The formation of beta-glucuronidase resistant glucuronides by the intramolecular rearrangement of glucuronic acid conjugates at mild alkaline pH

Dose-Dependent Increase in Unconjugated Cinnamic Acid Concentration in Plasma Following Acute Consumption of Polyphenol Rich Curry in the Polyspice Study

Spices that are rich in polyphenols are metabolized to a convergent group of phenolic/aromatic acids. We conducted a dose-exposure nutrikinetic study to investigate associations between mixed spices intake and plasma concentrations of selected, unconjugated phenolic/aromatic acids. In a randomized crossover study, 17 Chinese males consumed a curry meal containing 0 g, 6 g, and 12 g of mixed spices. Postprandial blood was drawn up to 7 h at regular intervals and plasma phenolic/aromatic acids were quantified via liquid chromatography tandem mass spectrometry (LC-MS/MS). Cinnamic acid (CNA, p < 0.0001) and phenylacetic acid (PAA, p < 0.0005) concentrations were significantly increased with mixed spices consumption, although none of the other measured phenolic/aromatic acids differ significantly between treatments. CNA displayed a high dose-exposure association (R² > 0.8, p < 0.0001). The adjusted mean area under the plasma concentration-time curve until 7 h (AUC_{0–7 h}) for CNA during the 3 increasing doses were 8.4 ± 3.4, 376.1 ± 104.7 and 875.7 ± 291.9 nM.h respectively. Plasma CNA concentration may be used as a biomarker of spice intake.

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.

Bioactivity of phenolic acids: metabolites versus parent compounds: a review

Phenolic acids are present in our diet in different foods, for example mushrooms. Due to their bioactive properties, phenolic acids are extensively studied and there is evidence of their role in disease prevention. Nevertheless, in vivo, these compounds are metabolized and circulate in the organism as glucuronated, sulphated and methylated metabolites, displaying higher or lower bioactivities. To clarify the importance of the metabolism of phenolic acids, knowledge about the bioactivity of metabolites is extremely important. In this review, chemical features, biosynthesis and bioavailability of phenolic acids are discussed, as well as the chemical and enzymatic synthesis of their metabolites. Finally, metabolite bioactive properties are compared with that of the corresponding parental compounds.

Antioxidant activity of phenolic acids and their metabolites: synthesis and antioxidant properties of the sulfate derivatives of ferulic and caffeic acids and of the acyl glucuronide of ferulic acid

The main metabolites of caffeic and ferulic acids (ferulic acid-4'-O-sulfate, caffeic acid-4'-O-sulfate, and caffeic acid-3'-O-sulfate), the most representative phenolic acids in fruits and vegetables, and the acyl glucuronide of ferulic acid were synthesized, purified, and tested for their antioxidant activity in comparison with those of their parent compounds and other related phenolics. Both the ferric reducing antioxidant power (FRAP) assay and the 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging method were used. Ferulic acid-4'-O-sulfate and ferulic acid-4'-O-glucuronide exhibited very low antioxidant activity, while the monosulfate derivatives of caffeic acid were 4-fold less efficient as the antioxidant than caffeic acid. The acyl glucuronide of ferulic acid showed strong antioxidant action. The antioxidant activity of caffeic acid-3'-O-glucuronide and caffeic acid-4'-O-glucuronide was also studied. Our results demonstrate that some of the products of phenolic acid metabolism still retain strong antioxidant properties. Moreover, we first demonstrate the ex vivo synthesis of the acyl glucuronide of ferulic acid by mouse liver microsomes, in addition to the phenyl glucuronide.

High Expression of Human CYP2C in Immortalized Human Liver Epithelial Cells

Cell lines stably expressing high levels of single isozymes of human CYP2C genes (CYP2C8, CYP2C9, CYP2C18 and CYP2C19) have been successfully generated by transfecting liver epithelial human cells (THLE) with an appropriate expression vector. To this aim, cDNAs encoding for each CYP2C gene were inserted by blunt-ended cloning into the unique insertion site of the singular expression vector pCMVneo. The recombinant pCMV2C8, pCMV2C9, pCMV2C18 and pCMV2C19 vectors were liposome-mediated transfected into THLE cells. The resulting transgenic cells, designated as T5-2C8, T5-2C9, T5-2C18 and T5-2C19, were cloned and the expression of the ectopic gene, mRNA and protein, was investigated by RT-PCR and Western blot analysis. The functionality of each expressed CYP2C was assessed by determining specific catalytic activities in these cells, that is, taxol-6-hydroxylation for CYP2C8; diclofenac-4'-hydroxylation for CYP2C9; S-mephenytoin-4'-hydroxylation for CYP2C18; S-mephenytoin-4'-hydroxylation for CYP2C19. As a result of the combined strategies used here, the transfected cells showed activities four to seven times higher than those of 24-hour cultured hepatocytes.

