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Biomedical subjects

W Lindner

Publications and source records attributed to W Lindner.

At least 37 records · Page 2Linked to original sources
PubMed2000

Enantioselective analysis of (R)- and (S)-atenolol in urine samples by a high-performance liquid chromatography column-switching setup.

An HPLC column-switching method for the enantioselective determination of (R,S)-atenolol in human urine was developed and validated. Diluted urine samples were injected onto a LiChrospher ADS restricted access column and atenolol was separated from most of the matrix components using 0.01 M Tris buffer. The atenolol peak was sharpened by a step gradient of 30% acetonitrile and the atenolol-containing fraction was switched onto an enantioselective column. Separation of the atenolol enantiomers was carried out on a Chirobiotic T (Teicoplanin) column using acetonitrile-methanol-acetic acid-triethylamine (55:45:0.3:0.2, v/v/v/v) as eluent. Detection of the effluent was performed by fluorescence measurement. Several experiments were carried out to suppress the high blank reading, which was efficiently achieved using Tris buffer in the first dimension. For the enantioselective analysis of (R)- and (S)-atenolol in plasma under the same conditions the sample capacity of the ADS column is considerably lower.

Adrenergic beta-Antagonists↗
PubMed2000

Direct high-performance liquid chromatographic determination of (R)- and (S)-propranolol in rat microdialysate using on-line column switching procedures.

Two different column-switching HPLC systems (CSWs), employing restricted access material for initial pretreatment of biological samples, were developed for the determination of propranolol enantiomers in microdialysate. CSW 1 was a single-pump set-up based on an initial sample clean-up step with a RP-18 ADS precolumn coupled with an ovomucoid analytical column for direct drug enantioseparation. For the two-pump column set-up (CSW 2), a teicoplanin analytical column was applied for the enantioselective assay after initial sample pretreatment using a RP-8 ADS precolumn. The inter-day precision of the CSW 1 ranged from 0.5 to 5.1% for (R)-propranolol and from 5.1 to 10.5% for (S)-propranolol. The limit of detection (LOD) was set at 10 ng/ml and 15 ng/ml for (R)- and (S)-propranolol, respectively. Inter-day relative standard deviation values of the CSW 2 ranged from 1.1 to 9.9% for (R)-propranolol and from 1.3 to 9.6% for (S)-propranolol. The LOD of the method was 3.0 ng/ml for (R)-propranolol and 2.5 ng/ml for (S)-propranolol. Both approaches were successfully applied for stereoselective monitoring of unbound propranolol levels in rat microdialysates.

Adrenergic beta-Antagonists↗
PubMed2000

Novel cinchona alkaloid carbamate C9-dimers as chiral anion-exchange type selectors for high-performance liquid chromatography.

Nine new quinine (QN) carbamate C9-dimers (QN-X-QN), with different aliphatic and cyclic spacers (X), have been synthesized and immobilized onto porous silica gel for HPLC. The chiral discriminating behavior of these "dimeric" anion-exchange type chiral stationary phases (CSPs) has been investigated in detail, to elucidate the role of the presence of a second QN subunit on the chiral selector (SO), as well as the influence of the structure and length of the spacer, on the overall chiral recognition of a set of N-derivatized amino acids and other acidic drugs. The bulkiness of the intermediate spacer tuned the chiral recognition abilities of these SOs, with the 1,3-adamantylen-derived CSP being the one that led to the best separations. Shorter spacers reduced the chiral discrimination abilities of the "dimeric" selectors, with the n-hexylen bridge being the most favorable distance to allow a nearly independent interaction of the two QN subunits with the racemic analytes. The comparison to five "monomeric" CSPs showed that the "dimeric" ones usually retain the chiral analytes more strongly, though the enantioseparation is not improved. Nevertheless, the exceptional resolution abilities of dimeric SOs with a trans- 1,2-diaminocyclohexylen-bridge for the separation of DNP-derivatives of amino acids and certain acidic drugs of therapeutical interest (e.g., profens) seemed to be superior to most of the other CSPs.

Anion Exchange Resins↗
PubMed2000

Determination of zearalenone and its metabolites alpha- and beta-zearalenol in beer samples by high-performance liquid chromatography-tandem mass spectrometry.

