Particular features of photolabile substances in tablets

Nifedipine and molsidomine tablets are extremely photolabile drug preparations, even at cool room light. Compared to solutions the light spectrum responsible for photodegradation is moved towards the long-wavelength range corresponding to the bathochromic shift of light absorption in the solid state. In the case of nifedipine tablets light up to 500 nm, especially the range between 400-420 nm, is degrading. Molsidomine tablets are affected only by ultraviolet light, but not by visible light. In both cases light penetrates less than 1 mm into the tablets. For nifedipine tablets the exact penetration depth could be determined due to the discolouration of the drug substance upon irradiation. It varied from 360 microm to 880 microm depending on the drug content. Since the decomposition products of nifedipine act as photostabilizers by spectral overlay, light penetration and photodegradation in nifedipine tablets are limited. The formation of gaseous and liquid decomposition products in molsidomine tablets enhances photodegradation. Changes of the tablet structure as well as dissolution and migration processes are discussed. Furthermore the degradation products donot photostabilize the drug substance due to the missing light absorption above 300 nm.