Direct high pressure liquid chromatographic analysis and preliminary pharmacokinetics of enantiomers of oxazepam and temazepam with their corresponding glucuronide conjugates

Metabolic-Hydroxy and Carboxy Functionalization of Alkyl Moieties in Drug Molecules: Prediction of Structure Influence and Pharmacologic Activity

Alkyl moieties-open chain or cyclic, linear, or branched-are common in drug molecules. The hydrophobicity of alkyl moieties in drug molecules is modified by metabolic hydroxy functionalization via free-radical intermediates to give primary, secondary, or tertiary alcohols depending on the class of the substrate carbon. The hydroxymethyl groups resulting from the functionalization of methyl groups are mostly oxidized further to carboxyl groups to give carboxy metabolites. As observed from the surveyed cases in this review, hydroxy functionalization leads to loss, attenuation, or retention of pharmacologic activity with respect to the parent drug. On the other hand, carboxy functionalization leads to a loss of activity with the exception of only a few cases in which activity is retained. The exceptions are those groups in which the carboxy functionalization occurs at a position distant from a well-defined primary pharmacophore. Some hydroxy metabolites, which are equiactive with their parent drugs, have been developed into ester prodrugs while carboxy metabolites, which are equiactive to their parent drugs, have been developed into drugs as per se. In this review, we present and discuss the above state of affairs for a variety of drug classes, using selected drug members to show the effect on pharmacologic activity as well as dependence of the metabolic change on drug molecular structure. The review provides a basis for informed predictions of (i) structural features required for metabolic hydroxy and carboxy functionalization of alkyl moieties in existing or planned small drug molecules, and (ii) pharmacologic activity of the metabolites resulting from hydroxy and/or carboxy functionalization of alkyl moieties.

Enzyme-assisted synthesis and structural characterization of pure benzodiazepine glucuronide epimers

The three hydroxybenzodiazepines oxazepam, temazepam, and lorazepam used for their anxiolytic, sedative, and anticonvulsant properties are metabolized by glucuronidation, which is the predominant pathway in the clearance mechanism of exogenous and endogenous substances during phase II metabolism. The aim of this study was the synthesis of benzodiazepine-O-glucuronides as analytical reference substances. All benzodiazepines are prescribed clinically as racemic formulations. The resulting conjugates from the coupling reactions with glucuronic acid are epimeric pairs of glucuronides. Due to the importance of stereochemical factors in drug disposition it is necessary to separate the diastereomeric forms after synthesis. An enzyme-assisted synthesis was developed and optimized by using microsomal UGT from fresh swine liver to receive multimilligram amounts of the benzodiazepine glucuronides, which were not accessible by standard synthetic procedures, like the Koenigs-Knorr- and Williamson-ether-synthesis. Swine liver microsomes were prepared by homogenization and differential centrifugation of liver tissue. In the presence of liver microsomes the benzodiazepines and cofactor UDPGA were incubated for 24h. After incubation the microsomes were removed by protein precipitation and the residual benzodiazepines by liquid-liquid extraction (dichloromethane). The epimeric pairs of benzodiazepine glucuronides were separated by preparative high performance liquid chromatography (HPLC) followed by solid phase extraction (SPE) to obtain the pure benzodiazepine glucuronide epimers. The synthesis products were characterized by mass spectroscopy and nuclear magnetic resonance (NMR) spectroscopy.

Separation of oxazepam, lorazepam, and temazepam enantiomers by HPLC on a derivatized cyclodextrin-bonded phase: application to the determination of oxazepam in plasma

The enantioselective high-performance liquid chromatography (HPLC) of three racemic 3-hydroxybenzodiazepines, oxazepam (Oxa), lorazepam (Lor), and temazepam (Tem), is a difficult operation because of the spontaneous chiral inversion in polar solvent. To solve this problem, we have developed an HPLC method based on a chiral Cyclobond I-2000 RSP column, maintained at 12 degrees C, and a reversed mobile phase (acetonitrile in 1% triethylamine acetate buffer, TEAA) at a flow rate of 0.4 ml/min. Peaks were detected by a photodiode-array detector at 230 nm for quantification and by an optical rotation detector for identification of (+) and (-) enantiomers. The results showed that peak resolutions of Oxa, Lor, and Tem enantiomers, analyzed under the same conditions, were 3.2, 2.0, and 1.8, respectively. For the determination of Oxa enantiomers in plasma of rabbits, extraction with diethyl ether at pH 1.5, a polar organic mobile phase, and a Cyclobond I-2000 SP column were used. Other analytical conditions were the same as previously described. Blood samples were immediately cooled at 4 degrees C and centrifuged at 0 degrees C for the collection of plasma. The results showed a difference in plasma S(+)- and R(-)-oxazepam concentrations in rabbits. No racemization of S(+)- or R(-)-Oxa enantiomers, added alone to blank plasma, was observed after extraction and enantioselective HPLC analysis.

