Characterization of CYP3A5 Selective Inhibitors for Reaction Phenotyping of Drug Candidates

CYP3A is one of the most important classes of enzymes and is involved in the metabolism of over 70% drugs. While several selective CYP3A4 inhibitors have been identified, the search for a selective CYP3A5 inhibitor has turned out to be rather challenging. Recently, several selective CYP3A5 inhibitors have been identified through high-throughput screening of ~ 11,000 compounds and hit expansion using human recombinant enzymes. We set forth to characterize the three most selective CYP3A5 inhibitors in a more physiologically relevant system of human liver microsomes to understand if these inhibitors can be used for reaction phenotyping studies in drug discovery settings. Gomisin A and T-5 were used as selective substrate reactions for CYP3A4 and CYP3A5 to determine IC50 values of the two enzymes. The results showed that clobetasol propionate and loteprednol etabonate were potent and selective CYP3A5 reversible inhibitors with selectivity of 24-fold against CYP3A4 and 39-fold or more against the other major CYPs. The selectivity of difluprednate in HLM is much weaker than that in the recombinant enzymes due to hydrolysis of the acetate group in HLM. Based on the selectivity data, loteprednol etabonate can be utilized as an orthogonal approach, when experimental fraction metabolized of CYP3A5 is greater than 0.5, to understand CYP3A5 contribution to drug metabolism and its clinical significance. Future endeavors to identify even more selective CYP3A5 inhibitors are warranted to enable accurate determination of CYP3A5 contribution to metabolism versus CYP3A4.

An Uncertainty-Guided Deep Learning Method Facilitates Rapid Screening of CYP3A4 Inhibitors

Cytochrome P450 3A4 (CYP3A4), a prominent member of the P450 enzyme superfamily, plays a crucial role in metabolizing various xenobiotics, including over 50% of clinically significant drugs. Evaluating CYP3A4 inhibition before drug approval is essential to avoiding potentially harmful pharmacokinetic drug-drug interactions (DDIs) and adverse drug reactions (ADRs). Despite the development of several CYP inhibitor prediction models, the primary approach for screening CYP inhibitors still relies on experimental methods. This might stem from the limitations of existing models, which only provide deterministic classification outcomes instead of precise inhibition intensity (e.g., IC50) and often suffer from inadequate prediction reliability. To address this challenge, we propose an uncertainty-guided regression model to accurately predict the IC50 values of anti-CYP3A4 activities. First, a comprehensive data set of CYP3A4 inhibitors was compiled, consisting of 27,045 compounds with classification labels, including 4395 compounds with explicit IC50 values. Second, by integrating the predictions of the classification model trained on a larger data set and introducing an evidential uncertainty method to rank prediction confidence, we obtained a high-precision and reliable regression model. Finally, we use the evidential uncertainty values as a trustworthy indicator to perform a virtual screening of an in-house compound set. The in vitro experiment results revealed that this new indicator significantly improved the hit ratio and reduced false positives among the top-ranked compounds. Specifically, among the top 20 compounds ranked with uncertainty, 15 compounds were identified as novel CYP3A4 inhibitors, and three of them exhibited activities less than 1 μM. In summary, our findings highlight the effectiveness of incorporating uncertainty in compound screening, providing a promising strategy for drug discovery and development.

Rationally Engineered CYP3A4 Fluorogenic Substrates for Functional Imaging Analysis and Drug-Drug Interaction Studies

Cytochrome P450 3A4 (CYP3A4) is a key xenobiotic-metabolizing enzyme-mediated drug metabolism and drug-drug interaction (DDI). Herein, an effective strategy was used to rationally construct a practical two-photon fluorogenic substrate for hCYP3A4. Following two-round structure-based substrate discovery and optimization, we have successfully constructed a hCYP3A4 fluorogenic substrate (F8) with desirable features, including high binding affinity, rapid response, excellent isoform specificity, and low cytotoxicity. Under physiological conditions, F8 is readily metabolized by hCYP3A4 to form a brightly fluorescent product (4-OH F8) that can be easily detected by various fluorescence devices. The practicality of F8 for real-time sensing and functional imaging of hCYP3A4 has been examined in tissue preparations, living cells, and organ slices. F8 also demonstrates good performance for high-throughput screening of hCYP3A4 inhibitors and assessing DDI potentials in vivo. Collectively, this study develops an advanced molecular tool for sensing CYP3A4 activities in biological systems, which strongly facilitates CYP3A4-associated fundamental and applied research studies.

