An Extension of Protocol PRO 140_CD01 Study

This study is a Phase 2b, multi-center, extension study designed to evaluate the long-term efficacy, safety, and tolerability of PRO 140 monotherapy for the maintenance of viral suppression in patients who were stable on combination antiretroviral therapy and completed 12 weeks of treatment under PRO 140\_CD 01 Treatment Substitution Study without experiencing virologic failure. Consenting patients will continue to receive PRO 140 monotherapy until investigational product (IP) receives marketing approval or investigational new drug (IND) is withdrawn by Sponsor. There is one week overlap of existing retroviral regimen and PRO 140 at the end of the treatment extension phase in subjects who do not experience virologic failure. PRO 140 will be administered as a 350 mg subcutaneous injection weekly during treatment extension phase. Study participants will be monitored for viral rebound on a weekly basis following initiation of PRO 140 monotherapy and will re-initiate their previous antiretroviral regimen if plasma HIV-1 RNA levels rise above 400 copies/ml on two consecutive blood draws at least 3 days apart. .