The calcium antagonists from the group of 1,4-dihydropyridine (DHP) derivatives are photolabile, and the products of their photochemical decomposition have no pharmacological activity. Like the other DHP derivatives, after exposure to light, manidipine undergoes decomposition leading to a weakening of the pharmacological activity.The protective influence of selected dyes on manidipine has been tested. The photodegradation of manidipine was performed according to the first version of the ICH Guideline for photostability testing. The solutions were illuminated by a high-pressure lamp HBO-200, and the methods used for evaluation of manidipine photodegradation were HPLC, HPLC-MS, and UV-vis spectrophotometry. Quantitative evaluation of photodegradation was made on the basis of the quantum yields of the processes with the use of Reinecke salt as a chemical actinometer.Results have shown that all the dyes studied, indigotine, azorubine, and cochineal red, slow down the process of manidipine photodegradation, and the most effective is azorubine. The role of the dyes has been determined on the basis of the Stern-Volmer dependence.The dyes studied, indigotine, azorubine, and cochineal red, can be potential photoprotectors of manidipine as they quench the excited states of the compound.
