Influence of phenytoin on theophylline clearance

Publisher Website
Google Scholar
Abstract
The influence of phenytoin [an anticonvulsant] on theophylline [an antiasthmatic] elimination was studied in 6 healthy, nonsmoking subjects not taking any other drugs. Subjects were given an i.v. infusion of theophylline (aminophylline), 5 mg/kg, and several blood specimens were drawn over the following 24 h for theophylline concentration determination. Subjects then began a 2-wk course on oral phenytoin, 300 mg/day, and on day 15 were again challenged with theophylline. For each subject theophyllline clearance (ClT) was greater when taking phenytoin (range 31-65%). Mean ClT of the 6 subjects was 45% greater when taking phenytoin than when not taking phenytoin. In 1 subject, 24-h urine collections were obtained in the untreated and treated state for analysis of theophylline metabolites. These results showed a general increase in excretion for each of the 3 major theophylline metabolites in the phenytoin pretreated state compared to the control state. Apparently, phenytoin increases theophylline elimination. Patients taking phenytoin may require larger theophylline doses than recommended to achieve therapeutic blood theophylline concentrations.

This publication has 1 reference indexed in Scilit: