Involvement of liver carboxylesterases in the in vitro metabolism of lidocaine

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

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

Carboxylesterase Activities and Protein Expression in Rabbit and Pig Ocular Tissues

Hydrolytic reactions constitute an important pathway of drug metabolism and a significant route of prodrug activation. Many ophthalmic drugs and prodrugs contain ester groups that greatly enhance their permeation across several hydrophobic barriers in the eye before the drugs are either metabolized or released, respectively, via hydrolysis. Thus, the development of ophthalmic drug therapy requires the thorough profiling of substrate specificities, activities, and expression levels of ocular esterases. However, such information is scant in the literature, especially for preclinical species often used in ophthalmology such as rabbits and pigs. Therefore, our aim was to generate systematic information on the activity and expression of carboxylesterases (CESs) and arylacetamide deacetylase (AADAC) in seven ocular tissue homogenates from these two species. The hydrolytic activities were measured using a generic esterase substrate (4-nitrophenyl acetate) and, in the absence of validated substrates for rabbit and pig enzymes, with selective substrates established for human CES1, CES2, and AADAC (d-luciferin methyl ester, fluorescein diacetate, procaine, and phenacetin). Kinetics and inhibition studies were conducted using these substrates and, again due to a lack of validated rabbit and pig CES inhibitors, with known inhibitors for the human enzymes. Protein expression levels were measured using quantitative targeted proteomics. Rabbit ocular tissues showed significant variability in the expression of CES1 (higher in cornea, lower in conjunctiva) and CES2 (higher in conjunctiva, lower in cornea) and a poor correlation of CES expression with hydrolytic activities. In contrast, pig tissues appear to express only CES1, and CES3 and AADAC seem to be either low or absent, respectively, in both species. The current study revealed remarkable species and tissue differences in ocular hydrolytic enzymes that can be taken into account in the design of esterase-dependent prodrugs and drug conjugates, the evaluation of ocular effects of systemic drugs, and in translational and toxicity studies.

Selective Inhibitors of Human Liver Carboxylesterase Based on a β-Lapachone Scaffold: Novel Reagents for Reaction Profiling

Carboxylesterases (CEs) are ubiquitous enzymes that are responsible for the metabolism of xenobiotics, including drugs such as irinotecan and oseltamivir. Inhibition of CEs significantly modulates the efficacy of such agents. We report here that β-lapachone is a potent, reversible CE inhibitor with K_i values in the nanomolar range. A series of amino and phenoxy analogues have been synthesized, and although the former are very poor inhibitors, the latter compounds are highly effective in modulating CE activity. Our data demonstrate that tautomerism of the amino derivatives to the imino forms likely accounts for their loss in biological activity. A series of N-methylated amino derivatives, which are unable to undergo such tautomerism, were equal in potency to the phenoxy analogues and demonstrated selectivity for the liver enzyme hCE1. These specific inhibitors, which are active in cell culture models, will be exceptionally useful reagents for reaction profiling of esterified drugs in complex biological samples.

Iron-Catalyzed Cα-H Oxidation of Tertiary, Aliphatic Amines to Amides under Mild Conditions

De novo syntheses of amides often generate stoichiometric amounts of waste. Thus, recent progress in the field has focused on precious metal catalyzed, oxidative protocols to generate such functionalities. However, simple tertiary alkyl amines cannot be used as starting materials in these protocols. The research described herein enables the oxidative synthesis of amides from simple, noncyclic tertiary alkyl amines under synthetically useful, mild conditions through a biologically inspired approach: Fe-catalyzed Cα-H functionalization. Mechanistic investigations provide insight into reaction intermediates and allow the development of a mild Cα-H cyanation method using the same catalyst system. The protocol was further applied to oxidize the drug Lidocaine, demonstrating the potential utility of the developed chemistry for metabolite synthesis.

Esterase Activity and Intracellular Localization in Reconstructed Human Epidermal Cultured Skin Models

Background

Reconstructed human epidermal culture skin models have been developed for cosmetic and pharmaceutical research.

