Utility of the carboxylesterase inhibitor bis-para-nitrophenylphosphate (BNPP) in the plasma unbound fraction determination for a hydrolytically unstable amide derivative and agonist of the TGR5 receptor

Biotransformation and brain distribution of the anti-COVID-19 drug molnupiravir and herb-drug pharmacokinetic interactions between the herbal extract Scutellaria formula-NRICM101

The aim of this study was to explore the effects of herbal drug pharmacokinetic interactions on the biotransformation of molnupiravir and its metabolite β-D-N4-hydroxycytidine (NHC) in the blood and brain. To investigate the biotransformation mechanism, a carboxylesterase inhibitor, bis(4-nitrophenyl)phosphate (BNPP), was administered. Not only molnupiravir but also the herbal medicine Scutellaria formula-NRICM101 is potentially affected by coadministration with molnupiravir. However, the herb-drug interaction between molnupiravir and the Scutellaria formula-NRICM101 has not yet been investigated. We hypothesized that the complex bioactive herbal ingredients in the extract of the Scutellaria formula-NRICM101, the biotransformation and penetration of the bloodbrain barrier of molnupiravir are altered by inhibition of carboxylesterase. To monitor the analytes, ultrahigh-performance liquid chromatography tandem mass spectrometry (UHPLCMS/MS) coupled with the microdialysis method was developed. Based on the dose transfer from humans to rats, a dose of molnupiravir (100 mg/kg, i.v.), molnupiravir (100 mg/kg, i.v.) + BNPP (50 mg/kg, i.v.), and molnupiravir (100 mg/kg, i.v.) + the Scutellaria formula-NRICM101 extract (1.27 g/kg, per day, for 5 consecutive days) were administered. The results showed that molnupiravir was rapidly metabolized to NHC and penetrated into the brain striatum. However, when concomitant with BNPP, NHC was suppressed, and molnupiravir was enhanced. The blood-to-brain penetration ratios were 2% and 6%, respectively. In summary, the extract of the Scutellaria formula-NRICM101 provides a pharmacological effect similar to that of the carboxylesterase inhibitor to suppress NHC in the blood, and the brain penetration ratio was increased, but the concentration is also higher than the effective concentration in the blood and brain.

Alcohol inhibits the metabolism of dimethyl fumarate to the active metabolite responsible for decreasing relapse frequency in the treatment of multiple sclerosis

Dimethyl fumarate (DMF) is a first-line prodrug for the treatment of relapsing-remitting multiple sclerosis (RRMS) that is completely metabolized to monomethyl fumarate (MMF), the active metabolite, before reaching the systemic circulation. Its metabolism has been proposed to be due to ubiquitous esterases in the intestines and other tissues, but the specific enzymes involved are unknown. We hypothesized based on its structure and extensive presystemic metabolism that DMF would be a carboxylesterase substrate subject to interaction with alcohol. We sought to determine the enzymes(s) responsible for the extensive presystemic metabolism of DMF to MMF and the effect of alcohol on its disposition by conducting metabolic incubation studies in human recombinant carboxylesterase-1 (CES1), carboxylesterase-2 (CES2) and human intestinal microsomes (HIM), and by performing a follow-up study in an in vivo mouse model. The in vitro incubation studies demonstrated that DMF was only metabolized to MMF by CES1. Consistent with the incubation studies, the mouse pharmacokinetic study demonstrated that alcohol decreased the maximum concentration and area-under-the-curve of MMF in the plasma and the brain after dosing with DMF. We conclude that alcohol may markedly decrease exposure to the active MMF metabolite in the plasma and brain potentially decreasing the effectiveness of DMF in the treatment of RRMS.

Insights Into Drug Repurposing, as Well as Specificity and Compound Properties of Piperidine-Based SARS-CoV-2 PLpro Inhibitors

The COVID-19 pandemic continues unabated, emphasizing the need for additional antiviral treatment options to prevent hospitalization and death of patients infected with SARS-CoV-2. The papain-like protease (PLpro) domain is part of the SARS-CoV-2 non-structural protein (nsp)-3, and represents an essential protease and validated drug target for preventing viral replication. PLpro moonlights as a deubiquitinating (DUB) and deISGylating enzyme, enabling adaptation of a DUB high throughput (HTS) screen to identify PLpro inhibitors. Drug repurposing has been a major focus through the COVID-19 pandemic as it may provide a fast and efficient route for identifying clinic-ready, safe-in-human antivirals. We here report our effort to identify PLpro inhibitors by screening the ReFRAME library of 11,804 compounds, showing that none inhibit PLpro with any reasonable activity or specificity to justify further progression towards the clinic. We also report our latest efforts to improve piperidine-scaffold inhibitors, 5c and 3k, originally developed for SARS-CoV PLpro. We report molecular details of binding and selectivity, as well as in vitro absorption, distribution, metabolism and excretion (ADME) studies of this scaffold. A co-crystal structure of SARS-CoV-2 PLpro bound to inhibitor 3k guides medicinal chemistry efforts to improve binding and ADME characteristics. We arrive at compounds with improved and favorable solubility and stability characteristics that are tested for inhibiting viral replication. Whilst still requiring significant improvement, our optimized small molecule inhibitors of PLpro display decent antiviral activity in an in vitro SARS-CoV-2 infection model, justifying further optimization.

