Properties of acyl glucuronides: implications for studies of the pharmacokinetics and metabolism of acidic drugs

Determination of the covalent adducts of the novel anti-cancer agent 5,6-dimethylxanthenone-4-acetic acid in biological samples by high-performance liquid chromatography

The reversed-phase HPLC methods were developed to determinate the covalently bound protein adducts of the novel anti-cancer drug 5,6-dimethylxanthenone-4-acetic acid (DMXAA) via its glucuronides after releasing aglycone by alkaline hydrolysis in human plasma and human serum albumin (HSA). An aliquot of 75 microl of the mixture was injected onto a Spherex C18 column (150x4.6 mm; 5 microm) at a flow-rate of 2.5 ml/min. The mobile phase comprising of acetonitrile:10 mM ammonium acetate buffer (24:76, v/v, pH 5.8) was used in an isocratic condition, and DMXAA was detected by fluorescence. The method was validated with respect to recovery, selectivity, linearity, precision, and accuracy. Calibration curves for DMXAA were constructed in the concentration range of 0.5-40 microM in washed blank human plasma or HSA prior to alkaline hydrolysis. The difference between the theoretical and calculated concentration and the relative standard deviation were less than 10% at all quality control (QC) concentrations. The limit of detection for the covalent adduct in human plasma or HSA is 0.20 microM. The methods presented good accuracy, precision and sensitivity for use in the preclinical and clinical studies.

Disposition of naproxen, naproxen acyl glucuronide and its rearrangement isomers in the isolated perfused rat liver

1. An isolated perfused rat liver (IPRL) preparation was used to investigate separately the disposition of the non-steroidal anti-inflammatory drug (NSAID) naproxen (NAP), its reactive acyl glucuronide metabolite (NAG) and a mixture of NAG rearrangement isomers (isoNAG), each at 30 microg NAP equivalents ml perfusate (n = 4 each group). 2. Following administration to the IPRL, NAP was eliminated slowly in a log-linear manner with an apparent elimination half-life (t 1/2) of 13.4 +/- 4.4h. No metabolites were detected in perfusate, while NAG was the only metablolite present in bile in measurable amounts (3.9 +/- 0.8% of the dose). Following their administration to the IPRL, both NAG and isoNAG were rapidly hydrolysed (t 1/2 in perfusate = 57 +/- 3 and 75 +/- 14 min respectively). NAG also rearranged to isoNAG in the perfusate. Both NAG and isoNAG were excreted intact in bile (24.6 and 14.8% of the NAG and isoNAG doses, respectively). 3. Covalent NAP-protein adducts in the liver increased as the dose changed from NAP to NAG to isoNAG (0.20 to 0.34 to 0.48% of the doses, respectively). Similarly, formation of covalent NAP-protein adducts in perfusate were greater in isoNAG-dosed perfusions. The comparative results suggest that isoNAG is a better substrate for adduct formation with liver proteins than NAG.

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.

Roles of glucuronidation and UDP-glucuronosyltransferases in xenobiotic bioactivation reactions

Glucuronide conjugates represent one of the major types of naturally occurring phase 2 metabolites of xenobiotics and endobiotics. The process underlying their formation, glucuronidation, is normally considered detoxifying, because glucuronides usually possess less intrinsic biological or chemical activity than their parent aglycones and they are rapid excreted. However, a number of glucuronide conjugates are known that are active and may contribute to pharmacological activities or toxicities associated with their parent compounds. These include two classes of glucuronides with electrophilic chemical reactivity (N-O-glucuronides of hydroxamic acids and acyl glucuronides of carboxylic acids) and several types of glucuronides that impart biological effects through non-covalent interactions (morphine 6-O-glucuronide, retinoid glucuronides, and D-ring glucuronides of estrogens). Glucuronides may thus contribute to clinically significant effects, including environmental arylamine-induced carcinogenesis, drug hypersensitivity and other toxicities associated with carboxylic acid drugs, morphine analgesia, and cholestasis from estrogens. This review summarizes the rat and human UDP-glucuronosyltransferases that may be involved in the formation of bioactive glucuronides, including their substrate- and tissue-specificity and genetic and environmental influences on their activity. This knowledge may be useful for enhancing the therapeutic efficacy and minimizing the risk of adverse effects associated with xenobiotics that undergo bioactivating glucuronidation reactions.

Determination of the Acyl Glucuronide Metabolite of Mycophenolic Acid in Human Plasma by HPLC and Emit

Background: The acyl glucuronide (AcMPAG) of mycophenolic acid (MPA) has been found to possess pharmacologic and potentially proinflammatory activity in vitro. To establish its pharmacologic and toxicologic relevance in vivo, a reversed-phase HPLC method was modified to simultaneously determine MPA, the phenolic MPA-glucuronide (7-O-MPAG), and AcMPAG. In addition, cross-reactivity of AcMPAG in the Emit assay for MPA was investigated.

Methods: The procedure used simple sample preparation, separation with a Zorbax Eclipse-XDB-C8 column, and gradient elution. AcMPAG was quantified as 7-O-MPAG-equivalents.

