A highly selective ratiometric fluorescent probe for in vitro monitoring and cellular imaging of human carboxylesterase 1

Design, synthesis, and evaluation of a carboxylesterase detection probe with therapeutic effects

In this study, a lysosomal targeting fluorescent probe recognition on CEs was designed and synthesized. The obtained probe BF2-cur-Mor demonstrated excellent selectivity, sensitivity, pH-independence, and enzyme affinity towards CEs within 5 min. BF2-cur-Mor could enable recognition of intracellular CEs and elucidate that the CEs content of different cancer cells follows the rule of HepG2 > HCT-116 > A549 > HeLa, and the CEs expression level of hepatoma cancer cells far exceeds that of normal hepatic cells, being in good agreement with the previous reports. The ability of BF2-cur-Mor to monitor CEs in vivo was confirmed by zebrafish experiment. BF2-cur-Mor exhibits some pharmacological activity in that it can induce apoptosis in hepatocellular carcinoma cells but is weaker in normal hepatocyte cells, being expected to be a potential "diagnostic and therapeutic integration" tool for the clinical diagnosis of CEs-related diseases.

Evaluation of the inhibitory effects of antigout drugs on human carboxylesterases in vitro

Gout is an immune-metabolic disease that frequently coexists with multiple comorbidities such as chronic kidney disease, cardiovascular disease and metabolic syndrome, therefore, it is often treated in combination with these complications. The present study aimed to evaluate the inhibitory effect of antigout drugs (allopurinol, febuxostat, topiroxostat, benzbromarone, lesinurad and probenecid) on the activity of the crucial phase I drug-metabolizing enzymes, carboxylesterases (CESs). 2-(2-benzoyl-3-methoxyphenyl) benzothiazole (BMBT) and fluorescein diacetate (FD) were utilized as the probe reactions to determine the activity of CES1 and CES2, respectively, through in vitro culturing with human liver microsomes. Benzbromarone and lesinurad exhibited strong inhibition towards CESs with Ki values of 2.16 and 5.15 μM for benzbromarone towards CES1 and CES2, respectively, and 2.94 μM for lesinurad towards CES2. In vitro-in vivo extrapolation (IVIVE) indicated that benzbromarone and lesinurad might disturb the metabolic hydrolysis of clinical drugs in vivo by inhibiting CESs. In silico docking showed that hydrogen bonds and hydrophobic interactions contributed to the intermolecular interactions of antigout drugs on CESs. Therefore, vigilant monitoring of potential drug-drug interactions (DDIs) is imperative when co-administering antigout drugs in clinical practice.

An ultrasensitive and selective near-infrared fluorescent probe for tracking carboxylesterases with large Stokes shift in living cells and mice

Carboxylesterases (CEs) play great role in CEs-related diseases and drug metabolism. Selectively monitoring its activity is important to explore its role in CEs-related diseases and drug combination. Herein, a new "turn-on" near-infrared (NIR) fluorescent probe (CHY-1) was reported with large Stokes shift (145 nm) for CEs detection. Dicyanoisophorone-based derivative was chosen as NIR fluorophore and 4-bromobutyrate was the identifying group. What's more, CHY-1 exhibited ultra-sensitivity (LOD ∼ 9.2 × 10-5 U/mL), high selectivity against Acetylcholinesterase (AChE), Butyrylcholinesterase (BChE) and Chymotrypsin for CEs fluorescence detection under physiological pH and temperature. Furthermore, CHY-1 showed little effect on cell viability at high concentration and featured good optical imaging character for the slight change of CEs activity induced by 5-Fu (5-Fluorouridine, anti-tumor drug) and CEs inhibitor in living cells. Moreover, CHY-1 was also used to detect the activity and distribution of CEs in mice. Taken together, CHY-1 had widely applicable value in the diagnosis of CEs-related diseases and drug combination.

