Absolute bioavailability of theophylline from a sustained-release formulation using different intravenous reference infusions.

The effect of different intravenous infusions on the absolute bioavailability of theophylline from a sustained-release formulation has been investigated. Oral administration of 750 mg theophylline (2 capsules Euphylong 375) was referenced to intravenous aminophylline infusions corresponding to 506 mg theophylline over 8 h (63 mg.h-1) in Study 1, and to 749 mg theophylline over 14 h in Study 2. A reduction in the infusion rate from 69 to 33 mg.h-1 was made in Study 2 after 8 h in order to mimic the concentration/time profile of the oral formulation as closely as possible. The absolute bioavailability was 100 (89, 115)% in Study 1 and 88 (73, 105)% in Study 2. The lower clearance values and, as a consequence, the lower bioavailability ratios observed with the higher intravenous dose, although not significant, indicate that the absolute bioavailability of theophylline might appear to depend on the choice of the intravenous reference standard.