In recent decades accurate delivery of radiation to the tumor site has greatly improved. However, it is not always feasible to eliminate a tumor without damaging the surrounding normal tissue. Tumor cells heavily rely on the activation of one or two pathways, a phenomenon known as oncogene addiction [49
], whereas normal cells use a broader range of molecular signals to overcome various cellular insults [51
] to maintain the normal genome. Switching off the pathways involved in cancer cell survival using chemicals before irradiation, while sparing normal cells will be an effective way to protect the normal cells. In contrast to normal cells, cancer cells mostly fail to activate damage sensor proteins involved in DNA repair pathways are often dysfunctional. DNA repair deficiency in cancer cells stimulates mutagenesis and further leads to tumorigenesis. However, at the same time these tumor cells are prone to the DNA damage by chemotherapy or radiotherapy treatments. From a clinical perspective, a good chemical radioprotectors must target this DNA repair pathways to kill the cancer cells differentially while protecting the normal cells. Therefore, a potential radio-protective agent should have more profound differential radiosensitizing effect on cancer cells include cell cycle arrest, apoptosis, direct and indirect effects on DNA bases, repair proteins and tumor vasculature. Understanding this problem at a molecular level, while reducing any unwanted deposition of radiation doses to the surrounding normal cells/tissues is a central pursuit in radiobiological research.
Repopulation of cells in critical normal tissues between individual dose fractions of either radiotherapy or chemotherapy is an important factor for the recovery or retention of normal organ function, thereby improving tolerance to treatment. Normal cells have a tremendous ability to repair DNA against radiation damage by activating “damage sensor” proteins such as activating ATM, and ATR, checkpoint kinase 1 and 2 (CHK1 and CHK2) or p53 [52
]. PI3K also plays a role in the integral functions for noncancerous (normal) cells repopulation [55
] along with the DNA repair proteins. Once DNA damage detected and sensor proteins activated, each lesion can be repaired by at least one of the six major DNA-repair pathways: BER (base excision repair); NER (nucleotide excision repair); DR (direct repair); MMR (mismatch repair); HR (homologous recombination) or NHEJ (non-homologous end joining) pathways [56
], if not, cell death occurs due to residual or misrepaired DNA double-strand breaks [58
]. In contrast to normal cells, in cancer cells DNA-repair path-ways are dysfunctional and this may make tumor cells prone to the DNA-damaging agents such as radiotherapy and/or chemotherapy [53
]. Furthermore understanding the biological mechanisms involved in signaling pathway(s) for resistance, cell cycle checkpoints, DNA damage and repair, antiangiogenesis could increase the therapeutic response of tumor microenvironment while sparing surrounding normal tissues. However, work on radioprotective chemicals started several decades ago in the USA, at the inception of the Manhattan Project and the available literature on the topic is enormous, this review focuses only on those relevant and potential agents which are of clinical importance with their mechanism of radioprotection.
In recent years, strategies to improve radiotherapy therefore aim to increase the effect on tumor while limiting the damage to the adjacent normal cells without sensitizing the normal tissues and ultimately without protecting the tumors to the radiation treatment. Despite the availability of various sophisticated technical improvements in radiotherapy [61
], treatment planning techniques and stereotactic body radiation therapy (SBRT) modalities, normal tissue toxicity remains a dose-limiting problem in therapeutic programs [19
]. The development of radioprotectors or radioprotectants that protect normal tissues surrounding the tumor cells against radiation damage is currently the subject of intense research [64
]. Radioprotectors are chemicals and most of the radioprotective compounds are antioxidants, can be given before or at the time of radiation treatment of cancer control. In general, radioprotectors are drugs that are designed with the intent of minimizing the risk of clonogenic death of normal (noncancerous) cells from the damage caused by radiation. These agents also promote the repair of normal cells that are exposed to radiation.
In recent years many chemical agents that have been postulated which protects the normal cells by targeting newly identified molecular and physiological pathways. Therefore, detailed understanding of the pathways influenced by the radioprotectors, as well as identification and characterization of the participating proteins will significantly advance our ability to unravel the complex processes leading to the development of new drugs that protect normal cells/tissues. Despite technical improvements, no radioprotective drug available today shows all the requisite qualities to be an ideal radioprotector and many patients still suffer from recurrent disease after radiotherapy. An ideal radioprotector is relatively non-toxic to normal cells and easy to administer without compromising the therapeutic effects of radiation treatment for cancer patients. For years, many radioprotective compounds have been developed, a majority of them designed to reduce the levels of radiation-induced free radicals within the cells. Indeed, after several decades of preclinical and clinical research, the first and only approved radioprotective drug by U.S. Food and Drug Administration (FDA) is amifostine, being used in clinical practice.
Amifostine is a prodrug (inactive form) which belongs to a general class of cytoprotective (cell -protecting) agents. In the body, amifostine is converted into an active thiol metabolite WR1065, which helps to protect the cells from DNA damage by scavenging free radicals [65
]. The conversion of amifostine to WR1065 is catalyzed by alkaline phosphatase which is a pH dependent and occurring more rapidly in alkaline pH, tumor cells are acidic in nature [66
]. Once dephosphorylated, it can freely diffuse mostly into normal cells and can act as a free radical scavenger. Furthermore, the lower concentration of membrane-bound alkaline phosphatase and lower pH (acidic) in tumor microenvironments contributes to a relatively low concentration of active chemoprotectant in malignant tissues and therefore providing a selective cytoprotection of normal tissues [68
] around the irradiation field. Since tumors are relatively in hypoxic condition due to the poor vasculature, thus resulting in comparative hypoxia and a low (acidic) interstitial pH than normal tissues [70
]. Therefore normal tissues can absorb greater level of amifostine than the tumor tissues, and furthermore cancer cells are not protected. The half-life of amifostine is approximately 9 minutes, whereas that of the active metabolite, WR1065, is approximately 15 minutes [74
]. Thus, the schedule of amifostine administration is potentially an important factor for optimum efficacy for the radiation treatment to the cancer patients.
