Characterization of amorphous ketoconazole using modulated temperature differential scanning calorimetry

Abstract

The objective of the present study was to characterize the glassy state of ketoconazole and to calculate its molecular mobility below the glass transition, with a view to further developing the use of modulated temperature differential scanning calorimetry (MTDSC) as a means of studying relaxation behavior. Particular emphasis is placed on identifying the influence of the choice of experimental parameters on the measured values of both the glass transition temperature (Tg) and the relaxation enthalpy magnitude. Amorphous ketoconazole was studied using an amplitude of ±0.212 K, a period of 40 s, and an underlying heating rate of 2 K/min. The correction required for the calculation of the relaxation endotherm magnitude (the ‘Tg shift effect’) was demonstrated and is discussed in terms of the mechanism underpinning this phenomenon. Similarly, the influence of the choice of MTDSC experimental parameters on the measured Tg was studied by varying the amplitude from ±0.011 to ±0.424 K and the period from 25 to 50 s. The influence of the cooling rate from the melt on the magnitude of the relaxation endotherm and position of the glass transition was investigated. It was noted that the magnitude of the relaxation endotherm increased with slower cooling rates, this being ascribed to a combination of annealing during the cooling and heating cycle and a further facet of the Tg shift effect. Annealing experiments were performed at aging temperatures Tg-12–Tg-42 K for periods ranging from 10 min up to 16 h. The relaxation behavior was characterized by fitting the calculated extent of relaxation to the Williams–Watts equation. Overall, the study has highlighted theoretical and experimental issues that need to be considered when using both DSC and MTDSC for the calculation of relaxation times. © 2001 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 90:996–1003, 2001