Potential for drug interactions mediated by polymorphic flavin-containing monooxygenase 3 in human livers

TMAO as a potential biomarker and therapeutic target for chronic kidney disease: A review

The gut microbiota and its metabolites have become a hotspot of recent research. Trimethylamine N-oxide (TMAO) metabolized by the gut microbiota is closely related to many diseases such as cardiovascular disease, chronic kidney disease, type 2 diabetes, etc. Chronic kidney disease (CKD) is an important contributor to morbidity and mortality from non-communicable diseases. Recently, increasing focus has been put on the role of TMAO in the development and progress of chronic kidney disease. The level of TMAO in patients with chronic kidney disease is significantly increased, and a high level of TMAO deteriorates chronic kidney disease. This article describes the relationship between TMAO and chronic kidney disease and the research progress of drugs targeted TMAO, providing a reference for the development of anti-chronic kidney disease drugs targeted TMAO.

Roles of selected non-P450 human oxidoreductase enzymes in protective and toxic effects of chemicals: review and compilation of reactions

This is an overview of the metabolic reactions of drugs, natural products, physiological compounds, and other (general) chemicals catalyzed by flavin monooxygenase (FMO), monoamine oxidase (MAO), NAD(P)H quinone oxidoreductase (NQO), and molybdenum hydroxylase enzymes (aldehyde oxidase (AOX) and xanthine oxidoreductase (XOR)), including roles as substrates, inducers, and inhibitors of the enzymes. The metabolism and bioactivation of selected examples of each group (i.e., drugs, "general chemicals," natural products, and physiological compounds) are discussed. We identified a higher fraction of bioactivation reactions for FMO enzymes compared to other enzymes, predominately involving drugs and general chemicals. With MAO enzymes, physiological compounds predominate as substrates, and some products lead to unwanted side effects or illness. AOX and XOR enzymes are molybdenum hydroxylases that catalyze the oxidation of various heteroaromatic rings and aldehydes and the reduction of a number of different functional groups. While neither of these two enzymes contributes substantially to the metabolism of currently marketed drugs, AOX has become a frequently encountered route of metabolism among drug discovery programs in the past 10-15 years. XOR has even less of a role in the metabolism of clinical drugs and preclinical drug candidates than AOX, likely due to narrower substrate specificity.

Different substrate elimination rates of model drugs pH-dependently mediated by flavin-containing monooxygenases and cytochromes P450 in human liver microsomes

The predicted contributions of flavin-containing monooxygenase 3 (FMO3) to drug candidate N-oxygenations can be estimated using classic base dissociation constants of the N-containing moiety. In this study, metabolic clearance values in human liver microsomes were experimentally determined for available model drugs. Typical metabolic clearance values (34-96 μL/min/mg protein) at pH 8.4 of trimethylamine, benzydamine, and itopride were two-to fourfold higher than those at pH 7.4. In contrast, the metabolic clearance of control drug midazolam at pH 8.4 was half that at pH 7.4. The ratios of clearance values at pH 8.4 to those at pH 7.4 and the substrate pKa (base) values of reported metabolic N-oxygenation sites of trimethylamine, benzydamine, clomipramine, chlorpromazine, tamoxifen, itopride, loxapine, xanomeline, tozasertib, dasatinib, and clozapine were significantly correlated (r = 0.60, p < 0.05, n = 11). These results suggested that the simple comparison of metabolic clearance values at pH 8.4 and at pH 7.4 could be useful for predicting the contributions of FMO3 to the N-oxygenations of new drug candidates. This method, along with in silico pKa (base) values > 8.4, could prove useful for predicting the contributions of FMO3 to N-oxygenations as part of drug development.

