Intrasubject variation in sustained-release theophylline absorption.

Intrasubject variability in the absorption characteristics of two sustained-release theophylline products was examined. Eight healthy, nonsmoking, adult volunteers received a single 250 mg Slo-Phyllin Gyrocap on two separate study days and a single 300 mg Theo-dur tablet on two other study days. Plasma samples were collected over a 24-hr period after each dose and were assayed for theophylline content. Absorption parameters were determined, including peak plasma concentration, peak time, absorption rate, time until 90% absorbed, and area under the plasma concentration time curve. Marked intrasubject variation in these absorption parameters was found by examination of duplicate plasma concentration times curves. These differences are not apparent from examination of mean data only. The implications of this dose-to-dose variation for the individual patient are discussed.