**1. Introduction**

496 Selected Topics in DNA Repair

Roots, R., Kraft, G., & Gosschalk, E. (1985). The formation of radiation-induced DNA breaks:

Salvi, V.P., Maurya, D.K., Kagiya, T.V., & Nair, C.K.K. (2007). Enhancement in repair of

Sasaki, Y.F., Ohta, T., Imanishi, H., Watanabe, M., Matsumoto, K., Kato, T., & Shirasu, Y.

Satyamitra, M., Devi ,P.U., Murase, H, & Kagiya, V.T. (2001). In vivo radioprotection by alpha-TMG: preliminary studies. *Mutation Research*, Vol. 479, No.1-2, pp.53-61 Serin, M., Gulbas, H., Gurses, I., Erkal, H.S., & Yucel, N. (2007). The histopathological

Spielberger, R., Stiff, P., Bensinger, W., Gentile, T., Weisdorf, D., Kewalramani, T., Shea, T.,

Spotheim-Maurizot, M., & Davidkova, M. (2011). Radiation damage to DNA in DNA-

Spothem-Maurizot, M., Mostafavi, M., Douki, T., & Belloni, J., editors. (2008). Radiation Chemistry: from basics to applications in material and life science: EDP Science. Stone, H.B., Moulder, J.E., Coleman, C.N., Ang, K.K., Anscher, M.S., Barcellos-Hoff, M.H.,

Tilak, J.C., & Devasagayam, T.P.A. (2003). Radioprotective property of baicalein. *BARC* 

Ueno, M., Inano. H., Onoda, M., Murase, H., Ikota, N., Kagiya, T.V., & Anzai, K. (2009).

Devi, P.U., Ganasoundari, A., Vrinda, B., Srinivasan, K.K., & Unnikrishnan, M.K. (2000).

Vijayalaxmi, Meltz, M.L., Reiter, R.J., Herman, T.S., & Kumar, K.S. (1999). Melatonin and

Vijayalaxmi, Reiter, R.J., Herman, T.S., & Meltz, M.L. (1996). Melatonin and radioprotection

Weiss, J.F., & Landauer, M.R. (2009). History and development of radiation-protective agents. International Journal of Radiation Biology, Vo. 85, No.7, pp.539-573

of action. *Radiation Research*, Vol 154, No.4, pp.455-460

Radiation Oncology Biology and Physics, Vol. 11, No.2, pp.259-265

*International Journal of Low Radiation*, Vol.4, No.1, pp.43-52

pp.153-159

No.4, pp.299-302

*Biology,* Vol. 83, No.3, pp.187-193

(doi:10.1088/1742-6596/261/1/012010)

*Research*,Vol. 162, No.6, pp.711-728

Research, Vol. 425, No.1, pp.21-27

*Research*, Vol. 371, No.3-4, pp.221-228

*Newsletter*, Vol. 249, pp.98–104

pp.519-524

351, No.25, pp.2590-2598

soluble derivative of vitamin E. *Journal of Radiation Research* (Tokyo), Vol. 43, No.2,

the ratio of double-strand breaks to single-strand breaks. International Journal of

radiation-induced DNA strand breaks in vivo by tocopherol monoglucoside.

(1990). Suppressing effects of vanillin, cinnamaldehyde, and anisaldehyde on chromosome aberrations induced by X-rays in mice. *Mutation Research*, Vol. 243,

evaluation of the effectiveness of melatonin as a protectant against acute lung injury induced by radiation therapy in a rat model. *International Journal of Radiation* 

Yanovich, S., Hansen, K., Noga, S., et al. (2004). Palifermin for oral mucositis after intensive therapy for hematologic cancers. N*ew England Journal of Medicine*, Vol.

protein complexes. Journal of Physics: Conference Series 261 (2011)

Dynan, W.S., Fike, J.R., Grdina, D.J., Greenberger, J.S., et al. (2004). Models for evaluating agents intended for the prophylaxis, mitigation and treatment of radiation injuries. Report of an NCI Workshop, December 3-4, 2003. *Radiation* 

Modification of mortality and tumorigenesis by tocopherol-mono-glucoside (TMG) administered after X irradiation in mice and rats. *Radiation Research*, Vol. 172, No.4,

Radiation protection by the ocimum flavonoids orientin and vicenin: mechanisms

protection from whole-body irradiation: survival studies in mice. Mutation

from genetic damage: in vivo/in vitro studies with human volunteers. *Mutation* 

For decades the world of radioprotectors has been dominated by the aminothiols, in particular WR1065 and its prodrug amifostine. These drugs emerged from an extensive programme of synthesis and evaluation under the auspices of the Walter Reed Army Institute of Research starting in the early 1950s (Sweeny, 1979). As discussed in detail in section 4 below, structure-activity studies on a series of aminothiols in John Ward's lab at the University of San Diego established a relationship between net charge and radioprotective activity. Positive charge conferred a DNA binding capability, by ionic interaction, and improved radioprotective activity. This was consistent with the fact that an important aspect of the mechanism of radioprotection by WR1065 is its radical scavenging activity. Given the limited range of diffusion of hydroxyl radicals generated from ionisation of water molecules, it makes sense that the radical scavengers will be most effective when located in the close vicinity of DNA. This basic rationale prompted the synthesis and evaluation of an aminothiol tethered to a DNA intercalating agent (Laayoun et al., 1994), but there is no evidence in the literature of a systematic follow-up.

