Amentoflavone is a potent broad-spectrum inhibitor of human UDP-glucuronosyltransferases

A high-affinity fluorescent probe for human uridine-disphosphate glucuronosyltransferase 1A9 function monitoring under environmental pollutant exposure

Uridine-disphosphate glucuronosyltransferase 1A9 (UGT1A9), an important detoxification and inactivation enzyme for toxicants, regulates the exposure level of environmental pollutants in the human body and induces various toxicological consequences. However, an effective tool for high-throughput monitoring of UGT1A9 function under exposure to environmental pollutants is still lacking. In this study, 1,3-dichloro-7-hydroxy-9,9-dimethylacridin-2(9H)-one (DDAO) was found to exhibit excellent specificity and high affinity towards human UGT1A9. Remarkable changes in absorption and fluorescence signals after reacting with UGT1A9 were observed, due to the intramolecular charge transfer (ICT) mechanism. Importantly, DDAO was successfully applied to monitor the biological functions of UGT1A9 in response to environmental pollutant exposure not only in microsome samples, but also in living cells by using a high-throughput screening method. Meanwhile, the identified pollutants that disturb UGT1A9 functions were found to significantly influence the exposure level and retention time of bisphenol S/bisphenol A in living cells. Furthermore, the molecular mechanism underlying the inhibition of UGT1A9 by these pollutant-derived disruptors was elucidated by molecular docking and molecular dynamics simulations. Collectively, a fluorescent probe to characterize the responses of UGT1A9 towards environmental pollutants was developed, which was beneficial for elucidating the health hazards of environmental pollutants from a new perspective.

Characterization of the glucuronidating pathway of pectolinarigenin, the major active constituent of the Chinese medicine Daji, in humans and its influence on biological activities

ETHNOPHARMACOLOGICAL RELEVANCE

The Chinese medicine Daji (the aerial part of Cirsium japonicum DC.) and its charred product (Cirsii Japonici Herba Carbonisata) have been widely used as hemostatic agents or diuretic agents to prepare a variety of Chinese herbal formula. Pectolinarigenin (PEC), one of the most abundant constituents in both Daji and its charred product, has been considered as the key effective substance responsible for the major pharmacological activities of Daji, including hemostasis, hepatoprotective, anti-tumor and anti-osteoporosis effects. However, the major metabolic pathways of PEC in humans and the influence of PEC metabolism on its biological activities are poorly understood.

AIM OF THE STUDY

To characterize the main metabolic pathway(s) and key enzymes of PEC in human biological systems, as well as to reveal the influence of PEC metabolism on its biological activities.

MATERIALS AND METHODS

The metabolic stability assays of PEC were investigated in human liver microsomes (HLM). The O-glucuronide of PEC was biosynthesized and characterized by nuclear magnetic resonance (NMR) spectroscopy. The key enzymes responsible for O-glucuronidation of PEC in humans were assigned by performing UGT reaction phenotyping, chemical inhibition and enzymatic kinetic assays. The agonist effects of PEC and its O-glucuronide on nuclear factor erythroid2-related factor 2 (Nrf2), Peroxisome proliferator activated receptors (PPARα and PPARβ) were tested at the cellular level.

RESULTS

PEC could be readily metabolized to form a mono-O-glucuronide in both human liver microsome (HLM) and human intestinal microsome (HIM). The mono-O-glucuronide was bio-synthesized by mouse liver S9 and its structure was fully characterized as PEC-7-O-β-D-glucuronide (PEC-O-7-G). UGT1A1, UGT1A3 and UGT1A9 are key enzymes responsible for PEC-7-O-glucuronidation in HLM, while UGT1A1, UGT1A9 and 1A10 may play key roles in this reaction in HIM. Biological tests revealed that PEC displayed strong agonist effects on Nrf2, PPARα and PPARβ, whereas PEC-7-O-glucuronide showed relatively weak Nrf2 agonist effect and very weak PPAR agonist effects, indicating that PEC-7-O-glucuronidation strongly weaken its agonist effects on Nrf2 and PPAR.

CONCLUSIONS

Our results demonstrate that 7-O-glucuronidation is the major metabolic pathway of PEC in human tissues, while UGT1A1, 1A3 and 1A9 are key contributing enzymes responsible for PEC-7-O-glucuronidation in human liver. It is also found that PEC 7-O-glucuronidation significantly weakens the Nrf2 and PPAR agonist effects. All these findings are very helpful for the pharmacologists to deep understand the metabolic rates of PEC in humans.

Garcinia kola: a critical review on chemistry and pharmacology of an important West African medicinal plant

Garcinia kola Heckel (Clusiaceae) is a tree indigenous to West and Central Africa. All plant parts, but especially the seeds, are of value in local folklore medicine. Garcinia kola is used in treatment of numerous diseases, including gastric disorders, bronchial diseases, fever, malaria and is used to induce a stimulating and aphrodisiac effect. The plant is now attracting considerable interest as a possible source of pharmaceutically important drugs. Several different classes of compounds such as biflavonoids, benzophenones, benzofurans, benzopyran, vitamin E derivatives, xanthones, and phytosterols, have been isolated from G. kola, of which many appears to be found only in this species, such as garcinianin (found in seeds and roots), kolanone (fruit pulp, seeds, roots), gakolanone (stem bark), garcinoic acid, garcinal (both in seeds), garcifuran A and B, and garcipyran (all in roots). They showed a wide range of pharmacological activities (e.g. analgesic, anticancer, antidiabetic, anti-inflammatory, antimalarial, antimicrobial, hepatoprotective and neuroprotective effects), though this has only been confirmed in animal models. Kolaviron is the most studied compound and is perceived by many studies as the active principle of G. kola. However, its research is associated with significant flaws (e.g. too high doses tested, inappropriate positive control). Garcinol has been tested under better conditions and is perhaps showing more promising results and should attract deeper research interest (especially in the area of anticancer, antimicrobial, and neuroprotective activity). Human clinical trials and mechanism-of-action studies must be carried out to verify whether any of the compounds present in G. kola may be used as a lead in the drug development.

