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.

[Construction and application of pharmacophore model of human carboxylesterase 2 inhibitors].

According to human carboxylesterase 2(hCE2) inhibitors reported in the literature, the pharmacophore model of hCE2 inhibitors was developed using HipHop module in Discovery Studio 2016. The optimized pharmacophore model, which was validated by test set, contained two hydrophobic, one hydrogen bond acceptor, and one aromatic ring features. Using the pharmacophore model established, 5 potential hCE2 inhibitors(CS-1,CS-2,CS-3,CS-6 and CS-8) were screened from 20 compounds isolated from the roots of Paeonia lactiflora, which were further confirmed in vitro, with the IC_(50) values of 5.04, 5.21, 5.95, 6.64 and 7.94 µmol·L~(-1), respectively. The results demonstrated that the pharmacophore model exerted excellent forecasting ability with high precision, which could be applied to screen novel hCE2 inhibitors from Chinese medicinal materials.

Chemoproteomics-Enabled De Novo Discovery of Photoswitchable Carboxylesterase Inhibitors for Optically Controlled Drug Metabolism.

Herein, we report arylazopyrazole ureas and sulfones as a novel class of photoswitchable serine hydrolase inhibitors and present a chemoproteomic platform for rapid discovery of optically controlled serine hydrolase targets in complex proteomes. Specifically, we identify highly potent and selective photoswitchable inhibitors of the drug-metabolizing enzymes carboxylesterases 1 and 2 and demonstrate their pharmacological application by optically controlling the metabolism of the immunosuppressant drug mycophenolate mofetil. Collectively, this proof-of-concept study provides a first example of photopharmacological tools to optically control drug metabolism by modulating the activity of a metabolizing enzyme. Our arylazopyrazole ureas and sulfones offer synthetically accessible scaffolds that can be expanded to identify specific photoswitchable inhibitors for other serine hydrolases, including lipases, peptidases, and proteases. Our chemoproteomic platform can be applied to other photoswitches and scaffolds to achieve optical control over diverse protein classes.

Investigation of the inhibitory effect of protostanes on human carboxylesterase 2 and their interaction: Inhibition kinetics and molecular stimulations.

Carboxylesterase 2 (CES 2), plays a pivotal role in endobiotic homeostasis and xenobiotic metabolism. Protostanes, the major constituents of the genus Alisma, display a series of pharmacological activities. Despite the extensive studies of pharmacological activities, the investigation on inhibitory effects of protostanes against CES 2 is rarely reported. In this study, the inhibitory activities of a library of protostanes (1-25) against human CES 2 were investigated for the first time, using 6,8-dichloro-9,9-dimethyl-7-oxo-7,9-dihydroacridin-2-yl benzoate (DDAB) as the specific fluorescent probe for human CES 2. Compounds 1, 2, 7, 8, 12, 13, 18, 19, and 25 showed strong inhibitory effects towards CES 2. For the most potent compounds 1, 7, 13, and 25, the inhibition kinetics were further investigated, and these four protostanes were all uncompetitive inhibitors against human CES 2 with the inhibition constant (Ki) values ranging from 0.89 µM to 2.83 µM. In addition, molecular docking and molecular dynamics stimulation were employed to analyze the potential interactions between these protostanes and CES 2, and amino acid residue Gln422 was identified to play a crucial role in the strong inhibition of protostanes towards CES 2.

Discovery of hCES2A inhibitors from Glycyrrhiza inflata via combination of docking-based virtual screening and fluorescence-based inhibition assays.