In vivo drug interactions of itopride and trimethylamine mediated by flavin-containing monooxygenase 3 in humanized-liver mice

Flavin-containing monooxygenase (FMO) catalyzes the oxygenation of a wide variety of medicines and dietary-derived compounds. However, little information is available regarding drug interactions mediated by FMO3 in vivo. Consequently, we investigated interactions between FMO substrates in humanized-liver mice. Trimethylamine-d₉ and itopride were, respectively, intravenously and orally administered to humanized-liver mice (n = 5-7). The pharmacokinetic profiles of itopride (the victim drug) in the presence of trimethylamine (the perpetrator drug) were determined for 24 h after co-administration using liquid chromatography/tandem mass spectrometry. Itopride (10 mg/kg) was extensively oxygenated in humanized-liver mice to its N-oxide. The plasma concentrations of itopride N-oxide after co-administration of itopride and trimethylamine (10 and 100 mg/kg) were significantly suppressed in a dose-dependent manner, but only during the early phase, i.e., up to 2 h after co-administration. With the higher dose of trimethylamine, the areas under the concentration-time curves of itopride and its N-oxide significantly increased (1.6-fold) and decreased (to 60%), respectively; modeling suggested that these modified pharmacokinetics resulted from suppression of the in vivo hepatic intrinsic clearance (to 67%). These results suggest that food-derived trimethylamine may result in interactions with FMO drug substrates immediately after administration; however, the potential for this to occur in vivo may be limited.

Ligand stabilization and effect on unfolding by polymorphism in human flavin-containing monooxygenase 3

Pharmacogenomics is a powerful tool to prevent adverse reactions caused by different response of individuals to drug administration. Single nucleotide polymorphisms (SNPs) represent up to 90% of genetic variations among individuals. Drug metabolizing enzymes are highly polymorphic therefore the kinetic parameters of their catalytic reactions can be significantly influenced. This work reports on the unfolding process of a phase I drug metabolizing enzyme, human flavin-containing monooxygenase 3 (hFMO3) and its single nucleotide polymorphic variants (SNPs) V257M, E158K and E308G. Differential scanning calorimetry (DSC) indicates that the thermal denaturation of the enzyme is irreversible. The melting temperature (T_m) for the (Wild Type) WT and its polymorphic variants is found to be in a range from 46 °C to 50 °C. Also the activation energies of unfolding (E_a) show no significant differences among all proteins investigated (290-328 KJ/mol), except for the E308G variant that showed a significantly higher E_a of 412 KJ/mol. The presence of the bound NADP⁺ cofactor is found to stabilize all the variants by shifting the main T_m by 4-5 °C for all the proteins, exception made for E308G where no changes are observed. Isothermal titration calorimetry (ITC) was used to characterize the interaction of the protein with NADP⁺ in terms of dissociation constant (K_d), enthalpy (ΔH) and entropy (ΔS). K_d values of 1.6 and 0.7 μM, ΔH of -13.9 Kcal/mol and -16.8 Kcal/mol, ΔS of -20.5 cal/mol/deg, and -28.5 cal/mol/deg were found for V257M and E158K respectively. E308G was found to be unable to bind the NADP⁺ cofactor, a result that is in line with the T_m results. Circular dichroism also confirmed an overall lower stability of E308G, while NADP⁺ was found to give a strong positive shift of the T_m stabilizing the structure of E158K (46.2 to 50.6 °C). Previous data highlighted significant differences in terms of activity among the SNPs of hFMO3. In this work a minor impact of the SNPs was found on the stability of the enzyme in the ligand free form, except for E308G, whereas the binding of NADP⁺ reveals major differences among WT and polymorphic variants that are all measurable in terms of heat capacity, enthalpy and secondary structure content. These data provide the first direct evidence of ligand stabilization effects on hFMO3 that can explain the differences observed in catalytic efficiencies and serve as the starting point for the development of inhibitors of this enzyme.

Drug metabolite synthesis by immobilized human FMO3 and whole cell catalysts

Background

Sufficient reference standards of drug metabolites are required in the drug discovery and development process. However, such drug standards are often expensive or not commercially available. Chemical synthesis of drug metabolite is often difficulty due to the highly regio- and stereo-chemically demanding. The present work aims to construct stable and efficient biocatalysts for the generation of drug metabolites in vitro.

