Impurity profiling of liothyronine sodium by means of reversed phase HPLC, high resolution mass spectrometry, on-line H/D exchange and UV/Vis absorption

For the first time, a comprehensive investigation of the impurity profile of the synthetic thyroid API (active pharmaceutical ingredient) liothyronine sodium (LT₃Na) was performed by using reversed phase HPLC and advanced structural elucidation techniques including high resolution tandem mass spectrometry (HRMS/MS) and on-line hydrogen-deuterium (H/D) exchange. Overall, 39 compounds were characterized and 25 of these related substances were previously unknown to literature. The impurity classification system recently developed for the closely related API levothyroxine sodium (LT₄Na) could be applied to the newly characterized liothyronine sodium impurities resulting in a wholistic thyroid API impurity classification system. Furthermore, the mass-spectrometric CID-fragmentation of specific related substances was discussed and rationalized by detailed fragmentation pathways. Moreover, the UV/Vis absorption characteristics of the API and selected impurities were investigated to corroborate chemical structure assignments derived from MS data.