Separation and Determination of Diastereomeric Flurbiprofen Acyl Glucuronides in Human Urine by LC/ESI-MS with a Simple Column-Switching Technique

Endogenous and exogenous compounds having a carboxyl group, such as alpha-arylpropionic acid derivatives, undergo a phase II metabolic reaction to produce an amino acid conjugate through the acyl CoA thioester as well as the acyl glucuronide. It was previously shown that flurbiprofen, one of the nonsteroidal anti-inflammatory drugs, is not subjected to activation of the carboxyl group by the CoA thioester ligase, suggesting that acyl glucuronidation is the main phase II metabolic pathway. Recent observations, however, have demonstrated that the nonenzymatic formation of a covalently protein-bound drug, which is produced by the action of the acyl glucuronide, may cause hypersensitive reactions. Accordingly, a reliable method to measure diastereomeric flurbiprofen glucuronides in human biological fluids is required. In this study, we describe a liquid chromatographic/mass spectrometric method with a simple column switching technique to determine diastereomeric flurbiprofen acyl glucuronides in human urine specimens. The optimal conditions for the electrospray ionization were established based on the effects of orifice and ring lens voltages as well as mobile phase additives. The proposed method applied to urine specimens demonstrates high accuracy and reproducibility for the determination of flurbiprofen glucuronides in a quantitative range from 0.74 to 146.5 microg/mL, with a detection limit of 7.4 pg (17.6 fmol)/injection of S-flurbiprofen glucuronide, at a signal-to-noise ratio of 10 under the selected ion-monitoring mode. The urinary concentration of R-flurbiprofen glucuronides in healthy subjects determined by the proposed method were 6.8-29.4 microg/mL, and those values were slightly higher than that of S-flurbiprofen glucuronides (3.9-18.0 microg/mL).

The pharmacokinetics of glycyrrhizic acid evaluated by physiologically based pharmacokinetic modeling

Glycyrrhizic acid is widely applied as a sweetener in food products and chewing tobacco. In addition, it is of clinical interest for possible treatment of chronic hepatitis C. In some highly exposed subjects, side effects such as hypertension and symptoms associated with electrolyte disturbances have been reported. To analyze the relationship between the pharmacokinetics of glycyrrhizic acid in its toxicity, the kinetics of glycyrrhizic acid and its biologically active metabolite glycyrrhetic acid were evaluated. Glycyrrhizic acid is mainly absorbed after presystemic hydrolysis as glycyrrhetic acid. Because glycyrrhetic acid is a 200-1000 times more potent inhibitor of 11-beta-hydroxysteroid dehydrogenase compared to glycyrrhizic acid, the kinetics of glycyrrhetic acid are relevant in a toxicological perspective. Once absorbed, glycyrrhetic acid is transported, mainly taken up into the liver by capacity-limited carriers, where it is metabolized into glucuronide and sulfate conjugates. These conjugates are transported efficiently into the bile. After outflow of the bile into the duodenum, the conjugates are hydrolyzed to glycyrrhetic acid by commensal bacteria; glycyrrhetic acid is subsequently reabsorbed, causing a pronounced delay in the terminal plasma clearance. Physiologically based pharmacokinetic modeling indicated that, in humans, the transit rate of gastrointestinal contents through the small and large intestines predominantly determines to what extent glycyrrhetic acid conjugates will be reabsorbed. This parameter, which can be estimated noninvasively, may serve as a useful risk estimator for glycyrrhizic-acid-induced adverse effects, because in subjects with prolonged gastrointestinal transit times, glycyrrhetic acid might accumulate after repeated intake.

The in vitro biosynthesis and stability measurement with agyl-glycuronide isoformes of the main metabolite of ipriflavone

The formation and stability of 1-beta-glucuronide conjugate of the main metabolite of ipriflavone [7-(1-carboxy-ethoxy)-isoflavone] (CI)--were studied by using liver microsomes, hepatocytes, and isolated perfused liver of untreated and 3-methylcholanthrene (MC) treated dog and rat, and human liver microsomes. MC treatment enhanced the rate of conjugation twice as much as that of the control in the microsomes of both dogs and rats. Conjugation of CI by microsomes results in two metabolites, both sensitive to pH and temperature. Other two glucuronide forms appeared in experiments with hepatocytes and perfused liver. Mass spectrometry supported. The conclusion, assumption that both metabolites produced by microsomes are glucuronide conjugate isoforms of CI, and that they could be distinguished according to the intensity of peaks on FAB-MIKE spectra. The beta-glucuronidase enzyme hydrolysed only the 1-beta-glucuronide isomer, the other, migrated form remained unchanged. D-saccharic-acid-1,4-lactone, a specific inhibitor of beta-glucuronidase enzyme, decreased the rate of enzymatic cleavage. Standard curves of CI were prepared by HPLC, and 1-beta-CI-glucuronide was quantified according to the amount of CI formed by hydrolysis. The stability of conjugates greatly depends on pH and temperature, and the rate of degradation and isomerization is sensitive to the value of both. Lowering the pH from 7.4 to 5.0 and the temperature from 37 degrees C to 18 degrees C increased the stability of glucuronides. Increasing the pH to 12.0 results in very rapid acyl migration and hydrolysis.

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.

