Background: Myotonic dystrophy type 1 (DM1) is a neuromuscular disease caused by an abnormal CTG repeat expansion in the 3’UTR region of the DMPK gene. In patients, the CTG repeat size ranges from 50 to thousands of CTG and usually increases over generations and time in the tissues. Larger expansions are associated with more severe symptoms and a decreasing age of onset. We hypothesize that the development of innovative therapeutic strategies, aimed at decreasing the CTG repeat length, and thus to stop or reverse the progression of the disease, may improve the quality of life of DM1 patients. Although many studies have provided insight into the mechanisms underlying the formation of expansions, how the contractions occur remains elusive. Aims: The specific objective of our work is to uncover bioactive molecules able to induce repeat contractions in trinucleotide repeat (TNR) models and to decipher the mechanisms promoting these contractions using efficient tools. Methods: We performed a large-scale screen for pharmacologically relevant chemical modulators of instability using the Prestwick Library (>1200 FDA-approved drugs) taking advantage of a chromosomal GFP reporter that can accurately measure both expansions and contractions in the same HEK293 cell population1. The effect of selected molecules directly on the dynamics of CTG.CAG repeat instability is studied in HEK293 cells as well as DM1 fibroblasts using targeted long-read sequencing developed by Pacific Biosciences2. Results: During the chemical screen, we identified several candidate molecules notably involved in epigenetic regulation pathways, that may change the size of CTG repeats. Some of these molecules induced stabilization or even contractions of CTG repeats in the HEK cell model and in DM1 fibroblasts. Conclusions: The next step is to better understand the mechanisms by which these molecules act on CTG repeat instability. The direct perspective of our work is to identify new small molecules and new druggable targets promoting CAG.CTG repeat contractions, thus offering new therapeutic perspectives for DM1 but also for other TNR diseases. 1Santillan et al. (2014). PloS One 9, e113952. 2Mangin et al. (2021). Int J Mol Sci 22, 2616.