Formation and reactivity of acyl glucuronides: the influence of chirality

Methyl tetra-O-acetyl-α-D-glucopyranuronate: crystal structure and influence on the crystallisation of the β anomer

Methyl tetra-O-acetyl-β-D-glucopyranuronate (1) and methyl tetra-O-acetyl-α-D-glucopyranuronate (3) were isolated as crystalline solids and their crystal structures were obtained. That of the β anomer (1) was the same as that reported by Root et al., while anomer (3) was found to crystallise in the orthorhombic space group P212121 with two independent molecules in the asymmetric unit. No other crystal forms were found for either compound upon recrystallisation from a range of solvents. The α anomer (3) was found to be an impurity in initially precipitated batches of β-anomer (1) in quantities <3%; however, it was possible to remove the α impurity either by recrystallisation or by efficient washing, i.e. the α anomer is not incorporated inside the β anomer crystals. The β anomer (1) was found to grow as prisms or needles elongated in the a crystallographic direction in the absence of the α impurity, while the presence of the α anomer (3) enhanced this elongation.

Acyl glucuronides: the good, the bad and the ugly

Acyl glucuronidation is the major metabolic conjugation reaction of most carboxylic acid drugs in mammals. The physiological consequences of this biotransformation have been investigated incompletely but include effects on drug metabolism, protein binding, distribution and clearance that impact upon pharmacological and toxicological outcomes. In marked contrast, the exceptional but widely disparate chemical reactivity of acyl glucuronides has attracted far greater attention. Specifically, the complex transacylation and glycation reactions with proteins have provoked much inconclusive debate over the safety of drugs metabolised to acyl glucuronides. It has been hypothesised that these covalent modifications could initiate idiosyncratic adverse drug reactions. However, despite a large body of in vitro data on the reactions of acyl glucuronides with protein, evidence for adduct formation from acyl glucuronides in vivo is limited and potentially ambiguous. The causal connection of protein adduction to adverse drug reactions remains uncertain. This review has assessed the intrinsic reactivity, metabolic stability and pharmacokinetic properties of acyl glucuronides in the context of physiological, pharmacological and toxicological perspectives. Although numerous experiments have characterised the reactions of acyl glucuronides with proteins, these might be attenuated substantially in vivo by rapid clearance of the conjugates. Consequently, to delineate a relationship between acyl glucuronide formation and toxicological phenomena, detailed pharmacokinetic analysis of systemic exposure to the acyl glucuronide should be undertaken adjacent to determining protein adduct concentrations in vivo. Further investigation is required to ascertain whether acyl glucuronide clearance is sufficient to prevent covalent modification of endogenous proteins and consequentially a potential immunological response.

Enantioselective immunorecognition of protein modification with optically active ibuprofen using polyclonal antibody

Formation of covalently bound protein adducts with 2-arylpropionic acids (2-APAs) has been proposed as a possible explanation for hypersensitivity and toxic responses to chiral carboxylic acid drugs. To identify the cellular proteins chemically modified with optically active (S)-ibuprofen, we generate polyclonal antibodies by immunizing rabbits with immunogen coupled to bovine serum albumin (BSA) via the spacer of 4-aminobutyric acid. The resulting antibodies largely cross-reacted with N-alpha-(t-butoxycarbonyl)--(S)-ibuprofenyl lysine as well as with the conjuguated (S)-ibuprofen with glycine and taurine and unconjugated (S)-ibuprofen, enabling enantioselective detection of (S)-ibuprofen residues anchored on ovalbumin molecules, introduced by the reaction of the ibuprofen p-nitrophenyl ester. Furthermore, immunoblotting with an antibody allows the enantioselective detection of (S)-ibuprofen-introduced glutathione-S-transferase (GST). These results indicate that the developed method will be useful for monitoring the generation and localization of protein covalently bound with (S)-ibuprofen, which may be the cause of ibuprofen-induced toxicity.

