A phase I evaluation of concomitant rifabutin and didanosine in symptomatic HIV-infected patients.

It has been suggested that didanosine (ddI) may undergo hepatic metabolism. Rifabutin is an inducer of drug metabolism. Fifteen human immunodeficiency virus-infected patients whose conditions were stabilized on twice-daily doses of ddI participated in a Phase I, open-label, pharmacokinetic and safety drug interaction study between rifabutin and ddI. Twelve patients completed the study. All patients received their regular ddI dose (167-375 mg) on day 1. On days 2-13 they received once-daily rifabutin (600 mg, three patients; 300 mg, nine patients) with their regular twice-daily ddI regimen. On days 14-16 they received rifabutin alone. Serial blood and urine samples were collected for 12 h on day 1 and for 24 h on days 13 and 16, and safety evaluations were made throughout the study. Average day 1/day 13 ddI pharmacokinetic ratios and 95% confidence interval values for Cmax, AUC0-infinity, Cls/F, and t 1/2, lambda z were 1.17 (0.96-1.38), 1.13 (0.99-1.27), 0.91 (0.81-1.01), and 0.97 (0.79-1.15), respectively (p > 0.05 for all comparisons; paired t test). A 20% difference in AUC0-infinity could be detected with 90% power. Also, there were no significant changes in laboratory values or electrocardiograms, or in rifabutin pharmacokinetic parameters when the two agents were coadministered. Based on the safety and pharmacokinetic assessments, rifabutin did not appear to interact with ddI.