A critical evaluation of the application of capillary electrophoresis to the detection and determination of 1,4-benzodiazepine tranquilizers in formulations and body materials

Studies of the capillary zone electrophoresis (CZE) and micellar electrokinetic capillary chromatography (MEKC) behaviour of 1,4-benzodiazepines have seen application in subject areas such as the development of pharmaceuticals, therapeutic drug monitoring and forensic toxicology. In the development of pharmaceuticals, pKa determinations by CZE can be used in preclinical studies whereas analytical data on the detection and determination of 1,4-benzodiazepines is of value primarily in raw material/formulation assay and in the analysis of body fluids in clinical studies. The capillary electrophoresis (CE) techniques, which generally have inferior limits of detection (LOD) to rival techniques such as gas chromatography (GC) and high-performance liquid chromatography (HPLC), are particularly applicable in forensic toxicology where reasonably high concentrations of these drugs can be encountered. It is anticipated that, with the interfacing of CZE and capillary electrochromatography (CEC) with mass spectrometry (MS) techniques, the excellent selectivity of CZE and particularly CEC will be effectively combined with the sensitivity of MS and the identification capabilities of tandem mass spectrometry (MS/MS) and MS hyphenated (MSn) techniques.

Simple high-performance liquid chromatographic separation of oxazepam and its diastereoisomeric glucuronides in serum. Applications in a pharmacokinetic study in sheep

This paper describes a highly specific and sensitive method for quantifying oxazepam and its diastereoisomeric glucuronides in serum. The method involves sample clean-up by solid-phase extraction on C18 cartridge followed by quantitation on a reversed-phase HPLC column. Diazepam is used as internal standard. Extraction recovery from serum proved to be more than 86%. Precision, expressed as C.V., was in the range 1.2-9.5%. The limits of quantification were 40, 400, and 200 nmol/l for oxazepam, S-(+)- and R-(-)-glucuronides, respectively. This method was applied to the determination of oxazepam and its diastereoisomeric glucuronides in serum collected during a pharmacokinetic study performed in sheep after oral administration of racemic oxazepam. S-(+)/R-(-) ratios were measured all along the sampling time collection and the pharmacokinetic parameters were determined.

Suppression of chiral recognition of 3-hydroxy-1,4-benzodiazepines during micellar electrokinetic capillary chromatography with bile salts

During the development of a micellar electrokinetic chromatographic screening method for 1,4-benzodiazepines, peak splitting and broadening were observed for some 3-hydroxy-1,4-benzodiazepines (oxazepam, lorazepam, temazepam and lormetazepam). This phenomenon occurred when the micellar phase consisted of bile salts and can be ascribed to the chiral nature of these surfactants. As the bile salts were applied in order to reduce the capacity factors to an appropriate level, enantiomer separation was not an objective and even disturbing. By increasing the analysis temperature, the chiral recognition of these compounds could be suppressed.

Direct gradient reversed-phase HPLC analysis and preliminary pharmacokinetics of nalidixic acid, 7-hydroxymethylnalidixic acid, 7-carboxynalidixic acid, and their corresponding glucuronide conjugates in humans

A gradient reversed-phase high pressure liquid chromatographic analysis was developed for the direct measurement of nalidixic acid with its acyl glucuronide, 7-hydroxymethylnalidixic acid with its acyl and ether glucuronides, and 7-carboxynalidixic acid in human plasma and urine. The glucuronides and 7-carboxynalidixic acid were not present in plasma after an oral dose of 1,000 mg nalidixic acid. The acyl glucuronides of 7-carboxynalidixic acid were not present in plasma and urine. The acyl glucuronides are stable in urine at pH 5.0-5.5. The subject's urine must therefore be acidified by the oral intake of 4 x 1 g of ammonium chloride per day. With acidic urine, hardly any nalidixic acid was excreted unchanged (0.2%). It was excreted as acyl glucuronide (53.4% of dose), 7-hydroxymethyl-nalidixic acid (10.0%), the latter's acyl glucuronide (30.9%), and 7-carboxynalidixic acid (4.2%).

Contribution of the human kidney to the metabolic clearance of drugs

OBJECTIVE

To demonstrate that the human kidney is capable not only of filtering and secreting drugs and their metabolites, but also of carrying out conjugation reactions such as acyl glucuronidation, N-glucuronidation, and glycination.