Human carboxylesterase 2 (hCES2A) is a key target to ameliorate the intestinal toxicity triggered by irinotecan that causes severe diarrhea in 50%-80% of patients receiving this anticancer agent. Herbal medicines are frequently used for the prevention and treatment of the intestinal toxicity of irinotecan, but it is very hard to find strong hCES2A inhibitors from herbal medicines in an efficient way. Herein, an integrated strategy via combination of chemical profiling, docking-based virtual screening and fluorescence-based high-throughput inhibitor screening assays was utilized. Following the screening of a total of 73 herbal products, licorice (the dried root of Glycyrrhiza species) was found with the most potent hCES2A inhibition activity. Further investigation revealed that the chalcones and several flavonols in licorice displayed strong hCES2A inhibition activities, while isoliquiritigenin, echinatin, naringenin, gancaonin I and glycycoumarin exhibited moderate inhibition of hCES2A. Inhibition kinetic analysis demonstrated that licochalcone A, licochalcone C, licochalcone D and isolicoflavonol potently inhibited hCES2A-mediated fluorescein diacetate hydrolysis in a reversible and mixed inhibition manner, with Ki values less than 1.0 µM. Further investigations demonstrated that licochalcone C, the most potent hCES2A inhibitor identified from licorice, dose-dependently inhibited intracellular hCES2A in living HepG2 cells. In summary, this study proposed an integrated strategy to find hCES2A inhibitors from herbal medicines, and our findings suggested that the chalcones and isolicoflavonol in licorice were the key ingredients responsible for hCES2A inhibition, which would be very helpful to develop new herbal remedies or drugs for ameliorating hCES2A-associated drug toxicity.

Limonoids with diverse structures of rings-A,B from the Thai mangrove, Xylocarpus moluccensis.

Nine new limonoids, named thaixylomolins S-Z (1-8) and 2-O-acetylthaixylomolin Z (9), were isolated from seeds of the mangrove, Xylocarpus moluccensis, collected in the mangrove swamp of Trang Province, Thailand. Thaixylomolin S (1) is the fourth member of the khayalactone class of limonoids containing a hexahydro-2H-2,5- propanocyclopenta[b]furan motif. Thaixylomolins T-Y (2-7) are structurally diverse mexicanolides; whereas thaixylomolin Z (8) and 2-O-acetylthaixylomolin Z (9) are phragmalin 8,9,30-orthoesters. The structures of these compounds were established by HRESIMS and extensive 1D and 2D NMR investigations. The absolute configurations of thaixylomolins S (1), U (3), and Z (8) were unambiguously established by single-crystal X-ray diffraction analyses, conducted with Cu Kα radiation; whereas that of 2-O-acetylthaixylomolin Z (9) was determined to be the same as that of thaixylomolin Z (8) by the accurate fit of their experimental electronic circular dichroism spectra. Thaixylomolin S (1), featuring the presence of a 30-(2'-methyl)butyryloxy group, is the first limonoid of the khayalactone class, whose constitution and absolute configuration are unequivocally determined by X-ray crystallography. The inhibitory activities of all the compounds, except for the epimers 4, were assayed against human carboxylesterase 2. All the tested compounds exhibited inhibition rates in the range of 16-65% at the concentration of 100.0 µM.

Pentacyclic triterpenoid acids in Styrax as potent and highly specific inhibitors against human carboxylesterase 1A.

Human carboxylesterase 1A1 (hCES1A) is a promising target for the treatment of hyperlipidemia and obesity-associated metabolic diseases. To date, the highly specific and efficacious hCES1A inhibitors are rarely reported. This study aims to find potent and highly specific hCES1A inhibitors from herbs, and to investigate their inhibitory mechanisms. Following large-scale screening of herbal products, Styrax was found to have the most potent hCES1A inhibition activity. After that, a practical bioactivity-guided fractionation coupling with a chemical profiling strategy was used to identify the fractions from Styrax with strong hCES1A inhibition activity and the major constituents in these bioactive fractions were characterized by LC-TOF-MS/MS. The results demonstrated that seven pentacyclic triterpenoid acids (PTAs) in two bioactive fractions from Styrax potently inhibit hCES1A, with IC50 values ranging from 41 nM to 478 nM. Among all the identified PTAs, epibetulinic acid showed the most potent inhibition activity and excellent specificity towards hCES1A. Both inhibition kinetic analyses and in silico analysis suggested that epibetulinic acid potently inhibited hCES1A in a mixed inhibition manner. Collectively, our findings demonstrate that some PTAs in Styrax are potent and highly specific inhibitors of hCES1A and these constituents can be used as promising lead compounds for the development of more efficacious hCES1A inhibitors.

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.

Construction and application of a high-content analysis for identifying human carboxylesterase 2 inhibitors in living cell system.