Objective

This study evaluated the total and carboxyl esterase activities (i.e., K_m and V_max, respectively) and localization in two reconstructed human epidermal culture skin models (LabCyte EPI-MODEL [Japan Tissue Engineering] and EpiDerm [MatTek/Kurabo]). The usefulness of the reconstruction cultured epidermis was also verified by comparison with human and rat epidermis.

Methods

Homogenized epidermal samples were fractioned by centrifugation. p-nitrophenyl acetate and 4-methylumbelliferyl acetate were used as substrates of total esterase and carboxyl esterase, respectively.

Results

Total and carboxyl esterase activities were present in the reconstructed human epidermal culture skin models and were localized in the cytosol. Moreover, the activities and localization were the same as those in human and rat epidermis.

Conclusion

LabCyte EPI-MODEL and EpiDerm are potentially useful for esterase activity prediction in human epidermis.

The catalytic mechanism of carboxylesterases: a computational study

The catalytic mechanism of carboxylesterases (CEs, EC 3.1.1.1) is explored by computational means. CEs hydrolyze ester, amide, and carbamate bonds found in xenobiotics and endobiotics. They can also perform transesterification, a reaction important, for instance, in cholesterol homeostasis. The catalytic mechanisms with three different substrates (ester, thioester, and amide) have been established at the M06-2X/6-311++G**//B3LYP/6-31G* level of theory. It was found that the reactions proceed through a mechanism involving four steps instead of two as is generally proposed: (i) nucleophilic attack of serine to the substrate, forming the first tetrahedral intermediate, (ii) formation of the acyl-enzyme complex concomitant with the release of the alcohol product, (iii) nucleophilic attack of a water or alcohol molecule forming the second tetrahedral intermediate, and (iv) the release of the second product of the reaction. The results agree very well with the available experimental data and show that the hydrolytic and the transesterification reactions are competitive processes when the substrate is an ester. In all the other studied substrates (thioester or amide), the hydrolytic and transesterification process are less favorable and some of them might not even take place under in vivo conditions.

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.

Lidocaïne test for easier and less time consuming assessment of liver function in several hepatic injury models

PURPOSE

In this study, we developed an ex vivo functional assay to assess liver metabolic capacity adapted from the lidocaïne test in rats.

METHODS

Animals used were subjected to different models of liver injury: hypothermic ischemia (H/I, n = 8), ischemia-reperfusion (I/R, n = 8) and CCl4 induced liver cirrhosis (n = 11), and compared with sham operated rats (n = 5). Livers were then extracted and a fragment of whole tissue was incubated with lidocaïne for 15, 30, 60, 120, 240, 360, and 720 min at which both lidocaïne and its major metabolite monoethylglycinexylidide (MEGX) were measured by high performance liquid chromatography (HPLC). A histological study and biochemical assays (transaminase levels) were also performed to further evaluate and confirm our data.

RESULTS

Pharmacokinetic profile of lidocaïne metabolism in sham-operated animals revealed that the maximum concentration of MEGX is achieved at 120 min. Both lidocaïne metabolism and MEGX formation levels were significantly altered in all three models of hepatic injury. The extent of hepatic damage was confirmed by increased levels of transaminase levels and alteration of hepatocyte's structure with areas of necrosis.

CONCLUSION

Our method provides reliable and reproducible results using only a small portion of liver which allows for a fast and easy assessment of liver metabolic capacity. Moreover, our method presents an alternative to the in vivo technique and seems more feasible in a clinical setting.

