Pharmacokinetics of levetiracetam and its enantiomer (R)-α-ethyl-2-oxo-pyrrolidine acetamide in dogs

Purpose: The new antiepileptic drug, levetiracetam (LEV, ucb LO59), is a chiral molecule with one asymmetric carbon atom whose anticonvulsant activity is highly enantioselective. The purpose of this study was to evaluate and compare the pharmacokinetics (PK) of LEV [(S)-alpha -ethyl-2-oxo-pyrrolidine acetamide] and its enantiomer (R)-alpha -ethyl-2-oxo-pyrrolidine acetamide (REV) after i.v. administration to dogs. This is the first time that the pharmacokinetics of both enantiomers has been evaluated. Methods: Optically pure LEV and REV were synthesized, and 20 mg/kg of individual enantiomers was administered intravenously to six dogs. Plasma and urine samples were collected until 24 h, and the concentrations of LEV and REV were determined by an enantioselective assay. The levels of 2-pyrrolidone-N-butyric acid, an acid metabolite of LEV and REV, were determined by high-performance liquid chromatography (HPLC). The data were used for PK analysis of LEV and REV. Results: LEV and REV had similar mean SD values for clearance; 1.5 +/- 0.3 ml/min/kg and volume of distribution; 0.5 +/- 0.1 L/kg. The half-life (t(1/2)) and mean residence time (MRT) of REV (t(1/2), 4.3 +/- 0.8 h, and MRT, 6.0 +/- 1.1 h) were, however, significantly longer than those of LEV (t(1/2), 3.6 +/- 0.8 h, and MRT, 5.0 +/- 1.2 h). The renal clearance and fraction excreted unchanged for LEV and REV were significantly different. Conclusions: In addition to the enantioselective pharmacodynamics, alpha -ethyl-2-oxo-pyrrolidine acetamide has enantioselective PK. The enantioselectivity was observed in renal clearance. Because REV has more favorable PK in dogs than LEV, the higher antiepileptic potency of LEV is more likely due to intrinsic pharmacodynamic activity rather than to enantioselective.