Photochemistry of a novel antimuscarinic drug fesoterodine and identification of its photodegradation products by LC–ESI–MS studies

Fesoterodine (FESO) is a novel muscarinic receptor antagonist for the treatment of overactive bladder syndrome. The aim of this work was to study the photodegradation of FESO, to determine its kinetics and to identify the photodegradation products. The photochemistry of FESO was investigated in sample solutions exposed to a UV-A (320–400 nm) and UV-C irradiation (100–280 nm) at room temperature. The photodegradation process was monitored by means of liquid chromatography method equipped with monolithic column and photodiode array detector. This drug is more photolabile under UV-C light. In methanol–water solution (1:1, v/v), approximately 62.5% of FESO decomposes after 60 min of UV-C irradiation, whereas under 6 h of UV-A light only 4.9% decomposes. FESO was shown to be photolabile and its photodegradation reaction followed the zero-order kinetics with the rate constant k = 0.5503 min−1, and t1/2 and t90% obtained were 46.92 min and 9.38 min, respectively. The main degradation products were isolated by semi-preparative liquid chromatography and identified by liquid chromatography coupled to an electrospray ionization mass spectrometry. The powdered and intact tablets were also exposed to UV-C irradiation. Based on the obtained results, a complete drug photodegradation pathway was proposed.