Studies on the reactivity of acyl glucuronides--II. Interaction of diflunisal acyl glucuronide and its isomers with human serum albumin in vitro

Mechanism of Acyl Group Migration in Carbohydrates

Acyl group migration has been the subject of several studies. Such migration processes may cause problems during synthesis, isolation, and purification of different acyl-bearing compounds, and have biological relevance, for example, in the metabolism of pharmaceuticals. Considering the recent evidence of acyl group migration being possible even over glycosidic bonds, it could be hypothesized to be involved also in the regulation of biological activity of natural polysaccharides in the host cells. Migrations are mostly observed in carbohydrates, typically having several hydroxyl groups near each other. Several studies have investigated the migration in a single or only a few different carbohydrate molecules, providing different suggestions for the mechanisms of migration, seldom supported by comprehensive computational investigations. In this concept article we discuss the recent progress on the mechanistic aspects of acyl group migration, with carbohydrates in particular focus.

Molecular mechanisms of hepatotoxicity induced by compounds occurring in Evodiae Fructus

Evodiae Fructus (EF) is a common herbal medicine with thousands of years of medicinal history in China, which has been demonstrated with many promising pharmacological effects on cancer, cardiovascular diseases and Alzheimer's disease. However, there have been increasing reports of hepatotoxicity associated with EF consumption. Unfortunately, in a long term, many implicit constituents of EF as well as their toxic mechanisms remain poorly understood. Recently, metabolic activation of hepatotoxic compounds of EF to generate reactive metabolites (RMs) has been implicated. Herein, we capture metabolic reactions relevant to hepatotoxicity of these compounds. Initially, catalyzed by the hepatic cytochrome P450 enzymes (CYP450s), the hepatotoxic compounds of EF are oxidized to generate RMs. Subsequently, the highly electrophilic RMs could react with nucleophilic groups contained in biomolecules, such as hepatic proteins, enzymes, and nucleic acids to form conjugates and/or adducts, leading to a sequence of toxicological consequences. In addition, currently proposed biological pathogenesis, including oxidative stress, mitochondrial damage and dysfunction, endoplasmic reticulum (ER) stress, hepatic metabolism disorder, and cell apoptosis are represented. In short, this review updates the knowledge on the pathways of metabolic activation of seven hepatotoxic compounds of EF and provides considerable insights into the relevance of proposed molecular hepatotoxicity mechanisms from a biochemical standpoint, for the purpose of providing a theoretical guideline for the rational application of EF in clinics.

Acyl glucuronide reactivity in perspective

Acyl glucuronidation is a common metabolic fate for acidic drugs and their metabolites and, because these metabolites are reactive, they have been linked to adverse drug reactions (ADRs) and drug withdrawals. However, alternative routes of metabolism leading to reactive metabolites (e.g., oxidations and acyl-CoA thioesters) mean that unambiguous proof that acyl glucuronides are toxic is lacking. Here, we review the synthesis and reactivity of these metabolites, and describe the use of molecular modelling and in vitro and in vivo reactivity assessment of acyl glucuronide reactivity. Based on the emerging structure-dependent differences in reactivity and protein adduction methods for risk assessment for acyl glucuronide-forming acid drugs or drug candidates in drug discovery/development are suggested.

Mass spectrometric characterization of circulating covalent protein adducts derived from a drug acyl glucuronide metabolite: multiple albumin adductions in diclofenac patients

Covalent protein modifications by electrophilic acyl glucuronide (AG) metabolites are hypothetical causes of hypersensitivity reactions associated with certain carboxylate drugs. The complex rearrangements and reactivities of drug AG have been defined in great detail, and protein adducts of carboxylate drugs, such as diclofenac, have been found in liver and plasma of experimental animals and humans. However, in the absence of definitive molecular characterization, and specifically, identification of signature glycation conjugates retaining the glucuronyl and carboxyl residues, it cannot be assumed any of these adducts is derived uniquely or even fractionally from AG metabolites. We have therefore undertaken targeted mass spectrometric analyses of human serum albumin (HSA) isolated from diclofenac patients to characterize drug-: derived structures and, thereby, for the first time, have deconstructed conclusively the pathways of adduct formation from a drug AG and its isomeric rearrangement products in vivo. These analyses were informed by a thorough understanding of the reactions of HSA with diclofenac AG in vitro. HSA from six patients without drug-: related hypersensitivities had either a single drug-: derived adduct or one of five combinations of 2-8 adducts from among seven diclofenac N-acylations and three AG glycations on seven of the protein's 59 lysines. Only acylations were found in every patient. We present evidence that HSA modifications by diclofenac in vivo are complicated and variable, that at least a fraction of these modifications are derived from the drug's AG metabolite, and that albumin adduction is not inevitably a causation of hypersensitivity to carboxylate drugs or a coincidental association.

Dissecting the reaction of Phase II metabolites of ibuprofen and other NSAIDS with human plasma protein

Blood-protein transacylation/glycosylation reactivity of glucuronides may distinguish beneficial (e.g., ibuprofen) and toxic (e.g., ibufenac) drugs.

Hepatic clearance of reactive glucuronide metabolites of diclofenac in the mouse is dependent on multiple ATP-binding cassette efflux transporters

Diclofenac is an important analgesic and anti-inflammatory drug that is widely used for the treatment of postoperative pain, rheumatoid arthritis, and chronic pain associated with cancer. Diclofenac is extensively metabolized in the liver, and the main metabolites are hydroxylated and/or glucuronidated conjugates. We show here that loss of multidrug resistance protein 2 (MRP2/ABCC2) and breast cancer resistance protein (BCRP/ABCG2) in mice results in highly increased plasma levels of diclofenac acyl glucuronide, after both oral and intravenous administration. The absence of Mrp2 and Bcrp1, localized at the canalicular membrane of hepatocytes, leads to impaired biliary excretion of acyl glucuronides and consequently to elevated liver and plasma levels. Mrp2 also mediates the biliary excretion of two hydroxylated diclofenac metabolites, 4'-hydroxydiclofenac and 5-hydroxydiclofenac. We further show that the sinusoidal efflux of diclofenac acyl glucuronide, from liver to blood, is largely dependent on multidrug resistance protein 3 (MRP3/ABCC3). Diclofenac acyl glucuronides are chemically instable and reactive, and in patients, these metabolites are associated with rare but serious idiosyncratic liver toxicity. This might explain why Mrp2/Mrp3/Bcrp1(-/-) mice, which have markedly elevated levels of diclofenac acyl glucuronides in their liver, display acute, albeit very mild, hepatotoxicity. We believe that the handling of diclofenac acyl glucuronides by ATP binding cassette transporters may be representative for the handling of acyl glucuronide metabolites of many other clinically relevant drugs.

