Exogenously supplemented antioxidants, or agents that stimulate endogenous antioxidant systems within cells, have shown promise in terms of suppressing the harmful effects of irradiation. If present in the cells at the time of radiation exposure, such antioxidants may protect cells from radiation damage by scavenging reactive oxygen species before they act on cellular components.
A variety of reducing agents, such as vitamin E analogs, polyphenols, thiols and superoxide dismutase mimetics have been described as potential radiation countermeasures in the recent past [ 8 , 10 ]. Natural products with human health benefits have become attractive targets for research [ 11 ]. By and large, such compounds are absorbed and processed easily by our body.
Vitamins are prominent among natural compounds considered beneficial for human health [ 12 ]. Vitamin E is well known for its established health benefits, including antioxidant, neuroprotective, and anti-inflammatory properties [ 13 ]. Vitamin E has emerged as an essential, fat-soluble nutrient that functions as an antioxidant in the human body. It is essential because the body cannot manufacture its own vitamin E, so foods and supplements must provide it.
Vitamin E represents a family of compounds that is divided into two subgroups called tocopherols and tocotrienols, which act as important antioxidants that regulate peroxidation reactions and control free-radical production within the body [ 17 , 18 ].
These eight components are collectively known as tocols. Tocopherols and tocotrienols share common structural features of a chromanol ring and a carbon tail at the C-2 position derived from homogenistate and phytyl diphosphate, respectively.
Tocotrienols differ structurally from tocopherols by the presence of three trans-double bonds in the hydrocarbon tail. The isomeric forms of tocopherol and tocotrienol are distinguished by the number and location of methyl groups on the chromanol rings. Recent studies suggest that both the molecular and therapeutic targets of the tocotrienols are distinct from those of the tocopherols. While the tocopherols have been investigated extensively, relatively little is known about the tocotrienols. The abundance of AT in the human body has led biologists to neglect the non-tocopherol vitamin E molecules as topics for basic and clinical research.
Recent developments suggest a serious reconsideration of this conventional wisdom is warranted. The latest developments in vitamin E research clearly indicate that members of the vitamin E family are not redundant with respect to their biological functions [ 13 ].
Life Sci. Mar 27;78(18) Epub Feb 3. Tocotrienols: Vitamin E beyond tocopherols. Sen CK(1), Khanna S, Roy S. Author information. In nature, eight substances have been found to have vitamin E activity: α-, β-, γ- and δ-tocopherol; and α-, β-, γ- and δ-tocotrienol. Yet, of all papers on vitamin E.
Current knowledge indicates prudent investigation of the less well-known isoforms is important, enabling intelligent selection of suitable molecules for studies. Although tocotrienols were discovered five decades ago, the majority of their biological properties have been revealed only in the last decade. The anti-inflammatory, antioxidant, and cholesterol-lowering properties of tocotrienols can prevent cancer, diabetes, and cardiovascular and neurodegenerative diseases [ 20 ]. The isoforms of tocotrienols, which differ in their number of methyl groups, also differ in their biological activities.
Tocols and their derivatives have been evaluated, sometimes comparatively, for their radioprotective properties at our institute as well as at several other institutions. The majority of these studies have been conducted with AT, the most commonly used vitamin E supplement and the most abundant vitamin E isoform in human and animal tissues [ 9 , 25 , 26 ]. During the last decade, tocotrienol research has gained substantial momentum. Here we discuss selected and promising tocols and their derivatives under development as radiation countermeasures.
Tocopherols and tocotrienols differ in that the latter contain three double bonds in their isoprenoid side chain while the former do not [ 21 ].
Although more than 30 papers have been published on tocopherols, less than papers deal with tocotrienols, with the majority of those published within the last decade. Although tocopherols and tocotrieniols can be found in, and isolated from, a wide variety of foods e. For example, tocopherols are relatively abundant in corn, wheat and soybeans, whereas tocotrienols show greater abundance in barley, oats, palm and rice bran Table 1.
Tocotrienols are present in a range of unrelated plant groups and are almost exclusively found in seeds and fruits. Tocotrienols possess better neureoprotective, anticancer and cholesterol lowering properties compared to tocopherols [ 32 ]. In two interesting studies, it has been shown that tocotrienols, but not tocopherols, inhibit the activation of pp60 c-Src, a key regulator of glutamate-induced neuronal cell death [ 21 ], and protect the neurons from glutamate-induced lipoxygenase Lox activation [ 33 ].
