Understanding the effect of UV light in systems containing clay minerals and tetracycline

Clay minerals are natural materials that can, due to their physicochemical properties and different structures, accommodate organic species and improve their photoprotection capacities. Tetracycline (TC) is a well-known antibiotic, but there is little research on its photostability. The objective of this work was to investigate the photostability of tetracycline incorporated into the following pharmaceutical grade clay minerals: montmorillonite (Mt), palygorskite (Pal), sepiolite (Sep) and montmorillonite (Veegum (c)). Photostability tests were performed by exposing samples to UV light for 200 h. The adsorption behavior of the antibiotic molecule on different clay minerals was observed. The drug/clay mineral systems were characterized by X-ray diffraction (XRD), carbon elemental analyses, N-2 adsorption/desorption isotherms, and thermogravimetry. The XRD results showed intercalation of tetracycline into the interlayer spacing of montmorillonite. In fibrous clay minerals, TC was located on the exterior surface as well as in the grooves and entrances of the structure's channels. Data from carbon elemental and specific surface area analyses indicated the incorporation of tetracycline into the clay mineral structures. Thermogravimetry showed differences in the profiles of thermal degradation in all irradiated and nonirradiated systems. For systems based on montmorillonite, the thermal results suggested hardening of the drug after UV radiation. For all analyzed samples, tetracycline incorporated into Veegum presented the highest stability under UV light.