Medical Need: Human immunodeficiency virus (HIV), the etiologic agent of acquired immune-deficiency syndrome (AIDS), continues to infect millions of people worldwide. Despite recent improvements in patient outcomes resulting from highly active antiretroviral therapy (HAART), there remains an urgent need for novel and improved methods of treatment for individuals living with AIDS. This need arises from limitations of existing treatment options, such as emergence of resistant viruses and adverse side effects. Viral entry into the host cell proceeds via formation of a gp41 six-helical coiled coil, which is the final step of HIV-1 membrane fusion and infection of the host cell. Due to the need for additional therapeutic modalities that are safe and well tolerated, inhibition of HIV-1 gp41 function is an innovative new treatment modality. Enfuvirtide (Fuzeon) is a safe and effective peptide inhibitor of gp41 function recently approved by the FDA for treating HIV-1 infection. However, manufacturing, cost and dosing difficulties limit the use of Fuzeon to a small portion of the HIV-1 infected population. A small molecule drug acting on HIV-1 gp41 that could be administered by oral dosing would constitute a significant advancement in the treatment of AIDS. Goal of Research: Using its innovative computational methodology, Locus Pharmaceuticals identified two chemical classes of small molecule HIV gp41 inhibitory compounds. Further improvement of these molecules as drug candidates requires the input of co-crystal structures as a basis for computational design. With the aid of a Phase I SBIR grant, Locus demonstrated the feasibility of generating such co-crystal structures. The focus of this proposal is to solve co-crystal structures of the compounds in complex with gp41 in order to enable computational and structure-based design. These structures will then be used as input for further computational design and optimization of gp41 inhibitory small molecules. The ultimate goal of this research is the identification of drug candidate small molecule gp41 inhibitory compounds for the treatment of AIDS caused by HIV-1 infection. Specific Aims: Obtain a co-crystal structure of a functionally relevant HIV-1 gp41 HR-1 domain in complex with a bound small molecule compound that inhibits gp41 function. Using these new co-crystal structures, apply computational and structure-based methods to design then chemical synthesis to generate additional small molecule gp41 inhibitors with improved potency.