"Clickable" Albumin Binders for Modulating the Tumor Uptake of Targeted Radiopharmaceuticals

The intentional binding of radioligands to albumin gains increasing attention in the context of radiopharmaceutical cancer therapy as it can lead to an enhanced radioactivity uptake into the tumor lesions and, thus, to a potentially improved therapeutic outcome. However, the influence of the radioligand's albumin-binding affinity on the time profile of tumor uptake has been only partly addressed so far. Based on the previously identified N^ε-4-(4-iodophenyl)butanoyl-lysine scaffold, we designed "clickable" lysine-derived albumin binders (cLABs) and determined their dissociation constants toward albumin by novel assay methods. Structure-activity relationships were derived, and selected cLABs were applied for the modification of the somatostatin receptor subtype 2 ligand (Tyr³)octreotate. These novel conjugates were radiolabeled with copper-64 and subjected to a detailed in vitro and in vivo radiopharmacological characterization. Overall, the results of this study provide an incentive for further investigations of albumin binders for applications in endoradionuclide therapies.

An update on the importance of plasma protein binding in drug discovery and development

Introduction: Plasma protein binding (PPB) remains a controversial topic in drug discovery and development. Fraction unbound (f_u) is a critical parameter that needs to be measured accurately, because it has significant impacts on the predictions of drug-drug interactions (DDI), estimations of therapeutic indices (TI), and developments of PK/PD relationships.  However, it is generally not advisable to change PPB through structural modifications, because PPB on its own has little relevance for in vivo efficacy.Areas covered: PPB fundamentals are discussed including the three main classes of drug binding proteins (i.e., albumin, alpha1-acid glycoprotein, and lipoproteins) and their physicochemical properties, in vivo half-life, and synthesis rate.  State-of-the-art methodologies for PPB are highlighted. Applications of PPB in drug discovery and development are presented.Expert opinion: PPB is an old topic in pharmacokinetics, but there are still many misconceptions. Improving the accuracy of PPB for highly bound compounds is an ongoing effort in the field with high priority. As the field continues to generate high quality data, the regulatory agencies will increase their confidence in our ability to accurately measure PPB of highly bound compounds, and experimental fu values below 0.01 will more likely be used for DDI predictions in the future.

Predicting blood-to-plasma concentration ratios of drugs from chemical structures and volumes of distribution in humans

Abstract

Despite their importance in determining the dosing regimen of drugs in the clinic, only a few studies have investigated methods for predicting blood-to-plasma concentration ratios (Rb). This study established an Rb prediction model incorporating typical human pharmacokinetics (PK) parameters. Experimental Rb values were compiled for 289 compounds, offering reliable predictions by expanding the applicability domain. Notably, it is the largest list of Rb values reported so far. Subsequently, human PK parameters calculated from plasma drug concentrations, including the volume of distribution (Vd), clearance, mean residence time, and plasma protein binding rate, as well as 2702 kinds of molecular descriptors, were used to construct quantitative structure–PK relationship models for Rb. Among the evaluated PK parameters, logVd correlated best with Rb (correlation coefficient of 0.47). Thus, in addition to molecular descriptors selected by XGBoost, logVd was employed to construct the prediction models. Among the analyzed algorithms, artificial neural networks gave the best results. Following optimization using six molecular descriptors and logVd, the model exhibited a correlation coefficient of 0.64 and a root-mean-square error of 0.205, which were superior to those previously reported for other Rb prediction methods. Since Vd values and chemical structures are known for most medications, the Rb prediction model described herein is expected to be valuable in clinical settings.

Graphical abstract

Supplementary informations

The online version of this article (10.1007/s11030-021-10186-7) contains supplementary material, which is available to authorized users.

Design, synthesis, and evaluation of liver-specific gemcitabine prodrugs for potential treatment of hepatitis C virus infection and hepatocellular carcinoma

Many successful anti-viral and anti-cancer drugs are nucleoside analogs, which disrupt RNA and/or DNA synthesis. Here, we present liver-specific prodrugs of the chemotherapy drug gemcitabine (2',2'-difluorodeoxycytidine) for the treatment of hepatitis C virus (HCV) infection and hepatocellular carcinoma. The prodrugs were synthesized by introducing aromatic functional moieties to the cytosine 4-NH₂ group of gemcitabine via amide bonds. The chemical modification was designed to i) enable passive diffusion across cellular membrane, ii) protect the prodrugs from inactivating deamination by cellular enzymes, and iii) allow release of active gemcitabine after amide hydrolysis by high levels of carboxylesterases in the liver. We found that many of our prodrugs exhibited similar toxicity as gemcitabine toward liver- and kidney-derived cancer cell lines but were 24- to 620-fold less cytotoxic than gemcitabine in breast- and pancreas-derived cancer cells, respectively. The prodrugs also inhibited an HCV replicon with IC₅₀ values ranging from 10 nM-1.7 μM. Moreover, many of the prodrugs had therapeutic index values of >10,000 and have synergetic effects when combined with other Food and Drug Administration-approved anti-HCV small molecule drugs. These characteristics support the development of gemcitabine prodrugs as liver-specific therapeutics.