Results: The assay was linear up to 50 mg/L for MPA, 250 mg/L for 7-O-MPAG, and 10 mg/L for AcMPAG (r >0.999). Detection limits were 0.01, 0.03, and 0.04 mg/L for MPA, 7-O-MPAG, and AcMPAG, respectively. The recoveries were 99–103% for MPA, 95–103% for 7-O-MPAG, and 104–107% for AcMPAG. The within-day imprecision was <5.0% for MPA (0.2–25 mg/L), <4.4% for 7-O-MPAG (10–250 mg/L), and ≤14% for AcMPAG (0.1–5 mg/L). The between-day imprecision was <6.2%, <4.5%, and ≤14% for MPA, 7-O-MPAG, and AcMPAG, respectively. When isolated from microsomes, purified AcMPAG (1–10 mg/L) revealed a concentration-dependent cross-reactivity in an Emit assay for the determination of MPA ranging from 135% to 185%. This is in accordance with the bias between HPLC and Emit calculated in 270 samples from kidney transplant recipients receiving mycophenolate mofetil therapy, which was greater (median, 151.2%) than the respective AcMPAG concentrations determined by HPLC. AcMPAG was found to undergo hydrolysis when samples were stored up to 24 h at room temperature or up to 30 days at 4 °C or −20 °C. Acidified samples (pH 2.5) were stable up to 30 days at −20 °C.

Conclusions: The HPLC and Emit methods for AcMPAG described here may allow investigation of its relevance for the immunosuppression and side effects associated with mycophenolate mofetil therapy.

Metabolism of CP-195,543, a leukotriene B4 receptor antagonist, in the Long-Evans rat and Cynomolgus monkey

1. The fate of [14C]CP-195,543, a novel leukotriene B4 receptor antagonist, was studied following oral administration to the Long-Evans rat and Cynomolgus monkey. 2. Most of the radioactivity was primarily excreted in the faeces, and urine was a minor route of excretion. 3. CP-195,543 was extensively metabolized in the two species, primarily by two metabolic pathways: glucuronidation of unchanged CP-195,543 and oxidative metabolism, presumably by cytochrome P450. 4. The sites of glucuronidation were the carboxylic acid moiety and the hydroxy group. The ester glucuronide was the predominant glucuronide conjugate detected in the rat, whereas the monkey generated the ether as well as the ester glucuronide. 5. The structures of oxidative metabolites were elucidated using mass spectrometry (in the positive- and negative-ion mode) and 1H-NMR. The sites of hydroxylation were the benzylic group and the 3-position of the benzopyran ring. 6. This study has indicated that CP-195,543 was mainly eliminated by Phase II metabolism in both species.

Determinaton of two major metabolites of the novel anti-tumour agent 5,6-dimethylxanthenone-4-acetic acid in hepatic microsomal incubations by high-performance liquid chromatography with fluorescence detection

High-performance liquid chromatographic methods have been developed and validated for the glucuronidated and oxidative metabolites of the novel anti-tumour agent 5,6-dimethylxanthenone-4-acetic acid (DMXAA), produced in human liver microsomal incubations. Calibration curves for DMXAA acyl glucuronide (DMXAA-Glu) and 6-hydroxymethyl-5-methylxanthenone-4-acetic acid (6-OH-MXAA) were constructed over the concentration ranges of 0.25 to 20 and 0.5 to 40 microM, respectively. Assay performance was determined by intra- and inter-day accuracy and precision of quality control (QC) samples. The difference between the theoretical and measured concentration, and the coefficient of variation, were less than 15% at low QC concentrations, and less than 10% at medium and high QC concentrations for both analytes. The methods presented good accuracy, precision and sensitivity for use in kinetic studies of the glucuronidated and oxidative metabolites of DMXAA in human liver microsomes.

Stereoselective pharmacokinetics of ketoprofen and ketoprofen glucuronide in end-stage renal disease: evidence for a 'futile cycle' of elimination

AIMS

To assess if futile cycling of ketoprofen occurs in patients with decreased renal function.

METHODS

Ketoprofen was administered to six haemodialysis-dependent patients with end-stage renal disease as single (50 mg) or multiple doses (50 mg three times daily, for 7 days). Plasma and dialysate concentrations of the unconjugated and glucuronidated R- and S-enantiomers of ketoprofen were determined using h.p.l.c. following the single and multiple dosing.

RESULTS

The oral clearance was decreased and terminal elimination half-lives of R- and S-ketoprofen and the corresponding acyl glucuronides were increased in functionally anephric patients compared with healthy subjects. In contrast with the R-isomers, S-ketoprofen and S-ketoprofen glucuronide exhibited an unexpected accumulation (2.7-3. 8 fold) after repeated dosing achieving S:R ratios of 3.3+/-1.7 and 11.2+/-5.3, respectively. The plasma dialysis clearances for R- and S-ketoprofen glucuronides were 49.4+/-19.8 and 39.0+/-15.9 ml min-1, respectively, and 10.8+/-17.6 and 13.3+/-23.5 ml min-1 for unconjugated R- and S-ketoprofen.