Glycerolysis of acyl glucuronides as an artifact of in vitro drug metabolism incubations

During an investigation of the in vitro glucuronidation of benoxaprofen by human liver S-9 fraction, an unusual drug-related entity possessing a protonated molecular ion that was 74 mass units greater than the parent drug was observed. It was identified as the glycerol ester of benoxaprofen. Formation of this entity required inclusion of uridine diphosphoglucuronic acid (UDPGA) in the incubation, suggesting the formation of benoxaprofen acyl glucuronide followed by transesterification with the glycerol present in the incubation due to its presence as a stabilizer for liver subcellular fractions. Formation occurred during the sample work-up procedure while the samples were subjected to evaporation in vacuo, which does not remove glycerol. Conversion of purified benoxaprofen acyl glucuronide to the glycerol ester was demonstrated in glycerol at 37 degrees C. Other drugs that are converted to acyl glucuronides in vitro (diclofenac, mefenamic acid, tolmetin, and naproxen) were also shown to form corresponding glycerol esters when incubated with human liver S-9 fraction and UDPGA. The potential formation of glycerol esters of carboxylic acid drugs undergoing acyl glucuronidation in vitro represents an experimental artifact to which drug metabolism scientists should be aware.

Metabolic activation of carboxylic acids

BACKGROUND

Carboxylic acids constitute a large and heterogeneous class of both endogenous and xenobiotic compounds. A number of carboxylic acid drugs have been associated with adverse reactions, linked to the metabolic activation of the carboxylic acid moiety of the compounds, i.e., formation of acyl-glucuronides and acyl-CoA thioesters.

OBJECTIVE

The objective is to give an overview of the current knowledge on metabolic activation of carboxylic acids and how such metabolites may play a role in adverse reactions and toxicity.

METHODS

Literature concerning the formation and disposition of acyl glucuronides and acyl-CoA thioesters was searched. Also included were papers on the chemical reactivity of acyl glutathione-thioesters, and literature concerning possible links between metabolic activation of carboxylic acids and reported cellular and clinical effects.

RESULTS/CONCLUSION

This review demonstrates that metabolites of carboxylic acid drugs must be considered chemically reactive, and that the current knowledge about metabolic activation of this compound class can be a good starting-point for further studies on the consequences of chemically reactive metabolites.

Investigation of acetyl migrations in furanosides

Standard reaction conditions for the desilylation of acetylated furanoside (riboside, arabinoside and xyloside) derivatives facilitate acyl migration. Conditions which favour intramolecular and intermolecular mechanisms have been identified with intermolecular transesterifications taking place under mild basic conditions when intramolecular orthoester formations are disfavoured. In acetyl ribosides, acyl migration could be prevented when desilylation was catalysed by cerium ammonium nitrate.

PRECLINICAL CHARACTERIZATION OF 2-[3-[3-[(5-ETHYL-4′-FLUORO-2-HYDROXY[1,1′-BIPHENYL]-4-YL)OXY]PROPOXY]-2-PROPYLPHENOXY]BENZOIC ACID METABOLISM: IN VITRO SPECIES COMPARISON AND IN VIVO DISPOSITION IN RATS

Assessment of the pharmacokinetics of [14C]2-[3-[3-[(5-ethyl-4'-fluoro-2-hydroxy[1,1'-biphenyl]-4-yl)oxy]propoxy]-2-propylphenoxy-]benzoic acid ([14C]LY293111), an experimental anti-cancer agent, suggested long-lived circulating metabolites in rats. In vivo metabolites of LY293111 were examined in plasma, bile, urine, and feces of Fischer 344 (F344) rats after oral administration of [14C]LY293111. Metabolites were profiled by high-performance liquid chromatography-radiochromatography, and identified by liquid chromatography (LC)/mass spectrometry and LC/NMR. The major in vivo metabolites of LY293111 identified in rats were phenolic (ether), acyl, and bisglucuronides of LY293111. Measurement of radioactivity in rat plasma confirmed that a fraction of LY293111-derived material was irreversibly bound to plasma protein and that this bound fraction increased over time. This was consistent with the observed disparity in half-lives between LY293111 and total radioactivity in rats and monkeys, and is likely due to covalent modification of proteins by the acyl glucuronide. In vitro metabolism of [14C]LY293111 in liver slices from CD-1 mice, F344 rats, rhesus and cynomolgus monkeys, and humans indicates that glucuronidation was the primary metabolic pathway in all species. The acyl glucuronide was the most prevalent radioactive peak (16% of total 14C) produced by F344 rat slices, whereas the ether glucuronide was the major metabolite in all other species (26-36% of total 14C). Several minor hydroxylated metabolites were detected in F344 rat slice extracts but were not observed in other species. The data presented suggest that covalent modification of proteins by LY293111 acyl glucuronide is possible in multiple species, although the relative reactivity of this metabolite appears to be low compared with those known to cause adverse drug reactions.

