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Biomedical subjects

S Riegelman

Publications and source records attributed to S Riegelman.

At least 19 recordsLinked to original sources
PubMed1984

Determination of picogram nitroglycerin plasma concentrations using capillary gas chromatography with on-column injection.

A specific, sensitive, and precise capillary gas chromatographic (GC) assay capable of analyzing picogram concentrations of nitroglycerin in human plasma was developed. The analytical procedure involves a double extraction of 1 mL of plasma with pentane, after the addition of internal standard (1 ng of 2,6-dinitrotoluene), followed by evaporation and reconstitution in 50 microL of heptane. The extract (1 microL) was injected onto a capillary column using the on-column injection technique. The GC oven temperature was programmed from 120 degrees C to 180 degrees C at a rate of 5 degrees C/min. The oven temperature was then programmed to 250 degrees C and was maintained for 10 min. The nitroglycerin and internal standard retention times were 8.6 and 11.4 min, respectively. The position of the end of the capillary column inside the detector is a critical determinant of sensitivity: the column exit must be positioned such that nitroglycerin adsorption to the detector is minimized (i.e., sensitivity maximized). The assay limit of quantitation was 25 pg/mL (CV = 7.6%) using 1 mL of plasma. This GC assay, specific for nitroglycerin in the presence of its metabolites, isosorbide dinitrate, and several other drugs, may be used to quantitate plasma levels obtained after therapeutic nitroglycerin doses.

Chromatography, Gas↗
PubMed1984

Induction of quinidine metabolism and plasma protein binding by phenobarbital in dogs.

Two porta-caval transposed mongrel dogs were studied for phenobarbital (PB) induction of quinidine disposition after separate quinidine infusions via normal intravenous route and via portal vein. The plasma concentrations of quinidine and of three metabolites measured (3-OH quinidine, quinidine N-oxide, quinidine 10,11-dihydrodiol) were quite similar between i.v. and portal vein infusions, suggesting that the liver extraction ratio for quinidine in dogs is very low. After PB pretreatment plasma quinidine concentrations at the end of a 10 hr infusion increased about twofold while the half-life decreased from a control value of about 16 hr to 6 hr. Plasma concentrations of the three major metabolites measured were also increased following PB treatment. Plasma protein binding for quinidine and two of its three measured metabolites (3-hydroxy quinidine and quinidine N-oxide) were increased after PB treatment. Pharmacokinetic analysis of the data showed a decrease in steady-state volume of distribution (Vdss) of quinidine from an average value of 153 L to 54 L after PB treatment, while the total clearance did not change (6.6 vs. 5.6 L/hr). This decrease in Vdss could be explained by an increase in plasma protein binding of quinidine after PB treatment. The unbound nonrenal clearance of quinidine was induced by PB treatment. The decrease in fraction free in plasma and increase in unbound nonrenal (hence total) clearance resulted in little or no change in total plasma clearance for quinidine. The formation rate constants calculated for two quinidine metabolites, 3-hydroxy quinidine and quinidine N-oxide, were increased after PB treatment, suggesting an induction in these two metabolic pathways. Only quinidine 10,11-dihydrodiol was found in the bile after quinidine infusion, and the biliary clearance of this metabolite was also induced after PB treatment.

Animals↗
PubMed1984

Nonlinear formation of propranolol metabolites in dogs after portacaval transpositions.

The formation of four major metabolites of propranolol by the liver was examined at steady state in three dogs that had undergone surgical portacaval transposition, following which injection of drug into the hindlimb delivers the total dose to the liver. Propranolol was infused directly into the liver via a hindlimb vein at dose rates ranging from 1.01 to 6.3 mg/min. In all dogs the formation of 4-hydroxypropranolol, alpha-naphthoxylactic acid, and propranolol glycol was saturable. Vmax and Km values were determined at steady state by relating the rate of excretion of each metabolite into bile and urine to the blood concentration of propranolol. The formation of propranolol glucuronide was a first order process. The use of a dog with a portacaval transposition has permitted development of a method to estimate, in vivo, the kinetic properties of enzymes responsible for hepatic first-pass metabolism of drugs.

Animals↗
PubMed1984

Pharmacokinetics of quinidine and three of its metabolites in man.