CONCLUSIONS

The selective accumulation of S-ketoprofen and its acyl glucuronide are consistent with amplification of chiral inversion subsequent to futile cycling between R-ketoprofen and R-ketoprofen glucuronide. Severe renal insufficiency, and possibly more modest decrements, results in a disproportionate increase in systemic exposure to the S-enantiomer which inhibits both pathologic and homeostatic prostaglandin synthesis.

Clinical consequences of the biphasic elimination kinetics for the diuretic effect of furosemide and its acyl glucuronide in humans

This review discusses the possibility of whether furosemide acyl glucuronide, a metabolite of furosemide, contributes to the clinical effect of diuresis. First an analytical method (e.g. HPLC) must be available to measure both parent drug and furosemide acyl glucuronide. Then, with correctly treated plasma and urine samples (light protected, pH 5) from volunteers and furosemide-treated patients, the kinetic curves of both furosemide as well as its acyl glucuronide can be measured. The acyl glucuronide is formed in part by the kidney tubules and it is possible that the compound is pharmacologically active through inhibition of the Na+/2Cl-/K+ co-transport system; up to now the mechanism of action has been solely attributed to furosemide. The total body clearance of furosemide occurs by hepatic and renal glucuronidation (50%) and by renal excretion (50%). Enterohepatic cycling of furosemide acyl glucuronide, followed by hydrolysis, results in a second and slow elimination phase with a half-life of 20-30 h. This slow elimination phase coincides with a pharmacodynamic rebound phase of urine retention. After each dosage of furosemide, there is first a short stimulation of urine flow (4 h), which is followed by a 3-day recovery period of the body. The following clinical implications arise from study of the elimination kinetics of furosemide. Repetitive dosing must result in accumulation of the recovery period. Accumulation of furosemide and its acyl glucuronide in patients with end-stage renal failure results from infinite hepatic cycling. Impaired kidney function may result in impaired glucuronidation and diuresis. While kidney impairment normally requires a dose reduction for those compounds which are mainly eliminated by renal excretion, for diuretics, a dose increment is required in order to maintain a required level of diuresis. The full clinical impact of the accumulation of furosemide and its acyl glucuronide in patients with end-stage renal failure has to be determined.

Limitations of hepatocytes and liver homogenates in modelling in vivo formation of acyl glucuronide-derived drug-protein adducts

The covalent binding of drugs or their metabolites to proteins is of increasing interest in the investigation of the toxicity of these compounds. Recent attention on biological consequences of protein adduct formation with carboxylate drugs, derived via their reactive acyl glucuronide metabolites, has focussed on liver tissue. Although the intact animal represents undisturbed hepatic physiology, other hepatic models can offer advantages, e.g., multiple experiments from a single liver. In this study we set out to compare the patterns of covalent binding of zomepirac (ZP) to proteins in the livers of intact rats, isolated rat hepatocytes (in culture or suspension), and in rat liver homogenates. Rats were dosed i.v. with 25 mg ZP/kg, and their livers were removed 3 h later. Isolated hepatocytes or liver homogenates were exposed to ZP at 100 microg/mL for 3 h at 37 degrees C. Liver homogenates were exposed to ZP and also zomepirac acyl glucuronide (ZAG) at 100 microg ZP equivalents/mL for 3 h at 37 degrees C. Covalent binding of ZP species was examined by SDS-PAGE and Western blotting with a polyclonal ZP antiserum. In livers from dosed animals, the strongest staining appeared at about 110120, 140, and 200 kDa. Few similarities existed with the results from isolated hepatocytes and, not surprisingly, liver homogenates. Only the 200-kDa band was common to all treatments. Many proteins seemed to be modified, at least to some extent. The differences in major bands are most likely caused by the loss of liver and hepatocyte architecture. The variability across different model systems in respect to covalent binding to hepatic proteins emphasizes the need for care in interpretation of results.

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.

Drug glucuronidation by human renal UDP-glucuronosyltransferases

The UDP-glucuronosyltransferases catalyse the conjugation of glucuronic acid to a wide variety of endobiotics and xenobiotics, representing one of the major conjugation reactions in the conversion of both exogenous (e.g. drugs and pesticides) and endogenous compounds (e.g. bilirubin and steroid hormones). The liver is the major site of glucuronidation, however a number of extrahepatic tissues exhibit particular UDP-glucuronosyltransferase activities. The present study was undertaken to assess the human renal UDP-glucuronosyltransferase system. Enzymatic analysis of human kidney showed that a limited number of UDP-glucuronosyltransferase isoforms were expressed in this tissue. However the level of renal UGT activity towards the anaesthetic propofol was higher compared with human liver. The glucuronidation of propofol is catalysed by UGT1A8/9 suggesting higher levels of this isoform in the kidney. Immunoblot analysis revealed two major UDP-glucuronosyltransferase immunopositive bands to be present in human kidney as compared to four major bands in human liver. The human kidney was capable of conjugating various structurally diverse drugs and xenobiotics.

