The influence of efavirenz on the pharmacokinetics of a twice-daily combination of indinavir and low-dose ritonavir in healthy volunteers

Objective: This study evaluated the effect of multiple-dose efavirenz on the steady-state pharmacokinetics of the combination of indinavir (800 mg) and low-dose ritonavir (100 mg) twice a day, in which ritonavir is used to increase indinavir plasma concentrations. Methods: Eighteen healthy male volunteers participated in this multiple-dose, 1-arm, 2-period interaction study. They took a combination of 800 mg indinavir and 100 mg ritonavir with food for 15 days. From days 15 to 29, a once-daily administration of 600 mg efavirenz was added to the combination. Pharmacokinetics of indinavir and ritonavir on days 15 and 29 were compared. Results: Fourteen volunteers completed the study. The addition of efavirenz resulted in significant reductions (P < .01) in indinavir area under the curve (AUC, -25%), trough concentration (C-min, -50%), and maximum concentration (C-max, -17%). All indinavir C-min levels on day 29 remained equivalent to or above the mean Cain value described for the regimen of 800 mg indinavir three times a day, without ritonavir (0.15 mg/L). Changes in ritonavir AUC, C-min, and C-max were -36%, -39%, and -34%, respectively. Pharmacokinetics of efavirenz on day 29 were comparable with published data. Conclusions: The addition of efavirenz to a combination of 800 mg indinavir and 100 mg ritonavir twice daily results in significant decreases in AUC, C-min and especially C-min of indinavir. The dose of indinavir or ritonavir should be increased to maintain similar indinavir drug levels after addition of efavirenz to the indinavir-ritonavir combination. Dose modifications may not be needed in antiretroviral-naive human immunodeficiency virus-infected patients if the reference C-min of the regimen of 800 mg indinavir 3 times a day is considered to be adequate.