Ebola (EBOV) and Marburg (MARV) viruses belong to the family Filoviridae and can cause fatal hemorrhagic fevers characterized by widespread tissue destruction with an incubation period of 4-14 days. Because of the safety concerns, these viruses are designated as biosafety level 4 agents. Currently there is no effective vaccine or therapeutic treatment against filoviral infection and pathogenesis in humans. Although several promising vaccine candidates have been shown to be effective in eliciting host immune responses and to protect primates against viral infection, the minimal time required for vaccination (at least one month) and the sporadic nature of outbreaks reinforce the urgent need to develop potent, small molecule inhibitors against filoviral infections. Thus it is imperative to identify and develop potent inhibitors against filoviral infection. These inhibitors are considered to be of paramount importance for use during filoviral outbreaks or bioterrorist attacks. This application defines a plan to develop potent small molecule inhibitors, which block entry of EBOV and MARV into host cells. Entry of EBOV and MARV is mediated by a single viral glycoprotein (GP), which is considered one of the major therapeutic targets. GP consists of two subunits, GP1 and GP2; GP1 is responsible for receptor binding and host tropism, while GP2 mediates viral/cell membrane fusion and viral entry. We have used an HTS protocol targeting GP-mediated viral entry to screen a small molecule library, and we have identified compounds that inhibit entry of infectious EBOV/MARV (IC50 values =25 M). These hit compounds exhibit selectivity for EBOV/MARV entry. The overall objective of this Phase I application is to develop these inhibitors as potential anti-filoviral therapeutics. This application will focus on the folloing three specific aims: (1) Synthesize structurally diverse analogs of the anti-Ebola SB699551 hit series based on structure-activity relationships (SARs) to improve potency and selectivity. (2) Validate the lead inhibitor candidates in the infectious assay and investigate the mechanism of action (MOA) of the EBOV/MARV inhibitors. (3) Select EBOV/MARV inhibitors with in vitro ADME properties suitable for i.v. and oral dosing.