Separation and characterization of carboxyl-linked glucuronides of bile acids in incubation mixture of rat liver microsomes

The carboxyl-linked 24-glucuronides of common bile acids have been identified by means of liquid chromatography (LC)/atmospheric pressure chemical ionization (APCI)-mass spectrometry (MS) in an incubation mixture with a male Wistar rat liver microsomal fraction. The authentic specimens of bile acid 24-glucuronide acetate-methyl esters were synthesized unequivocally using the Mitsunobu reaction, and the APCI-mass spectrometric properties of these glucuronide derivatives were also characterized. After incubation of common unconjugated bile acids with hepatic microsomes, glucuronides were extracted and purified with a Sep-Pak C18 cartridge and lipophilic ion exchange gel, piperidino-hydroxypropyl Sephadex LH-20, and then derivatized into the acetate-methyl esters. Subsequent resolution into alpha- and beta-isomers at the glucuronosyl linkage was attained by LC on Cosmosil 5C8 and Sumichiral OA-2500 columns using 200 mM ammonium acetate (pH 7.0)-methanol (1:4, v/v), where 24-glucuronides were monitored with characteristic positive ions [M + NH4]+. The 24-glucuronides of lithocholic, chenodeoxycholic, deoxycholic, ursodeoxycholic and cholic acid were definitely characterized, in contrast to no formation of corresponding 3-glucuronides.

Hepatobiliary transport of diflunisal conjugates and taurocholate by the perfused rat liver: the effect of chronic exposure of rats to diflunisal

Acyl glucuronides are reactive electrophilic metabolites of carboxylate drugs which can form covalent adducts with endogenous macromolecules such as serum albumin and hepatic proteins. Such adducts have been suggested as initiating factors in certain immune and toxic responses to acidic drugs. In the present study, pretreatment of rats with high daily doses (50 mg/kg orally) of the non-steroidal anti-inflammatory drug (NSAID) diflunisal (DF) for 35 days, followed by perfusion of the isolated liver with 3 mg DF for 3 hr, resulted in appreciable concentrations of covalent adducts of DF with hepatic tissue (3.68 microg DF/g liver). Immunoblotting using a rabbit polyclonal DF antiserum showed the major DF-modified bands at about 110, 140 and 200 kDa. A vehicle-pretreated control group achieved adduct concentrations of only 0.37 microg DF/g liver, with the 200 kDa band not detectable in immunoblots. Elimination of DF from perfusate of the isolated perfused rat liver (IPRL) preparation was the same (t1/2 about 3.4 hr) in both DF- and vehicle-pretreated groups. Appearance of the sulfate (DS) conjugate, the major metabolite in perfusate, was also similar. However, higher concentrations of the acyl glucuronide (DAG) and phenolic glucuronide (DPG) conjugates were found in perfusate at later times, though a statistically significant difference in area under the concentration-time curve was found only in the case of DAG. At 3 hr, recoveries of dose as DAG and DPG were significantly higher in perfusate, but not in bile. No significant differences in uptake and biliary excretion of taurocholate were found between the two groups. The finding of higher perfusate concentrations of DAG and DPG could signal a minor compromise to biliary excretion processes for the glucuronides, though whether such a result is simply coincident with or attributable to DAG-derived covalent DF-protein adducts in liver remains indeterminate.

Stereospecific analysis of the major metabolites of ibuprofen in urine by sequential achiral-chiral high-performance liquid chromatography

A sequential achiral-chiral HPLC method has been developed for the stereospecific analysis of the two major urinary metabolites of ibuprofen, namely hydroxyibuprofen and carboxyibuprofen. Achiral analysis was carried out using a Partisil column (250x4.6 mm, 5 microm) and a mobile phase of hexane:ethanol (98.2:1.8, v/v) containing trifluoroacetic acid (TFA; 0.05%, v/v) at a flow-rate of 2.0 ml/min. The HPLC eluate containing the two metabolites was separately collected, evaporated under nitrogen and the residue dissolved in the mobile phase used for chiral chromatography. Chiral-phase analysis was carried out using a Chiralpak AD CSP (250x4.6 mm, 10 microm) with a mobile phase of hexane:ethanol (92:8, v/v) containing TFA (0.05%, v/v) at a flow-rate of 1.0 ml/min. In both assays the analytes were quantified by ultraviolet detection at a wavelength of 220 nm. Modification of the mobile-phase composition allowed the resolution of all six analytes in a single chromatographic run but with an increase in run time and consequent band broadening. The analytical method described allows the direct quantitation of the stereoisomers of both metabolites of ibuprofen in urine following the administration of therapeutic doses of the racemic drug to man.

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.

Simultaneous analysis of several non-steroidal anti-inflammatory drugs in human urine by high-performance liquid chromatography with normal solid-phase extraction

A practical and reproducible high-performance liquid chromatographic method using normal solid-phase extraction has been developed for the simultaneous analysis of twelve non-steroidal anti-inflammatory drugs (NSAIDs) in human urine. A urine specimen mixed with acetate buffer pH 5.0 was purified by solid-phase extraction on a Sep-Pak Silica cartridge. The analyte was chromatographed by a reversed-phase Inertsil ODS-2 column using a phosphate buffer-acetonitrile at pH 5.0 as the mobile phase, and the effluent from the column was monitored at 230 or 320 nm. Absolute recoveries were greater than 73% for all of the twelve NSAIDs. The present method enabled simple manipulation and isocratic HPLC with UV analysis as well as high sensitivity of 0.005 microg/ml for naproxen, and 0.05 microg/ml for sulindac, piroxicam, loxoprofen, ketoprofen, felbinac, fenbufen, flurbiprofen, diclofenac, ibuprofen and mefenamic acid as the quantitation limit in human urine using indomethacin as an internal standard.

