Characterization of enalapril maleate: An approach using thermoanalytical, thermokinetic and spectroscopic techniques

Abstract

Enalapril maleate is a widely used drug for the treatment of cardiovascular diseases. Its mechanism of action is to inhibit the angiotensin-converting enzyme selectively. Therefore, it is metabolized to enalaprilat by liver cells. The thermal behaviour of enalapril maleate was investigated by simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC), as well as with evolved gas analysis by simultaneous thermogravimetry and differential scanning calorimetry coupled infrared spectroscopy (TG-DSC–FTIR). The results provided information on thermal stability, purity, thermal decomposition steps and the main products formed in the heating. The enalapril maleate was found to be stable up to 148 °C. Above this temperature causes thermal degradation of the substance, which occurs in two stages in an inert atmosphere (N2) and three stages in an oxidizing atmosphere (air). Through the TG-DSC–FTIR the released gases were identified as maleic anhydride as a thermal decomposition intermediate. DSC analysis showed that the material obtained 99.5 % purity, which indicates high purity. Employing both the Kissinger and Friedman equations, alongside model fitting methods, the study reveals key insights. The Kissinger method unveils an apparent activation energy of 47.07±15.45 kJ mol-1 for the complete thermal breakdown, a finding corroborated by the Friedman method. Model fitting methods, the article applies them, yielding an apparent activation energy of 55.7±3.4 kJ mol-1 with a three-dimensional diffusion thermal degradation model.

Keywords

• thermal stability
• thermal decomposition
• kinetic characterization.