New Near-Infrared Fluorescence Imaging Platform with Large Stokes Shift for Carboxylesterase 2 Detection in Thyroid Cancer and Inflammatory Diseases Diagnosis

Development of new near-infrared fluorophores is one of the eternal themes in the field of biosensing and biological imaging. In this work, we constructed a novel fluorophore platform MOR by replacing methylindole of hemicyanine fluorophore (CyR) with benzoxazole to acquire better fluorescence characteristics. Based on the platform, a near infrared (NIR) fluorescent probe MOR-CES2 was synthesized for the specific "off-on" response to carboxylesterase 2 (CES2). The probe exhibited excellent properties including near-infrared emission (735 nm), large Stokes shift (105 nm), high sensitivity (LOD, 0.3 ng/mL), and rapid response (15 min). The successful application of MOR-CES2 in biological imaging of CES2 in mice with thyroid cancer and inflammatory bowel disease demonstrated that the probe could identify cancer cells and tissues and sensitively respond to inflammation. The results proved the potency of MOR-CES2 as an efficient imaging tool to assist in the surgical resection of CES2-related tumors.

Inhibitory effects of phthalate esters (PAEs) and phthalate monoesters towards human carboxylesterases (CESs)

Phthalate esters (PAEs), accompanied by phthalate monoesters as hydrolysis metabolites in humans, have been widely used as plasticizers and exhibited disruptive effects on the endocrine and metabolic systems. The present study aims to investigate the inhibition behavior of PAEs and phthalate monoesters on the activity of the important hydrolytic enzymes, carboxylesterases (CESs), to elucidate the toxicity mechanism from a new perspective. The results showed significant inhibition on CES1 and CES2 by most PAEs, but not by phthalate monoesters, above which the activity of CES1 was strongly inhibited by DCHP, DEHP, DiOP, DiPP, DNP, DPP and BBZP, with inhibition ratios exceeding 80%. Kinetic analyses and in vitro-in vivo extrapolation were conducted, revealing that PAEs have the potential to disrupt the metabolism of endogenous substances catalyzed by CES1 in vivo. Molecular docking results revealed that hydrogen bonds and hydrophobic contacts formed by ester bonds contributed to the interaction of PAEs towards CES1. These findings will be beneficial for understanding the adverse effect of PAEs and phthalate monoesters.

An endoplasmic reticulum-specific ratiometric fluorescent probe for imaging esterase in living cells

Esterase is primarily distributed in the endoplasmic reticulum (ER) and often overexpressed in cancer cells. Therefore, the detection of esterase in ER is significant for monitoring the metabolic process of various esters and evaluating the efficacy of chemotherapeutic prodrugs. However, only few fluorescent probes can detect esterase in the ER due to the lack of ER-specificity. More seriously, these probes are often limited by low pearson's colocalization coefficient and one single wavelength emission. To solve those problems, an ER-specific ratiometric fluorescent probe (ER-EST) is designed for detecting esterase in living cells. The ER-EST shows a ratiometric and red-shifted emission (125 nm) from 435 to 560 nm after hydrolysis by esterase. The fluorescence intensity ratio of ER-EST displays quantitative response to the esterase activity (0-0.5 U/mL) with low detection limit of 1.8 × 10^-4 U/mL. Importantly, the ER-EST with good biocompatibility and excellent ER-targeted ability was successfully employed to ratiometric image the endogenous endoplasmic reticulum esterase in living cells.

