Differential binding of disopyramide and warfarin enantiomers to human α₁‐acid glycoprotein variants

Abstract

Aims The F1S and A genetic variants of a 1 -acid glycoprotein (AAG) change under various physiological and pathological conditions. They also vary in their drug binding abilities. We have studied the stereoselective binding ability of each of the AAG variants using enantiomers of disopyramide (DP) and warfarin (WR). Methods The AAG variants were separated by hydroxyapatite chromatography. Binding of drug enantiomers to the AAG variants was studied by the HummelDreyer method. The characteristics of the binding activities were examined by Scatchard plot analysis. The first five amino-terminal amino acids (residues 112-116) of the cyanogen bromide (CNBr) fragment (residues 112-181) of each of the separated AAG fractions were elucidated by Edman degradation. Results Commercial AAG was separated into two main fractions. Residues 112-116 of fraction 2 were identical to the amino acid sequences predicted from the AAG A gene, LAFDV, and encode the F1S variant. In fraction 3, the deduced amino acid sequence of the AAG B gene, FGSYL, was established, and encodes the A variant. The binding affinities of both DP enantiomers in fraction 3 were significantly higher than those in fraction 2. The differences between dissociation constants (Kd) in fractions 2 and 3 were 5.2-fold for (S)-DP ( P < 0.05) and 3.7-fold for (R)-DP ( P < 0.001). The dissociation constant of (S)-DP (0.39 ± 0.08 m M ) was lower than that of (R)-DP (0.53 ± 0.10 m M ) in fraction 3 [95% confidence interval (CI) -0.282, -0.010; P < 0.05], although the binding activities of the DP enantiomers were almost the same in fraction 2. By contrast WR enantiomers had a higher binding affinity in fraction 2 than in fraction 3, the differences in dissociation constants between fractions 2 and 3 being 12.6-fold for (S)-WR ( P < 0.001) and 8.3-fold for (R)-WR ( P < 0.001). The dissociation constant of (S)-WR (0.28 ± 0.10 m M ) was significantly lower than that of (R)-WR (0.48 ± 0.08 m M ) in fraction 2 (95% CI -0.369, -0.028; P < 0.05), but there were no significant differences between the binding activities of WR enantiomers in fraction 3. Conclusions DP and WR enantiomers bind preferentially to fraction 3 and fraction 2, respectively. Fractions 2 and 3 are encoded by the AAG A and the AAG B genes, respectively.

Microseparation techniques for the study of the enantioselectivity of drug–plasma protein binding

“…Warfarin is a selective ligand of ORM 1 variant of human AGP and the possibility of high binding stereoselectivity was raised (Hervé et al, 1998). Nakagawa et al (2003) determined preference in binding of (S)-warfarin to ORM 1 variant in contrast with the higher affinity of (S)-disopyramide towards ORM 2 variant.…”

Pharmacogenetics of warfarin: current status and future challenges

“…The latter are encoded by ORM1 (orosomucoid 1 gene or alpha-1-acid glycoprotein 1 gene) and ORM2 (orosomucoid 2 gene or alpha-1-acid glycoprotein 2 gene). 16,17 It has been shown that warfarin binds preferentially to certain genetic variants of alpha-1-acid glycoproteins that can be separated by chromatography. 17 Whether this has any effect on warfarin dose requirement seems rather unlikely given the binding to albumin.…”

Selective Binding of Tamsulosin to Genetic Variants of Human .ALPHA.1-Acid Glycoprotein

“…The extents of nonspecific binding of tamsulosin and warfarin to the F1/S variants were also comparable (0.09 vs. 0.11), suggesting that tamsulosin and warfarin have similar characteristics of binding to these variants. Warfarin is available as a racemate and shows stereoselective binding to AGP, 6,8) suggesting that only a mixed affinity constant for warfarin enantiomers is obtained if racemic warfarin is used for a binding study.…”

“…However, warfarin is supplied as a racemate and its binding to AGP shows stereoselectivity. 8) The affinity constant of S-warfarin for human AGP is higher than that of the R-isomer. Furthermore, warfarin shows wide inter-individual variation in its pharmacokinetics, and binds mainly to albumin.…”