Recent Advances in Molecular Fluorescent Probes for CYP450 Sensing and Imaging

Cytochrome P450 (CYP450) is a major drug-metabolizing enzyme system mainly distributed in liver microsomes and involved in the metabolism of many endogenous substances (such as fatty acids and arachidonic acids), and exogenous compounds (such as drugs, toxicants, carcinogens, and procarcinogens). Due to the similarity in structures and catalytic functions between CYP450 isoforms, the lack of effective selective detection tools greatly limits the understanding and the research of their respective physiological roles in living organisms. Until now, several small-molecular fluorescent probes have been employed for selective detection and monitoring of CYP450s (Cytochrome P450 enzymes) in vitro or in vivo owing to the tailored properties, biodegradability, and high temporal and spatial resolution imaging in situ. In this review, we summarize the recent advances in fluorescent probes for CYP450s (including CYP1, CYP2, and CYP3 families), and we discuss and focus on their identification mechanisms, general probe design strategies, and bioimaging applications. We also highlight the potential challenges and prospects of designing new generations of fluorescent probes in CYP450 studies, which will further enhance the diversity, practicality, and clinical feasibility of research into CYP450.

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

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

Herb-drug interaction between Styrax and warfarin: Molecular basis and mechanism

BACKGROUND

Styrax, one of the most famous folk medicines, has been frequently used for the treatment of cardiovascular diseases and skin problems in Asia and Africa. It is unclear whether Styrax or Styrax-related herbal medicines may trigger clinically relevant herb-drug interactions.

PURPOSE

This study was carried out to investigate the inhibitory effects of Styrax on human cytochrome P450 enzymes (CYPs) and to clarify whether this herb may modulate the pharmacokinetic behavior of the CYP-substrate drug warfarin when co-administered.

STUDY DESIGN

The inhibitory effects of Styrax on CYPs were assayed in human liver microsomes (HLM), while the pharmacokinetic interactions between Styrax and warfarin were investigated in rats. The bioactive constituents in Styrax with strong CYP3A inhibitory activity were identified and their inhibitory mechanisms were carefully investigated.

METHODS

The inhibitory effects of Styrax on human CYPs were assayed in vitro, while the pharmacokinetic interactions between Styrax and warfarin were studied in rats. Fingerprinting analysis of Styrax coupled with LC-TOF-MS/MS profiling and CYP inhibition assays were used to identify the constituents with strong CYP3A inhibitory activity. The inhibitory mechanism of oleanonic acid (the most potent CYP3A inhibitor occurring in Styrax) against CYP3A4 was investigated by a panel of inhibition kinetics analyses and in silico analysis.

RESULTS

In vitro assays demonstrated that Styrax extract strongly inhibited human CYP3A and moderately inhibited six other tested human CYPs, as well as potently inhibited warfarin 10-hydroxylation in liver microsomes from both humans and rats. In vivo assays demonstrated that compared with warfarin given individually in rats, Styrax (100 mg/kg) significantly prolonged the plasma half-life of warfarin by 2.3-fold and increased the AUC_(0-inf) of warfarin by 2.7-fold when this herb was co-administrated with warfarin (2 mg/kg) in rats. Two LC fractions were found with strong CYP3A inhibitory activity and the major constituents in these fractions were characterized by LC-TOF-MS/MS. Five pentacyclic triterpenoid acids (including epibetulinic acid, betulinic acid, betulonic acid, oleanonic acid and maslinic acid) present in Styrax were potent CYP3A inhibitors, and oleanonic acid was a competitive inhibitor against CYP3A-mediated testosterone 6β-hydroxylation.

CONCLUSION

Styrax and the pentacyclic triterpenoid acids occurring in this herb strongly modulate the pharmacokinetic behavior of warfarin via inhibition of CYP3A.

A dual functional probe for assessing human CYP450 3A5 and 3A enzymes bioactivities

Aim: CYP3A5 plays a vital role in the drug metabolism, it displays varied expression levels among individuals and is easily influenced by genetic polymorphisms and some diseases. Methodology & results: A dual function probe isobutyryl-11-keto-β-boswellic acid (IKBA) was developed; it possessed a high selectivity toward CYP3A5 and CYP3A enzymes for its two individual metabolites, respectively. The probe has the high accuracy and wide applicability in measuring the real activity of CYP3A5. Finally, IKBA was successfully used for the evaluation of the activity of CYP3A5 and CYP3A enzymes in various bio samples. Conclusion: IKBA could serve as a useful tool for exploring the physiology and pathology functions of CYP3A5 and give some useful guidance for the rational use of clinical drugs.

