Partially selective semiconductor redox electrodes.

Redox electrodes made of passivated metals of the subgroups IV to VI of the periodic system with a surface coating of n-conducting metal oxides, generated by oxidizing agents or anodic oxidation, are well suited for measuring the concentration of oxidizing agents. Neither oxygen nor reducing agents affect the measurements. There is a correlation between the mean conductivity of the passive layer and its selectivity. The aged n-conducting passive layers permit reproducible potential settings, and the occurring changes in potential are greater than what is to be expected according to the Nernst equation. The potentiometric measuring of nitrosyl ions and chlorine is used as an example to demonstrate the selectivity of these electrodes.

Stereoselective disposition of carprofen, flunoxaprofen, and naproxen in rats.

The stereoselective dispositions of carprofen, flunoxaprofen, and naproxen were studied in rats after i.v. administration of racemate (11 mumol/kg) or enantiomer (5.5 mumol/kg). The total clearances of the (R)-enantiomers of carprofen and flunoxaprofen were significantly greater than those of the (S)-enantiomers. The clearance of (S)-naproxen was similar to the value for (R)-naproxen. There were no marked differences in steady-state volume of distribution between (R)- and (S)-enantiomers for carprofen, flunoxaprofen, or naproxen. The (R)- to (S)-enantiomer inversion ratio for flunoxaprofen in rats was 0.54. The ratios for naproxen and carprofen were 0.02 and 0.003, respectively. Biliary excretion of (R)-carprofen and of its glucuronide were higher than those of the (S)-enantiomer and its glucuronide. In contrast, biliary excretion of the (S)-enantiomers of flunoxaprofen, naproxen, and of their glucuronides were greater than those of their antipodes. Insignificant amounts of the parent enantiomers and of the glucuronides of these three drugs were excreted in urine. These results indicate that there is a wide variation in the extent of inversion at a chiral center for these three 2-arylpropionates and in the stereoselective disposition of their acyl glucuronides.

The loop diuretic torasemide in chronic renal failure. Pharmacokinetics and pharmacodynamics.

The pharmacodynamic effect of a diuretic agent is essentially dependent on its renal elimination characteristics. The influence of renal function on the pharmacodynamic and pharmacokinetic characteristics of a diuretic should therefore be considered. The results of a study with torasemide given intravenously in healthy subjects and in patients with stable chronic renal failure of various degrees are reported and discussed. After a single dose of torasemide 20mg, a marked diuresis was observed and electrolyte excretion was increased, whereas the glomerular filtration rate was unchanged. Throughout the duration of action (tau) of torasemide, drug-induced excretion of Cl-, Na+, K+, Ca++ and Mg++ was linearly related to the creatinine clearance (CLcr), and excretion of K+ and Ca++ were closely related to that of Na+ over the entire range of CLcr. Similarly, excretion of Mg++ was related to that of K+. As occurs with furosemide (frusemide), torasemide induced a kaliuresis which amounted to 12% of natriuresis. This kaliuretic effect of loop diuretics is less than that of thiazides. Beyond tau, e.g. over a 24-hour period, kaliuresis was no longer correlated with natriuresis. The extent to which a rebound effect occurred was diminished with increasing renal impairment. tau averaged 6 hours and was independent of CLcr. The mean half-life (t1/2) of torasemide was approximately 5 hours and was independent of renal function, since renal clearance accounted for only around 25% of total body clearance. In contrast to the parent drug, however, the active minor metabolite M1 and the inactive main metabolite, M5, were found to accumulate in patients with chronic renal failure.

Xipamide disposition in liver cirrhosis.

The pharmacokinetics of the sulfonamide-type diuretic xipamide was studied in patients with liver cirrhosis and ascites and compared with healthy control subjects. After oral administration of 40 mg xipamide, the diuretic was rapidly distributed in the blood and the ascites. The ratio of the area under the concentration-time curve (AUC) of plasma and ascitic fluid was 7:2, as was the protein content in the respective compartments. The AUC in plasma of cirrhotic patients was significantly greater than in control subjects (p less than 0.001). The most striking finding was the increase of the amount (Ae) of parent drug and main metabolite excreted into the urine (p less than 0.001). The renal clearance of xipamide was only moderately reduced in patients with liver cirrhosis. Both AUC and Ae were positively correlated to the plasma concentration of direct bilirubin of the patients (p less than 0.05). We concluded that nonrenal drug clearance in patients with liver cirrhosis was reduced as a result of the blockade of hepatobiliary excretion during cholestatic conditions.

Chiral amines derived from 2-arylpropionic acids: novel reagents for the liquid chromatographic (LC) fluorescence assay of optically active carboxylic acid xenobiotics.