Human carboxylesterase 2 (hCE2), one of the most principal drug-metabolizing enzymes, catalyzes the hydrolysis of a variety of endogenous esters, anticancer agents, and environmental toxicants. The significant roles of hCE2 in both endobiotic and xenobiotic metabolism sparked great interest in the discovery and development of efficacious and selective inhibitors. However, the safe and effective inhibitors of hCE2 are scarce, due to the lack of efficient screening and evaluation systems for complex biological systems. To offer a solution to this problem, a high-content analysis (HCA)-based cell imaging and multiparametric assay method was constructed for evaluating the inhibitory effect and safety of hCE2 inhibitors in living cell system. In this study, we first established a cell imaging-based method for identifying hCE2 inhibitors at the living cell level with hCE2 fluorescent probe NCEN. Meanwhile, two nuclear probes, Hoechst 33342 and PI, were integrated to evaluate the potential cytotoxicity of compounds simultaneously. Then, the accuracy of the HCA-based method was verified by the LC-FD-based method with a positive inhibitor BNPP, and the results showed that the HCA-based method exhibited excellent precision, robustness, and reliability. Finally, the newly established HCA-based multiparametric assay panel was successfully applied to re-evaluate a series of reported hCE2 inhibitors in living cells. In summary, the HCA-based multiparametric method could serve as an efficient tool for the accuracy measurement inhibitory effect and cytotoxicity of compounds against hCE2 in living cell system. Graphical abstract.

Demethylbellidifolin isolated from Swertia bimaculate against human carboxylesterase 2: Kinetics and interaction mechanism merged with docking simulations.

In this study, forty-nine kinds of traditional Chinese medicines (TCMs) were evaluated for their inhibitory activities against human carboxylesterase 2 (HCE 2) using a human liver microsome (HLM) system. Swertia bimaculata showed significant inhibition on HCE 2 at 10 µg/mL among forty-nine kinds of TCMs. The extract of Swertia bimaculata was separated by preparative HPLC to afford demethylbellidifolin (1) identified by MS, 1H NMR, and 13C NMR spectra. Demethylbellidifolin (1) was assayed for its inhibitory HCE 2 effect by HCE 2-mediated DDAB hydrolysis, and its potential IC50 value was 3.12 ±â€¯0.64 µM. Demethylbellidifolin (1) was assigned as a mixed-type competitive inhibitor with the inhibiton constant Ki value of 6.87 µM by Lineweaver-Burk and slope plots. Living cell imaging was conducted to corroborate its inhibitory HCE 2 activity. Molecular docking indicated potential interactions of demethylbellidifolin (1) with HCE 2 through two hydrogen bonds of the C-3 and C-5 hydroxy groups with amino acid residues Glu227 and Ser228 in the catalytic cavity, respectively.

A natural inhibitor from Alisma orientale against human carboxylesterase 2: Kinetics, circular dichroism spectroscopic analysis, and docking simulation.

As a part of our searching for natural human carboxylesterase 2 (human CES 2) inhibitors from traditional Chinese medicine, we found that the extract of Alisma orientale significantly inhibited human CES 2 in vitro. The investigation on A. orientale led to the isolation of a new protostane-type triterpenoid alismanin I (1). Its structure was determined according to HRESIMS, 1D and 2D NMR spectra. Alismanin I (1) displayed significantly inhibitory activity against human CES 2 with IC50 value of 1.31 ±â€¯0.09 µM assayed by human CES 2-mediated DDAB hydrolysis. According to its inhibition kinetic result, compound 1 was a noncompetitive type inhibitor, and its Ki was 3.65 µM. Its inhibitory effect was confirmed in living cell level through a visual manner. The potential interaction mechanism of compound 1 with human CES 2 was also analyzed by circular dichroism (CD) spectrum and molecular docking.

Evaluation of the inhibition of human carboxylesterases (CESs) by the active ingredients from Schisandra chinensis.

