Identification and characterization of naturally occurring inhibitors against human carboxylesterase 2 in White Mulberry Root-bark

Design, synthesis, and biological evaluation studies of novel carboxylesterase 2 inhibitors for the treatment of irinotecan-induced delayed diarrhea

Human carboxylesterase 2 (hCES2A), one of the most important serine hydrolases distributed in the small intestine and colon, plays a crucial role in the hydrolysis of various prodrugs and esters. Accumulating evidence has demonstrated that the inhibition of hCES2A effectively alleviate the side effects induced by some hCES2A-substrate drugs, including delayed diarrhea caused by the anticancer drug irinotecan. Nonetheless, there is a scarcity of selective and effective inhibitors that are suitable for irinotecan-induced delayed diarrhea. Following screening of the in-house library, the lead compound 01 was identified with potent inhibition on hCES2A, which was further optimized to obtain LK-44 with potent inhibitory activity (IC50 = 5.02 ± 0.67 μM) and high selectivity on hCES2A. Molecular docking and molecular dynamics simulations indicated that LK-44 can formed stable hydrogen bonds with amino acids surrounding the active cavity of hCES2A. The results of inhibition kinetics studies unveiled that LK-44 inhibited hCES2A-mediated FD hydrolysis in a mixed inhibition manner, with a Ki value of 5.28 μM. Notably, LK-44 exhibited low toxicity towards HepG2 cells according to the MTT assay. Importantly, in vivo studies showed that LK-44 significantly reduced the side effects of irinotecan-induced diarrhea. These findings suggested that LK-44 is a potent inhibitor of hCES2A with high selectivity against hCES1A, which has potential as a lead compound for the development of more effective hCES2A inhibitors to mitigate irinotecan-induced delayed diarrhea.

Mulberry Diels-Alder-type adducts: isolation, structure, bioactivity, and synthesis

Mulberry Diels-Alder-type adducts (MDAAs) are unique phenolic natural products biosynthetically derived from the intermolecular [4 + 2]-cycloaddition of dienophiles (mainly chalcones) and dehydroprenylphenol dienes, which are exclusively distributed in moraceous plants. A total of 166 MDAAs with diverse skeletons have been isolated and identified since 1980. Structurally, the classic MDAAs characterized by the chalcone-skeleton dienophiles can be divided into eight groups (Types A - H), while others with non-chalcone dienophiles or some variations of classic MDAAs are non-classic MDAAs (Type I). These compounds have attracted significant attention of natural products and synthetic chemists due to their complex architectures, remarkable biological activities, and synthetic challenges. The present review provides a comprehensive summary of the structural properties, bioactivities, and syntheses of MDAAs. Cited references were collected between 1980 and 2021 from the SciFinder, Web of Science, and China National Knowledge Internet (CNKI).

Optical substrates for drug-metabolizing enzymes: Recent advances and future perspectives

Drug-metabolizing enzymes (DMEs), a diverse group of enzymes responsible for the metabolic elimination of drugs and other xenobiotics, have been recognized as the critical determinants to drug safety and efficacy. Deciphering and understanding the key roles of individual DMEs in drug metabolism and toxicity, as well as characterizing the interactions of central DMEs with xenobiotics require reliable, practical and highly specific tools for sensing the activities of these enzymes in biological systems. In the last few decades, the scientists have developed a variety of optical substrates for sensing human DMEs, parts of them have been successfully used for studying target enzyme(s) in tissue preparations and living systems. Herein, molecular design principals and recent advances in the development and applications of optical substrates for human DMEs have been reviewed systematically. Furthermore, the challenges and future perspectives in this field are also highlighted. The presented information offers a group of practical approaches and imaging tools for sensing DMEs activities in complex biological systems, which strongly facilitates high-throughput screening the modulators of target DMEs and studies on drug/herb‒drug interactions, as well as promotes the fundamental researches for exploring the relevance of DMEs to human diseases and drug treatment outcomes.

Recent Development of Flavonoids with Various Activities

Flavonoids, a series of compounds with C6-C3-C6 structure, mostly originate from plant metabolism. Flavonoids have shown beneficial effects on many aspects of human physiology and health. Recently, many flavonoids with various activities have been discovered, which has led to more and more studies focusing on their physiological and pharmacodynamic activities. The anti-cancer and anti-viral activities especially have attracted the attention of many researchers. Therefore, the discovery and development of flavonoids as anti-disease drugs has great potential and may make significant contribution to fighting diseases. This review focus on the discovery and development of flavonoids in medicinal chemistry in recent years.

