Mechanism-based inactivation of cytochrome P450 enzymes by natural products based on metabolic activation

Cytochrome P450 enzymes (P450 enzymes) are the most common and important phase I metabolic enzymes and are responsible for the majority of the metabolism of clinical drugs and other xenobiotics. Drug-drug interactions (DDIs) can occur when the activities of P450 enzymes are inhibited. In particular, irreversible inhibition of P450 enzymes may lead to severe adverse interactions, compared to reversible inhibition. Many natural products have been shown to be irreversible inhibitors of P450 enzymes. The risks for intake of naturally occurring irreversible P450 enzyme inhibitors have been rising due to the rapid growth of the global consumption of natural products. Irreversible inhibition is usually called mechanism-based inactivation, which is time-, concentration- and NADPH- dependent. Generally, the formation of electrophilic intermediates is fundamental for the inactivation of P450 enzymes. This review comprehensively classifies natural P450 enzyme inactivators, including terpenoids, phenylpropanoids, flavonoids, alkaloids, and quinones obtained from herbs or foods. Moreover, the structure - activity correlations according to the IC₅₀ (or K_i) values reported in the literature as well as the underlying mechanisms based on metabolic activation are highlighted in depth.

Numerical analysis of time-dependent inhibition kinetics: comparison between rat liver microsomes and rat hepatocyte data for mechanistic model fitting

Time-dependent inhibition (TDI) may confound drug interaction predictions. Recently, models were generated for an array of TDI kinetic schemes using numerical analysis of microsomal assays. Additionally, a distinct terminal inactivation step was identified for certain mechanism based inhibitors (MBI) following reversible metabolite intermediate complex (MIC) formation. Longer hepatocyte incubations potentially allow analysis of slow TDI and terminal inactivation. In the experiments presented here, we compared the quality of TDI parameterization by numerical analysis between hepatocyte and microsomal data. Rat liver microsomes (RLM), suspended rat hepatocytes (SRH) and sandwich-cultured rat hepatocytes (SCRH) were incubated with the prototypical CYP3A MBI troleandomycin and the substrate midazolam. Data from RLM provided a better model fit as compared to SRH. Increased CYP3A expression after dexamethasone (DEX) induction improved the fit for RLM and SRH. A novel sequential kinetic scheme, defining inhibitor metabolite production prior to MIC formation, improved the fit compared to direct MIC formation. Furthermore, terminal inactivation rate constants were parameterized for RLM and SRH samples with DEX-induced CYP3A. The low expression of CYP3A and experimental error in SCRH resulted in poor data for model fitting. Overall, RLM generated data better suited for elucidation of TDI mechanisms by numerical analysis.

Piperine Is a Mechanism-Based Inactivator of CYP3A

Piperine (PPR) is the representative alkaloid component of the piper species (family: Piperaceae). Our rapid screening study found PPR caused time-dependent inhibition of cytochrome P450s (CYP) 3A and 2D6, and CYP3A was inactivated the most. Further study demonstrated that PPR is a time-, concentration-, and NADPH-dependent inhibitor of CYP3A, and significant loss (49.5% ± 3.9%) of CYP3A activity was observed after 20minute incubations with 80 μM PPR at 37°C. The values of K _I and k _inact were 30.7 μM and 0.041 minutes^-1, respectively. CYP3A competitive inhibitor ketoconazole showed protective effect against the enzyme inactivation. Superoxide dismutase/catalase and GSH displayed minor protection against the PPR-caused enzyme inactivation. Ferricyanide partially reduced the enzyme inhibition by PPR. Additionally, NADPH-dependent formation of reactive metabolites from PPR were found in human liver microsomal incubation mixtures. An ortho-quinone intermediate was trapped by NAC in microsomal incubations with PPR. DM-PPR, demethylene metabolite of PPR, showed weak enzyme inactivation relative to that caused by PPR. It appears that both carbene and ortho-quinone intermediates were involved in the inactivation of CYP3A caused by PPR. SIGNIFICANCE STATEMENT: CYP3A subfamily members (mainly CYP3A4 and CYP3A5) play a critical role in drug metabolism. Piperine (PPR), a methylenedioxyphenyl derivative combined with an unsaturated ketone, is the major active ingredient of pepper. PPR revealed time-, concentration-, and NADPH-dependent inhibitory effect on CYP3A. Carbene and quinone metabolites were both involved in the observed CYP3A inactivation by PPR. Apparently, the unsaturated ketone moiety did not participate in the enzyme inactivation. The present study sounds an alert of potential risk for food-drug interactions.