In Silico Design and Evaluation of Carboxylesterase Inhibitors

Carboxylesterases (CEs) are important enzymes that catalyze biological detoxification, hydrolysis of certain pesticides, and metabolism of many esterified drugs. The development of inhibitors for CE has many potential uses, including increasing drug lifetime and altering biodistrubution; reducing or abrogating toxicity of metabolized drugs; and reducing pest resistance to insecticides. In this review, we discuss the major classes of known mammalian CE inhibitors and describe our computational efforts to design new scaffolds for development of novel, selective inhibitors. We discuss several strategies for in silico inhibitor development, including structure docking, database searching, multidimensional quantitative structure activity analysis (QSAR), and a newly-used approach that uses QSAR combined with de novo drug design. While our research is focused on design of specific inhibitors for human intestinal carboxylesterase (hiCE), the methods described are generally applicable to inhibitors of other enzymes, including CE from other tissues and organisms.

Improved, selective, human intestinal carboxylesterase inhibitors designed to modulate 7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin (Irinotecan; CPT-11) toxicity

CPT-11 is an antitumor prodrug that is hydrolyzed by carboxylesterases (CE) to yield SN-38, a potent topoisomerase I poison. However, the dose limiting toxicity delays diarrhea that is thought to arise, in part, from activation of the prodrug by a human intestinal CE (hiCE). Therefore, we have sought to identify selective inhibitors of hiCE that may have utility in modulating drug toxicity. We have evaluated one such class of molecules (benzene sulfonamides) and developed QSAR models for inhibition of this protein. Using these predictive models, we have synthesized a panel of fluorene analogues that are selective for hiCE, demonstrating no cross reactivity to the human liver CE, hCE1, or toward human cholinesterases, and have K(i) values as low as 14 nM. These compounds prevented hiCE-mediated hydrolysis of the drug and the potency of enzyme inhibition correlated with the clogP of the molecules. These studies will allow the development and application of hiCE-specific inhibitors designed to selectively modulate drug hydrolysis in vivo.

Investigation of the species-dependent in vitro metabolism of BAL30630 by stable isotope labeling and isotope exchange experiments analyzed by capillary liquid chromatography coupled to mass spectrometry

The in vitro metabolic profile of BAL30630, an antifungal piperazine propanol derivative, which inhibits the 1,3-beta-D-glucansynthase, was investigated by incubation with microsomes of several species and with rat hepatocytes. For the spotting of the metabolites, mixtures of BAL30630 with a stable isotope (deuterium) labeled analogue were incubated. The metabolic pattern comprises several oxidized metabolites. Based on isotope exchange experiments, their structures could be assigned to epoxide- and hydroxylated metabolites. In hepatocyte incubations, several glucuronides formed from these oxidized metabolites could be observed. From the analysis of the metabolic pattern in microsomes, products of carbamate hydrolysis were characterized. This hydrolysis was highly species dependent. In activated incubations and in rat hepatocytes, those metabolites were further oxidized. In incubations without NADPH activation, the resulting hydrolytic metabolites could be enriched without the subsequent oxidation. Final structural elucidation of the metabolites was performed using accurate mass determination and isotope exchange experiments, in which incubations were analyzed by deuterium exchange and capillary HPLC-QTof-MS and MS/MS. The use of non-radioactive, stabile isotope labeled drug analogues in combination with isotope exchange studies was essential in particular for a defined assignment of the functional groups in the structures of the investigated metabolites.

Protection of cellular and mitochondrial functions against liver ischemia by N-benzyl-N'-(2-hydroxy-3,4-dimethoxybenzyl)-piperazine (BHDP), a sigma1 ligand

