Acetylator phenotyping via analysis of four caffeine metabolites in human urine by micellar electrokinetic capillary chromatography with multiwavelength detection

NAT2 and CYP1A2 phenotyping with caffeine: head-to-head comparison of AFMU vs. AAMU in the urine metabolite ratios

AIMS

(i) To compare the phenotyping of healthy subjects for NAT2 and CYP1A2 activities with caffeine, by the simultaneous assay of the urinary metabolites AFMU and AAMU, and (ii) to ascertain whether NAT2 and CYP1A2 phenotyping is influenced by the use of AFMU or AAMU in the metabolite ratio.

METHODS

Thirty-five healthy subjects (16 men, 19 women) participated to the study. Caffeine metabolite concentrations were measured in urine collected 8 h after 2.5 mg kg-1 caffeine intake using a new validated h.p.l.c. method. The metabolite ratios AFMU/1X, AFMU/(AFMU+1X+1U), AAMU/1X, AAMU/(AAMU+1X+1 U), and (AFMU+1U+1X)/17U, (AAMU+1U+1X)/17U were determined as indices of NAT2 and CYP1A2 activity, respectively.

RESULTS

Slow and rapid acetylators were similarly identified using the four NAT2 metabolite ratios in 139 out of 140 measurements. An appreciable amount of AAMU was present in urine that was immediately acidified and analysed. Consequently, the ratio using AFMU was lower than that using total AAMU following transformation of AFMU in basic conditions. The proportion of AFMU in urine analysed immediately expressed as AFMU/(AFMU+AAMU) ratio did not correlate with urine pH, but was a function of the acetylation phenotype, with a low intergroup variability (64 +/- 3% and 32 +/- 5%, for rapid and slow acetylators, respectively; P < 0.00001, anova). Regarding CYP1A2 activity, a good correlation (r = 0.99) was observed between the metabolite ratios calculated from AFMU and AAMU, although the ratios calculated from AFMU were proportionately and systematically lower P < 0.00001, paired t-test, slope 1.2).

CONCLUSIONS

This study demonstrates that both AFMU and AAMU can be used for NAT2 and CYP1A2 metabolite ratio determinations. The reported conversion of AFMU into AAMU is unlikely to explain the large amount of AAMU in urine that was acidified and analysed immediately after voiding. The results suggest that AAMU is formed not solely through a nonenzymatic hydrolysis in urine, but in vivo by a NAT2 phenotype-dependent pathway.

Determination of N-acetylation phenotype using caffeine as a metabolic probe and high-performance liquid chromatography with either ultraviolet detection or electrospray mass spectrometry

A rapid, sensitive method using liquid chromatography-electrospray mass spectrometry (LC-ES-MS) was developed and evaluated for the simultaneous quantitative determination of caffeine metabolites 1U, 1X and AAMU in human urine. This method involved a simple dilution of urine samples. The chromatographic separation was achieved on a C18 reversed-phase column using a gradient of acetonitrile in 2 mM, pH 3.0 ammonium formate as mobile phase. After ionisation in an electrospray source, mass spectrometric detection was performed in the negative ion, selected ion monitoring mode. This method yielded acceptable accuracy and precision within the range 0.25-50 microg/ml. This analytical method was applied to investigate the N-acetylator phenotype of HIV-infected patients and compared with high-performance liquid chromatography with UV detection. Its specificity was better, which appeared to be absolutely necessary to prevent errors in metabolic ratios and phenotype interpretation.

Extractionless method for the simultaneous high-performance liquid chromatographic determination of urinary caffeine metabolites for N-acetyltransferase 2, cytochrome P450 1A2 and xanthine oxidase activity assessment

