Development of a simple liquid chromatographic method for the separation of mixtures of positional isomers and anomers of synthetic 2-, 3- and 4-fluorobenzoic acid glucuronides formed via acyl migration reactions

Many drugs containing carboxylate groups form beta-1-O-acyl glucuronides as their major phase II metabolites in vivo. These ester glucuronides are potentially reactive due to the susceptibility of the acyl group to nucleophilic reactions resulting in hydrolysis, acyl migration or covalent adduct formation. In the present study, a number of synthetic fluorobenzoic acid glucuronide conjugates were chosen as models for chromatographic studies. A high-performance liquid chromatography method is presented for the simultaneous determination of the 1-, 2-, 3- and 4-positional isomers of the acyl glucuronides, and their alpha- and beta-anomers for the 2-, 3- and 4-fluorobenzoic acids as well as each aglycone formed as a result of hydrolysis. The same elution order was found for the acyl migrated glucuronide isomers of the three fluorobenzoic acids in their equilibrium mixtures. The alpha-4-O-acyl isomer eluted first followed by the beta-4-O-acyl isomer, then the beta-1-O-acyl, the beta-3-O-acyl, the alpha-3-O-acyl, the alpha-2-O-acyl and finally the beta-2-O-acyl isomer eluted. The method was used to determine the overall degradation rates, the acyl migration rates and the hydrolysis rates of 1-O-(2-fluorobenzoyl)-beta-D-glucopyranuronic acid 1-O-(3-fluorobenzoyl)-beta-D-glucopyranuronic acid and 1-O-(4-flurobenzoyl)-beta-D-glucopyranuronic acid in a buffer system pH 7.4 at 25 degrees C. It was found that the order of beta-1-glucuronide acyl migration rates was 2-fluorobenzoyl > 3-fluorobenzoyl > 4-fluorobenzoyl. Both the acyl migration rates and the elution order were interpreted in terms of electronic effect of the fluorine substituent on the carbonyl carbon.

Direct characterization of drug glucuronide isomers in human urine by HPLC-NMR spectroscopy: application to the positional isomers of 6,11-dihydro-11-oxodibenz[b,e]oxepin-2-acetic acid glucuronide

In this work, 400 and 600 MHz 1H HPLC-NMR spectroscopic methods were developed and applied to separate and identify the positional glucuronide isomers and anomers of the model nonsteroidal antiinflammatory drug, 6,11-dihydro-11-oxodibenz[b,e]oxepin-2-acetic acid, in whole human urine. The HPLC methods utilized either an isocratic system, comprising 30% acetonitrile in water at pH 2.5, or a gradient elution system increasing from 30% to 60% acetonitrile, in order to achieve improved separation of the 2-, 3-, and 4-O-acylglucuronide isomers from the faster eluting endogenous urinary metabolites. Directly coupled stop-flow 1H HPLC-NMR spectroscopic measurements were made at the retention times indicated by the UV-monitored chromatographic peaks. The glucuronide isomers were identified from the 1H NMR spectra on the basis of their chemical shifts and spin-spin coupling patterns. The elution order was 4-O-acyl-, 3-O-acyl-, and finally 2-O-acylglucuronide, with tR values of 10.04, 11.68, and 12.64 min, respectively. Although the alpha- and beta-anomers of each of the positional isomers could not be separated in these solvent systems, they could be identified in the individual 1H NMR spectra. This work shows for the first time that directly coupled HPLC-NMR spectroscopy can be used directly to isolate and characterize acyl-migrated isomers of drug glucuronides in whole urine. This approach will be of value in the study of glucuronide acyl migration reactions of nonsteroidal antiinflammatory drugs and other xenobiotic ester glucuronides in whole biofluids.

Thin-layer chromatography: a neglected technique

Thin-layer (or planar) chromatography (TLC) is critically reviewed from the point of view of drug analysis in biological fluids. The capabilities of the various techniques of TLC are described and their advantages and disadvantages are discussed. An example of the use of high-performance TLC with scanning densitometry for the quantitative determination of antipyrine in human plasma is provided. The use of TLC-mass spectrometry and TLC-tandem mass spectrometry, directly from the sorbent, in the identification of the compounds separated by TLC is discussed.