A fast, robust and sensitive LC-MS-MS method for the determination of zearalenone (ZON) and its metabolites alpha-zearalenol (alpha-ZOL) and beta-zearalenol (beta-ZOL) in beer samples is described. Sample preparation was performed by direct RP-18 solid-phase extraction of undiluted beer samples followed by selective determination of analytes by LC-MS-MS applying an atmospheric pressure chemical ionization (APCI) interface. Using the negative ion mode limits of determination of 0.03-0.06 microg l(-1) beer and limits of quantification of 0.07-0.15 microg l(-1) beer were achieved, which was distinctly more sensitive than in the positive ion mode. Twenty-three beer samples from different countries, produced from different grains and under different brewing conditions, were investigated by this method, but only in one sample could beta-ZOL and ZON be detected. Independently of the type of beer, relative standard deviations between 2.1% and 3.3%, a linear working range of 0.15 microg l(-1) to 500 microg l(-1) beer and recovery rates around 100% could be achieved when zearalanone (ZAN) was used as internal standard.

Beer↗
PubMed2000

Enantiodiscrimination by a quinine-based chiral stationary phase: a computational study.

A detailed computational study of a derivatized quinine chiral stationary phase (CSP) interacting with enantiomeric 3, 5-dinitrobenzoyl derivatives of leucine was carried out to understand where and how chiral discrimination takes place. The most stable structure of the CSP derived from a conformer search gave a structure whose geometry agrees with an X-ray structure (rmsd 0.6 A). The computed retention order and enantiodiscriminating free energy differences also agree with chromatographic data. The location and characteristics of the analyte binding site were assessed. An evaluation of total energies and intermolecular energies responsible for complex formation and for chiral discrimination was performed. Molecular dynamics trajectories of those intermolecular forces as well as distributions of the stabilizing and destabilizing forces are presented. A partitioning of the CSP into molecular fragments and the role each fragment plays in complexation and chiral recognition is also described.

Binding Sites↗
PubMed2000

Comparative molecular field analysis of quinine derivatives used as chiral selectors in liquid chromatography: 3D QSAR for the purposes of molecular design of chiral stationary phases.

A comparative molecular field analysis (CoMFA) was carried out on a set of aligned quinine-based stationary phase molecules used in enantioselective chromatography. The best QSAR derived has a cross-validated (predictive) r(2)(cv) = 0.671 and a normal r(2) = 0.998. For CoMFAs using both steric and electrostatic fields as descriptors, the steric field descriptors explained more than 91% of the variance while the electrostatic descriptors explained less than 9% of the variance. It is concluded that the long-range electrostatic potential surrounding the positively charged CSPs are not enantiodiscriminating, while the van der Waals and local electrostatic surface features of these CSPs are highly discriminating. Quantum mechanical calculations back up this claim by showing a relatively symmetric electrostatic iso-contour surface. From the QSAR derived here, a region around the carbamate moiety was located where placement of steric bulk is predicted to enhance chiral discrimination. A set of possible synthetic target molecules is presented.

Chromatography, Liquid↗
PubMed2000

Time-dependent interactions of platinum(II) complexes with 5'-GMP under simulated physiological conditions studies by capillary electrophoresis.

The binding behaviour as well as the time-dependent reaction of five platinum(II) complexes with 5'-GMP have been investigated by capillary electrophoresis under simulated physiological conditions referred to chloride concentration, pH and temperature. Different amine ligands influenced the binding properties towards 5'-GMP and resulted in different half-times of the overall reaction. Complexes with bidentate ligands reacted faster with the monophosphate compared to complexes with monodentate ligands. Complexes consisting of two monodentate hydroxyethylamine ligands reacted very slowly owing to a competitive intramolecular reaction of the hydroxyethyl residues, which was proven by NMR investigations. Reducing the number of hydroxyethyl residues increased the half-times of the reactions. Moreover, the major adducts formed with 5'-GMP were identified by MALDI-MS analysis.

Cisplatin↗
PubMed1999

Determination of zearalenone in grains by high-performance liquid chromatography-tandem mass spectrometry after solid-phase extraction with RP-18 columns or immunoaffinity columns.

In this paper a robust, sensitive and selective LC-MS-MS method for the determination of zearalenone (ZON) in several cereals is described. Sample preparation was performed by extraction of the commodities with a mixture of acetonitrile and water followed by solid-phase extraction with RP-18 columns or immunoaffinity columns. The selective determination of ZON was achieved with an atmospheric pressure chemical ionization interface. Using the negative ion mode a detection limit of 0.5 microg/kg and a determination limit of 1 microg/kg grain was achieved, which is by a factor of 100 more sensitive than the positive ion mode. Zearalanone (ZAN), which does not occur in nature, was used as internal standard for quantification. A linear working range from 1.0 microg/kg to 1000 microg/kg could be achieved in grains with a standard deviation of 4% and recovery rates around 100%. All these results were independent from the grain matrices (maize, barley, oats, wheat) when ZAN was used as internal standard. Sample preparation with RP-18 and immunoaffinity materials gave comparable results. In addition, the method was successfully used for the investigation of naturally contaminated maize samples in the course of an interlaboratory comparison test.

