Cimetidine-theophylline interaction: effects of age and cimetidine dose.

Influences of cimetidine dose and age on the cimetidine-theophylline interaction were evaluated. Group Y consisted of nine young adults, aged 22-35 years, and Group O of nine elderly adults, aged 60-74 years. Each subject completed three study phases in this randomized crossover study. During Phase A, oral dosing of 5 mg/kg theophylline was followed by 14 serial blood samples drawn over 36 h. Phases B and C involved the same procedures, but with oral cimetidine treatment of either 200 or 300 mg every 6 h, respectively. Theophylline pharmacokinetic parameters for Group Y, Group O, and Groups Y + O were calculated. Analyses of variance (ANOVA) for crossover design were performed for each variable. Intragroup interphase ANOVA results were interpreted using multiple range tests (Tukey's Q). Comparisons between groups were performed using two-sided Student's t tests (alpha = 0.05). Within each phase, the area under the concentration-time curve (AUC), elimination half-life (t1/2 el), clearance (Clp), and volume of distribution (Vd) of theophylline for the elderly subjects were not significantly different from those of the younger adults. Mean changes in AUC, t1/2 el, and Clp between Phases A and B for both Groups Y and O were highly significant (29.2 vs. 40.4, 36.7 vs. 44.8, and 25.9 vs. 29.8%, respectively). Further significant changes in those parameters were associated with 1.2 g/day of cimetidine (Phase C). Alterations of theophylline pharmacokinetics by cimetidine appear to be dose-related and are of similar magnitude in elderly and young healthy adults.