Photostabilization of Solid and Semisolid Dosage Forms

Institutfur Pharmazie—Zentrumfiir Pharmaforschung, Lehrstuhl fur Pharmazeutische Technologie, Ludwig-Maximilians-Universitat, Munich, Germany 

The majority of drug photostability studies focus on the photodegradation of drugs in solution rather than the solid state. In both cases, photoreactions are based on the absorption of energy in the form of radiation. 

In solutions, absorption can be described by the Lambert-Beer Law and depend on the coefficient of absorption, solution thickness and concentration of the solution 

Due to absorption of incident radiation in upper layers, the drug itself may have a photostabilizing effect. Photodegradation often leads to the production of colored products which themselves absorb and further shield the sample. Consequentially, degradation rates decrease with increasing concentrations as shown by the example of nifedipine solutions (Fig. 1) (3). 

A similar effect was observed for the highly photosensitive nifedipine and molsidomine tablets where the loss of content decreased with concentration (Fig. 2) (Thoma K, Aman W. In preparation) (4). 

---

The various means of photostabilization of solid and semisolid dosage forms, based on the principle of spectral overlay and opacification, have been reviewed and presented. Selection of one particular technique will depend on the particular drug substance or ingredients causing the photodegradation and its particular spectral characteristics. 

---