Plasma theophylline concentrations following the oral administration of microcrystalline theophylline and a new sustained-release theophylline preparation to normal subjects.

Abstract

Plasma theophylline concentrations have been measured in 14 normal subjects following the oral administration of a microcrystalline theophylline preparation (MT) 187.5 mg every 6 h and a sustained-release theophylline preparation (SRT) 375 mg every 12 h for 5 days. During the 5 days, blood samples were drawn before and 2 h after each morning dose with MT, and before and 4.5 h after each morning dose of SRT. On days 1 and 5, more frequent samples were taken during the dose interval. With both preparations, steady-state plasma concentrations were achieved by the third day. The trough levels were significantly higher with SRT than with MT on days 3 and 4, and the levels at 4.5 h after SRT were significantly higher than those measured 2 h after MT on days 3, 4 and 5. Over the terminal 3 days of the study, mean theophylline concentrations with SRT ranged between 11.2 and 15.5 µg/ml at measured trough and peak times, whereas the mean trough levels with MT were always below 10 µg/ml. With adjustment for the dosage differences, the mean ratio of the areas under the plasma concentration/time curves for the final dosage interval for the two formulations (AUCSRT/AUCMT) was 1.29±0.56, suggesting that the SRT preparation was well absorbed. The “mean” steady-state plasma theophylline concentrations (AUC/dose interval) on day 5 were 11.5±4.7 µg/ml with MT and 13.7±5.7 µg/ml with SRT. Nine subjects experienced a total of 35 side-effects whilst taking MT, compared with 10 subjects complaining of 23 side-effects on SRT. These results indicate that, in normal subjects, SRT 375 mg every 12 h exhibited satisfactory sustained-release properties and achieved adequate mean plasma theophylline concentrations during chronic administration. It produced higher plasma levels and a lower incidence of side-effects than the same daily dose of MT.