Miscibility and interactions between 17β-estradiol and Eudragit® RS in solid dispersion

Miscibility of 17β-estradiol and Eudragit® RS in solid dispersions was determined by modulated temperature differential scanning calorimetry (MTDSC). A reduction of the 17β-estradiol melting point in Eudragit® RS solid dispersions was observed by MTDSC using heating program I in which the maximum temperature in the first heating run was lower than the 17β-estradiol melting point. The melting point depression of 17β-estradiol in solid dispersions as a function of composition could be explained by the Nishi-Wang equation indicating an interaction between 17β-estradiol and Eudragit® RS in the system. A variation of glass transition temperature (Tg) of 17β-estradiol in Eudragit® RS solid dispersion was observed by MTDSC using heating program II in which the maximum temperature in the first heating run reached the 17β-estradiol melting point. In the second heating run of heating program II, 17β-estradiol was in an amorphous form blended with Eudragit® RS. The variation in Tg of amorphous 17β-estradiol blended with Eudragit® RS could be explained by the Kwei equation, a modified version of the Gordon-Taylor equation. The parameter estimates from the Kwei equation were consistent with an interaction between 17β-estradiol and Eudragit® RS, which was due to inter-associated hydrogen bonding as deduced from the FTIR spectra of the blends.