Preparative chromatography of furosemide 1-O-acyl-glucuronide from urine using micronized amberiite XAD-2 and its application to other 1-O-acyl-glucuronides

Furosemide 1-O-acyl glucuronide (Fgnd) was extracted from the urine following oral administration of furosemide. The crude Fgnd was applied to micronized Amberlite XAD-2 column (2.5 cm i.d. x 90 cm length, 75-500 microns particle size). The purified Fgnd was identified by mass spectrometry and beta-glucuronidase treatment. This method was also applicable to the purification of glucuronide of tolmetin (nonsteroidal anti-inflammatory drug, NSAID), suggesting that it was applicable to the other NSAIDs, most of which were known to be metabolized to acyl-glucuronides.

Ion-pair high-performance liquid chromatographic separation of two thyroxine glucuronides formed by rat liver microsomes

A simple reversed-phase ion-pair high-performance liquid chromatographic separation method has been developed for thyroxine (T4) and its glucuronide metabolites formed by liver microsomes of untreated and 3-methylcholanthrene-treated rats. Besides the phenol-T4-glucuronide, another, probably acyl-T4-glucuronide, formation has been detected. The effect of pH and temperature on the stability of the acyl-T4-glucuronide was also investigated. The lowering of pH to 2 and cooling the samples to 5 degrees C is necessary to prevent the hydrolysis of acylglucuronide, while both pH and temperature do not affect the stability of the phenol-T4-glucuronide. The retention times of T4 and phenol-T4-glucuronide are highly influenced by the pH of the mobile phase, but not that of acyl-T4-glucuronide.

Pharmacokinetics of trovafloxacin (CP-99,219), a new quinolone, in rats, dogs, and monkeys

The pharmacokinetics of trovafloxacin [CP-99,219; 7-(3-azabicyclo[3.1.0]hexyl)-naphthyridone] were studied in rats, dogs, and monkeys following oral and intravenous administration. After intravenous dosing, the systemic clearances of trovafloxacin in rats, dogs, and monkeys were 12.5, 11.1, and 7.2 ml/min/kg of body weight, respectively, and the respective volumes of distribution were 0.9, 1.7, and 4.3 liters/kg, with corresponding elimination half-lives of 0.7, 1.8, and 7.0 h. After the administration of oral doses of 50, 20, and 20 mg/kg to rats, dogs, and monkeys serum trovafloxacin concentrations reached a maximum at 0.6, 2.3, and 2.3 h, respectively, with respective maximum concentrations of trovafloxacin in serum of 11.5, 3.5, and 5.2 micrograms/ml; the corresponding elimination half-lives were 2.2, 2.5, and 7.5 h. The oral bioavailability of trovafloxacin was 68, 58, and 85% in rats, dogs, and monkeys, respectively. The binding of trovafloxacin to serum proteins was concentration independent, averaging 92, 75, and 66% for rats, dogs, and monkeys, respectively. Trovafloxacin penetrated well into tissues in dogs. The urinary recoveries of unchanged drug were less than 5% in dogs and monkeys, with or without incubation with alkali or Glusulase (beta-glucuronidase and sulfatase). In rats, 99.8% of the orally administered radioactivity was recovered in feces, while 20.6, 3.4, and 67.1% of the radioactive dose in bile duct-cannulated rats were recovered in feces, urine, and bile, respectively. These results suggest that the elimination of trovafloxacin from rats, and possibly from dogs and monkeys, is primarily through biliary excretion.

Chromatographic fractionation and analysis by mass spectrometry of conjugated metabolites of bis(2-ethylhexyl)phthalate in urine

Mono(2-ethylhexyl)phthalate (MEHP), the primary metabolite of the plasticizer bis(2-ethylhexyl)phthalate (DEHP), was given to guinea pigs and mice and the methods for the isolation, separation and analysis of its metabolites in urine were developed. Following solid-phase extraction with octadecylsilane-bonded silica, individual metabolites were purified and separated using a combination of ion-exchange chromatography on lipophilic gels and reversed-phase high-performance liquid chromatography. Analysis of intact conjugates, as well as nonconjugated metabolites, was performed by fast atom bombardment mass spectrometry (FAB-MS) and, after derivatization, by gas chromatography-mass spectrometry. Enzymatic methods were used for further characterization. The study confirms glucuronidation as the major conjugation pathway for MEHP in the investigated species. Although less important quantitatively, glucosidation is shown to be an alternative conjugation pathway in mice. The methods developed were applied to a sample of urine from a hyperbilirubinemic newborn infant subjected to DEHP-exposure in conjunction with an exchange transfusion. It was demonstrated that metabolites of DEHP were excreted in amounts which could be analyzed by FAB-MS.

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.

