Abstract

The effect of plasma protein binding of warfarin on its transfer into the brain and salivary gland was investigated using alpha-1-acid glycoprotein and human serum albumin (HSA) in combination or not with palmitic acid. The tissue extraction of [14C] warfarin relative to [3H]water was determined by intracarotid injection technique in male Wistar rats. The tissue extraction of warfarin varied inversely with the concentration of added serum protein, (HSA and alpha-1-acid glycoprotein), and addition of palmitic acid to HSA diminished the extraction. The fraction of drug uptaked by tissue (tissue available fraction) was always dramatically greater than the in vitro free drug fraction, and this was interpreted as an enhanced in vivo drug dissociation from the binding protein. The fraction of drug uptake by salivary gland was closer to the in vitro free fraction than the fraction of drug uptake by brain tissue. The addition of palmitic acid to HSA induced parallel changes in the in vitro free fraction of warfarin and in the brain tissue or salivary gland extraction of warfarin. These data indicate that a part of protein-bound warfarin (as determined in vitro) is available for tissue extraction via an enhanced in vivo dissociation of the drug-protein complex in the tissue microcirculation. The in vitro data were fitted to a saturable model of binding whereas the in vivo data could satisfactorily fit a model dealing with a nonsaturable model of binding, and this is probably the result of the several-fold increase in the in vivo dissociation constant.