Disposition parameters of quinidine and three of its metabolism, 3-hydroxy quinidine, quinidine N-oxide, and quinidine 10,11-dihydrodiol, were determined in five normal healthy volunteers after prolonged intravenous infusion and multiple oral doses. The plasma concentrations of individual metabolites after 7 hr of constant quinidine infusion at a plasma quinidine level of 2.9 +/- (SD) 0.3 mg/L were: 3-hydroxy quinidine, 0.32 +/- 0.06 mg/L; quinidine N-oxide, 0.28 +/- 0.03 mg/L; and quinidine 10,11-dihydrodiol, 0.13 +/- 0.04 mg/L. Plasma trough levels after 12 oral doses of quinidine sulfate every 4 hr averaged: quinidine, 2.89 +/- 0.50 mg/L; 3-hydroxy quinidine, 0.83 +/- 0.36 mg/L; quinidine N-oxide, 0.40 +/- 0.13 mg/L; and quinidine 10,11-dihydrodiol, 0.38 +/- 0.08 mg/L. Relatively higher plasma concentrations of 3-hydroxy quinidine metabolite after oral dosing probably reflect first-pass formation of this quinidine metabolite. A two-compartment model for quinidine and a one-compartment model for each of the metabolites described the plasma concentration-time curves for both i.v. infusion and multiple oral doses. Mean (+/- SD) disposition parameters for quinidine from individual fits, after i.v. infusion were as follows: Vl, 0.37 +/- 0.09 L/kg; lambda 1, 0.094 +/- 0.009 min-1; lambda 2, 0.0015 +/- 0.0002 min-1; EX2, 0.013 +/- 0.002 min-1; clearance (ClQ), 3.86 +/- 0.83 ml/min/kg. Both plasma and urinary data were used to determine metabolic disposition parameters. Mean (+/- SD) values for the metabolites after i.v. quinidine infusion were as follows: 3-hydroxy quinidine: formation rate constant kmf, 0.0012 +/- 0.0005 min-1, volume of distribution, Vm, 0.99 +/- 0.47 L/kg; and elimination rate constant, kmu 0.0030 +/- 0.0002 min-1. Quinidine N-oxide: kmf, 0.00012 +/- 0.00003 min-1; Vm, 0.068 +/- 0.020 L/kg; and kmu, 0.0063 +/- 0.0008 min-1. Quinidine 10,11-dihydrodiol: kmf, 0.0003 +/- 0.0001 min-1; Vm, 0.43 +/- 0.29 L/kg; and kmu, 0.0059 +/- 0.0010 min-1. Oral absorption of quinidine was described by a zero order process with a bioavailability of 0.78. Concentration dependent renal elimination of 3-hydroxy quinidine was observed in two out of five subjects studied.

Administration, Oral↗
PubMed1983

Dependence of renal clearance on urine flow: a mathematical model and its application.

A mathematical model is developed to explain the dependence of renal clearance on urine flow rate. The model is tested using human data from the literature on compounds that are neither secreted nor reabsorbed by active or pH-sensitive mechanisms. The physiologically derived model explains and predicts the relationship between renal clearance and urine flow for a broad spectrum of compounds (i.e., butabarbital, chloramphenicol, creatinine, ethanol, theophylline, and urea) for which appropriate data are available.

Absorption↗
PubMed1983

Estimation of absolute bioavailability assuming steady state apparent volume of distribution remains constant.

The limitations of using estimates of extent of bioavailability (F) based on the assumption that either clearance (CL) or Varea remain, constant are discussed in relation to the situation where CL changes between doses. When estimates of F assume CL to remain constant, the extent of the error is the same for all drugs where the percentage change in CL is the same. Assuming Varea to remain constant, the error in F will vary between drugs for similar percentage changes in CL and is related to the extent to which the kinetics of the disposition process deviate from a one compartment body model. A noncompartmental method is described where, provided the reference dose is given intravenously, F can be estimated based on the assumption that Vss remains constant between doses. This method is more accurate than those based on the assumption that either CL or Varea remain, constant when CL changes between doses, but is subject to error when the terminal log-linear slope of Cp vs. time better reflects the process of absorption rather than elimination.

