safety, especially in the context of combination therapy.