Urinary metabolic ratios of caffeine are used in humans to assess the enzymatic activities of cytochrome P450 isoenzyme 1A2 (CYP1A2), xanthine oxidase (XO) and for phenotyping individuals for the bimodal N-acetyltransferase 2 (NAT2), all of them involved in the activation or detoxification of various xenobiotic compounds. Most reported analytical procedures for the measurement of the urinary metabolites of caffeine include a liquid-liquid extraction of urine samples prior to their analysis by reversed-phase HPLC. At neutral to basic pH however, 5-acetylamino-6-formylamino-3-methyluracil (AFMU), a metabolite of caffeine, spontaneously decomposes to 5-acetylamino-6-amino-3-methyluracil (AAMU). Since AAMU is not extracted in most organic solvents, the extent of AFMU decomposition cannot be precisely assessed. Although the decomposition reaction can be minimized by immediate acidification of the urine, accurate results can only be obtained when both AAMU and AFMU are monitored, or alternatively, if AAMU is measured after complete transformation of AFMU into AAMU in basic conditions. We report a liquid chromatographic method for the simultaneous quantitative analysis of the five urinary metabolites of caffeine used for the CYP1A2, XO and NAT2 phenotyping studies: AAMU, AFMU, 1-methylxanthine, 1-methyluric acid and 1,7-dimethyluric acid. These metabolites are satisfactory separated from all other known caffeine metabolites as well as endogenous urinary constituents. Sample treatment does not require any liquid-liquid extraction procedure. Urine samples are diluted and centrifuged before being injected (10 microl) onto a YMC-Pack Polyamine II (250x4.6 mm) column. A step-wise gradient elution program is applied using acetonitrile-0.75% (v/v) formic acid: (91:9) at 0 min-->(75:25) at 25 min-->(65:35) at 35 min-->(65:35) at 45 min, followed by a re-equilibration step to the initial solvent composition. The flow-rate is 1.0 ml/min and the separations are monitored by UV absorbance at 260 and 280 nm. The procedure described here represents a substantial improvement over previous methods: a single analysis and a minimal urine sample treatment enables the simultaneous quantitation of five caffeine metabolites, notably AFMU and AAMU, used for the determination of CYP450 1A2, XO and NAT2 enzyme activity. Importantly enough, phenotyping individuals for the bimodal NAT2 is made possible without the uncertainty associated with the deformylation of AFMU, which is likely to happen at all steps prior to the analysis, during sample storage and even in the bladder of the subjects.

Determination of illicit and/or abused drugs and compounds of forensic interest in biosamples by capillary electrophoretic/electrokinetic methods

The application of capillary electrophoresis (CE) methods in forensic toxicology for the determination of illicit and/or misused drugs in biological samples is reviewed in the present paper. Sample pretreatments and direct injection modes used in CE for analysis of drugs in biological fluids are briefly described. Besides, applications of separation methods based on capillary zone electrophoresis or micellar electrokinetic chromatography with UV absorbance detection to (i) analysis of drugs of abuse, (ii) analysis of other drugs and toxicants of potential forensic interest and (iii) for metabolism studies are reviewed. Also, alternative CE methods are briefly discussed, including capillary isotachophoresis and separation on mixed polymer networks. High sensitivity detection methods used for forensic drug analysis in biological samples are then presented, particularly those based on laser induced fluorescence. A glimpse of the first examples of application of CE-mass spectrometry in forensic toxicology is finally given.

Determination of p-aminosalicylic acid and its N-acetylated metabolite in human urine by capillary zone electrophoresis as a measure of in vivo N-acetyltransferase 1 activity

A capillary zone electrophoresis method has been developed for the determination of p-aminosalicylic acid (PAS) and its metabolite, N-acetyl-p-aminosalicylic acid (N-acetyl-PAS), in urine. A linear relationship was observed between time-normalized peak area and the concentration of the parent and metabolite with correlation coefficients greater than 0.9990. The method could be applied to the determination of PAS and N-acetyl-PAS in human urine without any sample pretreatment. A good separation of the analytes is achieved in a run time of 12 min (15 min total, including capillary wash). Using PAS as a probe for N-acetyltransferase 1 activity, 20 healthy volunteers were phenotyped after oral administration of a 1 g dose. The preliminary results seem to indicate a bimodal distribution of N-acetyl-PAS/PAS molar ratios.

