Supplemental application U54 CA231630 PI: Angela Koehler, Co-PIs: Sara Buhrlage, Jarrod Marto Abstract Alveolar rhabdomyosarcoma (ARMS) is a lethal pediatric cancer driven by the oncogenic transcription factor fusion protein PAX3-FOXO1 (P3F). While genetic data demonstrate P3F is a potential therapeutic target, transcription factors are often considered challenging pharmalogical targets. The goal of this U54 supplement is to expand drug identification strategies past our current small-molecule microarrays (SMMs) and implement covalent ligand screening against P3F protein Cys residues, thus increasing the pool of potential drug candidates. To achieve this goal, team will collaborate to pursue three objectives. First, Dr. Koehler’s team will increase production of purified full-length P3F as well as truncated forms of the protein which contain Cys-793, a residue identified as a critical mediator of cofactor CBP. Second, Dr. Koehler’s team will work with Drs. Buhrlage and Marto to optimize screening conditions for a full cys-reactive library utilizing a pooling-approach to increase throughput. Individual hits will be validated through LC-MS/MS to confirm biochemical results, such as covalent modifications and stoichiometry. Lastly, covalent ligand screen hits will be subjected to secondary assays, such as cellular thermal shift or reporter assays, outlined in the parent U45 for SMM drug discovery. This collaboration between Dr. Koehler and Drs. Buhrlage and Marto will expand the U54 team’s drug discovery capabilities, and enable a more thorough and efficient assessment of candidate drugs to target P3F. In the future, this technological approach could be extended to other transcription factors for drug candidate identification.