Abstract Sarcomas are rare and heterogenous cancers arising from bone, soft and connective tissue. Developing effective therapies remains a challenge, since many types are understudied and poorly characterized at the molecular level. Here, we propose to investigate the role of protein misfolding in sarcoma, starting from the tumor suppressor p53. Wild type p53 is a tumor suppressor that regulates many cellular processes and prevents malignant transformation. Missense mutations promote p53 misfolding and convert the protein into a cancer-promoting oncogene. Mutant p53 is found in ~80% of all osteosarcoma cases, ~50% of leiomyosarcoma and angiosarcomas. It is also mutated at lower frequencies in other sarcoma subtypes. We investigated whether p53 mutations found in different types of sarcoma cause p53 misfolding and aggregation and can be targeted by ReACp53, a p53 aggregation-targeting peptide with anti-tumor activity in ovarian cancer and prostate cancer (Soragni et al, 2016, Zhang et al, 2019). We established tumor organoids from four sarcoma lines, which include osteosarcoma, fibrosarcoma, leiomyosarcoma and rhabdomyosarcoma. The cells carry either p53 mutations or WT p53. To characterize the p53 aggregation status and elucidate the response to ReACp53, we performed staining and quantification of folded/unfolded p53 pools in the organoids. Our results demonstrate the presence of overexpression and aggregation of p53 in the nuclei as well in the cytosol of different types of sarcoma lines. We found a marked susceptibility to ReACp53 in all lines, including cells bearing WT p53. We also explored whether similar p53 conformational defects are found in tumor tissue from sarcoma patients and if the activity of ReACp53 is confirmed in patient-derived tumor organoids. Overall, the discovery of structural alterations of p53 in sarcoma could provide an opportunity for the development of novel therapeutic strategies. Citation Format: Sara Sartini, Hillary Le, Alice Soragni. Protein aggregation in sarcoma as a target for therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2582.