A novel, fast and economic chiral HPLC method was developed and validated for the resolution of the four isomers of tofisopam. The separation capacity of eleven different chiral columns: six polysaccharide-type including three amylose-based (Chiralpak AD, Chiralpak AD-RH and Chiralpak AS) and three cellulose-based (Chiralcel OD, Chiralcel OJ and Lux Cellulose-4); three cyclodextrin- (Quest-BC, Quest-C2 and Quest-CM) and two macrocyclic glycopeptide antibiotic-type (Chirobiotic T and Chirobiotic TAG) were screened using polar organic or reversed-phase mode. Chiralpak AD, based on amylose tris(3,5-dimethylphenylcarbamate) as chiral selector with neat methanol was identified as the most promising system. In order to improve resolution, an orthogonal experimental design was employed, altering the concentration of 2-propanol, column temperature, and flow rate in a multivariate manner. Using the optimized method (85/15 v/v methanol/2-propanol, 40°C, flow rate: 0.7 mL/min) we were not only able to separate the four isomers but also detect 0.1% S-enantiomer as chiral impurity in R-tofisopam. This is important since the latter is under development as a single enantiomeric agent. Thermodynamic investigation revealed an unusual entropy and enthalpy-entropy co-driven controlled enantioseparation on Chiralcel OJ and on Chiralpak AD column, respectively. Our newly developed HPLC method was validated according to the ICH guidelines and its application was tested on a pharmaceutical formulation containing the racemic mixture of the drug. As a further novelty, a separate circular dichroism method was applied for the investigation of the interconversion kinetics of tofisopam conformers, which proved to be crucial for sample preparation and method validation.
Keywords: Benzodiazepine; Chiral separation; Chiral switch; Enantioseparation; Polysaccharide-type chiral selector.
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.