Consideration of the Unbound Drug Concentration in Enzyme Kinetics

The study of enzyme kinetics in drug metabolism involves assessment of rates of metabolism and inhibitory potencies over a suitable concentration range. In all but the very simplest in vitro system, these drug concentrations can be influenced by a variety of nonspecific binding reservoirs that can reduce the available concentration to the enzyme system(s) under investigation. As a consequence, the apparent kinetic parameters, such as K_m or K_i, that are derived can deviate from the true values. There are a number of sources of these nonspecific binding depots or barriers, including membrane permeation and partitioning, plasma or serum protein binding, and incubational binding. In the latter case, this includes binding to the assay apparatus as well as biological depots, depending on the characteristics of the in vitro matrix being used. Given the wide array of subcellular, cellular, and recombinant enzyme systems utilized in drug metabolism, each of these has different components which can influence the free drug concentration. The physicochemical properties of the test compound are also paramount in determining the influential factors in any deviation between true and apparent kinetic behavior. This chapter describes the underlying mechanisms determining the free drug concentration in vitro and how these factors can be accounted for in drug metabolism studies, illustrated with case studies from the literature.

Rapid bioluminescence assay for monitoring rat CES1 activity and its alteration by traditional Chinese medicines

In traditional Chinese medicine herbs (TCM), including Radix Salviae Miltiorrhizae (Danshen), Radix Puerariae Lobatae (Gegen), Radix Angelicae Sinensis (Danggui), and Rhizoma Chuanxiong (Chuanxiong) are widely used for the prevention and treatment of cardiovascular diseases and also often co-administered with Western drugs as a part of integrative medicine practice. Carboxylesterase 1 (CES1) plays a pivotal role in the metabolisms of pro-drugs. Since (S)-2-(2-(6-dimethylamino)-benzothiazole)-4,5-dihydro-thiazole-4-carboxylate (NLMe) has recently been identified by us as a selective CES1 bioluminescent sensor, we developed a rapid method using this substrate for the direct measurement of CES1 activity in rats. This bioluminescence assay was applied to determine CES1 activity in rat tissues after a two-week oral administration of each of the four herbs noted above. The results demonstrated the presence of CES1 enzyme in rat blood and all tested tissues with much higher enzyme activity in the blood, liver, kidney and heart than that in the small intestine, spleen, lung, pancreas, brain and stomach. In addition, the four herbs showed tissue-specific effects on rat CES1 expression. Based on the CES1 biodistribution and its changes after treatment in rats, the possibility that Danshen, Gegen and Danggui might alter CES1 activities in human blood and kidney should be considered. In summary, a selective and sensitive bioluminescence assay was developed to rapidly evaluate CES1 activity and the effects of orally administered TCMs in rats.

Norepinephrine-Transporter-Targeted and DNA-Co-Targeted Theranostic Guanidines

High risk neuroblastoma often recurs, even with aggressive treatments. Clinical evidence suggests that proliferative activities are predictive of poor outcomes. This report describes syntheses, characterization, and biological properties of theranostic guanidines that target norepinephrine transporter and undergo intracellular processing, and subsequently their catabolites are efficiently incorporated into DNA of proliferating neuroblastoma cells. Radioactive guanidines are synthesized from 5-radioiodo-2'-deoxyuridine, a molecular radiotherapy platform with clinically proven minimal toxicities and DNA-targeting properties. The transport of radioactive guanidines into neuroblastoma cells is active as indicated by the competitive suppression of cellular uptake by meta-iodobenzylguanidine. The rate of intracellular processing and DNA uptake is influenced by the agent's catabolic stability and cell population doubling times. The radiotoxicity is directly proportional to DNA uptake and duration of exposure. Biodistribution of 5-[¹²⁵I]iodo-3'-O-(ε-guanidinohexanoyl)-2'-deoxyuridine in a mouse neuroblastoma model shows significant tumor retention of radioactivity. Neuroblastoma xenografts regress in response to the clinically achievable doses of this agent.

