PO-464 Endoglin-targeted therapy demonstrates strong preclinical anti-tumour activity in ewing sarcoma using antibody-drug conjugates

Abstract

IntroductionEwing sarcoma (ES) is a bone/soft tissue neoplasia putatively originated from mesenchymal stem cells (MSC). Among the proteins that define the MSC signature, endoglin (ENG, CD105) is considered one of the highly expressed molecules. This protein is a transmembrane co-receptor of the TGF-β family. The poor outcome experienced by ES patients with disseminated disease highlights the necessity of developing new therapeutic strategies. The generation of two novel anti-ENG monoclonal antibody-drug conjugates, linked to nigrin-b A chain (OMTX503) or cytolysin (OMTX703), appeared as an appealing scenario to test the possible role of ENG as a key target in ES.Material and methods mRNA levels were evaluated by qRT-PCR. Protein levels were studied by western blot, flow cytometry and immunohistochemistry (IHC). OMTX503 and OMTX703 activity was evaluated by MTT and WST1 assays. OMTX503 and OMTX703 were tested in ES cell line-derived (RM82 and ES8) xenografts and patient-derived xenografts (PDXs). Hematoxylin and eosin staining was performed to assess cell viability. IHC analyses for ENG, MMP14, and Ki67 expression were performed to assess ENG/MMP14 expression and tumour proliferation in samples from ES xenografts and patients.Results and discussionsWe evaluated the ENG expression in a set of ES cell lines, related xenografts, PDXs and patient tumours. The consistent heterogeneous ENG expression found among ES sets suggested that the patients presenting high ENG expression in their tumour cells could benefit from anti-ENG treatments. In order to confirm this hypothesis pre-clinically, we assessed the anti-tumoral activity of OMTX503 and OMTX703. Firstly, a significant ENG-dependent anti-proliferating effect was observed in ES cell lines in vitro. In two ES cell line-derived xenograft models (RM82 and ES8), we identified a tumour growth impairment in OMTX503-treated animals. Moreover, a significant response to treatment was observed in OMTX703-treated mice, followed by a significantly increased median time of survival (p<0.019). Taking into consideration these results, we evaluated OMTX703 in an ES PDX model with high ENG expression. The complete response and increased survival observed in this PDX model treated with 3 weekly doses of 60 mg/kg OMTX703 strengthened the significant anti-tumoral effect of this drug.ConclusionThese results support the role of ENG as an effective target in ES, and suggest the inclusion of novel ADCs —like OMTX703— in the ES therapeutic arsenal for patients with high ENG expression in their tumours.