Direct determination of ibuprofen and ibuprofen acyl glucuronide in plasma by high-performance liquid chromatography using solid-phase extraction

A method for the simultaneous determination of ibuprofen and its labile, reactive metabolite, ibuprofen acyl glucuronide, in plasma is described. Reversed-phase high-performance liquid chromatography (HPLC) employed a C18 column using methanol-10 mM trifluoroacetic acid as the mobile phase with ultraviolet detection at 225 or 214 nm. It is essential that blood is handled rapidly and plasma is acidified upon collection prior to freezing. Plasma samples first are deproteinated with acetonitrile, the supernatant is diluted with phosphate buffer, and ibuprofen, ibuprofen glucuronide, and ibufenac (internal standard) are initially isolated by solid-phase extraction on C18 cartridges. Upon elution, the residue is evaporated, dissolved and injected onto the HPLC system. Recovery is 94 +/- 8 and 70 +/- 9% for ibuprofen glucuronide and ibuprofen, respectively. The measurable concentration range is linear from 0.1 to 10 micrograms/ml for ibuprofen glucuronide and from 0.5 to 100 micrograms/ml for ibuprofen. The method is satisfactory for the analysis of ibuprofen and ibuprofen glucuronide from pharmacokinetic studies in humans. The direct determination of ibuprofen glucuronide allows accurate measurement of this conjugate at low levels relative to the parent compound, ibuprofen, a distinct advantage compared to previously employed indirect methods.

Capacity-limited renal glucuronidation of probenecid by humans. A pilot Vmax-finding study

Probenecid shows dose-dependent pharmacokinetics. When in one volunteer the dose is increased from 250 to 1,500 mg orally, the t1/2 increased from 3 to 6 h. The Cmax was 14 micrograms/ml with a dosage of 250 mg, 31 micrograms/ml with 500 mg, 70 micrograms/ml with 1,000 mg and 120 micrograms/ml with 1,500 mg. The tmax remained 1 h for all four dosages. The AUC/dose ratio increased with the dose, indicating nonlinear elimination. The total body clearance declined from 64.5 ml/min for 250 mg to 26.0 ml/min for 1,500 mg. The renal clearance of probenecid remained constant, 0.6-0.8 ml/min. Protein binding of probenecid is high (91%) and independent of the dose. The phase I metabolites show lower protein binding values (34-59%). The protein binding of probenecid glucuronide in vitro (spiked plasma) is 75%. Probenecid is metabolized by cytochrome P-450 to three phase I metabolites. Each of the metabolites accounts for less than 10% of the dose administered; the percentage recovered in the urine is independent of the dose. The main metabolite probenecid glucuronide is only present in urine and not in plasma. The renal excretion rate--time profile of probenecid glucuronide shows a plateau value of approximately 700 micrograms/min (46 mg/h) with acidic urine pH. The duration of this plateau value depends on the dose: 2 h at 500 mg, 10 h at 1,000 mg and 20 h at 1,500 mg. It is demonstrated that probenecid glucuronide must be formed in the kidney during its passage of the tubule. The plateau value in the renal excretion rate of probenecid value reflects its Vmax of formation.

Clearance of indomethacin occurs predominantly by renal glucuronidation

In this report we describe the conditions of collection, storage and handling of urine samples, collected after oral dosing with indomethacin in man, in order to maintain the integrity of the labile glucuronide formed. We found that the body clearance occurs predominantly by renal metabolism, due to glucuronidation in the human kidney. These glucuronides may be converted to isomeric glucuronides and/or the parent compound indomethacin during the residence time in the bladder.

Stereoselective interactions of ketoprofen glucuronides with human plasma protein and serum albumin

A clearance pathway common to many aryl alkanoic acids is the generation of renally eliminated ester glucuronides. These metabolites are susceptible to systemic hydrolysis which generates the parent aglycone. We have conducted in vitro studies with biosynthetic R- and S-ketoprofen glucuronides to elucidate the mechanism of this phenomenon. These conjugates were incubated in human plasma, various concentrations of human serum albumin (HSA) and protein-free buffer. It was apparent that albumin, rather than plasma esterases, catalysed the hydrolysis of the glucuronides. The albumin-catalysed hydrolysis of ketoprofen glucuronides was highly stereoselective. The mean (+/- SD) hydrolysis half-life of R-ketoprofen glucuronide in plasma (N = 4) at physiological pH and temperature was 1.37 (+/- 0.30) hr. The corresponding value for S-ketoprofen glucuronide, 3.46 (+/- 0.84) hr, was significantly different (P less than 0.005). In contrast, synthetic ethyl esters of R- and S-ketoprofen were hydrolysed by plasma esterases, but not by HSA, and with little stereoselectivity. The reversible protein binding of ketoprofen glucuronides was determined at physiological pH and temperature by a rapid ultra-filtration method. The binding of R- and S-ketoprofen glucuronide to human plasma protein was independent of concentration (P greater than 0.05) over the range of 1-20 micrograms/mL. The mean (+/- SD) percentage unbound in plasma (N = 4) of R-ketoprofen glucuronide was 12.6 (+/- 1.4)%. The corresponding value for S-ketoprofen glucuronide, 9.12 (+/- 0.54)%, was significantly different (P less than 0.005). S-Ketoprofen glucuronide was also more avidly protein bound in physiological concentrations of HSA. However, this stereoselectivity decreased in more dilute HSA solutions. Based on the hydrolysis and protein binding data for ketoprofen glucuronides, we propose the existence of separate binding and catalytic sites on the albumin molecule for these metabolites.