Pharmaceutical applications of micelles in chromatography and electrophoresis

This review surveys the use of micelles as separation media in chromatography and electrophoresis. Applications to pharmaceuticals whose molecular masses are relatively small are focused on in this review. In high-performance liquid chromatography (HPLC), chromatography using micelles and reversed-phase stationary phases such as octadecylsilylized silica gel (ODS) columns is known as micellar liquid chromatography (MLC). The main application of MLC to pharmaceutical analysis is the same as in ion-pair chromatography using alkylsulfonate or tetraalkylammonium. In most cases, selectivity is much improved compared with other short alkyl chain ion-pairing agents such as pentanesulfonate or octanesulfonate. Direct plasma/serum injection can be successful in MLC. Separation of small ions is also successful by using gel filtration columns and micellar solutions. In electrophoresis, especially capillary electrophoresis (CE), micelles are used as pseudo-stationary phases in capillary zone electrophoresis (CZE). This mode is called micellar electrokinetic chromatography (MEKC). Most of the drug analysis can be performed by using the MEKC mode because of its wide applicability. Enantiomer separation, separation of amino acids and closely related peptides, separation of very complex mixtures, determination of drugs in biological samples etc. as well as separation of electrically neutral drugs can be successfully achieved by MEKC. Microemulsion electrokinetic chromatography (MEEKC), in which surfactants are also used in forming the microemulsion, is successful for the separation of electrically neutral drugs as in MEKC. This review mainly describes the typical applications of MLC and MEKC for the analysis of pharmaceuticals.

Micelles as pseudo-stationary phases in micellar electrokinetic chromatography

This review article describes some general comments on micellar electrokinetic chromatography (MEKC) from the viewpoint of pseudo-stationary phases and presents a compiled list of surfactants used for MEKC, prepared from published papers. We tried to give comments on some typical surfactants from the practical point of view.

Capillary electrophoresis in clinical chemistry

Since its introduction, capillary electrophoresis has diversified, spreading out into different specialized fields covering solutions for almost any analytical questions arising in research laboratories. In the context of clinical chemistry, results must be provided at low costs and in a clinically relevant time frame; however, the attributes which have made capillary electrophoresis such a successful tool in basic research are identical to those attracting clinical laboratories: speed (more efficient, less labor-intensive), low costs (minimal buffer consumption), small sample volume (reduced blood collection volume from patient), increased selectivity (determination of multiple solutes in one run), and versatility (detection of analytes over the wide range of molecular masses and chemical composition). Nevertheless, it should be mentioned that there are still some drawbacks at this stage to be solved in the near future, such as lack of sensitivity for many clinical applications or the constraint to measure in a sequential mode. The aim of this survey is to familiarize clinical chemists, as well as chemists, with a short introduction to capillary electrophoresis, followed by chapters reviewing prominent fields of applications and the latest developments in clinical chemistry.

New approaches in clinical chemistry: on-line analyte concentration and microreaction capillary electrophoresis for the determination of drugs, metabolic intermediates, and biopolymers in biological fluids

The use of capillary electrophoresis (CE) for clinically relevant assays is attractive since it often presents many advantages over contemporary methods. The small-diameter tubing that holds the separation medium has led to the development of multicapillary instruments, and simultaneous sample analysis. Furthermore, CE is compatible with a wide range of detectors, including UV-Vis, fluorescence, laser-induced fluorescence, electrochemistry, mass spectrometry, radiometric, and more recently nuclear magnetic resonance, and laser-induced circular dichroism systems. Selection of an appropriate detector can yield highly specific analyte detection with good mass sensitivity. Another attractive feature of CE is the low consumption of sample and reagents. However, it is paradoxical that this advantage also leads to severe limitation, namely poor concentration sensitivity. Often high analyte concentrations are required in order to have injection of sufficient material for detection. In this regard, a series of devices that are broadly termed 'analyte concentrators' have been developed for analyte preconcentration on-line with the CE capillary. These devices have been used primarily for non-specific analyte preconcentration using packing material of the C18 type. Alternatively, the use of very specific antibody-containing cartridges and enzyme-immobilized microreactors have been demonstrated. In the current report, we review the likely impact of the technology of capillary electrophoresis and the role of the CE analyte concentrator-microreactor on the analysis of biomolecules, present on complex matrices, in a clinical laboratory. Specific examples of the direct analysis of physiologically-derived fluids and microdialysates are presented, and a personal view of the future of CE in the clinical environment is given.