For the enantiospecific analysis of optically active carboxylic acids, the availability of readily detectable coupling components is desirable, but highly fluorescent chiral amines are rare. From activated enantiomers of fluorescent 2-arylpropionic acids fluorescent chiral amines were synthesized via Curtius degradation, i.e., under formation of the acyl azide, the isocyanate, and finally, the amine. The formation of isocyanates and of amine hydrochlorides led to an inversion of the direction of rotation of polarized light. Amines derived from R- and S-flunoxaprofen, R- and S-naproxen, and R/S-benoxaprofen were characterized. The amines were found to be applicable for the chiral separation of carboxylic acids (such as 2-arylpropionic acids) as diastereomeric derivatives via high-performance liquid-chromatographic (normal and reversed-phase) and thin-layer chromatographic techniques.

Measurement of carvedilol enantiomers in human plasma and urine using S-naproxen chloride for chiral derivatization.

The enantiospecific procedure for assaying carvedilol includes the extraction of the drug from plasma or urine with diisopropylether after alkalization of the sample with pH 9.8 buffer. After evaporation of the org. solvent a chiral derivatization is performed using S-(+)-naproxen chloride. The HPLC separation of the diastereomeric amides is possible on a silica gel stationary phase with a mixture of n-hexane, dichloromethane, and ethanol as mobile phase. Detection of the products is performed by fluorescence measurement at 285/355 nm. Preliminary pharmacokinetic studies after i.v. infusion of racemic compound to healthy volunteers showed that the concentrations of the R-(+)-enantiomer exceeded those of the S-(-)-enantiomer. Overall, both carvedilol enantiomers exhibited a high clearance with preference for the S-enantiomer. The difference was even more expressed after p.o. dosage indicating a stereoselective first-pass effect with higher extraction of the levorotatory enantiomer, which is more potent with respect to beta-adrenoceptor antagonistic activity.

Fluorescence assay for small peptides and amino acids: high-performance liquid chromatographic determination of selected substrates using activated S-flunoxaprofen as a chiral derivatizing agent.

The applicability of the fluorescent S-flunoxaprofen, activated to the corresponding acyl chloride, as a chiral derivatizing agent for amino acids and small peptides was evaluated. The amino acid or peptide solution was evaporated to dryness. The carboxylic moieties were esterified with 3 M hydrochloric acid in 2-propanol. The solvent and hydrochloric acid were evaporated, and the residue was dissolved in aqueous sodium heptanesulphonate and extracted with benzene-butanol (85:15, v/v). The organic phase was evaporated and S-flunoxaprofen chloride reagent solution (in dichloromethane) added, together with 20 mg of anhydrous sodium carbonate. After 60 min at 40 degrees C the solvent was evaporated and the residue reconstituted in mobile phase (n-hexane-dichloromethane-ethanol). Resolution of the diastereomeric derivatives was accomplished on a Zorbax Sil high-performance liquid chromatography column. The eluate was monitored either fluorimetrically at 305/355 nm or by measuring the UV absorption at 305 nm. The procedure leads to highly fluorescent derivatives of amino acids or small peptides, which are resolvable on silica gel stationary phases.

Simultaneous determination of propranolol and 4-hydroxypropranolol enantiomers after chiral derivatization using reversed-phase high-performance liquid chromatography.

A reversed-phase high-performance liquid chromatographic method is described, which allows the simultaneous quantification of propranolol and 4-hydroxypropranolol enantiomers in human plasma. After extraction from plasma (pH 10.5) using ethyl acetate, the enantiomers are derivatized with R-(+)-phenylethylisocyanate as chiral derivatization reagent and triethylamine as basic catalyst in chloroform. Ascorbic acid is used to prevent 4-hydroxypropranolol from oxidation during the extraction. Chromatographic separation on ODS columns and fluorescence detection (228 nm/greater than 340 nm) allows sensitive quantitation of all derivatives. Incubation of the plasma samples with beta-glucuronidase/arylsulfatase and the use of the specific beta-glucuronidase inhibitor saccharo-1,4-lactone allows the quantitation of both the sulfate and glucuronide conjugates of the enantiomers. The method was applied to human plasma samples from a subject after administration of 60 mg racemic propranolol three times daily.

Stereoselective binding of the glucuronide conjugates of carprofen enantiomers to human serum albumin.

The stereoselective binding of carprofen enantiomers and carprofen glucuronide diastereomers to human serum albumin (HSA) was studied using an ultrafiltration method. Carprofen glucuronides exhibit a considerable and stereoselective affinity to HSA, although less than that seen for the parent enantiomers. The (S)-glucuronide showed a higher binding affinity to HSA than the (R)-glucuronide. The (S)-enantiomer of unmetabolized carprofen was bound to fatty acid free HSA to a much greater extent than the (R)-enantiomer. Warfarin reduced the binding of the glucuronides to a greater extent than did diazepam, but diazepam displaced the unconjugated enantiomers to a greater extent than did warfarin. These results suggest differences in binding region between the carprofen enantiomers and their glucuronides on the albumin molecule.