Acyl glucuronide reactivity in perspective: biological consequences

The metabolic conjugation of exogenous and endogenous carboxylic acid substrates with endogenous glucuronic acid, mediated by the uridine diphosphoglucuronosyl transferase (UGT) superfamily of enzymes, leads to the formation of acyl glucuronide metabolites. Since the late 1970s, acyl glucuronides have been increasingly identified as reactive electrophilic metabolites, capable of undergoing three reactions: intramolecular rearrangement, hydrolysis, and intermolecular reactions with proteins leading to covalent drug-protein adducts. This essential dogma has been accepted for over a decade. The key question proposed by researchers, and now the pharmaceutical industry, is: does or can the covalent modification of endogenous proteins, mediated by reactive acyl glucuronide metabolites, lead to adverse drug reactions, perhaps idiosyncratic in nature? This review evaluates the evidence for acyl glucuronide-derived perturbation of homeostasis, particularly that which might result from the covalent modification of endogenous proteins and other macromolecules. Because of the availability of acyl glucuronides for test tube/in vitro experiments, there is now a substantial literature documenting their rearrangement, hydrolysis and covalent modification of proteins in vitro. It is certain from in vitro experiments that serum albumin, dipeptidyl peptidase IV, tubulin and UGTs are covalently modified by acyl glucuronides. However, these in vitro experiments have been specifically designed to amplify any interference with a biological process in order to find biological effects. The in vivo situation is not at all clear. Certainly it must be concluded that all humans taking carboxylate drugs that form reactive acyl glucuronides will form covalent drug-protein adducts, and it must also be concluded that this in itself is normally benign. However, there is enough in vivo evidence implicating acyl glucuronides, which, when backed up by in vivo circumstantial and documented in vitro evidence, supports the view that reactive acyl glucuronides may initiate toxicity/immune responses. In summary, though acyl glucuronide-derived covalent modification of endogenous macromolecules is well-defined, the work ahead needs to provide detailed links between such modification and its possible biological consequences.

Implication of P450-metabolite complex formation in the nonlinear pharmacokinetics and metabolic fate of (+/-)-(1'R*,3R*)-3-phenyl-1-[(1',2',3',4'-tetrahydro-5',6'-methylene-dioxy-1'-naphthalenyl) methyl] pyrrolidine methanesulfonate (ABT-200) in dogs

Following a single oral or intravenous administration of the R,R(+) and S,S(-) (14)C-pseudoracemate of (+/-)-(1'R*,3R*)-3-phenyl-1-[(1',2',3',4'-tetrahydro-5',6'-methylene-dioxy-1'-naphthalenyl) methyl] pyrrolidine methanesulfonate (ABT-200/I) to dogs, a total of six (R,R[+]) and eight (S,S[-]) metabolites were identified by high-pressure liquid chromatography/mass spectral techniques. Greater than 99% of the dose was eliminated as metabolites indicating that the clearance of I was predominantly metabolic. The catechol was the major excreted metabolite (fecal), whereas the urine and bile predominantly contained metabolites resulting from secondary biotransformation of the catechol via O-methylation, glucuronidation, and sulfation. After a single 12 mg/kg oral dose of racemic I to dogs, the mean area under the plasma curve (AUC(0-24h)) averaged 4.55 micro g. h/ml, with an apparent plasma clearance value of 2.70 l/h. kg. After 14 daily doses, the apparent plasma clearance was 3.5-fold lower (0.78 l/h. kg) and the AUC(0-24h) about 4-fold higher (18.58 micro g. h/ml). Isolation of liver microsomes from these animals indicated that a cytochrome p450 (p450)-metabolite complex (MI complex) was formed in the liver after both single and multiple dosing. The mean concentration of the MI complex 24 h after a single dose averaged 31 pmol/mg of microsomal protein, whereas the amount in the animals given multiple doses of I averaged 163 pmol/mg. There was a positive correlation (R(2) = 0.993) between the plasma AUC for I and the amount of the MI complex found in the liver of each animal. Formation of the MI complex was demonstrated in vitro in control dog liver microsomes, was NADPH-dependent, and was dissociated from p450 with 2-methylbenzimidazole. In vitro preincubation studies indicated that I was a mechanism-based inhibitor and that formation of the complex inhibited catechol formation. These results demonstrate that the liver p450s that metabolize I form an inhibitory MI complex after in vivo administration in dogs. Formation of the complex increases during multiple dosing and inhibits the enzymes from further metabolism of I resulting in nonlinear pharmacokinetics.

Rapid internal acyl migration and protein binding of synthetic probenecid glucuronides

Internal acyl migration reactions of drug 1-O-acyl-beta-D-glucopyranuronates (1beta-acyl glucuronides) are of interest because of their possible role in covalent binding to proteins and consequent adverse effects. The reactivity of the synthetic probenecid 1beta-acyl glucuronide (PRG), the principal metabolite of probenecid (PR) in humans, has been investigated in terms of acyl migration, hydrolysis, and covalent binding to proteins in phosphate buffer (pH 7.4) and human plasma at 37 degrees C. PRG primarily degraded by acyl migration according to apparent first-order kinetics and the 2-, 3-, and 4-acyl isomers sequentially appeared as both alpha- and beta-anomeric forms. In addition, small amounts of PRG and extremely labile 1alpha-acyl isomer existed in the equilibrated mixture favoring the 2alpha/beta-acyl isomer, that provided significant information regarding the mechanism of acyl migration. All of the positional isomers and anomers were characterized using preparative HPLC and NMR spectroscopy. Acyl migration was observed to predominate over hydrolysis in both media although the extent of hydrolysis in plasma was larger than that in the buffer. The overall degradation half-lives (h) in the buffer and plasma were 0.27 +/- 0.003 and 0.17 +/- 0.007, respectively. The covalent binding rapidly proceeded mainly via the Schiff's base mechanism and reached a plateau after 2 h of incubation. The maximal binding was 146 +/- 4.8 pmol/mg of protein, and ca. 10% of the initial concentration of PRG. These results indicated that PRG is most labile and susceptible to acyl migration of all the drug acyl glucuronides reported to date in the physiological conditions, and highly reactive to plasma proteins, that could provide a possible explanation for the immunologically based adverse effects of PR.