Ratiometric Two-Photon Near-Infrared Probe to Detect DPP IV in Human Plasma, Living Cells, Human Tissues, and Whole Organisms Using Zebrafish

DPP IV, otherwise known as CD26 lymphocyte T surface antigen, is a transmembrane glycoprotein also found in circulation in the blood. It plays an important role in several processes like glucose metabolism and T-cell stimulation. Moreover, it is overexpressed in renal, colon, prostate, and thyroid human carcinoma tissues. It can also serve as a diagnostic in patients with lysosomal storage diseases. The biological and clinical importance of having readouts for the activity of this enzyme, in physiological and disease conditions, has led us to design a near-infrared (NIR) fluorimetric probe that also has the characteristics of being ratiometric and excitable by two simultaneous NIR photons. The probe consists of assembling an enzyme recognition group (Gly-Pro) (Mentlein, 1999; Klemann et al., 2016) on the two-photon (TP) fluorophore (derivative of dicyanomethylene-4H-pyran, DCM-NH₂) disturbing its NIR characteristic internal charge transfer (ICT) emission spectrum. When the dipeptide group is released by the DPP IV-specific enzymatic action, the donor-acceptor DCM-NH₂ is restored, forming a system that shows high ratiometric fluorescence output. With this new probe, we have been able to detect, quickly and efficiently, the enzymatic activity of DPP IV in living cells, human tissues, and whole organisms, using zebrafish. In addition, due to the possibility of being excited by two photons, we can avoid the autofluorescence and subsequent photobleaching that the raw plasma has when it is excited by visible light, achieving detection of the activity of DPP IV in that medium without interference.

A fast fluorescent probe for tracing endoplasmic reticulum-located carboxylesterase in living cells

Carboxylesterase (CEs), mainly localized in endoplasmic reticulum (ER), are responsible for hydrolyzing compounds containing various ester bonds. They have been closely associated with drug metabolism and cellular homeostasis. Although some CE fluorescent probes have been developed, there are still a lack of probes that could target to the ER. Here, we developed a novel fluorescent probe CR with a specific ER anchor for monitoring CEs. In CR, p-toluenesulfonamide was chosen for precise ER targeting. A simple acetyl moiety was used as the CE response site and fluorescence modulation unit. During the spectral tests, CR displayed a fast response speed (within 10 s) towards CEs. In addition, it showed high sensitivity [limit of detection (LOD) = 5.1 × 10^-3 U/ml] and high selectivity with CEs. In biological imaging, probe CR could especially locate in the ER in HepG2 cells. After cells were treated with orilistat, CR succeeded in monitoring the changes in the CEs. Importantly, CR also had the ability to trace the changes in CEs in a tunicamycin-induced ER stress model. Therefore, probe CR could be a powerful molecular tool for further investigating the functions of CEs in the ER.

Deciphering the species differences in CES1A-mediated hydrolytic metabolism by using a bioluminescence substrate

Carboxylesterases 1A (CES1A) is a key enzyme responsible for the hydrolytic metabolism of a great deal of endogenous and exogenous substrates bearing ester- or amide-bond(s). This study aimed to decipher the species difference in CES1A-mediated hydrolytic metabolism by using a newly developed bioluminescence CES1A sensor (termed NLMe) as the probe substrate, while the liver microsomes from six different mammalian species (human, cynomolgus monkey, dog, minipig, rat and mouse) were used as the enzyme sources. Metabolite profiling demonstrated that all tested liver microsomes from various species could catalyze NLMe hydrolysis, but significant difference in hydrolytic rate was observed. Kinetic plots of NLMe hydrolysis in liver microsomes from different species showed that the inherent clearance rates (C_lint) of NLMe in human liver microsomes (HLM), cynomolgus monkey liver microsomes (CyLM), and pig liver microsome (PLM) were comparable, while the C_lint values of NLMe in dog liver microsomes (DLM), mouse liver microsomes (MLM), and rat liver microsomes (RLM) were relatively small. Moreover, chemical inhibition assays showed that NLMe hydrolysis in all tested liver microsomes could be competently inhibited by BNPP (a potent broad-spectrum inhibitor of CES), but CUA (a selective inhibitor of human CES1A) only inhibited NLMe hydrolysis in human liver microsomes and dog liver microsomes. In summary, the species differences in CES1A-catalyzed NLMe hydrolysis were carefully investigated from the views of the similarities in metabolite profile, hydrolytic kinetics and inhibitor response. All these findings provide new insights into the species differences in CES1A-mediated hydrolytic metabolism and suggest that it is necessary for the pharmacologists to choose appropriate animal models to replace humans for evaluating the in vivo effects of CES1A inhibitors.