Target Enzyme-Activated Two-Photon Fluorescent Probes: A Case Study of CYP3A4 Using a Two-Dimensional Design Strategy

The rapid development of fluorescent probes for monitoring target enzymes is still a great challenge owing to the lack of efficient ways to optimize a specific fluorophore. Herein, a practical two-dimensional strategy was designed for the development of an isoform-specific probe for CYP3A4, a key cytochrome P450 isoform responsible for the oxidation of most clinical drugs. In first dimension of the design strategy, a potential two-photon fluorescent substrate (NN) for CYP3A4 was effectively selected using ensemble-based virtual screening. In the second dimension, various substituent groups were introduced into NN to optimize the isoform-selectivity and reactivity. Finally, with ideal selectivity and sensitivity, NEN was successfully applied to the real-time detection of CYP3A4 in living cells and zebrafish. These findings suggested that our strategy is practical for developing an isoform-specific probe for a target enzyme.

Solving the interactions of steroidal ligands with CYP3A4 using a grid-base template system

Using over fifty steroidal ligands, CYP3A4 Template system established in our previous study (DMPK 34: 113-125, 2019) has been evaluated for the applicability for prediction of regioselective metabolisms of steroids in the present study. Plural regional interactions near Site of Oxidation of CYP3A4 (Slide-down and Adaptation) are newly defined for steroid ligands in addition to previously characterized Trigger- and IJL-interactions on Template. Interaction of steroids at ring-A with CYP3A4 residue (Front-residue), at the facial side of Ring B of Template, determined the availability of ligand sitting at Rings A and B of Template. Steroids having 3-one-4-ene structures, which are not stacked on Front-residue, thus slide down for their 6-oxidations. Some steroids with 3β-ol structures undergo the further right-side movement (Adaptation) for their 7-oxidations. Similar overpassing phenomena are also expected for steroid 15/16-oxidations and 2/1-oxidations. Allowable width on ligand accommodation was also defined as Width-gauge of Template. Reciprocal comparison of sittings of steroids on Template with experimental data offered idea of CYP3A4-mediated oxidations of steroids through seven distinct types of placements on Template and of the relationship with their usage abundance. The present system would offer practical way for structural identification and verification of CYP3A4-mediated metabolisms of various types of steroids.

Rational Design of a Long-Wavelength Fluorescent Probe for Highly Selective Sensing of Carboxylesterase 1 in Living Systems

Rational design of practical probes with excellent specificity and improved optical properties for a particular enzyme is always a big challenge. Herein, a practical and highly specific fluorescent probe for carboxylesterase 1 (CES1) was rationally designed using meso-carboxyl-BODIPY as the basic fluorophore based on the substrate preference and catalytic properties of CES1. Following molecular docking-based virtual screening combined with reaction phenotyping-based experimental screening, we found that MMB (probe 7) exhibited the optimal combination of sensitivity and specificity toward human CES1 in contrast to other ester derivatives. Under physiological conditions, MMB could be readily hydrolyzed by CES1 and release MCB; such biotransformation brought great changes in the electronic properties at the meso position of the fluorophore and triggered a dramatic increase in fluorescence emission around 595 nm. Moreover, MMB was cell membrane permeable and was successfully applied to monitor the real activities of CES1 in various biological samples including living cells, tissue slices, organs, and zebrafish. In summary, this study showed a good example for constructing specific fluorescent probe(s) for a target enzyme and also provided a practical and sensitive tool for real-time sensing of CES1 activities in complicated biological samples. All these findings would strongly facilitate high-throughput screening of CES1 modulators and the studies on CES1-associated physiological and pathological processes.