Biological Availability↗
PubMed1983

Disposition of caffeine and its metabolites in man.

The disposition of caffeine and its metabolites was studied in six healthy subjects by use of sensitive and specific assays. The primary degradation of caffeine in man was found to be N-demethylation and/or ring oxidation to theophylline, paraxanthine, theobromine and 1,3,7-trimethyluric acid. These compounds were further degraded to dimethylated uric acids, monomethylxanthines and monomethyluric acids. About 3 and 6% of the drug was converted to theophylline and theobromine, respectively. The elimination of paraxanthine after its formation did not follow linear kinetics. A large urine recovery of 1-methylxanthine after caffeine administration in comparison with the amount recovered after administration of theophylline suggests an inhibitory effect on the degradation of this metabolite by either caffeine itself or another metabolite of caffeine. Caffeine and its primary metabolites, dimethylxanthines, were extensively reabsorbed in the renal tubule. Their renal clearances were highly urine flow-dependent and their urinary excretion varied with urine output during the study. About 70% of the dose was recovered in the urine. Postulated degradation pathways of caffeine are discussed.

Adult↗
PubMed1982

Stereoselective disposition and glucuronidation of propranolol in humans.

Following oral dosing to steady state, the disposition of S(-)- and R(+)-propranolol and their corresponding glucuronide conjugates was studied in 4 healthy adults using doses from 40 to 320 mg/day of the racemate. Steady -state plasma concentrations of S(-)-propranolol and its corresponding glucuronide conjugate were greater than that for R(+)-propranolol and its corresponding conjugate. The average steady-state concentration of both enantiomers increased disproportionately to dose. There was a 52+/- 7 (mean +/- SD) % decrease in the intrinsic clearance (clint) of S(-)-propranolol and a 65 +/- 22% decrease in the Clint of R(+)-propranolol over the dosing range studied. The terminal elimination half-lives of S(-)-propranolol and its glucuronide conjugate were longer than for the R(+)-enantiomer at all doses. The formation of glucuzonide conjugates of S(-)- and R(+)-propranolol was best described by a saturable process in all subjects. Within individuals, the ratio of Vmax/Km for the glucuronide conjugate of S(-)-propranolol was from 2.1-to 4.9-fold greater than for the conjugate of the R(+)-enantiomer. These studies demonstrate for the first time, that propranolol undergoes stereoselective disposition in humans.

Adult↗
PubMed1982

A method for estimating within-individual variability in clearance and in volume of distribution from standard bioavailability studies.

Bioavailability studies are commonly undertaken, and most, because they involve subjects taking repeated doses of a drug, contain information on intraindividual variability in pharmacokinetics. However, because in such studies bioavailability itself is unknown, it is difficult to resolve which pharmacokinetic parameters vary within individuals. A mathematical model is presented which permits estimation of variability in clearance and in volume of distribution. When applied to pooled data arising from five theophylline bioavailability studies, this model has given statistical evidence that clearance of theophylline is inherently more variable within individuals (coefficient of variation, 13%) than volume of distribution (8%). As a result, use of the measurement AUC-beta rather than AUC as a more precise index of bioavailability is justified in studies where beta is measured with reasonable precision. The model could be applied to estimation of within-batch within-person variability in bioavailability.

Biological Availability↗
PubMed1982

Urine flow-dependence of theophylline renal clearance in man.

Theophylline renal clearance is highly dependent on urine flow rate and is neither concentration nor dose related. To examine the flow dependency, theophylline was administered in single doses (4.3 mg/kg to 8.6 mg/kg) to 14 volunteers. Seven of these volunteers participated in studies in which theophylline and metabolite concentrations were held constant at six different levels. Due to the diuretic effect of theophylline, its renal clearance contributed up to 70% of the time-averaged total clearance, dose/total area, in the first hour after a single dose. The contribution then dropped to 5% of the time-averaged total clearance when the normal urine flow rate was restored. As a consequence of extensive tubular reabsorption, the urine/plasma concentration ratio of theophylline varied with urine flow rate and approached the value of the unbound fraction in plasma. On assumption that the reabsorption is passive, a mathematical model was used to explain the urine flow dependence of reabsorption and, therefore, the renal clearance of theophylline.

