Contribution of thermal methods and related techniques to the rational development of pharmaceuticals—Part 2

Bernardi L, R. Oliveira P. Characterization, Purity Determination and Decomposition Kinetics of Ezetimibe Under Non-Isothermal Conditions

Background:

Ezetimibe is a lipid-lowering agent used therapeutically alone or in combination of other drugs. The properties of the solid-state of drugs are critical factors in the pharmaceutical formulation development. Several instrumental techniques can be employed in the analysis of new formulations, but the thermoanalytical techniques, provide a fast and careful evaluation of physicochemical properties of a compound. </P><P> Objective: To carry out the physicochemical characterization, purity evaluation and non-isothermal kinetic studies of ezetimibe raw material.

Methods:

A combination of the following different analytical technics was employed: Differential Scanning Calorimetry, Thermogravimetric, Scanning Electron Microscopy, X-ray powder diffraction.

Results:

The results evidenced the crystalline characteristic of ezetimibe. The sample purity was 99.06 % ± 0.02 and the thermal decomposition followed a zero order kinetic, with activation energy of 96.56 kJ mol–1 and Arrhenius frequency factor of 3,442 x 109 min-1.

Conclusion:

The characterization of ezetimibe together with the non-isothermal kinetic degradation represents important studies for the pharmaceutical area, since it provides crucial information for the pharmacotechnical/quality control/production areas that should establish the specifications necessary to standardize the requirements of the raw material acquire to ensure the batch-to-batch reproducibility.

Thermal stability and decompositions kinetics under non-isothermal conditions of imatinib mesylate α form

The thermal decomposition and kinetic parameters of synthetized imatinib mesylate α form α form were determined by thermogravimetry (TGA/DTG) under non-isothermal conditions. The experiments were performed at a 25-940°C temperature range at five different heating rates: 2.5Kmin(-1), 5Kmin(-1), 10Kmin(-1), 15Kmin(-1) and 20Kmin(-1) per minute in a nitrogen atmosphere. Imatinib mesylate α form presents one-step mass loss during the degradation process. The thermal stability of the examined material, the melting temperature (Tonset=220.6°C) and ΔH fusion=-95.74Jg(-1) at a heating rate of 10°Cmin(-1) was established. The values of activation energies have been estimated using Kissinger, Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) methods.

Thermal behavior and decomposition kinetics of rifampicin polymorphs under isothermal and non-isothermal conditions

The thermal behavior of two polymorphic forms of rifampicin was studied by DSC and TG/DTG. The thermoanalytical results clearly showed the differences between the two crystalline forms. Polymorph I was the most thermally stable form, the DSC curve showed no fusion for this species and the thermal decomposition process occurred around 245 ºC. The DSC curve of polymorph II showed two consecutive events, an endothermic event (Tpeak = 193.9 ºC) and one exothermic event (Tpeak = 209.4 ºC), due to a melting process followed by recrystallization, which was attributed to the conversion of form II to form I. Isothermal and non-isothermal thermogravimetric methods were used to determine the kinetic parameters of the thermal decomposition process. For non-isothermal experiments, the activation energy (Ea) was derived from the plot of Log β vs 1/T, yielding values for polymorph form I and II of 154 and 123 kJ mol-1, respectively. In the isothermal experiments, the Ea was obtained from the plot of lnt vs 1/T at a constant conversion level. The mean values found for form I and form II were 137 and 144 kJ mol-1, respectively.

Determination of the melting temperature, heat of fusion, and purity analysis of different samples of zidovudine (AZT) using DSC

The determination of chemical purity, melting range, and variation of enthalpy in the process of characterizing medicines is one of the principal requirements evaluated in quality control of the pharmaceutical industry. In this study, the method of purity determination using DSC was outlined, as well as the application of this technique for the evaluation of commercial samples of zidovudine (AZT) (raw material) supplied by different laboratories. To this end, samples from six different laboratories (A, B, C, D, E, and F) and the standard reference (R) from the United States Pharmacopeia (USP) were analyzed. The DSC curves were obtained in the temperature range of 25 to 200 ºC under the dynamic atmosphere of N2 (50 mL min-1), heating rate of β=2 ºC min-1, using an Al capsule containing approximately 2 mg of sample material. The results demonstrated that the standard reference presented a proportion of 99.83% whereas the AZT samples presented a variation ranging from 97.59 to 99.54%. In addition, the standard reference was found to present a temperature of onset of melting point of 122.80 °C. Regarding the samples of active agents provided by the different laboratories, a variation ranging from 118.70 to 122.87 °C was measured. In terms of ΔHm, the samples presented an average value of 31.12 kJ mol-1.

A new approach to accelerated drug-excipient compatibility testing

The purpose of this study was to develop a method of qualitatively predicting the most likely degradants in a formulation or probing specific drug-excipient interactions in a significantly shorter time frame than the typical 1 month storage testing. In the example studied, accelerated storage testing of a solid dosage form at 50 degrees C, the drug substance SB-243213-A degraded via the formation of two oxidative impurities. These impurities reached a level of 1% PAR after 3 months. Various stressing methods were examined to try to recreate this degradation and in doing so provide a practical and reliable method capable of predicting drug-excipient interactions. The technique developed was able to mimic the 1-month's accelerated degradation in just 1 hr. The method was suitable for automated analysis, capable of multisample stressing, and ideal for use in drug-excipient compatibility screening.

Dynamics of pharmaceutical amorphous solids: the study of enthalpy relaxation by isothermal microcalorimetry

The structural relaxation time is a measure of the molecular mobility involved in enthalpy relaxation, and thus, is a measure of the dynamics of amorphous (glassy) pharmaceutical solids that determines physicochemical properties and reactivity of drugs in amorphous formulations. In this article we describe a novel method for characterization of structural relaxation using isothermal microcalorimetry, which directly measures the rate of heat release during the relaxation processes. The structural relaxation time is then obtained from a fit of the power data to the derivative version of the Kohlrausch-Williams-Watts (KWW) equation. The relaxation times of quenched and lyophilized samples of saccharides were studied using an isothermal microcalorimeter, the Thermal Activity Monitor (TAM). In addition to the KWW derivative function, a derivative equation of the modified stretched exponential function (MSE) was employed to evaluate TAM data. The later (MSE) appeared to have numerical advantages over the KWW equation. The data demonstrate, as expected, that structural relaxation times of amorphous solids depend on a number of variables, including nature of material, temperature, moisture content, thermal history, etc. Isothermal microcalorimetry with the TAM provides a very fast and reliable way to characterize the dynamics of glassy materials, which in many respects is superior to the conventional DSC approach. To the extent stability and structural relaxation dynamics in the glass are correlated, structural relaxation parameters derived by isothermal microcalorimetry may provide data useful for rational development of stable peptide and protein formulations and for the control of their processing.