Dose-Response Study of Ibalizumab (Monoclonal Antibody) Plus Optimized Background Regimen in Patients With HIV-1

The primary objectives of this study are to: * Evaluate the dose-response relationship of antiviral activity of the ibalizumab dose regimens at Week 24 in order to determine the optimal dose and regimen. The primary evaluation of effectiveness will be based on the proportion of patients achieving undetectable viral loads at Week 24. * Evaluate the safety and tolerability of two dose regimens of ibalizumab for dose selection The secondary objectives of this study are to: * Evaluate changes from Baseline in viral load, CD4+ cell counts, and time to loss of virologic response (TLOVR) * Characterize HIV-1 sensitivity/susceptibility changes associated with ibalizumab administration in combination with OBR * Determine the presence and significance of anti-ibalizumab antibodies, if any (immunogenicity of ibalizumab) * Assess CD4 receptor density and occupancy * Determine the impact of ibalizumab on quality of life as assessed by patient-reported outcomes on questionnaires * Evaluate the pharmacokinetic profile of two dose regimens of ibalizumab at steady state