In-vitro dissolution behavior of some sustained-release theophylline dosage forms.

In-vitro dissolution of theophylline from six commercially sustained-release (SR) preparations (Euphyllin retard, Euphyllin retard mite, Theovent L.A. 125 mg, Theovent L.A. 250 mg, Phyllocontin and Uniphyllin) was determined by using rotating basket and rotating bottle apparatus. The effect of pH and rotating speed on the dissolution rate of these SR products was also studied. Scanning electron microscope was used to observe the change of surface topograph and inner texture of these products before, during and after dissolution. The results indicates that the dissolution behavior was pH-dependent which corresponding to the types of SR products. The rotating bottle method was only suitable for the evaluation of release rate of spansule-type SR products and was not suitable for matrix-type SR tablets due to the sticking occurred on the bottle surface leading to faster dissolution and larger variation. The release mechanism of these six SR products can be deduced into three models. Fickian transport (Phyllocontin), non-Fickian transport (Euphyllin, Theovent L.A. 250 mg and Uniphyllin) and zero order transport (Theovent L.A. 125 mg).