Novel regulatory roles of carboxylesterase 3 in lipid metabolism and browning in 3T3-L1 white adipocytes

The role of carboxylesterase 3 (Ces3) in the lipolysis of adipocytes has been overlooked, as 2 major lipolytic enzymes, hormone-sensitive lipase and adipose triglyceride lipase, play more powerful roles in lipolysis. In this study, we explored the effects of Ces3 in lipid metabolism by activating and inhibiting, as well as silencing, Ces3-encoding gene in 3T3-L1 cell model. Our results demonstrated that activation of Ces3 increased adipogenesis, and attenuated lipogenesis, whereas it promoted lipolysis and fatty acid oxidation. In addition, activated Ces3 led to enhanced expression of core fat browning marker genes and proteins, suggesting that Ces3 may play a pivotal role in fat browning and thermogenesis. In contrast, deficiency of Ces3 nullified the browning effect in white adipocytes, along with decreased adipogenesis in 3T3-L1 adipocytes. Interestingly, the expression pattern of adipose triglyceride lipase was in line with Ces3, whereas hormone-sensitive lipase was independently regulated irrespective of Ces3 expression levels, suggesting that Ces3 may play an important and compensatory role in the breakdown of triglycerides in white adipocytes. In conclusion, we provide the first evidence that activation of Ces3 contributes in the browning of white adipocytes, and maintains a balance in lipid metabolism, which could be a potential strategy in fighting against obesity.

Traditional Chinese medicine Gegen Qinlian decoction ameliorates irinotecan chemotherapy-induced gut toxicity in mice

BACKGROUND

Gegen Qinlian decoction (GQT), is a classic traditional Chinese medicine formula chronicled in Shang Han Lun, and is widely used to treat diarrhea and inflammation symptoms in various gastrointestinal disorders. Although it has been found to inhibit delayed-onset mice diarrhea resulted from irinotecan (CPT-11) administration in preliminary experiments, the underlying mechanisms and chemical components remain elusive.

METHODS

The effective fraction of GQT by macroporous resin elution was obtained and screened using a diarrhea mouse model induced by CPT-11 and quantified by UPLC analysis. The protective effect of GQT extract towards alleviating diarrhea in mice following CPT-11 administration was further investigated. The levels of inflammatory cytokines and intestinal tight junction related proteins in colonic tissues were determined. The inhibitory effect of GQT extract against hCE2 was evaluated by a fluorescence-based method. Lastly, the synergistic effect of GQT extract combined with CPT-11 against tumor growth in a colorectal tumor mouse model, induced by HT-29 colon cancer cells xenograft subcutaneously, was investigated.

RESULTS

The obtained GQT extract, which profoundly ameliorated the gut toxicity induced by CPT-11, contained puerarin, liquiritin, berberine, and baicalin of 27.2 mg/g, 4.6 mg/g, 491.4 mg/g, and 304.2 mg/g, respectively. After 5 days of administration of GQT extract to mice with diarrhea induced by CPT-11, aberrantly elevated levels of pro-inflammatory cytokines, including IL-1β, COX-2, ICAM-1, and TNF-α, were significantly decreased. Meanwhile, GQT extract also exhibited a remarkable anti-oxidative stress effect, involving activating the Keap1/Nrf2 pathway, and up-regulating the intestinal barrier function by enhancing the expression of tight junction proteins ZO-1, HO-1, and occludin. Additionally, a potent inhibitory effect of GQT extract against hCE2 was observedin vitro, with its IC₅₀ value of 0.187 mg/ml, suggesting alleviating activity on hCE2-mediated severe diarrhea in patients suffered from CPT-11. Moreover, GQT extract was shown to improve inhibition of the colonic tumor growth synergistically with CPT-11.

CONCLUSION

The present study indicates that GQT extract can ameliorate CPT-11 induced gut toxicity in mice and improve CPT-11 efficacy in colorectal cancer treatment.

A rapid solution-based method for determining the affinity of heroin hapten-induced antibodies to heroin, its metabolites, and other opioids

We describe for the first time a method that utilizes microscale thermophoresis (MST) technology to determine polyclonal antibody affinities to small molecules. Using a novel type of heterologous MST, we have accurately measured a solution-based binding affinity of serum antibodies to heroin which was previously impossible with other currently available methods. Moreover, this mismatch approach (i.e., using a cross-reactive hapten tracer) has never been reported in the literature. When compared with equilibrium dialysis combined with ultra-performance liquid chromatography/tandem mass spectrometry (ED-UPLC/MS/MS), this novel MST method yields similar binding affinity values for polyclonal antibodies to the major heroin metabolites 6-AM and morphine. Additionally, we herein report the method of synthesis of this novel cross-reactive hapten, MorHap-acetamide—a useful analog for the study of heroin hapten–antibody interactions. Using heterologous MST, we were able to determine the affinities, down to nanomolar accuracies, of polyclonal antibodies to various abused opioids. While optimizing this method, we further discovered that heroin is protected from serum esterase degradation by the presence of these antibodies in a concentration-dependent manner. Lastly, using affinity data for a number of structurally different opioids, we were able to dissect the moieties that are crucial to antibody binding. The novel MST method that is presented herein can be extended to the analysis of any ligand that is prone to degradation and can be applied not only to the development of vaccines to substances of abuse but also to the analysis of small molecule/protein interactions in the presence of serum.

