Riboflavin as a Coloring Agent of Tablets Affects the Photostability of Manidipine after the Change of Dosage Forms

Manidipine (MP) is widely used for reducing high blood pressure. Calslot® (CALS) tablets, which are the original MP medicines, and their generic medicines have been used for patients in clinical situations. The authors hypothesized that the photodegradability of MP drug substance in CALS tablets might be enhanced when the tablets were photo-exposed after the change of the dosage form by the presence of riboflavin (RF), which is utilized as a coloring agent and a well-known photosensitizer. The present study clarified that RF enhanced the photodegradation of MP when the powders and the suspensions of CALS tablets were ultraviolet light (UV) irradiated. The addition of RF to the suspension of MP standard substances also promoted MP photodegradation along with the increase of the generation rate of its main photoproduct, benzophenone. Finally, the authors performed the photostabilization of MP suspensions based on the addition of quercetin (QU), which is one of polyphenols and has both the antioxidative potency and the UV filtering potency. It is summarized that QU has a protective potency for MP’s own photodegradation, and it partially suppresses the photocatalytic effect of RF. Further studies focused on the photochemical behaviors of utilized additives for medicines are needed for their safe use.