Role of unstirred water layer in the exsorption of quinidine

Intestinal ++exsorption of salicylic acid, thiopentone, theophylline, and quinidine was measured during perfusion of the intestinal lumen with Tyrode solution. The effect of pectin or bovine serum albumin added to the perfusate on intestinal clearance (CLi) was investigated. Increasing pectin concentration from 0.0 to 0.5, 1.0 and 1.5% gave CLi values for quinidine of 499 +/- 18, 363 +/- 35, 237 +/- 56, and 300 +/- 28 mL h-1 kg-1, respectively. One per cent of pectin in the perfusate also decreased the CLi of thiopentone, but had no effect on the CLi of salicylic acid or theophylline. Pectin may have increased the thickness of the unstirred water layer on the mucous membrane and the resistance of drug exsorption for some drugs. When bovine serum albumin was added, drug binding in the perfusate increased, and the CLi of salicylic acid, thiopentone, and theophylline increased; the CLi of quinidine was unaltered. Co-administration of theophylline with quinidine decreased the CLi of quinidine without affecting quinidine binding in serum or in the perfusate. The CLi theophylline was not affected by quinidine. These observations are consistent with the hypothesis that the exsorption of quinidine is rate-limited by diffusion through the unstirred water layer on the mucous membrane. The CLi of quinidine is affected by the microclimate-pH in the unstirred water layer. An alternative possibility is that quinidine exsorption is mediated by a carrier-transport pathway.

Comparative drug exsorption in the perfused rat intestine

The factors affecting drug exsorption into the gastrointestinal tract are uncertain. In this study, the intestinal clearance (CLi) of compounds which vary in their lipophilicity, serum protein binding, molecular weight and ionic charge at physiological pH, has been measured. Male Sprague-Dawley rats with ligated bile ducts were infused with the test compounds through the jugular vein. The small intestine was intubated and perfused with Tyrode solution at 20 mL h-1. The CLi of the compounds investigated (urea, polyethylene glycol, inulin, albumin, dextran, barbituric acid, salicylic acid, thiobarbital, thiopental, thioseconal, theophylline, S-disopyramide and quinidine) was determined under anaesthesia by dividing the rate of a component's appearance rate in the perfusate by its carotid arterial concentration. Serum protein binding of the compounds was determined by equilibrium dialysis. The n-octanol-water partition coefficients of the compounds were measured as indices of lipophilicity. The CLi values of dextran, albumin, inulin, polyethylene glycol and urea were 0.56, 1.03, 4.5, 4.8 and 12.0 mL h-1, respectively. The larger the molecular weight of a compound, the smaller its CLi. The molecular weight is apparently one of the major determinants of CLi. Thiobarbital, thiopental and thioseconal are compounds of similar structure with increasing lipophilicity and serum protein binding. The CLi of thiobarbital, thiopental and thioseconal was proportional to the unbound fraction in serum. The unbound clearance (CLui) of three thiobarbiturates were similar (approximately 11 mL h-1). The unbound fraction of drug in serum appears to be a factor determining their CLi. Barbituric acid and salicylic acid, two acidic compounds, showed a low CLi (less than 1 mL h-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Single- and multiple-dose pharmacokinetics of ceftazidime in infected patients with varying degrees of renal function

To determine the single- and multiple-dose ceftazidime kinetics, we administered ceftazidime, 2 gm intravenous bolus every 12 hours, to 14 infected Chinese patients with various degrees of renal function. Blood samples were drawn in serial after the first and 7th dose and serum ceftazidime concentrations were measured by high pressure liquid chromatography. Ceftazidime concentration-time data were fitted to a two-compartment model with a nonlinear regression program. Ceftazidime kinetics was unaltered by repeated dosing. Both total body clearance and elimination rate constant of ceftazidime decrease significantly in proportion to the creatinine clearance estimated by Bjornsson's method. Renal insufficiency did not modify the steady-state volume of distribution (Vdss) of ceftazidime which, however, appeared to be larger than those reported previously. This larger Vdss may be explained by acute infection process, confinement to bed, and increased extracellular fluid volume as a result of hypoalbuminemia. Our study indicates the estimated creatinine clearance as a useful guide to ceftazidime dosage adjustment and also emphasizes the clinical relevance of conducting kinetic studies of antibiotics in infected patients.

