Theophylline and caffeine - Pharmacokinetics
Theophylline has some pharmacokinetic characteristics which deserve to be pointed out.
The bioavailability of theophylline administered by oral route is close to 90%. Its rate of absorption depends primarily on the galenic form administered: normal or sustained. Sustained-release formulation is intended to reduce the number of daily intakes necessary to maintain the effective therapeutic plasma concentration, which is comprised between 8 and 15 mg/L.
Hepatic biotransformations of xanthines are carried out according to three catabolic routes: by oxidation to 1-3-dimethyluric acid, by demethylation to methyl-3-xanthine and by demethylation and oxidation to 1-methyluric acid. The different rates of these transformations explains the fact that the plasma half-life of theophylline differs according to age and to hepatic function: in adults, the half-life is about 6 hours; in premature neonate: 20 to 30 hours; in adult smokers: 3 hours; in adults with hepatic impairment: 25 hours.
Drugs, such as troleandomycine, erythromycin, cimetidine, nilutamide, can slow down the hepatic catabolism of theophylline and elicit poisoning.
Theophylline and its metabolites cross the placenta and are found in milk.
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