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.

Carboxylesterases and arylacetamide deacetylase comparison in human A549, H460, and H727 pulmonary cells

AIMS

Human carboxylesterases (CESs) and arylacetamide deacetylase (AADAC) are serine-esterase enzymes catalyzing the hydrolysis of many compounds containing esters, amides, thioesters, or acetyl groups. This study aimed to investigate the presence, kinetic parameters, and inhibition of CES1, CES2, and AADAC in A549, H460, and H727 pulmonary cells in both living cells and S9 fractions.

MATERIALS AND METHODS

The p-nitrophenyl acetate (pNPA) and 4-methylumbelliferyl acetate (4-MUA) were used as non-selective esterase substrates, whereas phenacetin as selective AADAC substrate. CESs activities were also investigated in living cells by cellular bioimaging using selective fluorescent probes.

KEY FINDINGS

AADAC gene was detected in A549 and H460 cells; nevertheless, arylesterase activity was not found in relative S9 fractions. Besides, CES1 and CES2 were expressed to a different extent by all lung cells, and enzymatic activities were quite overlapping each other. All enzymes exhibited a typical Michaelis-Menten saturation curve and, regarding 4-MUA, similar K_m values were found in both living cells and S9 fractions. Conversely, kinetic parameters relative to the pNPA hydrolysis by S9 fractions were significantly lower than those detected in living cells. Inhibition studies revealed that 4-MUA hydrolysis was inhibited by bis-p-nitrophenyl phosphate and phenylmethanesulfonyl fluoride more than loperamide; on the contrary, pNPA hydrolysis inhibition was limited with similar inhibition profiles being obtained in both living cells and S9 fractions. The presence of carboxylesterases was definitely confirmed by cellular bioimaging.

SIGNIFICANCE

These findings add information to esterase knowledge in pulmonary cells that could be used as in vitro models for toxicological and pharmacological studies.

Lysosome targeting metal-organic framework probe LysFP@ZIF-8 for highly sensitive quantification of carboxylesterase 1 and organophosphates in living cells

Herein, a lysosomal targeting LysFP@ZIF-8 metal-organic framework (MOF) was fabricated using fluorescent protein chromophore-based probe (LysFP) for selectively detection of carboxylesterase 1 (CES1) in living cells. Unlike the regular small molecule fluorescent probes, LysFP@ZIF-8 showed wide range pH tolerabiligy, high selectivity and sensitivity to CES1 in bio-samples, and was successfully applied to achieve the visual monitoring of CES1 activity in living cells. Low detection limit and high fluorescence quantum yield was calculated as 79 ng/mL and 0.76 for LysFP@ZIF-8, respectively. Furthermore, LysFP@ZIF-8 can also serve as a fluorescence indicator of organophosphates pesticide exposure in the way of hydrolyzing the carboxylic acid ester group in LysFP. This type of probe can inspire the development of fluorescent tools for further explore many pathological processes.

Advances in Activity-Based Sensing Probes for Isoform-Selective Imaging of Enzymatic Activity

Until recently, there were no generalizable methods for assessing the effects of post-translational regulation on enzymatic activity. Activity-based sensing (ABS) has emerged as a powerful approach for monitoring small-molecule and enzyme activities within living systems. Initial examples of ABS were applied for measuring general enzymatic activity; however, a recent focus has been placed on increasing the selectivity to monitor a single enzyme or isoform. The highest degree of selectivity is required for differentiating between isoforms, where the targets display significant structural similarities as a result of a gene duplication or alternative splicing. This Minireview highlights key examples of small-molecule isoform-selective probes with a focus on the relevance of isoform differentiation, design strategies to achieve selectivity, and applications in basic biology or in the clinic.

