The photochemical stability of cis- and trans-isomers of tricyclic neuroleptic drugs.

The irradiation of the tranquillizers flupenthixol, clopenthixol and chlorprothixene has been found to induce rapid cis-trans isomerization. The composition of the photosatitionary mixture is not that of the batch drug and hence this process may affect the activity. Further decomposition to a thioxanthone derivative occurs rapidly in the presence of air. Exclusion of oxygen, however, does not prevent further degradation and a slower secondary isomerization is observed on prolonged irradiation. Doxepin and dothiepin also undergo analogous reactions but the isomerizations are much slower and the oxidative degradation yields many products.