Nuclear magnetic resonance and high-performance liquid chromatography-nuclear magnetic resonance studies on the toxicity and metabolism of ifosfamide

A combination of high-resolution nuclear magnetic resonance (NMR) and high-performance liquid chromatography (HPLC)-NMR spectroscopic methods has been used to analyse urine from humans and rats treated with the anticancer drug ifosfamide. It was possible to detect a range of abnormal endogenous metabolites in urine after ifosfamide administration to human subjects undergoing cancer therapy and to relate the metabolic perturbations to the nephrotoxic effects of the drug. Changes observed by 1H NMR included increases in levels of urinary glucose, glycine, alanine, histidine, lactate, acetate, succinate, and trimethylamine-N-oxide and decreases in the levels of hippurate and citrate. Additional evidence was gained that ifosfamide-induced nephrotoxicity might be related to the level of oxidation of the coadministered drug mesna. By using both directly coupled continuous-flow 31P HPLC-NMR spectroscopy to determine the retention times of the phosphorus-containing metabolites and, subsequently, stop-flow 1H HPLC-NMR of the urine, it was possible to isolate and identify on-line the metabolites ifosfamide mustard, 4-hydroxy-ifosfamide, 2-dechloroethylifosfamide, and the parent compound itself. These studies illustrate the potential of combining 1H NMR spectroscopy of biofluids and HPLC-NMR spectroscopy for the investigation of drug metabolism and toxicity in humans and animals.

1H and 19F-nmr spectroscopic studies on the metabolism and urinary excretion of mono- and disubstituted phenols in the rat

1. 1H and 19F-nmr spectroscopy was used to investigate quantitatively the urinary excretion of the metabolites of 15 substituted phenols in the rat. The compounds studied were: 2-, 3-, and 4-fluorophenols; 2-, 3-, and 4-trifluoromethylphenol; 2,4-, 2,6- and 3,4-difluorophenol; 2-fluoro-5-trifluoromethylphenol, 3-fluoro-5-trifluoromethylphenol, 2-trifluoromethyl-4-fluorophenol; 3-chloro-4-fluorophenol, 3-fluoro-4-chlorophenol, and 3-methyl-4-fluorophenol. All compounds were dosed to the Sprague-Dawley rat (10 mg/kg i.p.) and urine was collected over the periods 0-8, 8-24 and 24-48 h post-dosing and analyzed using nmr spectroscopy. 2. The compounds were excreted in the urine mainly as glucuronide or sulphate conjugates or as the unchanged parent compound. There was considerable variation in the urinary excretion of the compounds over 48 h ranging from 22.1 to 93.6% of the dose. There was no apparent relationship between the molecular weight of compounds or their metabolites and the percentage molar recovery of each in the urine. 3. Ortho-substituted phenols in general showed a greater propensity for glucuronidation than did either meta- or para-substituted compounds, irrespective of the substituent group. The molar glucuronide-to-sulphate ratio for ortho-substituted compounds was found to be 2.2 +/- 0.9 whereas the ratio for both meta- and para-substituted compounds was 0.8 +/- 0.2 (p < 0.0001). 4. There were characteristic substituent effects of phenolic glucuronidation or sulphation on the 19F-nmr chemical shifts for both F- and CF3-substituted phemols and these substituent effects were a useful aid to metabolite signal assignment. 5. These studies show that nmr spectroscopy provides a rapid and convenient approach to the construction of metabolic databases of simple xenobiotics for the investigation of structure-metabolism relationships.

An investigation of the properties of a "shielded" phase for the solid phase extraction of acidic and basic compounds

The extraction properties of Hisep, a novel bonded phase for solid phase extraction, have been investigated using a number of acidic and basic analytes and compared with those of a conventional ODS bonded material. Marked differences in extraction properties for the test analytes propranolol, anisic acid and ICI 128436 from aqueous buffer were noted between the two materials. Thus, extraction onto the ODS material was by both ionic and reversed-phase mechanisms whilst retention on the Hisep phase appeared to be mediated predominantly by reversed-phase hydrophobic interactions. The presence of dog plasma had no effect on the extraction of propranolol but did reduce the extraction efficiencies observed for anisic acid and ICI 128436 in a pH dependent manner.

Quantitative structure-metabolism relationships for substituted benzoic acids in the rabbit: prediction of urinary excretion of glycine and glucuronide conjugates

1. Quantitative relationships between molecular physicochemical properties of 24 substituted benzoic acids and their metabolic fate in the rabbit have been investigated using computational chemistry and multivariate statistical methods. 2. A total of 34 molecular properties were calculated for each compound using computational chemistry and were related statistically to the % molar recovery of glycine and glucuronide conjugates excreted in the urine of rabbits for the 24 compounds. 3. Compounds were successfully classified according to their dominant metabolic fate based on urinary excretion data, where stepwise linear regression analysis of the theoretical property data achieved good predictive fits for observed versus predicted % molar urinary recovery as glucuronide conjugates (r2 = 0.79) and % molar urinary recovery as glycine conjugates (r2 = 0.66). 4. Quantitative prediction of the urinary excretion of glucuronide and glycine conjugates of the parent compounds was achieved based on a statistical model using calculated molecular physicochemical parameters. Interpretation of the molecular properties, which are important for predicting the metabolic class, should give new insights into basic mechanisms of drug metabolism and underlying molecular recognition events that determine disposition and metabolism.

