Netropsin, a Minor Groove Binding Ligand: A Potential Radioprotective Agent

Chemical and biological basis for development of novel radioprotective drugs for cancer therapy

Ionizing radiation (IR) causes chemical changes in biological systems through direct interaction with the macromolecules or by causing radiolysis of water. This property of IR is harnessed in the clinic for radiotherapy in almost 50% of cancers patients. Despite the advent of stereotactic radiotherapy instruments and other advancements in shielding techniques, the inadvertent deposition of radiation dose in the surrounding normal tissue can cause late effects of radiation injury in normal tissues. Radioprotectors, which are chemical or biological agents, can reduce or mitigate these toxic side-effects of radiotherapy in cancer patients and also during radiation accidents. The desired characteristics of an ideal radioprotector include low chemical toxicity, high risk to benefit ratio and specific protection of normal cells against the harmful effects of radiation without compromising the cytotoxic effects of IR on cancer cells. Since reactive oxygen species (ROS) are the major contributors of IR mediated toxicity, plethora of studies have highlighted the potential role of antioxidants to protect against IR induced damage. However, owing to the lack of any clinically approved radioprotector against whole body radiation, researchers have shifted the focus toward finding alternate targets that could be exploited for the development of novel agents. The present review provides a comprehensive insight in to the different strategies, encompassing prime molecular targets, which have been employed to develop radiation protectors/countermeasures. It is anticipated that understanding such factors will lead to the development of novel strategies for increasing the outcome of radiotherapy by minimizing normal tissue toxicity.

DNA binding and antiradical potential of ethyl pyruvate: Key to the DNA radioprotection

DNA is the store house of all necessary hereditary information for growth of cells and tissues. Physiological functionality of DNA depends on its 3D helical structure and any distortion in a structure may lead to mutation and genomic instability that may translate into disease like cancer. In order to prevent DNA damage, an exogenous compound is required that can either scavenge the excess free radicals or enhance the structural integrity of DNA through binding. In the present study, the binding mechanism of ethyl pyruvate (EP) with DNA models using different spectroscopic techniques was investigated for their structural integrity. Besides, 2, 2-diphenyl-1-picrylhydrazyl (DPPH) and ferric reducing antioxidant power (FRAP) assays were performed to determine the antioxidant scavenging of EP. Plasmid DNA relaxation assay was performed to assess the radioprotection efficacy of EP in the plasmid DNA. Circular dichroism (CD) and UV-Vis absorbance spectroscopic data confirmed the conformation change in ctDNA upon binding with EP. The molecular docking visualized that EP stacks between the DNA bases with a glide score of -2.117 kcalmol while EP binds in the minor groove region of DNA with the glide score of -1.414 kcalmol . DPPH and FRAP data confirmed that EP scavenges significantly radicals at higher concentrations. In vitro radioprotection study in plasmid DNA pBR322 showed that EP retained the supercoiled form of plasmid DNA at 50 Gy radiation dose.

Radioprotection and Radiomitigation: From the Bench to Clinical Practice

The development of protective agents against harmful radiations has been a subject of investigation for decades. However, effective (ideal) radioprotectors and radiomitigators remain an unsolved problem. Because ionizing radiation-induced cellular damage is primarily attributed to free radicals, radical scavengers are promising as potential radioprotectors. Early development of such agents focused on thiol synthetic compounds, e.g., amifostine (2-(3-aminopropylamino) ethylsulfanylphosphonic acid), approved as a radioprotector by the Food and Drug Administration (FDA, USA) but for limited clinical indications and not for nonclinical uses. To date, no new chemical entity has been approved by the FDA as a radiation countermeasure for acute radiation syndrome (ARS). All FDA-approved radiation countermeasures (filgrastim, a recombinant DNA form of the naturally occurring granulocyte colony-stimulating factor, G-CSF; pegfilgrastim, a PEGylated form of the recombinant human G-CSF; sargramostim, a recombinant granulocyte macrophage colony-stimulating factor, GM-CSF) are classified as radiomitigators. No radioprotector that can be administered prior to exposure has been approved for ARS. This differentiates radioprotectors (reduce direct damage caused by radiation) and radiomitigators (minimize toxicity even after radiation has been delivered). Molecules under development with the aim of reaching clinical practice and other nonclinical applications are discussed. Assays to evaluate the biological effects of ionizing radiations are also analyzed.

Repurposing sodium diclofenac as a radiation countermeasure agent: A cytogenetic study in human peripheral blood lymphocytes

We assessed the radioprotective and mitigative actions of sodium diclofenac, a non-steroidal anti-inflammatory drug using cultured human peripheral blood as a model. Both pre- and post-irradiation treatments with the drug reduced gamma radiation-induced formation of dicentric chromosome, cytochalasin-blocked micronuclei and γ-H2AX foci in human peripheral blood lymphocytes. This work supports the concept that sodium diclofenac may be a useful radiation countermeasure agent.

Appraisal of biochemical classes of radioprotectors: evidence, current status and guidelines for future development

The search for efficient radioprotective agents to protect from radiation-induced toxicity, due to planned or accidental radiation exposure, is still ongoing worldwide. Despite decades of research and development of widely different biochemical classes of natural and derivative compounds, a safe and effective radioprotector is largely unmet. In this comprehensive review, we evaluated the evidence for the radioprotective performance of classical thiols, vitamins, minerals, dietary antioxidants, phytochemicals, botanical and bacterial preparations, DNA-binding agents, cytokines, and chelators including adaptogens. Where radioprotection was demonstrated, the compounds have shown moderate dose modifying factors ranging from 1.1 to 2.7. To date, only few compounds found way to clinic with limited margin of dose prescription due to side effects. Most of these compounds (amifostine, filgratism, pegfilgrastim, sargramostim, palifermin, recombinant salmonella flagellin, Prussian blue, potassium iodide) act primarily via scavenging of free radicals, modulation of oxidative stress, signal transduction, cell proliferation or enhance radionuclide elimination. However, the gain in radioprotection remains hampered with low margin of tolerance. Future development of more effective radioprotectors requires an appropriate nontoxic compound, a model system and biomarkers of radiation exposure. These are important to test the effectiveness of radioprotection on physiological tissues during radiotherapy and field application in cases of nuclear eventualities.

DNA recognition by Escherichia coli CbpA protein requires a conserved arginine-minor-groove interaction

Curved DNA binding protein A (CbpA) is a co-chaperone and nucleoid associated DNA binding protein conserved in most γ-proteobacteria. Best studied in Escherichia coli, CbpA accumulates to >2500 copies per cell during periods of starvation and forms aggregates with DNA. However, the molecular basis for DNA binding is unknown; CbpA lacks motifs found in other bacterial DNA binding proteins. Here, we have used a combination of genetics and biochemistry to elucidate the mechanism of DNA recognition by CbpA. We show that CbpA interacts with the DNA minor groove. This interaction requires a highly conserved arginine side chain. Substitution of this residue, R116, with alanine, specifically disrupts DNA binding by CbpA, and its homologues from other bacteria, whilst not affecting other CbpA activities. The intracellular distribution of CbpA alters dramatically when DNA binding is negated. Hence, we provide a direct link between DNA binding and the behaviour of CbpA in cells.