Acyl glucuronide reactivity in perspective: biological consequences

Bioactivation and reactivity research advances - 2023 year in review

Advances in the field of bioactivation have significantly contributed to our understanding and prediction of drug-induced liver injury (DILI). It has been established that many adverse drug reactions, including DILI, are associated with the formation and reactivity of metabolites. Modern methods allow us to detect and characterize these reactive metabolites in earlier stages of drug development, which helps anticipate and circumvent the potential for DILI. Improved in silico models and experimental techniques that better reflect in vivo environments are enhancing predictive capabilities for DILI risk. Further, studies on the mechanisms of bioactivation, including enzyme interactions and the role of individual genetic differences, have provided valuable insights for drug optimizations. Cumulatively, this progress is continually refining our approaches to drug safety evaluation and personalized medicine.

Role of microsomal metabolism in bromfenac-induced cytotoxicity

This study delves into the intricate mechanisms underlying drug-induced liver injury (DILI) with a specific focus on bromfenac, the withdrawn nonsteroidal anti-inflammatory drug. DILI is a pervasive concern in drug development, prompting market withdrawals and posing significant challenges to healthcare. Despite the withdrawal of bromfenac due to DILI, the exact role of its microsomal metabolism in inducing hepatotoxicity remains unclear. Herein, employing HepG2 cells with human liver microsomes and UDP-glucuronic acid (UDPGA), our investigation revealed a substantial increase in bromfenac-induced cytotoxicity in the presence of UDPGA, pointing to the significance of UDP-glucuronosyltransferase (UGT)-dependent metabolism in augmenting toxicity. Notably, among the recombinant UGTs examined, UGT2B7 emerged as a pivotal enzyme in the metabolic activation of bromfenac. Metabolite identification studies disclosed the formation of reactive intermediates, with bromfenac indolinone (lactam) identified as a potential mediator of hepatotoxic effects. Moreover, in cytotoxicity experiments, the toxicity of bromfenac lactam exhibited a 34-fold increase, relative to bromfenac. The toxicity of bromfenac lactam was mitigated by nicotinamide adenine dinucleotide phosphate-dependent metabolism. This finding underscores the role of UGT-dependent metabolism in generating reactive metabolites that contribute to the observed hepatotoxicity associated with bromfenac. Understanding these metabolic pathways and the involvement of specific enzymes, such as UGT2B7, provides crucial insights into the mechanisms of bromfenac-induced liver injury. In conclusion, this research sheds light on the metabolic intricacies leading to cytotoxicity induced by bromfenac, especially emphasizing the role of UGT-dependent metabolism and the formation of reactive intermediates like bromfenac lactam. These findings offer insight into the mechanistic basis of DILI and emphasize the importance of understanding metabolism-mediated toxicity.

Unraveling the versatility of human serum albumin - A comprehensive review of its biological significance and therapeutic potential

Human serum albumin (HSA) is a multi-domain macromolecule with diverse ligand binding capability because of its ability to allow allosteric modulation despite being a monomeric protein. Physiologically, HSA act as the primary carrier for various exogenous and endogenous compounds and fatty acids, and alter the pharmacokinetic properties of several drugs. It has antioxidant properties and is utilized therapeutically to improve the drug delivery of pharmacological agents for the treatment of several disorders. The flexibility of albumin in holding various types of drugs coupled with a variety of modifications makes this protein a versatile drug carrier with incalculable potential in therapeutics. This review provides a brief outline of the different structural properties of HSA, and its various binding sites, moreover, an overview of the genetic, biomedical, and allosteric modulation of drugs and drug delivery aspects of HSA is also included, which may be helpful in guiding advanced clinical applications and further research on the therapeutic potential of this extraordinary protein.

Optimization of Physicochemical Properties of Pyrrolidine GPR40 AgoPAMs Results in a Differentiated Profile with Improved Pharmacokinetics and Reduced Off-Target Activities

GPR40 AgoPAMs are highly effective antidiabetic agents that have a dual mechanism of action, stimulating both glucose-dependent insulin and GLP-1 secretion. The early lipophilic, aromatic pyrrolidine and dihydropyrazole GPR40 AgoPAMs from our laboratory were highly efficacious in lowering plasma glucose levels in rodents but possessed off-target activities and triggered rebound hyperglycemia in rats at high doses. A focus on increasing molecular complexity through saturation and chirality in combination with reducing polarity for the pyrrolidine AgoPAM chemotype resulted in the discovery of compound 46, which shows significantly reduced off-target activities as well as improved aqueous solubility, rapid absorption, and linear PK. In vivo, compound 46 significantly lowers plasma glucose levels in rats during an oral glucose challenge yet does not demonstrate the reactive hyperglycemia effect at high doses that was observed with earlier GPR40 AgoPAMs.

The Prevalence of Several Treatments in Preventing the Back Conversion of Acyl Glucuronide Metabolites in Abbreviated New Drug Applications

The bioanalysis of drugs that undergo acyl glucuronidation presents an analytical challenge due to poor stability of acyl glucuronide metabolites in biological matrices. The objective of this study was to investigate the impact of back conversion of acyl glucuronide metabolites on drug concentration measurement in bioequivalence (BE) studies submitted to Abbreviated New Drug Applications (ANDAs). The prevalence of several treatments for preventing the back conversion of acyl glucuronide metabolites and the results of incurred sample reanalysis (ISR) were analyzed. In total, 322 ANDAs for 26 drugs known to generate acyl glucuronide metabolites were surveyed. Many studies have applied multiple preventive treatments during the clinical and bioanalytical phases. More than two-thirds (67.2%) of the studies utilized procedures of lowering the temperature for sample collection during clinical phase. Fewer studies have utilized procedures for lowering the pH of plasma samples (12.3%) or adding enzyme inhibitors (4.4%) in the clinical phase. A small fraction (16.9%) validated the pre-study method in the presence of the acyl glucuronide metabolites. The majority (62.2%) of the studies employed the procedure of lowering the pH during the sample extraction process in the bioanalytical phase. Among the studies that had significantly higher (p-value < 0.01 by sign test) ISR results than the corresponding original concentration values, 41 BE studies did not carry out any preventive treatments during the bioanalysis phase, suggesting that back conversion of acyl glucuronide metabolites to parent drugs may be present in these studies. The awareness of appropriate treatments of study samples for possible back-conversions of acyl glucuronide metabolites is expected to assist generic drug applicants in improving the quality of their future applications.