Computational insights into the different catalytic activities of CYP3A4 and CYP3A5 toward schisantherin E

The cytochromes CYP3A4 and CYP3A5 share 84% sequence identity, but they exhibit different catalytic activities toward some substrates. Schisantherin E (SE) was recently identified as a selective substrate of CYP3A5, which exhibited catalytic efficiency that was more than 23 times higher than CYP3A4. At present, however, the structural determinants responsible for the different catalytic activities of the two enzymes toward SE have not been fully understood. In this study, a combination of molecular docking, molecular dynamic simulations, and binding free energy calculation was performed on the CYP3A4/CYP3A5-SE systems to investigate the issue. The results demonstrate that Ser119 in CYP3A4 and Glu374 in CYP3A5 formed direct hydrogen bonding with SE, respectively. Additionally, one water molecule located between the B-C loop and the I helix mediated different hydrogen-bonding networks between CYP3A4/3A5 and SE. The residue differences (Phe/Leu108 and Leu/Phe210) triggered the distinct conformational changes of the Phe-cluster residues, especially Phe213 and Phe215, which formed stronger hydrophobic interactions with SE in CYP3A5. The calculated binding free energies were consistent with the experimental results.

Cytochrome P450 3A Enzymes Are Key Contributors for Hepatic Metabolism of Bufotalin, a Natural Constitute in Chinese Medicine Chansu

Bufotalin (BFT), one of the naturally occurring bufodienolides, has multiple pharmacological and toxicological effects including antitumor activity and cardiotoxicity. This study aimed to character the metabolic pathway(s) of BFT and to identify the key drug metabolizing enzyme(s) responsible for hepatic metabolism of BFT in human, as well as to explore the related molecular mechanism of enzymatic selectivity. The major metabolite of BFT in human liver microsomes (HLMs) was fully identified as 5β-hydroxylbufotalin by LC-MS/MS and NMR techniques. Reaction phenotyping and chemical inhibition assays showed that CYP3A4 and CYP3A5 were key enzymes responsible for BFT 5β-hydroxylation. Kinetic analyses demonstrated that BFT 5β-hydroxylation in both HLMs and human CYP3A4 followed the biphasic kinetics, while BFT 5β-hydroxylation in CYP3A5 followed substrate inhibition kinetics. Furthermore, molecular docking simulations showed that BFT could bind on two different ligand-binding sites on both CYP3A4 and CYP3A5, which partially explained the different kinetic behaviors of BFT in CYP3A4 and CYP3A5. These findings are very helpful for elucidating the phase I metabolism of BFT in human and for deeper understanding the key interactions between CYP3A enzymes and bufadienolides, as well as for the development of bufadienolide-type drugs with improved pharmacokinetic and safety profiles.

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.

A Practical and High-Affinity Fluorescent Probe for Uridine Diphosphate Glucuronosyltransferase 1A1: A Good Surrogate for Bilirubin

This study aimed to develop a practical and high-affinity fluorescent probe for uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1), a key conjugative enzyme responsible for the elimination and detoxification of many potentially harmful compounds. Several substrates derived from N-butyl-4-phenyl-1,8-naphthalimide were designed and synthesized on the basis of the substrate preference of UGT1A1 and the principle of photoinduced electron transfer (PET). Following the preliminary screening, substrate 2 was found with a high specificity and high affinity toward UGT1A1, while such biotransformation brought remarkable changes in fluorescence emission. Both inhibition kinetic analyses and molecular docking simulations demonstrated that 2 could bind on UGT1A1 at the same ligand-binding site as bilirubin. Furthermore, this newly developed probe was successfully used for sensing UGT1A1 activities and the high-throughput screening of UGT1A1 modulators in complex biological samples. In conclusion, a practical and high-affinity fluorescent probe for UGT1A1 was designed and well-characterized, which could serve as a good surrogate for bilirubin to investigate UGT1A1-ligand interactions.

The prenylated phenolic natural product isoglycycoumarin is a highly selective probe for human cytochrome P450 2A6

Prenylated phenolic compounds are an important class of bioactive natural products. One major in vivo metabolic pathway of these compounds is hydroxylation at terminal methyl of the isoprenyl group. This study aims to identify the P450 isozyme catalyzing this metabolic reaction. In human liver microsomes, 16 out of 24 screened compounds could be metabolized into their hydroxylated derivatives. Chemical inhibition assays using 11 isozyme specific inhibitors indicated the hydroxylation reactions of 12 compounds were primarily catalyzed by cytochrome P450 2A6 (CYP2A6). In particular, CYP2A6 was the major enzyme participating in the metabolism of isoglycycoumarin (IGCM). The product of IGCM was obtained and identified as licopyranocoumarin (4″-hydroxyl isoglycycoumarin) using NMR spectroscopic analysis. The K_m values for human liver microsomes and recombinant human CYP2A6 were 7.98 and 10.14μM, respectively. According to molecular docking analysis, the catalytic mechanism may involve cyclized isoprenyl group of IGCM entering the active cavity of CYP2A6. These results demonstrate that IGCM could serve as an ideal isozyme selective probe to evaluate CYP2A6 activities.

