The emergence of drug resistance is a major factor contributing to the failure of antiretroviral therapy in HIV-infected patients. Drug resistance can be detected by genotypic or phenotypic assays, both having distinct advantages and disadvantages. Results from genotypic and phenotypic testing are helpful in excluding from the subsequent regimen drugs to which the resistance is identified, and both tests predict virologic response to salvage therapy in patients who have failed a previous regimen. Resistance testing is likely to be beneficial as an aid in selecting a salvage regimen.

At entry, patients are randomized to Arm A (sequencing) or Arm B (phenotyping) and have a resistance test drawn while still receiving the current regimen even though regimen failure is suspected. The test results are available between Weeks 1 and 4, inclusive. There are weekly visits for the first 4 weeks after entry to monitor viral load and maintenance of the current failing (prestudy) regimen. If virologic failure is confirmed, a new regimen is chosen and prescribed at the first visit after resistance test results are available. [AS PER AMENDMENT 12/6/00: If the resistance assay fails to yield results, another regimen is chosen and prescribed based on the patient's medical and medication history.] If virologic failure is not confirmed, the current drug regimen is not changed. Otherwise, on-site study visits occur every 4 weeks until Week 24 and then every 8 weeks thereafter through Week 48. [AS PER AMENDMENT 12/6/00: on-site study visits occur every 4 weeks until Week 24 and then every 8 weeks thereafter]. Medical resource use is assessed at baseline and then every 8 weeks through Week 48. Quality of life is assessed at baseline and then every 16 weeks through Week 48.

Inclusion Criteria

Patients may be eligible for this study if they:

• Are HIV-positive.
• Have failed 2 to 4 anti-HIV regimens containing 3 or more combinations of drugs. A patient has failed his/her current regimen if he/she has, within 30 days of study entry, either a viral load (level of HIV in the blood) of at least 10,000 copies/ml or 2 tests which show a viral load between 1,000 and 10,000 copies/ml.
• Have taken 3 or more anti-HIV drugs for 8 or more weeks before the study.
• Are at least 14 years old.
• Have consent of parent or guardian if less than 18 years old.

Exclusion Criteria

Patients will not be eligible for this study if they:

• Have failed only 1 anti-HIV drug combination.
• Have failed 5 or more anti-HIV drug combinations, each containing 3 to 5 drugs.
• Have had and received the results of prior resistance tests.
• Have had treatment with a combination of 6 or more anti-HIV drugs.
• Have problems absorbing food in the intestine.
• Have had HIV vaccines.
• Have taken drugs that affect the immune system or investigational drugs.
• Are taking medications not allowed with protease inhibitors (PIs) if PIs would be part of their anti-HIV treatment during the study.
• Have failed anti-HIV therapy due to nonadherence to medication.