dl-alpha tocopheryl succinate (aTOS) is an analogue of alpha-tocopherol (vitamin E) with unique biological properties. Unlike its parent compound, aTOS does not have a redox potential and is therefore not an antioxidant. Its ability to prolong cell cycle arrest, induce apoptosis, and act as a radiosensitizer make aTOS a compound of great interest in integrative cancer care. In vitro and in vivo evidence suggests it is capable of simultaneously protecting normal cells from chromosomal damage while potentiating cytotoxicity of conventional therapies. Application of aTOS to chemopreventative and cancer treatment strategies should take into account these known actions as well as conversion back to alpha tocopherol (aTOH) systemically.
Naturally occurring vitamin E is not one compound, but a family of 8 different isomers. Tocopherol or tocotrienol form the backbone structures and consist of a chromanol head and a phytyl or farnesyl tail, respectively. To either tocopherol or tocotrienol, there are 4 possible variations of methylation of the chromanol head, designated alpha, beta, delta, and gamma. All the members of the family of naturally occurring vitamin E isomers are well-established antioxidant compounds. They all derive their redox (antioxidant) capacity through the hydroxyl group at the C6 position of the chromanol ring. Analogues of vitamin E can be formed with a substitution at the site of the C6 hydroxyl group. Such analogues are popularly used in nutritional supplements due to their greater stability. However, regardless of the bond or group used for the substitution, loss of the hydroxyl moiety means there is no longer any redox (antioxidant) capacity.1
The most common vitamin E analogues are made through substitution at C6 with an ester linkage of either acetate or succinate to form what is commonly known as “dry vitamin E.” These analogues of vitamin E are unique, semi-synthetic compounds with their own biological activities. One such analogue, dl-alpha tocopheryl succinate (a-TOS) has undergone extensive research as an anticancer compound both alone and with chemotherapy and/or radiation; studies suggest it has great promise as an anticancer agent.2
“Vitamin E” is a generic term that can represent any of the naturally occurring vitamers (tocopherols/ tocotrienols) or any of the semi-synthetic analogues of these vitamers. Further, medical literature does not always clearly distinguish between redox-sensitive tocopherols/tocotrienols and the redox-silent analogues such as aTOS. The lack of distinction between the various molecular forms may be a confounder when comparing various data sets. Further, extrapolation of information between the divergent compounds may lead to misinformation, as the redox-silent analogues are distinctly different from their redox-sensitive isomers, both in structure and biological activity. Even a single analogue such as aTOS can be variably referred to in medical literature as “alpha-tocopheryl succinate,” “vitamin E succinate (VES),” “vitamin E,” “alpha-tocopherol,” or simply “tocopherol.”
The first publication of aTOS’s potential as a direct antiproliferative agent was in 1982, when Prasad and Prasad tested several analogues of alpha-tocopherol, all using substitution of the hydroxyl group at the C6 position, for uptake and growth inhibition of melanoma cells in vitro. They found aTOS was the only analogue able to inhibit growth of melanoma cells.3 Over the ensuing decades, in vitro studies of various cell lines confirmed the antiproliferative/pro-apoptotic effects of a-TOS. Neuroblastoma,4 glioma,5 gastric,6 lymphoma,7 breast,8 prostate,9 pancreatic, colon, leukemic, and oral squamous carcinoma cell lines all showed growth inhibition with aTOS.10 Several in vivo studies have corroborated the growth-inhibitory effects for a variety of cancers, including oral, breast, melanoma, lung, and prostate.11,12,13,14,15
aTOS has the potential to result in improved cytotoxicity to malignant cells while lessening the toxicity on normal cells.
Unfortunately, clinical trials confirming this preliminary data still need to be done. Lack of such trials may be due to the assumption that oral aTOS does not circulate as such. It has been proposed aTOS acts as a pro-vitamin whose beneficial actions are due to the conversion into alpha-tocopherol (aTOH).16,17 However, it has also been stated that aTOS and aTOH reach an equimolar balance in peripheral circulation.18 The possibility that a significant amount of aTOS may reach malignant cells warrants follow-up, since the therapeutic potential of aTOS is impressive both as a single agent and when combined with radiation or chemotherapy. aTOS has the potential to result in improved cytotoxicity to malignant cells while lessening the toxicity on normal cells—a duality not found in many compounds. If the in vitro and animal data are supported by human trials, outcomes of combining conventional treatments with aTOS would be expected to improve significantly.
aTOS and Radiation
Ionizing radiation leads to DNA oxidation through the generation of reactive oxygen species (ROS), specifically superoxide anion and hydroxyl radicals. This leads to chromosomal breaks and either survival of chromosomally damaged cells or death to those cells.19 This is how radiation can result in either a carcinogenic or cytotoxic action on cells, the net effect largely dependent on the cumulative dose of radiation.
