Separation and purification of methadone enantiomers by continuous- and interval-flow electrophoresis.

Continuous- or free-flow electrophoresis is based upon a thin film of fluid flowing between two parallel plates. The electrolytes and the sample are continuously admitted at one end of the electrophoresis chamber and are fractionated by an array of outlet tubes at the other. Using the Octopus apparatus in a horizontal position, continuous preparative separation of methadone enantiomers in the presence of (2-hydroxypropyl)-β-cyclodextrin as a chiral selector was investigated under conditions of continuous-flow zone electrophoresis and continuous-flow isotachophoresis. The enantiomeric composition of methadone in the collected fractions was assessed by chiral capillary electrophoresis and circular-dichroism spectroscopy. In both electrophoretic modes, partial separation of the two enantiomers with an enrichment of about 80% and a throughput of 10-20 mg of racemic methadone per hour was obtained. Operating the Octopus apparatus with interrupted buffer flow during electrophoresis, a process termed interval-flow electrophoresis, resulted in complete separation of milligram quantities of the two methadone enantiomers. Furthermore, commencing with racemic methadone, continuous multistage isotachophoretic processing is shown to be suitable to purify (R)-(-)-methadone, the enantiomer with higher pharmacological activity, on a mg/h scale and at a mM concentration in the collected product stream.