Study of forced degradation behavior of enalapril maleate by LC and LC–MS and development of a validated stability-indicating assay method

In the present study, comprehensive stress testing of enalapril maleate was carried out according to ICH guideline Q1A(R2). The drug was subjected to acid (0.1N HCl), neutral and alkaline (0.1N NaOH) hydrolytic conditions at 80 °C, as well as to oxidative decomposition at room temperature. Photolysis was carried out in 0.1N HCl, water and 0.1N NaOH at 40 °C. Additionally, the solid drug was subjected to 50 °C for 60 days in a dri-bath, and to the combined effect of temperature and humidity, with and without light, at 40 °C/75% RH. The products formed under different stress conditions were investigated by LC and LC–MS. The LC method that could separate all degradation products formed under various stress conditions involved a C18 column and a mobile phase comprising of ACN and phosphate buffer (pH 3). The flow rate and detection wavelength were 1 ml min−1 and 210 nm, respectively. The developed method was found to be precise, accurate, specific and selective. It was suitably modified for LC–MS studies by replacing phosphate buffer with water, where pH was adjusted to 3.0 with formic acid. The drug showed instability in solution state (under acidic, neutral, alkaline and photolytic stress conditions), but was relatively stable in the solid-state, except formation of minor products under accelerated conditions. Primarily, maximum degradation products were formed in acid conditions, though the same were also produced variably under other stress conditions. The LC–MS m/z values and fragmentation patterns of two of the five products matched with enalaprilat and diketopiperazine derivative, previously known degradation products of enalapril. Also, m/z value of another product matched with an impurity listed in the drug monograph in European Pharmacopoeia. Rest two were hitherto unknown degradation products. The products were characterized through LC–MS fragmentation studies. Based on the results, a more complete degradation pathway for the drug could be proposed.