Study on the stability and compatibility mechanism of enalapril maleate based on pKa and pH microenvironment.

The difference of stability and compatibility between the reference listed drug (RLD) and abbreviated new drug application (ANDA) formulations of enalapril maleate (EM) was evaluated using three thermoanalytical methods, including thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and thermomechanical analysis (TMA). TGA results showed high thermostability but no difference between formulations. DSC demonstrated an endothermic reaction between EM and NaHCO3 excipient, and indicating an incomplete reaction for the RLD formulation. TMA results showed differences between formulations. The most suitable pH or microenvironmental pH range that improved the stability of the EM formulation was also determined, finding that neutral pH promoted the highest stability, but that acidic conditions at pH 3 also stabilized the formulation. The mechanism for the stability of EM compounds based on their physical and chemical properties and the pKa of the compound itself was further proposed by us. These results can be used to improve the stability of other angiotensin-converting enzyme inhibitors with similar chemical structures. [ABSTRACT FROM AUTHOR]