DATA SOURCES

Plasma concentrations and renal excretion rates of drugs are measured and renal clearance is calculated in a series of selected pharmacokinetic studies in healthy human volunteers (some studies were conducted in the authors' laboratory and others were reported in the literature). BACKGROUND THEORY: It is generally agreed that the liver plays the dominant role in drug metabolism, and that the function of the kidneys is limited to excretion of parent drug and metabolites. This can be easily understood when a metabolite is present in both plasma and urine. When the metabolite is present in urine but is not measurable in plasma, then the possibility exists that the metabolite is formed by the kidneys.

RESULTS

"Simple" excretion by the kidneys is demonstrated for sulfatroxazole/sulfamethoxazole. Ether glucuronides of codeine are formed in the liver, and the resulting glucuronide is excreted by the kidneys. Possible formation of N1- and N2-glucuronides by the kidneys is demonstrated for sulfadimethoxine, sulfametomidine, and sulfaphenazole. Acyl glucuronidation of probenecid and nalidixic acid is carried out by the kidneys. The acyl glucuronidation of probenecid shows a capacity-limited formation/excretion rate of 46 mg/h, which is subject dependent. During this process, the acyl glucuronidation of co-administered nalidixic acid is reduced from 53 to 16 percent compared with that of nalidixic acid alone. Probenecid and its acyl glucuronidation do not inhibit the ether glucuronidation of codeine in the liver, but only interfere with the active tubular secretion process. The acyl glucuronidation of the nonsteroidal antiinflammatory drug naproxen and its metabolite, O-desmethylnaproxen, may be carried out by the liver and kidneys. Glycination of benzoic acid and salicylic acid is carried out in both the liver and kidneys.

CONCLUSIONS

It is difficult to recognize renal drug metabolism in the intact human body (in vivo); the glucuronides or conjugates must be measured via direct HPLC analysis. In cases where the metabolite is present in high concentrations in urine but not in blood, there may be an indication that the kidneys are responsible for the formation of the metabolite. Impaired kidney function not only affects renal excretion but may also affect renal metabolism.

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

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

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

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

Pharmacokinetics, N1-glucuronidation and N4-acetylation of sulfamethomidine in humans

Sulfamethomidine metabolism was studied in 6 volunteers. In humans, only N1-glucuronidation and N4-acetylation take place, leading to the final double conjugate N4-acetylsulfamethomidine N1-glucuronide. The N1-glucuronides were directly measured by high pressure liquid chromatography. Fast and slow acetylators show a similar half-life for sulfamethomidine (26 +/- 6 h) and its conjugates sulfamethomidine (26 +/- 6 h) and N4-acetylsulfamethomidine (36 +/- 16 h). Approximately 50-60% of the oral dose of sulfamethomidine is excreted in the urine, leaving 40-50% for excretion into bile and faeces. The main metabolite of sulfamethomidine is its N1-glucuronide, which accounts for 36 +/- 7% of the dose, followed by N4-acetylsulfamethomidine (16 +/- 8%). N1-glucuronidation results in a 75% decrease in protein binding of sulfamethomidine. N4-acetylsulfamethomidine and its N1-glucuronide showed the same high protein binding of 99%. The renal clearance of N4-acetylsulfamethomidine is 7.9 +/- 2.2 ml/min and approximately 20 times as high as that of the parent drug (0.46 +/- 0.16 ml/min). Total body clearance of sulfamethomidine is 4.5 +/- 0.9 ml/min and the volume of distribution in steady state 10.6 +/- 1.7 1. No measurable plasma concentrations of the N1-glucuronides from sulfamethomidine are found in plasma. This may be explained by renal glucuronidation after active tubular reabsorption.

Direct determination of codeine, norcodeine, morphine and normorphine with their corresponding O-glucuronide conjugates by high-performance liquid chromatography with electrochemical detection

A high-performance liquid chromatographic method has been developed for the detection, separation and measurement of codeine and its metabolites norcodeine, morphine and normorphine, with their glucuronide conjugates. The glucuronidase Escherichia coli type VIIA hydrolyses codeine-6-glucuronide completely and is used for the construction of the calibration curves of codeine-6-glucuronide. Enzymic hydrolysis of codeine-6-glucuronide depends on the specific activity of the glucuronidase applied. Examples are shown of a volunteer who is able to form morphine from codeine and one who is unable to do so.

Comparison of the metabolism of four sulphonamides between humans and pigs

Pigs are unable to form N1-glucuronides of sulphadimethoxine and sulphamethomidine, while humans are able to do so. Pigs and humans are able to oxidise sulphapyridine and form the O-glucuronide. The double conjugate N4-acetylsulphapyridine-O-glucuronide is formed in humans but not in pigs. Sulphadiazine is mainly acetylated in both humans and pigs. A hypothesis about N1-glucuronidation is presented.