A rapid Dilute-and-Shoot LC-MS/MS method for quantifying THC-COOH and THC-COO(Gluc) in urine

Cannabis remains one of the most commonly used psychotropes. Cannabis use is frequently evaluated via the testing of suspected patient samples. Thus, there is a high demand for simple, accurate and fast assays to support the increasing needs for testing. This report highlights a reliable, simple and fast liquid chromatography - tandem mass spectrometry assay that quantifies the cannabis metabolites THC-COOH and THC-COO(Gluc) in human urine. The assay employs a direct dilute-and-shoot approach, whereby urine samples are diluted 10X before being directly injected on the liquid chromatography and mass spectrometer. The assay quantification is based on an internal calibration approach that used deuterated analogues for THC-COOH and THC-COO(Gluc) as internal standards. The assay's analysis time was 5 min. The quantification was valid over a wide linear range (25 - 8,000 ng/mL) for both analytes and was free of matrix interferences. The within-day and between-day precision was determined to be ≤ 15 % CV for both analytes. The assay was validated based on the College of American Pathologists (CAP) and Clinical Laboratory Standards Institute (CLSI) guidelines.

Improved Predictability of Hepatic Clearance with Optimal pH for Acyl-Glucuronidation in Liver Microsomes

The purpose of this study was to investigate the optimal pH for acyl-glucuronidation formation with carboxylic acid-containing compounds in human and rat liver microsomes to improve the predictability of their hepatic clearance. The optimal pH for acyl-glucuronidation of all 17 compounds was around pH 6.0 in human and rat liver microsomes. Correlation analysis was done with the predicted in vitro intrinsic clearance (CL_{int,in vitro}) and in vivo intrinsic clearance (CL_{int,in vivo}) calculated from available reported data of total clearance (CL_tot) of 11 compounds in humans. For 8 of the 11 compounds, under the pH 6.0 condition, the CL_{int,in vitro} were within 1/3 to 3-fold error of the observed CL_{int,in vivo} whereas, the error was within 1/3 to 3-fold of the observed CL_{int,in vivo} for only 3 of the 11 under the pH 7.4 condition. The intracellular pH in human and rat hepatocytes decreased in the presence of a carboxylic acid-containing compound. These findings suggest that acyl-glucuronidation in liver microsomes at pH 6.0 is closer to physiological conditions in the presence of carboxylic acid compounds, and thus, use of this pH condition is important for physiological interpretation and predictability of intrinsic clearance.

Acyl Glucuronide and Coenzyme A Thioester Metabolites of Carboxylic Acid-Containing Drug Molecules: Layering Chemistry with Reactive Metabolism and Toxicology

Glucuronidation and CoA (coenzyme A) conjugation are common pathways for the elimination of carboxylic acid-containing drug molecules. In some instances, these biotransformations have been associated with toxicity (such as idiosyncratic hepatic injury, renal impairment, hemolytic anemia, gastrointestinal inflammation, and bladder cancer) attributed to, in part, the propensity of acyl glucuronides and acyl CoA thioesters to covalently modify biological macromolecules such as proteins and DNA. It is to be noted that, while acyl glucuronidation and CoA conjugation are indeed implicated in adverse effects, there are many safe drugs in the market that are cleared by these reactive pathways. It is therefore important that new molecular entities with carboxylic acid groups are evaluated for toxicity in a manner that is not unreasonably risk-averse. In the absence of truly predictable methods, therefore, the general approach is to apply a set of end points to generate a weight-of-evidence evaluation. In practice, the focus is to identify structural liabilities and provide structure-activity recommendations early in the program, at a stage where an attempt to improve reactive metabolism does not deoptimize other critical drug-quality criteria. This review will present a high-level overview of the chemistry of glucuronidation and CoA conjugation and provide a discussion of the possible mechanisms of adverse effects that have been associated with these pathways, as well as how such potential hazards are addressed while delivering a new chemical entity for clinical evaluation.

Bioactivation and Reactivity Research Advances - 2021 year in review

This year's review on bioactivation and reactivity began as a part of the annual review on biotransformation and bioactivation led by Cyrus Khojasteh (Khojasteh et al., 2021, 2020, 2019, 2018, 2017; Baillie et al., 2016). Increased contributions from experts in the field led to the development of a stand alone edition for the first time this year focused specifically on bioactivation and reactivity. Our objective for this review is to highlight and share articles which we deem influential and significant regarding the development of covalent inhibitors, mechanisms of reactive metabolite formation, enzyme inactivation, and drug safety. Based on the selected articles, we created two sections: (1) reactivity and enzyme inactivation, and (2) bioactivation mechanisms and safety (Table 1). Several biotransformation experts have contributed to this effort from academic and industry settings.

