Steady-State Pharmacokinetic Interaction Study of Indinavir and Rifabutin

Currently, rifabutin is the only rifamycin that can be administered with indinavir. ACTG 365 is the first formal study of the pharmacokinetics of this dosing combination regimen in HIV seropositive patients. It is hypothesized that staggered administration of rifabutin and indinavir might minimize their pharmacokinetic interaction. If the intestinal tract plays a significant role in the presystemic clearance of rifabutin, the inhibitory activity of indinavir on rifabutin could depend on either luminal concentrations of indinavir, systematic concentrations of indinavir, or both. If luminal concentrations are important, then the interaction between these 2 drugs will be maximal when administered simultaneously, and minimal when their oral administration is staggered. Finally, since indinavir has a half-life of 1.8 hours, its effects on rifabutin's systematic clearance may be much less when administration of these drugs is staggered by 4 hours as compared with simultaneous administration with rifabutin. If the interaction on rifabutin is minimized, then the rifabutin levels may be suboptimal for treatment of tuberculosis in patients who are not administered the 2 drugs simultaneously. It is, therefore, important to define the magnitude of the effect of staggered vs simultaneous drug administration in order to clarify dose and regimen recommendations in HIV-infected patients with tuberculosis who also require protease inhibitor therapy.

Study Arm A is a multiple-dose, 3-period, sequential study in 18 evaluable HIV-infected indinavir-naive male and female volunteers [AS PER AMENDMENT 11/16/98: Arm A will be assessed in 18 evaluable HIV-seronegative patients]. Patients receive 3 different treatments consisting of 14 days of administration: rifabutin alone (Period IA); indinavir plus rifabutin (Period IIA); and indinavir plus rifabutin (Period IIIA). Study Arm B is a multiple-dose, 2-period, sequential study in 10 evaluable HIV-infected male and female volunteers. Patients receive 2 different treatments, each consisting of 14 days of administration; indinavir alone (Period IB); and indinavir plus rifabutin (Period IIB). Patients on both arms take each dose of their study medications with water. [AS PER AMENDMENT 8/8/97: Patients treated on Arm A are randomized, following Period IA therapy, to Period IIA or IIIA therapy for 14 days, then are crossed over to the alternate regimen for 14 days.] [AS PER AMENDMENT 4/17/98: After completion of therapy on Arm A or B, patients continue therapy with indinavir alone for 7 days.] [AS PER AMENDMENT 11/16/98: The final 7 days of indinavir dosing has been eliminated for patients on Arm A. Also per this amendment, to ensure compliance, Arm A patients' rifabutin supply will be dispensed in containers fitted with an electronic monitoring cap device.]