The utility of the bile-exteriorized rat as a source of reactive acyl glucuronides: studies with zomepirac

Acyl glucuronide conjugates of acidic drugs are chemically unstable metabolites, able to undergo a number of reactions including covalent binding interactions with proteins. The question of whether any toxicological or immunological responses result from such covalent modification of native proteins in vivo is topical. Study of acyl glucuronide reactivity thus requires a convenient source of these metabolites. The utility of the bile-exteriorized rat for this purpose is highlighted herein using the formerly marketed nonsteroidal antiinflammatory agent zomepirac. Zomepirac was injected i.v. at 60 mg/kg four times into bile-exteriorized rats at 6-h intervals. The 24-h bile samples contained ca. 24% of zomepirac doses as zomepirac acyl glucuronide (ZAG). Purification was achieved by washing of the acidified bile with etherhexane, extraction into ethyl acetate, semipreparative HPLC, and crystallization. Overall recovery through the purification procedure was ca. 50%. Identity as ZAG was confirmed by mass spectrometry. The approach takes advantage of the robust glucuronidation capacity of the rat, especially at higher drug doses, and of its ability to preferentially excrete hepatically formed drug glucuronides into bile rather than into urine via blood. Prior to this work, ZAG was presumed to be only a minor metabolite of zomepirac in rats, based on early urinary recovery studies. Thus, measurement of urinary acyl glucuronide conjugates in the rat may severely underestimate their true formation in this species.

Vesico-hepato-renal cycling of acidic drugs via their reactive acyl glucuronide metabolites? Studies with diflunisal in rats

1. Deconjugation-reconjugation cycling of acidic drugs is known to occur in vivo via the hydrolysis of their reactive acyl glucuronide metabolites during their circulation in the blood (systemic cycling) or during their passage through the gut after biliary excretion (enterohepatic cycling). Whether such cycling occurs after renal excretion via hydrolysis in the urinary bladder followed by absorption of liberated drug (vesico-hepato-renal cycling) was investigated in rats using diflunisal (DF) and its acyl glucuronide (DFAG) as model compounds. 2. After administration of DF (1 mg/0.5 mL buffer, pH 7) into the bladder of anaesthetized bile-exteriorized rats, DF appeared rapidly in plasma, achieving peak concentrations of 7 micrograms/mL at 1 h. At 4 h, 30% of the dose was recovered as metabolites, mainly DFAG and DF phenolic glucuronide (DFPG) in bile, while 30% was recovered as unchanged DF from the bladder. 3. By contrast, after intravesical administration of an equimolar amount of DFAG at pH 7 or 5, DFAG itself was not detectable in plasma. Plasma concentrations of DF were barely detectable, with only approximately 1% of the administered dose recovered as metabolites in bile. 4. The data thus show that, although DF itself undergoes facile absorption from the urinary bladder of healthy rats, vesico-hepato-renal cycling of DF via DFAG appears to be of only minor quantitative importance.

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.

Direct analysis of salicylic acid, salicyl acyl glucuronide, salicyluric acid and gentisic acid in human plasma and urine by high-performance liquid chromatography

A method for the simultaneous direct determination of salicylate (SA), its labile, reactive metabolite, salicyl acyl glucuronide (SAG), and two other major metabolites, salicyluric acid and gentisic acid in plasma and urine is described. Isocratic reversed-phase high performance liquid chromatography (HPLC) employed a 15-cm C18 column using methanol-acetonitrile-25 mM acetic acid as the mobile phase, resulting in HPLC analysis time of less than 20 min. Ultraviolet detection at 310 nm permitted analysis of SAG in plasma, but did not provide sensitivity for measurement of salicyl phenol glucuronide. Plasma or urine samples are stabilized immediately upon collection by adjustment of pH to 3-4 to prevent degradation of the labile acyl glucuronide metabolite. Plasma is then deproteinated with acetonitrile, dried and reconstituted for injection, whereas urine samples are simply diluted prior to injection on HPLC. m-Hydroxybenzoic acid served as the internal standard. Recoveries from plasma were greater than 85% for all four compounds over a range of 0.2-20 micrograms/ml and linearity was observed from 0.1-200 micrograms/ml and 5-2000 micrograms/ml for SA in plasma and urine, respectively. The method was validated to 0.2 microgram/ml, thus allowing accurate measurement of SA, and three major metabolites in plasma and urine of subjects and small animals administered salicylates. The method is unique by allowing quantitation of reactive SAG in plasma at levels well below 1% that of the parent compound, SA, as is observed in patients administered salicylates.