Also, the new DNA binding radioprotector methylproamine emerged not from a rational design premise, but rather, from the serendipitous discovery of radioprotective activity of a minor groove binder Hoechst 33342 synthesized by the Hoechst company as part of a program aimed at developing antihelminthics. From that starting point, a modest lead optimisation program guided by a mechanistic hypothesis showed that radioprotective activity was enhanced by the introduction of more electron-rich substituents into the phenyl ring of the molecule.

Thus, this article links two groups of radioprotectors with the common feature of DNAbinding, albeit with quite different affinities. The dissociation constant for the WR1065-DNA interaction is in the mM range (Smoluk et al., 1986), whereas that for methylproamine is a few hundred nM (Martin et al., 2004). Accordingly, the relative radioprotective potency of WR1065 and methylproamine differs by more than 2-orders of magnitude. In contrast to this focus, other publications review a much wider range of radioprotectors (Hosseinimehr, 2007; Weiss & Landauer, 2009; Citrin, 2010).

DNA-Binding Radioprotectors 499

2002; Georgakilas, 2008). An attempt to process by the base excision repair (BER) a modified base that constitute an OCDL with a SSB in opposite strand may result in formation of a DSB, which also can be formed when an unprocessed OCDL interferes with either DNA replication and transcription (Bonner et al., 2008; Sedelnikova et al., 2010). Although DNA DSB can be repaired in cells by non-homologous end joining (NHEJ) and homologous recombination (HR) (Matsumoto et al., 1994; Memisoglu & Samson, 2000; Wilson et al., 2003; Cadet et al., 2010; Hinz, 2010; Lieber, 2010; Mladenov & Iliakis, 2011), DSB are among the most toxic IR-induced DNA lesions. If not properly repaired, both accumulated DSB and OCDL lead to cytotoxicity, genome instability and carcinogenesis (Jeggo & Lobrich, 2007;

It is established that a substantial level of oxidative DNA lesions may be present in normal cells and tissues, usually a few isolated oxidative DNA lesions per Mbp (Nakamura & Swenberg, 1999; De Bont & van Larebeke, 2004). These lesions are believed to be generated by free radicals that originate from endogenous reactive oxygen species (ROS) (Riley, 1994; Mikkelsen & Wardman, 2003). Two of the biologically important endogenous ROS are

produced in cells mainly as a result of mitochondrial respiration (Mikkelsen & Wardman, 2003), and then is efficiently converted to hydrogen peroxide by cellular superoxide dismutase (SOD). Although the superoxide and hydrogen peroxide are relatively long lived species and are able to diffuse in cells over considerable distance (Riley, 1994), these endogenous species are produced in cytoplasm and also they are not able to damage DNA directly. The genotoxic effect of the endogenous ROS is mainly mediated by their ability to give rise to hydroxyl radicals from hydrogen peroxide by a redox reaction with traces of reduced transitional metal ions, mainly ferrous via Fenton chemistry (Mikkelsen & Wardman, 2003). Since ROS represent potential risk for cells, an antioxidant defense system

Exogenous cytotoxic agents can lead to the increase above the steady state in the ROS level thus creating an oxidative stress that can result in induction of additional oxidative DNA damage (Sedelnikova et al., 2010). Exposure to IR is also known to cause the oxidative stress however, interestingly, the level of ROS generated directly from radiolysis of water at biologically relevant doses is much less that the level of the endogenous ROS. This follows from the estimation that for example a 100 Gy radiation dose would be required to double the endogenous level of one of the major types of DNA base damage 7,8-dihydro-8 oxoguanine (8-oxoG) (Ward, 1994b). There is also experimental evidence that the transient increase in the cellular ROS level following irradiation is dependent on mitochondria respiration, however it is dose independent in the range of biologically relevant doses (1 – 10 Gy) with the fraction of cells exhibiting the increased ROS level being dose dependent (Leach et al., 2001). These observations underline the minimal impact of isolated DNA lesions and the critical role of clustered lesions for cyto- and genotoxic consequences of IR. The major difference between endogenous ROS and those generated by IR is that while the spatial distribution of hydroxyl radicals produced from endogenous ROS is random, IR is also able to generate clusters of hydroxyl radicals within a nanometre scale resulting in the multiple radical attack on DNA within small volume from a single track of a charged particle (Goodhead, 1994; Goodhead & Nikjoo, 1997; Nikjoo et al., 1997; Nikjoo et al., 1998). As a result, the relative frequency of OCDL is much higher for damage induced by IR as compared to endogenous oxidative DNA damage for which OCDL are very rare. The frequency of endogenous OCDL is estimated to be a few per Gbp in normal tissues (Bennett

has been developed in cells to maintain a steady state level of ROS.

• and hydrogen peroxide H2O2. Endogenous superoxide is

McKinnon & Caldecott, 2007).

superoxide anion radical O2-