In vivo mechanistic studies on the metabolic activation of 2-phenylpropionic acid in rat

Two alternative metabolic pathways, acyl glucuronidation and acyl-CoA formation, are implicated in the generation of reactive acylating metabolites of carboxylic acids. Here, we describe studies that determine the relative importance of these two pathways in the metabolic activation of a model substrate, 2-phenylpropionic acid (2-PPA), in vivo in rats. Male Sprague-Dawley rats were pretreated with and without (-)-borneol (320 mg/kg i.p.), an inhibitor of acyl glucuronidation, or trimethylacetic acid (TMA, 500 mg/kg i.p.), an inhibitor of acyl-CoA formation, before receiving 2-PPA (racemic, 130 mg/kg). After administration of 2-PPA, livers were collected over a 2-h period and analyzed for 2-PPA acyl glucuronidation and 2-PPA-CoA formation by high-performance liquid chromatography. Covalent binding was measured by scintillation counting of washed liver protein precipitates. Results showed that pretreatment with TMA led to a 49% decrease in covalent binding of 2-PPA to liver proteins, when a 64% decrease in the exposure of 2-PPA-CoA was observed. Conversely, 95% inhibition of acyl glucuronidation by (-)-borneol, led to a 23% decrease in covalent binding to protein. These results suggest that metabolic activation by 2-PPA-CoA formation contributes to covalent adduct formation to protein in vivo to a greater extent than metabolic activation by acyl glucuronidation for this model substrate.

Self-micellization of gemfibrozil 1-O-beta acyl glucuronide in aqueous solution

PURPOSE

Phase II metabolism involves the conjugation of a polar moiety, such as sulfate or glucuronic acid, to a (relatively) nonpolar xenobiotic. Although it might be expected that such conjugates may exhibit amphiphilic character (e.g., surface activity and potential to form micelles), no detailed study of the micellization characteristics of any drug-glucuronide conjugates has yet been reported. Therefore, the aim of this study was to investigate the solution behavior and amphiphilic characteristics of gemfibrozil 1-O-beta glucuronide (GG), a model drug-glucuronide conjugate.

METHODS

Crude GG was extracted from the urine of volunteers dosed with 600 mg of gemfibrozil, and this material was then purified by reversed-phase high-performance liquid chromatography to yield a white solid. The amphiphilic properties of GG within the bulk aqueous phase were studied by isothermal titration microcalorimetry and 1H-NMR spectrometry, whereas those at the aqueous/air interface were studied by surface tensiometry.

RESULTS

The results of each independent analytical technique were consistent with GG in aqueous solution exhibiting amphiphilic properties typical of a hydrophilic surfactant. The titration microcalorimetry and 1H-NMR spectrometry data were in excellent agreement with each other, yielding critical micellization concentrations (cmc) for GG in 0.1 M acetate buffer of 18.1 +/- 0.4 mM and 18.3 +/- 0.3 mM, respectively. The profile and results of the surface tension measurements were consistent with GG localizing at the aqueous/air interface.

CONCLUSIONS

These results confirm the hypothesis that a glucuronide conjugate of a relatively nonpolar xenobiotic, such as gemfibrozil, behaves as an amphiphile in aqueous solution. The implications of this observation include a likely basis for the previously observed concentration-dependence in the degradation rate of the acyl glucuronides of 2-phenylpropionic acid, as well as identifying a possible broader contributory effect to the structural dependencies in biliary choleresis of different glucuronide conjugates of xenobiotics.

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.