In keeping, there are 2 areas of concern regarding ionizing radiation in human health and disease: accidental overexposure to the carcinogenic effects of ionizing radiation and the use of therapeutic radiation, in which high doses are administered with cytotoxic intent. In either scenario, protection of normal cells is desirable. In the case of therapeutic radiation, protection of normal cells must not occur at the cost of protecting the target of toxicity, the cancerous cells themselves. It is in this unique dichotomy of action—both enhancing cytotoxic tumor effect and protecting surrounding normal cells—that aTOS may have a distinct advantage over any naturally occurring forms of tocopherols.
Rodents given a lethal dose of gamma radiation had significant reduction in lethality and pancytopenia when given aTOS injections at a high dose (400 mg/kg).20 The mechanism of rescue in this study was hypothesized to be protection of bone marrow by increasing granulocyte colony stimulating factor (G-CSF). Using aTOS and whole-body radiation in rodents, a second study corroborated the attenuation of lethal damage through bone marrow protection.21 In addition to its support of the bone marrow compartment, rodent studies suggest that there is an increase in antioxidant enzyme activity and inhibition of oncogene expression.22
aTOS has also been shown to selectively increase the therapeutic apoptotic effect of radiation while protecting nearby normal cells from the damaging effects of radiation. aTOS led to increased chromatid breakage in cervical cancer (HeLa) cells and ovarian cancer (OVG1 and SCOV3) cells undergoing gamma radiation, while not affecting normal fibroblasts in vitro.23 The cytotoxicity of radiation on neuroblastoma cells in vitro was also potentiated in the presence of aTOS.24
aTOS and Chemotherapy Agents
The use of antioxidants in patients undergoing chemotherapy has been largely avoided due to concerns of reducing cytotoxic effects of therapy. Specifically, many chemotherapeutic drug classes induce apoptosis of cancer cells through the generation of oxidative damage, including anthracyclines (eg, doxorubicin), platinum-based drugs (eg, cisplatin, oxaliplatin), alkylating agents (eg, ifosfamide, cyclophosphamide), and antibiotic-based cytotoxics (eg, bleomycin, mitomycin-c). While a theoretical risk of interfering with such agents exists, no clinical evidence demonstrates such interference with either isomers or analogues of vitamin E, including aTOS.25
aTOS has been shown to potentiate the apoptotic effects of doxorubicin through a combined increase in influx and decrease of efflux of the drug in cancerous cells, rendering a higher concentration of doxorubicin intracellularly.26In vitro, aTOS showed enhanced prostate cancer cell toxicity through additive or synergistic effects with doxorubicin, depending on the dose of aTOS used.27
In vitro use of aTOS at lower-than-therapeutic doses needed for its own antiproliferative effects synergized with the apoptotic effects of paclitaxel on non-small cell lung cancer (NSCLC).28 This appeared to be through an increase in pro-apoptotic caspase 8 activity. Separately, aTOS increased cytotoxicity of paclitaxel on bladder cancer cells in vitro and in vivo, presumably through down-regulation of nuclear factor kappa B (NF-kB).29
In one study, peripheral lymphoid cells were collected from patients receiving bleomycin for head and neck cancers. Exposure of these cells to aTOS in vitro showed a lessening of genotoxic effects of bleomycin, suggesting a possible role for protection of white blood cells during bleomycin therapy.30 Cyclophosphamide is a chemotherapeutic with anti-gonadal effects, including lowering circulating testosterone and spermatogenic disruption. Rodents given aTOS along with cyclophosphamide had a complete reversion of these anti-gonadal effects.31
Synergism with other natural anticancer agents is also possible. Calcitriol (1,25 dihydroxycholecalciferol) has direct antiproliferative/cytotoxic effects but only at high doses, which potentially induce calcium toxicity. Both in vitro and in mice, aTOS potentiated the antitumor effects of calcitriol on prostate cancer cells.32 Such synergism may allow for safer doses of calcitriol to be used while maintaining its antiproliferative effects.
