Comparative pharmacokinetics of theophylline and ethylenediamine following single and repeated doses of a sustained-release aminophylline preparation to volunteers.

Plasma concentrations of theophylline and ethylenediamine have been measured by HPLC after a single dose and also after a further four consecutive doses at 12 h intervals of a tablet containing 225 mg aminophylline in a sustained release matrix (Phyllocontin) had been taken by volunteers. A dissolution study of the product showed the release of theophylline to follow zero-order kinetics, to be independent of bath pH, and to be complete in 6.5 h, while the release of ethylenediamine depended upon the pH of the medium and was more extensive in acid solutions. After one tablet, the plasma theophylline concentrations reached a maximum of 4 micrograms ml-1 at 5 h and fell slightly at 7 h. Immediately before the fifth dose. The areas under the plasma level-time curves (AUC) were significantly (P less than 0.001) increased by the chronic regimen, and although the individual values were highly variable, the increase in AUC from the first to the fifth dose was significantly (P less than 0.001) correlated within each subject. Ethylenediamine concentrations after a single dose reached a peak of 0.16 micrograms ml-1 at 1 h, and returned to baseline values in 5-7 h. After the fifth dose, the plasma levels and kinetics were no different from those obtained with the first dose indicating that ethylenediamine did not accumulate as a result of chronic administration of aminophylline in a form designed to give steady-state levels of theophylline.