Graphical abstract

Approaches to measure protein binding of enzymatically unstable compounds in plasma

AIM

To develop approaches to measure plasma protein binding (PPB) of enzymatically unstable compounds.

METHODOLOGY

Bis-para-nitrophenyl phosphate (BNPP) was used to inhibit enzyme activity and stabilize two model compounds (diltiazem and oseltamivir) that are subject to enzyme-catalyzed hydrolysis in plasma. Protein binding of the compounds in BNPP-treated rat plasma was measured using equilibrium dialysis or ultrafiltration.

CONCLUSION

PPB measurement of unstable compounds was improved by using enzyme inhibitor to stabilize the compounds in plasma during the assay. The effect of BNPP concentration on drug-protein binding appeared to be compound dependent. Given the compound's nonspecific binding to the assay device can be accounted for in the unbound fraction measurement, ultrafiltration can be a viable alternative or complementary approach for PPB assay of unstable compounds while minimizing the potential impact of enzyme inhibitor on drug-protein binding.

1-Aminobenzotriazole: A Mechanism-Based Cytochrome P450 Inhibitor and Probe of Cytochrome P450 Biology

1-Aminobenzotriazole (1-ABT) is a pan-specific, mechanism-based inactivator of the xenobiotic metabolizing forms of cytochrome P450 in animals, plants, insects, and microorganisms. It has been widely used to investigate the biological roles of cytochrome P450 enzymes, their participation in the metabolism of both endobiotics and xenobiotics, and their contributions to the metabolism-dependent toxicity of drugs and chemicals. This review is a comprehensive evaluation of the chemistry, discovery, and use of 1-aminobenzotriazole in these contexts from its introduction in 1981 to the present.

Diterpenoids from the roots of Euphorbia ebracteolata and their inhibitory effects on human carboxylesterase 2

A chemical investigation of the roots of Euphorbia ebracteolata identified eighteen diterpenoids and glycosides. On the basis of spectroscopic data, they were determined to be ent-kauranes, ent-atisanes, tigliane derivatives, ingenane, and ent-abietanes, among which were eleven previously undescribed diterpenoids. The inhibitory effects of the isolated compounds against human carboxylesterase 2 (hCE-2) were evaluated in vitro, which revealed moderate inhibitory effects with IC₅₀ values < 50 μM. Next, the inhibitory kinetics were evaluated for the putative hCE-2 inhibitor 4β,9α,16,20-tetrahydroxy-14(13 → 12)-abeo-12αH-1,6-tigliadiene-3,13-dione (IC₅₀ 3.88 μM), and results indicated competitive inhibition with K_i 4.94 μM. Additionally, none of the diterpenoids showed cytotoxic effects against five human tumor cell lines as determined by MTT assays.

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.

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

White Mulberry Root-bark (WMR) is an edible Chinese herbal used for the treatment of inflammation, nephritis and asthma. This study aimed to investigate the inhibitory effects of ethanol extract from WMR against human carboxylesterase 2 (hCE2), as well as to identity and character natural hCE2 inhibitors in this herbal. Our results demonstrated that the ethanol extract of WMR displayed potent inhibitory effects against hCE2, while three major bioactive constitutes in WMR were identified on the basis of LC fingerprinting combined with activity-based screening of LC fractions. Three bioactive compounds including SD, KG and SC were efficiently identified by comparison of LC retention times, UV and MS spectral data, with the help of authentic standards. The inhibition potentials and inhibition types of these natural compounds against hCE2 were further investigated in human liver microsomes. The results demonstrated that these bioactive compounds are potent non-competitive inhibitors against hCE2, with the K_i values ranging from 0.76μM to 1.09μM. All these findings suggested that three abundant natural compounds in WMR displayed potent inhibitory effects against hCE2, which could be used as lead compounds to develop more potent hCE2 inhibitors for the alleviation of hCE2-mediated severe delayed-onset diarrhea.

