Routes of Photochemical Degradation

In order to be able to suggest methods of protection against drug photodecomposition, one needs to be aware of the routes of photodegradation involved. Photochemical degradation occurs by two mechanisms primary photochemical decomposition and/or secondary or photosensitizer photochemical decomposition (24). 

Photons, therefore, provide the activation energy needed for the initiation of a reaction. Whether or not the drug itself will absorb energy can be deduced from an examination of its UV-VIS spectrum. Since the incident photonic energy may be converted to heat, a photolytic reaction may be accompanied by a thermal reaction, resulting in a photothermal effect. Once a thermal reaction is initiated, it may proceed even in the dark (25). 

A photosensitizer may behave as a catalyst if not consumed or regenerated in the reaction. Ferrous ions are photooxidized to ferric ions in the presence of acid. Therefore, lack of absorption bands in the UV-VIS region of the spectrum neither ensures photostability (24) nor indicates that a product will be photostable. The presence of chlorominoxidil as an impurity in minoxidil has been reported to act as a sensitizer in the photodecomposition of minoxidil (personal communication from the Pharmacia Upjohn Company, Kalamazoo, Michigan). It is essential, therefore, that the purity of the drug substance should be determined to ensure freedom from photosensitizing impurities. 

Erythrosine was also found to promote the decomposition of phenylbutazone, when irradiated with unfiltered light from a 300 W projector bulb. In the absence of the dye, the drug is stable (27). 

The photolytic degradations of drugs are complex, and include reactions such as oxidations, reductions, cis-trans isomerizations, structural rearrangements, hydrolyses, dechlorinations, etc. Oxidation is one of the major pathways of photochemical reactions. Table 1 lists the pathways of photochemical decomposition of some therapeutic and nontherapeutic agents. 

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There are four generalized routes of photochemical degradation ... 

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