Background/Rationale: An alternative HIV prophylaxis/treatment modality of growing interest is based on the premise that the disadvantages stemming from cART use for the prevention/treatment of HIV can be mitigated by passive immunization with broadly neutralizing anti-HIV antibodies (bNAbs) against the HIV envelope. A pan- neutralizing antibody could provide a feasible means to treat or prevent HIV infection worldwide. Objectives: Through systematic deconvolution of circulating plasma anti-HIV envelope responses in HIV+ humans, we identified and characterized unique families of extremely broad and potent anti-CD4 receptor binding site (CD4bs) bNAbs with distinct CDR domain structural characteristics. Modifications of these bNAbs generated one iteration, N49P9.6-FR that neutralizes 97% of 117 viruses in a standardized, multi-tier, cross- subtype panel, with an overall potency that surpasses all other anti-CD4bs bNAbs and equals that of anti-glycan bNAbs. Further, because of its breadth, N49P9.6-FR covers viruses that are resistant to other anti-CD4bs bNAbs and is not polyreactive with human antigens in standard tests. Finally, we have generated an “LS” variant (N49P9.6-FR/LS) with mutations in the Fc domain that prolong circulating half-life. As such, N49P9.6-FR/LS provides a new opportunity to fully realize the utility of bNAb-based antivirals. However, the path forward demands demonstrations of efficacy in animal models. Accordingly, the goal of this project is to generate such data. Our hypothesis is that N49P9.6-FR will exhibit potent prevention and suppression of HIV infection, superior to related bNAbs now in clinical development. The specific aims of this proposal are 1) Establish the efficacy of bNAb N49P9.6-FR/LS in humanized mouse models; 2) Establish the protective efficacy of bNAb N49P9.6-FR/LS in the rhesus macaque/SHIV infection model. Methods: We will test the ability of N49P9.6-FR/LS to prevent and treat HIV infection in humanized mouse models and to prevent HIV infection in rhesus macaques. Immunodeficient NSG mice will be reconstituted with either HIV-1 infected human cells (Hu-PBL mice) or human CD34+ stem cells (Hu-CD34 mice) to examine preventive and therapeutic efficacy of the bNAb. Rhesus macaques will be challenged with SHIV to examine the preventive efficacy of the bNAb. In addition, we will complete PK studies with N49P9.6-FR/LS in the mice and rhesus macaques. Impact: Upon completion of the project, we expect to have determined that bNAb N49P9.6-FR has the capacity to provide safe, single-dose, potent, escape-resistant and durable HIV prevention and therapy, superior overall to other bNAbs. Success in this regard should position bNAb N49P9.6-FR for straightforward translational development into clinical applications with significant impact.