Frusemide and its acyl glucuronide show a short and long phase in elimination kinetics and pharmacodynamic effect in man

The pharmacokinetics of 80 mg frusemide given orally were investigated in normal subjects using a direct HPLC method for parent drug and its acyl glucuronide conjugate. Two half-lives could be distinguished in the plasma elimination of both frusemide and its conjugate, with values of 1.25 +/- 0.75 and 30.4 +/- 11.5 h for frusemide and 1.31 +/- 0.60 and 33.2 +/- 28.0 h for the conjugate. The renal excretion rate-time profile showed two phases; the rapid elimination phase lasted from 0-15 h and the second and slow phase, from 15-96 h. During the first 15 h, 33.3 +/- 4.8% of the dosed frusemide was excreted; in the remaining period 15-96 h, 4.6 +/- 1.5% was excreted. In the same two periods the excretion of the glucuronide was 13.4 +/- 4.7 and 1.9 +/- 1.1%, respectively. The mean renal clearance of frusemide was 90.2 +/- 16.9 mL min-1 during the first period and 91.5 +/- 29.3 mL min-1 in the remaining period, during which the stimulation of urine production was absent. The renal clearance of the acyl glucuronide was 702 +/- 221 mL min-1 in the first period, but only 109 +/- 51.0 mL min-1 in the second period. The stimulated urine production in the first 6 h after administration amounted to 2260 +/- 755 mL (measured urine production minus baseline value of 1 mL min-1 (360 mL). During the second or rebound period (6-96 h after drug administration), the quantity of urine was 990 +/- 294 mL lower than what would have been expected from the baseline production of 5400 mL. This reduced production (0.82 mL min-1) is equivalent to an 18% reduction in the average urine flow rate of 1 mL min-1.

New specific and sensitive HPLC-assays for ethacrynic acid and its main metabolite--the cysteine conjugate--in biological material

A reversed-phase high-performance liquid chromatography (HPLC) method was developed and validated for the determination of the loop diuretic ethacrynic acid and its potentially active main metabolite, the ethacrynic acid-cysteine conjugate, in biological material. Simple and rapid sample preparation procedures were established using solid-phase extraction for the parent drug and direct injection after one washing step for the metabolite. HPLC separation was performed on a Spherisorb ODS II (3 microns) analytical column using isocratic elution with different mixtures of mobile phases (phosphoric acid-methanol-acetonitrile-tetrahydrofuran or triethylamine buffer-methanol, respectively). The analytes were detected by measuring the UV absorption of the eluate at 275 nm. Stability studies revealed that considerable amounts of ethacrynic acid may be released from the cysteine conjugate unless the urine samples are pH stabilized (pH 3-4). The assay provided high sensitivity with limits of quantification of 20 ng ml-1 for ethacrynic acid in plasma and urine, and 240 ng ml-1 for the cysteine conjugate in urine. All validation parameters were within the required limits. For the presented assays, the applicability to pharmacokinetic studies and routine analyses was proved.

Complete reversion and prevention of rectal adenomas in colectomized patients with familial adenomatous polyposis by rectal low-dose sulindac maintenance treatment. Advantages of a low-dose nonsteroidal anti-inflammatory drug regimen in reversing adenomas exceeding 33 months

PURPOSE

This nonrandomized, controlled Phase II pilot study aims at the lowest effective dose of rectally applied sulindac to achieve and maintain adenoma reversion in colectomized patients with familial adenomatous polyposis (FAP).

METHODS

The study group (n = 15) underwent proctoscopic and laboratory follow-up for polyp reversion every 6 to 12 weeks. Polyp reversion was followed by dose reduction in predefined steps. Proliferating cell nuclear antigen/cyclin (PCNA) and KI-67 proliferation indices (PI) were performed by point counting. Prostaglandin (PG)E2 and PGF2 alpha were quantified by time-resolved competitive fluorescence immunoassay.

RESULTS

All patients responded to therapy within 6 to 24 weeks. Sixty and 87 percent of patients achieved complete adenoma reversion after 48 weeks at 53 and 67 mg of sulindac per day per patient on average, respectively. Reversion was evident compared with the control group. Dose reduction by one-sixth to one-eighth of the usual oral dose was significant (Mann's trend test, P < 0.05). PCNA and KI-67 PIs of adenomatous and flat mucosa were significantly reduced (Wilcoxon's test, P < 0.05). Correlation of PCNA and KI-67 PIs indicate similar reaction of different tissue structures (Spearman's rank correlation test, P < 0.01). Nonsteroidal anti-inflammatory drug-induced redifferentiation from high-grade to low-grade dysplasia occurred in all but two patients. Tissue-PGE2 levels were greatly reduced. Unwanted, curable side effects were rare (gastritis, n = 2), and laboratory controls are within detection limits.

CONCLUSIONS

Low-dose rectal sulindac maintenance therapy is highly effective in achieving complete adenoma reversion without relapse in 87 percent of patients after 33 months. Rectal FAP phenotype should be crucial for the surgical decision. Colectomy with ileorectal anastomosis and regular chemoprevention might proceed to be a promising alternative to pouch procedures. Chemoprevention with lower incidence of FAP-related tumors via dysplasia reversion may be possible in the future.

