CK2 inhibitor CX-4945 targets EWS-FLI1 protein abundance and shows anti-tumor activity in metastatic mouse models of Ewing Sarcoma

Ewing sarcoma (ES) is a highly aggressive bone cancer. EWS-FLI oncogenic fusion protein is detected in more than 85% of cases and is indispensable for ES tumor survival and progression. Casein kinase II (CK2) is a serine/threonine kinase that plays an essential role in apoptosis, DNA damage repair, and the cell cycle. CSNK2A1 is highly expressed in ES and associated with metastatic disease and poor 5-year overall survival. CK2 is shown to phosphorylate and stabilize several transcription factors, including EWS-FLI1. We demonstrate that genetic and pharmacological inhibition of CK2 decreases protein abundance of EWS-FLI1 by increased ubiquitination and proteasomal degradation. Clinical-grade inhibitor of CK2, CX-4945 (silmitasertib), shows in vitro cytotoxic activity in a series of ES tumor organoids and patient-derived xenograft cells. We also demonstrate anti-tumor activity of single agent CX-4945 using metastatic xenograft models of Ewing sarcoma. We also show decreased lung metastases in mice treated with CX-4945. CX-4945 showed synergistic cytotoxic activity with Temozolamide and Irinotecan. CX-4945 is currently being tested in a Phase 1 study to evaluate the safety and tolerability in combination with chemotherapy for the treatment of pediatric colloid tumors, including Ewing sarcoma. The preclinical studies reported here support the clinical studies evaluating the efficacy of CX-4945 for the treatment of Ewing sarcoma.