Time-dependent absorption of theophylline in man.

Sixteen healthy volunteers were given either oral or intravenous doses of aminophylline (125 mg) at 9:00 A.M. and 9:00 P.M. under controlled food conditions. Measured at regular time intervals by homogeneous enzyme immunoassay, the plasma theophylline concentrations 1.5 and 2 hours after oral aminophylline were significantly higher in the morning than in the evening (P less than 0.05). Also, the mean peak plasma concentration was significantly higher (P less than 0.05) and the time to peak concentration was faster (P = 0.02) after the morning dose. Neither the morning mean elimination half-life nor the morning mean area under the plasma concentration-time curve differed significantly from those after the evening dose. After intravenous aminophylline, no significant differences were found in the plasma theophylline concentrations and in the elimination half-life between morning and evening. Therefore, the small but statistically significant time-dependent differences in theophylline kinetics must be due to changes in absorption from the gastrointestinal tract and not to changes in distribution or elimination of the drug.