Simultaneous determination of curcumin diethyl disuccinate and its active metabolite curcumin in rat plasma by LC-MS/MS: Application of esterase inhibitors in the stabilization of an ester-containing prodrug

Four esterase inhibitors, ethylenediamine tetraacetic acid disodium (Na2EDTA), sodium fluoride (NaF), bis(4-nitrophenyl) phosphate (BNPP) and phenylmethanesulfonyl fluoride (PMSF), were evaluated for their inhibitory effects on enzymatic hydrolysis of labile phenolate esters in curcumin diethyl disuccinate (CDD), a prodrug of curcumin (CUR), in rat plasma. BNPP and PMSF at 10mM exhibited stabilization by preventing degradation of CDD. BNPP at a final concentration of 10mM was subsequently selected to prevent ex vivo metabolism of CDD throughout LC-MS/MS analysis of CDD and CUR in rat plasma. A simple protein precipitation technique using acetonitrile as a precipitating agent was used to extract CDD, CUR and dimethylcurcumin (DMC), an internal standard, from rat plasma. Chromatographic separation was performed on a Halo C8 column (4.6×50mm, 2.7μm) using an isocratic mobile phase containing acetonitrile-0.2% formic acid in water (73:27v/v) with a flow rate of 0.4mLmin(-1). An AB SCIEX QTRAP(®) 6500 mass spectrometer was operated using a positive ion electrospray mode for ionization and detection of analytes and internal standard. Calibration curves for CDD and CUR were established using 50μL of rat plasma over the concentration range of 1-500ngmL(-1). The developed method was fully validated according to US Food and Drug Administration (FDA) guidelines for selectivity, sensitivity, linearity, accuracy, precision, dilution integrity, recovery, matrix effect, and stability. The validated method was applied to evaluate the pharmacokinetics of CDD and CUR in rats after a single intravenous dose of 40mgkg(-1). The method using BNPP as an esterase inhibitor was successful in determining the remaining CDD in rat plasma. The pharmacokinetic results indicate that CDD in rats is converted instantaneously to CUR after intravenous administration and a higher CUR plasma concentration at 5min is achieved in comparison with direct intravenous injection of CUR.

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 simple nonradioactive method for the determination of the binding affinities of antibodies induced by hapten bioconjugates for drugs of abuse

The accurate analytical measurement of binding affinities of polyclonal antibody in sera to heroin, 6-acetylmorphine (6-AM), and morphine has been a challenging task. A simple nonradioactive method that uses deuterium-labeled drug tracers and equilibrium dialysis (ED) combined with ultra performance liquid chromatography/tandem mass spectrometry (UPLC/MS/MS) to measure the apparent dissociation constant (K d) of antibodies to 6-AM and morphine is described. The method can readily detect antibodies with K d in the low nanomolar range. Since heroin is rapidly degraded in sera, esterase inhibitors were included in the assay, greatly reducing heroin hydrolysis. MS/MS detection directly measured the heroin in the assay after overnight ED, thereby allowing the quantitation of % bound heroin in lieu of K d as an alternative measurement to assess heroin binding to polyclonal antibody sera. This is the first report that utilizes a solution-based assay to quantify heroin-antibody binding without being confounded by the presence of 6-AM and morphine and to measure K d of polyclonal antibody to 6-AM. Hapten surrogates 6-AcMorHap, 6-PrOxyHap, MorHap, DiAmHap, and DiPrOxyHap coupled to tetanus toxoid (TT) were used to generate high affinity antibodies to heroin, 6-AM, and morphine. In comparison to competition ED-UPLC/MS/MS which gave K d values in the nanomolar range, the commonly used competition enzyme-linked immunosorbent assay (ELISA) measured the 50% inhibition concentration (IC50) values in the micromolar range. Despite the differences in K d and IC50 values, similar trends in affinities of hapten antibodies to heroin, 6-AM, and morphine were observed by both methods. Competition ED-UPLC/MS/MS revealed that among the five TT-hapten bioconjugates, TT-6-AcMorHap and TT-6-PrOxyHap induced antibodies that bound heroin, 6-AM, and morphine. In contrast, TT-MorHap induced antibodies that poorly bound heroin, while TT-DiAmHap and TT-DiPrOxyHap induced antibodies either did not bind or poorly bound to heroin, 6-AM, and morphine. This simple and nonradioactive method can be extended to other platforms, such as oxycodone, cocaine, nicotine, and methamphetamine for the selection of the lead hapten design during substance abuse vaccine development.

Protostane Triterpenoids from the Rhizome of Alisma orientale Exhibit Inhibitory Effects on Human Carboxylesterase 2

Twelve new and 10 known protostane triterpenoids were isolated from the rhizome of Alisma orientale. Their structures were elucidated based on physical data analyses, including UV, HRESIMS, NMR experiments ((1)H, (13)C NMR, (1)H-(1)H COSY, HSQC, HMBC, and NOESY), and induced electronic circular dichroism. New compounds 1-12 were classified as protostanes (1-10), 29-norprotostane (11), and 24-norprotostane (12) by structure analyses. Furthermore, the inhibitory effects on human carboxylesterases (hCE-1, hCE-2) of compounds 1-22 were evaluated. Compounds 2, 6, 9, and 11 showed moderate inhibitory activities and were selective toward hCE-2 enzymes, with IC50 values of 8.68, 4.72, 4.58, and 2.02 μM, respectively. The inhibition kinetics of compound 11 toward hCE-2 were established, and the Ki value was determined as 1.76 μM using a mixed inhibition model. The interaction of bioactive compound 11 with hCE-2 was shown using molecular docking.

