This is a two phase study (randomised and non-randomised phase). The randomised phase will initially examine 4 blinded doses of GW640385 boosted with rtv (with continuation of current background therapy) in comparison to an ongoing, open-labeled rtv-boosted protease inhibitor (PI) regimen for 15 days. At the Day 15 visit, all subjects will optimize background therapy. Additionally, subjects receiving the lowest dose of GW640385 will be re-randomised to one of the higher doses and subjects in the control arm will receive a new rtv-boosted PI based on resistance testing at screening. Subjects will remain in the randomized phase on one of these 4 continuing treatment arms for at least 48 weeks. An interim analysis will occur during the randomised phase to select for a dose of GW640385 to evaluate further in Phase III studies. After dose selection subjects will move to the non-randomised phase of the study. In the non-randomised phase subjects who are receiving GW640385 will be assigned to final selected dose for assessment of long term safety, tolerability, pharmacokinetics, and antiviral activity.

A Phase IIB, Randomized, Multicenter, Parallel Group Study to Evaluate the Short-Term Safety, PK and Antiviral Activity of Four Dosing Regimens of GW640385/rtv Therapy Compared to Open-label Current Protease Inhibitor (PI) Therapy in HIV-1, PI-Experienced Adults for 2 wks with Long-Term Evaluation (>48 wks) of Safety, PK and Antiviral Activity of Selected GW640385/rtv Dosing Regimen(s) vs. a RTV-boosted, PI Containing Regimen

Inclusion criteria:

• 18+ years of age (or =16 years of age for non-EU countries, according to local requirements).
• HIV-1 infected subjects.
• Females must be of either non-childbearing potential or have a negative pregnancy test at Screening and agree to use a protocol approved method of contraception.
• Plasma HIV-1 RNA (viral load) =1,000 copies/mL at Screening.
• Evidence of at least 2 multi-PI resistant mutations at Screening or within 3 months of Screening.
• Subjects must have been receiving the same anti-HIV medicines that they are on currently for at least 8 weeks prior to Screening; these anti-HIV medicines will include a single protease inhibitor (PI) in combination with a low dose of ritonavir (i.e., a ritonavir-boosted PI). However, the current PI cannot be tipranavir.
• Able to understand and follow protocol requirements, instructions and protocol-stated restrictions.
• Be willing and able to provide signed and dated written informed consent prior to study entry.

Exclusion criteria:

• Subjects cannot change their anti-HIV medicines between Screening and Day 1 Visit.
• Subjects can not be receiving dual ritonavir-boosted PIs, non-nucleoside reverse transcriptase inhibitors (NNRTIs) or Tipranavir at Screening.
• Active CDC Class C disease at screening.
• Pregnant or breastfeeding women.
• Protocol-specified laboratory abnormalities at Screening.