AbstractThe Rac1 signaling pathway is known to be hyperactivated in numerous solid tumors. We developed a Rac1 AlphaLISA assay that is amenable to high-throughput screening (HTS) to screen large libraries of small molecules. Based on molecular modeling of a putative allosteric binding site for several of the hits, we optimized a series of compounds that lead to RP-102124. RP-102124 engages with Rac1 in both biochemical and cellular target engagement assays. RP-102124 has greater potency than previously reported Rac1 inhibitors in biochemical assays, with potent inhibition of phenotypic characteristics of Rac signaling such as cell migration and cytoskeletal rearrangements. RP-102124 is a Racmulti inhibitor with activity against oncogenic Rac1b. RP-102124 is orally bioavailable and shows dose dependent inhibition of tumor growth in xenograft models at doses that are well-tolerated. RP-102124 is a first-in-class allosteric Racmulti inhibitor that shows promising therapeutic potential.Citation Format: Robert Silverman, Alan Cooper, Ronodip Kar, Mayuri V. Raman, Ramu Ravirala, Mubarak Hasan Shah, Sandeep Mahankali, Erik T. Goka. Discovery of a first-in-class allosteric inhibitor of the small GTPase Rac1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 677.