Stereoselective, competitive, and nonlinear plasma protein binding of ibuprofen enantiomers as determined in vivo in healthy subjects

Abstract

The plasma protein binding and competitive inhibition parameters of R(−)- and S(+)-ibuprofen were determined in vivo in 12 healthy subjects. Subjects participated in a 4×4 Latin square design in which oral solutions of drug were administered as 300 mg R (−)-ibuprofen, 300 mg S (+)-ibuprofen, 300 mg R (−)-+300 mg S (+)-ibuprofen, and 300 mg R(−)-+600 mg S (+)-ibuprofen. Unlabeled ibuprofen enantiomers were quantitated using a stereospecific reversed-phase HPLC assay, and plasma protein binding experiments were performed using radiolabeled 14 C-enantiomers and an ultrafiltration method at 37C. At therapeutic drug concentrations, the protein binding of each enantiomer was greater than 99%. Furthermore, the binding of ibuprofen enantiomers was Stereoselective and mutually competitive, as well as nonlinear. The bound-free data were fitted to a model in which the non-linearity of plasma protein binding and competition between enantiomers for binding sites could be accommodated. There were substantial differences in the affinity of ibuprofen enantiomers for protein binding sites (RP2=0.358±0.185 vs. SP2=0.979 ±0.501 μg/ml; X±SD) but no differences in their binding capacity (RP1=160±86 vs. SP1=161 ±63 μg/ml). Although statistically significant, the differences in competitive inhibition parameters were more modest (SKI=0.661 ±0.363 vs. RKI=0.436 ±0.210 μg/ml). As a result, the intrinsic binding (i.e.), P1/P2J of R(−)-ibuprofen was greater than S(±)-ibuprofen, and the unbound fraction was significantly greater for S-enantiomer vs. R-enantiomer after a given dose of R-ibuprofen or racemate.