The Effect of Raising Gastric pH with Ranitidine on the Absorption and Elimination of Theophylline from a Sustained-Release Theophylline Tablet

Prior to evaluating the effect of ranitidine on theophylline absorption from a sustained-release theophylline tablet, the effect of ranitidine on the time course of gastric pH in 12 healthy subjects was evaluated with an encapsulated radio-telemetry device (Heidelberg capsule). Gastric pH was measured hourly from 7 AM to 1 PM prior to beginning ranitidine treatment at 2 PM (150 mg every 4 hr for eight doses). The next day, pH was again measured hourly from 7 AM to 7 PM. Subjects fasted overnight and remained fasted until lunch at 11 AM. Prior to ranitidine treatment, the mean morning gastric pH remained between 1.5 and 2.2. After lunch, the pH increased to 2.2–2.3. During ranitidine treatment the mean morning gastric pH measurements were 5.5 to 5.8, decreasing after lunch to 3.1 by 4 PM and increasing to 3.9 at 7 PM. One week later the subjects participated in a three-way crossover theophylline bioavailability study receiving at weekly intervals, single doses at 7 AM of (a) 5 × 100-mg immediate-release tablets, (b) 2 × 300-mg sustained-release theophylline tablets, and (c) 2 × 300-mg sustained-release theophylline tablets after ranitidine pretreatment of 150 mg every 4 hr beginning at 2 PM the previous day. The increase in gastric pH with ranitidine had no effect (P > 0.05) on the rate and extent of absorption or on the elimination rate of theophylline.