Flavonoids and saponins: What have we got or missed?

BACKGROUND

Flavonoids and saponins are important bioactive compounds that have attracted wide research interests. This review aims to summarise the state of the art of the pharmacology, toxicology and clinical efficacy of these compounds.

METHODS

Data were retrieved from PubMed, Cochrane Library, Web of Science, Proquest, CNKI, Chongqing VIP, Wanfang, NPASS and HIT 2.0 databases. Meta-analysis and systematic reviews were evaluated following the PRISMA guideline. Statistical analyses were conducted using SPSS23.0.

RESULTS

Rising research trends on flavonoids and saponins were observed since the 1990s and the 2000s, respectively. Studies on pharmacological targets and activities of flavonoids and saponins represent an important area of research advances over the past decade, and these important resources have been documented in open-access specialised databases and can be retrieved with ease. The rising research on flavonoids and saponins can be attributed, at least in part, to their links with some highly investigated fields of research, e.g., oxidative stress, inflammation and cancer; i.e., 6.88% and 3.03% of publications on oxidative stress cited by PubMed in 1990 - 2021 involved flavonoids and saponins, respectively, significantly higher than the percentage involving alkaloids (1.88%). The effects of flavonoids concern chronic venous insufficiency, cervical lesions, diabetes, rhinitis, dermatopathy, prostatitis, menopausal symptoms, angina pectoris, male pattern hair loss, lymphocytic leukaemia, gastrointestinal diseases and traumatic cerebral infarction, etc, while those of saponins may have impact on venous oedema in chronic deep vein incompetence, erectile dysfunction, acute impact injuries and systemic lupus erythematosus, etc. The volume of in vitro research appears way higher than in vivo and clinical studies, with only 10 meta-analyses and systematic reviews (involving 290 interventional and observational studies), and 36 clinical studies on flavonoids and saponins. Data are sorely needed on pharmacokinetics, in vitro pan-assay interferences, purity of tested compounds, interactions in complex herbal extracts, real impact of anti-oxidative strategies, and mid- and long-term toxicities. To fill these important gaps, further investigations are warranted. On the other hand, drug interactions may cause adverse effects but might also be useful for synergism, with the goals of enhancing effects or of detoxifying. Furthermore, the interactions between phytochemicals and the intestinal microbiota are worth investigating as the field may present a promising potential for novel drug development.

Inhibitory effect of Selaginella doederleinii hieron on human cytochrome P450

Introduction: Selaginella doederleinii Hieron is a traditional Chinese herbal medicine, the ethyl acetate extract from Selaginella doederleinii (SDEA) showed favorable anticancer potentials. However, the effect of SDEA on human cytochrome P450 enzymes (CYP450) remains unclear. To predict the herb-drug interaction (HDI) and lay the groundwork for further clinical trials, the inhibitory effect of SDEA and its four constituents (Amentoflavone, Palmatine, Apigenin, Delicaflavone) on seven CYP450 isoforms were investigated by using the established CYP450 cocktail assay based on LC-MS/MS. Methods: Appropriate substrates for seven tested CYP450 isoforms were selected to establish a reliable cocktail CYP450 assay based on LC-MS/MS. The contents of four constituents (Amentoflavone, Palmatine, Apigenin, Delicaflavone) in SDEA were determined as well. Then, the validated CYP450 cocktail assay was applied to test the inhibitory potential of SDEA and four constituents on CYP450 isoforms. Results: SDEA showed strong inhibitory effect on CYP2C9 and CYP2C8 (IC₅₀ ≈ 1 μg/ml), moderate inhibitory effect against CYP2C19, CYP2E1 and CYP3A (IC₅₀ < 10 μg/ml). Among the four constituents, Amentoflavone had the highest content in the extract (13.65%) and strongest inhibitory effect (IC₅₀ < 5 μM), especially for CYP2C9, CYP2C8 and CYP3A. Amentoflavone also showed time-dependent inhibition on CYP2C19 and CYP2D6. Apigenin and Palmatine both showed concentration-dependent inhibition. Apigenin inhibited CYP1A2, CYP2C8, CYP2C9, CYP2E1 and CYP3A. Palmatine inhibited CYP3A and had a weak inhibitory effect on CYP2E1. As for Delicaflavone, which has the potential to develop as an anti-cancer agent, showed no obvious inhibitory effect on CYP450 enzymes. Conclusion: Amentoflavone may be one of the main reasons for the inhibition of SDEA on CYP450 enzymes, the potential HDI should be considered when SDEA or Amentoflavone were used with other clinical drugs. On the contrast, Delicaflavone is more suitable to develop as a drug for clinical use, considering the low level of CYP450 metabolic inhibition.

A review of recent progress in improving the bioavailability of nutraceutical-loaded emulsions after oral intake

Increasing awareness of the health benefits of specific constituents in fruits, vegetables, cereals, and other whole foods has sparked a broader interest in the potential health benefits of nutraceuticals. Many nutraceuticals are hydrophobic substances, which means they must be encapsulated in colloidal delivery systems. Oil-in-water emulsions are one of the most widely used delivery systems for improving the bioavailability and bioactivity of these nutraceuticals. The composition and structure of emulsions can be designed to improve the water dispersibility, physicochemical stability, and bioavailability of the encapsulated nutraceuticals. The nature of the emulsion used influences the interfacial area and properties of the nutraceutical-loaded oil droplets in the gastrointestinal tract, which influences their digestion, as well as the bioaccessibility, metabolism, and absorption of the nutraceuticals. In this article, we review recent in vitro and in vivo studies on the utilization of emulsions to improve the bioavailability of nutraceuticals. The findings from this review should facilitate the design of more efficacious nutraceutical-loaded emulsions with increased bioactivity.