The oral administration of tocotrienols but not tocopherols block tumor-induced angiogenesis [ 34 ]. Tocotrienols are converted to tocopherols in vivo and the rate of conversion is species dependent. Though the conversion of tocotrienols to tocopherols is higher in human and chicken compared to swine, only a small percentage of tocotrienols is converted to tocopherols, which may not be biologically relevant [ 35—37 ].
A selected number of studies have demonstrated that tocotrienols have superior antioxidant activity compared with tocopherols [ 24 , 38—40 ]. One study reported that the antioxidant potential of tocotrienols is times more than that of AT [ 41 ]. Other reports suggesting that the tocotrienol's superior antioxidant efficacy may lie in the unsaturation in the aliphatic tail, which facilitates easier penetration into the tissue. Although the above studies suggest that tocotrienols have stronger antioxidant properties than tocopherols, other studies were not able to demonstrate a difference between tocopherols and tocotrienol in antioxidant potential [ 42 ].
There is no unanimity regarding whether the intrinsic antioxidant abilities of tocotrienols are different from those of tocopherols, or whether the different effects of tocopherols and tocotrienols in tissue may be due to differences in uptake. The data from the existing literature suggest that the reducing ability and radical chain-breaking activity of tocols depend on the circumstances under which the assays are performed. Tocopherol succinate TS , DT3 and GT3 have comparable radioprotective efficacy, but all three appear to be better than AT, thus suggesting that better antioxidant activity may not necessarily translate into better radioprotective efficacy Table 2.
The alternative explanation might be that these tocols might not be significantly different in terms of their inherent antioxidant activities. In laboratory animals, supplementation of tocopherols in the diet raises blood serum levels of both total and low-density lipoprotein cholesterol levels, whereas supplementation of tocotrienols lowers serum total and low-density lipoprotein cholesterol levels [ 43 ]. This is a mechanism different from common HMG-CoA reductase inhibitors such as statins that inhibit the rate-limiting step in the sterol biochemical pathway and the conversion of HMG-CoA to mevalonate [ 47 ].
These results provide a plausible mechanism for the hypercholesterolemic activity of tocotrienols that has been observed in both animals and humans. Several mechanisms could account for potency differences in efficacy of tocopherols and tocotrienols:. When administered orally to mice, tocotrienols appear faster in the blood plasma but at lower levels compared with tocopherols [ 49 ].
Faster appearance of a drug in circulation and sustained release at lower levels suggest the early- and long-acting nature of the drug. These important attributes may make tocotrienols better therapeutic agents than tocopherols. This situation has prompted intensified investigation to identify a new generation of countermeasures. In this communication we discuss promising radiation countermeasures being developed from tocols. Radioprotectors are agents administered before exposure to prevent radiation-induced cellular and molecular damage [ 50 ].
Radiation mitigators are drugs administered shortly after radiation exposure that accelerate recovery or repair injury caused by radiation. All agents in the above three categories are collectively known as radiation countermeasures [ 8 , 9 , 51 ]. Numerous candidate countermeasures have been identified and investigated, and are at various stages of development [ 8—10 , 25 , 26 ]. Current research in radiation countermeasures focuses on radioprotectors as well as on mitigators and treatments.
Radioprotectants will be useful for personnel expected to be at risk of exposure, such as military personnel Special Operations or National Guard individuals preparing for a mission with a possibility of radiation exposure and first responders, as well as civilians exposed to fallout fields during evacuation from radiological disaster areas. Given the likely breakdown in transportation during a mass-casualty scenario, such exposures may be significant.
Radiomitigators and therapeutics will be useful in mass-casualty scenarios resulting from nuclear accidents or terrorist attacks involving nuclear or radiological devices. The route of administration and the time of administration relative to time of radiation exposure are important parameters for comparing various drugs and selecting a radiation countermeasure for advance development. Optimally, the radiation countermeasure should be effective orally and can be administered as late as possible after radiation exposure.
All tocols tested for radioprotective efficacy have demonstrated protection against radiation exposure when administered subcutaneously 24 h before radiation exposure [ 28 , 52—54 ]. Tocotrienols may show higher cell uptake, but lower oral bioavailability [ 49 ]. High concentrations of tocopherols may reduce the uptake of tocotrienols. The magnitude of protection against radiation damage is expressed as the dose reduction factor DRF or dose modification factor DMF. The LD 50 is based on a probit analysis, typically using at least three radiation doses that do not result in all or none mortality.
