Steady-state pharmacokinetic interaction of modified-dose indinavir and rifabutin.

Background: Combined administration of the human immunodeficiency virus protease inhibitor indinavir (800 mg every 8 hours) with the antimycobacterial rifabutin (300 mg daily) results in a significant decrease in indinavir concentrations with subsequent risk of treatment failure, as well as a significant increase in rifabutin concentrations with increased toxicity. Therefore this study was designed to evaluate alternative dosing regimens.

Methods: Eighteen healthy volunteers received 300 mg rifabutin daily alone for 14 days and then 1000 mg indinavir every 8 hours plus rifabutin at a reduced dose of 150 mg daily, given at 8 am or noon in a randomized crossover sequence for 14 days. Ten human immunodeficiency virus-infected subjects received 800 mg indinavir every 8 hours for 14 days and then 1000 mg indinavir every 8 hours plus 150 mg rifabutin daily at 8 am for 14 days. Twenty-four-hour pharmacokinetic sampling was performed at the end of each 14-day study period.

Results: Indinavir, 1000 mg every 8 hours, coadministered with 150 mg rifabutin daily produced an area under the concentration-time curve similar to that of 800 mg indinavir every 8 hours. The mean area under the concentration-time curve values of rifabutin and 25-desacetyl rifabutin, when 150 mg rifabutin every morning was coadministered simultaneously with 1000 mg indinavir every 8 hours, were 70% and 120% higher than with 300 mg rifabutin daily alone. Drug concentrations were not different when rifabutin and indinavir were administered simultaneously at 8 am or staggered by 4 hours.

Conclusions: Increasing indinavir's dose to 1000 mg every 8 hours when coadministered with rifabutin at a reduced dose of 150 mg daily compensates for rifabutin induction of indinavir metabolism. Rifabutin concentrations were still higher than with rifabutin alone despite a 50% reduction of rifabutin dose, which is the current recommendation when these 2 drugs are combined. The clinical significance of the increase in rifabutin and 25-desacetyl rifabutin concentrations is not known.