The defining molecular feature of Ewing sarcoma is
The defining molecular feature of Ewing sarcoma is the characteristic EWSR1/ETS fusion protein, most commonly involving EWSR1 and FLI1 (Delattre et al., 1992, May et al., 1993). EWS–FLI1 causes global changes in gene expression both by directly regulating gene expression and by causing changes in chromatin structure influenced by EWS–FLI1 downstream targets (Bertolotti et al., 1996, Owen et al., 2008, Kinsey et al., 2009, Patel et al., 2012). The end result is a transcriptional program that mediates malignant transformation, maintains the Paxilline and in a de-differentiated state and helps to evade the toxicity associated with DNA damaging agents (Riggi and Stamenkovic, 2007, Ban et al., 2008, Riggi et al., 2008, Awad et al., 2010, von Levetzow et al., 2011, Lawlor and Thiele, 2012).
In addition, silencing of EWS–FLI1 using antisense DNA, siRNA or dominant negative methods markedly impairs Ewing sarcoma cell growth (Maksimenko & Malvy, 2005).
From a therapeutic standpoint, the challenge is integrating this data into the preclinical and clinical development of new agents that target the biological drivers of this disease. In order to accomplish this, work continues to understand the mechanism of drugs that have proven to be active in the clinic, such as the camptothecins. Conversely, other preclinical data is aimed at understanding the reasons for disappointing clinical results with promising agents that have a strong preclinical rationale such as ET-743 and Insulin like Growth Factor (IGF) pathway inhibitors (Scotlandi et al., 2002, Lau et al., 2005, Grohar, Griffin, et al., 2011, Pappo et al., 2011, Wagner, 2011, Malempati and Hawkins, 2012). Finally, the clinical translation of therapies that directly target EWS–FLI1, associated epigenetic changes as well as the DNA damage response presents unique challenges to both preclinical and clinical investigators (Stegmaier et al., 2007, Erkizan et al., 2009, Grohar, Woldemichael, et al., 2011, Lawlor and Thiele, 2012).
The camptothecins As with many tumors, the favorable survival rates for patients with localized Ewing sarcoma are the direct result of decades of skillful clinical research. In recent years, these efforts have focused on identifying and developing other active agents in both the relapsed and up-front setting. Among the most well studied compounds in the clinic in Ewing sarcoma are the camptothecins, irinotecan and topotecan (reviewed in Wagner, 2011). The camptothecins are a class of compounds originally isolated from the bark of a Chinese tree called Camptotheca acuminate. The primary mechanism of action of these compounds involves stabilization of the topoisomerase I cleavage complex leading to replication fork collision, DNA damage and cell death (Pommier, 2006). In Ewing sarcoma, the camptothecin derivatives, topotecan and irinotecan have been investigated both as up-front windows in high-risk patients and in the relapsed setting. These compounds are structural derivatives of the parent compound substituted at the 7, 9 and 10 positions of the camptothecin backbone (Fig. 1). The importance of these compounds is suggested by a series of clinical studies that show good activity in Ewing sarcoma both as single agents and in combination therapies with either cyclophosphamide or temozolamide (Pappo et al., 2011, Wagner, 2011). Topotecan is hydroxylated at the 10 position and contains an alkylamine at the 9 position of the camptothecin backbone. This agent has been evaluated in a broad range of both pediatric and adult tumors. The Pediatric Preclinical Testing Program (PPTP) did a thorough investigation of this compound across 45 different xenograft models of pediatric cancer and found an increase in event free survival in 86% of the solid tumors evaluated with complete responses observed in Ewing sarcoma, Wilm\'s tumor and rhabdomyosarcoma (Carol et al., 2010). In Ewing sarcoma patients, a COG phase II evaluated topotecan as an up-front window in high-risk patients as a single agent (n=36) and in combination with cyclophosphamide (n=40) and showed partial response rates of 8% and 57% respectively (Bernstein et al., 2006). It is notable that these response rates are higher than what has been observed in the relapsed setting where partial response rates of 4% as a single agent and 33% as a combination have been seen although a 21-day infusion may offer slightly better results (Blaney et al., 1998, Hawkins et al., 2006, Hunold et al., 2006). The major dose limiting toxicity of this drug has been myelosuppression, occurring in more than 50% of patients treated with cyclophosphamide and topotecan (Saylors et al., 2001).