Degradation of lisinopril: A physico-chemical study

Thermal analysis, IR spectroscopy and X-ray diffraction, at low and high temperatures, studies were carried out in order to study the degradation of lisinopril. TGA analyses indicating weight losses were observed when this powder was heated up to the fusion temperatures. To corroborate these data Differential Scanning Calorimetry (DSC) was applied, endothermic transitions were found at 88.8, 110.4 and 179.4 °C; the absorbed energies were ΔH = 256 J/g (first + second transitions) and ΔH = 91.63 J/g (third transition). X-ray powder diffraction (XRPD) was used to characterize lisinopril crystalline powder at 25, 125 and 190 °C. The analysis of the X-ray diffraction pattern obtained at room temperature showed that the unit cell of the lisinopril crystal corresponds to monoclinic system (space group, P1211), with cell parameters a = 94.912 Å, b = 121.223 Å, c = 94.74 Å and β = 99.39°; whereas the diffraction pattern obtained at 125 °C indicated that the unit cell of the lisinopril crystal corresponds to the monoclinic system with identical parameters as before, but different β angle (β = 105.16°). No crystallinity was found in the diffraction pattern obtained at 190 °C. In order to determine the possible cyclic process to a further heterocycle within the lisinopril structure, IR-spectra and theoretical studies were carried out.