Kinetic Investigation of the Oxidative Thermal Decomposition of Levonorgestrel

In this study, we have focused on studying the heterogenous degradation kinetics regarding the decomposition of the emergency contraceptive agent levonorgestrel (LNG), which is a second-generation synthetic progestogen that is the active component of the racemic mixture of norgestrel. The degradation processes of the active pharmaceutical ingredient (API) were compared with the ones obtained from a model system containing the API along with the excipients that are found in a commercialized pharmaceutical formulation in a mass ratio of 1:1 (LNGMIX), in order to observe if the excipients have a stabilizing or destabilizing effect on the degradation of this progestogen. To achieve this, the following investigational methods were used: FTIR (Fourier transform infrared) spectroscopy and thermal analysis (TG/DTG/DSC analysis). For the kinetic analysis, the data obtained from two main decomposition processes observed on the DTG curves were used and processed with a preliminary method, namely ASTM E698, and two isoconversional methods: Friedman and Flynn-Wall-Ozawa. The isoconversional study revealed that the decomposition mechanisms of both LNG and LNGMIX are complex, and the excipients have a stabilizing effect on the decomposition of the API in tablet.