No influence of doxycycline on theophylline pharmacokinetics

The influence of the antibiotic drug doxycycline on steady-state pharmacokinetics of theophylline was studied in nine healthy adults by comparing the pharmacokinetic parameters measured during a 9-day course of theophylline alone and during comedication with doxycycline. Theophylline plasma concentrations were measured by means of high performance liquid chromatography analysis. Trough theophylline plasma concentrations were measured on days 1-8. On day 9 of each of the two periods of drug administration, a plasma concentration-time curve was evaluated. No influence of doxycycline on absorption, elimination, and volume of distribution of theophylline was found. Mean steady-state plasma concentrations were not significantly different during the two treatments. It is concluded that the drugs can be given concomitantly without any dosage adjustment of theophylline.