18O-Enabled High-Throughput Acyl Glucuronide Stability Assay

Acyl glucuronides (AGs) are common metabolites of carboxylic acid-containing compounds. In some circumstances, AGs are suspected to be involved in drug toxicity due to formation of acyl migration products that bind covalently to cellular components. The risk of this adverse effect has been found to be correlated with the chemical stability of the AG, and assays have been described that monitor acyl migration by liquid chromatography coupled with mass spectrometry (LC-MS). This analysis can be challenging as it requires baseline chromatographic separation of the unmigrated 1-β-acyl glucuronide from the migrated isomers and thus needs to be individually optimized for each aglycone. Therefore, a high-throughput assay that eliminates LC method development is desirable. Herein, we report an improved acyl glucuronide stability assay based on the rate of ¹⁸O-incorporation from [¹⁸O] water, which is compatible with high-throughput bioanalytical LC-MS workflows. Synthetic AGs with shorter migration half-lives showed faster incorporation of ¹⁸O. The level of differential incorporation of ¹⁸O following a 24 h incubation correlates well with the migration tendency of AGs. This assay was developed further, exploring in situ generation of AGs by human hepatic microsomal fraction. The results from 18 in situ-formed acyl glucuronides were similar to those obtained using authentic reference standards. In this format, this new ¹⁸O-labeling method offers a simplified workflow, requires no LC method development or AG reference standard, and thus facilitates AG liability assessment in early drug discovery.

Mechanistic Study on the Effect of Renal Impairment on the Pharmacokinetics of Vildagliptin and its Carboxylic Acid Metabolite

PURPOSE

To clarify the mechanism of renal impairment leading to different degrees of increased plasma exposure to dipeptidyl peptidase 4 inhibitor vildagliptin and its major metabolite, M20.7.

METHODS

The 5/6 nephrectomized (5/6 Nx) rat model, to simulate chronic renal failure (CRF) patients, combined with kidney slices and transporter studies in vitro were used to assess this pharmacokinetic differences.

RESULTS

After intragastric administration to 5/6 Nx rats, vildagliptin showed increased plasma levels by 45.8%, and M20.7 by 7.51 times, which was similar to patients with severe renal impairment. The recovery rate of M20.7 in urine and feces increased by less than 20%, showing limited effect of renal impairment on vildagliptin metabolism. In vitro studies found M20.7 to be the substrate for organic anion transporter 3 (OAT3). However, the active uptake of M20.7 in renal slices showed no difference between the 5/6 Nx and normal rats. In OAT3 overexpressed cells, the protein-bound uremic toxins, 3-carboxy-4-methyl-5propyl-2-furanpropionate (CMPF), hippuric acid (HA) and indoxyl sulfate (IS), which accumulate in CRF patients, inhibited M20.7 uptake with IC₅₀ values of 5.75, 29.0 and 69.5 μM respectively, far lower than plasma concentrations in CRF patients, and showed a mixed inhibition type.

CONCLUSIONS

The large increase in plasma exposure of M20.7 could be attributed to the accumulation of uremic toxins in CRF patients, which inhibited OAT3 activity and blocked renal excretion of M20.7, while vildagliptin, with high permeability, showed a slight increase in plasma exposure due to reduced glomerular filtration.

Investigation of Human in vivo Metabolism of SEP-227900 Using the Samples from a Randomized First-in-Human Study by LC-UV/HRMS and NMR

OBJECTIVE

To explore the human in vivo metabolism of SEP-227900 (4H-furo[3, 2-b]pyrrole-carboxylic acid, m.w 151.03), a D-amino-acid oxidase (DAAO) inhibitor by using plasma and urine samples from first-in-human study.

METHODS

The human plasma and urine samples were from a single dose cohort that consisted of 9 healthy male volunteers each received 80-mg dose of SEP-227900 orally. The pooled pre-dose urine and the pooled 0-24 h urine sample were created across 9 subjects by equal volume. Plasma samples were pooled by equal volume across 9 subjects to obtain 0-12 h plasma for metabolite searching, and also pooled by timepoints across 9 subjects to obtain 0.5-, 5-, and 12-h plasma for semi-quantitation. The plasma was de-proteinized by acetonitrile (1:3 v/v plasma-acetonitrile) then the supernatant was dried down, reconstituted and injected for LC-HRMS/UV analysis. The urine sample was just simply centrifuged before analysis. LC-HRMS/UV was utilized to search predictable and unknown metabolites and estimate their relative abundances. Accurate mass measurement by Orbitrap-MS and MS/MS were used for metabolite identification. Chromatographic separation was achieved on a MACMOD AQ C8 column (250 × 4.6 mm, 5-µm) with a gradient mobile phase (A: 10 mM NH4Ac; B: acetonitrile; flowrate: 0.700 ml/min) for a total run-time of 65 min. The definite position in the molecule for the glucuronidation metabolism was characterized by detected migration phenomenon, methylation with diazomethane (CH2N2), and NMR.

RESULTS

Unchanged parent drug and four metabolite peaks were detected in humans: M1 was a mono-oxidative metabolite of SEP-227900; M2 was a glucuronide conjugate of SEP-227900; M3 was a glycine conjugate of SEP-227900; and M4 was a glycine conjugate of M1. The specific position of the oxidation in M1 solely based on the mass spectral (MS and MS/MS) data was not identified. However, for the major metabolite M2, the acyl glucuronidation was unambiguously determined through multiple pieces of experimental evidence such as the observation of a migration pattern, mono-methylation by diazomethane, and NMR measurement. This determination is of significance related to the safety evaluation of an investigational new drug development. The glycine conjugate of SEP-227900, i.e. M3 was found to be the most abundant metabolite in human urine (approximately 3-fold higher level as the glucuronide level). All together (mainly glycine-conjugate and glucuronide), it resulted in greater than 80% of the dosed amount in urine excretion (a separate measurement showed 23% of the dosed amount in urine excretion as the glucuronide).

CONCLUSION

Four metabolites were found in humans: SEP-227900-glycine conjugate, SEP227900-glucuronide, mono-oxidative metabolite and its consequent glycine conjugate. The glucuronide metabolite was identified as the acyl glucuronide. Greater than 80% of the dosed amount of SEP-227900 was excreted in urine mainly in the forms of glycine- and glucuronide- conjugates.