1. Schisandra chinensis, also called wuweizi in Chinese, is the fruit of Schisandra chinensis (Turcz.) Baill., and has been officially utilized as an astringent tonic for more than two thousand years in China. This study aims to evaluate the inhibition of carboxylesterases (CESs) by the major ingredients isolated from Schisandra chinensis, including Anwuligan, Schisandrol B, Schisanhenol, deoxyschizandrin, and Schisandrin B. 2. In vitro human liver microsomes (HLMs)-catalyzed hydrolysis of 2-(2-Benzoyl-3-methoxyphenyl) benzothiazole (BMBT) and fluorescein diacetate (FD) was employed as the probe reaction for CES1 and CES2, respectively. Initial screening, inhibition kinetics determination (inhibition type and parameters (Ki)), and in silico docking method were carried out. 3. Schisandrin B showed strong inhibition on the activity of CES1, and the activity of CES2 was strongly inhibited by Anwuligan and Schisandrin B. Schisandrin B exhibited noncompetitive inhibition towards CES1 and CES2. Anwuligan showed competitive inhibition towards CES2. The inhibition kinetic parameters (Ki) were calculated to be 29.8, 0.6, and 8.1 uM for the inhibition of Schisandrin B on CES1, Anwuligan on CES2, and Schisandrin B on CES2. In silico docking showed that hydrogen bonds and hydrophobic interactions contributed to the inhibition of Schisandrin B on CES1, Anwuligan on CES2, and Schisandrin B on CES2. All these information will be helpful for understanding the adverse effects of Schisandra chinensis due to the inhibition of CESs-catalyzed metabolism.

Phytochemical constituents from Uncaria rhynchophylla in human carboxylesterase 2 inhibition: Kinetics and interaction mechanism merged with docking simulations.

BACKGROUND: Carboxylesterases (CEs) belong to the serine hydrolase family, and are in charge of hydrolyzing chemicals with carboxylic acid ester and amide functional groups via Ser-His-Glu. Uncaria rhynchophylla (Miq.) Miq. ex Havil. is a famous traditional Chinese medicine used in managing hyperpyrexia, epilepsy, preeclampsia, and hypertension in China. HYPOTHESIS/PURPOSE: To discover the potential natural human carboxylesterase 2 (hCE 2) inhibitors from U. rhynchophylla. METHODS: Compounds were obtained from the hooks of U. rhynchophylla by silica gel and preparative HPLC. Their structures were elucidated by using HRESIMS, 1D and 2D NMR spectra. Their inhibitory activeties and inhibition kinetics against hCE 2 were assayed by the fluorescent probe, and potential mechanisms were also investigated by molecular docking. RESULTS: Twenty-three compounds, including a new phenolic acid uncariarhyine A (1), eight known triterpenoids (2-9), and ten known aromatic derivatives (10, 13-16, and 19-23), were isolated from U. rhynchophylla. Compounds 1-5, 7, 9, and 15 showed significant inhibitory activities against hCE 2 with IC50 values from 4.01  ±â€¯0.61 µM to 18.60 ±â€¯0.21 µM, and their inhibition kinetic analysis results revealed that compounds 1, 5, 9, and 15 were non-competitive; compounds 3 and 4 were mixed-type, and compounds 2 and 7 were uncompetitive. Molecular docking studies indicated inhibition mechanisms of compounds 1-5, 7, 9, and 15 against hCE 2. CONCLUSION: Our present findings highlight potential natural hCE 2 inhibitors from U. rhynchophylla.

Effects of Panax Notoginseng Saponins on Esterases Responsible for Aspirin Hydrolysis In Vitro.

Herb⁻drug interactions strongly challenge the clinical combined application of herbs and drugs. Herbal products consist of complex pharmacological-active ingredients and perturb the activity of drug-metabolizing enzymes. Panax notoginseng saponins (PNS)-based drugs are often combined with aspirin in vascular disease treatment in China. PNS was found to exhibit inhibitory effects on aspirin hydrolysis using Caco-2 cell monolayers. In the present study, a total of 22 components of PNS were separated and identified by UPLC-MS/MS. Using highly selective probe substrate analysis, PNS exerted robust inhibitory potency on human carboxylesterase 2 (hCE2), while had a minor influence on hCE1, butyrylcholinesterase (BChE) and paraoxonase (PON). These effects were also verified through molecular docking analysis. PNS showed a concentration-dependent inhibitory effect on hydrolytic activity of aspirin in HepaRG cells. The protein level of hCE2 in HepaRG cells was suppressed after PNS treatment, while the level of BChE or PON1 in the extracellular matrix were elevated after PNS treatment. Insignificant effect was observed on the mRNA expression of the esterases. These findings are important to understand the underlying efficacy and safety of co-administration of PNS and aspirin in clinical practice.

