Summary Myotonic dystrophy 1 (DM1) is an autosomal dominant disorder resulting from the expansion of a CTG repeat tract in the 3’ untranslated region of the DMPK gene. The primary therapeutic target in DM1 is the mutant DMPK RNA encoding expanded CUG repeats (CUGexp), which forms toxic intra-nuclear aggregates or CUGexp foci in patient cells. We have developed an RNA-based screening strategy to identify small molecules that selectively modulate the DMPK CUGexp RNA without affecting the normal DMPK transcript. In a pilot screen of 2,500 compounds we identified a prototype small molecule MDI16, which effectively reverses critical DM1 pathological features in both patient cells and in the HSALR mouse model of DM1. As identification of multiple leads greatly enhances the probability of a small molecule therapy for DM1, we used this screening strategy to identify 30 novel hits from 40,000 diverse drug-like small molecules of the MSSR-UCLA library, which has undergone extensive filtering against liabilities. This panel of hits show better safety and efficacy in reducing CUGexp foci when compared to MDI16 in patient cells. Importantly, comparative analysis demonstrates that our hits perform on par with antisense oligonucleotides directed against CUGexp and better than DM1 small molecule therapeutics published in the literature. In this application we propose to identify lead compounds by rank-ordering the efficacy, potency, selectivity and safety of our hit panel in reversing key DM1 pathological features including the formation of CUGexp foci, aberrant RNA splicing, SHARP mis-localization, elevated CUGBP1 and GSK3b levels in DM1 patient myoblasts. Biochemical assays will be used to determine the mechanism of action of top-ranked hits, which will be tested for their in vivo efficacy in reversing DM1 skeletal muscle disease in the HSALR DM1 mouse model. Blood brain barrier penetrance of the hits will be tested in a novel bi- transgenic mouse model that expresses CUGexp foci in the brain. As there is no effective treatment for DM1, the identification of candidate therapeutic compounds offers hope for patients with this debilitating disease.