A cysteine trapping assay for risk assessment of reactive acyl CoA metabolites

1. Some drugs with carboxylic acid moieties can potentially cause rare but severe hepatotoxicity. The reactive chemical species generated by drug metabolism are thought to be one reason for this event. Although the phase II conjugation metabolism of carboxylic acids generally renders a compound more polar and inactive, it is also responsible for the formation of reactive metabolites.2. This study aimed to provide a new approach toward the risk assessment of carboxylic acids in the aspect of reactive acyl CoA metabolites.3. Although acyl CoA metabolites have been concerned, it is difficult to detect them because of its instability. We investigated the trapping agents for acyl CoA metabolites. We found that cysteine is a good trapping agent and developed an assay method for the reactivity of acyl CoA metabolites. We evaluated 17 drugs with carboxylic acid moieties, all drugs concerned with hepatotoxicity displayed reactive potential. With a consideration of the exposure of each parent drug, the correlation between drug labels and the calculated risk of carboxylic drugs was improved.4. These evaluations can be conducted without radiochemical reagents or the authentic standards of metabolites. We believe that the method will be beneficial for drug discovery.

Serum Albumin: A Multifaced Enzyme

Human serum albumin (HSA) is the most abundant protein in plasma, contributing actively to oncotic pressure maintenance and fluid distribution between body compartments. HSA acts as the main carrier of fatty acids, recognizes metal ions, affects pharmacokinetics of many drugs, provides the metabolic modification of some ligands, renders potential toxins harmless, accounts for most of the anti-oxidant capacity of human plasma, and displays esterase, enolase, glucuronidase, and peroxidase (pseudo)-enzymatic activities. HSA-based catalysis is physiologically relevant, affecting the metabolism of endogenous and exogenous compounds including proteins, lipids, cholesterol, reactive oxygen species (ROS), and drugs. Catalytic properties of HSA are modulated by allosteric effectors, competitive inhibitors, chemical modifications, pathological conditions, and aging. HSA displays anti-oxidant properties and is critical for plasma detoxification from toxic agents and for pro-drugs activation. The enzymatic properties of HSA can be also exploited by chemical industries as a scaffold to produce libraries of catalysts with improved proficiency and stereoselectivity for water decontamination from poisonous agents and environmental contaminants, in the so called “green chemistry” field. Here, an overview of the intrinsic and metal dependent (pseudo-)enzymatic properties of HSA is reported to highlight the roles played by this multifaced protein.

Enzyme Kinetics of Uridine Diphosphate Glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bisubstrate reaction that requires the aglycone and the cofactor, UDP-GlcUA. Accumulating evidence suggests that the bisubstrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modeling of glucuronidation reactions in vitro, UDP-GlcUA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for during experimental design and data interpretation. While the assessment of drug-drug interactions resulting from UGT inhibition has been challenging in the past, the increasing availability of UGT enzyme-selective substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of drug-drug interaction potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often underpredicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation. Physiologically based pharmacokinetic (PBPK) modeling has also shown to be of value for predicting PK of drugs eliminated by glucuronidation.

Identification of an isoform catalyzing the CoA conjugation of nonsteroidal anti-inflammatory drugs and the evaluation of the expression levels of acyl-CoA synthetases in the human liver

Nonsteroidal anti-inflammatory drugs (NSAIDs) containing carboxylic acid are conjugated with coenzyme A (CoA) or glucuronic acid in the body. It has been suggested that these conjugates are associated with toxicities, such as liver injury and anaphylaxis, through their binding via trans-acylation to cellular proteins. Although studies on glucuronidation have progressed, studies on CoA conjugation of drugs catalyzed by acyl-CoA synthetase (ACS) enzymes are still in the early stages. This study aimed to clarify the human ACS isoforms responsible for CoA-conjugation of NSAIDs through consideration of the hepatic expression levels of ACS isoforms. We found that among 10 types of NSAIDs, propionic acid-class NSAIDs, namely, alminoprofen, flurbiprofen, ibuprofen, ketoprofen, and loxoprofen, were conjugated with CoA in the human liver, whereas NSAIDs in the other classes, including diclofenac and mefenamic acid, were not. qRT-PCR revealed that among the 26 ACS isoforms, ACSL1 was the most highly expressed in the human liver, followed by ACSM2B. The propionic acid-class NSAIDs were conjugated with CoA by recombinant human ACSL1. The protein binding abilities of the CoA conjugates and the glucuronide forms of propionic acid-class NSAIDs were compared as an index of toxicity. The CoA conjugates had stronger adduct formation with liver microsomal proteins than glucuronides for all 5 propionic acid-class NSAIDs. In conclusion, we found that propionic acid-class NSAIDs could be conjugated to CoA by ACSL1 in the human liver to form CoA conjugates, which likely cause toxicity by protein adduct formation.

Unearthing novel thiazolidinone building blocks as carboxylic acid bioisosteres

Aim: Thiazolidinones were prepared as building blocks for the replacement of carboxylic acids. Materials & methods: Chemical syntheses of thiazolidinones were developed. In addition, the drug-likeness of the target compounds was evaluated in silico. Results: The prepared compounds included the novel structure 4; 5-(3-Iodophenylmethylene)-2,4-thiazolidinedione. Conclusion: Exploration of the methods required to synthesize thiazolidinone building blocks was completed. This work allows future generation of bioisosteric analogs of drugs.

