A photodegradation study on anthelmintic benzimidazoles

Mebendazole and Albendazole, two benzimidazole drugs widely used in the treatment of helminthes infections, are photosensitive, as light catalyzes the hydrolysis of the carbamic group, with the lack of therapeutic effect. Light-sensitivity of these drugs represents a big drawback in tropical regions, where strong sunlight and high temperatures are usual and worms infections represent a serious health problem. R = Ph-CO- Mebendazole R = H7C3-S- Albendazole Samples of drug solutions were subjected to thermo- and light-exposure and analyzed by UV spectrophotometry and HPLC. The photodegradation process was monitored according to the ICH Guideline for photostability testing, by using a light cabinet equipped with a Xenon lamp, able to closely simulate sunlight. Samples were irradiated in a  range between 300 and 800 nm, by means of a glass filter, maintaining the radiant power to 350 W/m2. Photostability was checked under several conditions of temperature and pH. Ethanol solutions showed a high photodegradation rate for both drugs at 20°C, compared to a good stability recorded in the dark even at higher temperatures. No degradation of the two drugs was observed when 0.1M HCl was used as a solvent under both light and high temperature (60°C) exposure. On the contrary, albendazole was almost completely degraded, and mebendazole only partially degraded, in alkaline solution, and the degradation rate was found to be dependent on the temperature. A derivative spectrophotometric procedure for the simultaneous determination of the drugs and their respective degradation products was defined successfully. The results were statistically compared with those obtained by a new HPLC reference method. Validation of the method was achieved by analysing synthetic mixtures. Each component was demonstrated to respond linearly, over a 190-450 nm wavelength range, for sets of concentration within 5.0 and 25.0 g mL-1 and between 0.25 and 2.00 g mL-1 for drugs and photoproducts, respectively. The mean recovery of the methods was calculated to lie between 99.51  2.16 for mebendazole and albendazole and 97.65  4.57 for their respective photoproducts. The proposed methods were applied successfully to the analysis of commercial drug formulations with results in agreement with the content of drug declared on confection labels. The photodegradation product content of assayed pharmaceutical specialties was found to be anyhow lower than 1.3%.