An Optimized Two-Photon Fluorescent Probe for Biological Sensing and Imaging of Catechol-O-Methyltransferase

A practical two-photon fluorescent probe was developed for highly sensitive and selective sensing of the activities of catechol-O-methyltransferase (COMT) in complex biological samples. To this end, a series of 3-substituted 7,8-dihydroxycoumarins were designed and synthesized. Among them, 3-BTD displayed the best combination of selectivity, sensitivity, reactivity, and fluorescence response following COMT-catalyzed 8-O-methylation. The newly developed two-photon fluorescent probe 3-BTD can be used for determining the activities of COMT in complex biological samples and bio-imaging of endogenous COMT in living cells and tissue slices with good cell permeability, low cytotoxicity, and high imaging resolution. All these findings suggest that 3-BTD holds great promise for developing therapeutic molecules that target COMT, as well as for exploring COMT-associated biological processes and its biological functions in living systems. Furthermore, the strategy also sheds new light on the development of fluorescent probes for other conjugative enzymes.

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.

A novel near-infrared fluorescent probe for sensitive detection of β-galactosidase in living cells

A novel near-infrared fluorescent probe for β-galactosidase has been developed based on a hemicyanine skeleton, which is conjugated with a d-galactose residue via a glycosidic bond. The probe serves as a substrate of β-galactosidase and displays rapid and sensitive turn-on fluorescent responses to β-galactosidase in aqueous solution. A 12.8-fold enhancement of fluorescence intensity at 703 nm was observed after incubation of 10 nM of β-galactosidase with 5 μM probe for 10 min. The probe can sensitively detect as little as 0.1 nM of β-galactosidase and shows linear responses to the enzyme concentration below 1.4 nM. The kinetic study showed that the probe has high binding affinity to β-galactosidase with K_m = 3.6 μM. The probe was used to detect β-galactosidase in living cells by employing the premature cell senescence model. The probe exhibited strong fluorescent signals in senescent cells but not in normal cells, which demonstrates that the probe is able to detect the endogenous senescence-associated β-galactosidase in living cells.

A practical strategy to design and develop an isoform-specific fluorescent probe for a target enzyme: CYP1A1 as a case study

The development of isoform-specific probe(s) for a target enzyme with multiple homologs is always challenging. Herein, a practical strategy was used to design and develop an isoform-specific probe for CYP1A1, a key cytochrome P450 isoenzyme involved in xenobiotic metabolism and bioactivation. On the basis of the subtle differences in 3D structure and substrate preference between CYP1A1 and its homolog CYP1A2, we proposed that it was possible to design a CYP1A1-specific probe via local modification of the reaction site on known CYP1A substrates. To validate this hypothesis, 4-hydroxy-1,8-naphthalimide (HN) was selected as the basic fluorophore due to its excellent optical properties, while a series of O-alkylated HN derivatives were prepared to evaluate their specificity towards CYP1A1. Our results revealed that the introduction of a chloroethyl to HN could get the best isoform selectivity towards CYP1A1 over other CYPs including CYP1A2. The newly developed probe NBCeN exhibited excellent specificity, high sensitivity, and a ratiometric fluorescence response following CYP1A1-catalyzed O-dechloroethylation. NBCeN was successfully used to real-time monitor the activity of CYP1A1 in complex biological samples and to rapidly screen CYP1A1 modulators in living systems. NBCeN could also be used for two-photon imaging of intracellular CYP1A1 in living cells and tissues with high ratiometric imaging resolution and deep tissue penetration. All these findings demonstrated that local modification of non-specific substrates was a practical strategy to develop an isoform-specific probe for a target isoenzyme, while NBCeN could serve as a specific imaging tool to explore the biological functions of CYP1A1 in complex biological systems.