Models of Idiosyncratic Drug-Induced Liver Injury

Drug-induced liver injury (DILI) is a leading cause of attrition during the early and late stages of drug development and after a drug is marketed. DILI is generally classified as either intrinsic or idiosyncratic. Intrinsic DILI is dose dependent and predictable (e.g., acetaminophen toxicity). However, predicting the occurrence of idiosyncratic DILI, which has a very low incidence and is associated with severe liver damage, is difficult because of its complex nature and the poor understanding of its mechanism. Considering drug metabolism and pharmacokinetics, we established experimental animal models of DILI for 14 clinical drugs that cause idiosyncratic DILI in humans, which is characterized by the formation of reactive metabolites and the involvement of both innate and adaptive immunity. On the basis of the biomarker data obtained from the animal models, we developed a cell-based assay system that predicts the potential risks of drugs for inducing DILI. These findings increase our understanding of the mechanisms of DILI and may help predict and prevent idiosyncratic DILI due to certain drugs.

PXR-mediated idiosyncratic drug-induced liver injury: mechanistic insights and targeting approaches

INTRODUCTION

The human liver is the center for drug metabolism and detoxification and is, therefore, constantly exposed to toxic chemicals. The loss of liver function as a result of this exposure is referred to as drug-induced liver injury (DILI). The pregnane X receptor (PXR) is the primary regulator of the hepatic drug-clearance system, which plays a critical role in mediating idiosyncratic DILI.

AREAS COVERED

This review is focused on common mechanisms of PXR-mediated DILI and on in vitro and in vivo models developed to predict and assess DILI. It also provides an update on the development of PXR antagonists that may manage PXR-mediated DILI.

EXPERT OPINION

DILI can be caused by many factors, and PXR is clearly linked to DILI. Although emerging data illustrate how PXR mediates DILI and how PXR activity can be modulated, many questions concerning the development of effective PXR modulators remain. Future research should be focused on determining the mechanisms regulating PXR functions in different cellular contexts.

Challenges and recommendations in developing LC–MS/MS bioanalytical assays of labile glucuronides and parent compounds in the presence of glucuronide metabolites

Glucuronides, especially acyl glucuronides, were often found to be unstable in vitro and in vivo. Acyl glucuronide metabolites can convert back to the parent drugs at physiological pH through hydrolysis. Glucuronides can also undergo in-source fragmentation during MS ionization to form the same ions as those of the parent compounds, which could cause interference to the analysis of the parent compounds. All of these may cause significant challenges in developing LC–MS/MS bioanalytical assays of labile glucuronides or parent compounds in the presence of glucuronide metabolites. In this manuscript, we will discuss these challenges and summarize recommended strategies and practices for fast and efficient method development. Critical considerations in assay development will also be discussed.

Non-clinical characterization of the disposition of EMA401, a novel small molecule angiotensin II type 2 receptor (AT2R) antagonist

EMA401, (the S-enantiomer of 5-(benzyloxy)-2-(2,2-diphenylacetyl)-6-methoxy-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid), also known as Olodanrigan, is an orally active selective angiotensin II type 2 receptor (AT₂ R) antagonist that is in Phase IIb clinical development as a novel analgesic for the relief of chronic pain. The main purpose of the present work was to investigate the disposition of a single ¹⁴ C- labeled EMA401 in non-clinical studies. The in vitro metabolism studies of EMA401 were undertaken to understand the hepatic biotransformation pathways in animal species used in toxicology studies and how they compare to human. Furthermore, investigation of EMA401's PK was carried out in vivo in rats. The study demonstrates the rapid absorption and distribution of drug-related material mainly to the tissues associated with absorption and elimination (GI tract, liver, and kidney). EMA401was then readily eliminated metabolically via the bile (95% of dose) predominantly in the form of the direct acylglucuronide (40% of dose), which was further hydrolysed by the intestinal flora to the active parent drug. Other metabolic pathways such as dealkylations and hydroxylation were also involved in the elimination of EMA401 to a lesser extent. EMA401 was metabolically unstable in hepatocytes of all species investigated and the key metabolites produced in the in vitro model were also detected in vivo. Independent of the dosing route, the S-enantiomer EMA401 showed a good in vivo chiral stability. Overall, the present study provides the first full characterization of the disposition of EMA401 in preclinical species.

Differentiation of Deprotonated Acyl-, N-, and O-Glucuronide Drug Metabolites by Using Tandem Mass Spectrometry Based on Gas-Phase Ion-Molecule Reactions Followed by Collision-Activated Dissociation

Glucuronidation, a common phase II biotransformation reaction, is one of the major in vitro and in vivo metabolism pathways of xenobiotics. In this process, glucuronic acid is conjugated to a drug or a drug metabolite via a carboxylic acid, a hydroxy, or an amino group to form acyl-, O-, and/or N-glucuronide metabolites, respectively. This process is traditionally thought to be a detoxification pathway. However, some acyl-glucuronides react with biomolecules in vivo, which may result in immune-mediated idiosyncratic drug toxicity (IDT). In order to avoid this, one may attempt in early drug discovery to modify the lead compounds in such a manner that they then have a lower probability of forming reactive acyl-glucuronide metabolites. Because most drugs or drug candidates bear multiple functionalities, e.g., hydroxy, amino, and carboxylic acid groups, glucuronidation can occur at any of those. However, differentiation of isomeric acyl-, N-, and O-glucuronide derivatives of drugs is challenging. In this study, gas-phase ion-molecule reactions between deprotonated glucuronide metabolites and BF₃ followed by collision-activated dissociation (CAD) in a linear quadrupole ion trap mass spectrometer were demonstrated to enable the differentiation of acyl-, N-, and O-glucuronides. Only deprotonated N-glucuronides and deprotonated, migrated acyl-glucuronides form the two diagnostic product ions: a BF₃ adduct that has lost two HF molecules, [M - H + BF₃ - 2HF]^-, and an adduct formed with two BF₃ molecules that has lost three HF molecules, [M - H + 2BF₃ - 3HF]^-. These product ions were not observed for deprotonated O-glucuronides and unmigrated, deprotonated acyl-glucuronides. Upon CAD of the [M - H + 2BF₃ - 3HF]^- product ion, a diagnostic fragment ion is formed via the loss of 2-fluoro-1,3,2-dioxaborale (MW of 88 Da) only in the case of deprotonated, migrated acyl-glucuronides. Therefore, this method can be used to unambiguously differentiate acyl-, N-, and O-glucuronides. Further, coupling this methodology with HPLC enables the differentiation of unmigrated 1-β-acyl-glucuronides from the isomeric acyl-glucuronides formed upon acyl migration. Quantum chemical calculations at the M06-2X/6-311++G(d,p) level of theory were employed to probe the mechanisms of the reactions of interest.