Pharmacokinetics of torasemide and its metabolites in healthy controls and in chronic renal failure.

The pharmacokinetics of 20 mg torasemide i.v. has been studied in 7 healthy controls and 9 patients with varying degrees of renal impairment. Torasemide had a t1/2 of about 4h which was independent of kidney function, as the nonrenal clearance of torasemide was 3-times greater than its renal clearance. The active metabolite M 1 and the main metabolite M 5 were accumulated in chronic renal failure. In contrast to liver function, therefore, kidney failure does not have an important effect on the pharmacokinetics of torasemide.

Excretion of azapropazone in human breast milk.

The excretion of azapropazone in breast milk was studied in four lactating women over one dosing interval following repeated doses of 600 mg b.d. for at least three days. Plasma and milk samples were collected up to 12 hours after drug intake. An average of 0.8 mg azapropazone was calculated as being excreted in breast milk in the 12 hour period. The breast fed neonate would ingest 0.2 mg/kg during this period.

Single- and multiple-dose pharmacokinetics of R-(-)-and S-(+)-prenylamine in man.

The pharmacokinetics of S-(+)- and R-(-)-prenylamine was studied in eight healthy volunteers given single and repeated oral doses of the racemic drug. Distinct differences in various pharmacokinetic parameters were found between the S- and R-enantiomer. The maximum plasma concentrations and AUCs of the R-enantiomer exceeded those of the S-enantiomer five-fold; the apparent oral clearance of the S-form was five-times and the renal clearance three-times higher than of the R-form. Acid catalyzed hydrolysis of urine samples released more S-prenylamine, indicating stereoselective glucuronidation of unchanged prenylamine. Plasma protein binding also differed between the two enantiomers, generally with a higher unbound fraction of the S-form, whereas analysis of the bound fractions showed that prenylamine was bound to different plasma proteins with inverse stereoselectivity.

Simultaneous determination of (R)- and (S)-celiprolol in human plasma and urine: high-performance liquid chromatographic assay on a chiral stationary phase with fluorimetric detection.

The quantitative enantiospecific determination of the beta 1-selective adrenergic antagonist (R,S)-celiprolol in human plasma and urine is described. It involves a two-step liquid-liquid extraction of celiprolol from biological material and separation of the underivatized enantiomers by high-performance liquid chromatography on a chiral stationary phase (cellulose tris-3,5-dimethylphenyl carbamate, coated on silica gel) with fluorimetric detection. R-(+)-Propranolol was used as an internal standard. The detection limits of 1.5 ng/ml enantiomer in plasma and 2.5 ng/ml enantiomer in urine at signal-to-noise ratios higher than 3 permit the performance of pharmacokinetic studies after therapeutic doses.

Enantioselective disposition of R-(-)- and S-(+)-prenylamine in the rat.

The disposition of R- and S-prenylamine was investigated in male Wistar rats after i.v. and p.o. dosage of 2 mg/kg racemic prenylamine. Concentrations of the enantiomers were determined in plasma, lung, heart, spleen, liver kidney and muscle tissue within a period of 5 h after dosage. In addition, plasma protein binding was assayed in vitro with racemic drug and found to be similar for the two enantiomers. Except for plasma samples after i.v. administration the concentrations of the S-enantiomer exceeded those of the R-enantiomer.

Direct determination of diastereomeric carprofen glucuronides in human plasma and urine and preliminary measurements of stereoselective metabolic and renal elimination after oral administration of carprofen in man.

A direct stereospecific HPLC assay for carprofen glucuronides in biologic fluids was developed that makes use of a reverse phase (C 18) gradient system, in which the mobile phase consisted of a mixture of acetonitrile and tetrabutylammonium hydroxide buffer (pH 2.5). Reference diastereomeric glucuronides of carprofen were purified from human urine after oral administration of the single enantiomers. When 0.1 ml of sample was used, the limit of detection for carprofen glucuronides was 50 ng/ml in plasma and 200 ng/ml in urine. Coefficients of variation did not exceed 12% for both intra and interday variability. This HPLC method is applicable to pharmacokinetic analysis for carprofen glucuronides in humans. After oral administration of the single enantiomers there was little indication of metabolic inversion. For the two enantiomers the apparent total and metabolic clearances were similar. The limited data available suggest that renal clearance of the (S)-glucuronide was greater than that of the (R)-glucuronide.

Probenecid-induced changes in the clearance of carprofen enantiomers: a preliminary study.