Result

In this work, using benzydamine as a model drug, two easy-to-perform approaches (whole cell catalysis and enzyme immobilization) were investigated for the synthesis of FMO3-generated drug metabolites. The whole cell catalysis was carried out by using cell suspensions of E. coli JM109 harboring FMO3 and E. coli BL21 harboring GDH (glucose dehydrogenase), giving 1.2 g/L benzydamine N-oxide within 9 h under the optimized conditions. While for another approach, two HisTrap HP columns respectively carrying His₆-GDH and His₆-FMO3 were connected in series used for the biocatalysis. In this case, 0.47 g/L benzydamine N-oxide was generated within 2.5 h under the optimized conditions. In addition, FMO3 immobilization at the C-terminal (membrane anchor region) significantly improved its enzymatic thermostability by more than 10 times. Moreover, the high efficiency of these two biocatalytic approaches was also confirmed by the N-oxidation of tamoxifen.

Conclusions

The results presented in this work provides new possibilities for the drug-metabolizing enzymes-mediated biocatalysis.

Electronic supplementary material

The online version of this article (10.1186/s12934-019-1189-7) contains supplementary material, which is available to authorized users.

Physiologically Based Pharmacokinetic Modeling Approach to Predict Drug-Drug Interactions With Ethionamide Involving Impact of Genetic Polymorphism on FMO3

The widely used second-line antituberculosis drug ethionamide shows wide interindividual variability in its disposition; however, the relevant factors affecting this phenomenon have not been characterized. We previously reported the major contribution of flavin-containing monooxygenase 3 (FMO3) in the reductive elimination pathway of ethionamide. In this study, ethionamide metabolism was potentially inhibited by methimazole in vitro. The drug-drug interaction leading to methimazole affecting the disposition of ethionamide mediated by FMO3 was then quantitated using a bottom-up approach with a physiologically based pharmacokinetic framework. The maximum concentration (C_max ) and area under the curve (AUC) of ethionamide were estimated to increase by 13% and 16%, respectively, when coadministered with methimazole. Subsequently, we explored the effect of FMO3 genetic polymorphism on metabolic capacity, by constructing 2 common functional variants, Lys¹⁵⁸ -FMO3 and Gly³⁰⁸ -FMO3. Compared to the wild type, recombinant Lys¹⁵⁸ -FMO3 and Gly³⁰⁸ -FMO3 variants significantly decreased the intrinsic clearance of ethionamide by 2% and 24%, respectively. Two prevalent functional variants of FMO3 were predicted to affect ethionamide disposition, with mean ratios of C_max and AUC of up to 1.5 and 1.7, respectively, in comparison with the wild type. In comparing single ethionamide administration with the wild type, simulations of the combined effects of comedications and FMO3 genetic polymorphism estimated that the C_max and AUC ratios of ethionamide increased up to 1.7 and 2.0, respectively. These findings suggested that FMO3-mediated drug-drug interaction and genetic polymorphism could be important determinants of interindividual heterogeneity in ethionamide disposition that need to be considered comprehensively to optimize the personalized dosing of ethionamide.

Mechanism of Rifampicin Inactivation in Nocardia farcinica

A novel mechanism of rifampicin (Rif) resistance has recently been reported in Nocardia farcinica. This new mechanism involves the activity of rifampicin monooxygenase (RifMO), a flavin-dependent monooxygenase that catalyzes the hydroxylation of Rif, which is the first step in the degradation pathway. Recombinant RifMO was overexpressed and purified for biochemical analysis. Kinetic characterization revealed that Rif binding is necessary for effective FAD reduction. RifMO exhibits only a 3-fold coenzyme preference for NADPH over NADH. RifMO catalyzes the incorporation of a single oxygen atom forming an unstable intermediate that eventually is converted to 2'-N-hydroxy-4-oxo-Rif. Stable C4a-hydroperoxyflavin was not detected by rapid kinetics methods, which is consistent with only 30% of the activated oxygen leading to product formation. These findings represent the first reported detailed biochemical characterization of a flavin-monooxygenase involved in antibiotic resistance.