In both normal and cancer cells, ionizing radiation has been shown to generate reactive oxygen species (ROS), mitochondrial respiratory chain is a major source of reactive oxygen species under various pathological conditions [75
]. Furthermore, mutation in mitochondrial DNA (mtDNA) increased the metastatic potential of tumor cells [78
]. ROS cause oxidative damage to DNA, proteins, lipids, and other cellular components and therefore pose a significant cellular stress [79
] and recent studies suggest that this biochemical property of cancer cells can be exploited for therapeutic benefits. Therefore targeting mitochondria is one of an essential step to therapeutic approach for the cancer control [80
]. However, in cancer cells aberrant metabolism and protein translation generate abnormally high levels of ROS [81
]. Agents that enhance ROS production, therefore, are expected to cause further stress overload in cancer cells by increase in DNA damage. Since ROS act as a mediator of the cellular damage induced by radiation, compounds that involved in the regulation of ROS may be of great interest in the protection of normal cells against radiation induced damage. For e.g. it has been shown that dichloroacetate, which inhibits pyruvate dehydrogenase kinase (PDK) and therefore stimulates mitochondrial oxidative phosphorylation and ROS production, such mechanisms can selectively increase DNA damage related apoptosis in cancer cells but not in normal cells [82
]. Similarly, reducing the cellular ROS buffering capacity through the inhibition of glutamate-cysteine ligase (a rate-limiting enzyme in cellular glutathione synthesis) can markedly increase the radiosensitivity of cancer cells [83
In humans, as in most animal cells, maintenance and expression of mtDNA are essential for the normal cell survival and metabolism. Manganese superoxide dismutase (MnSOD) exists exclusively in the mitochondria and scavenges toxic superoxide radicals produced by radiation. Therefore it has been proposed that MnSOD play a role in protecting cells against ROS damage during the radiation exposure. Lee et al. [84
] showed mitochondrial damage such as altered permeability transition, increase in accumulation of reactive oxygen species, reduction of ATP production and morphological change induced by radiation in cells and mice were protected by the treatment of MnSOD. Preclinical studies in mouse have demonstrated that the expression of human MnSOD transgene confers protection of normal tissues from ionizing irradiation damage and also radiosensitizing the tumor cells [85
]. Administration of manganese superoxide dismutase plasmid liposomes (MnSOD-PL) carrying DNA damage control genes has been demonstrated to provide local radiation protection to the lung, esophagus, oral cavity, urinary bladder and intestine [88
]. Therefore, over expression of MnSOD has been shown to sensitize the cancer cells to radiation, while differentially over expression in normal cells protect from irradiation [90
In cancer patients undergoing radiation treatment, highly proliferating organs like intestine and bone marrow limits its beneficial effects. Using mouse model, Burdelya and colleagues recently tested a new approach to protect the intestine from irradiation-induced injury. Burdelya et al. [91
] developed an NF-κB-activating CBLB502 a polypeptide drug by modifying a small fragment of a Salmonella flagella
. CBLB502 is a bioengineered derivative of a microbial protein derived from flagellin. Human immune response to flagellin can be explained by two facts: 1. flagellin is an extremely abundant protein in flagellated bacteria, 2. a specific innate immune receptor that recognizes flagellin is toll like receptor-5 (TLR-5). Most TLRs activates NF-κB, which transcri-ptionally regulates a diverse array of genes. These include cytokines, chemokines and their respective receptors, which are associated with the important role of NF-κB in the inflammatory response, together with genes regulating cell survival, proliferation, cell adhesion in the cellular microenviroment [32
]. All these events activated by TLR-NF-κB protect normal cells by preventing death of the cell. Although the NF-κB gene targets may be similar between normal and cancer cells, the difference is the ‘appropriateness’ of the signal and their regulation [95
]. For example, in tumor cell NF-κB targets may show sustained induction of their expression, resulting from the loss of negative feedback control mechanisms. Furthermore, NF-κB-transcription dependent gene expressions can either promote growth and survival of cancer cells or contribute towards tumor suppressor mechanisms, depending on the status of tumor cell for e.g loss of key tumor suppressors such as p53 or PTEN can drive NF-κB towards oncogenic and tumor-promoting activity [96
]. Therefore, NF-κB pathway can be exploited as a target for normal cell survival from radiation injury.
Burdelya et al. [91
] injected CBLB502 into mice before total body irradiation. The treatment prevented radiation induced mortality or completely protected the animals against radiation induced damage. Another interesting application of the drug is that it protects normal cells without diminishing the therapeutic antitumor effect of radiation and further without promoting radiation induced carcinogenicity in the tumor cell injected mice. CBLB502 uniquely showed combined properties of supportive care (radiotherapy adjuvant) and anticancer agent, both mediated via the activation of TLR-5 signaling in normal tissues or in tumor, respectively [96
]. Thus, TLR-5 agonists CBLB502 could potentially protect normal cells, while improving the therapeutic index of radiotherapy when large and actively proliferating organs like intestine or bone marrow are considered as a dose limiting factors. MnSOD gene therapy and CBLB502 are still in the early stages of development but in the available radioprotectors it could allow for safer and more effective radiation treatment while protecting the normal cells.