Characteristics of serum protein binding of felodipine.

The protein binding of felodipine in concentrations ranging from 3 to 65 nmol L-1 has been characterized in pooled serum, and in isolated human plasma proteins: albumin (HSA) and a1-acid glycoprotein (AAG). Protein binding was determined by an ultrafiltration technique using an Amicon Micropartition System. Serum protein binding of felodipine (16 nmol L-1) was measured in five groups of individuals: I: healthy subjects (n = 16). II: patients with chronic renal disease before and after dialysis (n = 10). III: patients with liver disease (n = 9). IV: diabetics Type I (n = 10) and Type II (n = 12) and V: cancer patients (n = 12). Concentrations of HSA, AAG, lipoproteins and non esterified fatty acids (NEFA) were also measured. The drug was extensively bound in pooled serum and the protein binding was essentially unchanged over the concentrations of felodipine studied (99.60 +/- 0.31% at 3 nmol L-1; 99.70 +/- 0.15% at 65 nmol L-1). In albumin solution (40 g L-1) felodipine was also highly bound. The mean of percentage bound was not significantly different from that in serum and was also independent of the felodipine concentrations (98.57 +/- 0.35 at 3 nmol L-1; 98.31 +/- 0.90 at 65 nmol L-1). The extent of binding to AAG was significantly lower than in serum (p < 0.01) and HSA (p < 0.01) and was independent of felodipine concentrations (85.64 +/- 2.25 at 3 nmol L-1; 85.68 +/- 2.3 at 65 nmol L-1). The percentage of bound felodipine in group II (before dialysis) was significantly lower than in group I (p < 0.001). The variability in the percentage of bound felodipine was greater in group II, before dialysis, than in the rest of the groups. After dialysis, protein binding was similar to that in group I. HSA did not change and AAG was increased. NEFA was significantly higher after dialysis when compared with group I (p < 0.01). In vitro carbamylation of serum did not change felodipine protein binding. HSA was decreased significantly in group III patients (p < 0.05). However, protein binding did not change. Binding of felodipine in the rest of the groups was not significantly different from that in group I. Linear regression analysis of the data for all individuals indicated that the binding of felodipine was related to serum lipoproteins and that age, HSA, AAG, and NEFA were not significant determinants of binding.