Physical and chemical changes which may occur during storage of the drug form often adversely affect its quality and in turn, the safety and efficacy of the treatment. The degradation of the active substance or excipients is caused in particular by sunlight. The development of modern techniques allows for the identification of changes occurring in the solid phase. There are no data on the use of hemispherical directional reflectance in the tablet stability. Recently, this method was used to distinguish the original Viagra® tablets from the counterfeit ones (Wilczynski et al. 2016). This is a fast, time-saving method of high sensitivity and specificity and no destruction of the drug form is required. The present study aimed to assess the possible usage of directional reflectance in terms of the stability of tablets during storage.

The expired (n=6; expire date 112019) and unexpired (n=6; expire date 112024) coating tablets containing nifuroxazide (Nifuroksazyd 200 Hasco, HASCO-LEK S.A., Poland) were analyzed. Directional reflectance was determined to assess the total reflectance in seven wavelength bands (i.e. ultraviolet, visible light, near infrared) using SOC-410 Directional Hemispherical Reflectometer (Surface Optics Corporation, USA). For each tablet 6 measurements within all bands were done. Statistical analysis was performed using Statistica software.

Significantly lower (p<0.001) mean directional reflectance was observed for expired tablets in relation to unexpired tablets for all the spectral bands apart from one of the infrared range (1000-1700 µm). The highest total directional reflectance was observed for one of the spectral band of near infrared (700-1100 µm) while the lowest – for the near ultraviolet band (335-380 µm) both for expired and unexpired tablets.

The presented method can be a useful tool for monitoring the stability and quality control of solid dosage forms. The results indicated that total reflectance lowered during the storage in all spectral bands except infrared.