We investigated the antiischemic properties of a new compound N-benzyl-N'-(2-hydroxy-3,4-dimethoxybenzyl)-piperazine (BHDP), having high affinity and selectivity for the sigma(1) receptor, in two different models of ischemia. The first was an experimental model of rat liver normothermic ischemia-reperfusion. Rats were pretreated with different doses of BHDP (0.5, 2.5 or 10 mg/kg/day, or solvent alone) and subjected to 90 min normothermic ischemia followed by either 30 or 120 min reperfusion. The second model was a hypothermic model of ischemia in which livers were incubated for 24 h at 4 degrees C in a preservation solution in the absence or presence of increasing BHDP concentrations (0.5, 2.5 or 10 microg/ml). These different ischemic conditions induced huge alterations in hepatocyte functions (membrane leakage of alanine aminotransferase and aspartate aminotransferase, decreased metabolic capacities evaluated by the ability of the liver to transform lidocaine, alterations of mitochondrial functions characterized by a decrease in ATP synthesis and the appearance of histological damages). Pretreatment of rats with BHDP alleviated these deleterious ischemia-reperfusion effects in a dose-dependent manner at both the cellular and mitochondrial levels. The protection of mitochondrial functions was almost complete at a dosage of 10 mg/kg/day during normothermic ischemia and 10 microg/ml in the preservation liquid during hypothermic ischemia. In addition, BHDP significantly reduced the histological damage. These data demonstrate that BHDP protects liver against the deleterious effects of ischemia-reperfusion and suggest that sigma(1) receptors play an important role in the protective effect.

The role of esterases in the metabolism of ciclesonide to desisobutyryl-ciclesonide in human tissue

Ciclesonide (CIC) is an inhaled glucocorticosteroid. This study aimed to identify esterases involved in the metabolism of CIC to the active metabolite desisobutyryl-ciclesonide (des-CIC), and to measure hydrolysis rates in human liver, lung and plasma and normal human bronchial epithelial (NHBE) cells in vitro. Ciclesonide (5 microM and 500 microM) was incubated with microsomal or cytosolic fractions from liver, lung and plasma (n=4 for each) and des-CIC formation was determined by reverse-phase high-performance liquid chromatography with U.V. detection. The roles of carboxylesterase, cholinesterase and A-esterase in CIC hydrolysis were determined using a range of inhibitors. Inhibitor concentrations for liver and NHBE cells were 100 microM and 5 microM, respectively. Liver tissue had a higher activity for 500 microM CIC hydrolysis (microsomes: 25.4; cytosol: 62.9 nmol/g tissue/min) than peripheral lung (microsomes: 0.089; cytosol: 0.915 nmol/g tissue/min) or plasma (0.001 nmol/mL plasma/min), corresponding with high levels of carboxylesterase and cholinesterase in the liver compared with the lung. CIC (5 microM) was rapidly hydrolyzed by NHBE cells (approximately 30% conversion at 4h), with almost complete conversion by 24h. In liver and NHBE cells, major involvement of cytosolic carboxylesterases, with some contribution by cholinesterases, was indicated. The highest level of conversion was found in the liver, the site of inactivation of des-CIC through rapid oxidation by cytochrome P450. Carboxylesterases in bronchial epithelial cells probably contribute significantly to the conversion to des-CIC in the target organ, whereas low systemic levels of des-CIC are a result of the high metabolic clearance by the liver following CIC inhalation.

Deacetylclivorine: A Gender-Selective Metabolite of Clivorine Formed in Female Sprague-Dawley Rat Liver Microsomes

Clivorine, a naturally occurring pyrrolizidine alkaloid, causes liver toxicity via its metabolic activation to generate toxic metabolite (pyrrolic ester). Female Sprague-Dawley (SD) rats are reported to be less susceptible to clivorine intoxication than male SD rats. However, the biochemical mechanism causing such gender difference is largely unknown. The present study investigated hepatic microsomal metabolism of clivorine in female rats to delineate the mechanism of the gender difference. Two pathways, which directly metabolize clivorine, were observed. First, the metabolic activation to produce the toxic pyrrolic ester followed by formations of bound pyrroles, dehydroretronecine, 7-glutathionyldehydroretronecine, and clivoric acid were found in female rats, and CYP3A1/2 isozymes were identified to catalyze the metabolic activation. Compared with male rats ( approximately 21%), the metabolic activation in female rats was significantly lower ( approximately 4%) possibly because of significantly lower CYP3A1/2 levels expressed in female rats. Second, a direct hydrolysis to generate the novel female rat-specific metabolite deacetylclivorine was shown as the predominant pathway ( approximately 16% clivorine metabolism) in female rat liver microsomes and was determined to be mediated by microsomal hydrolase A. Furthermore, when the metabolic activation was completely inhibited by ketoconazole, the amount of deacetylclivorine formed in a 1-h incubation significantly increased from 19.44 +/- 3.00 to 54.87 +/- 9.30 nmol/mg protein, suggesting that the two pathways compete with each other. Therefore, the lower susceptibility of female SD rats to clivorine intoxication is suggested to be caused by the significantly higher extent of the direct hydrolysis and a lower degree of the metabolic activation.

