Protein binding and the metabolism of thiamylal enantiomers in vitro

Thiamylal, a chiral thiobarbiturate, is marketed as a racemic product. We s tudied the serum protein binding and microsomal metabolism of thiamylal ena ntiomers in vitro. The unbound fraction of R(+)-thiamylal was greater than that of S(-)-thiamylal. The analysis of binding data revealed that both ena ntiomers bound to human serum albumin through only one site. In displacemen t studies with site-specific probes, dansylsarcosine, but not warfarin, sig nificantly decreased the binding of both enantiomers. The bindings of enant iomers were also decreased by octanoate and a large concentration of oleate . These findings suggest that both enantiomers bind to Site II of albumin w ith higher affinity for S(-)-enantiomer. R(+)-thiamylal was metabolized mor e rapidly than S(-)-enantiomer by human liver microsomes. An experiment wit h isoform-selective inhibitors and cytochrome P-450 (CYP) isoforms showed t hat CYP2C9 had the highest activity for the metabolism of both enantiomers, the activity being 7 to 10 times that of CYP2E1 and CYP3A4. CYP2C9 showed a significantly rapid metabolism of R(+)-enantiomer, suggesting that CYP2C9 is mainly involved in the enantioselective metabolism of thiamylal.