Disposition and covalent binding of ibuprofen and its acyl glucuronide in the elderly

Ibuprofen is an over-the-counter nonsteroidal anti-inflammatory drug with a low incidence of severe adverse reactions. It is metabolized by oxidation to carboxyibuprofen and hydroxyibuprofen and by conjugation to an acyl glucuronide. In vitro studies have indicated that ibuprofen glucuronide is labile and reactive, forming covalent adducts with proteins. To verify the formation of ibuprofen-protein adducts in vivo, the pharmacokinetics of ibuprofen glucuronide and its covalent binding to plasma proteins were studied in five elderly patients who received long-term administration of oral doses of ibuprofen. Plasma levels of ibuprofen glucuronide were low relative to those of ibuprofen; the ratio of area under the plasma concentration versus time curve for the glucuronide relative to the parent drug was only 4%. Covalent binding of ibuprofen to plasma protein was observed in all patients, correlating well with the area under the plasma concentration versus time curve of ibuprofen glucuronide (r = 0.966). Compared with reports for other nonsteroidal anti-inflammatory drugs that form acyl glucuronides, plasma levels of ibuprofen-protein adduct are low during long-term administration. The observed lower reactivity in vivo is probably attributable to the greater stability of ibuprofen glucuronide relative to other acyl glucuronides.

Covalent binding of suprofen to renal tissue of rat correlates with excretion of its acyl glucuronide

1. Dosing rat with suprofen produces suprofen equivalents that are covalently bound to plasma and tissue proteins in vivo. 2. Suprofen acyl glucuronide is reactive in vitro, resulting in suprofen equivalents covalently bound to proteins of plasma and tissues in a time-dependent manner. 3. Bile duct ligation of rat increases exposure to suprofen acyl glucuronide in vivo, which leads to enhanced covalent binding of suprofen equivalents to plasma proteins and to kidney tissue. 4. Covalent binding of suprofen equivalents to kidney tissue correlates with excretion of suprofen and suprofen glucuronide by the kidney.

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.

Stereoselective binding of the glucuronide of ketoprofen enantiomers to human serum albumin

Since acyl glucuronides are known to undergo deconjugation, especially in the presence of human serum albumin (HSA), only a few reports have described their reversible binding to plasma proteins. The aim of this study was to investigate the reversible binding of R and S ketoprofen glucuronides to HSA by a rapid technique, such as ultraviolet circular dichroism. Binding of R ketoprofen glucuronide only induced an extrinsic Cotton effect at 340 nm. Scatchard plot analysis revealed that R ketoprofen and its glucuronide are bound to one site of albumin with an association constant of 28.1 x 10(4) and 6.1 x 10(4) M-1, respectively. Modification of one tyrosine residue by diisopropylfluorophosphate prevented the access of ligands to sites I and II of albumin, and also fully inhibited the binding of R ketoprofen and that of its conjugate. Displacement experiments with specific probes of albumin binding sites suggested that R ketoprofen and the glucuronide are bound to site II rather than site I. However, R ketoprofen was not displaced by its conjugate. S ketoprofen glucuronide is also bound to HSA, since it decreased the binding of the antipode conjugate. However, the binding of this metabolite to albumin did not induce an extrinsic Cotton effect large enough to determine the binding constants. D-Glucuronic acid did not bind to sites I or II of albumin. This moiety is likely responsible for the lower affinity of HSA for the R ketoprofen glucuronide when compared to that for R ketoprofen, due to the hydrophilicity and/or the bulkiness of this group.

Disposition of the novel antitumour agent xanthenone-4-acetic acid in the mouse: identification of metabolites and routes of elimination

1. Xanthenone-4-acetic acid (XAA) is an experimental antitumour agent which resembles flavone-8-acetic acid in its induction of cytokine synthesis, nitric oxide production and tumour haemorrhagic necrosis. We have investigated the excretion and metabolic fate of XAA in the BDF1 mouse. 2. XAA was administered intravenously at the maximal tolerated dose (1090 mumol/kg). Urine, plasma and bile were collected and subjected to analysis by hplc. Urine samples demonstrated labile metabolites which released XAA following incubation with beta-glucuronidase/sulphatase or at pH 9.0. The structures of isolated XAA metabolites were characterized by ms or 1H-NMR spectra at 400 MHz. 3. The major metabolite pathway of XAA involves conjugation with glucuronic acid, since the resulting metabolite, XAA acyl glucuronide, accounts for 25% of the dose excreted in the urine. Other metabolite pathways include alpha-oxidation of the acetic acid side chain and aromatic hydroxylation of the xanthenone ring.