Comparative pharmacology of S(+)-ibuprofen and (RS)-ibuprofen

Racemic ibuprofen, which contains equal quantities of R(-)-ibuprofen and S(+)-ibuprofen, has been used as an anti-inflammatory and analgesic agent for over 30 years. Although the S(+)-enantiomer is capable of inhibiting cyclooxygenase (COX) at clinically relevant concentrations, R(-)-ibuprofen is not a COX inhibitor. The two enantiomers of ibuprofen are therefore different in terms of their pharmacological properties and may be regarded as two different 'drugs'. They also differ in terms of their metabolic profiles. For example, R(-)-ibuprofen becomes involved in pathways of lipid metabolism and is incorporated into triglycerides along with endogenous fatty acids. S(+)-Ibuprofen does not appear to become involved in these unusual metabolic reactions, which is why S(+)-ibuprofen is regarded as being metabolically 'cleaner' than racemic ibuprofen. When racemic ibuprofen is given to humans, a substantial fraction of the dose of R(-)-ibuprofen (50%-60%) undergoes 'metabolic inversion' to yield S(+)-ibuprofen. On this basis, it has been argued that to obtain clinical effects that are comparable to those of a given dose of racemic ibuprofen, the dose of S(+)-ibuprofen would need to be about 75% of the dose of the racemate. However, this 'pharmacokinetic' rationale does not take into account the fact that inversion is not instantaneous, that there is variability in the extent of inversion between individuals, and that the kinetics of inversion may differ depending on the dosing situations. For example, the extent of inversion appears to be reduced when the racemate is given to patients experiencing acute pain. Recent studies have demonstrated that the clinical benefits of racemic ibuprofen can be derived from the administration of the single S(+)-enantiomer at a dose that is half that of the racemate. For example, 200 mg of S(+)-ibuprofen has been found to be superior or at least equivalent to 400 mg of the racemate in the relief of dental pain. Possible explanations for this higher than expected efficacy of S(+)-ibuprofen are considered.

The enantioselective immunoaffinity extraction of an optically active ibuprofen-modified peptide fragment

Acyl glucuronides are known to produce the covalently bound protein adducts which may be the cause of hypersensitivity and toxic responses to acidic drugs. The structural analysis of the drug-protein adducts is therefore needed. From this point of view, we developed an enantioselective immunoaffinity extraction method, which employs an immobilized antibody to specifically isolate peptide fragments that have been modified with optically active ibuprofen. Rabbits were immunized with (S)-ibuprofen coupled to bovine serum albumin through a beta-alanine group. The elicited antibody strongly recognizes the asymmetric center and the isobutylphenyl moiety of (S)-ibuprofen and its conjugates but has a low affinity for their anti podes. A 0.5-mL aliquot of the immunosorbent (11.5 mg of IgG/mL gel) prepared by immobilization of the antibody was capable of retaining up to 1 microg of (S)-ibuprofen. When a mixture of substance P with (R)- and (S)-ibuprofen-modified substance P was loaded on the immunosorbent, the (S)-ibuprofen-modified substance P was selectively retained. The modified peptide was quantitatively recovered by elution with 10 mM ammonium acetate buffer (pH 5.0)/methanol (5:95, v/v). The proposed method would be useful for the structural characterization of optically active ibuprofen-modified human serum albumin.

Enantioselectivity in the metabolism of mexiletine by conjugation in female patients with the arrhythmic form of chronic Chagas' heart disease

