Intraindividual relationships between serum protein binding of drugs in normal human subjects, patients with impaired renal function, and rats.

The serum protein binding of phenytoin, salicylic acid, sulfisoxazole, and warfarin was determined in normal human adults, in patients with impaired renal function (kidney donor and recipient), and in adult male Sprague--Dawley rats. The free fraction values for salicylate and sulfisoxazole were significantly correlated in all three groups. The other correlations were statistically significant in only one or two of these groups. There was a statistically significant negative correlation between albumin concentration and the free fraction values of salicylic acid and sulfisoxazole (but not of phenytoin and only under special circumstances with warfarin) in normal subjects and of phenytoin, salicylic acid, and sulfisoxazole (but not warfarin) in rats. No such correlation was observed for any of the drugs in patients with impaired renal function. These observations show that no single weakly acidic drug can serve as an index for quantitatively determining the effect of disease or species differences on the serum protein binding of other weakly acidic drugs.