Photoreactivity of biologically active compounds. XVIII. Photostability of ofloxacin in the solid state and in a tablet formulation

The photostability of ofloxacin in the solid state has been investigated. The change in colour of uncoated and film coated ofloxacin tablets and compressed ofloxacin was studied as a function of irradiance level and total exposure energy. The degradation of ofloxacin in the various preparations was quantified by HPLC and the antimicrobial activity was determined for selected tablets. The structure of two main degradation products from ofloxacin in the solid state has been postulated from LC-MS analysis. Both products have an absorption cut-off below 400 nm and cannot explain the observed change in tablet colour. There was no apparent relationship between the change in colour and the loss of active substance or antibacterial activity for the preparations investigated. The change in colour was easily detectable at rather low exposure levels. Apparently, there was a difference in light sensitivity between the two film-coated tablet batches investigated. The results obtained were partly dependent on the conditions within the radiation chamber (e.g. exposure time and irradiance level), which emphasizes the importance of testing the samples under various conditions unless the results are unequivocal. The tablets were sensitive to visible light although ofloxacin only has a neglectible absorption above 400 nm. The film coated ofloxacin tablets did, however, absorb above 400 nm with a cut-off at approximately 520 nm. A change in tablet coating to include a component that filters visible light in addition to UV radiation might provide a solution to the discolouration problem and prevent batch to batch variations with respect to light sensitivity.