Studies on Chemical Characterization of Ginkgo Amillaria Oral Solution and Its Drug–Drug Interaction With Piceatannol 3′-O-β-D-Glucopyranoside for Injection

Ginkgo Amillaria oral solution (GAO) is commonly used for the treatment of cardiovascular and cerebrovascular diseases in China. Piceatannol-3′-O-β-D-glucopyranoside for injection (PGI) is mainly used for the prevention and treatment of ischemic cerebrovascular diseases. With the spread of cerebrovascular disease, the possibility of combining the two drugs has increased; however, there is no research on the drug–drug interaction (DDI) between these two medicines. In this paper, an ultrahigh-performance liquid chromatography/quadrupole–orbitrap mass spectrometry (UHPLC/Q-Orbitrap MS) method was established to characterize the chemical constituents of GAO first; 62 compounds were identified or tentatively identified based on their retention time (RT), MS, and MS/MS data. Nine main compounds were determined by ultrahigh-performance liquid chromatography/triple quadrupole mass spectrometry (UPLC-QQQ-MS). Furthermore, incubation with liver microsomes in vitro was fulfilled; the results showed that GAO had a significant inhibitory effect on UGT1A9 and UGT2B7 (p < 0.05), and PGI was mainly metabolized by UGT1A9. The identification results of in vivo metabolites of PGI showed that PGI mainly undergoes a phase II binding reaction mediated by UDP-glucuronosyltransferase (UGT) and sulfotransferase (SULT) in vivo. Therefore, pharmacokinetic studies were performed to investigate the DDI between GAO and PGI. The results showed that the AUC (p < 0.05) and T_1/2 (p < 0.05) of PGI in vivo were significantly increased when administered together with GAO, whereas the CL was significantly decreased (p < 0.05). The exploration of in vitro and in vivo experiments showed that there was a DDI between GAO and PGI.

Biflavonoids: Important Contributions to the Health Benefits of Ginkgo (Ginkgo biloba L.)

Ginkgo (Ginkgo biloba L.) is one of the most distinctive plants, characterized by excellent resistance to various environmental conditions. It is used as an ornamental plant and is recognized as a medicinal plant in both traditional and Western medicine. Its bioactive potential is associated with the presence of flavonoids and terpene trilactones, but many other compounds may also have synergistic effects. Flavonoid dimers-biflavonoids-are important constituents of ginkgophytopharmaceuticals. Currently, the presence of 13 biflavonoids has been reported in ginkgo, of which amentoflavone, bilobetin, sciadopitysin, ginkgetin and isoginkgetin are the most common. Their role in plants remains unknown, but their bioactivity and potential role in the management of human health are better investigated. In this review, we have provided an overview of the chemistry, diversity and biological factors that influence the presence of biflavonoids in ginkgo, as well as their bioactive and health-related properties. We have focused on their antioxidant, anticancer, antiviral, antibacterial, antifungal and anti-inflammatory activities as well as their potential role in the treatment of cardiovascular, metabolic and neurodegenerative diseases. We also highlighted their potential toxicity and pointed out further research directions.

Insights Into Amentoflavone: A Natural Multifunctional Biflavonoid

Amentoflavone is an active phenolic compound isolated from Selaginella tamariscina over 40 years. Amentoflavone has been extensively recorded as a molecule which displays multifunctional biological activities. Especially, amentoflavone involves in anti-cancer activity by mediating various signaling pathways such as extracellular signal-regulated kinase (ERK), nuclear factor kappa-B (NF-κB) and phosphoinositide 3-kinase/protein kinase B (PI3K/Akt), and emerges anti-SARS-CoV-2 effect via binding towards the main protease (Mpro/3CLpro), spike protein receptor binding domain (RBD) and RNA-dependent RNA polymerase (RdRp) of SARS-CoV-2. Therefore, amentoflavone is considered to be a promising therapeutic agent for clinical research. Considering the multifunction of amentoflavone, the current review comprehensively discuss the chemistry, the progress in its diverse biological activities, including anti-inflammatory, anti-oxidation, anti-microorganism, metabolism regulation, neuroprotection, radioprotection, musculoskeletal protection and antidepressant, specially the fascinating role against various types of cancers. In addition, the bioavailability and drug delivery of amentoflavone, the molecular mechanisms underlying the activities of amentoflavone, the molecular docking simulation of amentoflavone through in silico approach and anti-SARS-CoV-2 effect of amentoflavone are discussed.

Investigating the Contribution of Drug-Metabolizing Enzymes in Drug-Drug Interactions of Dapivirine and Miconazole

Dapivirine (DPV) is a potent NNRTI used to prevent the sexual transmission of HIV. In a phase 1 trial (IPM 028), the concomitant use of a DPV vaginal ring and an antifungal miconazole (MIC) vaginal capsule was found to increase the systemic exposure to DPV in women, suggesting a potential for drug-drug interactions. This study's objective was to investigate the mechanism of DPV-MIC interactions using drug-metabolizing enzymes (DMEs; CYPs and UGTs) that are locally expressed in the female reproductive tract (FRT). In vitro studies were performed to evaluate the metabolism of DPV and its inhibition and induction potential with DMEs. In addition, the impact of MIC on DPV metabolism and the inhibitory potential of DPV with DMEs were studied. Our findings suggest that DPV is a substrate of CYP1A1 and CYP3A4 enzymes and that MIC significantly decreased the DPV metabolism by inhibiting these two enzymes. DPV demonstrated potent inhibition of CYP1A1 and moderate/weak inhibition of the six CYP and eight UGT enzymes evaluated. MIC showed potent/moderate inhibition of seven CYP enzymes and weak/no inhibition of eight UGT enzymes. The combination of DPV and MIC showed potent inhibition of seven CYP enzymes (1A1, 1A2, 1B1, 2B6, 2C8, 2C19, and 3A4) and four UGT enzymes (1A3, 1A6, 1A9, and 2B7). DPV was not an inducer of CYP1A2, CYP2B6, and CYP3A4 enzymes in primary human hepatocytes. Therefore, the increased systemic concentrations of DPV observed in IPM 028 were likely due to the reduced metabolism of DPV because of CYP1A1 and CYP3A4 enzymes inhibition by MIC in the FRT.

