Lack of clinically important interaction between erythromycin and theophylline

In 11 healthy volunteers the kinetics of theophylline and the plasma levels and the urinary excretion of its metabolites were studied before and after treatment with erythromycin for 10 days. Theophylline was administered as an intravenous bolus injection (280 mg) followed by a constant intravenous infusion (23.8 +/- 4.1 mg/h) for 6 hours. The total clearance of theophylline at steady-state (63.4 +/- 9.9 vs 63.8 +/- 14.4 ml/min, before vs after erythromycin treatment) and the elimination half-life after cessation of the infusion (6.7 +/- 2.6 vs 7.5 +/- 1.8 h, before vs after treatment) did not change during the treatment with erythromycin. No difference in the formation of metabolites before and after treatment with erythromycin was detected; the findings in urine were 40.4 +/- 5.0 vs 42.1 +/- 5.4% 1,3-dimethyluric acid, 29.6 +/- 4.6 vs 30.1 +/- 5.9% 1-methyluric acid and 13.4 +/- 3.5 vs 12.5 +/- 2.2% 3-methylxanthine before and after erythromycin treatment, respectively. It is concluded that a clinically relevant interaction between erythromycin and theophylline does not occur.