Stereoselective gastrointestinal clearance of disopyramide in rabbits treated with activated charcoal

The ability of activated charcoal to enhance the drug elimination of two enantiomers, R- and S-disopyramide, was compared in rabbits. Orally administered activated charcoal significantly decreased the area under the serum concentration curve of R-disopyramide, whereas the same treatment had no effect on that of S-disopyramide. The difference could be explained by the difference in the hepatic extraction ratio of two enantiomers in rabbits. S-Disopyramide is a drug of high hepatic extraction ratio in rabbits and R-disopyramide is of intermediate hepatic extraction ratio. Equations were derived to illustrate the influence of the hepatic extraction ratio on the apparent gastrointestinal clearance. A higher hepatic extraction ratio decreases the apparent gastrointestinal clearance of a drug. This phenomenon may explain some unsuccessful experiments of gastrointestinal dialysis. When considering the use of activated charcoal to enhance systemic drug elimination, the hepatic extraction ratio of the drug should be included.

Kinetics of theophylline clearance in gastrointestinal dialysis with charcoal

The pharmacokinetic response of theophylline following the oral administration of activated charcoal was investigated in rabbits. Rabbits were continuously infused with a theophylline solution at a rate of 2.12 mg/h. At the fourth hour of theophylline infusion, 20 g of activated charcoal was administered by intubation to the rabbit (n = 12). The concentration of theophylline in serum gradually decreased after the charcoal treatment. The magnitude of the change in theophylline concentration induced by charcoal varied among animals. By comparing the steady-state theophylline concentration in the control and treated animals, the total body clearance was found to increase from 94.4 +/- 7.5 to 210 +/- 27 mL/h/kg (mean +/- SE). The rate of change of theophylline concentration in serum following charcoal treatment was fitted with a simulation curve by assuming a stepwise increase in clearance immediately following the charcoal treatment. The results indicated that activated charcoal exerted its maximum effect in increasing theophylline clearance immediately after its administration.

Pulmonary elimination of 5-fluorouracil in anesthetized rats

The pharmacokinetics of 5-fluorouracil (5-FU) was investigated in tumor-free anesthetized rats. Blood samples in the carotid artery and in the right atrium were sampled by cannulation during the steady-state infusion of 5-FU. Serum and blood samples were assayed by HPLC. After comparing the steady-state concentration of 5-FU in the right carotid artery with that in the right atrium, it was concluded that there was no pulmonary elimination of 5-FU in the anesthetized rat.

Influence of serum protein binding on hepatic clearance of S-disopyramide in the rabbit

The effect of changing the protein binding of S-disopyramide, a compound that is highly extracted by the liver on its hepatic clearance in rabbits is described. Using indocyanine green clearance as a means of measuring hepatic blood flow, qualitative changes were observed which are in agreement with the results predicted by both the 'parallel tube' and the 'well-stirred' models. However, neither model alone could explain the quantitative changes. Suggestions are put forward to explain these results.

Hepatic elimination of drugs with concentration-dependent serum protein binding

For a drug with concentration-dependent serum protein binding, the unbound fraction of drug decreases during the drug elimination process. The clearance of the drug at a given blood flow rate is lower than would be expected from the observed unbound fraction in venous blood from a noneliminating organ. Based on both the "well-stirred" and "parallel tube" models, simulations demonstrated that consideration of concentration-dependent binding during drug elimination is important when the intrinsic clearance is higher than the blood flow and when the unbound drug concentration is much greater than the dissociation equilibrium constant of the binding complex.

