Determination of zonisamide (3-sulphamoylmethyl-1,2-benzisoxazole) in plasma at therapeutic concentrations by high-performance liquid chromatography.

A selective and sensitive liquid chromatographic method for the determination of zonisamide in small (0.1 ml) plasma samples is described. After adding internal standard, a direct solvent extract of the sample is examined by reversed-phase liquid chromatography with ultra-violet spectrophotometric detection. The method is rapid, simple and capable of determining plasma levels after therapeutic ingestion of zonisamide. Some results from a dose-ranging clinical trial are presented.