Comparison of the in vitro dissolution properties and in vivo steady-state pharmacokinetics of two sustained-release theophylline preparations.

The sustained-release properties and relative bioavailability of Theolin Retard and Pharphylline Retard were studied in eight healthy adults after treatment for five days with twice daily 450 mg, respectively 425 mg. During the day-time dosing interval on the fourth and fifth day theophylline plasma concentrations were assayed by HPLC. After intake of Theolin Retard, minimum theophylline plasma concentrations were significantly higher, fluctuations in theophylline plasma concentrations were significantly smaller and t75 (the period within a dosing interval during which the plasma concentration exceeds 75% of the maximal concentration) was significantly longer than after Pharphylline Retard. Maximal concentrations and AUC values were not significantly different. For both products the plasma concentration time-curves on day 5 were significantly lower than on day 4. In vitro dissolution tests confirmed the more sustained release of theophylline from Theolin Retard. These results indicate an equal extent of absorption from the two products but better sustained-release properties for Theolin Retard.