Chem. Pharm. Bull. 53(9) 1092—1096 (2005)

pigment and is widely used as a coating material for solid pharmaceutical preparations. Recently, many studies have been performed, from a material science perspective, to modify the function of TiO2 as a strong photocatalyst. In some studies the photocatalytic activity of TiO2 has been linked to oxidation-reduction reactions. With regard to pigment use, TiO2 is usually surface-treated to prevent photocatalytic activity. On the other hand, for pharmaceutical use, TiO2 is not surface-treated, and even if the surface is treated, the amount of surface coating substance would be very small, because of its strictly regulated purity. Thus, TiO2 for pharmaceutical use may exhibit considerable photocatalytic activity. Therefore, when TiO2 is used in pharmaceutical preparations, some drugs may often be easily decomposed by light irradiation. However, on pharmaceutical research field, almost studies were reported for the effect of light resistance of TiO2 or the reaction related to TiO2 in the solution system. 7—11) We focused the solid-state reaction between TiO2 and drug substance. In our previous work, it was revealed that famotidine, known as an H2blocker, with TiO2 as an additive, was easily discolored by only light, and the photocatalytic activity of TiO2 was controlled by relative humidity. Therefore, the purpose of this preformulation study is to clarify the relationship between the addition of TiO2 and drug decomposition, so that from a thorough investigation, consistently high quality in pharmaceutical products may be assured. Mequitazine, known as a histamine H1-receptor antagonist, is slightly discolored under irradiation with light. On the basis of this fact, mequitazine may be unstable under light irradiation, and therefore it may be presumed that the photostability of mequitazine will be influenced by the photocatalytic activity of TiO2. In this study, mequitazine was employed as a model drug for fundamental study and the effect of photocatalytic activity of TiO2 on the solid-state photostability of mequitazine under various relative humidity conditions was investigated. The photodegradation products of irradiated mequitazine containing TiO2 were also investigated.