The Study of Isolated Alkane Compounds and Crude Extracts From Sphagneticola trilobata (Asterales: Asteraceae) as a Candidate Botanical Insecticide for Lepidopteran Larvae.

The antifeedant and contact toxicity of Sphagneticola trilobata L. (Asterales: Asteraceae) extracts and isolated alkane compounds were investigated. Leaves of S. trilobata were sequentially extracted with hexane, dichloromethane, ethyl acetate, and methanol. Each extract and the compounds isolated were evaluated against the third instars of Spodoptera litura (F.) (Lepidoptera: Noctuidae), Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), and Plutella xylostella L. (Lepidoptera: Yponomeutidae). Ethyl acetate extract and isolated alkanes were feeding deterrents as well as contact toxins against all the three species evaluated (FI50 ~ 0.27-2.34 mg/ml; LD50 ~ 0.88-4.2 µg/larvae for ethyl acetate extract, and FI50 ~ 0.06-4.35 mg/ml; LD50 ~ 0.72-3.54 Ethyl acetate extract for isolated alkane). Impact on detoxifying enzymes was variable. The ethyl acetate crude extract reduced carboxylesterase activity in S. litura and P. xylostella while in S. exigua the enzyme was induced. In contrast, glutathione-S-transferase activity was induced in S. exigua but no significant difference in P. xylostella and S.litura was observed. Our results suggest that the S. trilobata extracts have multiple biological activities that contribute to the toxicity in lepidopterans. Variable enzyme responses to the products evaluated in different lepidopteran species also confirm that some species-specific inductions do occur, suggesting the possibility of resistance development in the future, which cannot be summarily ignored. However, for this detailed biochemical studies are required. Multiple bioefficacies of S. trilobata makes it a potential botanical for further exploitation on larger scale so that field potential can be established in any integrated pest management (IPM) system.

Bysspectin A, an unusual octaketide dimer and the precursor derivatives from the endophytic fungus Byssochlamys spectabilis IMM0002 and their biological activities.

Bysspectin A (1), a polyketide-derived octaketide dimer with a novel carbon skeleton, and two new precursor derivatives, bysspectins B and C (2 and 3), were obtained from an organic extract of the endophytic fungus Byssochlamys spectabilis that had been isolated from a leaf tissue of the traditional Chinese medicinal plant Edgeworthia chrysantha, together with a known octaketide, paecilocin A (4). Their structures were determined by HRMS, 1D and 2D NMR spectroscopic analysis. A plausible route for their biosynthetic pathway is proposed. Compounds 1-3 were tested for their antimicrobial activities. Only compound 3 was weakly active against Escherichia coli and Staphyloccocus aureus with MIC values of 32 and 64 µg/mL, respectively. Further, the inhibitory effects on human carboxylesterases (hCE1, hCE2) of compounds 1 and 4 were evaluated. The results demonstrated that bysspectin A (1) was a novel and highly selective inhibitor against hCE2 with the IC50 value of 2.01 µM. Docking simulation also demonstrated that active compound 1 created interaction with the Ser-288 (the catalytic amino-acid in the catalytic cavity) of hCE2 via hydrogen bonding, revealing its highly selective inhibition toward hCE2.

Role of geraniol against lead acetate-mediated hepatic damage and their interaction with liver carboxylesterase activity in rats.

In this study, the effect of geraniol (50 mg/kg for 30 d), a natural antioxidant and repellent/antifeedant monoterpene, in a rat model of lead acetate-induced (500 ppm for 30 d) liver damage was evaluated. Hepatic malondialdehyde increased in the lead acetate group. Reduced glutathione unchanged, but glutathione S-transferase, glutathione reductase, as well as carboxylesterase activities decreased in geraniol, lead acetate and geraniol + lead acetate groups. 8-OhDG immunoreactivity, mononuclear cell infiltrations and hepatic lead concentration were lower in the geraniol + lead acetate group than the lead acetate group. Serum aspartate aminotransferase and alanine aminotransferase activities increased in the Pb acetate group. In conclusion, lead acetate causes oxidative and toxic damage in the liver and this effect can reduce with geraniol treatment. However, we first observed that lead acetate, as well as geraniol, can affect liver carboxylesterase activity.