Stereospecific pH-dependent degradation kinetics of R- and S-naproxen-beta-l-O-acyl-glucuronide

The hydrolysis and acyl migration of biosynthetic S-naproxen-beta-l-O-acyl glucuronide (I) and R-naproxen-beta-l-O-acyl glucuronide (II) was followed by HPLC. Nine first-order kinetic rate constants for the hydrolysis and acyl migration between the beta-l-O-acyl glucuronide, its alpha/beta-2, alpha/beta-3-, alpha/beta-4-, and alpha-1-O-acyl isomers and naproxen aglycone were determined for I and II at pH 7.00, 7.40 and 8.00 at 37 degrees C by kinetic simulation. For I the 3-O-acyl isomer was the most stable isomer as the pseudo-equilibrium ratio for the major acyl-migrated isomers was 1:1.5:0.9 (2-O-acyl isomer:3-O-acyl isomer:4-O-acyl isomer). The 3- and 4-O-acyl isomers of II were equally stable as the pseudo-equilibrium ratio for the major acyl-migrated isomers was 1:1.4:1.4 (2-O-acyl isomer:3-O-acyl isomer:4-O-acyl isomer). For both I and II, the pseudo-equilibrium ratio between the major 2-O-acyl isomer and the minor alpha-l-O-acyl isomer was 10:1 (2-O-acyl isomer:alpha-l-O-acyl isomer). The pseudo-equilibrium found for the major acyl-migrated isomers of I and II in the present study corresponds with the pattern previously published for R- and S-ketoprofen-beta-l-O-acyl glucuronide acyl-migrated isomers, suggesting that these findings may be general for acyl-migrated beta-l-O-acyl glucuronides of enantiomeric 2-arylpropionic acids.

Kinetics of intramolecular acyl migration of 1beta-O-acyl glucuronides of (R)- and (S)-2-phenylpropionic acids

The stereoselective acyl migration of diastereomeric 1beta-O-acyl glucuronides of (R)- and (S)-2-phenylpropionic acid [(R)-1PG and (S)-IPG, respectively] in phosphate buffer (pH 7.4) at 310K was investigated using HPLC. The disappearance of (R)-1PG was faster than that of (S)-1PG according to pseudo first-order kinetics. A kinetic model describing the degradation reactions was constructed. The rate constant for acyl migration from the 1beta-O-isomer to the 2-O-acyl isomer (k12) was about one order magnitude larger than that for hydrolysis from 1beta-O-acyl isomer to aglycone (k10). The k12 of (R)-IPG (0.377 +/- 0.005 h(-1)) was about two times larger than that of (S)-IPG (0.184 +/- 0.003 h(-1)). The results indicated that the stereoselectivity in the degradation of 1PG was apparently governed by the acyl migration from 1-isomer to 2-isomer. The kinetic parameters for acyl migration from 1-isomer to 2-isomer were estimated from temperature-dependent experiments using the transition state theory. The value of the free energy of activation at 310 K for (R)-1PG (99.67 kJ/mol) was smaller than that of (S)-IPG (101.60kJ/mol), suggesting that (R)-IPG showed thermodynamically higher reactivity in acyl migration than (S)-1PG.

In vitro stereoselective degradation of carprofen glucuronide by human serum albumin. Characterization of sites and reactive amino acids

Acyl glucuronides formed from carboxylic acids can undergo hydrolysis, acyl migration, and covalent binding to proteins. In buffers at physiological pH, the degradation of acylglucuronide of a chiral NSAID, carprofen, consisted mainly of acyl migration. Acidic pH reduced hydrolysis and acyl migration, thus stabilizing the carprofen acyl glucuronides. Addition of human serum albumin (HSA) led to an increased hydrolysis of the conjugates of both enantiomers. This protein protected R-carprofen glucuronide from migration and therefore improved its overall stability. Hydrolysis was stereoselective in favor of the S conjugate. The protein domains and the amino acid residues likely to be responsible for the hydrolytic activity of HSA were deduced from the results of various investigations: competition with probes specific of binding sites, effects of pH and of chemical modifications of albumin. Dansylsarcosine (DS), a specific ligand of site II of HSA, impaired the hydrolysis, whereas dansylamide (DNSA) and digoxin, which are specific ligands of sites I and III, respectively, had no effect. The extent of hydrolysis by HSA strongly increased with pH, indicating the participation of basic amino acids in this process. The results obtained with chemically modified HSA suggest the major involvement of Tyr and Lys residues in the hydrolysis of glucuronide of S-carprofen, and of other Lys residues for that of its diastereoisomer.

In vitro regioselective stability of beta-1-O- and 2-O-acyl glucuronides of naproxen and their covalent binding to human serum albumin

beta-1-O- (NAG) and 2-O-glucuronides (2-isomer) of (S)-naproxen (NA) were prepared to determine which positional isomer(s) of the acyl glucuronide of NA is responsible for forming covalent adducts with human serum albumin (HSA). Their comparative stability and covalent binding adduct formation with HSA were investigated at pH 7.4 and at 37 degreesC. NA and its acyl glucuronides were simultaneously determined by HPLC. Three positional isomers were formed successively after incubation of NAG in the buffer only. However, when NAG was incubated with HSA (30 mg/mL), isomers other than the 2-isomer were formed in little or negligible quantities. In HSA solution, NAG (kd = 2.08 +/- 0.08 h-1) was four times less stable than 2-isomer (kd = 0.51 +/- 0.02 h-1). NAG was degraded by hydrolysis (khyd = 1.01 +/- 0.10 h-1) and isomerization (kiso = 1.07 +/- 0.07 h-1) to the same extent; however, hydrolysis was predominant for the 2-isomer (kd = 0.51 +/- 0.02 h-1). The incubation of both NAG and 2-isomer with HSA led to the formation of a covalent adduct; however, the adduct formation from the 2-isomer proceeded more slowly than that from NAG. The present results suggest that the covalent binding of NA to HSA via its acyl glucuronides proceeds through both transacylation (direct nucleophilic displacement) and glycation mechanisms; NAG rapidly forms an adduct that may be unstable, and the protein adduct from the 2-O-acyl glucuronide is as important for the covalent binding as those from the 1-O-acyl glucuronides.

Diclofenac acyl glucuronide, a major biliary metabolite, is directly involved in small intestinal injury in rats

BACKGROUND & AIMS

Enterohepatic recirculation of nonsteroidal anti-inflammatory drugs is a critical factor in the pathogenesis of intestinal injury, but the underlying mechanism of toxicity remains obscure. The aim of this study was to examine the role of diclofenac acyl glucuronide, which is the major biliary metabolite and is chemically reactive, in the precipitation of small intestinal ulceration.

