Procedures to characterize in vivo and in vitro enantioselective glucuronidation properly: studies with benoxaprofen glucuronides

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.

Kinetic modelling of acyl glucuronide and glucoside reactivity and development of structure–property relationships

Detailed kinetic and transition structure modelling to rationalise the differences in reactivity observed between the acyl glucuronide and glucoside metabolites of a series of phenylacetic acid analogues.

Species differences in the elimination of a peroxisome proliferator-activated receptor agonist highlighted by oxidative metabolism of its acyl glucuronide

A species difference was observed in the excretion pathway of 2-[[5,7-dipropyl-3-(trifluoromethyl)-1,2-benzisoxazol-6-yl]oxy]-2-methylpropanoic acid (MRL-C), an alpha-weighted dual peroxisome proliferator-activated receptor alpha/gamma agonist. After intravenous or oral administration of [14C]MRL-C to rats and dogs, radioactivity was excreted mainly into the bile as the acyl glucuronide metabolite of the parent compound. In contrast, when [14C]MRL-C was administered to monkeys, radioactivity was excreted into both the bile and the urine as the acyl glucuronide metabolite, together with several oxidative metabolites and their ether or acyl glucuronides. Incubations in hepatocytes from rats, dogs, monkeys, and humans showed the formation of the acyl glucuronide of the parent compound as the major metabolite in all species. The acyl glucuronide and several hydroxylated products, some which were glucuronidated at the carboxylic acid moiety, were observed in incubations of MRL-C with NADPH- and uridine 5'-diphosphoglucuronic acid-fortified liver microsomes. However, metabolism was more extensive in the monkey microsomes than in those from the other species. When the acyl glucuronide metabolite of MRL-C was incubated with NADPH-fortified liver microsomes, in the presence of saccharo-1,4-lactone, it underwent extensive oxidative metabolism in the monkey but considerably less in the rat, dog, and human liver microsomes. Collectively, these data suggested that the oxidative metabolism of the acyl glucuronide might have contributed to the observed in vivo species differences in the metabolism and excretion of MRL-C.

Glucuronidation of nonsteroidal anti-inflammatory drugs: identifying the enzymes responsible in human liver microsomes

Nonsteroidal anti-inflammatory drugs (NSAIDs), used for the treatment of pain and inflammation, are eliminated primarily through conjugation with polar sugar moieties to form glucuronides. Glucuronidation is catalyzed by the UDP-glucuronosyltransferases (UGT) superfamily. An inverse relationship may exist between glucuronidation activity and NSAID efficacy; however, specific UGTs catalyzing conjugation of the structurally diverse NSAIDs have yet to be identified systematically. Therefore, NSAID glucuronidation activity by 12 individually expressed UGTs was investigated by liquid chromatography-tandem mass spectrometry. The relative rates of NSAID glucuronidation varied among UGT enzymes examined, demonstrating specificity of the individual UGTs toward selected NSAIDs. Kinetic parameters were determined for expressed UGT Supersomes and compared with parameters determined in pooled human liver microsomes (HLMs). Comparison of K(m) values suggested roles for UGTs 1A3 and 2B7 in indene glucuronidation and UGTs 1A9, 2B4, and 2B7 in profen glucuronidation. Inhibitory studies in pooled HLMs support the role of UGTs 1A1, 1A3, 1A9, 2B4, and 2B7 in the glucuronidation of ibuprofen, flurbiprofen, and ketoprofen. Bilirubin did not inhibit indomethacin or diclofenac glucuronidation, suggesting that UGT1A1 was not involved in catalysis. Imipramine did not inhibit glucuronidation of sulindac, sulindac sulfone, indomethacin, or naproxen in pooled HLMs, suggesting that UGT1A3 was not a principal hepatic catalyst. Nevertheless, multiple UGT enzymes, most notably UGTs 1A1, 1A9, 2B4, and 2B7, seem to be involved in the hepatic catalysis of NSAID glucuronidation.