The phenomenon of enantioselectivity in the metabolism of mexiletine (MEX) conjugation was investigated in eight female patients with the arrhythmic form of chronic Chagas' heart disease treated with racemic mexiletine hydrochloride (two 100 mg capsules every 8 hr). Blood samples were collected up to 24 hr after the administration of the morning dose, with discontinuation of the subsequent doses during the study period. Plasma concentrations of N-hydroxymexiletine glucuronide were calculated as the difference between the concentrations of unchanged and total (unchanged + conjugated) MEX enantiomers. Total plasma MEX concentrations were analyzed by HPLC after enzymatic hydrolysis with beta-glucuronidase, the formation of diastereomeric derivatives with the chiral reagent N-acetyl-L-cysteine/o-phthalaldehyde, and fluorescence detection. The differences in the pharmacokinetic parameters of the enantiomers were evaluated by the paired t-test. The plasma concentrations of the (+)-(S)-MEX did not differ before and after enzymatic hydrolysis. The pharmacokinetic parameters calculated for (-)-(R)-N-hydroxymexiletine glucuronide are presented as means (95% confidence interval): maximum plasma concentration Cmax = 194.0 ng.ml-1 (154.3-233.7), time to maximum plasma concentration tmax = 1.4 hr (0.3-2.5), area under the plasma concentration versus time curve AUC0-24 = 2099.2 ng.h.ml-1 (1585.6-2612.6), elimination half-life t1/2 beta = 12.8 hr (9.9-15.6) and extent of conjugation of 31.6% (24.3-38.9%). The present data indicate stereospecific conjugation of (-)-(R)-N-hydroxymexiletine in the female patients with the arrhythmic form of Chagas' heart disease.

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.

Separatory determination of diastereomeric ibuprofen glucuronides in human urine by liquid chromatography/electrospray ionization-mass spectrometry

A method for the separatory determination of diastereomeric isomers of glucuronic acid conjugates of ibuprofen having a carboxyl group at the chiral center by liquid chromatography (LC)/electrospray ionization (ESI)-mass spectrometry (MS) has been developed. The authentic specimens of acyl glucuronides of R(-)- and S(+)-ibuprofen were chemically synthesized by the Mitsunobu reaction. In the ESI mode, the glucuronides were characterized by an abundant quasi-molecular ion [M-H]-, and the formation of the negative ion was markedly influenced by a drift voltage. The resolution of diastereomeric isomers was achieved on a Develosil ODS-HG-5 column with 20 mM ammonium acetate (pH 5.0):acetonitrile (5:2, v/v) as a mobile phase where diastereomers were monitored with a corresponding quasi-molecular ion. After oral administration of racemic ibuprofen, a preferential excretion of (S)-ibuprofen glucuronide into the urine was observed.

Selective protein adduct formation of diclofenac glucuronide is critically dependent on the rat canalicular conjugate export pump (Mrp2)

Previous work demonstrates that the reactive acyl glucuronide of the nonsteroidal antiinflammatory drug diclofenac forms selective protein adducts in the liver, which may play a causal role in the pathogenesis of diclofenac-associated liver toxicity. Because glucuronide conjugates can be exported into the bile, we explored the role of diclofenac glucuronide hepatobiliary transport in the formation of site-specific protein adducts. Specifically, to analyze intracellular (hepatocytes) versus extracellular (biliary tree) targeting of proteins, we have compared the pattern of diclofenac binding in normal Wistar rats with that in mutant transport-deficient (TR-) rats which lack the functional canalicular isoform of the conjugate export pump, Mrp2. In bile duct-cannulated normal rats, >50% of an iv injected dose of [14C]diclofenac appeared in the bile over a 90-min period. In contrast, in TR- rats virtually no hepatobiliary excretion of diclofenac glucuronide was found. After administration of diclofenac (30 mg/kg/day, ip for 3 days) to rats of both genotypes, a major protein adduct of an apparent molecular mass of 118 kDa was selectively detected by immunoblotting in isolated canalicular, but not in basolateral, membrane subfractions of wild-type rats, whereas no plasma membrane adducts could be found in the livers of TR- rats. Furthermore, immunohistochemical analysis using an anti-diclofenac antibody revealed the presence of distinct diclofenac-modified proteins on canalicular membranes of liver sections from diclofenac-treated normal rats, whereas no adducts could be identified in livers of TR- rats. In Western blots, the major diclofenac-modified canalicular membrane protein did not comigrate with Mrp2, indicating that the glucuronide carrier itself was unlikely to be a target. Collectively, the results demonstrate that the reactive diclofenac glucuronide is selectively transported into bile via Mrp2 and that hepatobiliary transport is critical for diclofenac covalent binding to proteins in the biliary tree.