Hydrolysis and photolysis of paroxetine, a selective serotonin reuptake inhibitor, in aqueous solutions

The hydrolysis and photolysis of paroxetine HCl, a selective serotonin reuptake inhibitor, in aqueous buffer solutions (pH 5, 7, and 9), in synthetic humic water, and in lake water were investigated at 25 C in the dark and in a growth chamber outfitted with fluorescent lamps simulating the ultraviolet (UV) output of sunlight. Paroxetine was degraded completely within 4 d by simulated sunlight in all aqueous media. Photolysis of paroxetine HCl was accelerated by increasing pH. The t(1/2) values at pH 5, 7, and 9 were 15.79, 13.11, and 11.35 h, respectively. The half-lives of paroxetine in synthetic humic water and two lake waters were slightly longer than in pH 7 buffer. Two photoproducts were detected and their structures were identified by liquid chromatography-mass spectrometry in positive mode. Photoproduct I was found to be photolytically unstable, being gradually degraded after 12 to 18 h of irradiation. However, photoproduct II was photolytically very stable throughout the experiment period, indicating that it was persistent to further photodegradation. In the dark, paroxetine in all aqueous solutions was found to be stable over a 30-d period. In conclusion, paroxetine is a relatively photolabile drug that has a possibility of photodegradation by sunlight in surface water.