Expression of drug metabolizing enzymes and transporters in the cochlea: Implications for drug delivery and ototoxicity

Inner ear drug delivery is a major area of research and development, but relatively little is known about basic drug metabolism in the cochlea. Additionally, the use of potentially ototoxic drugs such as NSAIDs, chemotherapeutics and aminoglycosides is common, but little is known about the role of metabolism in ototoxicity of those drugs. To address those issues, we compared expression of major Cytochromes P450 (Cyps), UDP-glucuronosyl-transferases (Ugts), sulfotransferases (Sults), and drug transporters between cochleae and liver, an organ with high expression, in mice using qPCR and enzyme kinetics. Together, the tested drug-metabolizing enzymes (DMEs) and transporters account for metabolism of approximately 70-80% of all medically important drugs in the body. Expression of most Cyps was low in the cochlea compared to liver, but three displayed similar expression levels to the liver, and one (Cyp2c65) had significantly higher levels of expression in the cochlea (1.9 ± 0.06 fold vs. liver). Enzyme kinetics revealed undetectable levels of p450 activity in the cochlea, especially as compared to the liver. Similar results were obtained for expression of Ugts and Sults. Interestingly, expression of most transporters was also low, with one major exception: Mdr1/P-glycoprotein (P-gp), which is generally thought to be highly expressed in liver and poorly expressed in most of the nervous system, was 3-fold greater in cochlea. Importantly, P-gp is known to protect other tissues from toxicity of cancer drugs by acting as an efflux pump. Our data demonstrate overall low levels of expression of DMEs and transporters in the cochlea, and identify a few that may be important to consider when designing and testing drugs for local delivery to the inner ear.

In Silico Evaluation of Ibuprofen and Two Benzoylpropionic Acid Derivatives with Potential Anti-Inflammatory Activity

Inflammation is a complex reaction involving cellular and molecular components and an unspecific response to a specific aggression. The use of scientific and technological innovations as a research tool combining multidisciplinary knowledge in informatics, biotechnology, chemistry and biology are essential for optimizing time and reducing costs in the drug design. Thus, the integration of these in silico techniques makes it possible to search for new anti-inflammatory drugs with better pharmacokinetic and toxicological profiles compared to commercially used drugs. This in silico study evaluated the anti-inflammatory potential of two benzoylpropionic acid derivatives (MBPA and DHBPA) using molecular docking and their thermodynamic profiles by molecular dynamics, in addition to predicting oral bioavailability, bioactivity and toxicity. In accordance to our predictions the derivatives proposed here had the potential capacity for COX-2 inhibition in the human and mice enzyme, due to containing similar interactions with the control compound (ibuprofen). Ibuprofen showed toxic predictions of hepatotoxicity (in human, mouse and rat; toxicophoric group 2-arylacetic or 3-arylpropionic acid) and irritation of the gastrointestinal tract (in human, mouse and rat; toxicophoric group alpha-substituted propionic acid or ester) confirming the literature data, as well as the efficiency of the DEREK 10.0.2 program. Moreover, the proposed compounds are predicted to have a good oral bioavailability profile and low toxicity (LD50 < 700 mg/kg) and safety when compared to the commercial compound. Therefore, future studies are necessary to confirm the anti-inflammatory potential of these compounds.

[Verification of Pharmacokinetic Approaches in Prior Drug Development]

In this review, 9 compounds with insufficient absorption characteristics, safety or efficacy were selected from among the compounds for which the author was in charge of development between 2000 and 2005, in order to evaluate the pharmacokinetic (PK) approaches used to develop these compounds. Optimization of the PK characteristics of a compound at the early stage of chemical design was found to be the most important factor for successful development. For example, (i) selecting class I or II drugs in the biopharmaceutical classification system, while avoiding efflux transporters, and introducing an appropriate dissociation moiety into a compound to make it soluble lead to sufficient drug absorption; (ii) designing compounds whose production of reactive metabolites, such as acyl glucuronide, does not largely affect total metabolism, yet helps to prevent abnormal PK caused by reactive metabolites. Other factors include (i) selection of a drug efficacy evaluation system based on the correct understanding of the relationship between PK and pharmacodynamics (PD) helps to solve species differences in PD; (ii) the establishment of a nonclinical study based on the identification of the involvement of specific cytochrome P450 molecules in the total metabolic clearance of a drug (f_m,CYPs) helps to solve species differences in PK; and (iii) PK analysis using the tube model for hepatic extraction kinetics, and knowledge of the f_m,CYPs of the victim drug, lead to successful drug-drug interaction (DDI) prediction. I hope that this review aids in future drug discovery or development.

Discovery of a GPR40 Superagonist: The Impact of Aryl Propionic Acid α-Fluorination

GPR40 is a G-protein-coupled receptor which mediates fatty acid-induced glucose-stimulated insulin secretion from pancreatic beta cells and incretion release from enteroendocrine cells of the small intestine. GPR40 full agonists exhibit superior glucose lowering compared to partial agonists in preclinical species due to increased insulin and GLP-1 secretion, with the added benefit of promoting weight loss. In our search for potent GPR40 full agonists, we discovered a superagonist which displayed excellent in vitro potency and superior efficacy in the Gαs-mediated signaling pathway. Most synthetic GPR40 agonists have a carboxylic acid headgroup, which may cause idiosyncratic toxicities, including drug-induced-liver-injury (DILI). With a methyl group and a fluorine atom substituted at the α-C of the carboxylic acid group, 19 is not only highly efficacious in lowering glucose and body weight in rodent models but also has a low DILI risk due to its stable acylglucuronide metabolite.

