Abstract
Tadalafil is one of the most potent inhibitors against Type V phosphodiesterase (PDE‐5) along with sildenafil and vardenafil exhibiting high efficacy in treatment for erectile dysfunction (ED). Due to their high ED efficacy, many illegal attempts to add their derivatives in dietary supplements have been made and LC‐ESI‐MS/MS has proven to be a powerful tool to expose such malfeasance. In contrast to sildenafil or vardenafil‐based analogues, tadalafil analogues have limited structural variation mainly localized at N2‐position to maintain their pharmacological activity. Therefore, it is probable that even the illegal yet unknown tadalafil analogues have similar structural aspect. After careful examination of ESI‐MS/MS spectra of tadalafil and its eight analogues, a detailed fragmentation mechanism with three major pathways was identified. Based on this mechanism, four distinct common ions [m/z 135 (C1), m/z 262 (C2), m/z 197 (C3), m/z 169 (C4)] and additional identifier ions (I1~I7) that reflect the structural variations were assigned.
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