Methylophiopogonanone A is a naturally occurring broad-spectrum inhibitor against human UDP-glucuronosyltransferases: Inhibition behaviours and implication in herb-drug interactions

Methylophiopogonanone A (MOA) is an abundant homoisoflavonoid in the Chinese herb Ophiopogonis Radix. Recent investigations revealed that MOA inhibited several human cytochrome P450 enzymes (CYPs) and stimulated OATP1B1. However, the inhibitory effects of MOA on phase II drug-metabolizing enzymes, such as human UDP-glucuronosyltransferases (hUGTs), have not been well investigated. Herein, the inhibition potentials of MOA on hUGTs were assessed. The results clearly demonstrated that MOA dose-dependently inhibited all tested hUGTs including UGT1A1 (IC₅₀  = 1.23 μM), one of the most important detoxification enzymes in humans. Further investigations showed that MOA strongly inhibited UGT1A1-catalysed NHPH-O-glucuronidation in a range of biological settings including hUGT1A1, human liver microsomes (HLM) and HeLa cells overexpressing UGT1A1. Inhibition kinetic analyses demonstrated that MOA competitively inhibited UGT1A1-catalysed NHPH-O-glucuronidation in both hUGT1A1 and HLM, with K_i values of 0.52 and 1.22 μM, respectively. Collectively, our findings expanded knowledge of the interactions between MOA and human drug-metabolizing enzymes, which would be very helpful for guiding the use of MOA-related herbal products in clinical settings.

Characterizations of Human UDP-Glucuronosyltransferase Enzymes in the Conjugation of p-Cresol

p-Cresol is a uremic toxin that is formed by intestinal microbiota and extensively conjugated by first-pass metabolism. p-Cresol glucuronide exerts various forms of cellular toxicity in vitro and is accumulated in the plasma of subjects with kidney disease, where associations with adverse cardiovascular and renal outcomes are evident. The objective of this study was to determine the contributions of human UDP-glucuronosyltransferase (UGT) enzymes in the formation of p-cresol glucuronide. Utilizing commonly expressed hepatic or renal human recombinant UGTs (ie, hrUGT1A1, 1A3, 1A4, 1A6, 1A7, 1A8, 1A9, 1A10, 2B4, 2B7, 2B10, 2B15, and 2B17), hrUGT1A6 and hrUGT1A9 exhibited the highest catalytic activities in the generation of p-cresol glucuronide. The kinetics of p-cresol glucuronide formation in hrUGT1A6 and pooled human liver microsomes were best described by the Hill equation and in hrUGT1A9 and pooled human kidney microsomes by substrate inhibition. Using inhibitory and selective UGT inhibitors (ie, acetaminophen or amentoflavone for UGT1A6 and niflumic acid for UGT1A9), UGT1A6 was identified the predominant enzyme responsible for p-cresol glucuronide production in pooled human liver (78.4%-81.3% contribution) and kidney (54.3%-62.9%) microsomes, whereas UGT1A9 provided minor contributions (2.8% and 35.5%, respectively). The relative contributions of UGT1A6 (72.6 ± 11.3%, mean ± SD) and UGT1A9 (5.7 ± 4.1%) in individual human liver microsomes from 12 adult donors were highly variable, where an inverse association (R = -.784, p = .003) between UGT1A6 contribution and UGT1A9 probe substrate activity (ie, mycophenolic acid) was evident. Our novel findings provide valuable tools for conducting further mechanistic studies and for designing clinical interventions to mitigate the toxicities associated with p-cresol glucuronide.

Potential of amentoflavone with antiviral properties in COVID-19 treatment

Amentoflavone is one of the flavonoids that are known for their antiviral effects and many of them are predicted to have inhibitory effects against severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome Coronavirus (MERS-CoV) enzymes 3-chymotrypsin-like protease (3CLpro) and papain-like protease (PLpro). Amentoflavone is a biflavonoid found in the herbal extracts of St. John's wort (Hypericum perforatum), Gingko biloba, Selaginella tamariscina, Torreya nucifera, and many other plants. Its pharmacological actions have been listed as antiviral, antibacterial, antioxidant, anti-inflammatory, antidiabetic, antidepressant, and neuroprotective. Molecular docking studies have found that amentoflavone binds strongly to the active site of the main protease (Mpro) of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). As conventional antiviral medications are met with limited success against coronavirus disease-2019 (COVID-19) and vaccines are one of the only weapons against COVID-19 in the pharmaceutical armamentarium, traditional medicines are being considered for the forefront battle against COVID-19. Clinical studies with Hypericum and Gingko extract as additional or alternative drugs/supplements are registered. Here we review the potential of amentoflavone, an active agent in both Hypericum and Gingko extract as an adjunct therapy for COVID-19. Its anti-inflammatory, antioxidant, and sepsis preventive actions could provide protection against the "cytokine storm." Compared with the herbal extracts, which induce cytochrome P450 (CYP) and uridine 5'-diphospho (UDP)-glucuronosyltransferases (UGT) activity producing a negative herb-drug interaction, amentoflavone is a potent inhibitor of CYP3A4, CYP2C9, and UGT. Further studies into the therapeutic potential of amentoflavone against the coronavirus infection are warranted.