Direct measurement of probenecid and its glucuronide conjugate by means of high pressure liquid chromatography in plasma and urine of humans

Probenecid with its phase-I metabolites, and phase-II glucuronide conjugate can be analysed by a gradient high pressure liquid chromatographic method. Probenecid glucuronide in plasma with pH 7.4 is not stable and declines to 10% of the original value within 6 h (t1/2 approximately 1 h). Probenecid glucuronide is stable in urine with pH 5.0, moderately unstable at pH 6.0 (t1/2 approximately 10 h), and unstable at pH 8.0 (t1/2 approximately 0.5 h). Probenecid glucuronide is stable in water and 0.01 mol/l phosphoric acid in the autosampler of the high pressure liquid chromatograph. The decrease in concentration in water is 5.5% during 9 h and 0% in diluted acid. Probenecid glucuronide and the phase-I metabolites were not detectable in plasma. The main compound in fresh urine is the phase-II conjugate probenecid glucuronide (62% of a 500 mg dose); the phase-I metabolites are present and only a trace of probenecid is present. The percentage of the dose of the phase-I metabolites varies between 5 and 10, while hardly any probenecid is excreted unchanged (0.33%).

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.

In vivo covalent binding of clofibric acid to human plasma proteins and rat liver proteins

Recent studies have shown that acyl-glucuronide conjugates are chemically reactive electrophilic metabolites that can undergo transacylation reactions resulting in intra-molecular rearrangement, hydrolysis and covalent binding of aglycone to albumin both in vitro and in vivo. The hypolipidaemic agent clofibrate is eliminated almost entirely as clofibric acid glucuronide in humans and rats. The formation of clofibric acid-protein adducts was investigated in 14 patients receiving 0.5-2.0 g/day of clofibrate for hypercholesterolaemia, and in liver homogenates from 20 rats administered 280 mg/kg/day of clofibric acid for up to 21 days. Total clofibric acid concentrations in the patients ranged from 0 to 114 mg/L. Covalently bound clofibric acid-protein adducts were detected in all patients, even in one subject in whom there was no measurable plasma clofibric acid. Concentrations ranged from 2.2 to 53.4 ng/mg protein and, in eight patients receiving 1.0 g/day of clofibrate, were correlated (P less than 0.05) with renal function as assessed by creatinine clearance. Clofibric acid-protein adducts were also present in rat liver homogenates, and increased with increasing duration of treatment (P less than 0.0001), from a mean (SE) of 10.1 (0.7) to 32.3 (1.6) ng/mg protein. The covalent binding of drugs to tissue macromolecules has traditionally been associated with toxicity. Further research is required to elucidate the role of acyl-glucuronide conjugates in the formation of drug-protein adducts and their biological consequences.

High-performance liquid chromatographic evaluation of Med 15 and its metabolites Med 5 and tolmetin in rat plasma

A simple and reliable high-performance liquid chromatographic method is described for the quantitative analysis of the new non-steroidal anti-inflammatory agent Med 15 and its metabolites Med 5 and tolmetin in rat plasma. After selective extraction the three analytes and an internal standard (p-phenyl-phenol) were separated on a reversed-phase Ultrasphere 5 micron column using potassium dihydrogenphosphate (0.05 M)-acetonitrile (52:48) (pH 4.7) as the mobile phase. The analytes were detected at 313 nm; the sensitivity of the method proved to be 0.05 microgram/ml for all three compounds. The method has been applied to investigate Med 15 pharmacokinetics in rats.

Contrasting systemic stabilities of the acyl and phenolic glucuronides of diflunisal in the rat

1. Diflunisal (DF) is metabolized in humans and rats primarily to its acyl glucuronide, phenolic glucuronide and sulphate conjugates. 2. After i.v. administration of DF acyl glucuronide to pentobarbitone-anaesthetized rats, DF and its phenolic glucuronide and sulphate conjugates appeared rapidly in plasma, indicating ready systemic hydrolysis of the acyl glucuronide and subsequent biotransformation of liberated DF. 3. Approximately 72% of the acyl glucuronide dose was recovered in bile and urine over 6 h: 52% as acyl glucuronide, 6% as phenolic glucuronide, 5% as sulphate, and 8% as isomers of the acyl glucuronide arising from intramolecular acyl migration. 4. Blockage of excretion routes by ligation of the ureters, bile duct, and both ureters and bile duct, decreased plasma clearance of the acyl glucuronide from 7.8 ml/min per kg to 6.0, 3.2 and 2.2 ml/min per kg respectively, and increased the apparent terminal plasma half-life of DF from 2.1 h to 2.6, 3.4 and 6.3 h, respectively. 5. By contrast, DF phenolic glucuronide was quite stable after i.v. administration at the same dose. 6. This study shows that systemic cycling between DF and its acyl glucuronide exists in the rat in vivo, with portions of each cycle of unstable acyl glucuronide through DF yielding stable phenolic glucuronide and (presumptively stable) sulphate conjugate.

