UDP-Glucuronosyltransferases 1A6 and 1A9 are the Major Isozymes Responsible for the 7-O-Glucuronidation of Esculetin and 4-Methylesculetin in Human Liver Microsomes

In vivo comprehensive metabolite profiling of esculetin and esculin derived from chicory in hyperuricemia rats using ultra-high-performance liquid chromatography coupled with quadrupole-orbitrap high-resolution mass spectrometry

Chicory, renowned for its multifaceted benefits, houses two vital coumarins, esculetin and esculin, both instrumental in reducing uric acid. This study emphasizes the metabolic pathways of esculetin and esculin under both standard and hyperuricemia conditions. Hyperuricemia was induced in Sprague-Dawley rats using oxonic acid potassium salt (300 mg·kg-1 ) and a 10% fructose water regimen over 21 days. Leveraging the ultra-high-performance liquid chromatography-Q Exactive hybrid quadrupole-orbitrap high resolution mass spectrometry, we analyzed the fragmentation behaviors of esculetin and esculin in rat bio-samples. Post oral-intake of esculetin or esculin, a notable dip in serum uric acid levels was observed in hyperuricemia rats. The investigation unveiled 24 esculetin metabolites and 14 for esculin. The metabolic pathways of both compounds were hydrolysis, hydroxylation, hydrogenation, dehydroxylation, glucuronidation, sulfation, and methylation. Interestingly, certain metabolites presented variations between standard and hyperuricemia rats, indicating that elevated levels of uric acid may affect enzyme activity linked to these metabolic reactions. This is the first systematic study on comparison of metabolic profiles of esculetin and esculin in both normal and hyperuricemia states, which was helpful to enrich our understanding of the complicated structure-activity relationships between esculin and esculetin and shed light to their action mechanism.

Comparison and identification of metabolic profiling of bicyclol in rats, dogs and humans in vitro and in vivo

Bicyclol, a novel hepatoprotective agent, has been widely used to treat chronic viral hepatitis and drug-induced liver injury (DILI). However, its metabolic characteristics remains to be explored, especially in humans. The current study aimed to identify major metabolites and specific metabolizing enzymes involved in bicyclol metabolism in vitro and in vivo using high performance liquid chromatography coupled with Q-Exactive orbitrap mass spectrometry (HPLC-Q-Exactive Orbitrap/MS). After incubation with liver microsomes and oral administration to rats, dogs and humans, a total of nine metabolites of bicyclol were identified including M1 (methyl ester hydrolysate product), M2-M3 (demethylated bicyclol), M4-M5 (demethoxy or dehydroxymethyl bicyclol), M6 (glucuronidated bicyclol) and M7-M9 (glucuronide conjugates of metabolites). Among these metabolites, M2 and M3 were the major phase I metabolites mainly mediated by CYP2C19 and CYP3A4, while M6 was the dominant phase II metabolite primarily catalyzed by UGT2B4. In this study, species-related metabolic difference among rats, dogs and humans were observed. In humans and dogs, M6 (glucuronidated bicyclol) was the most abundant circulating metabolite (higher than the parent drug) in the blood after oral administration, while the parent drug was the highest in rats. M4 and M5 were rats-specific metabolites whereas M1 and M9 were absent in dogs in vivo. The metabolism of bicyclol was demonstrated as demethylation and glucuronidation mediated by multiple drug metabolizing enzymes in different species. Our findings systematically elucidated the metabolic sites and routes of bicyclol in human for the first time, which may be helpful for rational combined application in clinic and further study of metabolites-related efficacy or toxicity.

Translational role of natural coumarins and their derivatives as anticancer agents

Natural coumarins and their derivatives isolated from various plants or microorganisms have inherent antioxidant, antibacterial, antifungal, antiviral and anticancer properties among many biological activities. Some of these coumarins and their derivatives lead to self-programmed cancer cell death (apoptosis) via different mechanisms, which will be discussed. The link between bacterial and viral infections to cancer compels us to highlight fascinating reports from coumarin isolation from microorganisms; comment on the recent bioavailability studies of natural or derived coumarins; and discuss our perspectives with respect to bioisosterism in coumarins, p-glycoprotein inhibition and covalent modification, and bioprobes. Overall, this review hopes to stimulate and offer in particular medicinal chemists and the reader in general an outlook on natural coumarins and their derivatives with potential for cancer therapy.