Construction of a red emission fluorescent protein chromophore-based probe for detection of carboxylesterase 1 and carbamate pesticide in culture cells

Designing fluorescent probe for detecting carboxylesterase 1 is remains challenging. Herein, a red emission human carboxylesterase 1 (CES1) probe (CAE-FP) was synthesized based on fluorescent protein chromophore. Probe CAE-FP can specific detect human CES1 with high selectively. The fluorescence quantum yield was calucated as 0.19. The carboxylic acid ester in CAE-FP could be easily hydrolyzed by CES1 under physiological conditions, and this process could induce the obvious fluorescence signal in red emission region. The detection limit of CES1 was calculated as 84.5 ng/mL. Due to the biological detoxification mechanism of carboxylesterase and the obvious inhibitory effect of pesticides on its activity, CAE-FP was applied to detect carbamate pesticide and have achieved good application results. Since fluorescent protein chromophore has excellent biocompatibility, probe CAE-FP with good cell membrane permeable and was successfully applied to monitor the real activities of CES1 in living cells. In summary, this is one of the few reported fluorescent probes that can specific detect the real-time activity of CES1 in biological samples. Besides, we first applied the fluorescent protein chromophore to construct the specific target enzyme probe. This work would contribute to further investigate CES1-associated physiological and pathological processe.

Macrophage imaging and subset analysis using single-cell RNA sequencing

Macrophages have been associated with drug response and resistance in diverse settings, thus raising the possibility of using macrophage imaging as a companion diagnostic to inform personalized patient treatment strategies. Nanoparticle-based contrast agents are especially promising because they efficiently deliver fluorescent, magnetic, and/or radionuclide labels by leveraging the intrinsic capacity of macrophages to accumulate nanomaterials in their role as professional phagocytes. Unfortunately, current clinical imaging modalities are limited in their ability to quantify broad molecular programs that may explain (a) which particular cell subsets a given imaging agent is actually labeling, and (b) what mechanistic role those cells play in promoting drug response or resistance. Highly multiplexed single-cell approaches including single-cell RNA sequencing (scRNAseq) have emerged as resources to help answer these questions. In this review, we query recently published scRNAseq datasets to support companion macrophage imaging, with particular focus on using dextran-based nanoparticles to predict the action of anti-cancer nanotherapies and monoclonal antibodies.

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 (K_i) 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.

Per- and polyfluoroalkyl substances exert strong inhibition towards human carboxylesterases

PFASs are highly persistent in both natural and living environment, and pose a significant risk for wildlife and human beings. The present study was carried out to determine the inhibitory behaviours of fourteen PFASs on metabolic activity of two major isoforms of carboxylesterases (CES). The probe substrates 2-(2-benzoyl-3-methoxyphenyl) benzothiazole (BMBT) for CES1 and fluorescein diacetate (FD) for CES2 were utilized to determine the inhibitory potentials of PFASs on CES in vitro. The results demonstrated that perfluorododecanoic acid (PFDoA), perfluorotetradecanoic acid (PFTA) and perfluorooctadecanoic acid (PFOcDA) strongly inhibited CES1 and CES2. The half inhibition concentration (IC₅₀) value of PFDoA, PFTA and PFOcDA for CES1 inhibition was 10.6 μM, 13.4 μM and 12.6 μM, respectively. The IC₅₀ for the inhibition of PFDoA, PFTA and PFOcDA towards CES2 were calculated to be 9.56 μM, 17.2 μM and 8.73 μM, respectively. PFDoA, PFTA and PFOcDA exhibited noncompetitive inhibition towards both CES1 and CES2. The inhibition kinetics parameters (K_i) were 27.7 μM, 26.9 μM, 11.9 μM, 4.04 μM, 29.1 μM, 27.4 μM for PFDoA-CES1, PFTA-CES1, PFOcDA-CES1, PFDoA-CES2, PFTA-CES2, PFOcDA-CES2, respectively. In vitro-in vivo extrapolation (IVIVE) predicted that when the plasma concentrations of PFDoA, PFTA and PFOcDA were greater than 2.77 μM, 2.69 μM and 1.19 μM, respectively, it might interfere with the metabolic reaction catalyzed by CES1 in vivo; when the plasma concentrations of PFDoA, PFTA and PFOcDA were greater than 0.40 μM, 2.91 μM, 2.74 μM, it might interfere with the metabolic reaction catalyzed by CES2 in vivo. Molecular docking was used to explore the interactions between PFASs and CES. In conclusion, PFASs were found to cause inhibitory effects on CES in vitro, and this finding would provide an important experimental basis for further in vivo testing of PFASs focused on CES inhibition endpoints.