Beta-Glucuronidase Inhibition by Constituents of Mulberry Bark

Intestinal bacterial β-glucuronidases, the key enzymes responsible for the hydrolysis of various glucuronides into free aglycone, have been recognized as key targets for treating various intestinal diseases. This study aimed to investigate the inhibitory effects and mechanisms of the Mulberry bark constituents on E. coli β-glucuronidase (EcGUS), the most abundant β-glucuronidases produced by intestinal bacteria. The results showed that the flavonoids isolated from Mulberry bark could strongly inhibit E. coli β-glucuronidase, with IC₅₀ values ranging from 1.12 µM to 10.63 µM, which were more potent than D-glucaric acid-1,4-lactone. Furthermore, the mode of inhibition of 5 flavonoids with strong E. coli β-glucuronidase inhibitory activity (IC₅₀ ≤ 5 µM) was carefully investigated by a set of kinetic assays and in silico analyses. The results demonstrated that these flavonoids were noncompetitive inhibitors against E. coli β-glucuronidase-catalyzed 4-nitrophenyl β-D-glucuronide hydrolysis, with Ki values of 0.97 µM, 2.71 µM, 3.74 µM, 3.35 µM, and 4.03 µM for morin (1: ), sanggenon C (2: ), kuwanon G (3: ), sanggenol A (4: ), and kuwanon C (5: ), respectively. Additionally, molecular docking simulations showed that all identified flavonoid-type E. coli β-glucuronidase inhibitors could be well-docked into E. coli β-glucuronidase at nonsubstrate binding sites, which were highly consistent with these agents' noncompetitive inhibition mode. Collectively, our findings demonstrated that the flavonoids in Mulberry bark displayed strong E. coli β-glucuronidase inhibition activity, suggesting that Mulberry bark might be a promising dietary supplement for ameliorating β-glucuronidase-mediated intestinal toxicity.

Carboxylesterase inhibitors from clinically available medicines and their impact on drug metabolism

Mammalian carboxylesterases (CES), the key members of the serine hydrolase superfamily, hydrolyze a wide range of endogenous substances and xenobiotics bearing ester or amide bond(s). In humans, most of identified CES are segregated into the CES1A and CES2A subfamilies. Strong inhibition on human CES (including hCES1A and hCES2A) may modulate pharmacokinetic profiles of CES-substrate drugs, thereby changing the pharmacological and toxicological responses of these drugs. This review covered recent advances in discovery of hCES inhibitors from clinically available medications, as well as their impact on CES-associated drug metabolism. Three comprehensive lists of hCES inhibitors deriving from clinically available medications including therapeutic drugs, pharmaceutical excipients and herbal medicines, alongside with their inhibition potentials and inhibition parameters, are summarized. Furthermore, the potential risks of hCES inhibitors to trigger drug/herb-drug interactions (DDIs/HDIs) and future concerns in this field are highlighted. Potent hCES inhibitors may trigger clinically relevant DDIs/HDIs, especially when these inhibitors are co-administrated with CES substrate-drugs with very narrow therapeutic windows. All data and knowledge presented here provide key information for the clinicians to assess the risks of clinically available hCES inhibitors on drug metabolism. In future, more practical and highly specific substrates for hCES1A/hCES2A should be developed and used for studies on CES-mediated DDIs/HDIs both in vitro and in vivo.

Discovery of Thai mangrove tetranortriterpenoids as agonists of human pregnane-X-receptor and inhibitors against human carboxylesterase 2

Human pregnane-X-receptor (hPXR) is considered to be the key target for the treatment of cholestasis and liver injury. Agonists of hPXR are potential drug leads. Potent and selective inhibitors of human carboxylesterase 2 (hCES2) could be utilized to alleviate the toxicity induced by ester drugs. In this work, fifteen new tetranortriterpenoids with structure diversity, named thaigranatins F-T (1-15), including four limonoids containing a C₁-O-C₂₉ bridge (1-4), four mexicanolides (5-8), three phragmalins (9-11), two limonoids belonging to the small group of trichiliton A (12-13), and two apotirucallanes (14-15), were isolated from seeds of the Thai mangrove, Xylocarpus granatum. The structures of these compounds were established by high resolution-electrospray ionization mass spectroscopy, extensive NMR spectroscopic investigations, single-crystal X-ray diffraction analyses, and the comparison of experimental electronic circular dichroism spectra. Most notably, thaigranatins L (7) and P (11) exhibited agonistic effects on hPXR at the concentration of 10.0 μM and 10.0 nM, respectively, whereas thaigranatins J (5), M (8), and T (15) showed inhibitory activities against hCES2 with IC₅₀ values of 6.63, 11.35, and 5.05 μM, respectively. The 8α,30α-epoxy moiety of mexicanolide and the Δ^8,14 double bond of phragmalin are pivotal for agonistic effects of these limonoids on hPXR, whereas the 6-OAc group of mexicanolide is crucial for its inhibitory activity against hCES2. Additionally, the flexible C-17-side-chain with appropriate hydroxy groups is considered to be important for the inhibitory activity of apotirucallane against hCES2.