Chromatography, Affinity↗
PubMed1999

Quinine versus carbamoylated quinine-based chiral anion exchangers. A comparison regarding enantioselectivity for N-protected amino acids and other chiral acids.

First, a chiral stationary phase was prepared by immobilization of the naturally occurring alkaloid quinine onto a 3-mercaptopropyl-modified silica gel via a radical addition reaction and it was evaluated for direct HPLC enantioseparation of acidic chiral compounds under buffered hydro-organic mobile phase conditions. Second, its enantioselectivity and retention characteristics for a representative set of N-derivatized alpha-amino acids and other chiral acids were compared with those of a similar weak chiral anion exchanger based on tert.-butyl carbamoylated quinine derivative as chiral selector. The results clearly indicate that the introduction of the carbamoyl functionality at the secondary hydroxyl group at C9 of quinine provides new and additional sites for intermolecular interactions with chiral analytes and this can profoundly change and improve chiral recognition ability, especially for amide, carbamate and sulfonamide derivatives of amino acids including DNB, Bz, Ac, For, Z, Fmoc, Boc and DNS-protected amino acids. The impact of this new and rigid hydrogen donor-acceptor group--directing stereoselective selector-selectand complexation by intermolecular hydrogen bonding--in comparison to the plain quinine selector is further evaluated by alkylation of the nitrogen atom of either the selector carbamate and/or of the amino function of leucine (N-methyl leucine) derivatized either by DNB or DNP groups.

Acids↗
PubMed1999

Capillary electrophoretic study of cisplatin interaction with nucleoside monophosphates, di- and trinucleotides.

cis-Diamminedichloroplatinum(II) (cisplatin) is applied against different kinds of cancer although toxic side effects are known. Screening systems for alternative compounds with higher effectiveness but minimizing toxic side effects are required. We investigated the adduct formation of cisplatin with nucleoside monophosphates, di- and trinucleotides. Capillary electrophoretic separations were performed in a sodium phosphate buffer using an instrument equipped with a diode array detector. Adduct formation results in a significant shift of lambda(max) to lower energy compared to free nucleotides. Therefore, UV spectra are an important tool for peak identification. We could separate and identify all four common nucleotides and their major platinum adducts in a single run demonstrating the suitability of CE for these kinds of investigations. Furthermore, kinetic studies of these reactions are performed.

Cisplatin↗
PubMed1999

Enantioselective anion exchangers based on cinchona alkaloid-derived carbamates: influence of C8/C9 stereochemistry on chiral recognition.

Four diastereomeric chiral stationary phases (CSPs) based on quinine, quinidine, epiquinine, and epiquinidine tert-butyl carbamate selectors were synthesized and evaluated under ion exchange HPLC conditions with a set of racemic N-acylated and N-oxycarbonylated alpha-amino acids as selectands. The enantioseparation potential of quinine- and quinidine-derived CSPs proved to be far superior to that of their C9-epimeric congeners. The absolute configuration of C9 stereogenic center of the cinchonan backbone of these selectors was identified as the structural feature controlling the elution order. Guided by an X-ray structure of a most favorable selector-selectand complex and the observed chromatographic enantioseparation data, a chiral recognition model was advanced. The contributions of ion-pairing, pi-pi donor-acceptor, hydrogen bonding and steric interactions were established as crucial factors.

Antimalarials↗
PubMed1999

Enantiomeric separation of N-protected amino acids by non-aqueous capillary electrophoresis using quinine or tert-butyl carbamoylated quinine as chiral additive.

A capillary electrophoretic (CE) method for the enantioseparation of N-protected chiral amino acids was developed using quinine and tert-butyl carbamoylated quinine as chiral selectors added to nonaqueous electrolyte solutions (NACE). A series of various N-derivatized amino acids were tested as chiral selectands, and in order to optimize the CE enantioseparation of these compounds, different parameters were investigated: the nature of the organic solvent, the combination of different solvents, the nature and the concentration of the background electrolyte, the selector concentration, the capillary temperature, and the applied voltage. The influence of these factors on the separation of the analyte enantiomers and the electroosmotic flow was studied. Generally, with tert-butyl carbamoylated quinine as chiral selector, better enantioseparations were achieved than with unmodified quinine. Optimum experimental conditions were found with a buffer made of 12. 5 mM ammonia, 100 mM octanoic acid, and 10 mM tert-butyl carbamoylated quinine in an ethanol-methanol mixture (60:40 v/v). Under these conditions, DNB-Leu enantiomers could be separated with a selectivity factor (alpha) of 1.572 and a resolution (Rs) of 64.3; a plate number (N) of 127,000 and an asymmetry factor (As) of 0.93 were obtained for the first migrating enantiomer.