Phenolic glycosides and monoterpenoids from the roots of Euphorbia ebracteolata and their bioactivities.

The bioactive substance investigation of Euphorbia ebracteolata obtained 17 compounds by various chromatographic techniques. Their structures were elucidated using widely spectroscopic data, including ESI-MS, HRESI-MS, CD, 1D- and 2D-NMR, which gave 5 new phenolic glucosides and 4 new monoterpenoids. The phenolic glucosides and monoterpenoids showed the inhibitory effect against the human carboxylesterase-2 (hCE-2) using a fluorescence bioassay in vitro, with the strongest inhibitor compound 4 (IC50 7.17µM). The antioxidant effects of these isolated compounds were evaluated using a DPPH scavenging assay. All of the phenolic acids displayed the DPPH scavenging effect, especially that eight compounds have better effect than vitamin C, with the IC50 values ranging from 4.52 to 7.52µM. Additionally, compounds 1-17 showed no cytotoxic effect against five human cancer cell lines by MTT assay.

Synthesis, molecular docking, and biological activity of polyfluoroalkyl dihydroazolo[5,1-c][1,2,4]triazines as selective carboxylesterase inhibitors.

To search for effective and selective inhibitors of carboxylesterase (CaE), a series of 7-hydroxy-7-polyfluoroalkyl-4,7-dihydroazolo[5,1-c][1,2,4]triazines has been synthesized. Their inhibitory activity against acetylcholinesterase, butyrylcholinesterase, and CaE were investigated using the methods of enzyme kinetics and molecular docking. It was shown that the tested compounds are reversible selective CaE inhibitors of mixed type. Elongation of the polyfluoroalkyl substituent and the presence of an ester, preferably the ethoxycarbonyl group, enhance inhibitory activity toward CaE. Furthermore, the compounds with a tetrazole ring are more active against CaE than their triazole analogues. The obtained kinetic data are well explained by the results of molecular docking, according to which there is a similar orientation of triazolo- and tetrazolotriazines in the active site of CaE and the opposite one for pyrazolotriazines. In the 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) assay, all of the studied tetrazolotriazines and some pyrazolotriazines demonstrated good antiradical activity comparable with a standard antioxidant, Trolox. The leading compounds were nonafluorobutyl substituted tetrazolo- and 7-phenylpyrazolotriazines, which possess effective and selective CaE inhibitory activity as well as additional useful radical-scavenging properties.

Recent progress in the discovery of natural inhibitors against human carboxylesterases.

Mammalian carboxylesterases (CEs) are important serine hydrolases catalyzing the hydrolysis of ester- or amide-containing compounds into the corresponding alcohols and carboxylic acids. In human, two primary carboxylesterases including hCE1 and hCE2 have been identified and extensively studied in the past decade. hCE1 is known to play crucial roles in the metabolism of a wide variety of endogenous esters, clinical drugs and insecticides, while hCE2 plays a key role in the metabolic activation of anticancer agents including irinotecan and capecitabine. The key roles of hCEs in both human health and xenobiotic metabolism arouse great interest in the discovery of potent and selective hCEs inhibitors to modulate endobiotic metabolism or to improve the outcomes of patients administrated with ester drugs. This review covers the significance and recent progress in the discovery of natural inhibitors against hCEs. The tools for screening and characterization of inhibitors against human CEs, including traditional LC-based approaches and the newly developed optical substrate-based assays, are summarized and discussed for the first time. Furthermore, the structural information and inhibitory capacities of all reported hCEs inhibitors including fatty acids, flavonoids, tanshinones and triterpenoids have been systematically summarized. All information and knowledge presented in this review will be very helpful for medicinal chemists to develop more potent and highly selective inhibitors against hCEs for potential biomedical applications.