Rapid high-performance liquid chromatographic assay for the simultaneous determination of probenecid and its glucuronide in urine. Irreversible binding of probenecid to serum albumin

A reversed-phase high-performance liquid chromatographic (HPLC) assay for the simultaneous determination of probenecid and its glucuronide in urine has been developed. The genuine glucuronide conjugate was isolated from urine by the use of solid-phase extraction on Amberlite XAD-2 and finally purified by the use of preparative HPLC on a Sepharon Hema 1000 RP-18 column. The purity of the product obtained was 88.9%. The isolated glucuronide was used as a standard sample. Of a p.o. dose of 500 mg to two volunteers, 26 and 29% were excreted as the ester glucuronide, while 1.0 and 2.7% were excreted unmetabolized. The stability of the ester glucuronide was investigated in aqueous buffers, buffered urine and human serum albumin solutions. The glucuronide was unstable in neutral and mildly alkaline solutions, and special precautions have to be taken during sampling and sample treatment in order to preserve the genuine glucuronide. The presence of human serum albumin in the solution stabilized the glucuronide against isomerization/rearrangements but catalysed the hydrolysis of the glucuronide. When incubating human serum albumin with the ester glucuronide, probenecid was shown to be covalently bound to the protein probably via a transacylation reaction.

Reactivity of diflunisal acyl glucuronide in human and rat plasma and albumin solutions

Diflunisal acyl glucuronide (DAG) is a major metabolite of diflunisal (DF) in rats and humans. We have investigated the reactivity of DAG, in purified albumin solutions and plasma from both rat and human sources, along three interrelated pathways: rearrangement via acyl migration to yield positional isomers of DAG, hydrolysis of DAG and/or its isomers to liberate DF, and formation of covalent adducts of DF (via DAG and/or its isomers) with plasma protein. Two initial concentrations of DAG (ca. 50 and 10 micrograms DF equivalents/mL) were used throughout. In all incubations, the order of quantitative importance of the reactions was: rearrangement greater than hydrolysis greater than covalent binding. At pH 7.4 and 37 degrees, degradation of DAG in albumin solutions (e.g. half-life ca. 95 min in fatty acid-free human serum albumin) was retarded in comparison to that found in buffer alone (half-life ca. 35 min). Degradation in unbuffered rat and human plasma containing heparin was comparable to that found in buffer. Maximal covalent binding to protein was achieved after 4-8 hr incubation, and was greatest for fatty acid-free human serum albumin (165 ng DF/mg albumin). Thereafter, slow degradation of the adducts was observed. Formation of DF-plasma protein adducts in vivo was also found in rats and humans dosed with DF.

Evaluation of a versatile reversed-phase high-performance liquid chromatographic system using cethexonium bromide as ion-pairing reagent for the analysis of glucuronic acid conjugates

An ion-pair high-performance liquid chromatographic technique has been developed to reduce the elution times of both glucuronic acid conjugates and their free aglycones. The chromatographic system combines the use of a reversed-phase column (LiChrospher CH-18; 5 microns) and a mobile phase of methanol (70-80%)-0.01 M phosphate buffer (pH 6.0) containing 2.5 mM cethexonium bromide as counter-ion at a flow-rate of 1 ml min-1. The hydrophobicity of this quaternary ammonium ion-pairing reagent and the high content of the organic modifier in the mobile phase provide close and short elution times for a wide structural variety of compounds (i.e. alcohols, phenols, steroids, carboxylic acids) and their conjugates with glucuronic acid (capacity factors lower than 7.5), without compromising the selectivity with respect to endogenous compounds of the microsomal incubation medium and urine. Advantages of cethexonium bromide over conventional tetrabutylammonium salts are clearly demonstrated, and the described system was applied to the simultaneous quantitation of clofibric acid and its acylglucuronide in human urine and validated for a pharmacogenetics purpose.

Isolation and identification of the rearrangement products of diflunisal 1-O-acyl glucuronide

A preparative reversed-phase high-performance liquid chromatographic method is described for the simultaneous separation of eight different isomers formed from the 1-O-acyl glucuronide of diflunisal. All isomers were formed when the acyl glucuronide was incubated under mildly alkaline conditions in aqueous solution. Various forms of two-dimensional NMR studies were performed in order to identify each isomer. Seven of the isomers were identified as alpha- and beta-forms of esters in which diflunisal forms an ester with one of the four alcohol groups in the glucupyranuronic acid. One isomer was identified as the ether glucuronide of diflunisal. To establish the exact chemical shift of the different protons, simulation of the one-dimensional NMR spectra and iterative analyses were performed.

Interactions between oxaprozin glucuronide and human serum albumin

1. The first step in the interaction between oxaprozin glucuronide and human serum albumin (HSA) is formation of a reversible complex which then leads to the following reactions; (a) acyl migration of the aglycone from position 1 to positions 2, 3 and 4 of the glucuronic acid moiety; (b) hydrolysis of the glycosidic bond; and (c) covalent binding of oxaprozin to the HSA molecule. The isomers of oxaprozin glucuronide formed in (a) and the covalently bonded drug in (c) are also hydrolyzed to oxaprozin. 2. Oxaprozin and ligands known to bind at Site II as classified by Sudlow et al. (1976), also called the benzodiazepine binding site (Müller and Wollert 1975), inhibit these reactions with oxaprozin glucuronide, while ligands which are known to bind at other sites on HSA do not. 3. Modification of a single tyrosine residue, located within Site II, with tetranitromethane, diisopropylfluorophosphate, and p-nitrophenylacetate causes significant reduction of the covalent binding of oxaprozin to HSA. 4. Tetranitromethane modification of HSA decreases all three reactions, while not inhibiting the formation of the reversible complex, indicating that the tyrosine located in Site II (tyr-411)acts as the nucleophile in these reactions. 5. Chemical modification of lysine residues has only a small effect on the reactions while modification of the lone free sulphhydryl (cys) in HSA has no effect.

