Abstract 2606: In vivo screening reveals therapeutic vulnerabilities in a patient-derived xenograft established from an intraosseous TFCP2-rearranged rhabdomyosarcoma

Abstract Primary intraosseous rhabdomyosarcoma (IORMS) is a rare, metastatic, spindle cell-like rhabdomyosarcoma (RMS) subset that occurs in craniofacial bones and less commonly in long bones, or pelvis. IORMS patients typically succumb to the disease within 15 months of diagnosis. This disease is distinguished by specific gene fusions, such as FUS-TFCP2 which blocks myogenic differentiation, inhibits DNA repair, and induces transcription of Anaplastic Lymphoma Kinase (ALK) and Telomerase Reverse Transcriptase (TERT). Although case reports of treatment with ALK inhibitors (ALKi) have shown modest results, resistance always emerges and no other treatment options beyond surgery and chemotherapy exist. The ability to understand disease pathogenesis and identify actionable targets for rare RMS subsets is hindered by the lack of appropriate pre-clinical in vivo models. Thus, there is a critical need for developing such models to identify therapeutic options that will block disease progression, enhance quality of life, and ultimately achieve a cure for these patients. In collaboration with the Pediatric Cancer Precision Genomics Program at the Riley Hospital for Children, we developed a set of FUS-TFCP2 fusion-containing IORMS patient-derived xenografts (PDXs). PDX174 was derived prior to ALKi treatment (lorlatinib) and PDX199 from the same patient following disease progression after 16 months of lorlatinib therapy. Consistent with IORMS characterization, PDX174 transcriptome shows significantly increased expression of ALK and TERT, 10.6 and 14.8-fold respectively, compared to normal human skeletal muscle cells. ALK overexpression was confirmed by western blot but was notably absent in the subsequent PDX199 model. To further investigate the PDX174 model, we conducted an in vivo screen of six different small molecule inhibitors (SMIs) based on previously prioritized biomarkers of therapeutic response. Bivalent BET inhibitor (BETi), AZD5153, resulted in an initial block in tumor growth (p<0.05) compared to the vehicle and other SMIs. However, resistance emerged after four weeks of AZD5153 treatment. In addition, while CDK4/6 inhibitor, palbociclib, and CHK1 inhibitor, SRA737, induced moderate inhibition of tumor growth, drug resistance quickly followed within 2 weeks. Global kinome analysis indicated a significant increase in the mitogen- and stress-activated kinase 1 (MSK1) in AZD5153-resistant tumors compared to the vehicle group, revealing a potential therapeutic vulnerability in the BETi-resistant tumors. IORMS models such as PDX174/PDX199 are crucial for revealing actionable molecular signatures before and after targeted therapy and will increase our mechanistic understanding of tumor adaptive responses and aid in designing therapies that mitigate the emergence of therapeutic resistance. Citation Format: Erika Dobrota, M. Reza Saadatzadeh, Barbara Bailey, Keiko Kreklau, Kathy Coy, Felicia Kennedy, Melissa Trowbridge, Anthony Sinn, Chris Davis, Michael Ferguson, Steve Angus, Pankita Pandya, Karen Pollok. In vivo screening reveals therapeutic vulnerabilities in a patient-derived xenograft established from an intraosseous TFCP2-rearranged rhabdomyosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2606.