Adult↗
PubMed1982

Nonlinear theophylline elimination.

Elimination kinetics of theophylline and its major metabolites were investigated in 14 healthy adults in single-dose studies and in a multiple-plateau study. The plasma concentrations of theophylline and the metabolites 3-methylxanthine (3-MX), 1-methyluric acid (1 MU), and 1,3-dimethyluric acid (13-MU) were monitored to about 0.020 mg/l and became convex descending at concentrations below 1 mg/l after single theophylline doses. Renal clearance values of 3-MX, 1-MU, and 13-MU were 12.0 +/- 1.3 l/hr, 22.5 +/-1.5 l/hr, and 22.6 +/- 1.6 l/hr. Metabolite formation of the three metabolites followed Michaelis-Menten kinetics and became capacity limited within the therapeutic range of theophylline. The apparent Michaelis-Menten parameters for each metabolite formation step were obtained by computer fitting. For the formation of 3-MX. 1-MU, and 13-MU, the approximate mean maximal rate of formation of metabolite (Vmax) values were 5 mg/hr, 13 mg/hr, and 34 mg/hr and the apparent concentration of theophylline at which metabolite formation rate is half of Vmax values were 2.7 mg/l, 9.3 mg/l, and 14.2 mg/l. The elimination of each of the metabolites was rate limited by the elimination of theophylline. Concomitant measurement of theophylline urinary excretion rate showed the renal clearance of the drug to be highly dependent on urine flow. The initial renal clearance, elevated due to diuresis, and the distribution phase tended to counterbalance the saturable metabolic formation clearance after a single therapeutic dose. Therefore, plasma theophylline concentration decayed roughly in a log-linear fashion and the convex-descending curve, characterized by capacity-limited elimination kinetics, was observed only at lower concentrations.

Administration, Oral↗
PubMed1982

An automated HPLC method for the assay of propranolol and its basic metabolites in plasma and urine.

An automated HPLC method is described for the simultaneous determination of propranolol, 4-hydroxypropranolol, and N-desisopropylpropranolol in plasma and urine before and after beta-glucuronidase/aryl sulfatase treatment. It involves extraction with ether at pH 10 in the presence of ascorbic acid, added to prevent oxidation of 4-hydroxypropranolol. The compounds are then back extracted into dilute acid and assayed on an HPLC using a fluorescence detector. Three HPLC columns have been used (a phenyl, an octyl, and an octadecyl column). The last column was found to be most reproducible with minimal intercolumn variation. The solvent system includes a combination of acetonitrile, methanol, and phosphoric acid. Concentrations as low as 0.2, 1.0, and 0.2 ng/ml of propranolol, 4-hydroxypropranolol, and N-desisopropylpropranolol, respectively, can be measured using 1 ml of plasma.

Chromatography, High Pressure Liquid↗
PubMed1982

An automated HPLC assay for simultaneous quantitation of methylated xanthines and uric acids in urine.

To investigate the elimination kinetics of caffeine and its metabolites, as well as the interaction between them, an automated HPLC method is described. This method involves a single extraction procedure, followed by a gradient elution. Fourteen methylated xanthines and uric acids are well separated with an assay sensitivity of 1 microgram/ml when one-half ml of urine is used. The assay is highly selective, from endogenous compounds, and reproducible. This method is recommended for accurate pharmacokinetic studies.

Caffeine↗
PubMed1982

A technique to study hepatic and intestinal drug metabolism separately in the dog.

A model to study hepatic and intestinal drug metabolism in the dog has been evaluated. The model is made by performing a portacaval transposition, cholecystectomy and inserting a Thomas cannula into the duodenum. The result is a healthy animal in which drugs can be infused either in a normal i.v. (forelimb) fashion, directly into the liver (hindlimb) or given orally. Blood can be sampled from a peripheral vein, peripheral artery or hepatic vein. Bile and urine can be collected continuously. Pharmacokinetics can be studied in awake, unmedicated animals. The model can be used to assess oral, i.v. or hepatic infusion of drugs and their hepatic, intestinal or pulmonary metabolism.

Animals↗