Binding of tienilic acid (Diflurex®) to human serum albumin and to plasma of normal subjects and uraemic patients

The binding of tienilic acid (TA) 2,3-dichloro-4-(2-thienyl-carbonyl) phenoxyacetic acid, a new diuretic, to human serum albumin (HSA) and to the plasma of 12 uraemic patients under treatment with haemodialysis was measured by equilibrium dialysis at 37°C and pH 7.4 using14C-TA. At a total concentration of 4,5 μg/ml (expected therapeutic concentration), the unbound fraction of TA to 4% HSA and to normal plasma was 0.49±0.01% and 0.6±0.12%, respectively. The degree of binding was approximately the same from 0.5 to 132 μg/ml of total drug concentration. The free fraction of TA increased about 2-fold when total concentration of the drug increased from 0.5 to 192 μg/ml. TA has two classes of binding sites (n1=1.61, k1=201900 M−1;n2=10.8, k2=3180 M−1). The effect of ioni strength and pH on the binding of TA to HSA indicate that the drug is bound mainly by hydrophobic interaction. Acetylsalicylic acid (300 μg/ml), bilirubin (10 mg%) and palmitic acid (2∶1 molar ratio with HSA) decreased TA binding. Reduced drug plasma protein binding was observed in uraemic patients. No correlation was found between the unbound fraction of TA and the uraemic serum albumin, creatinine or blood urea concentrations. Following an intravenous injection of heparin (10.000 IU), the free fraction of TA in uraemic plasma rose from 1.7% to 4.5% after 10 min. of haemodialysis. This could be explained, at least partially, by the increased uraemic plasma free fatty acids to albumin molar ratio.