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The new metabolites of hongdenafil, homosildenafil, and hydroxyhomosildenafil were determined using LC-Q TOF-MS/MS.Biological samples of rats for in vivo study and human liver microsome for in vitro study were determined and compared.Major metabolites were identified at m/z 461.1966 or 439.2455 by piperazine N-dehydroxyethylation or N-deethylation.From five to seven metabolites were identified in hongdenafil, homosildenafil and hydroxy homosildenafil treated samples.These new metabolites could be fundamental data for the toxicity study of sildenafil analogues and forensic science fields.Recently, illegal sildenafil analogues have emerged, causing serious social issues. In spite of the importance of sildenafil analogues, their metabolic profiles or clinical effects have not been reported yet. In this study, new metabolites of illegal sildenafil analogues such as hongdenafil, homosildenafil, and hydroxyhomosildenafil were determined using liquid chromatography quadrupole-time of flight mass spectrometry (LC-Q-TOF-MS) and tandem mass spectrometry (LC-Q-TOF-MS/MS). To prepare metabolic samples, in vitro and in vivo studies were performed. For in vivo metabolites analysis, urine and feces samples of rats treated with sildenafil analogues were analyzed. For in vitro metabolites analysis, human liver microsomes incubated with sildenafil analogues were extracted and analyzed. All metabolites were characterized by LC-Q-TOF-MS and LC-Q-TOF-MS/MS. As a result, five, six, and seven metabolites were determined in hongdenafil, homosildenafil, and hydroxyhomosildenafil treated samples, respectively. These results could be applied to forensic science and other analytical fields. Moreover, these newly identified metabolites could be used as fundamental data to determine the side effect and toxicity of illegal sildenafil analogues.