A major challenge in drug development is that the majority of drugs are water insoluble, and a powerful method to conquer this obstacle is to transfer a crystalline drug into its amorphous phase (AP) or coamorphous phase (CAP) with a coformer. In the present study, the physical and chemical stabilities of an AP and a CAP based on the dihydropyridine calcium ion antagonist azelnidipine (AZE) were investigated using thermal analysis and a solution chemistry method. The identification of two APs (named α-AP and β-AP, from crystalline α-AZE and β-AZE, respectively) and one AZE-piperazine CAP was attempted using powder X-ray diffraction, temperature modulated differential scanning calorimetry and Fourier-transform infrared spectroscopy. The transition thermodynamics from the two APs and the CAP to stable crystalline β-AZE (β-Cry) were investigated using a solubility method. The solubility of the two APs, the CAP and β-Cry in 0.01 M HCl medium at 298, 304, 310, 316 and 322 K was investigated; the values obtained were used to calculate the thermodynamic parameters of the transition reaction. The transition temperatures of α-AP, β-AP and the CAP to form β-Cry in 0.01 M HCl were 237.7, 400.3, and 231.4 K, respectively. The glass transition temperature (T g) values of α-AP, β-AP and the CAP were 365.5, 358.9 and 347.6 K, respectively, indicating a high physical stability for α-AP. However, β-AP proved to be the most thermodynamically stable form at room temperature compared with α-AP and CAP in the 0.01 M HCl medium. As evidenced by those observations, no general relationship occurred between the solid physical stability and the solution chemical stability for AP and CAP. The kinetics of the solid-state decomposition, studied using DSC analysis, showed that the activation energies for decomposition of α-AP, β-AP and CAP at high temperatures were 133.0, 114.2 and 131.6 kJ mol-1, respectively.
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