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Background: Synthetic lethality is one of the most innovative approaches for selective targeting of cancer cells with defined genetic background. SMARCA2 and SMARCA4 are two mutually exclusive helicase/ATPase catalytic subunits belonging to SWI/SNF chromatin remodeling complex. It is estimated that one in five tumors possess a mutation in proteins of this complex. The Cancer Genome Atlas (TCGA) and the Catalog of Somatic Mutation in Cancer (Cosmic) shows that SMARCA4 is one of the most mutated genes in lung, colorectal, breast, melanoma and CNS tumors. Herein, we present development of a potent and selective SMARCA2 degrader, structurally unrelated to known chemotypes with first in class potential selectively targeting SMARCA4 mutant cells. Methods: ADP-Glo assay was used to identify SMARCA2 inhibitors through HTS. Binding was confirmed using MST, FTS and SPR methods. Biochemical and biophysical investigation guided rational optimization. Identified ligands were used to build a hybrid PROTAC by linking to a ligand for the E3 ligase. Western blotting was used to assess degradation efficiency and selectivity. Target engagement was confirmed using qPCR and AlphaLISA methods. On target activity was confirmed using SMARCA4 isogenic cells and a panel of SMARCA4 WT and mutant cell lines. Results: High throughput screening allowed identification of novel inhibitors of SMARCA2 ATPase activity. Medicinal chemistry efforts improved potency and affinity to the target over 100 fold. Target engagement was confirmed using biophysical methods and biomarker modulation. Proprietary ligand was used to build a hybrid PROTAC. Western blotting confirmed selective and long-lasting degradation of SMARCA2. Co-treatment with Epoxomicin confirmed proteasomal dependent degradation of targeted protein. Specific SMARCA2 depletion in SMARCA4 mutated cancer cell lines induced apoptosis, growth inhibition and cell death. Observed mechanism of action is consistent with a phenotype seen with perturbation through inhibition of ATPase activity of SMARCA2 and genetic knock-down. Conclusion: Treatment with Ryvu's PROTACs led to selective, proteasomal dependent degradation of SMARCA2 protein and in consequence to a targeted cell death of SMARCA4 mutated cancers. Fine-tuning of available compounds will allow for proof-of-concept experiments in animal models as a single agent or in combinations with radio- or immuno-therapies. Citation Format: Andrzej Mazan, Anna Wróbel, Agnieszka Dreas, Adam Radzimierski, Kinga Michalik, Anna Kowal, Katarzyna Wiklik, Katarzyna Wnuk-Lipińska, Paulina Niedziejko, Kamila Kozłowska, Magdalena Łośko, Joanna Szczęśniak, Agnieszka Sroka-Porada, Justyna Martyka, Urszula Kulesza, Karol Zuchowicz, Magdalena Zastawna, Jose Alvarez, Luigi Stasi, Peter Littlewood, Tomasz Rzymski, Krzysztof Brzózka. Development of novel, selective SMARCA2 (BRM) degraders for treatment of SMARCA4 (BRG1) mutated tumors [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3656.