METHODS

Hepatocanalicular conjugate export pump-deficient (TR-) rats were used to selectively block diclofenac enterohepatic circulation without interrupting bile flow. Bile from diclofenac-treated normal rats was orally transferred to wild-type and TR- rats, and the extent of ulcer formation was compared with that induced by control bile containing free diclofenac. The effect of induction of hepatic diclofenac glucuronosyltransferase on the severity of diclofenac-induced ulceration was also determined.

RESULTS

TR- rats were refractory to diclofenac given either intraperitoneally or perorally. However, transfer of bile containing diclofenac glucuronide significantly increased the extent of ulcer formation in both normal and TR- rats. Moreover, induction of glucuronosyltransferase aggravated intestinal ulceration.

CONCLUSIONS

The reactive acyl glucuronide of diclofenac, or the acyl glucuronide of one of its oxidative metabolites, is directly involved in the pathogenesis of small intestinal injury.

Zomepirac acyl glucuronide covalently modifies tubulin in vitro and in vivo and inhibits its assembly in an in vitro system

Drugs possessing a carboxylate functional group usually form acyl glucuronides as major metabolites. These electrophilic metabolites can undergo several spontaneous reactions, including covalent adduct formation with proteins. The present study examined whether covalent adducts were formed with microtubular protein (MTP, 85%, alpha/beta-tubulin) and whether this influenced its ability to assemble into microtubules. Bovine brain microtubular protein (MTP) was purified by assembly-disassembly cycles and incubated with the nonsteroidal anti-inflammatory drug (NSAID) zomepirac (ZP), its acyl glucuronide (ZAG) and rearrangement isomers (iso-ZAG) at various concentrations for 2 h at room temperature and pH 7.5. Assembly was monitored by change in turbidity (increase in absorbance at 340 nm). Both ZAG and iso-ZAG caused dose-dependent inhibition of assembly (50% inhibition at about 1 mM), while ZP caused modest inhibition (< 50% inhibition at 4 mM). In a slightly different system, incubation of performed microtubules with 4 mM ZAG caused about 35% inhibition of reassembly ability, while modification of MTP under similar conditions resulted in about 85% reduction of assembly ability. Immunoblotting with a ZP antiserum showed that ZAG and iso-ZAG covalently modified MTP in a dose-dependent manner, while ZP itself caused no modification. Tubulin and many minor proteins comprising MTP were modified. ZP-modified tubulin was shown to be present in the cytosol of livers from rats dosed twice daily for 3 days with ZP at 50 mg/kg, using a sandwich ELISA with ZP and tubulin antisera. Whether any perturbation of microtubule assembly occurs in vivo as a result of this in vivo modification is currently under investigation.

Direct detection of the internal acyl migration reactions of benzoic acid 1-O-acylglucuronide by 13C-labeling and nuclear magnetic resonance spectroscopy

1-O-Acyl-beta-D-glucopyranuronates can undergo irreversible binding to proteins mainly through internal acyl migration reactions, which may have toxicological significance. A new method based on the 13C-labeling and nuclear magnetic resonance (NMR) spectroscopy has been developed to study the reactivity of the 1-O-acyl-beta-D-glucopyranuronate of benzoic acid. In phosphate buffer (pH 7.4) solution at 37 degrees C, the glucuronide showed apparent first-order degradation kinetics (T1/2, 125 min), and concurrent and sequential appearance of 2-, 3- and 4-O-acyl isomers as both alpha- and beta-anomers was observed. The isomeric glucuronides were identified by two-dimensional NMR of the reaction mixture. The direct approach using 13C-labeling and NMR could also provide insights into the reactivities of other labile drug acylglucuronides and their isomeric glucuronides.

Rat serum albumin modified by diflunisal acyl glucuronide is immunogenic in rats

Acyl glucuronide metabolites of carboxylic acid drugs such as the salicylate derivative diflunisal (DF) have been shown to react with proteins in vitro and in vivo to produce covalent adducts. Such attachment of foreign compounds to endogenous molecules could be associated with toxic and/or immune consequences. In this study we have injected rats with rat serum albumin (RSA) modified (a) by DF using a carbodiimide reagent (-->DF-RSA-I, 4.9 micrograms DF/mg RSA) and (b) by incubation with DF acyl glucuronide (DAG) and its rearrangement isomers (iso-DAG) (-->DF-RSA-II, 0.34 micrograms DF/mg RSA). All of the six rats injected with DF-RSA-I produced antibodies reactive with DF-modified keyhole limpet hemocyanin (KLH), the coating protein used in the ELISA. Three out of six animals injected with DF-RSA-II generated similar antibodies. Cross-reactivity with other non-steroidal anti-inflammatory drugs (NSAIDs) such as naproxen and ketoprofen (as the free drugs) was not observed. This study shows that a self protein covalently modified by incubation with DAG and iso-DAG is immunogenic in rats. The data thus support the hypothesis that covalent modification of macromolecules by acyl glucuronide metabolites of acidic drugs in vivo can lead to the production of circulating antibodies which may be involved in aberrant immune responses such as drug hypersensitivity.

Studies on the reactivity of acyl glucuronides--VIII. Generation of an antiserum for the detection of diflunisal-modified proteins in diflunisal-dosed rats

Acyl glucuronide metabolites of carboxylic drugs such as the salicylate derivative diflunisal (DF) have been shown to react with proteins to produce covalent adducts. To aid in the study of the formation and distribution of these adducts in both humans and rats, we raised an antiserum against human serum albumin modified by covalent attachment of DF via an amide bond, using a carbodiimide reagent. This antiserum had wide reactivity, reacting with all types of DF-modified proteins tested and with free DF (albeit at a lower affinity). It did not cross-react with other salicylates or other non-steroidal anti-inflammatory drugs. The antiserum has been used in immunoblotting to detect proteins covalently modified by DF in the plasma and livers of rats treated with the drug for 7 days. Although some cross-reactivity was apparent on the blots, a series of DF-modified proteins was found in cytosolic, mitochondrial and mixed membrane fractions of hepatocytes, with molecular weights ranging from 28 to 130 kDa.