Amino Acids↗
PubMed1999

Influence of beta-blockers on melatonin release.

OBJECTIVE: Melatonin is a mediator in the establishment of the circadian rhythm of biological processes. It is produced in the pineal gland mainly during the night by stimulation of adrenergic beta1- and alpha1-receptors. Sleep disturbances are common side-effects of beta-blockers. The influence of specific beta-blockade as well as that of combined alpha-and beta-blockade on melatonin production has not been investigated in humans before. METHODS: We performed a randomized, double-blind, placebo-controlled, cross-over study in 15 healthy volunteers. Subjects received single oral doses of 40 mg (R)-propranolol, 40 mg (S)-propranolol, 50 mg (R)-atenolol, 50 mg (S)-atenolol, 25 mg (R,S)-carvedilol, 120 mg (R,S)-verapamil or placebo at 1800 hours. Urine was collected between 2200 hours and 0600 hours, and 6-sulfatoxy-melatonin (aMT6s), the main metabolite of melatonin which is almost completely eliminated in urine, was determined by radioimmunoassay (RIA). RESULTS: Mean nocturnal excretion of aMT6s in urine after intake of the drugs was as follows (in microg): placebo 26; (R)-propranolol 24 (-7%, NS); (S)-propranolol 5 (-80%, P < 0.001); (R)-atenolol 27 (+7%, NS); (S)-atenolol 4 (-86%, P < 0.01); (R,S)-carvedilol 23 (-10%, NS); (R,S)-verapamil 29 (+14%, NS). These data show that only the specifically beta-blocking (S)-enantiomers of propranolol and atenolol decrease the nocturnal production of melatonin whereas the non-beta-blocking (R)-enantiomers have no effect. Unexpectedly, (R,S)-carvedilol which inhibits both alpha- and beta-adrenoceptors does not decrease melatonin production. CONCLUSION: These findings indicate that beta-blockers decrease melatonin release via specific inhibition of adrenergic beta1-receptors. Since lower nocturnal melatonin levels might be the reason for sleep disturbances, further clinical studies should investigate whether or not oral administration of melatonin might avoid this well-known side-effect of beta-blockers. The reason why (R,S)-carvedilol does not influence melatonin production remains to be determined.

Adrenergic beta-Antagonists↗
PubMed1999

A severely affected infant with absence of cysteinyl leukotrienes in cerebrospinal fluid: further evidence that leukotriene C4-synthesis deficiency is a new neurometabolic disorder.

Leukotrienes are potent oxygenated metabolites derived from the 5-lipoxygenase pathway of arachidonic acid metabolism. They comprise the cysteinyl leukotrienes (LTC4, LTD4, LTE4) and LTB4. The rate limiting step in the formation of cysteinyl leukotrienes is the conversion of LTA4 to LTC4 catalyzed by the enzyme LTC4 synthase. Recently, the first inborn error of leukotriene synthesis, LTC4-synthesis deficiency, has been identified in a patient with a fatal developmental syndrome. We report on an additional infant presenting with severe muscular hypotonia, symmetrical extension in the lower extremities and psychomotor retardation who died at the age of 6 months. Despite intensive investigations no specific diagnosis could be made. Leukotrienes were subsequently analyzed in the cerebrospinal fluid. Concentrations of LTC4, LTD4 and LTE4 were below the detection limit (< 5 pg/ml) whereas LTB4 was found to be in the upper normal range. The absence of cysteinyl leukotrienes with normal LTB4 concentration in cerebrospinal fluid is unique and seems to be pathognomonic for LTC4-synthesis deficiency. Our patient most likely represents the second case described so far with this condition. This report provides further evidence that LTC4-synthesis deficiency represents a new neurometabolic disorder.

Fatal Outcome↗
PubMed1999

Delivery room management of extremely low birth weight infants: spontaneous breathing or intubation?