750 MHz HPLC-NMR spectroscopic studies on the separation and characterization of the positional isomers of the glucuronides of 6,11-dihydro-11-oxodibenz[b,e]oxepin-2-acetic acid

Ester glucuronides (beta-1-O-acyl-D-glucopyranuronates) of many drugs can undergo a series of acyl migration reactions, resulting in positional isomers and anomers which can react with serum proteins with possible toxicological consequences. We have investigated the acyl migration of the ester glucuronides of the model drug 6,-11-dihydro-11-oxodibenz[b,e]oxepin-2-acetic acid in pH 7.4 buffer using directly coupled 750 MHz stopped-flow HPLC-NMR spectroscopy. Using a reversed phase isocratic HPLC method with 21% acetonitrile and 79% D2O in the mobile phase, it was possible to separate and hence identify the individual positional isomers of the model drug glucuronide by 750 MHz HPLC-NMR. The order of elution of the isomers from the C18 column was 4alpha-, 4beta-, aglycon, 1beta-, 3beta-, 3alpha-, 2alpha-, 2beta- (alpha- and beta- referring to the anomerization state at C1 on the glucuronide ring and the numbers referring to the carbon number on the glucuronide ring to which the drug moiety has migrated). It is shown that directly coupled ultra-high-field HPLC-NMR spectroscopy offers a unique analytical advantage for obtaining structural information of interconverting compounds in equilibrium mixtures, and this method will be of value in the study of reactive drug glucuronides of toxicological importance.

The detection and characterization of analgesics and anti-inflammatory drugs on high performance thin-layer chromatography plates using tandem mass spectrometry: application to drugs and metabolites in urine

The application of tandem mass spectrometry to the analysis and identification of analgesics and non-steroidal anti-inflammatory drugs such as paracetamol, ibuprofen and indomethacin following thin-layer chromatography (TLC) is described. TLC was combined successfully with mass spectrometry and with tandem mass spectrometry using silica gel and diol-bonded silica gel high performance TLC plates. The diol-bonded phase was found to be superior for use with biological samples and enabled the identification of paracetamol, ibuprofen and salicylhippuric acid (the major metabolite of acetylsalicylic acid) in human urine extracts following normal therapeutic doses.

Prediction of urinary sulphate and glucuronide conjugate excretion for substituted phenols in the rat using quantitative structure-metabolism relationships

1. The quantitative urinary excretion of the sulphate and glucuronide metabolites of 15 substituted phenols dosed to rat has been determined using high resolution 19F-nmr spectroscopy. 2. The urinary metabolic fate of each of the compounds was related to a series of calculated physicochemical properties for each compound to produce quantitative structure-metabolism relationships (QSMRs). Using these calculated molecular properties it was possible to predict the urinary recovery of xenobiotic material as a percentage of the administered dose, to classify the compounds according to their 'dominant' metabolite pattern and to predict quantitatively the proportions of glucuronide and sulphate conjugates in the urine by the use of multiple linear regression. 3. The quantitative predictions were tested by cross-validation and good prediction of total xenobiotic urinary recovery as a percentage of the administered dose was achieved based on an equation involving the electrophilic superdelocalizability at C4 (para to the hydroxyl function), the smallest principal ellipsoid axis dimension and the heat of formation. The largest moment of inertia and the electrophilic superdelocalizability at C3 were found to be the most significant factors for the prediction of the percentage glucuronide in the urine, and the urinary excretion of sulphate conjugates as a percentage of total urinary recovery was negatively correlated with the glucuronide excretion as little parent compound was excreted.

Solid state NMR and extraction studies on "phenyl"-bonded stationary phases used for solid phase extraction

The solid phase extraction (SPE) and elution of [14C]-propranolol from aqueous buffer has been studied for a range of phenyl-bonded SPE materials. Differences were noted in the recovery of the analyte using methanol-water eluents depending upon the manufacturer and whether or not phase had been endcapped. Efficient recoveries of [14C]-propranolol were only achieved when triethylamine was added to the eluting solvent as a competing base. Solid state cross polarisation/magic angle spinning (CP/MAS) NMR spectroscopy was used to characterise the phases further, which revealed differences in endcapping between materials as well as differences in the type and extent of cross-linking.