Pyroglutamate aminopeptidase 1 may be an indicator of cellular inflammatory response as revealed using a sensitive long-wavelength fluorescent probe

Pyroglutamate aminopeptidase 1 (PGP-1) can remove pyroglutamic acid from the N-terminus of a polypeptide, including some important anti-inflammatory proteins. Detecting the change and distribution of cellular PGP-1 in an inflammation process would be helpful to better understand the role of this enzyme. However, no report has been found on this subject, mainly due to the lack of a proper research approach. Herein, we develop such a new method by preparing a sensitive long-wavelength fluorescent probe combined with confocal fluorescence imaging. The probe, consisting of l-pyroglutamic acid and cresyl violet, exhibits high selectivity and sensitivity for PGP-1 under physiological conditions. With this probe, the up-regulation of PGP-1 in LO-2 cells under the stimulation of Freund's incomplete adjuvant and lipopolysaccharide (two main immunopotentiators) is revealed for the first time, and this up-regulation is also observed in typical phagocytic RAW264.7 cells, as evidenced by western blot and inhibition assays. Studies on the distribution of PGP-1 in cells using our probe showed that most PGP-1 is located in the cytoplasm, which is further supported by an immunofluorescence assay. Moreover, the inflammatory response induced by the immunopotentiators in either RAW264.7 or LO-2 cells is confirmed by measuring tumor necrosis factor alpha (a common inflammatory factor). The above findings indicate that cellular inflammation is accompanied by an increase in PGP-1, and PGP-1 may serve as a new indicator of cellular inflammatory response.

Characterization of regio- and stereo-selective sulfation of bufadienolides: exploring the mechanism and providing insight into the structure–sulfation relationship by experimentation and molecular docking analysis

Bufadienolides are a major class of bioactive compounds derived from amphibian skin secretion.

A bioluminescent sensor for highly selective and sensitive detection of human carboxylesterase 1 in complex biological samples

A highly selective and sensitive bioluminescent sensor (DME) for real-time monitoring of human carboxylesterase 1 (hCE1) activities in complex biological samples and bio-imaging of endogenous hCE1 in living cells.

A highly selective marker reaction for measuring the activity of human carboxylesterase 1 in complex biological samples

NMHN hydrolysis was found to be a highly selective marker reaction for sensing the activity of human carboxylesterase 1 (hCE1).

A Two-Photon Ratiometric Fluorescent Probe for Imaging Carboxylesterase 2 in Living Cells and Tissues

In this study, a two-photon ratiometric fluorescent probe NCEN has been designed and developed for highly selective and sensitive sensing of human carboxylesterase 2 (hCE2) based on the catalytic properties and substrate preference of hCE2. Upon addition of hCE2, the probe could be readily hydrolyzed to release 4-amino-1,8-naphthalimide (NAH), which brings remarkable red-shift in fluorescence (90 nm) spectrum. The newly developed probe exhibits good specificity, ultrahigh sensitivity, and has been successfully applied to determine the real activities of hCE2 in complex biological samples such as cell and tissue preparations. NCEN has also been used for two-photon imaging of intracellular hCE2 in living cells as well as in deep-tissues for the first time, and the results showed that the probe exhibited high ratiometric imaging resolution and deep-tissue imaging depth. All these findings suggested that this probe holds great promise for applications in bioimaging of endogenous hCE2 in living cells and in exploring the biological functions of hCE2 in complex biological systems.

Identification of the hydroxylated derivatives of bufalin: phase I metabolites in rats

Bufalin was a typical bioactive bufadienolide, existed in the traditional Chinese medicine Chan Su with the high content of 1-5%. The in vivo metabolites (1-5) of bufalin were prepared by various chromatographic techniques from the bile samples of SD rats, which were administrated with bufalin orally. Their structures were determined on the basis of the widely spectroscopic data, including HRESIMS, 1D-, and 2D NMR. And 1-3, 5 were new compounds. In the in vitro cytotoxicity assay, metabolites (1-5) showed weaker cytotoxic effects than bufalin against human cancer cell lines A549 and H1299, which indicated that the metabolism was a significant pathway for the detoxification of bufalin. Structures analyses indicated that metabolites 1-5 were hydroxylated derivatives of bufalin. This study suggested that Phase I metabolism catalyzed by CYP450 enzymes was one of the metabolic ways of bufalin, which may promote the excretion of bufalin.

