The use of microcalorimetry and HPLC for the determination of degradation kinetics and thermodynamic parameters of Perindopril Erbumine in aqueous solutions

Perindopril Erbumine (PER) is one of the newly used angiotensin-converting enzyme inhibitors (ACE inhibitors) and is used for the treatment of patients with hypertension and symptomatic heart failure. It has two main degradation pathways, i.e. the degradation by hydrolysis and the degradation by cyclization. An isothermal heat conduction microcalorimetry (MC) and high pressure liquid chromatography (HPLC) were used for the characterization of aqueous solutions of PER and its stability properties. The rates of heat evolved during degradation of perindopril were measured by MC as a function of temperature and pH and from these data rate constant and change in enthalpy of the reactions were determined. With the HPLC method the concentration of perindopril and its degradation products were measured as a function of time in aqueous solutions of different pH that were stored at different temperatures. We demonstrated that reactions of degradation of perindopril at observed conditions follow the first order kinetics. The Arrhenius equation for each pH was determined. At pH 6.8 only one degradation pathway is present, i.e. the degradation by hydrolysis. Degradation constants for this pathway calculated from MC data are in good agreement with those obtained from HPLC. MC as a non-specific technique was shown to be useful in studies of PER when one reaction was present in the sample and also when more chemical and physical processes were simultaneously running.

Use of microcalorimetry in determination of stability of enalapril maleate and enalapril maleate tablet formulations

The stability properties of enalapril maleate (EM) and of different tablet formulations including EM were studied by isothermal microcalorimetry and by high performance liquid chromatography (HPLC). It was shown that water content of the sample and elevated temperature have a high impact on stability properties of the substance itself and of the formulations including this substance. The degradation is more extensive at higher water content and at elevated temperature. The type of the tablet formulation (5 or 20mg EM tablet formulation) also has an impact: the 5 EM tablet formulation is the less stable one. The heat output of individual tablet formulations was used to evaluate the enthalpy changes and to calculate the difference in the amount of degraded EM between various samples. These results agreed satisfactorily with those obtained by HPLC. Isothermal microcalorimetry proved to be a fast and predictive method that could be used in preformulation studies to accelerate the pharmaceutical development and shorten the time before launching the product to the market.