Mechanism of Action of aTOS
Since the first discovery of aTOS as a potential anticancer agent, there have been numerous studies on its mechanisms of action. The mechanisms of greatest interest influence either proliferation or apoptosis. Of late, its ability to trigger apoptosis through actions on both intrinsic and extrinsic apoptotic pathways has been a topic of intense interest.
aTOS appears to have several mechanisms of antiproliferation. It is able to arrest cells at the S-phase or the G0/G1 phase through increased expression of tumor suppressor p21 and/or a reduction in the cell cycle regulatory proteins of cyclins.33 aTOS also attenuates the proliferative effects of Ras oncogene overexpression.34 It is also able to reduce activation of NF-kB, leading to decreased levels of several growth-promoting cytokines, such as Interleukin-6 (IL-6), IL-8, and vascular endothelial growth factor (VEGF).35
Extrinsic pathways of apoptosis involve extracellular binding to transmembrane receptors, leading to intracellular signal transduction that ultimately results in cell death. One well-characterized transmembrane receptor is Fas, whose binding leads to the Fas death receptor pathway. aTOS is able to influence cellular sensitivity to the Fas death receptor pathway, leading to increased apoptosis in several cancer cell lines.36,37 Another extrinsic pathway involves the transmembrane receptor for transforming growth factor (TGF), a cytokine that induces epithelial cell apoptosis. aTOS was able to restore TGF signal transduction in cancer cells with non-functional TGF receptors.38 This may have been through increased expression of TGF receptors.
The intrinsic apoptotic pathways involve mitochondria-driven events. The mitochondria have become a target for anticancer compounds, with an explosion of interest in the last decade or so.39 The term “mitocan” has emerged and refers to any compounds that induce cell death by targeting the mitochondria specifically.40,41 There are 7 classes of mitocans, each designating a particular molecular means of destabilizing the mitochondria and triggering apoptosis.42 aTOS has been designated a class II mitocan, which include agents that act on the apoptotic protein system Bcl2/Bcl-xL. aTOS has also shown interference with complex II of the respiratory chain, making it a class V mitocan as well.43,44 While various analogues of vitamin E are capable of triggering apoptosis, aTOS is considered the prototypical analogue.45
Of note, the extrinsic and intrinsic pathways of apoptosis are not exclusive of one another. Mitogen-activated protein kinases (MAPKs) and protein kinase C (PKC) can trigger apoptosis of the intrinsic pathways through effects on the balance of antiapoptotic Bcl2 and proapoptotic Bax proteins. The migration of the proapoptotic protein Bax from the cytoplasm to mitochondria alters the permeability of the mitochondria, ultimately leading to apoptosis. aTOS is able to influence phosphorylation or dephosphorylation by MAPK or PKC on the Bcl2 family of proteins so that antiapoptotic Bcl2 is suppressed while proapoptotic Bax in increased.46
If aTOS induces mitochondrial destabilization, how are normal cells spared? First, it is possible that the aTOS builds up specifically in cancer cells but not in normal cells. This may be due to higher esterase activity in normal cells, which cleaves the esterified succinate moiety from tocopherol to give alpha tocopherol (aTOH) intracellularly.47 Another possibility is that superoxide anions accumulate due to increased generation by aTOS. While normal cells have efficient superoxide dismutase enzymes to remove the anions, many tumor cell types lack this enzyme. This mechanism is similar to several conventional therapies in that the tumor kill depends on the increase in ROS accumulation. Another hypothesis has suggested that the more acidic interstitium of tumor versus normal tissue allows for the more efficient diffusion of aTOS into tumor cells.48
At least some of the confusion regarding antioxidants in cancer care is from the use of imprecise terminology. For example, without a thorough understanding of aTOS as a unique compound, its use in integrative cancer care will blend in with the extrapolation of data of the entire family of “vitamin E” compounds. One prominent example of this is the Selenium and Vitamin E Cancer Prevention (SELECT) trial, in which no effect on prostate cancer prevention was found with selenium and/or vitamin E supplementation in men.49 This particular study is well known throughout the medical oncology community. In this trial, the analogue alpha tocopherol acetate (aTOA) was used. The authors explain in a later publication that this form of tocopherol was “easily chosen” for several reasons: 1) a prior cancer prevention trial suggested benefit, 2) it is the form that is often used in nutritional supplements, and 3) there was insufficient data on aTOH [DB1]supplements and cancer incidence to justify its use.50 Nonetheless, the conclusion of the study was generally interpreted as “vitamin E does not prevent prostate cancer,” implying naturally occurring vitamers of E are not helpful.