Safety Assessment of Acyl Glucuronides-A Simplified Paradigm

While simple O- (ether-linked) and N-glucuronide drug conjugates generally are unreactive and considered benign from a safety perspective, the acyl glucuronides that derive from metabolism of carboxylic acid-containing xenobiotics can exhibit a degree of chemical reactivity that is dependent upon their molecular structure. As a result, concerns have arisen over the safety of acyl glucuronides as a class, several members of which have been implicated in the toxicity of their respective parent drugs. However, direct evidence in support of these claims remains sparse, and due to frequently encountered species differences in the systemic exposure to acyl glucuronides (both of the parent drug and oxidized derivatives thereof), coupled with their instability in aqueous media and potential to undergo chemical rearrangement (acyl migration), qualification of these conjugates by traditional safety assessment methods can be very challenging. In this Commentary, we discuss alternative (non-acyl glucuronide) mechanisms by which carboxylic acids may cause serious adverse reactions, and propose a novel, practical approach to compare systemic exposure to acyl glucuronide metabolites in humans to that in animal species used in preclinical safety assessment based on relative estimates of the total body burden of these circulating conjugates.

Neutrophil depletion protects against zomepirac-induced acute kidney injury in mice

Acyl glucuronide (AG) metabolites of carboxylic acid-containing drugs have been implicated in drug toxicity. Zomepirac (ZP) is a non-steroidal anti-inflammatory drug that was withdrawn from the market because of anaphylactic reactions and renal injury. We recently established a novel mouse model of ZP-induced kidney injury by increasing zomepirac acyl-glucuronide (ZP-AG) concentration via pretreatment with tri-O-tolyl phosphate, a nonselective esterase inhibitor, and l-buthionine-(S,R)-sulfoximine, a glutathione synthesis inhibitor. Although we have shown that ZP-AG is responsible for ZP-induced kidney injury in mice, the exact pathogenic mechanisms of ZP-induced kidney injury have not been investigated yet. In this study, we aimed to investigate the role of immune cells in the pathogenesis of ZP-induced kidney injury, as a representative of AG toxicity. We found that the counts of neutrophils and inflammatory monocytes increased in the blood of mice with ZP-induced kidney injury. However, clodronate liposome- or GdCl₃-induced monocyte and/or macrophage depletion did not affect blood urea nitrogen and plasma creatinine levels in mice with ZP-induced kidney injury. Neutrophil infiltration into the kidneys was observed in mice with ZP-induced kidney injury, whereas anti-lymphocyte antigen 6 complex, locus G (Ly6G) antibody pretreatment prevented the renal neutrophil infiltration and partially protected against ZP-induced kidney injury. The mRNA expression of neutrophil-infiltrating cytokines and chemokines, interleukin-1α and macrophage inflammatory protein-2α, increased in mice with ZP-induced kidney injury, whereas pretreatment with anti-Ly6G antibody resulted in a marked reduction of their expression. These results suggest that ZP-AG might be involved in kidney injury, partly via induction of neutrophil infiltration. Therefore, this study may provide an important understanding on toxicological role of ZP-AG in vivo that helps to understand toxicity of AG metabolites.

ADP-Ribosylation, a Multifaceted Posttranslational Modification Involved in the Control of Cell Physiology in Health and Disease

Posttranslational modifications (PTMs) regulate protein functions and interactions. ADP-ribosylation is a PTM, in which ADP-ribosyltransferases use nicotinamide adenine dinucleotide (NAD⁺) to modify target proteins with ADP-ribose. This modification can occur as mono- or poly-ADP-ribosylation. The latter involves the synthesis of long ADP-ribose chains that have specific properties due to the nature of the polymer. ADP-Ribosylation is reversed by hydrolases that cleave the glycosidic bonds either between ADP-ribose units or between the protein proximal ADP-ribose and a given amino acid side chain. Here we discuss the properties of the different enzymes associated with ADP-ribosylation and the consequences of this PTM on substrates. Furthermore, the different domains that interpret either mono- or poly-ADP-ribosylation and the implications for cellular processes are described.

Clinical Pharmacokinetics and Pharmacodynamics of Febuxostat

Febuxostat is a xanthine oxidoreductase inhibitor that has been developed to treat chronic gout. In healthy subjects, the pharmacokinetic parameters of febuxostat after multiple oral dose administration include an oral availability of about 85 %, an apparent oral clearance (CL/F) of 10.5 ± 3.4 L/h and an apparent volume of distribution at steady state (V _ss/F) of 48 ± 23 L. The time course of plasma concentrations follows a two-compartment model. The initial half-life (t _½) is approximately 2 h and the terminal t _½ determined at daily doses of 40 mg or more is 9.4 ± 4.9 h. Febuxostat is administered once daily. The maximum (peak) plasma concentrations are approximately 100-fold greater than the trough concentrations. Consequently, there is no significant accumulation of the drug during multiple dose administration. There are few data on the pharmacokinetics of febuxostat in patients with gout. While the pharmacokinetic parameters are not affected by mild to moderate hepatic impairment, there is no consensus on whether renal impairment has any effect on the pharmacokinetics of febuxostat. Febuxostat is extensively metabolised by oxidation (approximately 35 %) and acyl glucuronidation (up to 40 %); febuxostat acyl glucuronides are cleared by the kidney. In healthy subjects treated with multiple doses of febuxostat 10-240 mg, the concentrations of serum urate are reduced by a maximum of about 80 %. The percentage reduction in the concentrations of serum urate is slightly less in gouty patients than in healthy subjects.