Idiosyncratic liver toxicity of nonsteroidal antiinflammatory drugs: molecular mechanisms and pathology

This review explores the clinical hepatic pathology associated with the use of nonsteroidal antiinflammatory drugs (NSAIDs), possible cellular and molecular mechanisms of injury, and future challenges. NSAIDs comprise a group of widely used compounds that have been associated with rare adverse reactions in the liver, including fulminant hepatitis and cholestasis. These reactions are idiosyncratic, mostly independent of the dose administered, and host-dependent. The mechanisms responsible for the initiation and perpetuation of NSAID-induced hepatotoxicity remain poorly understood and have been largely inferred from clinical manifestation. A mounting body of evidence, however, indicates that many acidic NSAIDs are metabolized to reactive acyl glucuronides that can form covalent adducts with plasma proteins and hepatocellular proteins. In hepatocytes cocultured with lymphocytes, these NSAID-altered proteins can become antigenic. Thus, long-lived, drug-altered proteins may act as immunogens and produce cytotoxic T-cell-mediated or antibody-dependent, cell-mediated toxicity in susceptible patients. Alternatively, individual abnormalities in metabolism or disposition of some NSAIDs may lead to the formation or accumulation of toxic metabolites. Additional work with transgenic animal models is needed to permit better understanding of the general and specific risk factors involved in the pathogenesis of the idiosyncratic liver injuries related to NSAIDs and other drugs.

Studies on the reactivity of acyl glucuronides--VII. Salicyl acyl glucuronide reactivity in vitro and covalent binding of salicylic acid to plasma protein of humans taking aspirin

Salicyl acyl glucuronide (SAG) is a significant metabolite of salicylic acid (SA) and aspirin. We have shown that, under physiological conditions in vitro, SAG undergoes rearrangement in a manner consistent with acyl migration to its 2-, 3- and 4-O-acyl positional isomers as the predominant pathway (T1/2 values were 1.4-1.7 hr in buffer at pH 7.4 and 37 degrees). Incubation of SAG or a mixture of its rearrangement isomers (iso-SAG) (each at approximately 50 micrograms SA equivalents/mL) with human serum albumin (HSA, at approximately 40 mg/mL) revealed the formation of covalent adducts with the protein, with peak concentrations of 1-2 micrograms SA equivalents/mL. The data support a role for the rearrangement/glycation mechanism of adduct formation. Covalent adducts of SA were also detected in the plasma of humans taking aspirin (at > or = 1200 mg/day), but the concentrations were low (<< 100 ng SA equivalents/mL). Reactivity of SAG thus provides a mechanism (though of uncertain quantitative importance) of covalent attachment of the salicyl moiety of aspirin to tissue macromolecules, which is in addition to its well-known acetylating capacity.

Studies on the reactivity of acyl glucuronides--VI. Modulation of reversible and covalent interaction of diflunisal acyl glucuronide and its isomers with human plasma protein in vitro

Acyl glucuronide conjugates are chemically reactive metabolites which can undergo hydrolysis, rearrangement (isomerization via acyl migration) and covalent binding reactions with protein. The present study was undertaken to identify factors modulating the reactivity of diflunisal acyl glucuronide (DAG) with human serum albumin (HSA) in vitro, by comprehensively evaluating the interplay of the three pathways above when DAG and a mixture of its 2-, 3- and 4-isomers (iso-DAG) were incubated with protein. Buffer, plasma, fraction V HSA, fatty acid-free HSA, globulin-free HSA and fatty acid- and globulin-free HSA were investigated at pH 7.4 and 37 degrees, each in the absence and presence of warfarin, diazepam and diflunisal (DF) as reversible binding competitors. DAG and iso-DAG were highly reversibly bound (ca. 98-99.5%) in plasma and HSA solutions. The binding was primarily at the benzodiazepine site, since displacement occurred in the presence of diazepam and fatty acids but not warfarin. DAG degradation, via rearrangement, hydrolysis and covalent adduct formation (in that order of quantitative importance), was retarded in plasma and HSA solutions compared to buffer. The protective effect of protein was afforded by the high reversible binding to the (non-catalytic) benzodiazepine site. The warfarin site appeared to be catalytic for DAG hydrolysis, whereas rearrangement appeared to be hydroxide ion-catalysed only. In contrast to DAG, iso-DAG degradation was greatly accelerated in the presence of protein, through both covalent binding and catalysis of hydrolysis. Covalent binding via DAG was increased in the presence of warfarin but decreased in the presence of diazepam, DF and fatty acids. The opposite effects were found for covalent binding via iso-DAG. The data suggest that covalent binding of DF to HSA via DAG and iso-DAG occurs by different mechanisms (presumably transacylation and glycation, respectively) at different sites (benzodiazepine and warfarin, respectively) whereas reversible binding occurs primarily at the same site (benzodiazepine).

Reversible binding of tolmetin, zomepirac, and their glucuronide conjugates to human serum albumin and plasma

Acyl glucuronides of drugs and bilirubin have been shown in the past decade to be reactive metabolites undergoing acyl migration and irreversible binding. The latter reaction has been hypothesized to be facilitated by or to proceed through the formation of a reversible complex. Furthermore, it has been suggested that the decreased binding seen in patients with compromised excretory function may be due to competition by elevated plasma concentrations of the glucuronides. In these reversible binding studies, we characterized the extent and the "site" of binding of tolmetin, zomepirac, their glucuronides and isomeric conjugates. We also examined the displacement between the parent drugs and their glucuronide conjugates using a rapid ultrafiltration method. Tolmetin exhibited three classes of binding sites with a primary association constant of 1.7 x 10(6) M-1 (Kd1 = 0.60 microM). The primary association constant of zomepirac (1.16 x 10(6) M-1, Kd1 = 0.86 microM) is similar to that of tolmetin. The beta 1 and alpha/beta 3 glucuronides of both compounds bind to a lesser extent than their parent aglycones. The isomeric glucuronide conjugates of both compounds showed much stronger binding than the beta/1 conjugates. Of the four glucuronides investigated, tolmetin glucuronide-alpha/beta 3 isomer was bound by fatty acid free human serum albumin with the highest affinity (4.6 x 10(5) M-1, Kd = 2.22 microM). Protein binding of the parent drugs and conjugates were decreased significantly at pH 5.0. In displacement studies, except for salicylate and acetylsalicylate, drugs known to bind to Sites I and II as well as the digitoxin and tamoxifen binding sites had little inhibitory effect on the binding of tolmetin, zomepirac, and their glucuronide conjugates.