Fructus Psoraleae contains natural compounds with potent inhibitory effects towards human carboxylesterase 2

Fructus Psoraleae (FP) is an edible Chinese herbal which is widely used in Asia for the treatment of various diseases including asthma, diarrhea, and osteoporosis. This study aimed to investigate the inhibitory effects of the crude ethanol extract from FP on human carboxylesterase 2 (hCE2), as well as to identity and characterize the naturally occurring inhibitors of hCE2 in FP. Our results demonstrated that the ethanol extract of FP displayed potent inhibitory effects towards hCE2, while five major bioactive constitutes in FP were efficiently identified by LC-DAD-ESI-MS/MS, with the aid of LC-based activity profiling. The identified bioactive compounds including neobavaisoflavone, isobavachalcone, bavachinin, corylifol A and bakuchiol were found to be naturally occurring potent inhibitors of hCE2, with low Ki values ranging from 0.62μM to 3.89μM. This is the first report of the chemical constitutes in FP as potent inhibitors of hCE2.

A highly selective long-wavelength fluorescent probe for the detection of human carboxylesterase 2 and its biomedical applications

A highly selective long-wavelength fluorescent probe for the detection of human carboxylesterase 2 (hCE2) has been developed and well characterized. The probe can be used for measuring the real activities of hCE2 in complex biological systems.

Consideration of the unbound drug concentration in enzyme kinetics

The study of enzyme kinetics in drug metabolism involves assessment of rates of metabolism and inhibitory potencies over a suitable concentration range. In all but the very simplest in vitro system, these drug concentrations can be influenced by a variety of nonspecific binding reservoirs that can reduce the available concentration to the enzyme system under investigation. As a consequence, the apparent kinetic parameters that are derived, such as K m or K i, can deviate from the true values. There are a number of sources of these nonspecific binding depots or barriers, including membrane permeation and partitioning, plasma or serum protein binding, and incubational binding. In the latter case, this includes binding to the assay apparatus, as well as biological depots, depending on the characteristics of the in vitro matrix being used. Given the wide array of subcellular, cellular, and recombinant enzyme systems utilized in drug metabolism, each of these has different components that can influence the free drug concentration. The physicochemical properties of the test compound are also paramount in determining the influential factors in any deviation between true and apparent kinetic behavior. This chapter describes the underlying mechanisms determining the free drug concentration in vitro and how these factors can be accounted for in drug metabolism studies, illustrated with case studies from the literature.

The bioanalytical challenge of determining unbound concentration and protein binding for drugs

Knowledge regarding unbound concentrations is of vital importance when exploring the PK and PD of a drug. The accurate and reproducible determination of plasma protein binding and unbound concentrations for a compound/drug is a serious challenge for the bioanalytical laboratory. When the drug is in equilibrium with the binding protein(s), this equilibrium will shift when physiological conditions are not met. Furthermore, the true unbound fraction/concentration is unknown, and there are numerous publications in the scientific literature reporting and discussing data that have been produced without sufficient control of the parameters influencing the equilibrium. In this Review, different parameters affecting the equilibrium and analysis are discussed, together with suggestions on how to control these parameters in order to produce as trustworthy results for unbound concentrations/fractions as possible.

The development of highly potent inhibitors for porcupine

Porcupine is a member of the membrane-bound O-acyltransferase family of proteins. It catalyzes the palmitoylation of Wnt proteins, a process required for their secretion and activity. We recently disclosed a class of small molecules (IWPs) as the first reported Porcn inhibitors. We now describe the structure-activity relationship studies and the identification of subnanomolar inhibitors. We also report herein the effects of IWPs on Wnt-dependent developmental processes, including zebrafish posterior axis formation and kidney tubule formation.

Dexamethasone regulates differential expression of carboxylesterase 1 and carboxylesterase 2 through activation of nuclear receptors

Carboxylesterases (CESs) play important roles in the metabolism of endogenous and foreign compounds in physiological and pharmacological responses. The aim of this study was to investigate the effect of dexamethasone at different doses on the expression of CES1 and CES2. Imidapril and irinotecan hydrochloride (CPT-11) were used as special substrates for CES1 and CES2, respectively. Rat hepatocytes were cultured and treated with different concentrations of dexamethasone. The hydrolytic activity of CES1 and CES2 was tested by incubation experiment and their expression was quantitated by real-time PCR. A pharmacokinetic study was conducted in SD rats to further evaluate the effect of dexamethasone on CESs activity in vivo. Western blotting was performed to investigate the regulatory mechanism related to pregnane X receptor (PXR) and glucocorticoid receptor (GR). The results showed that exposure of cultured rat hepatocytes to nanomolar dexamethasone inhibited the imidapril hydrolase activity, which was slightly elevated by micromolar dexamethasone. For CES2, CPT-11 hydrolase activity was induced only when dexamethasone reached micromolar levels. The real-time PCR demonstrated that CES1 mRNA was markedly decreased by nanomolar dexamethasone and increased by micromolar dexamethasone, whereas CES2 mRNA was significantly increased by micromolar dexamethasone. The results of a complementary animal study showed that the concurrent administration of dexamethasone significantly increased the plasma concentration of the metabolite of imidapril while the ratio of CPT-11 to its metabolite SN-38 was significantly decreased. PXR protein was gradually increased by serial concentrations of dexamethasone. However, only nanomolar dexamethasone elevated the level of GR protein. The different concentrations of dexamethasone required suggested that suppression of CES1 may be mediated by GR whereas the induction of CES2 may result from the role of PXR. It was concluded that dexamethasone at different concentrations can differentially regulate CES1 and CES2.

