Background: Insulin-like growth factor-I receptor (IGFR) signaling promotes tumorigenic properties which makes targeting IGFR an attractive therapeutic target. IGF signaling has been linked to proliferation, survival, angiogenesis, metastasis and resistance to anticancer therapies, and contributes to pediatric malignancies including Ewing sarcoma and rhabdomyosarcoma. Recently, monoclonal antibodies targeting IGFR have shown promise in a limited number of sarcoma patients. A substantial proportion of patients demonstrate intrinsic resistance, and those that do respond will often lose clinical benefit (acquired resistance). Intrinsic and acquired resistance ultimately results in failure to respond to treatment and continued growth of the tumor. Understanding these resistance mechanisms will identify combinations of targeted therapy to optimize therapeutic efficiency of this targeted agent.

Methods: To test intrinsic and acquired mechanisms of resistance, we utilized Receptor Tyrosine Kinase (RTK) Arrays to analyze the phosphorylation status of 49 RTKs. For intrinsic resistance, we used a panel of Ewing Sarcoma (EWS) cell lines that are unresponsive to TZ1, a monoclonal antibody that targets IGFR. For acquired resistance, we developed a TZ1 resistant rhabdomyosarcoma (RMS) cell line from TZ1 sensitive RH41 cells. The cell lines were treated with TZ1, an anti-IGFR monoclonal antibody. Signaling changes were assessed by comparing phosphorylation status of the RTKs in the untreated versus treated cells in the inherent resistance model and in the parental versus resistant cells in the acquired resistance model. Results: TZ-1 treatment of the EWS cell lines with intrinsic resistance decreased pIGFR, but these cells were characterized by expression of multiple phosphorylated RTKs. In the Rh41 acquired resistance model, increased activation of signaling pathways was detected in the resistance cells including: AXL, PDGFR, FGFR4 and ALK. Treatment of parental Rh41 cells with TZ1 also rapidly induced several activated RTKs. In TZ1-resistant Rh41 cells combined treatment of TZ1 with the PDGFR inhibitor, crenolanib, and the pan-FGFR inhibitor LY2874455 resulted in significant decrease in cellular proliferation.

Conclusions: Intrinsic resistance to TZ1 in EWS cell lines appears to be due to redundancy through multiple alternative activated RTKs. Systematic knockdown to determine which activated RTKs confer resistance is being undertaken. In Rh41 RMS cells acquired resistance to TZ1 is characterized by induction of several alternative activated RTKs, some of which are induced rapidly after TZ1 exposure. Combined treatment with TZ1 and drugs targeting PDGFR and FGFR was effective at overcoming resistance. These results suggest that identification of signaling pathways that compensate for IGFR inhibition can lead to effective potent multi-target combination therapy. Supported by USPHS Grant CA165995.

Citation Format: Terry J. Shackleford, Seethalakshmi Hariharan, Hemant K. Bid, Peter J. Houghton. Mechanisms of resistance to IGFR-targeted therapy in pediatric sarcomas. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2472.