The overall goal of this project is to develop host cell proteasetargeted drugs for use against a range of respiratory viruses of significant public health interest and pandemic concern—including coronaviruses, orthomyxoviruses (influenza), paramyxoviruses and pneumoviruses. The surface glycoproteins of such enveloped viruses are activated by one or more proteolytic priming steps, dependent on host cell proteases that cleave at specific amino acid residues of the viral glycoprotein. Without proteolytic priming, infection cannot be initiated. Proteases are now well-recognized drug targets for many diseases, including viral proteases, but targeting of host cell proteases involved in virus entry has received comparatively little attention. Host cell proteases offer significant advantages for therapeutic development, including the opportunity for enhanced efficacy against a broad range of viruses, as well as the lack anti-microbial resistance (AMR). Here, we propose to develop therapeutics that can be used to treat infections from different respiratory viruses by targeting the common set of host cell proteases that are critical for their cell entry. We have outstanding preliminary data demonstrating the antiviral efficacy for a lead small-molecule peptidomimetic (N-0385) that targets type II transmembrane serine proteases (TTSPs) as an early-acting therapeutic for SARS-CoV-2, and as a lead candidate for influenza treatment. The overall scope of our team covers medicinal chemistry, molecular pharmacology and in vitro characterization (Drs. LeDuc and Boudreault, Université de Sherbrooke, Quebec) live virus experiments in cell culture (Whittaker Lab) and in vivo studies using nasal delivery in animal challenge models (Drs. Aguilar-Carreno and Diel, Cornell University). We propose a vertical approach to develop, characterize, and validate TTSP inhibitors as broad-acting indirect antivirals for respiratory viruses. Importantly, our lead compound (N-0385) has an extremely high therapeutic index, making it very safe, and outstanding efficacy, especially when used prior to infection. These properties make possible the use of use of broad-based pre-exposure prophylactic for wide-spread use as a way to block infection of the upper airways of people—as an “invisible mask”.