Stereoselective Taurine Conjugation of (R)-Benoxaprofen Enantiomer in Rats: In Vivo and in Vitro Studies Using Rat Hepatic Mitochondria and Microsomes

PURPOSE

Identify (R)-BOP-T in rat bile after administration of (R)-BOT over a 12 h period.

METHODS

Each benoxaprofen (BOP) enantiomer was administered i.v. to bile duct-cannulated rats at a dose of 5 mg/kg. The optical isomers of BOP and its metabolites in plasma, urine, and bile were quantified using a chiral HPLC column. The amounts of BOP glucuronide (BOP-G), BOP taurine conjugate (BOP-T), and BOP enantiomers excreted into the bile over 12 h after administration of (R)-BOP were as follows: (R)-BOP-G and (S)-BOP-G, 2.1 +/- 0.5 and 6.2 +/- 1.4% of the dose; (R)-BOP-T and (S)-BOP-T, 5.6 +/- 1.8 and 0.7 +/- 0.3% of the dose; (R)-BOP and (S)-BOP, 0.7 +/- 0.1 and 1.7 +/- 0.2% of the dose, respectively, whereas after (S)-BOP administration, (S)-BOP-G and (S)-BOP were mainly excreted into the bile (14.3 +/- 1.8 and 3.0 +/- 0.4% of the dose, respectively). Only after (R)-BOP administration was the taurine conjugate of BOP found in the bile, and the configuration was R. BOP-T could not be found in the bile after (S)-BOP administration. To investigate the stereoselectivity of the conjugation enzymes responsible for BOP-T formation, in vitro studies were performed using rat hepatic organelles.

RESULTS

When (R)-BOP was used as a substrate, rat hepatic mitochondrial and microsomal fractions exhibited stereoselective BOP-T formation activity, with microsomal activity approximately 3.0 times greater than that of the mitochondria. That of (S)-BOP was approximately 2.1. Mean (R)/(S) ratios of BOP enantiomer for BOP-T formation in the mitochondrial and microsomal incubations were approximately 1.7 and 2.4, respectively.

CONCLUSION

Although in the in vivo studies, only (R)-BOP-T originated from (R)-BOP was found in the bile, the configuration of BOP-T formed by the incubations of (R)-BOP or (S)-BOP with rat hepatic mitochondria or microsomes was S for both.

Acyl glucuronide drug metabolites: toxicological and analytical implications

Although glucuronidation is generally considered a detoxification route of drug metabolism, the chemical reactivity of acyl glucuronides has been linked with the toxic properties of drugs that contain carboxylic acid moieties. It is now well documented that such metabolites can reach appreciable concentrations in blood. Furthermore, they are labile, undergo hydrolysis and pH-dependent intramolecular acyl migration to isomeric conjugates of glucuronic acid, and may react irreversibly with plasma proteins, tissue proteins, and with nucleic acids. This stable binding causes chemical alterations that are thought to contribute to drug toxicity either through changes in the functional properties of the modified molecules or through antigen formation with subsequent hypersensitivity and other immune reactions. Whereas in vitro data on the toxicity of acyl glucuronides have steadily accumulated, direct evidence for their toxicity in vivo is scarce. Acyl glucuronides display limited stability, which is dependent on pH, temperature, nature of the aglycon, and so on. Therefore, careful sample collection, handling, and storage procedures are critical to ensure generation of reliable pharmacologic and toxicologic data during clinical studies. Acyl glucuronides can be directly quantified in biologic specimens using chromatographic procedures. Their adducts with plasma or cell proteins can be determined after electrophoretic separation, followed by blotting. ELISA techniques have been used to assess the presence of antibodies against acyl glucuronide-protein adducts. This review summarizes the most recent evidence concerning biologic and toxicologic effects of acyl glucuronide metabolites of various drugs and discusses their relevance for drug monitoring. A critical evaluation of the available methodology is included.

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.

