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.