|Ewing sarcoma is the second most common malignant bone tumor of childhood with a poor overall survival rate of 55%. This tumor is dependent on the activity of the oncogenic transcription factor EWS-FLI1 for cell survival. Therefore, we have identified mithramycin as an inhibitor of EWS-FLI1 and translated the compound to the clinic. Overall, the drug was well tolerated by patients; however, the activity of mithramycin was modest due to liver toxicity that limited patient plasma drug levels to concentrations that are insufficient to block EWS-FLI1 activity.
In order to achieve EWS-FLI1 blockade in patients, the goal of this work is to increase the potency and/or decrease the toxicity of mithramycin. To increase the potency of mithramycin, we demonstrate that the CDK9 inhibitor, PHA-767491, improves the mithramycin-mediated blockade of EWS-FLI1 activity to suppress cell growth. Furthermore, we show that two additional CDK9 inhibitors that have been used in the clinic for other indications also potentiate mithramycin activity. To decrease the toxicity of mithramycin, we screened more than 80 known liver toxicity genes in immortalized hepatocytes to identify the candidate gene, BTG2 (BTG family, member 2). We show that the induction of BTG2 is context dependent, occurring in immortalized hepatocytes, but not in Ewing sarcoma cells. In addition, this induction is associated with the accumulation of reactive oxygen species, which may drive mithramycin liver toxicity. These studies provide the basis for future work to develop these approaches in order to make the mithramycin-mediated blockade of EWS-FLI1 achievable in patients.