Dipolar Reorientations in Amorphous Nimesulide: A TSDC and DSC Study

Background: The preparation of APIs in the amorphous solid form can be a means of circumventing problems arising from poor solubility and low dissolution rate of the crystalline drugs. However, molecular mobility can be responsible for the glass instability, so that the kinetic characterization of the different relaxations that subsist in the amorphous solid is useful to allow define the conditions for greater stability of the glassy pharmaceutical. Our purpose is to use the experimental techniques of differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC) to study the thermal behavior of the pharmaceutical drug nimesulide and its slow molecular mobility in the amorphous solid state. Methods: DSC provides us a general view of the thermal behaviour of nimesulide and allows a general kinetic characterization of its glass transition relaxation. TSDC allows isolating the individual modes of motion present in nimesulide (in the temperature range between -150 degrees C and + 15 degrees C). From the experimental output of the TSDC experiments, the kinetic parameters associated with the different mobility modes of motion were obtained, which allowed a detailed characterisation of the distribution of relaxation times of the complex relaxations. Results: No molecular mobility was detected below similar to -30 degrees C. A sub-T-g relaxation, or secondary process, was found by TSDC in the temperature region between similar to -15 degrees C and similar to + 7 degrees C; this is a local mobility that is affected by physical aging, and was attributed to a slow beta-relaxation (Johari-Goldstein). The analysis by DSC and TSDC of the a-relaxation showed that nimesulide is a moderately fragile glass former. The dynamic fragility obtained by DSC was m(DSC) = 52 while that obtained by TSDC was m(TSDC) = 70. Conclusions: From the DSC study of the thermal behaviour we concluded that nimesulide has a moderate glass forming ability and a week glass stability. The fact that the cold crystallization occurs only some few tens of degrees above the glass transition temperature, and shows a slow kinetics, allowed the study of the mobility by TSDC. TSDC thus proved to be an adequate technique to study the molecular mobility in the amorphous nimesulide. However, the study by spectroscopic dielectric relaxation is probably impossible under these conditions.