OBJECTIVE: To study the effect of two different delivery room (DR) policies on the rate of endotracheal intubation and mechanical ventilation (EI/MV) and short term morbidity in extremely low birth weight infants (ELBWI; <1000 g, >/=24 weeks). METHODS: Retrospective cohort study of 123 inborn ELBWIs born in 1994 and in 1996. DR policies have changed. Until 1994, ELBWIs were intubated immediately after delivery when presenting the slightest signs of respiratory distress or asphyxia after initial resuscitation using a face mask and a handbag. During 1995, the guidelines for respiratory support were changed. In 1996, continuous (15 to 20 seconds), pressure controlled (20 to 25 cm H2O) inflation of the lungs using a nasal pharyngeal tube, followed by continuous positive airway pressure (CPAP; 4 to 6 cm H2O) was applied to all ELBWIs immediately after delivery to establish a functional residual capacity and perhaps to avoid EI/MV. In addition to the changes in respiratory support, the prevention of conductive and evaporative heat loss was improved in 1996. For analysis of morbidity and mortality, infants were matched for gestational age and birth weight. RESULTS: The rate of EI/MV in the DR decreased from 84% in 1994 to 40% in 1996. In 1996, 25% of the ELBWIs were never intubated (7% in 1994), but 35% of the ELBWIs needed secondary EI/MV, primarily because of respiratory distress syndrome (RDS). Initial ventilator settings, ventilator days, mortality, and morbidity were not different between ELBWIs with EI/MV in the DR and infants with secondary EI/MV attributable to RDS in the intensive care unit. ELBWIs with no EI/MV that was caused by RDS had a lower morbidity (ie, bronchopulmonary dysplasia, intraventricular hemorrhage >grade 2 and/or periventricular leukomalacia), mortality, and fewer hospital days (mean: 79 vs 105 days). The incidence of gastrointestinal adverse effects like feeding intolerance or necrotizing enterocolitis was not increased in 1996. PaCO2 was significantly higher at admission to the neonatal unit in ELBWIs with CPAP in 1996 (54 +/- 15 mm Hg, 7.2 +/- 2.0 kPa) compared with infants with EI/MV in 1994 (38 +/- 11 mm Hg, 5.1 +/- 1. 5 kPa. A total of 26% of spontaneously breathing infants had hypercapnia (PaCO2 >/=60 mm Hg [8.0 kPa]), compared with 7% of infants with EI/MV in 1994. Within the first few hours of life, PaCO2 decreased to 46 (32 to 57) mm Hg (6.1 [4.3 to 7.6] kPa) in never intubated ELBWIs (n = 17), but increased to 70 (57 to 81) mm Hg (9.3 [7.6 to 10.8] kPa) in ELBWIs (n = 14) with RDS and secondary EI/MV (age 5.5 [1 to 44] hours). CONCLUSIONS: In our setting, the individualized intubation strategy in the DR restricted EI/MV to those ELBWIs who ultimately needed it, without increasing morbidity or mortality in infants with secondary EI/MV attributable to RDS. We speculate that an individualized intubation strategy of the ELBWI is superior to immediate intubation of all ELBWIs with slight signs of respiratory distress after birth.

Cohort Studies↗
PubMed1998

On-column formation of arsenic-glutathione species detected by size-exclusion chromatography in conjunction with arsenic-specific detectors.

The 'retention analysis method', which is based on size-exclusion chromatography (SEC) in conjunction with an arsenic-specific detector (graphite furnace atomic absorption spectrometer, GFAAS), was used to study the effect of pH (range 2.0-10.0), temperature (4, 25 and 37 degrees C), and the concentration of glutathione in the mobile phase (0.5-7.5 mM) on the formation of arsenic-glutathione species after injection of sodium arsenite using phosphate-buffered saline solutions as mobile phases. The formation of arsenic-GSH species was facilitated by low temperatures (4 degrees C), pH 6.0-8.0 and high concentrations of glutathione (7.5 mM) in the mobile phase. Simulating the physicochemical parameters found inside human red blood cells (approximately 3.0 mM glutathione, 37 degrees C, pH 7.4) and hepatocytes (approximately 7.5 mM glutathione, 37 degrees C, pH 7.4), SEC-GFAAS provided evidence for the formation of arsenic-glutathione species under these conditions. In addition, the 'chelating agent', sodium DL-2,3-dimercapto- -propanesulfonate (1.0 and 2.0 mM) was demonstrated to bind arsenous acid stronger in the presence of glutathione (7.5 mM) under these conditions (PBS buffer, pH 7.4, 37 degrees C).

Arsenic↗