Glucuronidation of diflunisal by rat liver microsomes. Effect of microsomal beta-glucuronidase activity

The in vitro formation rates of the phenolic (DPG) and acyl (DAG) glucuronides of diflunisal were investigated using rat liver microsomes. Preliminary studies showed that DAG hydrolysed rapidly (T1/2 = 12 min) when incubated in the presence of rat liver microsomes at pH 7.4 and 37 degrees. DPG was much more stable under the same conditions (T1/2 = 35 hr). Hydrolysis of DAG and DPG by rat liver microsomes was inhibited by 4 mM saccharolactone, a beta-glucuronidase inhibitor. The apparent Km and Vmax values for the formation of DAG in the absence and presence of 4 mM D-saccharic acid-1,4-lactone (saccharolactone) were the following: Km = 0.05 +/- 0.02 vs 0.08 +/- 0.02 mM and Vmax = 0.20 +/- 0.06 vs 0.43 +/- 0.07 nmol/min/mg protein (0 and 4 mM saccharolactone, respectively). The significant increase in apparent Vmax for DAG formation in the presence of saccharolactone can be explained by the inhibition of beta-glucuronidase-catalysed hydrolysis of DAG. Apparent Km and Vmax values for the formation rate of DPG were not affected by addition of saccharolactone to the incubation medium. These results indicate that beta-glucuronidase-catalysed hydrolysis of certain glucuronides formed during microsomal incubations may significantly affect the apparent glucuronidation rate due to the presence of a glucuronidation-deglucuronidation cycle.

Studies on the reactivity of acyl glucuronides--V. Glucuronide-derived covalent binding of diflunisal to bladder tissue of rats and its modulation by urinary pH and beta-glucuronidase

Acyl glucuronide conjugates of acidic drugs have been shown to be reactive metabolites capable of undergoing non-enzymic hydrolysis, rearrangement (isomerization via acyl migration) and covalent binding reactions with plasma protein. In an earlier study (King and Dickinson, Biochem Pharmacol 45: 1043-1047, 1993), we documented formation of covalent adducts of diflunisal (DF), a salicylate derivative which is metabolized in part to a reactive acyl glucuronide (DAG), with liver, kidney, skeletal muscle and small and large intestine (in addition to plasma protein) of rats given the drug i.v. twice daily at 50 mg DF/kg for 7 days. The present study shows that covalent adducts of DF were also formed with urinary bladder tissue of these rats, achieving concentrations (ca. 5 micrograms DF equivalents/g tissue) higher than those found in the other tissues noted above. After cessation of dosing, the adduct concentrations declined with an apparent T 1/2 value of ca. 20 hr. Adducts were also formed ex vivo in excised rat bladders in which DAG or a prepared mixture of its acyl migration isomers (iso-DAG) were incubated at pH 5.0, 6.5 and 8.0. After 8 hr incubation, the highest concentrations (ca. 11 micrograms DF equivalents/g) were produced with iso-DAG at pH 5.0, and the lowest (ca. 2.3 micrograms DF equivalents/g) with DAG at pH 5.0. However, a major competing reaction for DAG (at least at pH 5.0) was hydrolysis by beta-glucuronidases originating from bladder tissue. By contrast, iso-DAG was quite resistant to such hydrolysis. The phenolic glucuronide conjugate, another important metabolite of DF, was hydrolysed only slowly. Similar results were obtained in fresh rat urine adjusted to pH 5.0. The results support covalent DF adduct formation in rat bladder originating from both DAG and iso-DAG as ultimate reactants, though the extent of binding is modulated by both urinary pH and beta-glucuronidases.

Studies on the in vitro reactivity of clofibryl and fenofibryl glucuronides. Evidence for protein binding via a Schiff's base mechanism

Clofibryl and fenofibryl acyl (ester) glucuronides (CAG and FAG) are major metabolites in humans of the hypolipidaemic drugs clofibrate and fenofibrate, respectively. We have investigated three inter-related aspects of the reactivity of CAG and FAG in human serum albumin (HSA) solution, human plasma and in buffer at pH 7.0: namely (a) rearrangement via acyl migration to glucuronic acid esters of clofibric acid (CA) and fenofibric acid (FA), (b) hydrolysis of the parent glucuronide and rearrangement products to yield CA and FA and (c) the formation of covalent adducts with albumin and plasma protein. CAG was more reactive than FAG in all media, especially the protein solutions. The reactivity of both glucuronides was accelerated in protein solution compared with buffer and this was more marked in plasma than in HSA solution. The predominant reaction during the initial stages of the incubation was formation of isomeric rearrangement products. In the protein solutions, CA and FA were the major reaction products after 24 hr, compared to the rearranged isomers in buffer. Protein binding of 14C to HSA was markedly higher after incubation of CAG and FAG labelled on the glucuronyl moiety compared with the label on the aglycone. This is consistent with the covalent binding of CAG and FAG to protein proceeding via the formation of a Schiff's base rather than by transacylation.

Studies on the renal excretion of the acyl glucuronide, phenolic glucuronide and sulphate conjugates of diflunisal

1. In five healthy male volunteers given multiple doses of diflunisal (DF), renal clearances (CLR) of the acyl glucuronide (DAG), phenolic glucuronide (DPG) and sulphate (DS) conjugates were about 42, 25 and 13 ml min-1, respectively. 2. These relatively low CLR values are probably due largely to the very high plasma protein binding of the conjugates, found in vitro to be 99.0%, 97.8% and 99.45%, respectively. 3. Thus glomerular filtration plays the minor and active tubular secretion the major role in renal excretion of the three conjugates. 4. This conclusion was supported by the effect of probenecid co-administration, which decreased CLR of DAG and DPG by about 70%. CLR for DS could not be calculated when probenecid was co-administered (because of interference by probenecid metabolites in the analysis of DS in urine). 5. Water-induced diuresis had no effect on CLR of the DF conjugates, consistent with tubular reabsorption being negligible.