This study is a prominent example of the use of the term “vitamin E” in the title and publication when it would have been more accurate to designate the compound used as aTOA throughout. This trial and others like it rely on 2 assumptions required to equate aTOA with aTOH: 1) that the analogue is fully hydrolyzed by esterases to form aTOH in systemic circulation, and 2) that the substituted moiety is inert and has no biological consequences of its own. These assumptions are necessary to make generalized conclusions about aTOH when any analogues are used. Interestingly, the assumption of full conversion to the parent compound, aTOH, has become convention in medical literature. Not only is this assumption confounding the data, it is possible that newer analogues or synergistic combinations of isomers of vitamin E may act more effectively in the prevention and treatment of cancer.51 The only means of delineating which vitamin E isomers or analogues are most effective is to treat them as distinct compounds in future studies.
In 2005, Bairati and colleagues published the results of a randomized trial of antioxidants alpha-tocopherol (aTOH) (400 IU/day po) and beta-carotene (30 mg/day po) in patients with head and neck cancers receiving radiotherapy.52 His group found that while acute side effects were significantly less in those taking the antioxidants, there tended to be higher local recurrence in the supplemental arm. These results were in keeping with a trial that used aTOH topically as a mouth rinse (400 mg, 5 minutes before and 8–12 hours after radiation treatment), versus primrose oil in head and neck cancer patients. It too found that acute toxicities were lessened, while the trend in overall survival (OS) at 2 years post treatment was reduced in those who used the mouth rinse.53 These trials are often cited as a cautionary tales for the use of vitamin E in radiotherapy; however, both trials used aTOH. In these trials, it is presumed that the decrease in survival is due to protection of the cancer cells by the antioxidant action of aTOH. If the assumption that vitamin E analogues are fully hydrolyzed to aTOH is true, then studies such as these justify caution.
So, where does this leave aTOS? This question cannot be definitely answered with the data we have to date. On the one hand, the compound itself is proven to selectively radiosensitize transformed cells while sparing normal cells. On the other hand, conversion to aTOH at any point before intracellular intake provides a note of caution in its use with radiation. Given the vastly disparate possibilities of either potentiation or interference, clinical trials are urgently needed to help guide decision-making. Of particular interest would be a duplication of the trial that used a mouth rinse, but using aTOS instead of aTOH. By completely avoiding systemic absorption, there is little reason to predict aTOH formation would occur. Such a study would afford a direct assessment of the effects of combining aTOS and radiotherapy.
Of note, even aTOH, an indisputable antioxidant, does not predictably interfere with radiation and chemotherapy. Several clinical trials using a-TOH during radiation or chemotherapy have shown either no interference or possible benefits to quality of life or overall survival.54,55, 56,57,58,59,60 This is most likely due to the pleomorphic actions of alpha tocopherol on cells—actions that go beyond its role in redox reduction. Nonetheless, there is vast disagreement on whether aTOH, and antioxidants in general, are safe for application during chemotherapy and radiation.
All practitioners involved in integrative oncology should be forthright in the application of aTOS in cancer care. aTOS is a semi-synthetic analogue of vitamin E without antioxidant capacity itself, and it may undergo significant conversion back to the parent compound, aTOH. While there is an assumption that this conversion to aTOH is nearly complete, there is also evidence that the 2 compounds circulate in equimolar amounts due to esterases in the bloodstream.61 While in vitro evidence indisputably shows benefit of aTOS, ingestion of aTOS adds an element of the unknown. Clinical trials that confirm or negate the many assumptions regarding the oral use of aTOS are needed for a better understanding of its place in integrative cancer care.
In the future, investigations of the vitamin E family of compounds should clearly identify the vitamin E isomer or analogue being studied. A prime example is aTOS, as a unique compound distinct from the redox-sensitive tocopherol/tocotrienols found in nature. In vitro and in vivo data for aTOS is impressive, and future publications may help shed light on the potential use of this mitocan as a chemopreventative agent, radiation and chemotherapy sensitizer, and radioprotectant. It has been nearly 30 years since the first experiments on aTOS suggested its benefits in cancer care. Clinical trials are still needed to substantiate what is a significant amount of In vitro and in vivo data regarding its benefit during radiation and chemotherapy and to determine how much aTOS is absorbed intact and how much is hydrolyzed back to aTOH. Until such trial data is obtained, practitioners should follow the cautionary principle and assume that aTOH also circulates in significant amounts.
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