Quantitative design for photostabilization of nifedipine by using titanium dioxide and/or tartrazine as colourants in model film coating systems.

The photostabilization of nifedipine by using film coating has been investigated in model systems in which a drug sample, dispersed on a glass plate, was covered with a free film and exposed to intensive light of mercury vapour lamp. The light transmission properties of films containing titanium dioxide or tartrazine alone as colourant were not always satisfactory. The tartrazine system exhibited superior light transmission properties to the titanium dioxide system at all additive concentrations. However, in combination both colourants gave much better light protection than did the colourants separately. The film-coated drug degraded following apparent first-order kinetics. The degradation rate constant decreased as both colourant concentration and film thickness increased; thus photostabilization was almost completely achieved by applying a film (thickness: 60 microns) of the binary mixture system containing only 0.7% of each of these colourants. The protective effectiveness of a film could be quantitatively evaluated by plotting the degradation rate constant against CL value of film formulation (C: concentration of colourant; L: film thickness). The degradation rate constant showed a good linear correlation with the average percent transmittance of a film in the wavelength range relating to the photolytic degradation of the drug. The average percent transmittance was thus proved to be an important and useful parameter for estimating the photostability of film-coated drugs.