A high performance liquid chromatographic assay of cis- and trans- isomers of tricyclic neuroleptic drugs.

Tricyclic neuroleptics based upon the thioxanthene nucleus exhibit geometrical isomerism and the major pharmacological activity resides in the Z-component. An h.p.l.c. procedure which enables the separation detection and quantification of these isomers is described. The method is applicable to the analysis of flupenthixol, clopenthixol, chlorprothixene, doxepin and dothiepin. Measurement of the isomer-ratios in various samples of flupenthixol has shown that small batch to batch variations are apparent. The determination of the isomer-ratio in formulations has been shown to rely upon the complete extraction of the medicament. This is due to the differential release of the components from the tablet matrix with the cis-isomer being favoured. There is little difference observed between the adsorption isotherms of the two components (onto charcoal) but in competition experiments differential adsorption may be demonstrated. This has clear implications for the pharmacokinetics of these drugs.