**2. Consequences of radiation exposure on cells/ tissues and possible role of radioprotectors**

Exposure of living cells/ tissues to ionizing radiation causes damages by transfer of energy to atoms and molecules in the cellular structure. Ionizing radiation causes either excitation or ionization or both to atoms and molecules. These excitations and ionizations can lead to following events inside the cells/ tissues;


To understand the mechanism of action of radioprotectors, an in-depth knowledge of fundamental radiobiological events happening during and shortly after irradiation in tissues and cells is essential. Scheme 1 depicts the series of events happening in cells/ tissues following radiation exposure. Radiation causes damage to cells/ tissues by both direct and indirect actions. During direct action, the radiation is directly causing irreparable damage to critical targets within the cell, such as DNA, RNA, proteins and lipids. In indirect action, radiation interacts with other molecules of the cell that are not critical targets but are close enough to pass on this damage, typically in the form of free radicals. Indirect action of ionizing radiation is due to free radicals, generated during radiolysis. Because body is composed of >80% water, indirect effect is important due to the radiolytic products, mainly the hydroxyl free radical, which is an effective oxidant capable of breaking chemical bonds, initiating lipid peroxidation, in the nano- to microsecond timeframe. After radiation exposures following changes are observed in DNA at the molecular level namely single- or double-strand breaks (DSB), base damage, and DNA-DNA or DNA-protein cross-links (Maurya et al. 2006). If different damages following radiation exposure not repaired, they affect the cell structure and function. After DNA damage has occurred, a number of processes occur in the damaged cell, tissue, or organism, including activation of DNA repair, activation of signal transduction, expression of radiation response genes and stimulation of proliferation etc. These pathways can be important for cell or tissue recovery after radiation exposure but may also play a role in the development of toxicity.

Radiation-induced DNA double-strand breaks are believed to be important lesions and the key trigger leading to a series of cellular consequences related to cell killing, gene mutation, induction of chromosome aberrations and carcinogenesis. There are two major cellular intrinsic factors deciding the extent of DNA damage in the irradiated cells, i.e. the activity of antioxidant systems and the capacity of DNA repair. There are two distinct but complementary mechanisms for DNA DSB repair namely; non-homologous end joining (NHEJ) and homologous recombination (HR) involving various repair proteins to execute the repair process. When discussing about a single or a group of radioprotectors, one has to keep in mind that radioprotective effect is an ability of radioprotectors to inhibit indirect effect and to repair direct and indirect damages occurred in the cells after radiation exposure. Discussions of all the molecular steps are out of the chapter's scope. In this chapter we are going to discuss a series of consequences happening after irradiation, types of damages induced, possible role of radioprotectors in preventing DNA damage and modulating DNA repair. At the end the future prospects for radioprotectors in mitigation of

**2. Consequences of radiation exposure on cells/ tissues and possible role of** 

Exposure of living cells/ tissues to ionizing radiation causes damages by transfer of energy to atoms and molecules in the cellular structure. Ionizing radiation causes either excitation or ionization or both to atoms and molecules. These excitations and ionizations can lead to

iv. Damage to biomolecules (e.g. DNA, RNA, lipids, proteins) which regulate vital cell

To understand the mechanism of action of radioprotectors, an in-depth knowledge of fundamental radiobiological events happening during and shortly after irradiation in tissues and cells is essential. Scheme 1 depicts the series of events happening in cells/ tissues following radiation exposure. Radiation causes damage to cells/ tissues by both direct and indirect actions. During direct action, the radiation is directly causing irreparable damage to critical targets within the cell, such as DNA, RNA, proteins and lipids. In indirect action, radiation interacts with other molecules of the cell that are not critical targets but are close enough to pass on this damage, typically in the form of free radicals. Indirect action of ionizing radiation is due to free radicals, generated during radiolysis. Because body is composed of >80% water, indirect effect is important due to the radiolytic products, mainly the hydroxyl free radical, which is an effective oxidant capable of breaking chemical bonds, initiating lipid peroxidation, in the nano- to microsecond timeframe. After radiation exposures following changes are observed in DNA at the molecular level namely single- or double-strand breaks (DSB), base damage, and DNA-DNA or DNA-protein cross-links (Maurya et al. 2006). If different damages following radiation exposure not repaired, they affect the cell structure and function. After DNA damage has occurred, a number of processes occur in the damaged cell, tissue, or organism, including activation of DNA repair, activation of signal transduction, expression of radiation response genes and stimulation of proliferation etc. These pathways can be important for cell or tissue recovery after radiation

iii. Formation of new chemical bonds and cross-linkage between macromolecules.