Flavonoids in Ampelopsis grossedentata as covalent inhibitors of SARS-CoV-2 3CLpro: Inhibition potentials, covalent binding sites and inhibitory mechanisms

Coronavirus 3C-like protease (3CL^pro) is a crucial target for treating coronavirus diseases including COVID-19. Our preliminary screening showed that Ampelopsis grossedentata extract (AGE) displayed potent SARS-CoV-2-3CL^pro inhibitory activity, but the key constituents with SARS-CoV-2-3CL^pro inhibitory effect and their mechanisms were unrevealed. Herein, a practical strategy via integrating bioactivity-guided fractionation and purification, mass spectrometry-based peptide profiling and time-dependent biochemical assay, was applied to identify the crucial constituents in AGE and to uncover their inhibitory mechanisms. The results demonstrated that the flavonoid-rich fractions (10-17.5 min) displayed strong SARS-CoV-2-3CL^pro inhibitory activities, while the constituents in these fractions were isolated and their SARS-CoV-2-3CL^pro inhibitory activities were investigated. Among all isolated flavonoids, dihydromyricetin, isodihydromyricetin and myricetin strongly inhibited SARS-CoV-2 3CL^pro in a time-dependent manner. Further investigations demonstrated that myricetin could covalently bind on SARS-CoV-2 3CL^pro at Cys300 and Cys44, while dihydromyricetin and isodihydromyricetin covalently bound at Cys300. Covalent docking coupling with molecular dynamics simulations showed the detailed interactions between the orthoquinone form of myricetin and two covalent binding sites (surrounding Cys300 and Cys44) of SARS-CoV-2 3CL^pro. Collectively, the flavonoids in AGE strongly and time-dependently inhibit SARS-CoV-2 3CL^pro, while the newly identified SARS-CoV-2 3CL^pro inhibitors in AGE offer promising lead compounds for developing novel antiviral agents.

Effects of p-Cresol on Oxidative Stress, Glutathione Depletion, and Necrosis in HepaRG Cells: Comparisons to Other Uremic Toxins and the Role of p-Cresol Glucuronide Formation

The toxicological effects of p-cresol have primarily been attributed to its metabolism products; however, very little human data are available in the key organ (i.e., liver) responsible for the generation of these metabolites. Experiments were conducted in HepaRG cells utilizing the following markers of cellular toxicity: 2′-7′-dichlorofluorescein (DCF; oxidative stress) formation, total cellular glutathione (GSH) concentration, and lactate dehydrogenase (LDH; cellular necrosis) release. Concentrations of p-cresol, p-cresol sulfate, and p-cresol glucuronide were determined using validated assays. p-Cresol exposure resulted in concentration- and time-dependent changes in DCF (EC₅₀ = 0.64 ± 0.37 mM at 24 h of exposure) formation, GSH (EC₅₀ = 1.00 ± 0.07 mM) concentration, and LDH (EC₅₀ = 0.85 ± 0.14 mM) release at toxicologically relevant conditions. p-Cresol was also relatively more toxic than 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, indole-3-acetic acid, indoxyl sulfate, kynurenic acid, and hippuric acid on all markers. Although the exogenous administration of p-cresol sulfate and p-cresol glucuronide generated high intracellular concentrations of these metabolites, both metabolites were less toxic compared to p-cresol at equal-molar conditions. Moreover, p-cresol glucuronide was the predominant metabolite generated in situ from p-cresol exposure. Selective attenuation of glucuronidation (without affecting p-cresol sulfate formation, while increasing p-cresol accumulation) using independent chemical inhibitors (i.e., 0.75 mM l-borneol, 75 µM amentoflavone, or 100 µM diclofenac) consistently resulted in further increases in LDH release associated with p-cresol exposure (by 28.3 ± 5.3%, 30.0 ± 8.2% or 27.3 ± 6.8%, respectively, compared to p-cresol treatment). These novel data indicated that p-cresol was a relatively potent toxicant, and that glucuronidation was unlikely to be associated with the manifestation of its toxic effects in HepaRG cells.

A fluorescence-based microplate assay for high-throughput screening and evaluation of human UGT inhibitors

Human UDP-glucuronosyltransferase enzymes (hUGTs), one of the most important classes of conjugative enzymes, are responsible for the glucuronidation and detoxification of a variety of endogenous substances and xenobiotics. Inhibition of hUGTs may cause undesirable effects or adverse drug-drug interactions (DDI) via modulating the glucuronidation rates of endogenous toxins or the drugs that are primarily conjugated by the inhibited hUGTs. Herein, to screen hUGTs inhibitors in a more efficient way, a novel fluorescence-based microplate assay has been developed by utilizing a fluorogenic substrate. Following screening of series of 4-hydroxy-1,8-naphthalimide derivatives, we found that 4-HN-335 is a particularly good substrate for a panel of hUGTs. Under physiological conditions, 4-HN-335 can be readily O-glucuronidated by ten hUGTs, such reactions generate a single O-glucuronide with a high quantum yield (Ф = 0.79) and bring remarkable changes in fluorescence emission. Subsequently, a fluorescence-based microplate assay is developed to simultaneously measure the inhibitory effects of selected compound(s) on ten hUGTs. The newly developed fluorescence-based microplate assay is time- and cost-saving, easy to manage and can be adapted for 96-well microplate format with the Z-factor of 0.92. We further demonstrate the utility of the fluorescence-based assay for high-throughput screening of two compound libraries, resulting in the identification of several potent UGT inhibitors, including natural products and FDA-approved drugs. Collectively, this study reports a novel fluorescence-based microplate assay for simultaneously sensing the residual activities of ten hUGTs, which strongly facilitates the identification and characterization of UGT inhibitors from drugs or herbal constituents and the investigations on UGT-mediated DDI.

