Combined dielectric spectroscopy and thermally stimulated currents studies of the secondary relaxation process in amorphous poly(ethylene terephthalate)

Stabilization of rasburicase and physico-chemical characterization of the resulting injectable formulation

Rasburicase (Fasturtec/Elitek) is a new generation of recombinant urate oxidase administred therapeutically by intravenous infusion for the prevention or treatment of hyperuricemia during chemotherapy. To ensure a long storage period, a freeze-dried formulation was developed to guarantee the molecular integrity and enzyme activity. Screening of potential excipients was the first stage of the preformulation study. The selection was based on stability results (rasburicase solution with excipient) obtained with the isoelectric focusing profiles and residual enzyme activity. The different excipients were classified as stabilising, neutral or destabilising. A stability study was then carried out on different freeze-dried formulations containing the usual bulking agents for freeze-drying, excipients with a high glass transition temperature or competitive enzyme inhibitors having a stabilising effect. A mannitol/alanine mixture in phosphate buffer was selected from these preliminary results. Finally, the optimal content of mannitol and alanine in the freeze-dried powder was determined by an experimental design study. The water content and the appearance of the "cake", the osmolality, pH, clarity, and enzyme activity of the reconstituted solution were assessed. The formula with a mannitol/alanine ratio of 0.7 was found to be the best composition. Differential scanning calorimetry and ThermoStimulated Current technique experiments were carried out to study the amorphous phase. A glass transition temperature of about 45-500 degrees C was found. Glassy state is known to preserve stability, which was verified by the real stability data. X-ray diffraction studies have shown that alanine is in a crystallised state and that mannitol remains amorphous. Crystallised excipients participate in forming the structure of the powder and therefore help to prevent any collapse. Amorphous mannitol creates a surrounding medium favourable to the stability of the protein.

Dielectric study of the molecular mobility and the isothermal crystallization kinetics of an amorphous pharmaceutical drug substance

During the development of new pharmaceutical products based on drug substances in their amorphous form, the molecular mobility of an amorphous active ingredient was characterized in detail within a very broad time-temperature range. The relation between the isothermal crystallization kinetics and the dynamics of this amorphous substance was investigated. First, dynamic dielectric spectroscopy (DDS) and the thermostimulated current (TSC) techniques were used to analyze the molecular mobility of the amorphous drug substance over a wide frequency and temperature range (the drug substance is referred to as SSR in this text and was chosen as a model glass-forming system). Two relaxation processes, corresponding to different molecular motions, were identified. The beta(a)-relaxation process, associated with intramolecular oscillation of small dipolar groups, followed Arrhenius temperature behavior over the entire time-temperature domain that was studied. However, the main alpha(a)-relaxation process, assigned to the dielectric manifestation of the dynamic glass transition of the amorphous phase, was described by Vogel-Fulcher-Tammann (VFT) and Arrhenius behavior above and below the glass transition temperature (T(g)) respectively. The physical meaning of these complex dynamics is explained in the context of the Adam and Gibbs (AG) model, by the temperature dependence of the size of cooperatively rearranging regions (CRR) that govern the time scale of delocalized molecular motions. The distinction between the molecular mobility and the structural relaxation of amorphous systems below T(g) is discussed. This work shows that the complementary nature of both DDS and TSC techniques is essential to directly analyze the intramolecular and molecular motions of disordered phases over a wide time-temperature range above and below the T(g). Second, real-time dielectric measurements were carried out to determine the isothermal crystallization kinetics of the SSR amorphous drug. Whatever the crystalline form obtained over time in the crystallization process, the decrease of the dielectric response of amorphous phase, which is characteristic of the isothermal crystallization, was studied to monitor the time dependence of the degree of crystallinity. The characteristic crystallization time, derived from Kohlrausch-Williams-Watt (KWW)-Avrami analyses performed at different temperatures, followed an Arrhenius temperature dependence. Behaviors specific to the molecular mobility of the amorphous drug substance were compared with the characteristic crystallization time. It was concluded that the crystal growth process of the SSR drug seems to be controlled by the intramolecular motions involving the beta(a)-relaxation mode and not by the molecular motions responsible for the alpha(a)-relaxation mode in the range of temperatures >T(g). Subsequent studies will focus on the crystallization process of the SSR drug in the glassy state (T < T(g)).

Molecular mobility study of amorphous and crystalline phases of a pharmaceutical product by thermally stimulated current spectrometry

Two crystalline forms and the amorphous state of irbesartan, a pharmaceutical drug chosen as a model, were analyzed by Thermally Stimulated Current (TSC) spectroscopy, a powerful technique currently used in polymer science to investigate the molecular dynamics of heterogeneous and complex materials. Whereas no specific dielectric response was noted for the B crystalline form, the A form of irbesartan exhibited molecular motions localized inside its channel structure. The dynamics involved in the dielectric glass transition of amorphous samples followed a compensation law characteristic of highly cooperative relaxation processes. Concerning the amorphous content in physical mixtures, a calibration curve and a limit of detection (2.5%) were established. The limit of detection could be improved by optimizing the TSC experimental parameters. The amorphous sample recrystallized at a single temperature was interpreted by the "idealized one-state model" defined here to describe systems composed of identical semicrystalline particles in which amorphous and crystalline phases are independent of each other (i.e., no chemical and physical interaction between the two phases). Therefore, the idealized one-state model may be simulated by a two-state model, which is representative of the two-phase model. Other samples recrystallized through a complex annealing stage were explained by the classical one-state model in agreement with the three-phase model used to describe bulk semicrystalline systems. These results demonstrate that, as for polymers, the semicrystalline state of pharmaceutical drugs should not be considered as a single state but as a more complex system that can be described as an idealized one-state model or a one-state model depending on the applied thermal treatment. These results give a new view that should be taken into account in the development of amorphous pharmaceutical drugs and formulations.