Breast Cancer Resistance Protein and Multidrug Resistance Protein 2 Determine the Disposition of Esculetin-7-O-Glucuronide and 4-Methylesculetin-7-O-Glucuronide

Esculetin (ET)-7-O-glucuronide (ET-G) and 4-methylesculetin (4-ME)-7-O-glucuronide (4-ME-G) are the main glucuronide of ET and 4-ME, respectively. The disposition mediated by efflux transporters for glucuronide has significant influence on the pharmacokinetic profile and efficacy of bioactive compounds. In the current study, transporter gene knockout mice and Caco-2 cells were used to explore the effects of breast cancer resistance protein (BCRP) and multidrug resistance-associated protein 2 (MRP2) on the disposition of ET-G and 4-ME-G. After oral or i.v. administration of ET and 4-ME, the area under the plasma concentration-time curve from time 0 to the last data point or infinity values of ET, 4-ME, and their glucuronides (ET-G and 4-ME-G) were remarkably and significantly increased in most Bcrp1^-/- and Mrp2^-/- mice compared with those in wild-type FVB mice (P < 0.05). These results were accompanied with a significant increase of maximum plasma concentration values (P < 0.05). In Caco-2 monolayers, the efflux and clearance rates of ET-G and 4-ME-G were markedly reduced by the BCRP inhibitor Ko143 and MRP2 inhibitor MK571 on the apical side (P < 0.05). In an intestinal perfusion study, the excretion of ET-G was significantly decreased in perfusate and increased in plasma in Bcrp1^-/- mice compared with those in wild-type FVB mice (P < 0.05). The 4-ME-G concentration was also decreased in the bile in transporter gene knockout mice. ET and 4-ME showed good permeability in both Caco-2 monolayers [apparent permeability (P_app ) ≥ 0.59 × 10^-5 cm/s] and duodenum (P_app ≥ 1.81). In conclusion, BCRP and MRP2 are involved in excreting ET-G and 4-ME-G. ET and 4-ME are most likely absorbed via passive diffusion in the intestines.

Interplay of Efflux Transporters with Glucuronidation and Its Impact on Subcellular Aglycone and Glucuronide Disposition: A Case Study with Kaempferol

Glucuronidation is a major process of drug metabolism and elimination that generally governs drug efficacy and toxicity. Publications have demonstrated that efflux transporters control intracellular glucuronidation metabolism. However, it is still unclear whether and how efflux transporters interact with UDP-glucuronosyltransferases (UGTs) in subcellular organelles. In this study, kaempferol, a model fluorescent flavonoid, was used to investigate the interplay of glucuronidation with transport at the subcellular level. Human recombinant UGTs and microsomes were utilized to characterize the in vitro glucuronidation kinetics of kaempferol. The inhibition of UGTs and efflux transporters on the subcellular disposition of kaempferol were determined visually and quantitatively in Caco-2/TC7 cells. The knockout of transporters on the subcellular accumulation of kaempferol in liver and intestine were evaluated visually. ROS and Nrf2 were assayed to evaluate the pharmacological activities of kaempferol. The results showed that UGT1A9 is the primary enzyme responsible for kaempferol glucuronidation. Visual and quantitative data showed that the UGT1A9 inhibitor carvacrol caused a significant rise in subcellular aglycone and reduction in subcellular glucuronides of kaempferol. The inhibition and knockout of transporters, such as P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), and multidrug resistance-associated proteins (MRPs), exhibited a marked increase in subcellular kaempferol and decrease in its subcellular glucuronides. Correspondingly, inhibition of UGT1A9 and transporters led to increased kaempferol and, consequently, a significantly enhanced ROS scavenging efficiency and nuclear translocation of Nrf2. In conclusion, the interplay of efflux transporters (P-gp, BCRP, and MRPs) and UGTs govern the subcellular exposure and corresponding pharmacological activity of kaempferol.

UGT-mediated metabolism plays a dominant role in the pharmacokinetic behavior and the disposition of morusin in vivo and in vitro