Amentoflavone from Selaginella tamariscina as a potent inhibitor of gut bacterial β-glucuronidase: Inhibition kinetics and molecular dynamics stimulation

Gut bacterial β-glucuronidase (GUS) plays a pivotal role in the metabolism and reactivation of a vast of glucuronide conjugates of both endogenous and xenobiotic compounds in the gastrointestinal tract of human, which has been implicated in certain drug-induced gastrointestinal tract (GI) toxicity in clinic. Inhibitors of gut microbial GUS exhibited great potentials in relieving the drug-induced GI toxicity. In this study, Selaginella tamariscina and its major biflavonoid amentoflavone (AMF) were evaluated for their inhibitory activity against Escherichia coli GUS. Two selective probe substrates for GUS (a specific fluorescent probe substrate for GUS, DDAOG and a classical drug substrate for GUS, SN38G) were used in parallel for charactering the inhibition behaviors. Both the extract of S. tamariscina and its major biflavonoid AMF displayed evident inhibitory effects on GUS, and the IC₅₀ values of AMF against GUS mediated DDAOG and SN-38G hydrolysis were 0.62 and 0.49 μM, respectively. Inhibition kinetics studies indicated that AMF showed mixed type inhibition for GUS-mediated DDAOG hydrolysis, while displayed competitive type inhibition against GUS-mediated SN-38G hydrolysis, with the K_i values of 0.24 and 1.25 μM, respectively. Molecular docking studies and molecular dynamics stimulation results clarified the role of amino acid residues Leu361, Ile363, and Glu413 in the inhibition of AMF on GUS. These results provided some foundations for the potential clinical utility of S. tamariscina and its major biflavonoid AMF for treating drug-induced enteropathy.

A broad-spectrum substrate for the human UDP-glucuronosyltransferases and its use for investigating glucuronidation inhibitors

Strong inhibition of the human UDP-glucuronosyltransferase enzymes (UGTs) may lead to undesirable effects, including hyperbilirubinaemia and drug/herb-drug interactions. Currently, there is no good way to examine the inhibitory effects and specificities of compounds toward all the important human UGTs, side-by-side and under identical conditions. Herein, we report a new, broad-spectrum substrate for human UGTs and its uses in screening and characterizing of UGT inhibitors. Following screening a variety of phenolic compound(s), we have found that methylophiopogonanone A (MOA) can be readily O-glucuronidated by all tested human UGTs, including the typical N-glucuronidating enzymes UGT1A4 and UGT2B10. MOA-O-glucuronidation yielded a single mono-O-glucuronide that was biosynthesized and purified for structural characterization and for constructing an LC-UV based MOA-O-glucuronidation activity assay, which was then used for investigating MOA-O-glucuronidation kinetics in recombinant human UGTs. The derived K_m values were crucial for selecting the most suitable assay conditions for assessing inhibitory potentials and specificity of test compound(s). Furthermore, the inhibitory effects and specificities of four known UGT inhibitors were reinvestigated by using MOA as the substrate for all tested UGTs. Collectively, MOA is a broad-spectrum substrate for the human UGTs, which offers a new and practical tool for assessing inhibitory effects and specificities of UGT inhibitors.

Glabrone as a specific UGT1A9 probe substrate and its application in discovering the inhibitor glycycoumarin

UDP-glucuronosyltransferase 1A9 (UGT1A9) is one of the most important UGT isoforms, and plays an important role in the metabolic elimination of therapeutic drugs via glucuronidation. Herbal medicines affecting the activity of UGT1A9 may influence the metabolism of related drugs, thus causing herb-drug interactions and even adverse effects. However, few methods are available to evaluate the activity of UGT1A9. In this study, a natural product glabrone was discovered as an isoform-specific probe substrate for UGT1A9. The V_max and K_m values of glabrone were 362.6 nmol/min/mg protein and 17.2 μM for human liver microsomes (HLMs), and 382.3 nmol/min/mg protein and 16.6 μM for recombinant human UGT1A9, respectively. Glabrone 7-O-glucuronide, the UGT1A9 metabolite of glabrone, was prepared by using a plant glucuronosyltransferase UGT88D1, and the structure was identified by NMR spectroscopy. Using glabrone as a probe, we established a rapid HPLC method to screen UGT1A9 inhibitors from 54 natural products isolated from the Chinese herbal medicine licorice. Among them, glycycoumarin was found as a potent UGT1A9 inhibitor with an IC₅₀ value of 6.04 μM. In rats, the pretreatment of glycycoumarin (4 mg/kg, i.p.) for 3 days could remarkably increase the plasma concentrations of dapagliflozin while decrease the concentrations of dapagliflozin-O-glucuronide after administration of dapagliflozin (1 mg/kg, i.v.), which is mainly metabolized by UGT1A9. The results indicated the potential risk of herb-drug interactions between licorice and UGT1A9-metabolizing drugs.

Bioactivity of natural biflavonoids in metabolism-related disease and cancer therapies

Natural biflavonoids, such as amentoflavone, bilobetin, ginkgetin, isoginkgetin, taiwaniaflavone, morelloflavone, delicaflavone, hinokiflavone, and other derivatives (~ 40 biflavonoids), are isolated from Selaginella sp., Ginkgo biloba, Garcinia sp., and several other species of plants. They are able to exert therapeutic benefits by regulating several proteins/enzymes (PPAR-γ, CCAAT/enhancer-binding protein α [C/EBPα], STAT5, pancreatic lipase, PTP1B, fatty acid synthase, α-glucosidase [AG]) and insulin signaling pathways (via PI3K-AKT), which are linked to metabolism, cell growth, and cell survival mechanisms. Deregulated insulin signaling can cause complications of obesity and diabetes, which can lead to cognitive disorders such as Alzheimer's, Parkinson's, and dementia; therefore, the therapeutic benefits of these biflavones in these areas are highlighted. Since biflavonoids have shown potential to regulate metabolism, growth- and survival-related protein/enzymes, their relation to tumor growth and metastasis of cancer associated with angiogenesis are highlighted. The translational role of biflavones in cancer with respect to the inhibition of metabolism-related processes/pathways, enzymes, or proteins, such as STAT3/SHP-1/PTEN, kinesins, tissue kallikreins, aromatase, estrogen, protein modifiers, antioxidant, autophagy, and apoptosis induction mechanisms, are discussed. Finally, considering their observed bioactivity potential, oral bioavailability studies of biflavones and related clinical trials are outlined.

