This study compares 2 different anti-HIV drug regimens to determine which is the most effective in lowering the amount of HIV in the blood. The anti-HIV drugs used in this study are 2 protease inhibitors (nelfinavir and ritonavir), 2 nucleoside reverse transcriptase inhibitors (stavudine and didanosine), and 1 nonnucleoside reverse transcriptase inhibitor (nevirapine).

These drug combinations have been previously studied in adults, but there is limited information on how well they work in HIV-infected children. It is important to develop drug combinations which are effective at suppressing the HIV virus in children.

The use of combination therapy with 2 or more antiretroviral agents has been strongly supported by recent studies in both children and adults. However, as of yet, few combinations of antiretrovirals have been studied in large cohorts of stable HIV-1 infected, antiretroviral-experienced children. Evidence suggests that viral suppression may be more difficult to achieve in children. Therefore, it is important to develop new drug combinations which can maximally suppress plasma HIV-1 RNA concentrations in children.

Patients are stratified by prior antiretroviral treatment (zidovudine [ZDV]/lamivudine [3TC] versus d4T/other treatment) and by age (under 24 months versus 24 months and older). Patients are then randomized to 1 of 4 treatment groups.

Arm A1: ddI/NFV/RTV (for prior ZDV/3TC-treated patients). Arm A2: ddI/NFV/RTV (for prior d4T/other-treated patients). Arm B1: d4T/NFV/NVP (for prior ZDV/3TC-treated patients). Arm B2: d4T/NFV/NVP (for prior d4T/other-treated patients). Treatment is administered for 48 weeks. At Weeks 2, 4, and then every 4 weeks thereafter, patients undergo physical examinations, and blood samples are drawn to measure viral load. [AS PER AMENDMENT 4/27/00: Patients in Arms A1 and A2 may continue to receive medication for an additional 24 weeks. While on the treatment extension, patients must continue their current schedule for study drug administration and completion of study visits. Patients in Arms A1 and A2 who have reached Week 44 participate in an enteric-coated ddI pharmacokinetic study as part of this 24-week extension. Patients who were enrolled in Arms A1 or A2 and who were taken off study after reaching Week 48 may be re-entered onto the study at Week 52 regardless of the number of weeks they have been off study.]

Inclusion Criteria

Patients may be eligible for this study if they:

• Are between 4 months and 21 years of age (consent of parent or guardian required if under 18).
• Are HIV-positive.
• Have a viral level of at least 4,000 copies/ml.
• Have a CD4 cell count of at least 750 (under 12 months of age), at least 500 (1 to 5 years of age), or at least 200 (6 years of age or older) cells/mm3 within the past 4 months or a CD4 percent of 15 percent or higher within the past 4 months.
• Have received the same continuous antiretroviral therapy for the past 16 weeks.

Exclusion Criteria

Patients will not be eligible for this study if they:

• Have an active opportunistic and/or serious bacterial infection.
• Have been diagnosed with a malignancy.
• Have received prior treatment with certain antiretroviral medications.
• Are pregnant or breast-feeding.