Concepts basic to pharmacokinetics

Protein binding and hepatic clearance: discrimination between models of hepatic clearance with diazepam, a drug of high intrinsic clearance, in the isolated perfused rat liver preparation

The influence of protein binding on the extraction ratio, and availability, of diazepam has been examined in the single-pass isolated perfused rat liver preparation. Binding of diazepam was varied by adjusting the concentration of albumin in the perfusate. In the absence of binding the extraction ratio of diazepam was high, 0.93-0.995. Extraction decreased dramatically as the degree of binding was increased. The data are more consistent with the "parallel-tube" model than with the "well-stirred" model, two perfusion models that have been used to describe hepatic drug elimination.

A physiologically based pharmacokinetic model for theophylline disposition in the pregnant and nonpregnant rat

There are numerous studies which examine the disposition of theophylline from a traditional point of view. Information about the behaviour of drugs, including theophylline, is, however, very scarce when investigating the kinetics by means of a physiological flow model. This study is concerned with the development of a predictive analytical model for the pharmacokinetics of theophylline in nonpregnant and pregnant rats. This model postulates that specific organ or tissue masses may be simulated by compartments whose elements have physiological properties, e.g., tissue volumes, blood flow, and metabolic activity. A model has been developed that has blood, brain, hepatic, muscular, pulmonary, renal, and fetal tissues. With few exceptions, the agreement was good between predicted and calculated tissue data in the pregnant and nonpregnant rats. Finally, model simulations were performed to investigate the impact of different pulmonary extraction ratios on the concentration-time profile of theophylline in a "hypothetical" human patient.

Pharmacokinetics of furosemide in man after intravenous and oral administration. Application of moment analysis

Furosemide 40 mg was administered to 8 healthy subjects as an i.v. bolus dose, as 1 tablet in the fasting state, and as 1 tablet and a solution after food intake. The i.v. data gave a total body clearance of 162 +/- 10.8 ml/min and a renal clearance of 117 +/- 11.3 ml/min; the volume of distribution at steady state was 8.3 +/- 0.61. Oral administration gave a bio-availability of the tablet (fasting) of 51%. Food intake slightly reduced the bioavailability, but not to a significant extent. There was no significant difference in availability between the tablet and the solution. Moment analysis gave a mean residence time after the i.v. dose, MRTi .v., of 51 +/- 1.5 min. The mean absorption times (MAT) for all oral doses were significantly longer than the MRTi .v., indicating absorption rate-limited kinetics of furosemide. On average, food delayed the absorption by 60 min. The MAT for the tablet in the postprandial state was significantly longer than for the solution, indicating dissolution rate-limited absorption of the tablet.

The effects of chronic renal insufficiency on the pharmacokinetics of doxycycline in man

The pharmacokinetics as well as erythrocyte and plasma protein binding of doxycycline were studied in fifteen patients with various renal function impairments after oral doxycycline polyphosphate single administration. Plasma half-life (t 1/2), area under the plasma concentration-time curve (AUC), urinary excretion, renal clearance, erythrocyte and plasma protein binding (%) were regressed vs creatinine clearance. No significant correlations were observed between t 1/2 or AUC and renal function nor plasma protein binding and plasma albumin concentrations. Significant correlations were obtained between urinary excretion, renal clearance, erythrocyte binding, plasma protein binding and creatinine clearance. Significant correlation was obtained between haematocrit and erythrocyte binding. Constancy of overall elimination parameters in renal failure is due to parallel increase in plasma free fraction of doxycycline.

Protein binding and hepatic clearance: studies with tolbutamide, a drug of low intrinsic clearance, in the isolated perfused rat liver preparation

The influence of altered drug binding on the hepatic elimination of tolbutamide, a drug of low intrinsic clearance, and on the formation of its metabolite, hydroxytolbutamide, was examined under linear conditions at steady state in the isolated in situ single-pass perfused rat liver preparation, with perfusate flow fixed at 15 ml/min. The fraction of tulbutamide unbound in the perfusate was varied (from 0.06 to 1.0) by either varying the perfusate concentration of albumin or using albumin of different animal species. The intrinsic clearance of tolbutamide varied fourfold between preparations (0.08-0.36 ml/min/g liver). Within each preparation the data were normalized to observations with a perfusate containing no protein. Both the extraction ratio (and clearance) of tolbutamide and the fraction of tulbutamide appearing as hydroxytolbutamide in effluent perfusate, a measure of hepatic metabolism, were directly proportional to the fraction of tolbutamide unbound in the perfusate.

Drug distribution and renal excretion in the elderly

Drug responsiveness in the elderly often differs from that in younger individuals. A causative factor is often thought to be age-related changes in the quantitative fashion in which the body handles the drug. It is speculated that changes in body size, composition, and tissue perfusion lead to differences in drug distribution. The limited supportive evidence for this is reviewed, along with the practical consequences in drug therapy. More definitive data is presented on the age-related impairment of renal function and its effects on the urinary elimination of drugs and metabolites. Methods are presented to permit rationale dosage regimen modifications for elderly patients with reduced renal function.

Effect of pirprofen on glibenclamide kinetics and response

Pirprofen (a new non-steroidal anti-inflammatory agent), 200 mg 8 hourly, or placebo was administered orally to eight normal volunteers to investigate its effect on the pharmacokinetics and glucose and insulin responses after 1 mg i.v. glibenclamide. No significant changes were produced in these measures and in vitro studies showed no displacement of glibenclamide by pirprofen from plasma protein binding.