A review of the methods of chemical synthesis of sulphate and glucuronide conjugates

1. Methods for the synthesis of drug conjugates with sulphuric acid have been reviewed. 2. Some analytical methods are presented for the analysis of sulphate conjugates. 3. The synthesis of several types of N, O and C beta-D-glucuronides is reviewed. Different beta-coupling reactions of protected glucuronides are presented. 4. Application of n.m.r. and mass spectrometry to the analysis of beta-D-glucuronides is discussed.

High-performance liquid chromatographic determination of tolmetin, tolmetin glucuronide and its isomeric conjugates in plasma and urine

A rapid and sensitive analytical procedure is described for the simultaneous measurement of tolmetin (T), tolmetin glucuronide (1 beta-TG) and the isomers of tolmetin glucuronide in plasma and urine. A reversed-phase liquid chromatographic system is used with an ion-pairing mobile phase of methanol-tetrabutylammonium hydrogensulfate buffered to pH 4.5 and kept at a constant temperature of 50 degrees C. Detection is by UV at 313 nm. Plasma (0.5 ml) and urine (0.1 ml) are collected in pre-cooled containers and immediately adjusted to pH 3.0 to minimize TG isomerization and hydrolysis. Samples are then deproteinated with acetonitrile, the supernatant is evaporated to dryness and reconstituted in an acetate buffer (pH 4.5), and 50 microliters are injected onto the system. Using zomepirac as the internal standard, the measurable, linear concentration ranges are 0.05-50 micrograms/ml for T in plasma and 0.025-50 micrograms/ml for T in urine. Chromatographic peaks representing T,1 beta-TG and three isomers of TG were identified, all with retention times less than 10 min. The need for special handling of biological samples is discussed.

Reversed-phase high-performance liquid chromatographic assay for the simultaneous determination of diflunisal and its glucuronides in serum and urine. Rearrangement of the 1-O-acylglucuronide

A reversed-phase ion-pair high-performance liquid chromatographic assay for the simultaneous determination of diflunisal and its ester and ether glucuronide in urine and serum has been developed. The determination of the ester glucuronide in serum has not been previously reported. The genuine glucuronide conjugates isolated from urine were used as standards. The ester glucuronide is found to be unstable, especially in neutral and basic solutions, and special precautions therefore have to be taken during sampling and sample treatment. Nine rearrangement/degradation products of the ester glucuronide were detected.

Liquid chromatographic assay for the measurement of glucuronidation of arylcarboxylic acids using uridine diphospho-[U-14C] glucuronic acid

A general method for the assay of UDP-glucuronosyltransferase activity towards arylcarboxylic acids (clofibric acid, 1- and 2-naphthylacetic acid) using UDP-[U-14C] glucuronic acid in liver microsomes is described. The 14C-labelled glucuronide was separated by high-performance liquid chromatography, identified by hydrolysis by beta-glucuronidase, characterized by laser desorption mass spectrometry and quantified by scintillation counting. The coefficient of variation of the enzyme activity for the inter-assay repeatability was below 4.5%. As little as 2.5 nmol of the arylcarboxylic acid glucuronides could be detected and precisely quantified. The method was applied to the determination of the apparent kinetic constants for glucuronidation of the acids. Clofibric acid was the best substrate for UDP-glucuronosyltransferase (Vmax/KM, the ratio of the maximum initial velocity and the Michaelis-Menten constant, is 12.3). The two isomers, 1- and 2-naphthylacetic acids, were transformed at a similar rate. However, they exhibited different enzymatic affinities, as the KM values were 1.0 mM and 5.6 mM for 1- and 2-naphthylacetic acid, respectively. This indicates that the spatial organization of the substrates played a critical role in this acyl glucuronoconjugation.

The metabolic sulfonation and side-chain oxidation of 3'-hydroxyisosafrole in the mouse and its inactivity as a hepatocarcinogen relative to 1'-hydroxysafrole

