Abstract
Mequitazine, a phenothiazine derivative having a quinuclidine ring, is a long-acting antihistamine drug which is sensitive to light. The colour of both a syrup and an isotonic buffer solution of the drug changed to rose-pink under irradiation with a D65 fluorescent lamp and then the mequitazine content of these preparations was measured by high-performance liquid chromatography. This showed that the mequitazine in both the syrup and the isotonic buffer solution was readily decomposed by light from the lamp. Next the photostability of the mequitazine syrup and isotonic buffer solution in various prescription bottles was investigated and it was found to decompose more quickly in a white prescription bottle. These mequitazine preparations were completely stable in a black-brown bottle. The photostability of mequitazine was closely related to the percentage transmittance of the containers.
The effectiveness of the light absorbers methyl p-hydroxybenzoate, propyl gallate, and vanillin in stabilizing the mequitazine isotonic buffer solution against light was then investigated. Vanillin exhibited an excellent photo-stabilizing effect and this was enhanced as the concentration of vanillin increased. The photostabilizing effect was closely related to the UV absorption spectrum of the additives and those having absorption spectra similar to that of mequitazine were more effective in this respect.
