Separation of amisulpride enantiomers in pharmaceutical formulations by means of capillary electrophoresis

Amisulpride (4-amino-N-[(1-ethyl-2-pyrrolidinyl)methyl]-5-(ethylsulfonyl)-2-methoxy-benzamide) is an atypical antipsychotic drug which is administered at low doses (50 mg/day) for the treatment of dysthymia and negative symptoms of schizophrenia, while at higher doses (400-1200 mg/day) it is efficacious in reducing the positive symptoms of schizophrenia. It has a chiral benzamidic structure and exerts its action by interacting with the D2 and D3 mesolimbic receptors. The main side effects of the drug are, hyperkinesia, anxiety and insomnia; minor extrapiramidal symptoms have been reported . Amisulpride is administered as the racemate, in the form of oral tablets (Deniban®, Solian® or Sulamid®), even if binding studies showed that the S(-) enantiomer is several times more potent than the R(+) enantiomer. However, only one HPLC method is currently available in the literature for the enantioselective determination of Amisulpride in biological fluids . The aim of this work is the development of a method based on capillary electrophoresis with diode-array detection for the enantiomeric separation of Amisulpride and its analysis in commercially available tablets. An uncoated fused silica capillary (effective length 8.5 cm, internal diameter 50 µm) was used, injecting the samples by pressure at the cathodic side. The effect of several parameters such as pH and buffer composition, as well as the applied voltage and the capillary temperature were investigated in order to obtain a complete separation in the shortest analysis time. The applied voltage was +15 kV, with the capillary thermostatted at 20°C and the detector set at 227 nm. Negatively charged beta-cyclodextrin was added to the BGE in order to separate Amisulpride enantiomers; the BGE consisted of a 10 mM pH 3.5 citrate buffer, containing beta-cyclodextrin sulfated (0.22%, w/v), which was sufficient to determine anodic migration of both enantiomers. Under these conditions a complete separation of Amisulpride enantiomers was obtained within a 10 min electrophoretic run, using Lamotrigine as the internal standard. A very simple procedure, consisting of a single extraction with MeOH, allows to quantitatively extract Amisulpride from the formulation. The method has been fully validated in terms of linearity (linearity range from 2.5 to 25 µg/mL), precision and accuracy (recovery percentage always ≥ 90%). Finally, it has been successfully applied to the quality control of commercially available tablets (Deniban®) containing racemic Amisulpride.