The chemically synthesized sulfuric acid esters of 1'-hydroxysafrole and 3'-hydroxyisosafrole, 1'-sulfooxysafrole and 3'-sulfooxyisosafrole, respectively, are both strong electrophiles. Each ester reacted with deoxyguanosine (dGuo) in aqueous solution to form both safrol-1'-yl- and isosafrol-3'-yl-deoxyguanosine adducts. Both 1'-hydroxysafrole and 3'-hydroxyisosafrole were also formed from each ester in the presence of water. When either 1'-[3H]hydroxysafrole or 3'-[3H]hydroxyisosafrole was incubated with mouse liver cytosols fortified with 3'-phosphoadenosine-5'-phosphosulfate (PAPS) and RNA, similar levels of RNA- and protein-bound adducts were formed; thus, the hepatic sulfotransferase activities for these two substrates appear to be similar. In contrast, the levels of hepatic nucleic acid and protein adducts formed after administration of 3'-[3H]hydroxyisosafrole to mice were only 2-4% and 8-14%, respectively, of those obtained after an equimolar dose of 1'-[3H]hydroxysafrole. Likewise, when 3'-hydroxyisosafrole was injected into 12-day-old male B6C3F1 mice at a level of 0.1 or 2.5 mumol/g body wt., the average numbers of hepatomas per mouse (0.2 and 0.4, respectively) were not significantly increased over the average number for mice treated only with the solvent (0.2). By contrast, mice that received 0.1 mumol of 1'-hydroxysafrole/g body wt. developed about 2 hepatomas per mouse. The metabolism of 3'-hydroxyisosafrole in the rat and mouse differed markedly from that of 1'-hydroxysafrole. 3'-Hydroxyisosafrole rapidly underwent side-chain oxidation to yield 3,4-methylenedioxycinnamic acid and 3,4-methylenedioxybenzoic acid. In the first 4 h, 3,4-methylenedioxybenzoyl glycine and 3,4-methylenedioxycinnamoyl glycine, the major urinary metabolites, together accounted for 39% and 63% of the dose administered to rats and mice, respectively. The glucuronide of 3'-hydroxyisosafrole was not detected in the urine, whereas urinary excretion of the glucuronide of 1'-hydroxysafrole at 2 h accounted for approx. 40% of a dose of 1'-hydroxysafrole.

The metabolism of aspirin in man: a population study

The metabolism of a 900 mg oral dose of aspirin has been investigated in 129 healthy volunteers. For this purpose, the 0-12 h urine was collected and analysed for the following excretion products: salicylic acid, its acyl and phenolic glucuronides, salicyluric acid, its phenolic glucuronide and gentisic acid. The total excretion of salicylate and metabolites was normally distributed within the population group studied, showing a 2.5-fold variation: a mean of 68.1% of the dose was recovered in 12 h. The excretion of salicylic acid was found to be highly variable within the study panel (1.3-31% of dose in 12 h), and was related to both urine volume and pH. Salicyluric acid was the major metabolite in the majority of the volunteers and its excretion was normally distributed amongst the study panel. The elimination of this metabolite ranged from 19.8 to 65% of the dose and was related to the total recovery of salicylate. The excretion of the two salicyl glucuronides was highly variable, ranging from 0.8 to 42% of the dose. The elimination of the glucuronides was inversely related to that of salicyluric acid. Gentisic acid and salicyluric acid phenolic glucuronide were minor metabolites of salicylate, accounting for 1 and 3% of the dose, respectively. The recovery of gentisic acid was statistically significantly greater in female subjects than in males, whilst the opposite was found for salicyluric acid and total salicylate. However, these differences were small in magnitude.

Rearrangement of valproate glucuronide in a patient with drug-associated hepatobiliary and renal dysfunction

Formation of beta-glucuronidase-resistant "glucuronides" of valproic acid (VPA) by intramolecular rearrangement of biosynthetic valproate glucuronide in vivo was investigated in a patient diagnosed with VPA-associated hepatobiliary and renal dysfunction. Plasma elimination half-life of VPA following cessation of the drug was 13.9 h. At the time of the toxicity, the concentration of conjugated VPA in plasma was very high (36-54% of nonconjugated VPA levels) relative to that in normal patients (2.9%). The fraction of conjugated VPA resistant to beta-glucuronidase hydrolysis was 0.28-0.47 in plasma and 0.15-0.42 in urine. The corresponding fraction in urine from normal patients receiving VPA therapy was 0.044. The evidence was consistent with retarded elimination of biosynthetic VPA glucuronide caused by renal and hepatobiliary dysfunction. Consequent prolongation of circulation of VPA glucuronide at the slightly alkaline pH of blood would permit extensive intramolecular rearrangement which is known to be pH-, temperature-, and time-dependent. The biological consequences of the presence of such beta-glucuronidase-resistant conjugated VPA in vivo are largely unknown.

Isolation and characterization of the four antipyrine glucuronides and determination of their urinary excretion pattern in man by a reversed-phase h.p.l.c. assay

A large-scale procedure for the isolation of four urinary glucuronides in antipyrine metabolism is described; the isolated compounds are used as standards in a direct h.p.l.c. assay. The four glucuronides were characterized by u.v. and 1H-n.m.r. spectroscopy, and after hydrolysis by a t.l.c. assay of the corresponding aglycones. A reversed-phase h.p.l.c. assay procedure has been developed for the direct quantification of the four antipyrine glucuronides; this separates 3-hydroxymethyl-antipyrine glucuronide, 4,4'-dihydroxy-antipyrine glucuronide, norantipyrine glucuronide and 4-hydroxy-antipyrine glucuronide in a single run. Urinary elimination patterns of these glucuronides have been determined in five female and five male volunteers after antipyrine (1200 mg) administration. The direct assay of urinary glucuronides enables the simultaneous determination of glucuronidation activities and four different phase-I metabolites of antipyrine in vivo.