For example, the DRF for a day survival in the mouse quantifies protection of the hematopoietic system and is probably the most useful measure for comparing the efficacy against ARS [ 55 , 56 ]. Because the probit regression lines are not always parallel, it is necessary to perform the complete DRF analysis. Even when using DRF, comparison may be difficult because the DRF can vary as a function of species, strain, age, gender, vehicle, route of administration, and the time of administration relative to time of radiation exposure.
In meaningfully comparing the DRF of different drugs, one needs to consider the relative toxicity of the drug doses being used. The therapeutics index of a drug is defined as the ratio of drug LD 50 to the effective dose of the drug. The larger the therapeutic index, the safer the drug will be for human [ 9 ]. Efficacy studies of radiation countermeasures in humans cannot be conducted because it would be unethical to deliberately expose healthy human volunteers to lethal or permanently damaging biological, chemical, radiological or nuclear substances.
Accordingly, the US FDA Animal Efficacy Rule 21CFR allows approval of new drug products that have been studied for their safety and efficacy in ameliorating or preventing serious or life-threatening conditions caused by exposure to lethal or permanently disabling toxic biological, chemical, radiological or nuclear substances, based on animal efficacy studies and phase I safety trials using healthy volunteers.
Animal models are used to define a developmental pathway for licensure, or to provide supportive data for drug approval or emergency use authorization of radiation countermeasures [ 57 , 58 ]. The animal model and the mechanism of action of radiation exposure on the specific organ system must be well characterized. How the radiation countermeasure affects that mechanism must be reasonably well understood. The effect of the countermeasure is demonstrated in more than one animal species expected to react to radiation with a response predictive of the human response to radiation and its treatment.
The experimental endpoint is clearly related to the desired benefit in humans, generally the enhancement of survival or prevention of major morbidity. The pharmacokinetics and pharmacodynamics of the countermeasure or other relevant information in the animal models and humans allows selection of an effective countermeasure dose in humans. There is significant interest in developing tocols as radioprotectors because of their potent antioxidant properties, free radical scavenging ability, lack of performance-degrading toxicity, and the presumed chemical benefit of suppressing chronic radiation-induced fibrosis in some organ systems.
Some of these agents have been shown to induce high levels of cytokines, particularly, granulocyte colony-stimulating factor [ 8 ]. There have been an ample number of previous reports demonstrating the radioprotective efficacy of tocols [ 54 , 59—65 ]. In this regard, we demonstrated that the administration of tocols subcutaneously, either 1 h before or 15 min after irradiation dose rate 0.
The DRF differences of the above two experiments may be due to differences in the dose rates of radiation exposure, in addition to differences in the treatment schedule i. There is a report that radiation-induced lipid peroxidation is greater after a lower dose rate of radiation compared to that after a higher dose rate of irradiation [ 61 ].
National Center for Biotechnology Information , U. The conversion to free tocopherol only occurred in the underlying skin layer, and the percentage of bioconversion was greater with occlusive application. As a part of its procedures, the Panel rereviews the conclusion of prior safety assessments, 15 years after the issuance of a final report. Because tocotrienols have a lower affinity for ATTP than tocopherols, they remain in the liver longer, increasing the chances they will be metabolized or excreted. Levin and D. Therefore, if tocotrienols can be made more bioavailable, they may prove effective radiation countermeasures.
These anti-cytoclastic effects of tocols administration probably have greater impact on late-arising, radiation-induced pathologies than on the manifestation of ARS. Administration of AT immediately after irradiation increased the number of hematopoietic colony-forming units in the spleen of mice [ 67 ]. In these studies, the DRF was estimated to be 1. The requirement of administration 24 h prior to irradiation suggests induction of other factors responsible for protection. Keeping in mind such mechanisms, various tocopherol analogs were studied for induction of radioprotective cytokines and growth factors in response to treatment, and TS was found to stimulate granulocyte colony-stimulating factor G-CSF to the greatest extent [ 69 ].
The radiomitigative potential of AT was first observed in when an aqueous preparation was used intraperitoneally [ 62 ].
AT's radiomitigative effect in mice was linked to enhanced cell-mediated immunity [ 63 ]. Due to TMG's long hydrophobic phytyl side chain, the mobility of tocols and its free radical scavenging activity are limited to a cell's lipid membrane. TMG is a water-soluble derivative of tocols and has been investigated for its radioprotective efficacy using various in vitro and in vivo models. The drug's LD 50 through the intraperitoneal route was 1.