Irreversible binding of tolmetin to macromolecules via its glucuronide: binding to blood constituents, tissue homogenates and subcellular fractions in vitro

1. The degradation of tolmetin glucuronide (TG) in biological fluids and tissue homogenates appears to follow first-order kinetics and is quite rapid in plasma. TG degradation was minimized upon the addition of phenylmethylsulphonyl fluoride (PMSF) and 1,4-saccharolactone, suggesting that the majority of the degradation may be enzymatic, rather than chemical hydrolysis. 2. Irreversible binding via TG was detected in all tissue preparations examined. Upon addition of an inhibitor of esterases (PMSF) to human serum albumin (HSA) and plasma, binding was extensive (2.5%) and the extent of binding was both time- and pH-dependent. Similar extents of binding were obtained with most tissue homogenates, except for spleen and intestine which exhibited much lower binding. 3. Incubation of TG with microsomal protein from sheep and rat yielded no significant differences. Incubations of tolmetin (T) and TG with microsomes, as well as tissue homogenates, indicates that irreversible binding occurs only in the presence of TG. 4. Irreversible binding occurred in all of the blood constituents, the highest extent with haemolyzed erythrocytes. The extent of binding was 15 times higher in disrupted versus intact red blood cells, suggesting a correlation between the extent of binding and the overall exposure of TG to the macromolecules to which it may bind irreversibly.

Determination of furosemide with its acyl glucuronide in human plasma and urine by means of direct gradient high-performance liquid chromatographic analysis with fluorescence detection. Preliminary pharmacokinetics and effect of probenecid

Furosemide is metabolized in humans by acyl glucuronidation to the 1-O-glucuronide (Fgluc). Furosemide (F) and the conjugate can be measured directly by gradient high-performance liquid chromatographic analysis without enzymic deglucuronidation. The glucuronide conjugate was isolated by preparative HPLC from human urine samples. Furosemide and its acyl glucuronide were present in plasma. No isoglucuronides were present in acidic urine of a volunteer. Calibration curves were constructed by enzymic deconjugation of samples containing different concentrations of isolated F-acyl glucuronide. The limit of quantitation of F in plasma is 0.007 microgram/ml, Fgluc 0.010 microgram/ml. The limits of quantitation in urine are respectively: F 0.10 microgram/ml, Fgluc 0.15 microgram/ml. A pharmacokinetic profile of furosemide is shown, and some preliminary pharmacokinetic parameters of furosemide obtained from one human volunteer are given. Probenecid does not inhibit the formation of the acyl glucuronide of F, but inhibits the renal clearance of both compounds.

Probenecid inhibits the glucuronidation of indomethacin and O-desmethylindomethacin in humans. A pilot experiment

Indomethacin is metabolized in humans by O-demethylation, and by acyl glucuronidation to the 1-O-glucuronide. Indomethacin, its metabolite, and their conjugates can be measured directly by gradient high-pressure liquid chromatographic analysis without enzymic deglucuronidation. The pharmacokinetic profile of indomethacin and some preliminary pharmacokinetic parameters of indomethacin obtained from one human volunteer are given. In plasma only the parent drug indomethacin is present, while in urine the acyl and ether glucuronides are present in high concentrations. This confirms other reports that indomethacin and O-desmethylindomethacin may be glucuronidated in the kidney. Probenecid is a known substrate for renal glucuronidation. If indomethacin is glucuronidated in the human kidney like probenecid, then this glucuronidation might be reduced or inhibited under probenecid co-medication. This pilot experiment shows that probenecid reduced the acyl glucuronidation of indomethacin by 50% and completely inhibited the formation of O-desmethylindomethacin acyl and ether glucuronide.

Direct gradient reversed-phase high-performance liquid chromatographic determination of salicylic acid, with the corresponding glycine and glucuronide conjugates in human plasma and urine

A gradient reversed-phase HPLC analysis for the direct measurement of salicylic acid (SA) with the corresponding glycine and glucuronide conjugates in plasma and urine of humans was developed. The glucuronides were isolated by preparative HPLC from human urine samples. The concentration of the glucuronides in the isolated fraction were determined after enzymatic hydrolysis. Salicylic acid acyl glucuronide (SAAG) was not present in plasma. No isoglucuronides were present in acidic and alkaline urine of the volunteer. The limits of quantitation in plasma are: SA 0.2 microgram/ml, salicyluric acid (SU) 0.1 microgram/ml, salicylic acid phenolic glucuronide (SAPG) 0.4 microgram/ml and salicyluric acid phenolic glucuronide (SUPG) 0.2 microgram/ml. The limit of quantitation in urine is for all compounds 5 micrograms/ml. Salicylic acid acyl glucuronide is stable in phosphate buffer pH 4.9 during 8 h at 37 degrees C; thereafter it declines to 80% after 24 h. The subject's urine was therefore acidified by the oral intake of 4 x 1.2 g of ammonium chloride/day. With acidic urine, hardly any salicylic acid is excreted unchanged (0.6%). It is predominantly excreted as salicyluric acid (68.7%).

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).

Variables affecting nicotine metabolism

Nicotine metabolism is exceedingly sensitive to perturbation by numerous host factors. To reduce the large variations and discrepancies in the literature pertaining to nicotine metabolism, investigators in future studies need to recognize and better control these host factors. Recent advances in the understanding of nicotine metabolism have suggested new approaches to elucidating underlying mechanisms of certain toxic effects associated with cigarette smoking.