Influence of free fatty acid anion on the binding of warfarin to cytoplasmic proteins from rat liver

Comparative pharmacokinetics of coumarin anticoagulants. XLIX: Nonlinear tissue distribution of S-warfarin in rats

The serum protein binding of the oral anticoagulant drug warfarin varies widely among rats and largely accounts for corresponding variations in the total serum clearance of the drug. The hepatic uptake of warfarin is concentration dependent despite the concentration independence of the free fraction of warfarin in serum over a wide concentration range. This investigation was designed to determine the distribution of the S enantiomer of warfarin in rats as a function of warfarin concentration, free fraction in serum, dose, and time. Two groups of rats, one with relatively low (0.0043) and the other with relatively high (0.0105) average serum free fraction values, were selected from a large number of adult male Sprague-Dawley rats. All animals received an iv injection of S-warfarin, either 0.25 or 1.0 mg/kg, and were sacrificed at intervals over a period of 10 d. Concentrations of S-warfarin in serum, liver, kidneys, muscle, and fat were determined by HPLC. The tissue:serum concentration ratio (T:S) of the drug was highly concentration dependent, but was independent of dose, time, and (except for fat) free fraction in serum. The T:S for fat was higher in animals with the larger serum free fraction values. The T:S of S-warfarin for the liver was greater than 10 at low concentrations and reached a limiting value of 0.25 at relatively high concentrations of the drug. In general, the T:S versus concentration profiles of S-warfarin are consistent with the presence of two classes of binding sites in the tissues, one with very high affinity and low capacity, the other with lower affinity and apparently unlimited capacity under the experimental conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic analysis of albumin-mediated uptake of warfarin by perfused rat liver

We previously found that the uptake of warfarin in the presence of albumin by perfused rat liver could not be explained simply by the unbound warfarin concentration. The aim of the present study is to develop a kinetic model to account for this albumin-mediated uptake of warfarin. Single circulation indicator dilution studies on warfarin uptake were carried out in the isolated perfused rat liver in the absence and presence of various concentrations of bovine serum albumin (BSA) in the perfusate. A distributed model was fitted to the dilution data and the estimates of the influx, efflux, and sequestration rate constants were obtained. The results showed that the predicted concentration of the unbound warfarin is not high enough to explain the observed uptake rate; the liver cell surface appears to reduce the binding affinity of warfarin for BSA to 1/20 of that observed in vitro. A kinetic model which considers the interaction between albumin and the liver cell surface was fitted to the uptake rates of warfarin over a wide range of BSA concentration. The model gave a dissociation constant of the cell surface for albumin of 160 microM, which is comparable with those reported by others for the hepatic extractions of free fatty acids and rose bengal. Based on this kinetic model, the contributions of the unbound and bound warfarin to its hepatic uptake were estimated, and the bound warfarin was found to contribute most in the physiological albumin concentration range.