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Theophylline directly relaxes the smooth muscle of the bronchial airways and pulmonary blood vessels, thus acting mainly as a bronchodilator and smooth muscle relaxant. It has also been demonstrated that aminophylline has a potent effect on diaphragmatic contractility in normal persons and may then be capable of reducing fatigability and thereby improve contractility in patients with chronic obstructive airways disease. The exact mode of action remains unsettled. Although theophylline does cause inhibition of phosphodiesterase with a resultant increase in intracellular cyclic AMP, other agents similarly inhibit the enzyme producing a rise of cyclic AMP but are unassociated with any demonstrable bronchodilation. Other mechanisms proposed include an effect on translocation of intracellular calcium; prostaglandin antagonism; stimulation of catecholamines endogenously; inhibition of cyclic guanosine monophosphate metabolism and adenosine receptor antagonism. None of these mechanisms has been proved, however.In vitro, theophylline has been shown to act synergistically with beta-agonists and there are now available data that do demonstrate an additive effect in vivo with combined use.
The half-life of theophylline is influenced by a number of known variables. It may be prolonged in chronic alcoholics, particularly those with liver disease (cirrhosis or alcoholic liver disease), in patients with congestive heart failure and in those patients taking certain other drugs (see DRUG INTERACTIONS).
Newborns and neonates have extremely slow clearance rates compared to older infants and children, i.e., those over one year. Older children have rapid clearance rates while most nonsmoking adults have clearance rates between these two extremes. In premature neonates the decreased clearance is related to oxidative pathways that have yet to be established.
|Theophylline Elimination Characteristics |
Half-life (in hours)
In cigarette smokers (1-2 packs/day) the mean half-life is 4-5 hours, much shorter than in nonsmokers. The increase in clearance associated with smoking is presumably due to stimulation of the hepatic metabolic pathway by components of cigarette smoke. The duration of this effect after cessation of smoking is unknown but may require 6 months to 2 years before the rate approaches that of the nonsmoker.
In single-dose bioavailability studies in normal volunteers, 300 mg Slo-Phyllin (theophylline, anhydrous) Syrup produced mean peak serum concentrations of 7.90 ± 1.67 pg/mL at a mean time of 1.43 ± 0.87 hours after dosing. At steady state in multiple-dose bioavailability studies with four times a day dosing (600-1000 mg/day), the mean peak-trough variation was 4.39 ± 0.77 pg/mL.
In a single-dose three-way cross-over bioavailability study, 400 mg each of the 100 mg Slo-Phyllin (theophylline, anhydrous) tablet, the 200 mg tablet and Slo-Phyllin (theophylline, anhydrous) Syrup were given to 19 volunteers. The respective means ± standard error for the bioequivalence parameters were: peak concentration (Cmax)11.18 ± 0.38, 10.91 ± 0.39 and 11.17 ± 0.40 pg/mL, time of peak concentration (Tmax) 1.67 ± 0.17, 1.84 ± 0.17 and 2.18 ± 0.21 hours; area under the curve, 0 to infinity (AUC0-)120.53 ± 9.80, 121.29 ± 9.61 and 127.37 ± 9.40. There were no statistical differences between the formulations with regard to Cmax, Tmax or AUC0-. Bioavailability relative to the Syrup was 95% for the 100 mg tablet and 96% for the 200 mg tablet.
Last reviewed on RxList: 12/8/2004
This monograph has been modified to include the generic and brand name in many instances.
Additional Slo-phyllin Information
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