A magnetic SERS immunosensor for highly sensitive and selective detection of human carboxylesterase 1 in human serum samples

Hepatocellular carcinoma (HCC) is a common and lethal cancer. New serum markers for detecting HCC are urgently needed. Human carboxylesterase 1 (hCE1) is an important member of the serine hydrolase superfamily and is closely related to the occurrence of HCC. It can be used as a good serum marker for early diagnosis of HCC. Here, we developed a surface enhanced Raman scattering (SERS)- based magnetic immunosensor that specifically recognizes and detects trace amounts of hCE1 in human serum via a sandwich structure consisting of a SERS tags, magnetic supporting substrates, and target antigen (hCE1). The SERS tags are 4-mercaptobenzoic acid (4-MBA)-labeled AgNPs, and the SERS supporting substrates are composed of a raspberry-like morphology of Fe₃O₄@SiO₂@AgNPs magnetic nanocomposites surface-functionalized with a hCE1 antibody. The prepared SERS magnetic immunosensor exhibits excellent selectivity and extremely high sensitivity for hCE1 detection. The SERS signal and logarithm of hCE1 concentration presented a wide linear response range of 0.1 ng mL^-1 to 1.0 mg mL^-1, and the detection limit of hCE1 was 0.1 ng mL^-1. The results indicate that the immunosensor can be used for the rapid determination of hCE1 in human serum without a complicated sample pre-treatment. Furthermore, the immunosensor has good reproducibility and stability, and has a promising prospect for the quantitative detection of other tumor markers in early clinical diagnosis.

Measuring human carboxylesterase 2 activity in pancreatic cancer patient-derived xenografts using a ratiometric fluorescent chemosensor

We report the development of a ratiometric fluorescent chemosensor that selectively measures human carboxylesterase 2 (hCES2) activity with application in pancreatic cancer patient-derived xenografts.

Irinotecan-based therapy is a common treatment for pancreatic cancer. To elicit its anticancer activity, the drug requires first the hydrolysis action of the enzyme human carboxylesterase 2 (hCES2). It has been established that pancreatic cancer patients have various levels of hCES2, whereby patients having low levels respond poorer to Irinotecan than patients with higher levels, suggesting that hCES2 can be used to predict response. However, current methods that measure hCES2 activity are inaccurate, complex or lengthy, thus being incompatible for use in a clinical setting. Here, we developed a small molecule ratiometric fluorescent chemosensor that accurately measures hCES2 activity in a single-step within complex mixtures. Our chemosensor is highly selective for hCES2 over hCES1, cell permeable and can measure hCES2 activity in pancreatic cancer patient-derived xenografts. Given the simplicity, accuracy and tissue compatibility of our assay, we anticipate our chemosensor can be used to predict patient response to Irinotecan-based therapy.