Multisite promiscuity in the processing of endogenous substrates by human carboxylesterase 1

Human carboxylesterase 1 (hCE1) is a drug and endobiotic-processing serine hydrolase that exhibits relatively broad substrate specificity. It has been implicated in a variety of endogenous cholesterol metabolism pathways including the following apparently disparate reactions: cholesterol ester hydrolysis (CEH), fatty acyl Coenzyme A hydrolysis (FACoAH), acyl-Coenzyme A:cholesterol acyltransfer (ACAT), and fatty acyl ethyl ester synthesis (FAEES). The structural basis for the ability of hCE1 to perform these catalytic actions involving large substrates and products has remained unclear. Here we present four crystal structures of the hCE1 glycoprotein in complexes with the following endogenous substrates or substrate analogues: Coenzyme A, the fatty acid palmitate, and the bile acids cholate and taurocholate. While the active site of hCE1 was known to be promiscuous and capable of interacting with a variety of chemically distinct ligands, these structures reveal that the enzyme contains two additional ligand-binding sites and that each site also exhibits relatively non-specific ligand-binding properties. Using this multisite promiscuity, hCE1 appears structurally capable of assembling several catalytic events depending, apparently, on the physiological state of the cellular environment. These results expand our understanding of enzyme promiscuity and indicate that, in the case of hCE1, multiple non-specific sites are employed to perform distinct catalytic actions.

Structural basis of heroin and cocaine metabolism by a promiscuous human drug-processing enzyme

We present the first crystal structures of a human protein bound to analogs of cocaine and heroin. Human carboxylesterase 1 (hCE1) is a broad-spectrum bioscavenger that catalyzes the hydrolysis of heroin and cocaine, and the detoxification of organophosphate chemical weapons, such as sarin, soman and tabun. Crystal structures of the hCE1 glycoprotein in complex with the cocaine analog homatropine and the heroin analog naloxone provide explicit details about narcotic metabolism in humans. The hCE1 active site contains both specific and promiscuous compartments, which enable the enzyme to act on structurally distinct chemicals. A selective surface ligand-binding site regulates the trimer-hexamer equilibrium of hCE1 and allows each hCE1 monomer to bind two narcotic molecules simultaneously. The bioscavenger properties of hCE1 can likely be used to treat both narcotic overdose and chemical weapon exposure.

Crystal structure of human carboxylesterase 1 complexed with the Alzheimer's drug tacrine: from binding promiscuity to selective inhibition

Human carboxylesterase 1 (hCE1) is a broad-spectrum bioscavenger that plays important roles in narcotic metabolism, clinical prodrug activation, and the processing of fatty acid and cholesterol derivatives. We determined the 2.4 A crystal structure of hCE1 in complex with tacrine, the first drug approved for treating Alzheimer's disease, and compare this structure to the Torpedo californica acetylcholinesterase (AcChE)-tacrine complex. Tacrine binds in multiple orientations within the catalytic gorge of hCE1, while it stacks in the smaller AcChE active site between aromatic side chains. Our results show that hCE1's promiscuous action on distinct substrates is enhanced by its ability to interact with ligands in multiple orientations at once. Further, we use our structure to identify tacrine derivatives that act as low-micromolar inhibitors of hCE1 and may provide new avenues for treating narcotic abuse and cholesterol-related diseases.