Characterization of stereoselectivity and genetic polymorphism of the debrisoquine hydroxylation in man via analysis of urinary debrisoquine and 4-hydroxydebrisoquine by capillary electrophoresis

Using capillary zone electrophoresis with a phosphate buffer at pH 2.5 containing 50 mM heptakis-(2,3,6-tri-O-methyl)-beta-CD as chiral selector, the separation of the enantiomers of the main metabolite of debrisoquine (DEB), 4-hydroxydebrisoquine (4-OHDEB), is reported. For extraction of underivatized urinary DEB, S-4-OHDEB and R-4-OHDEB, a procedure using disposable cartridges containing a polystyrene-based polymer was developed. A few nL of the extracts were analyzed in a 60 cm fused-silica capillary of 50 microns ID and solute detection was effected at 195 nm. For all three compounds, a mean (n = 5) recovery of about 73% and a detection limit of about 150 ng/mL were noted. Data obtained with urines that were received for routine phenotyping with DEB and mephenytoin confirmed the almost exclusive formation of S-4-OHDEB. Under the described conditions, no R-4-OHDEB could be detected. With these data and those obtained employing no chiral selector in the buffer, differentiation between extensive metabolizer phenotypes (EM) and poor metabolizer phenotypes (PM) for DEB was unambiguously possible by the presence of a significant peak and no (or minor) peak for 4-OHDEB, respectively. Data obtained for ten EM subjects and five PM subjects were found to agree with those generated by the routine assay based on gas chromatography. The capillary electrophoretic assays described are simple, reproducible (relative standard deviation of peak area ratios < 3%), require no sample derivatization, consume no halogenated organic solvents, and operate with inexpensive separation columns as well as small amounts of chemicals.

Separation and quantitation of debrisoquine and 4-hydroxydebrisoquine in human urine by capillary electrophoresis and high-performance liquid chromatography

A comparative study on the use of reversed-phase high-performance liquid chromatography (RP-HPLC) and capillary electrophoresis (CE) for the determination of debrisoquine (D) and its metabolite, 4-hydroxydebrisoquine (4-HD), in human urine is presented. Four different urine pre-treatments are compared for purification of samples prior to their injection in HPLC and CE. The use of a solid-phase extraction with a C18 cartridge provides the best results for the urine sample treatment, with good recoveries, i.e., 94.5% for D and 93.4% for 4-HD, and high reproducibility, i.e., R.S.D. N = 10 values of 1.7% and 1.2%, respectively. Under our separation conditions it is shown that CE is twice as fast and provides slightly better analysis time reproducibility than HPLC for this type of sample. Both the sensitivity and peak area reproducibility are better when HPLC is used. The two techniques show good agreement when employed for determination of phenotypes for hydroxylation, which seems to corroborate the usefulness of CE for this type of study.

Capillary electrophoresis for drug analysis in body fluids

Capillary zone electrophoresis (CZE) and micellar electrokinetic capillary chromatography (MECC) represent attractive methods for the determination of drugs and metabolites in body fluids. In CZE, minute (nanoliter) quantities of samples are applied to the beginning of a fused-silica capillary filled with buffer. On application of a high-voltage DC field, charged solutes begin to separate and are swept through the capillary by the combined action of electrophoresis and electroosmotic bulk flow and are on-column detected toward the capillary end. In MECC, the buffer contains charged micelles (e.g., dodecyl sulfate micelles) and both uncharged and charged solutes separate based on differential partitioning between the micelles and the surrounding buffer and, if charged, also by differential charge effects, including electrophoresis. Based on validated MECC drug assays developed in our laboratory, key aspects of measuring drug levels by MECC, including sample preparation, solute detection and identification, quantitation, reproducibility, and quality assurance are discussed. Drug levels determined by MECC are shown to be in good agreement with those obtained by nonisotopic immunoassays and/or high-performance liquid chromatography (HPLC). Using on-column multi-wavelength detection, this technology is also well suited for toxicological drug screening and confirmation and for the exploration of drug metabolism. Compared with HPLC and gas chromatography, capillary electrophoresis has distinct advantages, including automation, small sample size, minimal sample preparation, use of very small amounts of organic solvents and inexpensive chemicals, ease of buffer change and method development, and low cost of capillary columns. Electrokinetic capillary assays are complementary to the widely employed immunoassays. The state of the art and the pros and cons of capillary electrophoresis for the determination of drugs in body fluids are discussed with the goal of encouraging newcomers to start using this emerging analytical methodology.

