Abstract Background: Ewing sarcoma (EWS) is the second most common bone malignancy in the pediatric population. Despite intensive treatment regimens, 5-year overall survival for patients with metastatic or relapsed disease is less than 20%, indicating a clear need for novel targeted therapies. The importance of IGF signaling in EWS is illustrated by activity of antagonistic targeting of IGF-1R in clinical trials, although this approach is plagued by rapid development of resistance. Pregnancy Associated Plasma Protein A (PAPP-A) anchors to cell surface proteoglycans and mediates metalloproteinase activity, that cleaves IGFs from IGFBP-4, IGFBP-2 and IGFBP-5, thus increasing local bioavailability of bioactive, free IGF. PAPP-A is expressed at high levels by placental trophoblasts, where it induces high levels of IGFs, which are required for fetal growth. Methods: In an attempt to identify novel targetable cell surface antigens in EWS, we performed ribosomal depleted RNA sequencing in 122 EWS samples (49 cell lines and 73 tumor samples) and 96 normal samples of variable tissues. Results: PAPP-A was identified as one of the top 5 membrane associated proteins overexpressed in EWS (> than 4-fold) compared to normal tissue (p = 3.89E-30). PAPP-A showed substantial expression in tumors (median log2FPKM = 3.933, n = 122) and minimal expression in normal tissue (median log2FPKM = -1.564, n = 96). We confirmed cell surface expression of PAPP-A by flow cytometry on 10 EWS cell lines tested, and identified PAPP-A in the supernatant of 12 EWS cell lines, measured by ELISA. Targeting the PAPP-A locus in the EWS cell line EW8 utilizing CRISPR/Cas9 completely abrogated PAPP-A secretion in single cell clones and significantly diminished bioactive IGF-1 levels in EWS cultures, while increasing IGF-1 bound to IGFBP-4, results which confirm absent PAPP-A metalloproteinase activity. Gene knockout of PAPP-A in EW8 resulted in decreased cell growth in vitro. Conclusions: We have identified PAPP-A as a novel potential cell surface target in Ewing sarcoma, characterized by high level tumor expression, limited normal tissue expression and biological significance in this tumor. Ongoing work is focused on studying growth of PAPP-A KO cell lines, and assessing whether PAPP-A neutralizing antibodies may mediate antitumor effects in EWS. Citation Format: Sabine Heitzeneder, John F. Shern, Javed Khan, Crystal L. Mackall. Pregnancy associated plasma protein A (PAPP-A) is a potential novel therapeutic target in Ewing sarcoma. [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 571.