High-performance liquid chromatographic assay for acetaminophen glucuronide in human liver microsomes

A rapid and specific high-performance liquid chromatographic assay was developed for the determination of acetaminophen glucuronide formed by human liver microsomes. In addition, incubation conditions were systematically evaluated. Conditions that yielded the optimal rate of acetaminophen glucuronide formation over various concentrations of acetaminophen (0.15-30 mM) consisted of the following: 0.1 M potassium phosphate buffer, 1 mM magnesium chloride, 30 microg/mg alamethicin, 4 mM uridine 5'-diphosphoglucuronic acid at a pH of 7.1. Alamethicin produced higher and more consistent APAPG formation rates compared to Brij-58. Adding saccharolactone to the incubation medium reduced the velocity of the reaction. Acetaminophen glucuronide, acetaminophen, and the internal standard (paraxanthine), were analyzed on a C18 column with UV detection at 250 nm. The mean correlation coefficient (r2) of the standard curves for acetaminophen glucuronide was >0.99 over the range of 0.1-25 nmol. The intra- and inter-day coefficients of variation were <4%. This method is suitable for in vitro studies using acetaminophen glucuronide formation as an index reaction for UGT activity.

Stereoselective binding properties of naproxen glucuronide diastereomers to proteins

The stability of naproxen glucuronide (NAP-G) diastereomers was investigated in buffer, 0.3% and 3% human serum albumin (HSA) solutions, and human plasma. R-NAP-G was found to be less stable in phosphate buffer than its S-diastereomer, whereas incubation media containing protein in general increased the degradation rate of NAP-G but also caused a change of the stereoselective stability where the R-NAP-G was more stable than S-NAP-G. Reversible binding of NAP-Gs to HSA (0.3%) was investigated and compared with the corresponding properties of naproxen (NAP) enantiomers. NAP-G diastereomers exhibited a considerable and stereoselective affinity to HSA, although less than that observed for the NAP enantiomers. In vitro irreversible binding of NAP-Gs to HSA, human and rat plasma proteins was also investigated. Irreversible binding was higher for R-NAP-G (50 microM) than for S-NAP-G (50 microM) in all incubation media. This stereoselective difference was observed with HSA containing medium as well as in rat and human plasma. Incubation with unconjugated NAP did not lead to irreversible binding. Preincubation of HSA with acetylsalicylic acid (approximately 11 mM) and glucuronic acid (50 mM) decreased the extent of irreversible binding suggesting involvement of lysine residues for covalent binding. Preincubation with S-NAP also decreased the irreversible binding yield.

Biosynthesis, characterisation and direct high-performance liquid chromatographic analysis of gemfibrozil 1-O-beta-acylglucuronide

Gemfibrozil 1-O-beta-acylglucuronide was purified from the urine of a volunteer administered gemfibrozil, and an isocratic reversed-phase HPLC method was developed for its direct measurement. Quantitation of gemfibrozil and gemfibrozil 1-O-beta-acylglucuronide was carried out from plasma, following extraction from acidified specimens into ethyl acetate, on a 5-microns CN reversed-phase column with a mobile phase (pH 3.5) containing acetonitrile, tetrabutylammonium sulphate and distilled water, using fluorescence detection at 284 nm excitation and 316 nm emission. Calibration curves were linear for both compounds over a concentration range of 0.1 to 40 mg/l, with intra-assay coefficients of variation < 5% at concentrations of 20.0, 2.0 and 0.2 mg/l, and inter-assay coefficients of variation < 10%. No degradation of gemfibrozil 1-O-beta-acylglucuronide was detected as a result of the analytical procedure. However, a preliminary application of the method indicates that gemfibrozil acylglucuronide is chemically unstable undergoing intra-molecular rearrangement and hydrolysis under physiological conditions.

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.