Probenecid inhibits the elimination of several acidic drugs. In this study, the influence of probenecid on the pharmacokinetics of carprofen was investigated in three healthy volunteers after single peroral administration of 150 mg of RS-(+/-)-carprofen. Carprofen enantiomers and their glucuronides (after cleavage with sodium hydroxide) were measured by use of a stereospecific procedure. The plasma concentrations of S-(+)-carprofen were higher than those of R-(-)-carprofen at most of the sampling points. Probenecid reduced apparent total and renal clearances for both enantiomers. It also reduced the clearances of the carprofen enantiomers to their glucuronides and the renal clearances of the glucuronides. The differences caused by probenecid were significant, but few stereoselective effects were observed.

Procedures to characterize in vivo and in vitro enantioselective glucuronidation properly: studies with benoxaprofen glucuronides.

The diastereoisomeric glucuronic acid conjugates of R/S-benoxaprofen are the major benoxaprofen metabolites and are found in urine at high concentrations. The conjugates of R- and S-benoxaprofen can be separated directly on a C18 reversed-phase column using a mixture of acetonitrile and tetrabutylammonium hydroxide buffer, pH 2.5 (28:72, v/v), as the mobile phase. The k' values of S- and R-benoxaprofen glucuronides are 57.5 and 63.0, respectively. Diluted urine or deproteinized plasma samples were injected without further treatment. With fluorescence detection at 313/365 nm, quantifiable limits of 50 ng equiv./ml were found for the conjugates. The intra- and interday variability was below 12%. Utilizing this analytical procedure it is possible to characterize enantioselective glucuronidation both in vivo and in vitro. For in vitro procedures, apparent rates of formation and the R/S ratio may be substrate (benoxaprofen) and cosubstrate (UDPGA) dependent. Moreover, enantioselective cleavage of the formed benoxaprofen glucuronides by alkaline hydrolysis, hydrolytic enzymes, and acyl migration must be controlled for both in vitro and in vivo studies since R-benoxaprofen glucuronide is degraded faster than the S-diastereomer under certain conditions.

Radioreceptor assay of metoprolol in human plasma: comparison with an enantiospecific high-performance liquid chromatographic (HPLC) procedure.

Plasma concentrations of metoprolol after acute and repetitive administration of R/S-metoprolol to healthy volunteers were measured by a beta-adrenoceptor subtype-specific radioreceptor assay (RRA) and by an enantiospecific high-performance liquid chromatographic (HPLC) method. In the RRA, R/S-metoprolol showed a 20-fold beta 1-subtype selectivity: the S-(-)-enantiomer was 35-fold more potent than the R-(+)-enantiomer. A comparison between S-(-)-metoprolol concentrations detected in the plasma samples by HPLC and those detected by RRA yielded a 1/1 relationship, indicating that active metabolites are not present to a significant extent. These results were independent of the widely scattering metabolic clearance of metoprolol (with the potential of differences in the rate and extent of formation of active metabolites) in the volunteers. In general, HPLC methods can be validated by comparison with RRA in order to clarify whether active metabolites are present and--on the basis of the Ki value from RRA--whether the detection limit of the physicochemical procedure is sufficient to cover the therapeutically relevant range.

(-)-(S)-flunoxaprofen and (-)-(S)-naproxen isocyanate: two new fluorescent chiral derivatizing agents for an enantiospecific determination of primary and secondary amines.

The synthesis and analytical testing of two new fluorescent chiral derivatizing agents (-)-(S)-flunoxaprofen and (-)-(S)-naproxen isocyanate, is described. In a few simple steps the free carboxylic acids [(S)-flunoxaprofen and (S)-naproxen] are activated with ethyl chloroformate/sodium azide and transformed to the corresponding isocyanates. The crystalline reaction products display high enantiomeric and chemical purity and stability. The direction of the optical rotation of both substances is inverse to that of the corresponding carboxylic acids. At ambient temperature the reagents swiftly react with primary and secondary amines, yielding highly fluorescent ureas. The applicability of the two reagents for the resolution of racemic amines was tested with a number of pharmaceuticals (antiarrhythmics, beta-adrenergic antagonists, calcium channel blockers, centrally acting antidepressants). The diastereoisomeric derivatives were efficiently resolved and separated from side-products by means of normal and reversed-phase high-performance liquid chromatography (HPLC). The use and sufficient sensitivity of the two reagents for pharmacokinetic studies were demonstrated with a determination of plasma levels of propranolol enantiomers after oral administration of the racemic drug [80 mg (R,S)-propranolol-HCl] to two volunteers.

Pharmacokinetics of triamterene in geriatric patients--influence of piretanide and hydrochlorothiazide.

Pharmacokinetics of triamterene (single oral dose of 50 mg) and its pharmacologically active metabolite (OH-TA-ester) were determined in 20 geriatric patients with multiple diseases. Mean peak concentration of triamterene was increased in the elderly patients compared with the data of young healthy volunteers. Coadministration of piretanide seems to lower the mean plasma concentration (AUC) and the mean concentration after 24 h of triamterene as compared with the coadministration of hydrochlorothiazide.