Druggability of the CK2 Inhibitor CX-4945 as an Anticancer Drug and Beyond
Protein kinase CK2 (formerly called casein kinase 2), a ubiquitous serine/threonine kinase, regulates a var- iety of cellular processes including the cell cycle, prolif- eration, and apoptosis (Allende et al., 1995; Litchfield, 2003; Meggio and Pinna, 2003; Wang et al., 2005; Homma and Homma, 2008; Fig. 1). CK2 is constitutive- ly active and elevated in cancers, and a positive corre- lation between its over-expression and tumorigenesis has been reported in several cellular and animal models (Prowald et al., 1984; Guerra and Issinger, 1999; Landesman-Bollag et al., 2001; Tawfic et al., 2001; Channavajhala and Seldin, 2002).
Because CK2 inhibition induces apoptotic cell death in tumor cells, this molecule has been considered a pro- mising druggable anticancer target (Slaton et al., 2004; Wang et al., 2005). Several studies suggests that decreasing CK2 activity using either a specific antisense
oligonucleotide or a selective small molecule inhibitor attenuates the growth of cancers (Wang et al., 2001; Slaton et al., 2004; Hessenauer et al., 2011; Pierre et al., 2011a, 2011b).
Recently, orally available CX-4945 (Fig. 2), a potent and selective small molecule inhibitor of CK2, has been developed and advanced to testing in human clinical trials. The biological importance of CK2 in the context of cancer as well as the suitability of its small ATP binding site for the design of selective inhibitors has led Cylene Pharmaceuticals to develop orally avail- able CX-4945 as an ATP-competitive inhibitor of CK2 for the treatment of cancer (Pierre et al., 2011). Mole- cular modeling of CK2 guided the optimization, leading to a form of CX-4945 with a high potency (Ki = 0.38 nM). CX-4945 displays an excellent kinase selectivity profile and is active against a broad range of cancer cell types, including breast, lung, and prostate cancer cells, in which CK2 is reported to be overexpressed (Siddiqui-Jain et al., 2010; Pierre et al., 2011). However, some discrepancies persist, such as the wide-margin difference between the IC50 values of CX-4945 in a kinase enzyme assay and in a cell-based assay. Enzymatic kinase screening data yielded an IC50 at nanomolarnM), but relative EC50 values in several cancer cell lines have been in the micromolar concentration range (1.1 to 13.1 M).
Fig. 1. Biological functions of CK2.
Fig 2. Schematic mode of action of CX-4945. CX-4945 inhibits the phosphorylation of PI3K/AKT and p21/p27 and transcriptional activation of HIF1 in the nucleus. These effects might influence cancer survival, angiogenesis, and pro- inflammatory cytokine production.
The anticancer activity of CX-4945 correlates with the suppression of CK2-regulated PI3K/Akt signaling, cell cycle arrest, and induction of apoptosis (Siddiqui- Jain et al., 2010); CX-4945 induces early dephosphory- lation of Akt (S129; CK2 phosphorylation site) and a subsequent reduction in the number of regulatory sites, Akt (T308) and Akt (S473). In addition, CX-4945 results in decreased phosphorylation of the cell cycle inhibitor protein p21 (T145) and in G2/M or G1 cell cycle arrest. These findings account for the consequent apoptotic response and induction of caspase-3/7 activity in CX-4945–treated cancer cells.
CK2 also regulates angiogenesis by driving the PI3K/ Akt signaling pathway (Shiojima and Walsh, 2002; Kramerov et al., 2008). CK2 inhibitors exhibit anti-angiogenic activity in human umbilical vein endothelial cells (HUVECs) and in neovascularization of mouse retina (Ljubimov et al., 2004; Kramerov et al., 2008). Of further interest, CX-4945 inhibits HUVEC prolif- eration, migration, and tube formation with the rapid dephosphorylation of Akt S129, S473, and T308 and the downstream Akt effector p21 (T145), suggesting an anti-angiogenic activity of CX-4945 via blocking Akt signaling (Siddiqui-Jain et al., 2010).
Additionally, CX-4945 inhibits hypoxia-driven hypoxia- inducible factor (HIF)-1 activity and prevents down- regulation of von Hippel-Lindau protein (pVHL) and p53; thus, another mechanism for the anti-angiogenic activity of CX-4945 could be regulation of HIF-1. Hyp- oxia increases CK2 activity by up-regulation of histone deacetylases (HDAC1 and HDAC2), which inhibit the expression of tumor suppressor genes, such as pVHL and p53, that have the potential to reduce HIF-1 activity (Mottet et al., 2005; Hubert et al., 2006; Pluemsampant et al., 2008).
These promising results in safety, pharmacokinetics, and efficacy studies have led to the current clinical evaluation of CX-4945 as a first-in-class CK2 inhibitor for the treatment of cancer (Pierre et al., 2011). CX- 4945 exhibits no significant inhibition of the hERG chan- nel and minimal inhibition of the principle cytochrome P450 isoforms, in addition to its non-mutagenic activity and non-genotoxic activity. Furthermore, it exhibits low to moderate clearance and a wide range of volume of distribution across species, with a terminal half-life ranging between 5 and 12 h and moderate oral bioavail- ability (range 20 to 51%) that could make CX-4945 effi- cacious in xenograft models. Indeed, in BT-474 or BxPC- 3-inoculated xenograft models, CX-4945 elicits dose- dependent decreases in tumor volume and reduced phosphorylation of p21 (T145).
Recently, a new potential indication for CX-4945 has been reported: It also exerts a regulatory effect on the secretion of interleukin-6 (IL-6) in inflammatory breast cancer (IBC; Drygin et al., 2011). CK2 activation regu- lates expression of cytokines associated with cancer cell survival and metastasis (Guerra and Issinger, 2008), and over-expression of pro-inflammatory cytokines is frequently found in IBC (van Golen et al., 2000; Ara and Declerck, 2010). Among these cytokines, IL-6 influ- ences tumor cell proliferation, inflammation, and migra- tion. Of special interest, CX-4945 reduces IL-6 produc- tion in IBC (Drygin et al., 2011). Thus, dual targeting of CK2 activation and IL-6 production could be a thera- peutic approach for various types of cancer.
In conclusion, the CK2 inhibitor CX-4945 exhibits anticancer activity by down-regulation of PI3K/Akt, p21, and HIF-1 activities, subsequently affecting the transcriptional regulation of molecules involved in can- cer proliferation, angiogenesis, and pro-inflammatory cytokine production (Fig. 2). The development of CX- 4945 and its warranted advancement into clinical development will allow therapeutic targeting of CK2 for the treatment of human cancers. Of importance, human clinical evaluation of CX-4945 as a single agent in solid tumors and multiple myeloma has established it as having promising pharmacokinetic, pharmaco- dynamic, and safety profiles (Padgett et al., 2010). Fur- thermore, beyond cancer, CX-4945 may be developed as a new therapeutic for inflammatory disorders, neuro- degenerative diseases, and viral or parasitic infections involving CK2 dysregulation (Guerra and Issinger, 2008; Drygin et al., 2011). In this way, its continued ad- vancement could open the Silmitasertib door to an entirely new class of anticancer therapeutics.