exposure but may also play a role in the development of toxicity.

radiation damage will be discussed.

following events inside the cells/ tissues;

i. Generation of free radicals ii. Breakage of chemical bonds

**radioprotectors** 

processes

Fig. 1. Chain of the cellular event occurring in the cell/ tissue after ionizing radiation exposure.

Role of Radioprotectors in the Inhibition of DNA Damage

**Radioprotectors/ mitigators and therapeutics** 

Amifostine (WR-2721)

Manganese (III) tetrakis (N-methyl-2-pyridyl)porphyrin

Growth factors such as Palifermin,

and Modulation of DNA Repair After Exposure to Gamma-Radiation 487

putative mechanism of radioprotection by plant and herbal radioprotectors may be mediated through several mechanisms, since they are complex mixtures of many chemicals (Jagetia 2007) but this may or may not be true with the pure compounds. Table 1 is a list of

> Administration of WR-2721 intramuscularly (IM) at 300 mg/kg in mice 15 min before radiation, a DRF of 2.7 was obtained with protection observed when injected up to 2 h before irradiation.

**Protective property Ref.** 

(Kuna 1983)

(Lee and Park 2004)

(Monobe et al. 2005);

(Vijayalaxmi et al. 1999); (Vijayalaxmi et al. 1996); (Serin et al. 2007)

(Charrier et al. 2004; Moulder and Cohen 2007)

(Johnson et al. 2010)

(Spielberger et al. 2004)

some radioprotective agents with their possible radioprotective properties.

This is the most effective thiol protector still today.

mice exposed to total body irradiation (TBI).

Melatonin Oral administration of 20 mg/ mouse (approximately 800

inflammation but promoted fibrosis.

ACE inhibitors They prevent the development of radiation-induced late effects

CDK4/6 inhibitors Entry into the cell cycle is mediated by cyclin-dependent kinase

irradiation, thus, it acted as a radiomitigator.

Administration for 14 days at a daily dosage of 5 mg/kg provided protection against oxidative damage and lethality in

mg/kg) of melatonin 30 min before irradiation significantly protected intestinal crypt cells. It inhibits chromosomal aberrations and micronuclei formation in lymphocytes. When melatonin treated mice were irradiated at an LD50/30 dose (8.15 Gy), survival increased to 85% in the group injected with 250 mg/kg melatonin administered 1 h before irradiation. In a model of acute lung injury in rats, administration of 100 mg/kg melatonin before radiation exposure resulted in decreased

including damage to the kidney and lung. When the ACE inhibitor perindopril was administered to mice for two days before and two days after a lethal dose of gamma-radiation, a significant increase in survival was observed ACE inhibition accelerated hematopoietic recovery and increased the number of hematopoietic stem cells. Protection was mediated by inhibition of the angiotensin II pathway through the AT1 receptor because similar effects were obtained with an AT1 receptor antagonist.

4/6 (CDK4/6) activation, followed by CDK2 activation. Johnson

(pharmacological quiescence) exclusively in Rb-positive and not Rb-deficient human cells. They also reported that in mice, oral administration of PD0332991 resulted in reversible inhibition of proliferation of different populations of bone marrow cells. Same group also found that PD0332991 markedly enhanced the survival of mice, when applied as late as 20 h after total body

They are FDA approved growth factors. Palifermin specifically decrease the incidence and duration of severe oral mucositis in

et al. have explored the role of cyclin-dependent kinase inhibitors (CDKIs) in the control of the transition from G1 to S phase in ionizing radiation–induced cell toxicity. They confirmed that inhibitors specific for CDK4/6, specifically PD0332991 and 2BrIC, caused reversible G1 arrest