Micellar electrokinetic chromatography perspectives in drug analysis

Micellar electrokinetic chromatography (MEKC) has become a popular mode among several capillary electromigration techniques. Most drug analyses can be performed by using MEKC because of its wide applicability. Enantiomer separation, separation of closely related peptides and isotopic compounds, separation of very complex mixtures, determination of drugs in the biological samples, etc., can be successfully achieved by MEKC. This review surveys typical applications of MEKC analysis. Recent advances in MEKC, especially with pseudo-stationary phases, are described. Modes of electrokinetic chromatography including MEKC, a separation theory of MEKC and selectivity manipulation in MEKC are also briefly mentioned.

Capillary electrophoretic analyses of drugs in body fluids: sample pretreatment and methods for direct injection of biofluids

A variety of strategies for the analysis of biological samples by capillary electrophoresis (CE) are described, with particular emphasis on the determination of drugs and metabolites. Analytical methods involving extensive sample pretreatment before CE analysis are considered, as well as strategies for directly injecting untreated biofluids. The application in CE of techniques common in liquid chromatography is first described, e.g. protein precipitation, liquid-liquid extraction and solid-phase extraction. On-capillary methods of sample concentration are considered. Approaches to performing CE assays of urine and plasma, without prior sample treatment, are described. The use of both capillary zone electrophoresis and micellar electrokinetic chromatography for direct-injection assays is compared for both urine and plasma analyses, and capillary washing strategies are discussed. Finally, direct-injection microanalyses are mentioned.

Characterization of the genetic polymorphism of dihydrocodeine O-demethylation in man via analysis of urinary dihydrocodeine and dihydromorphine by micellar electrokinetic capillary chromatography

The genetic polymorphism of dihydrocodeine O-demethylation in man via analysis of urinary dihydrocodeine (DHC) and dihydromorphine (DHM) by micellar electrokinetic capillary chromatography is described. Ten healthy subjects which are known to be extensive metabolizers for debrisoquine ingested 60 mg of DHC and collected their 0-12 h urines. In these samples, about 1% of the administered DHC equivalents are shown to be excreted as DHM. Premedication of 50 mg quinidine sulfate to the same subjects is demonstrated to significantly reduce (3-4 fold) the amount of O-demethylation of DHC, a metabolic step which is thereby demonstrated to co-segregate with the hydroxylation of debrisoquine. Thus, in analogy to codeine and other substrates, extensive and poor metabolizer phenotypes for DHC can be distinguished. Using the urinary DHC/DHM metabolic ratio to characterize the extent of O-demethylation, the metabolic ratio ranges of extensive and poor metabolizers in a frequency histogram are shown to partially overlap. Thus, classification of borderline values is not unequivocal and DHC should therefore not be employed for routine pharmacogenetic screening purposes. Nevertheless, the method is valuable for metabolic research and preliminary data demonstrate that the same assay could also be used to explore the metabolism of codeine.

A competitive enzyme linked immunosorbent assay for the determination of N-acetyltransferase (NAT2) phenotypes

The ratio of 5-acetylamino-6-amino-3-methyluracil (AAMU) to 1-methylxanthine (1X) in urine samples after caffeine ingestion can be used to indicate human N-acetyltransferase (NAT2) phenotypes. In previous studies, this ratio has been determined by LC or capillary electrophoresis. The possibility that this ratio could be determined by competitive antigen enzyme linked immunosorbent assays (ELISAs) has been investigated. Polyclonal antibodies were raised in rabbits against synthetic derivatives of AAMU and 1X, and competitive antigen ELISAs were developed after isolation of the IgGs by ion-exchange chromatography. The competitive antigen ELISA correctly identified previously determined NAT2 phenotypes and gave the expected distribution of slow and fast N-acetylators within a group of 48 individuals.