Strong and Selective Inhibitory Effects of the Biflavonoid Selamariscina A against CYP2C8 and CYP2C9 Enzyme Activities in Human Liver Microsomes

Like flavonoids, biflavonoids, dimeric flavonoids, and polyphenolic plant secondary metabolites have antioxidant, antibacterial, antiviral, anti-inflammatory, and anti-cancer properties. However, there is limited data on their effects on cytochrome P450 (P450) and uridine 5'-diphosphoglucuronosyl transferase (UGT) enzyme activities. In this study we evaluate the inhibitory potential of five biflavonoids against nine P450 activities (P450s1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, and 3A) in human liver microsomes (HLMs) using cocktail incubation and liquid chromatography-tandem mass spectrometry (LC-MS/MS). The most strongly inhibited P450 activity was CYP2C8-mediated amodiaquine N-dealkylation with IC50 ranges of 0.019~0.123 μM. In addition, the biflavonoids-selamariscina A, amentoflavone, robustaflavone, cupressuflavone, and taiwaniaflavone-noncompetitively inhibited CYP2C8 activity with respective K_i values of 0.018, 0.083, 0.084, 0.103, and 0.142 μM. As selamariscina A showed the strongest effects, we then evaluated it against six UGT isoforms, where it showed weaker inhibition (UGTs1A1, 1A3, 1A4, 1A6, 1A9, and 2B7, IC50 1.7 μM). Returning to the P450 activities, selamariscina A inhibited CYP2C9-mediated diclofenac hydroxylation and tolbutamide hydroxylation with respective K_i values of 0.032 and 0.065 μM in a competitive and noncompetitive manner. However, it only weakly inhibited CYP1A2, CYP2B6, and CYP3A with respective K_i values of 3.1, 7.9, and 4.5 μM. We conclude that selamariscina A has selective and strong inhibitory effects on the CYP2C8 and CYP2C9 isoforms. This information might be useful in predicting herb-drug interaction potential between biflavonoids and co-administered drugs mainly metabolized by CYP2C8 and CYP2C9. In addition, selamariscina A might be used as a strong CYP2C8 and CYP2C9 inhibitor in P450 reaction-phenotyping studies to identify drug-metabolizing enzymes responsible for the metabolism of new chemicals.

Glucuronidation and its effect on the bioactivity of amentoflavone, a biflavonoid from Ginkgo biloba leaves

OBJECTIVES

Ginkgo biloba leaves contain amentoflavone (AMF), a dietary flavonoid that possesses antioxidant and anticancer activity. Flavonoids are extensively subjected to glucuronidation. This study aimed to determine the metabolic profile of AMF and the effect of glucuronidation on AMF bioactivity.

METHODS

A pharmacokinetic study was conducted to determine the plasma concentrations of AMF and its metabolites. The metabolic profile of AMF was elucidated using different species of microsomes. The antioxidant activity of AMF metabolites was determined using DPPH/ABTS radical and nitric oxide assays. The anticancer activity of AMF metabolites was evaluated in U87MG/U251 cells.

KEY FINDINGS

Pharmacokinetic studies indicated that the oral bioavailability of AMF was 0.06 ± 0.04%, and the area under the curve of the glucuronidated AMF metabolites (410.938 ± 62.219 ng/ml h) was significantly higher than that of AMF (194.509 ± 16.915 ng/ml h). UGT1A1 and UGT1A3 greatly metabolized AMF. No significant difference was observed in the antioxidant activity between AMF and its metabolites. The anticancer activity of AMF metabolites significantly decreased.

CONCLUSIONS

A low AMF bioavailability was due to extensive glucuronidation, which was mediated by UGT1A1 and UGT1A3. Glucuronidated AMF metabolites had the same antioxidant but had a lower anticancer activity than that of AMF.

Potential hepatic and renal toxicity induced by the biflavonoids from Ginkgo biloba

Evidence continues to grow on potential health risks associated with Ginkgo biloba and its constituents. While biflavonoid is a subclass of the flavonoid family in Ginkgo biloba with a plenty of pharmacological properties, the potential toxicological effects of biflavonoids remains largely unknown. Thus, the aim of this study was to investigate the in vitro and in vivo toxicological effects of the biflavonoids from Ginkgo biloba (i.e., amentoflavone, sciadopitysin, ginkgetin, isoginkgetin, and bilobetin). In the in vitro cytotoxicity test, the five biflavonoids all reduced cell viability in a dose-dependent manner in human renal tubular epithelial cells (HK-2) and human normal hepatocytes (L-02), indicating they might have potential liver and kidney toxicity. In the in vivo experiments, after intragastrical administration of these biflavonoids at 20 mg·kg^-1·d^-1 for 7 days, serum biochemical analysis and histopathological examinations were performed. The activity of alkaline phosphatase was significantly increased after all the biflavonoid administrations and widespread hydropic degeneration of hepatocytes was observed in ginkgetin or bilobetin-treated mice. Moreover, the five biflavonoids all induced acute kidney injury in treated mice and the main pathological lesions were confirmed to the tubule, glomeruli, and interstitium injuries. As the in vitro and in vivo results suggested that these biflavonoids may be more toxic to the kidney than the liver, we further detected the mechanism of biflavonoids-induced nephrotoxicity. The increased TUNEL-positive cells were detected in kidney tissues of biflavonoids-treated mice, accompanied by elevated expression of proapoptotic protein BAX and unchanged levels of antiapoptotic protein BCL-2, indicating apoptosis was involved in biflavonoids-induced nephrotoxicity. Taken together, our results suggested that the five biflavonoids from Ginkgo biloba may have potential hepatic and renal toxicity and more attentions should be paid to ensure Ginkgo biloba preparations safety.