Significant species difference in amide hydrolysis of GDC-0834, a novel potent and selective Bruton's tyrosine kinase inhibitor

(R)-N-(3-(6-(4-(1,4-dimethyl-3-oxopiperazin-2-yl)phenylamino)-4-methyl-5-oxo-4,5-dihydropyrazin-2-yl)-2-methylphenyl)-4,5,6,7-tetrahydrobenzo[b]thiophene-2-carboxamide (GDC-0834) is a potent and selective inhibitor of Bruton's tyrosine kinase (BTK), investigated as a potential treatment for rheumatoid arthritis. In vitro metabolite identification studies in hepatocytes revealed predominant formation of an inactive metabolite (M1) via amide hydrolysis in human. The formation of M1 appeared to be NADPH-independent in human liver microsomes. M1 was found in only minor to moderate quantities in plasma from preclinical species dosed with GDC-0834. Human clearance predictions using various methodologies resulted in estimates ranging from low to high. In addition, GDC-0834 exhibited low clearance in PXB chimeric mice with humanized liver. Uncertainty in human pharmacokinetic prediction and high interest in a BTK inhibitor for clinical evaluation prompted an investigational new drug strategy, in which GDC-0834 was rapidly advanced to a single-dose human clinical trial. GDC-0834 plasma concentrations in humans were below the limit of quantitation (<1 ng/ml) in most samples from the cohorts dosed orally at 35 and 105 mg. In contrast, substantial plasma concentrations of M1 were observed. In human plasma and urine, only M1 and its sequential metabolites were identified. The formation kinetics of M1 was evaluated in rat, dog, monkey, and human liver microsomes in the absence of NADPH. The maximum rate of M1 formation (V(max)) was substantially higher in human compared with that in other species. In contrast, the Michaelis-Menten constant (K(m)) was comparable among species. Intrinsic clearance (V(max)/K(m)) of GDC-0834 from M1 formation in human was 23- to 169-fold higher than observed in rat, dog, and monkey.

Carboxylesterase inhibitors

INTRODUCTION

Carboxylesterases play major roles in the hydrolysis of numerous therapeutically active compounds. This is, in part, due to the prevalence of the ester moiety in these small molecules. However, the impact these enzymes may play on drug stability and pharmacokinetics is rarely considered prior to molecule development. Therefore, the application of selective inhibitors of this class of proteins may have utility in modulating the metabolism, distribution and toxicity of agents that are subjected to enzyme hydrolysis.

AREAS COVERED

This review details the development of all such compounds dating back to 1986, but principally focuses on the very recent identification of selective human carboxylesterases inhibitors.

EXPERT OPINION

The implementation of carboxylesterase inhibitors may significantly revolutionize drug discovery. Such molecules may allow for improved efficacy of compounds inactivated by this class of enzymes and/or reduce the toxicity of agents that are activated by these proteins. Furthermore, since lack of carboxylesterase activity appears to have no obvious biological consequence, these compounds could be applied in combination with virtually any esterified drug. Therefore, inhibitors of these proteins may have utility in altering drug hydrolysis and distribution in vivo. The characteristics, chemical and biological properties and potential uses of such agents are discussed here.

Detection and quantification of carboxylesterase 2 activity by capillary electrophoresis

The purpose of this study was to develop an analytical method to quantify the relative activities of carboxylesterases (CESs) in biological samples. Taking the advantage of loperamide, a specific carboxylesterase 2 (CES2) inhibitor, and bis-p-nitrophenyl phosphate (BNPP), an irreversible CESs inhibitor, we propose for the first time a capillary electrophoresis (CE) method that enables detecting and distinguishing CES2 activity from other CESs in complex biological samples. The capillary electrophoresis method proved to be fast, simple, repeatable, and applicable to the measurement of the specific activity of CESs. The method was successfully applied to the evaluation of human cells overexpressing human carboxylesterase 2 (hCE-2) and to several mammalian sera, using extremely small amounts of samples in comparison with traditional spectrophotometric methods. The same rationale can be applied to establish methods for determining the activity of other isoenzymes, using the appropriate specific inhibitors.