Pharmacokinetic stereoselectivity of troglitazone, an antidiabetic agent, in the KK mouse.

Troglitazone, an oral antidiabetic agent, is an equal mixture of four stereoisomers involving two asymmetric centres. In the present study, the stereoselectivity of in vitro epimerization in plasma and organ homogenate and in vivo plasma disposition in the KK mouse, an animal model of non-insulin-dependent diabetes, was examined. In the incubation experiments at 37 degrees C, there was a fivefold to eightfold acceleration of epimerization at the 5 position of the thiazolidine ring in KK mouse plasma compared with that in buffer. However, no inversion at the 2 position of the chroman ring was observed. In addition, there was an approximately 1.3-fold difference in the epimerization rates among stereoisomers at the 2 position of the chroman ring. However, there were no differences in the values of the equilibrium constants of epimerization, and the ratio of epimerization among stereoisomers at the 5 position of thiazolidine ring was almost unity. The acceleration of epimerization is thought to be due to the high degree of protein binding because of the relationship between the initial epimerization rate and the dilution ratio of the plasma. Although acceleration of epimerization was also observed in the 20% homogenates of liver, kidney, and intestine of the KK mouse, the degree of stereoselectivity was lower than in plasma. The analysis of the plasma disposition after intravenous administration of troglitazone stereoisomers, using a kinetic model, indicated that the metabolic clearance in the liver showed a 2.5-fold maximum difference among stereoisomers and that the stereoselectivity of epimerization was low.

Determination of protein binding of troglitazone stereoisomers by fluorometric titration.

Determination of the protein binding of troglitazone is difficult because of its high adsorption to filters and membranes and the instability of the stereoisomers. We attempted to assess the protein binding of four stereoisomers of troglitazone in the plasma and albumin from several species by the method using fluorescent probes. The inhibition constants (Ki) for the stereoisomers of troglitazone were obtained from the decreases in fluorescence intensity of dansylsarcosine caused by competitive inhibition. Each stereoisomer of troglitazone displaced dansylsarcosine, a typical specific fluorescent probe for the diazepam binding site on human serum albumin (HSA). The highest binding affinity for dansylsarcosine was observed with HSA (dissociation constant, Kd,1 = 0.5 microM), while it was lowest in the mouse (Kd,1 = 18 microM). The Ki values for KK and ddY mouse plasma and mouse and rat albumin were in the range 2-15 microM, and there were no large variations among stereoisomers, the maximum differences being twofold. For human plasma and albumin, the displacement could not be accounted for by a simple competitive inhibition. Comparison between unbound fraction (fu) values calculated from thus obtained Ki values and those of a mixture of the four stereoisomers determined by high-performance frontal analysis showed that the fu values obtained by fluorometric titration were higher, while the relative differences among the stereoisomers in terms of animal species and strain were comparable for the two methods. Small differences in protein binding among stereoisomers of troglitazone may not be the major reason for their stereoselective pharmacokinetics.

Stereoselective determination of the active metabolites of a new anti-inflammatory agent (CS-670) in human and rat plasma using antibody-mediated extraction and high-performance liquid chromatography.

The main metabolites of (+-)-2-[4-(2-oxocyclohexylidenemethyl)phenyl]propionic acid (CS-670), a new pro-drug anti-inflammatory agent of the 2-arylpropionic acid type, have one or two chiral centres arising from reduction of the oxocyclohexylidene moiety in addition to an original chiral centre in the propionic acid moiety. To determine these metabolites stereoselectively, antibody-mediated extraction was investigated as a stereoselective clean-up method prior to chiral HPLC. Immunoglobulin G, which recognizes each stereoisomeric cyclohexanol moiety, was coupled to cyanogen bromide-activated Sepharose 4B to prepare re-usable immobilized antibody, and its specificity was improved by examination of a washing process after charging of samples. Plasma extracted with the immobilized antibody column was derivatized with a chiral reagent to separate the enantiomers of the propionic acid moiety by HPLC. This newly developed analytical method clarified the stereoselective biotransformation of the pro-drug to pharmacologically active forms in humans and rats related to reduction of the oxocyclohexylidene moiety and chiral inversion in the propionic acid moiety.