Characterization of glassy itraconazole: a comparative study of its molecular mobility below Tg with that of structural analogues using MTDSC

Abstract

The objective of the present study was to estimate the molecular mobility of glassy itraconazole below the glass transition, in comparison with structural analogues (i.e. miconazole and ketoconazole). Glassy itraconazole and miconazole were prepared by cooling from the melt. The glassy state of the drug was investigated with modulated temperature DSC using the following conditions: amplitude ±0.212 K, period 40 s, underlying heating rate 2 K/min. The glass transition was determined from the reversing heat flow and occurred at 332.4 (±0.5) K and 274.8 (±0.4) K for itraconazole and miconazole, respectively. The jump in heat capacity at the glass transition was 303.42 (±3.43) J/mol K for itraconazole and 179.35 (±0.89) J/mol K for miconazole. The influence of the experimental conditions on the position of the glass transition of itraconazole was investigated by varying the amplitude from ±0.133 to ±0.292 K and the period from 25 to 55 s, while the underlying heating rate was kept constant at 2 K/min. Glass transition temperature, T g, was not significantly influenced by the frequency of the modulation nor by the cooling rate. However, the relaxation enthalpy at the glass transition increased with decreasing cooling rate indicating relaxation during the glass formation process. To estimate the molecular mobility of the glassy materials, annealing experiments were performed from T g−10 to T g−40 K for periods ranging from 15 min to 16 h. Fitting the extent of relaxation of glassy itraconazole to the Williams–Watts decay function and comparing the obtained values with those of amorphous miconazole and ketoconazole indicated that the molecular mobility is influenced by the complexity of the molecular structure. The more complex the structure, the more stable the amorphous state.