The relatively low overall response
The relatively low overall response rates suggest the need for better predictive markers for response. The patient demographics in the SARC trial did not reflect typical Ewing sarcoma demographics as the patients tended to be older and had primarily soft tissue tumors. In addition, although accrual favored older children with soft tissue primary tumors, bone based primary tumors, more common in younger children, appeared to have a more robust response to the inhibitor. Finally, the dose and schedule of the drug in the early part of the study were designed to favor AUC pharmacokinetics, while later data suggested that drug peak was more important to activity. This led to a trial design that allowed both schedules. Unfortunately, the trial was forced to close prior to completing accrual due to a loss of corporate support (Pappo et al., 2011).
There remains considerable enthusiasm for IGF therapy due to the dramatic responses observed and the plurality of preclinical data. Therefore, efforts moving forward have been focused on addressing mechanisms of resistance as well as developing methods to try and identify those patients that are likely to respond to the drug. These efforts have met with some success. It is known that downstream blockade of mTOR leads to feedback activation of AKT via the IGF Alfacalcidol originally described in myeloma and rhabdomyosarcoma (Shi et al., 2005, Wan et al., 2007). Therefore, combining IGF inhibition and mTOR inhibitors in preclinical sarcoma models has led to improved activity of both agents (Wan et al., 2007). This improved activity has also been observed in the clinic. A recent study that combined the IGF-1R inhibitor cixutumumab with temsirolimus found tumor regressions in 30% of Ewing sarcoma patients treated with this combination including a complete response in a patient previously refractory to IGF-1R targeting (Naing et al., 2012).
Finally, there have been a number of important studies that attempt to identify those patients that might respond to the antibody. For example, proteomic profiling has led to the development of a proteomic signature of likely responders that could be used to preselect patients once validated (Subbiah et al., 2011). In addition, a recent report suggests that nuclear staining of IGF may reflect pathway activation and predict those patients that might respond to the antibody (Asmane et al., 2012). Other important investigations have shown a wide range of IGF receptor expression that predicts response in rhabdomyosarcoma perhaps detectable with imaging methods (Cao et al., 2008, Fleuren et al., 2011). Finally, an analysis of over 200 ES patients has shown that patients with Ewing sarcoma have variable expression of circulating IGF and IGFBP3 (Borinstein et al., 2011). It is not known how these variable levels would predict a response to IGF blockade but it is notable that in preclinical models circulating IGF has been shown to block the anti-angiogenic effects of the drug (Bid et al., 2012).
Epigenetic targeting Since Ewing sarcoma is a transcription factor driven disease, agents that target chromatin structure would be expected to have activity in this tumor type. In addition, a variety of protein complexes involved in the regulation of chromatin structure have been shown to be dysregulated in Ewing sarcoma (reviewed in Lawlor & Thiele, 2012). The resulting changes in the epigenome play a major role in the biology of ES and have been linked to alterations in gene expression, malignant transformation and even drug resistance (Lawlor & Thiele, 2012). It is known that EWS–FLI1 blocks the expression of more genes then it induces (Kauer et al., 2009). This suppression is a critical component of the tumorigenicity of EWS–FLI1 as it likely silences critical tumor suppressor genes. The mechanism of EWS–FLI1 gene suppression is both direct, as with TGFBR2, and indirect by driving the expression of genes that in turn blocks expression of portions of the EWS–FLI1 gene signature, an effect that is at least partially mediated by changes in the epigenome (Hahm et al., 1999, Owen et al., 2008, Kinsey et al., 2009). For example, it has been shown that EWS–FLI1 drives the expression of NKX2.2, a gene that silences a portion of the EWS–FLI1 repressed signature (Owen et al., 2008). NKX2.2 binds to the promoters of target genes and facilitates TLE associated HDAC recruitment to suppress expression of these genes. Treatment of A673 ES cells with the HDAC inhibitor vorinostat reverses this suppression and leads to a dose dependent inhibition of Ewing sarcoma cell growth (Owen et al., 2008). This marked sensitivity to HDAC inhibitors mirrors an earlier report that evaluated the HDAC inhibitor, MS-275, in the TC71 xenograft model of Ewing sarcoma and found an impressive response with single agent treatment by oral gavage (Jaboin et al., 2002).