Investigation on the photostability of tretinoin in creams

In this investigation, the photodegradation of some tretinoin cream formulations was evaluated. Several oils were selected to prepare the cream formulations: olive oil, maize oil, castor oil, isopropyl myristate and Miglyol 812. A solubility study showed that tretinoin is best soluble in castor oil (0.60g/100ml), followed by isopropyl myristate, maize oil, Miglyol 812 and olive oil, respectively, 0.35, 0.30, 0.29 and 0.22g/100ml. The photostability of tretinoin in oils is comparable with the photostability of a tretinoin lotion (ethanol/propylene glycol 50/50), castor oil and olive oil giving slightly better results than the other oils. Investigation of the photodegradation of tretinoin in o/w creams, prepared with the same oils as mentioned above, revealed that tretinoin is far more stable in the cream formulations than in the respective oils, however it is not clear whether this is due to the formulation or due to a different irradiation technique. Tretinoin seemed to be most stable in the olive oil cream, followed by the castor oil cream. However microscopic investigation revealed the presence of tretinoin crystals in the olive oil cream, while the other creams were free of it. As a conclusion, one can say that the cream prepared with castor oil seems to be the most suitable one, in terms of solubility of tretinoin and in terms of photostability.