Mechanism-based inactivation of cytochrome P450 2D6 by chelidonine

Chelidonine (CHE) is a major bioactive constituent of greater celandine, a plant used in traditional herbal medicines. CHE has widely been used as an analgesic in clinical settings. We evaluated the inhibitory effects of CHE on human cytochrome P450 enzymes. CHE produced time-, concentration-, and NADPH-dependent inhibition of CYP2D6, with K _I and k _inact values of 20.49 μM and 11.05 min ^-1 , respectively. Approximately 76% of CYP2D6 activity was suppressed after 9 minute incubation with CHE (50 μM). The loss of enzyme activity was not restored following dialysis. The estimated partition ratio of the inactivation was about 156. Quinidine, a competitive inhibitor of CYP2D6, attenuated the CHE-mediated enzyme inactivation, while glutathione and catalase/superoxide dismutase did not markedly ameliorate the inhibitory effect. Upon oxidation using potassium ferricyanide, the 15.1% activity of CYP2D6 was restored. These findings indicate that CHE acted as a mechanism-based inactivator of CYP2D6 and the observed effects may induce potential drug-drug interactions.

Nitidine Chloride Is a Mechanism-Based Inactivator of CYP2D6

Nitidine chloride (NC) is a benzophenanthridine alkaloid isolated from the roots of Zanthoxylum nitidum (Roxb.) DC, a widely used traditional herbal medicine. Several reports have revealed NC's multiple pharmacologic properties. The inhibitory effects of NC on human cytochrome P450 enzymes were investigated in the present study. We found that NC caused time- and concentration-dependent inhibition of CYP2D6, and more than 50% of CYP2D6 activity was suppressed after a 15-minute incubation with NC at 100 μM in the primary incubation mixtures, with K_I of 4.36 μM, k_inact of 0.052 minute^-1, and a partition ratio of approximately 290. Moreover, the loss of CYP2D6 activity required the presence of NADPH. Superoxide dismutase/catalase and glutathione showed minor protection against the NC-induced enzyme inhibition. Quinidine as a competitive inhibitor of CYP2D6 slowed down the inactivation by NC. Trapping experiments using N-acetylcysteine demonstrated that quinone and/or carbene intermediate(s) were/was generated in human liver microsomal incubations with NC. In addition, potassium ferricyanide prevented the enzyme from the inactivation mediated by NC, which provided evidence that inhibition of CYP2D6 resulted from heme destruction by the formation of a carbene-iron complex. CYP1A2 was found to be the major enzyme involved in the generation of NC quinone metabolites. In conclusion, NC is a mechanism-based inactivator of CYP2D6. The generation of a carbene intermediate might be mainly responsible for the enzyme inactivation.

Time-Dependent Inhibition of CYP2C19 by Isoquinoline Alkaloids: In Vitro and In Silico Analysis

The cytochrome P450 2C19 (CYP2C19) enzyme plays an important role in the metabolism of many commonly used drugs. Relatively little is known about CYP2C19 inhibitors, including compounds of natural origin, which could inhibit CYP2C19, potentially causing clinically relevant metabolism-based drug interactions. We evaluated a series (N = 49) of structurally related plant isoquinoline alkaloids for their abilities to interact with CYP2C19 enzyme using in vitro and in silico methods. We examined several common active alkaloids found in herbal products such as apomorphine, berberine, noscapine, and papaverine, as well as the previously identified mechanism-based inactivators bulbocapnine, canadine, and protopine. The IC50 values of the alkaloids ranged from 0.11 to 210 µM, and 42 of the alkaloids were confirmed to be time-dependent inhibitors of CYP2C19. Molecular docking and three-dimensional quantitative structure-activity relationship analysis revealed key interactions of the potent inhibitors with the enzyme active site. We constructed a comparative molecular field analysis model that was able to predict the inhibitory potency of a series of independent test molecules. This study revealed that many of these isoquinoline alkaloids do have the potential to cause clinically relevant drug interactions. These results highlight the need for studying more profoundly the potential interactions between drugs and herbal products. When further refined, in silico methods can be useful in the high-throughput prediction of P450 inhibitory potential of pharmaceutical compounds.

Influence of sesamin on CYP2C-mediated diclofenac metabolism: in vitro and in vivo analysis