The influence of renal function on the enantioselective pharmacokinetics and pharmacodynamics of ketoprofen in patients with rheumatoid arthritis

1. Single oral doses of 100 mg racemic ketoprofen were given to 15 patients (age range: 51-79 years) with rheumatoid arthritis and a range of creatinine clearances (CLCR) from 26 to 159 ml min-1. 2. The fractions unbound of (R)- and (S)-ketoprofen in plasma were determined for each subject after in vitro addition of rac-ketoprofen (enantiomer range: 1.00-6.00 micrograms ml-1) to pre-dose plasma. 3. An index of the antiplatelet effect of ketoprofen in vitro was measured as inhibition of platelet thromboxane B2 (TXB2) generation during the controlled clotting of whole blood (pre-dose) spiked with rac-ketoprofen. 4. In vivo studies revealed significant associations (P < 0.05) between the reciprocal of AUC for both unbound and total (bound plus unbound) (S)-ketoprofen and CLCR. Corresponding relationships were also observed for the (R)-enantiomer of ketoprofen. In addition, the half-life of each enantiomer was negatively correlated with CLCR. There was a positive relationship between the 24 h urinary recovery of combined non-conjugated and conjugated (R)-ketoprofen and CLCR while that for the (S)-stereoisomer failed to reach statistical significance (P > 0.05). 5. There was no difference between AUC for (R)- and (S)-ketoprofen for either unbound or total drug. 6. The mean +/- s.d. percentage unbound of (S)-ketoprofen in plasma (0.801 +/- 0.194%) exceeded (P < 0.05) the corresponding value for its optical antipode (0.724 +/- 0.149%). The percentage unbound of the (S)-enantiomer was higher at 6.00 micrograms ml-1 than that at enantiomer concentrations of 3.50 micrograms ml-1 and below, where it was invariant. The percentage unbound of (R)-ketoprofen was independent of plasma concentration up to 6.00 micrograms ml-1. There were no correlations between the percentage unbound of each enantiomer and either serum albumin concentration or CLCR. 7. The relationship between the serum concentration of unbound (S)-ketoprofen and the percentage inhibition of platelet TXB2 generation was described by a sigmoidal Emax equation for each patient. There was no correlation between the unbound concentration of (S)-ketoprofen in serum required to inhibit platelet TXB2 generation by 50% (EC50) and CLCR. The mean +/- s.d. EC50 was 0.216 +/- 0.143 ng ml-1. 8. These data indicate that diminished renal function is associated with an increased exposure to unbound (S)-ketoprofen, presumably due to regeneration of parent aglycone arising from the hydrolysis of accumulated acyl-glucuronide conjugates. The apparent sensitivity of platelet cyclo-oxygenase to the inhibitory effect of (S)-ketoprofen was not influenced by renal function.

Capillary gas chromatographic method for the determination of the thromboxane A2 receptor antagonist S-1452 and its metabolites in human urine

A capillary gas chromatographic method using a sulphur-specific detector (Hall's electrolytic conductivity detector) was established to determine the thromboxane A2 antagonist S-1452 and its metabolites in human urine. The target species were the free acid (+)-S-145 of the drug and its nine metabolites: the three hydroxyl forms of (+)-S-145 (I, II and III), bis-nor-(+)-S-145 (IV) the hydroxylated forms of IV (V and VI), tetranor-(+)-S-145 (VII) and the hydroxylated forms of VII (VIII and IX). These ten compounds, which have the same sulphur-containing functional group in common, were determined simultaneously. Their conjugated forms, which were assumed to be glucuronides, were also assayed after hydrolysis. The first derivatization was esterification with diazomethane. The second, for the hydroxylated compounds, was trimethylsilylation with bis(trimethylsilyl)trifluoroacetamide. The ten analytes appeared as separate peaks without mutual interference during 5 min. Hall's detector distinguished the ten analytes selectively from the other urinary components, which removed the need for complex clean-up procedures and led to higher sensitivity with a lower noise level. The method is sensitive enough for the assay of substances present at more than 0.1 micrograms/ml of urine. All the compounds could be determined with a high level of precision and accuracy, with 2-5% relative standard deviation and within +/- 5% deviation from the actual value. Day-to-day measurements verified the reproducibility of the method. Recovered substances were quantified by following the time course, and the analytical data together with previously obtained plasma data clarified the metabolism pharmacokinetically.