Drug metabolism by flavin-containing monooxygenases of human and mouse

INTRODUCTION

Flavin-containing monooxygenases (FMOs) play an important role in drug metabolism. Areas covered: We focus on the role of FMOs in the metabolism of drugs in human and mouse. We describe FMO genes and proteins of human and mouse; the catalytic mechanism of FMOs and their significance for drug metabolism; differences between FMOs and CYPs; factors contributing to potential underestimation of the contribution of FMOs to drug metabolism; the developmental and tissue-specific expression of FMO genes and differences between human and mouse; and factors that induce or inhibit FMOs. We discuss the contribution of FMOs of human and mouse to the metabolism of drugs and how genetic variation of FMOs affects drug metabolism. Finally, we discuss the utility of animal models for FMO-mediated drug metabolism in humans. Expert opinion: The contribution of FMOs to drug metabolism may be underestimated. As FMOs are not readily induced or inhibited and their reactions are generally detoxifications, the design of drugs that are metabolized predominantly by FMOs offers clinical advantages. Fmo1^(-/-),Fmo2^(-/-),Fmo4^(-/-) mice provide a good animal model for FMO-mediated drug metabolism in humans. Identification of roles for FMO1 and FMO5 in endogenous metabolism has implications for drug therapy and initiates an exciting area of research.

Identification of structural determinants of NAD(P)H selectivity and lysine binding in lysine N(6)-monooxygenase

l-lysine (l-Lys) N(6)-monooxygenase (NbtG), from Nocardia farcinica, is a flavin-dependent enzyme that catalyzes the hydroxylation of l-Lys in the presence of oxygen and NAD(P)H in the biosynthetic pathway of the siderophore nocobactin. NbtG displays only a 3-fold preference for NADPH over NADH, different from well-characterized related enzymes, which are highly selective for NADPH. The structure of NbtG with bound NAD(P)(+) or l-Lys is currently not available. Herein, we present a mutagenesis study targeting M239, R301, and E216. These amino acids are conserved and located in either the NAD(P)H binding domain or the l-Lys binding pocket. M239R resulted in high production of hydrogen peroxide and little hydroxylation with no change in coenzyme selectivity. R301A caused a 300-fold decrease on kcat/Km value with NADPH but no change with NADH. E216Q increased the Km value for l-Lys by 30-fold with very little change on the kcat value or in the binding of NAD(P)H. These results suggest that R301 plays a major role in NADPH selectivity by interacting with the 2'-phosphate of the adenine-ribose moiety of NADPH, while E216 plays a role in l-Lys binding.

Cytochrome P450 and Non-Cytochrome P450 Oxidative Metabolism: Contributions to the Pharmacokinetics, Safety, and Efficacy of Xenobiotics

The drug-metabolizing enzymes that contribute to the metabolism or bioactivation of a drug play a crucial role in defining the absorption, distribution, metabolism, and excretion properties of that drug. Although the overall effect of the cytochrome P450 (P450) family of drug-metabolizing enzymes in this capacity cannot be understated, advancements in the field of non-P450-mediated metabolism have garnered increasing attention in recent years. This is perhaps a direct result of our ability to systematically avoid P450 liabilities by introducing chemical moieties that are not susceptible to P450 metabolism but, as a result, may introduce key pharmacophores for other drug-metabolizing enzymes. Furthermore, the effects of both P450 and non-P450 metabolism at a drug's site of therapeutic action have also been subject to increased scrutiny. To this end, this Special Section on Emerging Novel Enzyme Pathways in Drug Metabolism will highlight a number of advancements that have recently been reported. The included articles support the important role of non-P450 enzymes in the clearance pathways of U.S. Food and Drug Administration-approved drugs over the past 10 years. Specific examples will detail recent reports of aldehyde oxidase, flavin-containing monooxygenase, and other non-P450 pathways that contribute to the metabolic, pharmacokinetic, or pharmacodynamic properties of xenobiotic compounds. Collectively, this series of articles provides additional support for the role of non-P450-mediated metabolic pathways that contribute to the absorption, distribution, metabolism, and excretion properties of current xenobiotics.