Our previous studies revealed that sesamin caused a mechanism-based inhibition (MBI) of CYP2C9 in human liver microsomes. Additionally, we observed a similar MBI of CYP2C by sesamin in the rat liver microsomes. Sesamin-induced difference spectra of rat or human liver microsomes in the presence of NADPH showed a peak at 459 nm, suggesting the formation of a metabolic–intermediate (MI) complex of cytochrome P450 and the methylenedioxyphenyl group of sesamin. However, the peak disappeared in both liver microsomes within 30 min after the termination of the metabolism. These results suggest that the MI complex of cytochrome P450 and sesamin is unstable, and the effects of sesamin on human CYP2C9- or rat CYP2C-mediated drug metabolism may be small. To confirm this, in vivo studies using rats were performed. The pharmacokinetics of diclofenac, which is mainly metabolized by CYP2C11 in male rats, were investigated after a 3-days administration of sesamin (0, 10, and 100 mg/kg bw). No significant differences were observed among the three groups in the pharmacokinetic parameters, C_max, T_max, and AUC. Furthermore, administration of sesamin to rats for 7 days had no significant effects on diclofenac hydroxylation activity in rat liver microsomes. These results demonstrate that no significant interaction occurs between diclofenac and sesamin in rats. Moreover, the results of these in vitro and in vivo studies suggest that no significant interaction may occur between sesamin and diclofenac when sesamin is administered to humans as a supplement, since the standard sesamin dose in humans is much lower than that administered to rats in this study.

Combining metabolomic analysis and microarray gene expression analysis in the characterization of the medicinal plant Chelidonium majus L

BACKGROUND AND OBJECTIVE

Even though herbal medicines have played an important role in disease management and health for many centuries, their present frequent use is challenged by the necessity to determine their complex composition and their multitarget mode of action. In the present study, modern methods were investigated towards their potential in the characterization of herbal substances. As a model the herbal substance Chelidonii herba was used, for which several reports on liver toxicities exist. Extracts of Chelidonii herba with different solvents were characterized phytochemically and functionally by experiments with HepG2 liver cells.

METHODS

Chelidonii herba was extracted with four solvents of different polarity (dichloromethane, water, ethanol, and ethanol 50% (V/V); four replicates each). The different extracts were characterized metabolomically by (1)H-NMR fingerprinting analysis and principal component analysis (PCA). The content of alkaloids was additionally determined by RP-HPLC. Functional characterization was achieved by the determination of cell proliferation and by transcriptomics techniques (Whole Genome Gene Expression Microarrays v2, Agilent Technologies) in HepG2 cells after exposure to the different extracts (four experimental replicates each).

RESULTS

Based on data from (1)H-NMR fingerprints and RP-HPLC analyses the different extracts showed a divergent composition of constituents depending on the solvent used. HepG2 liver cells responded differentially to the four extracts. Microarray analysis revealed a significant regulation of genes and signal cascades related to biotransformation. Also liver-toxic signal cascades were activated. Neither the activated genes nor the proliferation response could be clearly related to the differing alkaloid content of the extracts.

CONCLUSION

Different manufacturing processes lead to different herbal preparations. A systems biology approach combining a metabolomic plant analysis with a functional characterization by gene expression profiling in HepG2 cells is an appropriate strategy to characterize variations in plant extracts. Safety assessments of herbal substances may benefit from such complementary analyses.

Irreversible enzyme inhibition kinetics and drug-drug interactions

This chapter describes the types of irreversible inhibition of drug-metabolizing enzymes and the methods commonly employed to quantify the irreversible inhibition and subsequently predict the extent and time course of clinically important drug-drug interactions.

Progress curve mechanistic modeling approach for assessing time-dependent inhibition of CYP3A4

A progress curve method for assessing time-dependent inhibition of CYP3A4 is based on simultaneous quantification of probe substrate metabolite and inhibitor concentrations during the experiment. Therefore, it may overcome some of the issues associated with the traditional two-step method and estimation of inactivation rate (k(inact)) and irreversible inhibition (K(I)) constants. In the current study, seven time-dependent inhibitors were investigated using a progress curve method and recombinant CYP3A4. A novel mechanistic modeling approach was applied to determine inhibition parameters using both inhibitor and probe metabolite data. Progress curves generated for clarithromycin, erythromycin, diltiazem, and N-desmethyldiltiazem were described well by the mechanistic mechanism-based inhibition (MBI) model. In contrast, mibefradil, ritonavir, and verapamil required extension of the model and inclusion of competitive inhibition term for the metabolite. In addition, this analysis indicated that verapamil itself causes minimal MBI, and the formation of inhibitory metabolites was responsible for the irreversible loss of CYP3A4 activity. The k(inact) and K(I) estimates determined in the current study were compared with literature data generated using the conventional two-step method. In the current study, the inactivation efficiency (k(inact)/K(I)) for clarithromycin, ritonavir, and erythromycin were up to 7-fold higher, whereas k(inact)/K(I) for mibefradil, N-desmethyldiltiazem, and diltiazem were, on average, 2- to 4.8-fold lower than previously reported estimates. Use of human liver microsomes instead of recombinant CYP3A4 resulted in 5-fold lower k(inact)/K(I) for erythromycin. In conclusion, the progress curve method has shown a greater mechanistic insight when determining kinetic parameters for MBI in addition to providing a more comprehensive experimental protocol.