Randomized, Double-Blind, Placebo-Controlled Trial, Followed by Single-Arm Treatment of PRO 140

This is a Phase 2b/3, multi-center, two part study, designed to evaluate the efficacy, safety, and tolerability of PRO 140 in conjunction with existing ART (failing regimen) for one week and Optimized Background Therapy (OBT) for 24 weeks respectively. Study population includes treatment-experienced HIV-infected patients with CCR5-tropic virus who demonstrates evidence of HIV-1 replication despite ongoing antiretroviral therapy with documented genotypic or phenotypic resistance to ART drugs within three drug classes (or within two or more drug classes with limited treatment options).The options may be limited as a result of drug antiviral class cross-resistance or documented treatment intolerance.