DNA-Dependent Protein Kinase Is a Molecular Target for the Development of Noncytotoxic Radiation–Sensitizing Drugs

Abstract

DNA-dependent protein kinase (DNA-PK)-defective severe combined immunodeficient (SCID) mice have a greater sensitivity to ionizing radiation compared with wild-type mice due to deficient repair of DNA double-strand break. SCID cells were therefore studied to determine whether radiosensitization by the specific inhibitor of DNA-PK, IC87361, is eliminated in the absence of functional DNA-PK. IC87361 enhanced radiation sensitivity in wild-type C57BL6 endothelial cells but not in SCID cells. The tumor vascular window model was used to assess IC87361-induced radiosensitization of SCID and wild-type tumor microvasculature. Vascular density was 5% in irradiated SCID host compared with 50% in C57BL6 mice (P < 0.05). IC87361 induced radiosensitization of tumor microvasculature in wild-type mice that resembled the radiosensitive phenotype of tumor vessels in SCID mice. Radiosensitization by IC87361 was eliminated in SCID tumor vasculature, which lack functional DNA-PK. Irradiated LLC and B16F0 tumors implanted into SCID mice showed greater tumor growth delay compared with tumors implanted into either wild-type C57BL6 or nude mice. Furthermore, LLC tumors treated with radiation and IC87361 showed tumor growth delay that was significantly greater than tumors treated with radiation alone (P < 0.01 for 3 Gy alone versus 3 Gy + IC87361). DNA-PK inhibitors induced no cytotoxicity and no toxicity in mouse normal tissues. Mouse models deficient in enzyme activity are useful to assess the specificity of novel kinase inhibitors. DNA-PK is an important target for the development of novel radiation-sensitizing drugs that have little intrinsic cytotoxicity. (Cancer Res 2005; 65(12): 4987-92)

Preclinical pharmacokinetics and metabolism of a novel prototype DNA-PK inhibitor NU7026

“…This compound has similar structural features to LY294002, in particular the morpholino group, but unlike LY294002 has no inhibitory effect on phosphoinositide 3-kinase (Veuger et al, 2003(Veuger et al, , 2004Griffin et al, 2005). A recent report on IC87361 shows a significant radiosensitisation in LCC and B16F0 cells with 6 Gy, which translates into a significant growth delay in xenograft models (Shinohara et al, 2005). Our study shows that at 10 mM, the compound alone has no intrinsic growth inhibition properties but it significantly potentiates the effect of radiation in CH1 human ovarian cancer cells.…”

“…administered four times at 1 h intervals should produce the required exposure to NU7026 for radiosensitisation in vivo. It will then be interesting to compare the results with other less-specific inhibitors such as LY294002 (Hu et al, 2000;Edwards et al, 2002;Semba et al, 2002;Gupta et al, 2003;Shinohara et al, 2005). If this dose is not tolerated, then it will be necessary to develop compounds that are either more potent or less readily metabolised.…”

Harnessing the complexity of DNA-damage response pathways to improve cancer treatment outcomes

“…Responses to the combination of DNA-PK inhibitors with DNA-damaging agents include reduced clonogenic survival and cellular proliferation, increased DSBs, G2 arrest and subtle increases in the sub-G1 population (indicative of apoptosis) (Willmore et al, 2004(Willmore et al, , 2008Shinohara et al, 2005), regardless of p53 status (Ismail et al, 2004;Zhao et al, 2006). Recently, Shang et al…”

DNA Protein Kinase–Dependent G2 Checkpoint Revealed following Knockdown of Ataxia-Telangiectasia Mutated in Human Mammary Epithelial Cells

“…Whereas ATM and ATR act as initiators of DNA damage checkpoint signaling (1), DNA-PKcs has a critical role in double-strand break repair via nonhomologous end-joining (2). The importance of each of these proteins in maintaining genomic stability is underscored by the increased sensitivity to DNA damage observed with reduced activity of any one of these kinases (3)(4)(5).…”