Gomisin A is a Novel Isoform-Specific Probe for the Selective Sensing of Human Cytochrome P450 3A4 in Liver Microsomes and Living Cells

Nearly half of prescription medicines are metabolized by human cytochrome P450 (CYP) 3A. CYP3A4 and 3A5 are two major isoforms of human CYP3A and share most of the substrate spectrum. A very limited previous study distinguished the activity of CYP3A4 and CYP3A5, identifying the challenge in predicting CYP3A-mediated drug clearance and drug-drug interaction. In the present study, we introduced gomisin A (GA) with a dibenzocyclooctadiene skeleton as a novel selective probe of CYP3A4. The major metabolite of GA was fully characterized as 8-hydroxylated GA by LC-MS and NMR. CYP3A4 was assigned as the predominant isozyme involved in GA 8-hydroxylation by reaction phenotyping assays, chemical inhibition assays, and correlation studies. GA 8-hydroxylation in both recombinant human CYP3A4 and human liver microsomes followed classic Michaelis-Menten kinetics. The intrinsic clearance values indicated that CYP3A4 contributed 12.8-fold more than CYP3A5 to GA 8-hydroxylation. Molecular docking studies indicated different hydrogen bonds and π-π interactions between CYP3A4 and CYP3A5, which might result in the different catalytic activity for GA 8-hydroxylation. Furthermore, GA exhibited a stronger inhibitory activity towards CYP3A4 than CYP3A5, which further suggested a preferred selectivity of CYP3A4 for the transformation of GA. More importantly, GA has been successfully applied to selectively monitor the modulation of CYP3A4 activities by the inducer rifampin in hepG2 cells, which is consistent with the level change of CYP3A4 mRNA expression. In summary, our results suggested that GA could be used as a novel probe for the selective sensing of CYP3A4 in tissue and cell preparations.

A Mechanism-Based Model for the Prediction of the Metabolic Sites of Steroids Mediated by Cytochrome P450 3A4

Early prediction of xenobiotic metabolism is essential for drug discovery and development. As the most important human drug-metabolizing enzyme, cytochrome P450 3A4 has a large active cavity and metabolizes a broad spectrum of substrates. The poor substrate specificity of CYP3A4 makes it a huge challenge to predict the metabolic site(s) on its substrates. This study aimed to develop a mechanism-based prediction model based on two key parameters, including the binding conformation and the reaction activity of ligands, which could reveal the process of real metabolic reaction(s) and the site(s) of modification. The newly established model was applied to predict the metabolic site(s) of steroids; a class of CYP3A4-preferred substrates. 38 steroids and 12 non-steroids were randomly divided into training and test sets. Two major metabolic reactions, including aliphatic hydroxylation and N-dealkylation, were involved in this study. At least one of the top three predicted metabolic sites was validated by the experimental data. The overall accuracy for the training and test were 82.14% and 86.36%, respectively. In summary, a mechanism-based prediction model was established for the first time, which could be used to predict the metabolic site(s) of CYP3A4 on steroids with high predictive accuracy.

Identifying and applying a highly selective probe to simultaneously determine the O-glucuronidation activity of human UGT1A3 and UGT1A4

Glucuronidation mediated by uridine 5′-diphospho (UDP)-glucuronosyltransferase is an important detoxification pathway. However, identifying a selective probe of UDP- glucuronosyltransferase is complicated because of the significant overlapping substrate specificity displayed by the enzyme. In this paper, desacetylcinobufagin (DACB) 3-O- and 16-O-glucuronidation were found to be isoform-specific probe reactions for UGT1A4 and UGT1A3, respectively. DACB was well characterized as a probe for simultaneously determining the catalytic activities of O-glucuronidation mediated by UGT1A3 and UGT1A4 from various enzyme sources, through a sensitive analysis method.

Mechanism-based inactivation of CYP450 enzymes: a case study of lapatinib

Mechanism-based inactivation (MBI) of CYP450 enzymes is a unique form of inhibition in which the enzymatic machinery of the victim is responsible for generation of the reactive metabolite. This precondition sets up a time-dependency for the inactivation process, a hallmark feature that characterizes all MBI. Yet, MBI itself is a complex biochemical phenomenon that operates in different modes, namely, covalent binding to apoprotein, covalent binding of the porphyrin group and also complexation of the catalytic iron. Using lapatinib as a recent example of toxicological interest, we present an example of a mixed-function MBI that can confound clinical drug-drug interactions manifestation. Lapatinib exhibits both covalent binding to the apoprotein and formation of a metabolite-intermediate complex in an enzyme-selective manner (CYP3A4 versus CYP3A5), each with different reactive metabolites. The clinical implication of this effect is also contingent upon genetic polymorphisms of the enzyme involved as well as the co-administration of other substrates, inhibitors or inducers, culminating in drug-drug interactions. This understanding recapitulates the importance of applying isoform-specific mechanistic investigations to develop customized strategies to manage such outcomes.

