Abstract 706: Advancing radiotherapy strategies for high-risk rhabdomyosarcoma: Unveiling BCL-xL inhibition as a potent sensitizer to radiotherapy

Abstract Pediatric soft-tissue sarcomas are a group of solid tumors that account for about 7% of childhood cancer cases. Rhabdomyosarcoma (RMS), the most common subtype, further stratifies into PAX3 or PAX7 and FOXO1 fusion-negative (FN) or fusion-positive (FP) RMS. FP RMS is the higher risk subtype and is more aggressive, metastatic, and less responsive to treatment. Front-line treatments for RMS FP patients have had limited progress in the recent years and existing treatments such as radiotherapy lacks tumor cell selectivity, resulting in toxic side effects. To address this, we conducted a high-throughput drug screen to identify radiosensitizers to enhance efficacy of RMS radiotherapy. Using three patient-derived FP RMS tumoroids, we screened a comprehensive drug library with and without radiation, comprising 528 drugs at varying stages of pre-clinical and clinical trials. A validation screen of the top 29 drugs was conducted on a larger cohort of samples including eight patient-derived FP RMS tumoroids and three non-cancer models. The screening revealed that drugs targeting the anti-apoptotic protein B-cell lymphoma-extra-large (BCL-xL) exhibited both potent radiosensitization and cytotoxicity selectively for FP RMS. Notably, the sensitivity to BCL-xL inhibitors correlated with elevated BCL2L1 expression and PAX3 fusion status. Additionally, drugs targeting polo-like kinase 1 (PLK1) and other cell cycle-related genes exhibited robust radioprotective effects across both RMS and non-cancer models. Our findings propose the therapeutic potential of BCL-xL inhibition, either as a standalone treatment or in combination with radiotherapy. In parallel, we show the protective potential of PLK1 inhibition to mitigate radiation toxicity. In the long term, we aspire to integrate the identified radiosensitizing and radioprotecting compounds with existing treatment regiments for RMS, maximizing precision and efficacy in RMS radiotherapy. Citation Format: Sammy Se Whee Park, Nona Struyf, Tom Erkers, Sören Lehmann, Michael T. Meister, Jarno Drost, Frank C. Holstege, Nikolas Herold, Päivi Östling, Jakob Stenman, Olli Kallioniemi, Jan-Inge Henter, Brinton Seashore-Ludlow, Kasper Karlsson. Advancing radiotherapy strategies for high-risk rhabdomyosarcoma: Unveiling BCL-xL inhibition as a potent sensitizer to radiotherapy [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 706.