Bioactivity-Guided Discovery of Human Carboxylesterase Inhibitors from the Roots of Paeonia lactiflora

In a continuing search for potential inhibitors against human carboxylesterases 1A1 and 2A1 (hCES1A1 and hCES2A1), an EtOAc extract of the roots of Paeonia lactiflora showed strong hCES inhibition activity. Bioassay-guided fractionation led to the isolation of 26 terpenoids including 12 new ones (1-5, 7-12, and 26). Among these, sesquiterpenoids 1 and 6, monoterpenoids 10, 11, and 13-15, and triterpenoids 18-20, 22, and 24-26 contributed to the hCES2A1 inhibition, in the IC₅₀ range of 1.9-14.5 μM, while the pentacyclic triterpenoids 18-26 were responsible for the potent inhibitory activity against hCES1A1, with IC₅₀ values less than 5.0 μM. The structures of all the compounds were elucidated using MS and 1D and 2D NMR data, and the absolute configurations of the new compounds were resolved via specific rotation, experimental and calculated ECD spectra, and single-crystal X-ray diffraction analysis. The structure-activity relationship analysis highlighted that the free HO-3 group in the pentacyclic triterpenoids is crucial for their potent inhibitory activity against hCES1A1.

Natural Flavones from Morus alba against Methicillin-Resistant Staphylococcus aureus via Targeting the Proton Motive Force and Membrane Permeability

The emergence and rapid spread of methicillin-resistant Staphylococcus aureus (MRSA) critically requires alternative therapeutic options. New antibacterial drugs and strategies are urgently needed to combat MRSA-associated infections. Here, we investigated the antibacterial activity of flavones from Morus alba and the potential mode of action against MRSA. Kuwanon G, kuwanon H, mulberrin, and morusin displayed high efficiency in killing diverse MRSA isolates. On the basis of structure-activity analysis, the cyclohexene-phenyl ketones and isopentenyl groups were critical to increase the membrane permeability and to dissipate the proton motive force. Meanwhile, mechanistic studies further showed that kuwanon G displayed rapid bactericidal activity in vitrowith difficulty in developing drug resistance. Kuwanon G targeted phosphatidylglycerol and cardiolipin in the cytoplasmic membrane through the formation of hydrogen bonds and electrostatic interactions. Additionally, kuwanon G promoted wound healing in a mouse model of MRSA skin infection. In summary, these results indicate that flavones are promising lead compounds to treat MRSA-associated infections through disrupting the proton motive force and membrane permeability.

Potential Pharmacokinetic Herb-Drug Interactions: Have we Overlooked the Importance of Human Carboxylesterases 1 and 2?

Background:

Herbal products have grown steadily across the globe and have increasingly been incorporated into western medicine for healthcare aims, thereby causing potential pharmacokinetic Herb-drug Interactions (HDIs) through the inhibition or induction of drug-metabolizing enzymes and transporters. Human Carboxylesterases 1 (CES1) and 2 (CES2) metabolize endogenous and exogenous chemicals including many important therapeutic medications. The growing number of CES substrate drugs also underscores the importance of the enzymes. Herein, we summarized those potential inhibitors and inducers coming from herbal constituents toward CES1 and CES2. We also reviewed the reported HDI studies focusing on herbal products and therapeutic agents metabolized by CES1 or CES2.

Methods:

We searched in PubMed for manuscript published in English after Jan 1, 2000 combining terms “carboxylesterase 1”, “carboxylesterase 2”, “inhibitor”, “inducer”, “herb-drug interaction”, “inhibitory”, and “herbal supplement”. We also searched specific websites including FDA and EMA. The data of screened papers were analyzed and summarized.

Results:

The results showed that more than 50 natural inhibitors of CES1 or CES2, including phenolic chemicals, triterpenoids, and tanshinones were found from herbs, whereas only few inducers of CES1 and CES2 were reported. Systemic exposure to some commonly used drugs including oseltamivir, irinotecan, and clopidogrel were changed when they were co-administered with herb products such as goldenseal, black cohosh, ginger, St. John’s Wort, curcumin, and some Chinese compound formula in animals.

Conclusion:

Nonclinical and clinical studies on HDIs are warranted in the future to provide safety information toward better clinical outcomes for the combination of herbal products and conventional drugs.

