Abstract 562: Frequent copy number gain of MCL1 is a therapeutic target for osteosarcoma

Abstract Osteosarcoma (OS) is the most common primary malignant bone tumor that predominantly occurs in children, adolescents, and young adults. The treatment for OS that combines surgery with chemotherapy, which consists of a four-drug combination of adriamycin (DOX), cisplatin (CDDP), high-dose methotrexate (MTX), and ifosfamide, was established in 1970s, and it is still used as a standard therapy. Oncogenic driver mutations and fusion genes common to OS have not been identified, which is one of the reasons for the lack of success in drug development for OS. Herein, to explore the characteristic vulnerability in OS, molecular targeted drugs were screened against OS cell lines and patient-derived cells; obatoclax, a pan-BH3 mimetic, and a concomitant drug that enhances obatoclax-induced apoptosis, OSI906, a dual inhibitor of IGF1R and IR, were found. Fluorescence in situ hybridization (FISH) analysis revealed that copy number gains of MCL1, an antiapoptotic Bcl-2 family protein, were observed in 46. 3% of formalin-fixed paraffin-embedded (FFPE) specimens from patients with OS (N = 41) and in 45. 5% of OS cell lines (N = 11), thereby causing vulnerability to BH3 mimetics. IHC staining also revealed high expression of Mcl-1 protein in specimens with MCL1-amplification in FISH analysis. Pharmacological or genetic inhibition of Mcl-1 induced potent apoptosis in MCL1-amplified OS cells, which was enhanced by the co-inhibition with Bcl-xL. Moreover, chromosome 1q21. 2-3, where MCL1 is located, contains several IGF1R/PI3K pathway-related genes, including PIP5K1A, TARS2, OUTD7B, and ENSA, whose copy numbers were also increased in MCL1-amplified OS cells, suggesting that 1q21. 2-3-amplified OS cells readily activate downstream pathways via IGF1R. IGF signaling activation in 1q21. 2-3-amplified OS cells negated the obatoclax-induced apoptosis, which was overcome by OSI906 treatment. Combined obatoclax and OSI906 administration remarkably suppressed MCL1-amplified OS tumor growth in the xenograft mouse model and resulted in tumor regression; however, none of the treatments reduced tumor growth of OS without MCL1 amplification. These results suggest that combining obatoclax with OSI906 could be a potential therapeutic intervention for 1q21. 2-3-amplified OS, and that prior stratification of OS based on the 1q21. 2-3 amplification status would be important for the success of this combination therapy. Thus, we elucidated the mechanism of action of drugs and their potential as a novel therapeutic strategy for approximately 50% of patients with OS. Moreover, a FISH analysis that can detect copy number gains of MCL1 in FFPE specimens from patients with OS was established to stratify patients eligible for this therapy. Citation Format: Satoshi Takagi, Ryohei Katayama. Frequent copy number gain of MCL1 is a therapeutic target for osteosarcoma 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 (6Suppl): Abstract nr 562.