Absorption, Distribution, Metabolism, and Excretion of the Oral Prostaglandin D2 Receptor 2 Antagonist Fevipiprant (QAW039) in Healthy Volunteers and In Vitro

Fevipiprant is a novel oral prostaglandin D₂ receptor 2 (DP₂; also known as CRTh2) antagonist, which is currently in development for the treatment of severe asthma and atopic dermatitis. We investigated the absorption, distribution, metabolism, and excretion properties of fevipiprant in healthy subjects after a single 200-mg oral dose of [¹⁴C]-radiolabeled fevipiprant. Fevipiprant and metabolites were analyzed by liquid chromatography coupled to tandem mass spectrometry and radioactivity measurements, and mechanistic in vitro studies were performed to investigate clearance pathways and covalent plasma protein binding. Biotransformation of fevipiprant involved predominantly an inactive acyl glucuronide (AG) metabolite, which was detected in plasma and excreta, representing 28% of excreted drug-related material. The AG metabolite was found to covalently bind to human plasma proteins, likely albumin; however, in vitro covalent binding to liver protein was negligible. Excretion was predominantly as unchanged fevipiprant in urine and feces, indicating clearance by renal and possibly biliary excretion. Fevipiprant was found to be a substrate of transporters organic anion transporter 3 (OAT3; renal uptake), multidrug resistance gene 1 (MDR1; possible biliary excretion), and organic anion-transporting polypeptide 1B3 (OATP1B3; hepatic uptake). Elimination of fevipiprant occurs via glucuronidation by several uridine 5'-diphospho glucuronosyltransferase (UGT) enzymes as well as direct excretion. These parallel elimination pathways result in a low risk of major drug-drug interactions or pharmacogenetic/ethnic variability for this compound.

Acyl glucuronide metabolites: Implications for drug safety assessment

Acyl glucuronides are important metabolites of compounds with carboxylic acid moieties and have unique properties that distinguish them from other phase 2 metabolites. In particular, in addition to being often unstable, acyl glucuronide metabolites can be chemically reactive leading to covalent binding with macromolecules and toxicity. While there is circumstantial evidence that drugs forming acyl glucuronide metabolites can be associated with rare, but severe idiosyncratic toxic reactions, many widely prescribed drugs with good safety records are also metabolized through acyl glucuronidation. Therefore, there is a need to understand the various factors that can affect the safety of acyl glucuronide-producing drugs including the rate of acyl glucuronide formation, the relative reactivity of the acyl glucuronide metabolite formed, the rate of elimination, potential proteins being targeted, and the rate of aglucuronidation. In this review, these factors are discussed and various approaches to de-risk the safety liabilities of acyl glucuronide metabolites are evaluated.

Toxicological role of an acyl glucuronide metabolite in diclofenac-induced acute liver injury in mice

The acyl glucuronide (AG) metabolites of carboxylic acid-containing drugs are potentially chemically reactive and are suggested to be implicated in toxicity, including hepatotoxicity, nephrotoxicity and drug hypersensitivity reactions. However, it remains unknown whether AG formation is related to toxicity in vivo. In this study, we sought to determine whether AG is involved in the pathogenesis of liver injury using a mouse model of diclofenac (DIC)-induced liver injury. Mice that were administered DIC alone exhibited significantly increased plasma alanine aminotransferase levels, whereas mice that were pretreated with the UDP-glucuronosyltransferase inhibitor (-)-borneol (BOR) exhibited suppressed alanine aminotransferase levels at 3 and 6 h after DIC administration although not significant at 12 h. The plasma DIC-AG concentrations were significantly lower in BOR- and DIC-treated mice than in mice treated with DIC alone. The mRNA expression levels of chemokine (C-X-C motif) ligand 1 (CXCL1), CXCL2 and the neutrophil marker CD11b were reduced in the livers of mice that had been pretreated with BOR compared to those that had been administered DIC alone, whereas mRNA expression of the macrophage marker F4/80 was not altered. An immunohistochemical analysis at 12 h samples revealed that the numbers of myeloperoxidase- and lymphocyte antigen 6 complex-positive cells that infiltrated the liver were significantly reduced in BOR- and DIC-treated mice compared to mice that were treated with DIC alone. These results indicate that DIC-AG is partly involved in the pathogenesis of DIC-induced acute liver injury in mice by activating innate immunity and neutrophils. Copyright © 2016 John Wiley & Sons, Ltd.

Structure, function and carcinogenicity of metabolites of methylated and non-methylated polycyclic aromatic hydrocarbons: a comprehensive review

The Unified Theory of PAH Carcinogenicity accommodates the activities of methylated and non-methylated polycyclic aromatic hydrocarbons (PAHs) and states that substitution of methyl groups on meso-methyl substituted PAHs with hydroxy, acetoxy, chloride, bromide or sulfuric acid ester groups imparts potent cancer producing properties. It incorporates specific predictions from past researchers on the mechanism of carcinogenesis by methyl-substituted hydrocarbons, including (1) requirement for metabolism to an ArCH2X type structure where X is a good leaving group and (2) biological substitution of a meso-methyl group at the most reactive center in non-methylated hydrocarbons. The Theory incorporates strong inferences of Fieser: (1) The mechanism of carcinogenesis involves a specific metabolic substitution of a hydrocarbon at its most reactive center and (2) Metabolic elimination of a carcinogen is a detoxifying process competitive with that of carcinogenesis and occurring by a different mechanism. According to this outlook, chemical or biochemical substitution of a methyl group at the reactive meso-position of non-methylated hydrocarbons is the first step in the mechanism of carcinogenesis for most, if not all, PAHs and the most potent metabolites of PAHs are to be found among the meso methyl-substituted hydrocarbons. Some PAHs and their known or potential metabolites and closely related compounds have been tested in rats for production of sarcomas at the site of subcutaneous injection and the results strongly support the specific predictions of the Unified Theory.