Pipequaline transport from blood to brain and liver: role of plasma protein-bound drug

Human alpha-1-glycoprotein and its interactions with drugs

For about half a century, the binding of drugs to plasma albumin, the "silent receptor," has been recognized as one of the major determinants of drug action, distribution, and disposition. In the last decade, the binding of drugs, especially but not exclusively basic entities, to another plasma protein, alpha 1-acid glycoprotein (AAG), has increasingly become important in this regard. The present review points out that hundreds of drugs with diverse structures bind to this glycoprotein. Although plasma concentration of AAG is much lower than that of albumin, AAG can become the major drug binding macromolecule in plasma with significant clinical implications. Also, briefly reviewed are the physiological, pathological, and genetic factors that influence binding, the role of AAG in drug-drug interactions, especially the displacement of drugs and endogenous substances from AAG binding sites, and pharmacokinetic and clinical consequences of such interactions. It can be predicted that in the future, rapid automatic methods to measure binding to albumin and/or AAG will routinely be used in drug development and in clinical practice to predict and/or guide therapy.

Effects of blood flow on [11C]raclopride binding in the brain: model simulations and kinetic analysis of PET data

To assess the stability of different measures of receptor occupancy from [11C]raclopride (a D2 antagonist) studies with positron emission tomography, we analyze data from five test/retest studies in normal volunteers in terms of individual model parameters from a three-compartment model, the distribution volume (DV) and the ratio of DVs from a receptor-containing region of interest to a non-receptor-containing region. Large variations were found in the individual model parameters, limiting their usefulness as an indicator of change in receptor systems. The DV ratio showed the smallest variation. Individual differences were reflected in the greater intersubject variation in DV than intrasubject variation. The potential effects of blood flow on these measurements were addressed both experimentally and by simulation studies using three models that explicitly incorporate blood flow into a compartmental model that also includes receptor-ligand binding. None of the models showed any variation in the DV with changes in blood flow as long as flow was held constant during the simulation. Experimentally, blood flow was significantly reduced by hyperventilation in a human subject. The DV was found to be reduced relative to baseline in the hyperventilation study, but the DV ratio remained unchanged. The effect of elevated and reduced flow was also tested in two baboon experiments in which PCO2 was varied. Some variability in the DV ratio was observed but was not correlated with changes in blood flow. This raises the possibility that other factors indirectly related to changes in blood flow (or PCO2) may cause changes in DV, and these effects need to be considered when evaluating experimental results.

Influence of albumin on the distribution and elimination kinetics of diclofenac in the isolated perfused rat liver: analysis by the impulse-response technique and the dispersion model

The impulse-response technique was used to investigate the influence of changes in the perfusate concentration of human serum albumin (HSA; 1.5-25 g/L) on the distribution and elimination kinetics of [14C]diclofenac in the isolated perfused rat liver. Output data were analyzed by a linear systems approach in combination with the axial dispersion model of hepatic elimination. This stochastic model is characterized by a dimensionless parameter (the dispersion number, DN) that quantifies the relative spreading of a substance as it passes through the liver. The two-compartment form of the axial dispersion model, which assumes that the radial transfer of a substance between the vascular and cellular spaces proceeds at a finite rate, was used to describe the output profiles for diclofenac, thereby providing estimates for DN and the first-order rate constants for the transfer of drug between the vascular and cellular compartments (k12 and k21) and its sequestration from the cellular compartment (kel). With a change in perfusate HSA concentration, the only one of these parameters to alter significantly (analysis of variance, p < 0.05) was the uptake rate constant (k12), which increased from 0.091 +/- 0.016 (mean +/- standard deviation) to 0.79 +/- 0.09 s-1 as HSA decreased from 25 to 1.5 g/L. Most of this change could be accounted for by an increase in the fraction of diclofenac unbound in perfusate, from 0.0030 to 0.0407 as HSA decreased from 25 to 1.5 g/L.(ABSTRACT TRUNCATED AT 250 WORDS)