Pharmacokinetic modeling of the sinusoidal efflux of anionic ligands from the isolated perfused rat liver: The influence of albumin

Abstract

This study contains a pharmacokinetic analysis on the efflux of organic anions from the liver into the bloodstream (sinusoidal efflux) with specific reference to the influence of albumin. The net sinusoidal efflux rate of dibromosulfophthalein (DBSP) from preloaded livers, being the resultant of sinusoidal efflux and reuptake of ligand by hepatocytes downstream the sinusoid, can be strongly increased by the presence of bovine serum albumin (BSA), a protein having multiple binding sites for DBSP. We previously attributed this effect to a reduction of reuptake through extracellular binding of the organic anion to the protein, rather than to an intrinsic stimulatory effect on the actual membrane transport process from the cells. In the present study we tested this hypothesis using a pharmacokinetic multicompartment liver model. This model resembles the parallel tube model in that the liver is described by several compartments placed in series instead of a single well-stirred compartment and it takes into account rates of dissociation and association in binding to proteins in the sinusoidal space. The model parameters were fitted from the sinusoidal efflux and biliary excretion data from efflux experiments measuring the stimulatory effect of various concentrations of BSA. Equilibrium binding of DBSP to albumin as well as the dissociation rate constant (koff) were determined in vitro with rapid filtration techniques. The experimental data could not be fitted satisfactorily when using the experimentally obtained values of the protein association and dissociation rate constants (kon and koff). However, they could be simulated accurately assuming 16 times higher values for the association and dissociation rate constant compared to those determined in vitro. Time constants of the perfusate flow, liver (re)uptake, and protein association and dissociation indicate that binding equilibrium does not exist within the sinusoids and that, in particular at low protein concentrations, the net sinusoidal efflux rate is association rate-limited: A large fraction of the ligand effluxed from the cell into the median is taken up by the hepatocyte before binding to the proteins occurs. Higher kon and koff values predicted by the model might indicate altered DBSP-albumin binding characteristics upon passage through the liver but alternatively can be explained by an intrinsic effect of albumin on the carrier-mediated efflux process. Efflux experiments showed a marked stimulatory effect of the protein on sinusoidal efflux but only a moderate effect on biliary excretion, despite a strong decrease in liver content. These patterns indicate that sinusoidal efflux and biliary excretion occur from two different intracellular compartments that equilibrate slowly.

Tissue Distribution and Pharmacodynamics: A Complicated Relationship

“…The results from this study indicated that an increase in protein binding decreased the rate of brain uptake for all benzodiazepines, regardless of whether they were poorly or efficiently extracted by the brain. On the other hand, the efflux rate of dibromosulfophthalein from preloaded rat livers was significantly increased by adding bovine serum albumin in perfusate [136]. Kinetic analysis indicated that the increased efflux rate was due to a reduction of re-uptake through extracellular binding of the drug to protein as well as a stimulatory effect of albumin on the efflux transporter systems.…”

Prediction of human pharmacokinetics—evaluation of methods for prediction of hepatic metabolic clearance

“…It has not been possible to find a model that takes all the descriptors of hepatic drug extraction into account. These models are most commonly used for analysis of animal liver perfusion data (Chiou 1984;Weisiger 1985 Evans et al 1993;Hussein et al 1993b;Piekoszewski et al 1993;Proost et al 1993;Xu et al 1994;Pang et al 1995;Mellick & Roberts 1996Chow et al 1997;Kwon & Morris 1997a, b;Ott et al 1997;Ott & Weisiger 1997;Hung et al 1998Hung et al , 2001Hung et al , 2004Anissimov et al 1999;Haddad & Funk 2001;Anissimov & Roberts 2002;Niro et al 2003;Sahin & Rowland 2004;Siebert et al 2004;Liu & Pang 2005). These creative and complex models are of great value for the understanding of the mechanisms determining the hepatic drug metabolism in animals, but have rarely been used for prediction of CL H .…”

Gd-BOPTA Transport Into Rat Hepatocytes: Pharmacokinetic Analysis of Dynamic Magnetic Resonance Images Using a Hollow-Fiber Bioreactor

“…In isolated perfused rat livers, modification of drug uptake, 40 metabolism 41 or excretion, 42 determination of the rate limiting steps in drug disposition and identification of sites where drugs interact 43 have been investigated with compartmental analysis. In addition, pharmacokinetic analysis of MR images obtained with extracellular CAs has emerged as a promising method for diagnostic and therapeutic monitoring of cancer, 30,44 for the measurement of blood brain-barrier permeability, 45,46 and for the assessment of organ perfusion such as the liver.…”