Assignment of the 750 MHz 1H NMR resonances from a mixture of transacylated ester glucuronic acid conjugates with the aid of oversampling and digital filtering during acquisition

Many drugs containing carboxylic acid functional groups are metabolised in vivo to ester glucuronides (1-O-acyl-beta-D-glucopyranuronates) and, of these, a number show a propensity to undergo internal isomerisation via a transacylation process, causing the carboxylic acid moiety to migrate successively to the 2-, 3- and 4-positions of the glucuronic acid. These products may be responsible, through reactions with plasma proteins, for some of the allergenic side effects in a number of non-steroidal anti-inflammatory drugs. It is important to understand those properties of the drug molecules which facilitate this reaction, and to this end we have studied the transacylation product formation and reaction kinetics in a series of aryl carboxylic acid glucuronides using NMR spectroscopy. However, the resulting 1H NMR spectra are very complex with much resonance overlap, and recourse to spectral simplification processes is necessary. Here, improvement in spectral resolution by oversampling and digital filtering to restrict the detection range of the spectrometer, thus yielding improved digital resolution, is demonstrated. The approach has been applied to the assignment of a mixture of transacylated ester glucuronides of 2-trifluoromethylbenzoic acid through the use of a two-dimensional 1H-1H TOCSY experiment.

1H and 2H NMR spectroscopic studies on the metabolism and biochemical effects of 2-bromoethanamine in the rat

Male Fischer 344 rats were dosed with 2-bromoethanamine hydrobromide (BEA, N = 6) or [1,2,2,-2H4]-bromoethanamine hydrobromide (BEA-d4, N = 6) at 150 mg/kg i.p. and urine was collected -24 to 0 hr pre-dose and at 0-2 hr, 2-4 hr, 4-8 hr and 8-12 hr post-dose (p.d.). Urine samples were analysed directly using 500 and 600 MHz 1H NMR and 92.1 MHz 2H NMR spectroscopy. The major observed effect of BEA treatment was the induction of transient elevations in urinary glutaric acid (GTA) and adipic acid (ADA) excretion lasting up to 24 hr p.d. Most of the GTA was excreted in the 0-8 hr p.d. with maximal rates of 100-120 microM/hr for each rat occurring between 4 and 8 hr p.d. in animals treated with BEA or BEA-d4. GTA and ADA were shown to be of endogenous origin as there was no detectable incorporation of the 2H label into either compound following treatment of rats with BEA-d4. Following BEA-treatment there was an initial decrease in the levels of urinary citrate, succinate, 2-oxoglutarate and trimethylamine-N-oxide. A subsequent recovery of citrate and succinate was noted following the onset of medullary nephropathy. The abnormal urinary metabolite profiles were similar to that observed in the urine of humans with glutaric aciduria type II (an inborn error of metabolism) caused by a lack of mitochondrial fatty acyl coenzyme A dehydrogenases indicating that BEA or its metabolites have similar metabolic consequences. The BEA metabolite aziridine was detected by 1H and 2H NMR spectroscopy of the urine 8 hr p.d. together with BEA itself and two novel metabolites 2-oxazolidone (OX) and 5-hydroxy-2-oxazolidone (HOX). The formation of OX requires the reaction of BEA with endogenous bicarbonate followed by a cyclisation reaction eliminating HBr. Dosing rats with authentic OX resulted in the excretion of HOX but did not cause glutaric or adipic aciduria indicating that either aziridine or BEA itself was responsible for the presumed defect in mitochondrial metabolism.

High resolution NMR spectroscopic studies on the metabolism and futile deacetylation of 4-hydroxyacetanilide (paracetamol) in the rat

