10-hydroxycarbazepine (MHD) is the active metabolite of the new antiepileptic drug oxcarbazepine. MHD is a chiral molecule with an asymmetric carbon at position 10. The purpose of this study was to evaluate the stereoselectivity in the pharmacokinetics of the enantiomers of MHD after oral administration of the individual MHD enantiomers and the racemic mixture to dogs.
A racemic mixture of MHD and the individual MHD enantiomers were administered to six dogs in a crossover design. Plasma and urine concentrations of R(-)- and S(+)-MHD were determined by a stereoselective high-performance liquid chromatography assay.
The area under the concentration-time curve of R(-)-MHD was significantly greater than that of S(+)-MHD after the administration of the individual enantiomers but not after the administration of MHD in a racemic form. The formation clearance of the S(+)-MHD glucuronide was approximately three times greater than that of R(-)-MHD glucuronide. No difference was found in the renal clearance and protein binding of R(-)- and S(+)-MHD enantiomers.
The pharmacokinetics of the MHD enantiomers was found to be stereoselective, mainly as a result of the stereoselectivity in the glucuronidation process. The difference in the pharmacokinetic parameters found after administration of individual MHD enantiomers compared with the administration of MHD in a racemic form suggests the possibility of interaction between the two enantiomers. Stereoselective pharmacokinetic and pharmacodynamic studies are needed to evaluate the rationale of developing MHD as a new antiepileptic drug, either in a stereospecific or racemic form.
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