Identification of novel agonists by high-throughput screening and molecular modelling of human constitutive androstane receptor isoform 3

Prediction of drug interactions, based on the induction of drug disposition, calls for the identification of chemicals, which activate xenosensing nuclear receptors. Constitutive androstane receptor (CAR) is one of the major human xenosensors; however, the constitutive activity of its reference variant CAR1 in immortalized cell lines complicates the identification of agonists. The exclusively ligand-dependent isoform CAR3 represents an obvious alternative for screening of CAR agonists. As CAR3 is even more abundant in human liver than CAR1, identification of its agonists is also of pharmacological value in its own right. We here established a cellular high-throughput screening assay for CAR3 to identify ligands of this isoform and to analyse its suitability for identifying CAR ligands in general. Proof-of-concept screening of 2054 drug-like compounds at 10 µM resulted in the identification of novel CAR3 agonists. The CAR3 assay proved to detect the previously described CAR1 ligands in the screened libraries. However, we failed to detect CAR3-selective compounds, as the four novel agonists, which were selected for further investigations, all proved to activate CAR1 in different cellular and in vitro assays. In primary human hepatocytes, the compounds preferentially induced the expression of the prototypical CAR target gene CYP2B6. Failure to identify CAR3-selective compounds was investigated by molecular modelling, which showed that the isoform-specific insertion of five amino acids did not impact on the ligand binding pocket but only on heterodimerization with retinoid X receptor. In conclusion, we demonstrate here the usability of CAR3 for screening compound libraries for the presence of CAR agonists.

Fluorogenic probes for disease-relevant enzymes

Traditional biochemical methods for enzyme detection are mainly based on antibody-based immunoassays, which lack the ability to monitor the spatiotemporal distribution and, in particular, the in situ activity of enzymes in live cells and in vivo. In this review, we comprehensively summarize recent progress that has been made in the development of small-molecule as well as material-based fluorogenic probes for sensitive detection of the activities of enzymes that are related to a number of human diseases. The principles utilized to design these probes as well as their applications are reviewed. Specific attention is given to fluorogenic probes that have been developed for analysis of the activities of enzymes including oxidases and reductases, those that act on biomacromolecules including DNAs, proteins/peptides/amino acids, carbohydrates and lipids, and those that are responsible for translational modifications. We envision that this review will serve as an ideal reference for practitioners as well as beginners in relevant research fields.

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.

Detection of carboxylesterase by a novel hydrosoluble near-infrared fluorescence probe

A novel hydrosoluble near-infrared fluorescence off–on probe has been developed for detecting carboxylesterase activity.

A red lysosome-targeted fluorescent probe for carboxylesterase detection and bioimaging

A red lysosome-targeting probe for carboxylesterase activity has been successfully applied in complex biological samples.

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.

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.

A fast responsive, highly selective and light-up fluorescent probe for the two-photon imaging of carboxylesterase in living cells

A fast responsive and two photon fluorescent probe (HCyNAc) for carboxylesterase (CaE) has been designed and used for the two-photon imaging of the endogenous CaE level in living HeLa cells under 800 nm NIR excitation.

Recent progresses in small-molecule enzymatic fluorescent probes for cancer imaging

An overview of recent advances in small-molecule enzymatic fluorescent probes for cancer imaging, including design strategies and cancer imaging applications.

Development of amide-based fluorescent probes for selective measurement of carboxylesterase 1 activity in tissue extracts

Carboxylesterases are well known for their role in the metabolism of xenobiotics. However, recent studies have also implicated carboxylesterases in regulating a number of physiological processes including metabolic homeostasis and macrophage development, underlying the need to quantify them individually. Unfortunately, current methods for selectively measuring the catalytic activity of individual carboxylesterases are not sufficiently sensitive to support many biological studies. In order to develop a more sensitive and selective method to measure the activity of human carboxylesterase 1 (hCE1), we generated and tested novel substrates with a fluorescent aminopyridine leaving group. hCE1 showed at least a 10-fold higher preference for the optimized substrate 4-MOMMP than the 13 other esterases tested. Because of the high stability of 4-MOMMP and its hydrolysis product, this substrate can be used to measure esterase activity over extended incubation periods yielding a low picogram (femtomol) limit of detection. This sensitivity is comparable to current ELISA methods; however, the new assay quantifies only the catalytically active enzyme facilitating direct correlation to biological processes. The method described herein may allow hCE1 activity to be used as a biomarker for predicting drug pharmacokinetics, early detection of hepatocellular carcinoma, and other disease states where the activity of hCE1 is altered.

