Project Summary/Abstract This R01 entitled, “Development and characterization of engineered therapeutic antibodies against SARS-CoV- 2”, builds on our project infrastructure, expertise, and experience in characterizing viral-host factor interactions in negative strand RNA viruses. Since originating in China, SARS-CoV-2 has since rapidly spread and is now a global pandemic. Significant concerns are that humans are immunologically naïve, and there are no available therapies. In the US, the disease has already overwhelmed the healthcare system in some states and have a serious knock-on effect in exacerbating the standard of care for other diseases. At the time of writing, nearly 5 million cases and >160,000 deaths have been attributed to COVID-19. The virus replicates in the lungs and causes a severe respiratory disease, COVID-19, which is fatal in >2% of cases. Neutralizing antibodies (nAbs) generated by natural infection or vaccines is known to control many infections and early studies in the current COVID-19 pandemic, including studies to test convalescent plasma treatments, are promising. These studies highlight the potential significance of nAb-based therapy. While IgG format of nAbs have long been the most extensively used format, early studies, including our own suggest that additional multivalent formats of nAbs may be more effective. This provides an innovative method to develop nAbs while acquiring potential benefits from effective neutralization at lower doses and lower likelihood of the emergence of resistance mutants. SARS-CoV- 2 is a single stranded, non-segmented, enveloped RNA virus. Viral infection requires interaction of the spike glycoprotein receptor binding domain (RBD) to the host receptor ACE2. Here, we will build on newly developed and established approaches that have been optimized through our work other systems to generate antibodies targeting spike and spike RBD using phage display technology and characterize their physical properties. We will engineer antibodies with increased valency and test for potency in in vitro neutralization assays and in vivo efficacy in a mouse model. At the completion, we expect to provide innovative and unique multivalent nAb leads with unique characteristics that will rival the best in class IgG drugs.