Paracetamol (4-hydroxyacetanilide, acetaminophen) was synthesized with the acetyl group labelled with C2H3 (paracetamol-C2H3), and dosed to rats i.p. at 25 mg/kg (N = 5) and 40 mg/kg (N = 3) body weight. Paracetamol, with a 13CH3 in the acetyl group (paracetamol-13CH3) was also synthesized and dosed to rats i.p. at 40 mg/kg (N = 3). The metabolism and excretion of the 2H-labelled compound was followed in the rat using 600 MHz 1H and 92.1 MHz 2H NMR spectroscopy of urine collected 0-8, 8-24, 24-32 and 32-48 hr post-dosing. The metabolism of paracetamol-13CH3 was also monitored using 600 MHz 1H NMR spectroscopy of urine collected 0-8, 8-24 and 24-48 hr post-dosing. For paracetamol-C2H3 the total recovery of the sulphate, glucuronide and N-acetyl cysteinyl metabolites via the urine accounted for 61.2 +/- 14.1% of the 25 mg/kg dose and 61.4 +/- 8.8% of the 40 mg/kg dose. For paracetamol-13CH3 the recovery was 102.7 +/- 3.7% indicating that the low % urinary recovery with the C2H3-labelled drug is the result of isotope effects on the disposition of paracetamol. In the case of the paracetamol-C2H3, quantitative 1H NMR analysis of urine showed that 13.3 +/- 0.5 and 10.0 +/- 1.2 mole % (25 and 40 mg/kg, respectively) of the urinary paracetamol sulphate recovered following dosing of the deuterium labelled drug had the C2H3 acetyl groups replaced by C1H3 acetyl groups from endogenous sources. In the case of the paracetamol-13CH3 8.9 +/- 0.7 mole % of the sulphate conjugate had also been transacetylated to paracetamol-12CH3. There was no significant difference between the level of futile deacetylation observed for the deuterated and 13C-labelled drug. Overall these data indicate a high level of deacetylation followed by reacetylation (i.e. futile deacetylation) prior to excretion of paracetamol via the nephrotoxic intermediate 4-aminophenol. The level of deacetylation is much higher than has previously been thought which may cast new light on the role of 4-aminophenol in the development of paracetamol induced nephrotoxicity.

Evaluation of liquid chromatography coupled with high-field 1H NMR spectroscopy for drug metabolite detection and characterization: the identification of paracetamol metabolites in urine and bile

The applicability of coupled reversed-phase high performance liquid chromatography (HPLC)-NMR spectroscopy for the detection and identification of paracetamol (N-(4-hydroxyphenyl)acetamide) and its sulfate, glucuronide and N-acetylcysteinyl metabolites in the unprocessed biological fluids, human urine, rat urine and rat bile, is investigated. Analysis of these samples was performed by gradient HPLC elution and directly coupled 500 MHz 1H NMR spectroscopy detection using a combination of one- and two-dimensional NMR methods in stopped-flow mode. The stopped-flow approach is demonstrated to be an efficient technique for identification of drug metabolites which have, for example, a UV-chromophore. Stopped-flow HPLC analysis with NMR detection is a viable technique and halting the chromatographic process several times during a run has a negligible effect on the separation and NMR characterization. The post-acquisition data processing method of 'quantified maximum entropy' is shown to provide a means of improving the quality of spectra for minor components, thus aiding NMR resonance assignments.

Packed-column supercritical fluid chromatography of beta-blockers

Packed-column subcritical and supercritical fluid chromatography (SFC) of a total of 10 beta-blocking drugs was investigated on four stationary phases using CO(2)-methanol based eluents. The stationary phases studied were all bonded silicas and included Diol, aminopropyl, cyanopropyl, and "C1". The SFC of some of these compounds was possible with simple CO(2)-methanol based mobile phases, but in general, it proved to be necessary to add triethylamine as a competing base in order to obtain acceptable peak shapes. The best chromatographic results were obtained on the aminopropyl bonded phase, with good results also seen on the Diol phase. Different selectivities were observed for the Diol and aminopropyl columns.

Increasing the pool of minority providers

The University of Arkansas has trained African-American medical students for 46 years. Today the school continues to place a high premium on recruiting and retaining minority medical students. Recruitment efforts involve comprehensive programs to target junior high schools in African-American communities throughout Arkansas, and initiatives to attract African-American faculty and residents. Through such retention-oriented activities as lenient first-year grade requirements, study groups, and a board-preparation course, African-American students have scored above the national average on Part I of the National Medical Board Examination despite matriculating in medical school with below-average scores on the Medical College Admission Test.

Coupling of HPLC with 19F- and 1H-NMR spectroscopy to investigate the human urinary excretion of flurbiprofen metabolites

Results of an on-line HPLC-NMR analysis of human urine from a volunteer administered the anti-inflammatory drug flurbiprofen are reported. The two major human urinary metabolites, namely the glucuronides of flurbiprofen and of 4'-hydroxyflurbiprofen, have been identified using 1H- and 19F-NMR spectroscopy. In vivo conjugation of the racemic drug and its metabolites with D-glucuronic acid results in diastereomeric molecules which give resolved NMR spectra thereby permitting the diastereomeric proportions to be evaluated. The cause of the observed deviation from equal proportions is discussed. This study represents the first use of both 19F- and 600 MHz 1H-NMR spectroscopy coupled to HPLC.