Comparative pharmacokinetic studies of racemic oxiracetam and its pure enantiomers after oral administration in rats by a stereoselective HPLC method

Highlights

• An enantioselective HPLC method was developed for oxiracetam (ORC) enantiomers.

• The pharmacokinetic profiles of (S)- and (R)-ORC were characterized and compared.

• Individual enantiomer and racemate of ORC have different pharmacokinetic profiles.

• The disposition of (S)-ORC in rats was influenced by the existence of (R)-antipode.

• (S)-ORC enanpure was better absorbed than the racemate.

Abstract

Oxiracetam (ORC), a nootropic drug used for improving the cognition and memory, has an asymmetric carbon in its structure and exists as (S)- and (R)-ORC. The pharmacokinetic profiles of racemic oxiracetam and its pure enantiomers in rats were evaluated and compared by enantioselective high-performance liquid chromatography, which was performed on a Chiralpak ID column with a mobile phase of hexane–ethanol–trifluoroacetic acid (78:22:0.1, v/v/v). The method was validated with respect to selectivity, linearity, accuracy and precision, stability and the limit of quantification. The validation acceptance criteria were met in all cases. A saturating phenomenon of (S)-ORC was observed when the dosage ranged from 200 mg/kg to 800 mg/kg. The two enantiomers showed similar profiles in the absorb phase, and reached the maximum concentration at 2 h after oral administration. However, compared with the racemate group, the AUC/dose and C_max/dose ratios of (S)-ORC were higher and Cl/f was lower in enanpure (S)-ORC group. The C_max of (S)-ORC decreased from 21.3 ± 5.0 μg/ml to 13.2 ± 4.2 when (R)-ORC was co-administrated at the dose of 200 mg/kg. AUC_0–t values of (S)-ORC were different after oral administration of 200 mg/kg (S)-ORC and 400 mg/kg racemic ORC (96.7 ± 15.5 and 50.1 ± 16.3 μg h/ml). The higher absorption and slower elimination suggest that enantiopure (S)-ORC could be a promising drug that efficiently reduces clinical dosage, improves therapeutic indices, decreases toxicology risks, and results in increased therapeutic ration.

Introduction

Oxiracetam (ORC), 4-hydroxy-2-oxo-1-pyrrolidine acetamide (Fig. 1), is a nootropic drug used clinically to improve cognition and memory and also has protective effect on ischemic stroke [1], [2], [3], [4]. ORC is a chiral drug with an asymmetric carbon at position 4 of the ring and exists as (S)- and (R)-ORC (Fig. 1). Clinically it is used in the form of racemic mixture. (S)-ORC is mainly responsible for the pharmacological activity of racemic ORC, and is more active than (R)-ORC in inducing long-term potentiation in rat hippocampal slices, potentiating glutamate stimulated Ca²⁺ uptake in cultured cerebellar granule cells and reverting the scopolamine including amnesia in rats [5].

Nowadays, nonstereoselective methods (such as HPLC-UV, HPLC-FLD and LC–MS/MS) have been developed and are applied to determine racemic oxiracetam in biological samples [6], [7], [8], [9]. However, drug enantiomers may have different pharmacokinetic, toxicological and pharmacodynamic properties due to biological stereoselectivity and potential inversion. Pharmacokinetic evaluations without chiral assays could be misleading when the disposition of enantiomers is different. Camilleri et al. reported a chiral HPLC method to separate ORC enantiomers directly. However, the analysis time of that method to achieve the baseline separation of ORC enantiomers in purified samples was very long (40–50 min) and endogenous interference from biological samples was not considered in the method [10], [11], [12]. Therefore, the aim of this study is to develop and validate a simultaneous and specific stereoselective HPLC method to determine the concentration of ORC enantiomers in rat plasma.

In this study, the stereoselective pharmacokinetic profiles of racemic ORC and its pure enantiomers were investigated and compared using a specific and accurate chiral HPLC method.