Hits from the primary screen were validated using the ISRE activity-luciferase reporter assay over a broad range of compound concentrations (0.01-25 mM) in the absence or presence of IFN-b (1, 5, and 15 U/ml). To determine drug potency, as defined by halfmaximal effective concentration (EC 50 ), this data was fit to a fourparameter concentration-response curve as described previously [58] using the log agonist concentration versus response, variable slope algorithm in GraphPad Prism 5 software (La Jolla, CA) where Y = Bottom + (Top-Bottom)/(1+10((LogEC50-X)*HillSlope)). To determine whether compound effect depended on IFN production, the ISRE activity-luciferase reporter assay was also performed in the presence of mouse anti-human IFN-a/b receptor chain 2 (IFNAR2) blocking mAb (clone MMHAE-2; Millipore, Billerica, MA) at a concentration of 4 mg/ml.