Modelling of molecular phase transitions in pharmaceutical inhalation compounds: an in silico approach

Molecular dynamic simulations have been successfully utilised with molecular modelling to estimate the glass transition temperature (T(g)) of polymers. In this paper, we use a similar approach to predict the T(g) of a small pharmaceutical molecule, beclomethasone dipropionate (BDP). Amorphous beclomethasone dipropionate was prepared by spray-drying. The amorphous nature of the spray-dried material was confirmed with scanning electron microscopy, differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD). Molecular models for amorphous BDP were constructed using the amorphous cell module in Discovery studio™. These models were used in a series of molecular dynamic simulations to predict the glass transition temperature. The T(g) of BDP was determined by isothermal-isobaric molecular dynamic simulations, and different thermodynamic parameters were obtained in the temperature range of -150 to 400°C. The discontinuity at a specific temperature in the plot of temperature versus amorphous cell volume (V) and density (ρ) was considered to be the simulated T(g.) The predicted T(g) from four different simulation runs was 63.8°C ± 2.7°C. The thermal properties of amorphous BDP were experimentally determined by DSC and the experimental T(g) was found to be ∼ 65°C, in good agreement with computational simulations.