Improvement of theophylline anhydrate stability at high humidity by surface-physicochemical modification

To improve the stability of theophylline anhydrate (THA) at high humidity and temperature, the surface of THA was modified by adsorption of n-butanol and phase transformation kinetics and dissolution kinetics were investigated. Hydration kinetics was evaluated using differential scanning calorimetry. The samples were stored in various saturated salt solutions at 30, 40 and 50 °C. A dissolution test was performed in distilled water at 37 °C, 100 rpm. Hydration of THA and surface-modified THA (STHA) at various levels of RH at 40 °C followed a two-dimensional growth of nuclei equation (Avrami equation) including an induction period (IP). The IP of STHA was significantly longer (p < 0.05) than that of THA at each RH, but the k of STHA was not significantly different (p > 0.05) from that at each relative humidity (RH). The IP of STMA was significantly longer at 40 and 50 °C, 93% RH (p < 0.05) than at 30 °C, but there was no significant difference between 40 and 50 °C. The k of THA and STHA increased with the increase in storage temperature at 30–50 °C. Arrhenius plots of the k of THA and STHA showed a straight line. The activation energy of hydration of THA and STHA was 17.60 ± 1.38 and 28.92 ± 5.56 J/mol, respectively. Dissolution of THA and STHA followed the Hixson–Crowell equation, and the dissolution rate constant, K′, was 0.0306 ± 0.0041 and 0.0269 ± 0.0034 mg1/3 s−1 (n = 3), respectively.