Studies on the thermal behavior of sildenafil citrate

Thermal behavior of sildenafil citrate (SC), a vasodilator used in the treatment of both erectile dysfunction (ED) and pulmonary arterial hypertension (PAH) treatment was investigated by thermogravimetry-differential thermal analysis (TGA-DTA), differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD) and evolved gas analysis by thermogravimetry coupled to infrared spectroscopy (TGA-FTIR). Under N₂ and air atmospheres SC remained thermally stable until 180.0 °C, and then decomposed in three steps under N₂ and four steps under air. DSC curves showed two endothermic events at first heating, one related to the dehydration around 63.5 °C and the second event is a concomitant melting-decomposition process (T_peak = 199.6 °C). Cold recrystallization was observed during the second heating, with peaks at 119.8 °C and 129.7 °C and melting at 179.7 °C. XRD results for powder collected at 138 °C suggested that the phase formed is the sildenafil free base. TGA-FTIR revealed that the thermal decomposition of SC initiates with release of water, carbon dioxide, and itaconic anhydride, resulting from the decomposition of the citrate counter-ion. Above 303 °C, CO, SO₂, benzene, ethanol, 1-methylpiperazine, isocyanic acid, and methylamine were identified in the gas phase. Based on these results, a thermal behavior mechanism for sildenafil citrate was proposed.