A substantial proportion of patients on antiretroviral therapy do not achieve sustained suppression of HIV viral load. Developing strategies to improve responses to subsequent regimens is an important objective for the management of patients with HIV infection. Increasing the potency of regimens by using a pharmacoenhancer such as RTV is of interest. RTV is used widely to increase plasma concentrations of PIs, but there is little efficacy and tolerability data about different RTV-enhanced PIs. The efficacy and tolerability of a triple PI regimen will be compared to dual PI regimens; dual PI regimens will also be compared to each other.

In Step 1, patients will be selectively randomized (based on prior exposure to the study drugs) and enrolled into 1 of 3 study arms. Patients in Arm A will receive lopinavir (LPV)/RTV in combination with TDF and 1 or 2 other NRTIs; patients in Arm B will receive fosamprenavir plus RTV in combination with TDF and 1 or 2 other NRTIs; Arm C patients were to receive LPV/RTV plus fosamprenavir in combination with TDF and 1 or 2 other NRTIs. Because interim study results indicated that mean PI levels for patients in Arm C were unacceptably low, Arm C patients will now either drop LPV/RTV and add RTV or drop fosamprenavir from their regimens.

The study will last 24 to 48 weeks. Medications and clinical assessment and blood collection will be performed at 2 weeks prior to entry, entry, and Weeks 2, 4, 8, 12, 16, 24, 32, 40, and 48. Blood samples to test for amprenavir (APV) and LPV pharmacokinetics will be collected at Weeks 12, 24, 48, and at confirmed virologic failure visits. In substudy A5147S, intensive 12-hour pharmacokinetic sampling for APV, LPV, and RTV will be conducted. The first 20-25 patients enrolled in each arm will be enrolled in the substudy 14-28 days after starting study treatment.