Vicriviroc (vye-kri-VYE-rock) is an investigational drug (not yet approved by Government Regulatory Authorities for commercial use) that belongs to a new class of drugs, called CCR5 receptor blockers. This group of drugs blocks one of the ways HIV enters T-cells (the cells that fight infection). Previous smaller studies in HIV treatment-experienced patients, have shown that vicriviroc is safe and effective. The purpose of this study is to investigate in subjects with detectable dual/mixed CCR5/CXCR4-tropic HIV whether vicriviroc when added to other appropriate HIV drugs can decrease the level of HIV (viral load) in the blood and that it is well tolerated.

This is a randomized, double-blind, placebo-controlled, parallel-group, multi-center study of vicriviroc maleate in HIV subjects infected with dual/mixed CCR5/CXCR4-tropic virus and who have documented resistance to at least 2 of the 3 antiretroviral drug classes (NRTI, NNRTI or PI) or at least 6 months experience with at least 2 of the following: one NRTI, one NNRTI, or one PI (excluding low-dose ritonavir) and failure on their current stable regimen. The study will compare the virologic benefit of adding vicriviroc to an optimized background regimen to a control group receiving placebo plus the new optimized background therapy. The optimized background regimen will be chosen by the investigator based on results of drug susceptibility tests performed at Screening, history of prior antiretroviral drug use by the patient, and drug toxicity. Primary efficacy analysis will be conducted when all subjects have completed 48 weeks of treatment. An interim analysis will be performed when all subjects have completed 24 weeks of treatment. If vicriviroc at the dose studied proves to be safe and efficacious, study participants who successfully complete 48 weeks of treatment or for whom vicriviroc is medically appropriate will be offered vicriviroc free of charge until the drug is commercially available.