Effect of intra-individual change in serum protein binding on the pharmacologic response of R- and S-disopyramide in the rabbit

The influence of altered serum protein binding on the pharmacologic response to R- and S-disopyramide was studied in the rabbit. Binding changes of disopyramide were achieved by the injection of human glycoprotein fraction VI. During constant infusion of R-disopyramide, the glycoprotein injection increased the total drug concentration in serum at steady state, while the unbound concentration was unaltered. The glycoprotein injection, however, decreased the unbound concentration and increased the total concentration during the steady-state infusion of S-disopyramide. In both experiments, the measured pharmacological response (delta QRS duration) reflected the change in the unbound drug concentration.

Comparison of electrocardiographic response and disposition of R- and S-disopyramide in the rabbit

Both R- and S-disopyramide prolong the QRS interval of the electrocardiogram in the rabbit. R-disopyramide is more potent than S-disopyramide at high concentrations (greater than 6 micrograms/ml), but shows similar potency at low concentrations (less than 6 micrograms/ml). R-disopyramide shows an estimated intermediate hepatic extraction ratio with a plasma clearance of 31.0 +/- 3.2 ml/min/kg that is only to a small degree concentration-dependent. S-disopyramide plasma clearance, on the other hand, is highly concentration-dependent, with a value larger than 50 ml/min/kg at concentrations below 4 micrograms/ml, that decreases to a value of approximately 30 ml/min/kg at 16 micrograms/ml. No differences in the binding of R- and S-disopyramide to rabbit serum or rabbit serum spiked with human alpha 1-acid glycoprotein were found.

Effect of altered disopyramide binding on its pharmacologic response in rabbits

This study was undertaken to determine whether pharmacologic response to disopyramide is related to unbound or to total plasma concentration. Pharmacologic response, measured as changes in QRS duration of the EKG, was determined in rabbits at various steady-state concentrations of disopyramide under normal and altered plasma protein binding conditions. Plasma protein binding was increased by injection of 40 mg/kg of human glycoprotein fraction VI. The concentration-response curve for total concentration was displaced toward higher concentrations after increasing the plasma protein binding of disopyramide. The concentration-response curves obtained for unbound drug with and without the increased binding were superimposable. The displacement of the concentration-response curve for total drug was identical to the increase in concentration of the bound drug in plasma. These results demonstrate that the unbound concentrations of disopyramide in plasma are a better measure of the pharmacologic response than is the total plasma concentration.

Disposition of quinidine in the rabbit

Quinidine shows two-compartment characteristics in rabbits with a terminal half-life of 67 min for total drug and 58 min for unbound drug. Statistically, the values are not significantly different from each other (p greater than 0.05). The clearances for total and unbound drug are 52 and 464 ml/min/kg, respectively, and the total and unbound apparent volumes of distribution at steady state are 4.2 and 27.3 liters/kg, respectively. The unbound clearance and unbound apparent volume of distribution were inversely related to the unbound fraction of quinidine in plasma. The total clearance and apparent volume of distribution showed no relationship to the binding. Approximately 0.5% of the dose was excreted as unchanged quinidine. Six identifiable metabolites were found in the urine, accounting for approximately 14% of the dose. Two unknown metabolites were also observed in the urine. With the exception of 2'-quinidinone, these metabolites were formed in the rate-limiting step in the metabolite kinetics. The quinidine unbound fraction ranged from 0.06 to 0.23 in the eight rabbits studied. The binding of the metabolites was less pronounced, and only 3-hydroxyquinidine showed a significant correlation with quinidine binding.

Influence of administration route on drug delivery to a target organ

Mathematical relationships describing the delivery of drug to a target organ after intra-arterial, intravenous, and oral administration are presented. This discussion clearly demonstrates that administration into a blood vessel leading to the target organ often is superior to intravenous administration. However, this superiority is not clear from traditional plasma concentration monitoring data.