Morusin is a prenylated flavone isolated from mulberry, the branch and root bark of various Morus species, which possesses diverse pharmacological activities. However, it lacks extensive studies about its absorption and disposition. This study investigated the pharmacokinetic behavior of morusin in rat, and its first-pass metabolism in situ. The metabolic pathway of morusin was further investigated by 12 human recombinant UDP-glucuronosyltransferases (UGTs), 9 CYP450s, as well as liver and intestinal microsomes. Four mono-glucuronide metabolites (M-5-G, M-4'-G, M-2'-G, and M_II-2) were identified in rat intestine and bile by LC-MS/MS, while three of them were also detected in plasma (M-5-G, M-4'-G, and M_II-2). M-4'-G was the principal conjugate. However, few CYP450 metabolites were found in rat intestine and bile. Only a small amount of M_I-1 could be detected in rat plasma. UGT1A1, 1A3, 1A7, and 2B7 were the major contributors to morusin glucuronidation. Morusin exhibited substrate inhibition kinetic characteristics in all UGTs. Clearance rates of M-4'-G in HLM, RLM, UGT1A1, UGT1A3, and UGT2B7 were 137.02, 127.55, 32.54, 41.18, and 35.07 ml/min/mg, respectively. Besides, CYP3A5, 3A4, and 2C19 primarily contributed to the oxidative metabolism of morusin. The pharmacokinetic curves of morusin and its conjugates presented double peaks, showing that an enterohepatic recycling may exist. In conclusion, glucuronidation was confirmed to be the crucial metabolic pathway for morusin in vivo, and M-4'-G was the main metabolite.

Metabolic Disposition of Luteolin Is Mediated by the Interplay of UDP-Glucuronosyltransferases and Catechol-O-Methyltransferases in Rats

Luteolin partially exerts its biologic effects via its metabolites catalyzed by UDP-glucuronosyltransferases (UGTs) and catechol-O-methyltransferases (COMTs). However, the interplay of UGTs and COMTs in mediating luteolin disposition has not been well clarified. In this study, we investigated the glucuronidation and methylation pathways of luteolin mediated by the interplay of UGTs and COMTs in vivo and in vitro. A total of nine luteolin metabolites was detected in rat plasma and bile by liquid chromatography-tandem mass spectrometry, namely, three glucuronides, two methylated metabolites, and four methylated glucuronides. Luteolin-3'-glucuronide (Lut-3'-G) exhibited the highest systemic exposure among these metabolites. Kinetics studies in rat liver S9 fractions suggested two pathways, as follows: 1) Luteolin was glucuronidated to luteolin-7-glucuronide, luteolin-4'-glucuronide, and Lut-3'-G by UGTs, and then Lut-7-G was methylated to chrysoeriol-7-glucuronide and diosmetin-7-glucuronide by COMTs. 2) Alternatively, luteolin was methylated to chrysoeriol and diosmetin by COMTs, and then chrysoeriol and diosmetin were glucuronidated by UGTs to their respective glucuronides. The methylation rate of luteolin was significantly increased by the absence of glucuronidation, whereas the glucuronidation rate was increased by the absence of methylation, but to a lesser extent. In conclusion, two pathways mediated by the interplay of UGTs and COMTs are probably involved in the metabolic disposition of luteolin. The glucuronidation and methylation of luteolin compensate for each other, although glucuronidation is the predominant pathway.

Regioselective Glucuronidation of Diosmetin and Chrysoeriol by the Interplay of Glucuronidation and Transport in UGT1A9-Overexpressing HeLa Cells

This study aimed to determine the reaction kinetics of the regioselective glucuronidation of diosmetin and chrysoeriol, two important methylated metabolites of luteolin, by human liver microsomes (HLMs) and uridine-5′-diphosphate glucuronosyltransferase (UGTs) enzymes. This study also investigated the effects of breast cancer resistance protein (BCRP) on the efflux of diosmetin and chrysoeriol glucuronides in HeLa cells overexpressing UGT1A9 (HeLa—UGT1A9). After incubation with HLMs in the presence of UDP-glucuronic acid, diosmetin and chrysoeriol gained two glucuronides each, and the OH—in each B ring of diosmetin and chrysoeriol was the preferable site for glucuronidation. Screening assays with 12 human expressed UGT enzymes and chemical-inhibition assays demonstrated that glucuronide formation was almost exclusively catalyzed by UGT1A1, UGT1A6, and UGT1A9. Importantly, in HeLa—UGT1A9, Ko143 significantly inhibited the efflux of diosmetin and chrysoeriol glucuronides and increased their intracellular levels in a dose-dependent manner. This observation suggested that BCRP-mediated excretion was the predominant pathway for diosmetin and chrysoeriol disposition. In conclusion, UGT1A1, UGT1A6, and UGT1A9 were the chief contributors to the regioselective glucuronidation of diosmetin and chrysoeriol in the liver. Moreover, cellular glucuronidation was significantly altered by inhibiting BCRP, revealing a notable interplay between glucuronidation and efflux transport. Diosmetin and chrysoeriol possibly have different effects on anti-cancer due to the difference of UGT isoforms in different cancer cells.