Pterostilbene supplements carry the risk of drug interaction via inhibition of UDP-glucuronosyltransferases (UGT) 1A9 enzymes

Pterostilbene (PT) is a natural stilbene common in small berries and food supplements, possessing numerous pharmacological activities. However, whether PT can affect the activities of UDP-glucuronosyltransferases (UGT) enzymes remains unclear. The aim of the present study was to investigate the effect of PT on UGT activities and to quantitatively evaluate the food-drug interaction potential due to UGT inhibition. Our data indicated that PT exhibited potent inhibition against HLM, UGT1A6, UGT1A9, UGT2B7, and UGT2B15, moderate inhibition against UGT1A1, UGT1A3, UGT1A8, and UGT2B4, negligible inhibition against UGT1A4, UGT1A7, UGT1A10, and UGT2B17. Further kinetic investigation demonstrated that PT exerted potent noncompetitive inhibition 4-MU glucuronidation by UGT1A9, with IC₅₀ and K_i values of 0.92 μM and 0.52 ± 0.04 μM, respectively. Quantitative prediction study suggested that coadministration of PT supplements at 100 mg/day or higher doses may result in at least a 50% increase in the AUC of drugs predominantly cleared by UGT1A9. Thus, the coadministration of PT supplements and drugs primarily cleared by UGT1A9 may result in potential drug interaction, and precautions should be taken when coadministration of PT supplements and drugs metabolized by UGT1A9.

Recent progress and challenges in screening and characterization of UGT1A1 inhibitors

Uridine-diphosphate glucuronosyltransferase 1A1 (UGT1A1) is an important conjugative enzyme in mammals that is responsible for the conjugation and detoxification of both endogenous and xenobiotic compounds. Strong inhibition of UGT1A1 may trigger adverse drug/herb-drug interactions, or result in metabolic disorders of endobiotic metabolism. Therefore, both the US Food and Drug Administration (FDA) and the European Medicines Agency (EMA) have recommended assaying the inhibitory potential of drugs under development on the human UGT1A1 prior to approval. This review focuses on the significance, progress and challenges in discovery and characterization of UGT1A1 inhibitors. Recent advances in the development of UGT1A1 probes and their application for screening UGT1A1 inhibitors are summarized and discussed in this review for the first time. Furthermore, a long list of UGT1A1 inhibitors, including information on their inhibition potency, inhibition mode, and affinity, has been prepared and analyzed. Challenges and future directions in this field are highlighted in the final section. The information and knowledge that are presented in this review provide guidance for rational use of drugs/herbs in order to avoid the occurrence of adverse effects via UGT1A1 inhibition, as well as presenting methods for rapid screening and characterization of UGT1A1 inhibitors and for facilitating investigations on UGT1A1-ligand interactions.

Inhibition of UGT1A1 by natural and synthetic flavonoids

Flavonoids are widely distributed phytochemicals in vegetables, fruits and medicinal plants. Recent studies demonstrate that some natural flavonoids are potent inhibitors of the human UDP-glucuronosyltransferase 1A1 (UGT1A1), a key enzyme in detoxification of endogenous harmful compounds such as bilirubin. In this study, the inhibitory effects of 56 natural and synthetic flavonoids on UGT1A1 were assayed, while the structure-inhibition relationships of flavonoids as UGT1A1 inhibitors were investigated. The results demonstrated that the C-3 and C-7 hydroxyl groups on the flavone skeleton would enhance UGT1A1 inhibition, while flavonoid glycosides displayed weaker inhibitory effects than their corresponding aglycones. Further investigation on inhibition kinetics of two strong flavonoid-type UGT1A1 inhibitors, acacetin and kaempferol, yielded interesting results. Both flavonoids were competitive inhibitors against UGT1A1-mediated NHPN-O-glucuronidation, but were mixed and competitive inhibitors toward UGT1A1-mediated NCHN-O-glucuronidation, respectively. Furthermore, docking simulations showed that the binding areas of NHPN, kaempferol and acacetin on UGT1A1 were highly overlapping, and convergence with the binding area of bilirubin within UGT1A1. In summary, detailed structure-inhibition relationships of flavonoids as UGT1A1 inhibitors were investigated carefully and the findings shed new light on the interactions between flavonoids and UGT1A1, and will contribute considerably to the development of flavonoid-type drugs without strong UGT1A1 inhibition.

Chemical Probes for Human UDP-Glucuronosyltransferases: A Comprehensive Review

UGTs play crucial roles in the metabolism and detoxification of both endogenous and xenobiotic compounds. The key roles of UGTs in human health have garnered great interest in the design and development of specific probes for human UGTs. However, in contrast to other human enzymes, the probe substrates for human UGTs are rarely reported, owing to the highly overlapping substrate specificities of UGTs and the lack of the integrated crystal structures of UGTs. Over the past decades, many efforts are made to develop specific probe substrates for UGTs and use them in both basic research and drug discovery. This review focuses on recent progress in the development of probe substrates for UGTs and their biomedical applications. A long list of chemical probes for UGTs, including non-fluorescent and fluorescent probes along with their structural information and kinetic parameters, are prepared and analyzed. Additionally, challenges and future directions in this field are highlighted in the final section. All information and knowledge presented in this review provide practical tools/methods for measuring UGT activities in complex biological samples, which will be very helpful for rapid screening and characterization of UGT modulators, and for exploring the relevance of UGT enzymes to human diseases.