Screening for hydroxylation and acetylation polymorphisms in man via simultaneous analysis of urinary metabolites of mephenytoin, dextromethorphan and caffeine by capillary electrophoretic procedures

Phenotypes for hydroxylation can be predicted by using mephenytoin and dextromethorphan as substrates, whereas phenotypes for acetylation can be determined with caffeine as probe drug. After single-dose administration of one of these drugs, of two of them simultaneously, or of the three drugs together, the major urinary metabolites (4-hydroxymephenytoin; dextrorphan, 3-methoxymorphinan, 3-hydroxymorphinan; 5-acetylamino-6-amino-3-methyluracil as decomposition product of 5-acetylamino-6-formylamino-3-methyluracil, 1-methylxanthine, respectively) of these substrates were analyzed by capillary electrophoretic techniques. No sample pretreatment other than enzymatic hydrolysis of the conjugated compounds was applied. Assays based on micellar electrokinetic capillary chromatography are shown to allow simultaneous and unambiguous phenotyping with mephenytoin and dextromethorphan or mephenytoin and caffeine. Simultaneous screening for all three polymorphisms with a single injection of a hydrolyzed urine is shown to be possible via use of multiwavelength absorption detection only. Phenotypes determined by electrokinetic capillary techniques are shown to agree with those obtained by analysis with customary assays based on high-performance liquid chromatography.

Biomedical applications of capillary electrophoresis

After having grown through the stages of technique development and instrumentation availability, capillary electrophoresis has reached the stage of applications. This review attempts to show the potential of this technique for biomedical analysis. Rather than going into a detailed description of the technical details of the separation conditions suitable for the separation of a particular category of compounds, the focus is on the general principles and areas in which this technique can be applied and the prospects for the future. Particular emphasis is placed on the separation of complex matrices and their simplification, a daily task in biomedical laboratories. In addition, methods for the optimization of separation conditions are considered. Considerable prospects for capillary electrophoresis lie in profiling. The applicability of the technique in peptide and protein mapping is discussed in some detail. Finally, three other topics are dealt with, namely enzymic activity microassays, drug-protein binding assays and monitoring of drugs in body fluids.

Clinical and forensic applications of capillary electrophoresis

This survey is aimed at giving the readers a short overview of the present state of the art of clinical and forensic applications of capillary electrophoresis. First, the principles associated with electrokinetic capillary separations and instrumentation, sample preparation and solute quantitation are briefly discussed. This is followed by chapters describing the determination of endogenous and exogenous compounds in body fluids and tissue extracts. Finally, a survey of major achievements including reference to fully developed electrokinetic capillary assays is provided. The paper concludes with a brief outlook.

Analysis of mephenytoin, 4-hydroxymephenytoin and 4-hydroxyphenytoin enantiomers in human urine by cyclodextrin micellar electrokinetic capillary chromatography: simple determination of a hydroxylation polymorphism in man

Using cyclodextrin micellar electrokinetic capillary chromatography (CD-MECC), baseline separation of mephenytoin, 4-hydroxymephenytoin and 4-hydroxyphenytoin enantiomers in urine was effected with beta-cyclodextrin. After single-dose administration of 100 mg of racemic mephenytoin, the 0-8 h urine was collected, and enzymatically hydrolyzed urine specimens were applied. For extensive metabolizers, a single peak for 4-hydroxymephenytoin was detected corresponding to the S-enantiomer. This peak was either very small or undetectable in samples of poor metabolizers. Typically, mephenytoin could not be detected in these samples. However, application of undeglucuronidated extracts revealed the presence of free S-4-hydroxymephenytoin and R,S-mephenytoin and thus permitted phenotyping via both the urinary S:R enantiomeric ratio of mephenytoin and the hydroxylated metabolite. Application of enzymatically hydrolyzed and extracted urines after phenytoin administration (100 mg; 0-8 h urine collection) revealed the presence of S-4-hydroxyphenytoin. Thus, CD- MECC is shown to be a simple and attractive approach for (i) the confirmation of the stereoselectivity of the aromatic hydroxylation of mephenytoin and phenytoin, (ii) the simple and rapid differentiation between extensive and poor metabolizers for mephenytoin, and (iii) assessment of compliance.