A Potent Inhibitor of Protein Sequestration by Expanded Triplet (CUG) Repeats that Shows Phenotypic Improvements in a Drosophila Model of Myotonic Dystrophy

Myotonic dystrophy is the most common form of adult‐onset muscular dystrophy, originating in a CTG repeat expansion in the DMPK gene. The expanded CUG transcript sequesters MBNL1, a key regulator of alternative splicing, leading to the misregulation of numerous pre‐mRNAs. We report an RNA‐targeted agent as a possible lead compound for the treatment of myotonic dystrophy type 1 (DM1) that reveals both the promise and challenges for this type of small‐molecule approach. The agent is a potent inhibitor of the MBNL1–rCUG complex with an inhibition constant (Ki) of 25±8 nm, and is also relatively nontoxic to HeLa cells, able to dissolve nuclear foci, and correct the insulin receptor splicing defect in DM1 model cells. Moreover, treatment with this compound improves two separate disease phenotypes in a Drosophila model of DM1: adult external eye degeneration and larval crawling defect. However, the compound has a relatively low maximum tolerated dose in mice, and its cell uptake may be limited, providing insight into directions for future development. Finish line in sight: A potential RNA‐targeted lead for the treatment of myotonic dystrophy type 1 (DM1) is described. Two bisamidinium ligands were linked using “click” chemistry. The heterodimer is a potent inhibitor of the MBNL1–rCUG complex (Ki=25±8 nm), relatively nontoxic to HeLa cells, dissolves nuclear foci, fully corrects the insulin receptor misregulated alternative splicing in DM1 model cells, and shows significant improvement of disease phenotypes in a DM1 Drosophila model.