Photostability of Terbinafine Under UVA Irradiation: The Effect of UV Absorbers.

The photostability of drugs administered topically on unprotected skin is a complex phenomenon that could be connected with the loss of activity or, rather rarely, the occurrence of toxic degradation products. In this study, an in-depth investigation of the photostability of terbinafine, in both solutions and formulations, was conducted, taking into account the presence of UV absorbers such as TiO2 , ZnO, avobenzone, 3-(4-methylbenzylidene)camphor, octocrylene, benzophenone-1 and benzophenone-2. The clear photocatalytic degradation of terbinafine in ethanol solution was observed in the presence of TiO2 and/or ZnO. In other cases, terbinafine was stable, with the exception of, in the presence of octocrylene. The presumed degradation products of terbinafine were identified for the first time using LC/MS/MS, and transformation pathways were proposed. In the case of a cream formulation, the percentage of initial terbinafine content was almost unchanged in the presence of the UV absorbers benzophenone-1, benzophenone-2 and 3-(4-methylbenzylidene)camphor. In vitro cytotoxicity risk assessment of terbinafine based on photostability under UVA irradiation was evaluated using the human skin fibroblast BJ (ATCC® CRL-2522™), and this showed no statistically significant difference in cell viability for all samples analyzed.