Abstract 5494: CX-4945, an inhibitor of protein kinase CK2, potentiates the antitumor activity of platinum chemotherapy in models of ovarian cancer by preventing phosphorylation of XRCC1 and MDC1 and disrupting DNA damage repair
Authors: Siddiqui-Jain Adam , Streiner Nicole , Bliesath Josh , Macalino Diwata , Omori Mayuko , Huser Nanni , Ho Caroline , O'Brien Sean , Drygin Denis , Anderes Kenna 
Published: June 2012

Abstract
Platinum-based chemotherapeutics are commonly used to treat solid tumors but may be restricted in their application by dose limiting toxicity and inherent or acquired resistance. Because efficient DNA damage repair mechanisms contribute to resistance, targeting components of the repair machinery has emerged as a strategy to increase the effectiveness of these and other DNA-damaging anti-cancer drugs. Protein kinase CK2 is a serine/threonine kinase that has emerged as an attractive molecular target due to its overexpression in cancer and regulatory role in key cellular processes including the cell cycle, apoptosis and DNA damage repair. Multiple lines of evidence suggest an increasingly important role for CK2 in the DNA damage response, including the phosphorylation and activation of the mediator/adaptor proteins XRCC1 and MDC1. XRCC1 is an essential component for nucleotide excision repair which is the major repair pathway responsible for removing platinum adducts. MDC1 is the predominant γ-H2AX recognition factor in mammalian cells and is essential for homologous recombination repair. CX-4945 is a first-in-class, selective, oral inhibitor of CK2 currently in Phase 1 clinical trials. We sought to determine if inhibiting CK2 activity with CX-4945 would prevent phosphorylation of XRCC1 and MDC1 and potentiate the activity of platinum-based drugs by preventing DNA damage repair. Treatment of the ovarian cancer cell lines A2780 and SKOV3 with CX-4945 led to decreased phosphorylation of XRCC1 at the CK2 specific phosphorylation sites and reduced total XRCC1 protein levels. Likewise, immunoprecipitation of MDC1 from SKOV3 cells treated with CX-4945 revealed significant reductions in phosphorylation at the CK2 specific sites, while in A2780 cells MDC1 protein levels were decreased. The reduction of MDC1 protein levels was reproduced by CK2 siRNA, confirming that CK2 activity supports MDC1 expression levels. Combined treatment of A2780 cells with CX-4945 and cycloheximide revealed a faster rate of MDC1 degradation than with cycloheximide alone but did not affect MDC1 mRNA levels, indicating that CK2 regulates MDC1 protein stability. When combined with cisplatin, CX-4945 enhanced the activation of CHK2 and increased levels of γ-H2AX and cleaved PARP. In antiproliferative experiments, CX-4945 exhibited synergy with cisplatin in A2780 and SKOV3 cell lines. The combination of CX-4945 with cisplatin or carboplatin significantly enhanced antitumor activity in ovarian xenograft models and was well tolerated. Thus, the inhibition of CK2 by CX-4945 enhanced the antitumor activity of platinum agents by preventing DNA damage repair and inducing apoptosis. These data provide compelling preclinical support for pursuing CX-4945 in combination with platinum chemotherapy in the clinic.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5494. doi:10.1158/1538-7445.AM2011-5494