Covalent binding of suprofen acyl glucuronide to albumin in vitro

1. Suprofen acyl glucuronide, a major metabolite of suprofen in man, is labile, undergoing acyl migration to isomeric conjugates which are not cleaved by beta-glucuronidase. 2. The pH-dependent degradation of diastereomeric suprofen glucuronides in aqueous buffer increases rapidly near physiological pH with an apparent first-order half life of 1.4 h at pH 7.4. 3. Suprofen glucuronide and its isomeric conjugates are reactive with albumin in a pH-dependent manner which corresponds to the stability of the acyl glucuronide. Several per cent of the conjugates added to albumin in solution become covalently bound. 4. The covalent binding of suprofen equivalents to albumin is greatly enhanced by the addition of either cyanide or cyanoborohydride anion, which supports the presence of an imine in the process of binding. Release of isomeric conjugates by treatment of the albumin adduct with dilute acid also supports covalent binding via an imine. 5. The covalent binding of suprofen to proteins through its reactive acyl glucuronide may be of toxicological importance and relevant to the acute renal toxicity observed for suprofen in man.

Studies on the reactivity of acyl glucuronides--IV. Covalent binding of diflunisal to tissues of the rat

Acyl glucuronides have been shown to be reactive electrophilic metabolites capable of undergoing hydrolysis, rearrangement (isomerization via acyl migration) and covalent binding reactions to plasma protein. The present study was undertaken to explore the occurrence and extent of in vivo formation of covalent adducts of diflunisal (DF), a salicylate derivative which forms a reactive acyl glucuronide, with tissues and plasma protein of rats. Groups of rats were given 50 mg DF/kg i.v. twice daily for periods of up to 7 days. Steady state plasma concentrations of reversibly bound DF and its conjugates (as measured 6 hr after a dose) were achieved by the third day of dosing. T 1/2 values after cessation of dosing were about 5-10 hr. By contrast, covalent DF-tissue adducts steadily accumulated over the 7-day dosing period. Maximum concentrations, measured 6 hr after the last dose, were 4.8 (liver), 1.0 (kidney), 0.74 (plasma), 0.26 (small intestine minus contents), 0.27 (large intestine minus contents) and 0.20 (skeletal muscle) microgram DF/g tissue or/mL plasma. T 1/2 values of about 50, 67, 18, 38 and 43 hr were obtained for liver, kidney, plasma and small and large intestine (respectively) after cessation of dosing. Thus, the study of acyl glucuronide reactivity and the question of any derived toxicity or immune responses should consider the formation of long-lived adducts in tissues as well as in plasma.

In vitro and in vivo irreversible plasma protein binding of beclobric acid enantiomers

Acyl glucuronides are known to be labile conjugates, which undergo hydrolysis and bind irreversibly to proteins. The lipid-regulating agent (+/-)-beclobrate is immediately converted to the free acid after oral administration. Further metabolism leads to formation of the corresponding diastereomeric acyl glucuronides. Beclobric acid glucuronides were quantified by indirect measurement with an HPLC method based on chiral fluorescent derivatization of the carboxylic acid and subsequent normal-phase chromatography. The renal clearance of unchanged drug is low, with almost all drug excreted into urine as glucuronic acid conjugates. Beclobric acid glucuronide is also detectable in plasma. In vitro degradation studies with beclobric acid glucuronide (at a concentration of 5 microM in 150 mM phosphate buffer pH 7.4) exhibited a minor tendency for acyl migration and hydrolysis, i.e., a higher stability than has been observed for the acyl glucuronides of most other drugs. The in vitro degradation half-lives of the two beclobric acid beta-1-O-acyl glucuronides were 22.7 and 25.7 h. After incubation with pooled plasma and human serum albumin in buffer pH 7.4 irreversible binding was measured in vitro. No significant difference between the two enantiomers was detected with respect to the magnitude of in vitro irreversible binding. In 3 healthy male volunteers the extent of irreversible binding of both beclobric acid enantiomers to plasma proteins was investigated after single and multiple oral doses of racemic beclobrate (100 mg once daily). Irreversible binding of both enantiomers was observed in all volunteers. The adduct densities for (-)- and (+)-beclobric acid after single 100 mg beclobrate doses were 0.147 x 10(-4) and 0.177 x 10(-4) mol/mol protein. Multiple dosing increased irreversible binding 3- to 4-fold.

Studies on the reactivity of acyl glucuronides--III. Glucuronide-derived adducts of valproic acid and plasma protein and anti-adduct antibodies in humans

The major metabolite of the anti-epileptic agent valproic acid (VPA) is its acyl glucuronide conjugate (VPA-G), which undergoes non-enzymic, pH-dependent rearrangement via acyl migration to a mixture of beta-glucuronidase-resistant forms (collectively VPA-G-R). We have compared the reactivity of VPA-G and VPA-G-R towards covalent VPA-protein adduct formation by incubation in buffer, human serum albumin (HSA) and fresh human plasma at pH 7.4 and 37 degrees. In all three media, the predominant reaction of VPA-G over 30 hr was rearrangement to VPA-G-R (ca. 24%). Hydrolysis was quite minor (ca. 2%) and covalent adduct formation negligible (when protein was present). On the other hand, both hydrolysis (ca. 27%) and adduct formation (ca. 7%) were extensive when VPA-G-R was incubated with HSA or plasma. These data do not support a transacylation mechanism for VPA-protein adduct formation, since this pathway should be much more highly favoured by VPA-G (an acyl-substituted acetal) than VPA-G-R (simple esters). VPA-protein adducts were found in the plasma of epileptic patients taking VPA chronically (mean 0.77 +/- SD 0.63 microgram VPA equivalents/mL, N = 17). An enzyme linked immunosorbent assay was developed, using HSA modified by incubation with VPA-G-R, to test the immunoreactivity of the patients' plasma. Of 57 patients tested, nine showed measurable levels of antibodies to these adducts, but the titres were very low, with no difference in response to modified and unmodified protein detectable at plasma dilutions of 1:16 or greater. These results suggest that the VPA-protein adducts have little immunogenicity, and are in agreement with clinical observations that drug hypersensitivity responses have not been associated with VPA therapy. Thus, although the in vitro data show that VPA-G is an example of a relatively unreactive acyl glucuronide, covalent VPA-plasma protein adducts and anti-adduct antibodies are nonetheless formed in vivo, at least in some patients on chronic therapy with the drug.