Enthalpy relaxation studies of celecoxib amorphous mixtures.

Abstract

The purpose of this study was to compare the structural relaxation and molecular mobility of amorphous celecoxib (CEL) with that of CEL amorphous mixtures consisting of various excipients and to study the effect of different excipients on the relaxation of high-energy amorphous systems. The measurement of glass transition temperatures (Tg) and enthalpy relaxation were performed using differential scanning calorimetry. The interactions between drug and excipients and the absence of crystalline forms were further confirmed by conducting Fourier transform infrared spectroscopic and X-ray powder diffraction studies on same samples. All samples exhibited a single Tg value. Polymers had a prominent effect on the lowering of the relaxation rate in amorphous CEL. The lowering of the rate of relaxation was directly dependent on the concentration and type of polymer used. The total enthalpy required for relaxation was same, although additives affected the rate of relaxation. In absence of any specific interactions during Fourier transform infrared studies, it was concluded that the antiplasticizing activity of polymers is responsible for the stabilization of CEL amorphous systems. Glassy amorphous dispersions of CEL exhibited a complex type of relaxation pattern, which failed to fit in Kohlrausch-Williams-Watts equation with respect to calculation of relaxation time constants.