A highly selective probe for UDP-glucuronosyltransferase 2B7 (UGT2B7) in human microsomes: isoform specificity, kinetic characterization, and applications

3-Epideacetycinobufagin (EDCB) was found to be a highly isoform-specific probe for 3-glucuronidation mediated by UDP-glucuronosyltransferase 2B7 (UGT2B7). The reaction was well-characterized, suggesting that EDCB can be used to measure the catalytic activity of UGT2B7.

Characterization of phase I metabolism of resibufogenin and evaluation of the metabolic effects on its antitumor activity and toxicity

Resibufogenin (RB), one of the major active compounds of the traditional Chinese medicine Chansu, has displayed great potential as a chemotherapeutic agent in oncology. However, it is a digoxin-like compound that also exhibits extremely cardiotoxic effects. The present study aimed to characterize the metabolic behaviors of RB in humans as well as to evaluate the metabolic effects on its bioactivity and toxicity. The phase I metabolic profile in human liver microsomes was characterized systemically, and the major metabolite was identified as marinobufagenin (5β-hydroxylresibufogenin, 5-HRB) by liquid chromatography-mass spectrometry and nuclear magnetic imaging techniques. Both cytochrome P450 (P450) reaction phenotyping and inhibition assays using P450-selective chemical inhibitors demonstrated that CYP3A4 was mainly involved in RB 5β-hydroxylation with much higher selectivity than CYP3A5. Kinetic characterization demonstrated that RB 5β-hydroxylation in both human liver microsomes and human recombinant CYP3A4 obeyed biphasic kinetics and displayed similar apparent kinetic parameters. Furthermore, 5-HRB could significantly induce cell growth inhibition and apoptosis in A549 and H1299 by facilitating apoptosome assembly and caspase activation. Meanwhile, 5-HRB displayed very weak cytotoxicity of human embryonic lung fibroblasts, and in mice there was a greater tolerance to acute toxicity. In summary, CYP3A4 dominantly mediated 5β-hydroxylation and was found to be a major metabolic pathway of RB in the human liver, whereas its major metabolite (5-HRB) displayed better druglikeness than its parent compound RB. Our findings lay a solid foundation for RB metabolism studies in humans and encourage further research on the bioactive metabolite of RB.

Regioselective glucuronidation of andrographolide and its major derivatives: metabolite identification, isozyme contribution, and species differences

Andrographolide (AND) and two of its derivatives, deoxyandrographolide (DEO) and dehydroandrographolide (DEH), are widely used in clinical practice as anti-inflammatory agents. However, UDP-glucuronosyltransferase (UGT)-mediated phase II metabolism of these compounds is not fully understood. In this study, glucuronidation of AND, DEO, and DEH was characterized using liver microsomes and recombinant UGT enzymes. We isolated six glucuronides and identified them using 1D and 2D nuclear magnetic resonance (NMR) spectroscopy. We also systematically analyzed various kinetic parameters (K m, V max, and CLint) for glucuronidation of AND, DEO, and DEH. Among 12 commercially available UGT enzymes, UGT1A3, 1A4, 2B4, and 2B7 exhibited metabolic activities toward AND, DEO, and DEH. Further, UGT2B7 made the greatest contribution to glucuronidation of all three anti-inflammatory agents. Regioselective glucuronidation showed considerable species differences. 19-O-Glucuronides were present in liver microsomes from all species except rats. 3-O-Glucuronides were produced by pig and cynomolgus monkey liver microsomes for all compounds, and 3-O-glucuronide of DEH was detected in mouse and rat liver microsomes (RLM). Variations in K m values were 48.6-fold (1.93-93.6 μM) and 49.5-fold (2.01-99.1 μM) for 19-O-glucuronide and 3-O-glucuronide formation, respectively. Total intrinsic clearances (CLint) for 3-O- and 19-O-glucuronidation varied 4.8-fold (22.7-110 μL min(-1) mg(-1)), 10.6-fold (94.2-991 μL min(-1) mg(-1)), and 8.3-fold (122-1,010 μL min(-1) mg(-1)), for AND, DEH, and DEO, respectively. Our results indicate that UGT2B7 is the major UGT enzyme involved in the metabolism of AND, DEO, and DEH. Metabolic pathways in the glucuronidation of AND, DEO, and DEH showed considerable species differences.