Inclusion Criteria:

• HIV-infected.
• Screening viral load at least 5000copies/mL.
• Total prior antiretroviral experience of at least 3 months and documented resistance to at least one drug in each of the following classes: nucleoside reverse transcriptase inhibitors (NRTI), non-nucleoside reverse transcriptase inhibitors (NNRTI), and protease inhibitors (PI), stable antiretroviral regimen (or no antiretroviral treatment) for at least 4 weeks before screening.
• Able to receive a ritonavir-boosted protease inhibitor during treatment studies.
• Women of childbearing potential must use specific forms of contraception.

Exclusion criteria:

• Acute laboratory abnormalities.
• History of pancreatitis or hepatitis, hepatitis B or hepatitis C coinfection, or any chronic liver disease. Screening liver function tests will be used to determine eligibility.
• Changes to antiretroviral therapy from 4 weeks prior to screening until Day 1 of treatment or observational studies.
• Pregnancy or breastfeeding women.
• Recent participation in an experimental drug trial.
• Prior use of a CCR5 or CXCR4 antagonist.
• Significant ECG abnormalities or significant history of active pancreatitis, hepatitis, opportunistic infections, malabsorption disorders, cancer, or severe illness.
• Current use of certain medications may exclude participation in this study.
• Additional qualifying criteria and laboratory test requirements to be assessed by study physician.