Comparative Pharmacokinetics of Coumarin Anticoagulants XLIX: Nonlinear Tissue Distribution of S-Warfarin in Rats

Abstract

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 >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. The high capacity sites, which appear to be on albumin in tissues, are responsible for the limiting T:S values of S-warfarin at high concentrations of the drug. Interindividual differences in serum protein binding of warfarin in normal rats are associated with corresponding differences in tissue binding of the drug.