Human carboxylesterases: a comprehensive review

Mammalian carboxylesterases (CEs) are key enzymes from the serine hydrolase superfamily. In the human body, two predominant carboxylesterases (CES1 and CES2) have been identified and extensively studied over the past decade. These two enzymes play crucial roles in the metabolism of a wide variety of endogenous esters, ester-containing drugs and environmental toxicants. The key roles of CES in both human health and xenobiotic metabolism arouse great interest in the discovery of potent CES modulators to regulate endobiotic metabolism or to improve the efficacy of ester drugs. This review covers the structural and catalytic features of CES, tissue distributions, biological functions, genetic polymorphisms, substrate specificities and inhibitor properties of CES1 and CES2, as well as the significance and recent progress on the discovery of CES modulators. The information presented here will help pharmacologists explore the relevance of CES to human diseases or to assign the contribution of certain CES in xenobiotic metabolism. It will also facilitate medicinal chemistry efforts to design prodrugs activated by a given CES isoform, or to develop potent and selective modulators of CES for potential biomedical applications.

Structure-Activity Relationships of Pentacyclic Triterpenoids as Potent and Selective Inhibitors against Human Carboxylesterase 1

Human carboxylesterase 1 (hCE1), one of the most important serine hydrolases distributed in liver and adipocytes, plays key roles in endobiotic homeostasis and xenobiotic metabolism. This study aimed to find potent and selective inhibitors against hCE1 from phytochemicals and their derivatives. To this end, a series of natural triterpenoids were collected and their inhibitory effects against human carboxylesterases (hCEs) were assayed using D-Luciferin methyl ester (DME) and 6,8-dichloro-9,9-dimethyl-7-oxo-7,9-dihydroacridin-2-yl benzoate (DDAB) as specific optical substrate for hCE1, and hCE2, respectively. Following screening of a series of natural triterpenoids, oleanolic acid (OA), and ursolic acid (UA) were found with strong inhibitory effects on hCE1 and relative high selectivity over hCE2. In order to get the highly selective and potent inhibitors of hCE1, a series of OA and UA derivatives were synthesized from OA and UA by chemical modifications including oxidation, reduction, esterification, and amidation. The inhibitory effects of these derivatives on hCEs were assayed and the structure-activity relationships of tested triterpenoids as hCE1 inhibitors were carefully investigated. The results demonstrated that the carbonyl group at the C-28 site is essential for hCE1 inhibition, the modifications of OA or UA at this site including esters, amides and alcohols are unbeneficial for hCE1 inhibition. In contrast, the structural modifications on OA and UA at other sites, such as converting the C-3 hydroxy group to 3-O-β-carboxypropionyl (compounds 20 and 22), led to a dramatically increase of the inhibitory effects against hCE1 and very high selectivity over hCE2. 3D-QSAR analysis of all tested triterpenoids including OA and UA derivatives provide new insights into the fine relationships linking between the inhibitory effects on hCE1 and the steric-electrostatic properties of triterpenoids. Furthermore, both inhibition kinetic analyses and docking simulations demonstrated that compound 22 was a potent competitive inhibitor against hCE1-mediated DME hydrolysis. All these findings are very helpful for medicinal chemists to design and develop highly selective and more potent hCE1 inhibitors for biomedical applications.

Comparative metabolism of DDAO benzoate in liver microsomes from various species

DDAB (6,8-dichloro-9,9-dimethyl-7-oxo-7,9-dihydroacridin-2-yl benzoate) is a newly developed near-infrared fluorescent probe for human carboxylesterase 2 (hCE2), exhibiting high specificity and good reactivity for real-time monitoring the enzymatic activities of hCE2 in complex biological systems. In order to explore the applicability of DDAB in commonly used animal species, the interspecies difference in DDAB hydrolysis was carefully investigated by using liver microsomes from human and five experimental animals including mouse, rat, dog, minipig and monkey. Metabolite profiling demonstrated that DDAB hydrolysis could be catalyzed by all tested liver microsomes from different animals but displayed significant difference in the reaction rate. Chemical inhibition assays demonstrated that carboxylesterases (CEs) were the major enzymes involved in DDAB hydrolysis in all tested liver microsomes, indicating that DDAB was a selective substrate of CEs in a variety of mammals. However, the differential effects of loperamide (LPA, a specific inhibitor against hCE2) on DDAB hydrolysis among various species were observed. The apparent kinetic parameters and the maximum intrinsic clearances (CL_max) for DDAB hydrolysis in liver microsomes from different animals were determined, and the order of CL_max values for the formation of DDAO was CyLM>MLM≈PLM>RLM>HLM≈DLM. These findings were helpful for the rational use of DDAB as an imaging tool for CE2 in different mammals, as well as for translational researches on the function of mammalian CEs and CE2-associated drug-drug interactions.