Synthesis of precipitating chromogenic/fluorogenic β-glucosidase/β-galactosidase substrates by a new method and their application in the visual detection of foodborne pathogenic bacteria

We developed a new efficient method for the synthesis of important indoxyl glycoside substrates for β-glucosidase and β-galactosidase by using 1-acetylindol-3-ones as intermediates. This method was used to synthesise novel precipitating fluorogenic substrates for β-glucosidase based on 2-(benzothiazol-2'-yl)-phenols. We also assessed the application of these substrates in the detection of foodborne pathogenic bacteria.

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.

A highly specific ratiometric two-photon fluorescent probe to detect dipeptidyl peptidase IV in plasma and living systems

In this study, a highly specific ratiometric two-photon fluorescent probe GP-BAN was developed and well-characterized to monitor dipeptidyl peptidase IV in plasma and living systems. GP-BAN was designed on the basis of the catalytic properties and substrate preference of DPP-IV, and it could be readily hydrolyzed upon addition of DPP-IV under physiological conditions. Both reaction phenotyping and inhibition assays demonstrated that GP-BAN displayed good reactivity and high selectivity towards DPP-IV over other human serine hydrolases including FAP, DPP-VIII, and DPP-IX. The probe was successfully used to monitor the real activities of DPP-IV in complex biological systems including diluted plasma, while it could be used for high throughput screening of DPP-IV inhibitors by using human plasma or tissue preparations as enzyme sources. As a two-photon fluorescent probe, GP-BAN was also successfully used for two-photon imaging of endogenous DPP-IV in living cells and tissues, and showed high ratiometric imaging resolution and deep-tissue penetration ability. Taken together, a ratiometric two-photon fluorescent probe GP-BAN was developed and well-characterized for highly selective and sensitive detection of DPP-IV in complex biological systems, which could serve as a promising imaging tool to explore the biological functions and physiological roles of this key enzyme in living systems.

Assessment of the inhibitory effects of pyrethroids against human carboxylesterases

Pyrethroids are broad-spectrum insecticides that widely used in many countries, while humans may be exposed to these toxins by drinking or eating pesticide-contaminated foods. This study aimed to investigate the inhibitory effects of six commonly used pyrethroids against two major human carboxylesterases (CES) including CES1 and CES2. Three optical probe substrates for CES1 (DME, BMBT and DMCB) and a fluorescent probe substrate for CES2 (DDAB) were used to characterize the inhibitory effects of these pyrethroids. The results demonstrated that most of the tested pyrethroids showed moderate to weak inhibitory effects against both CES1 and CES2, but deltamethrin displayed strong inhibition towards CES1. The IC₅₀ values of deltamethrin against CES1-mediated BMBT, DME, and DMCB hydrolysis were determined as 1.58μM, 2.39μM, and 3.3μM, respectively. Moreover, deltamethrin was cell membrane permeable and capable of inhibition endogenous CES1 in living cells. Further investigation revealed that deltamethrin inhibited CES1-mediated BMBT hydrolysis via competitive manner but noncompetitively inhibited DME or DMCB hydrolysis. The inhibition behaviors of deltamethrin against CES1 were also studied by molecular docking simulation. The results demonstrated that CES1 had at least two different ligand-binding sites, one was the DME site and another was the BMBT site which was identical to the binding site of deltamethrin. In summary, deltamethrin was a strong reversible inhibitor against CES1 and it could tightly bind on CES1 at the same ligand-binding site as BMBT. These findings are helpful for the deep understanding of the interactions between xenobiotics and CES1.