Clinical trials of antioxidants as cancer prevention agents: Past, present, and future

Abstract

The purpose of this review is to summarize the most important human clinical trials of antioxidants as cancer prevention agents conducted to date, provide an overview of currently ongoing studies, and discuss future steps needed to advance research in this field. To date there have been several large (at least 7000 participants) trials testing the efficacy of antioxidant supplements in preventing cancer. The specific agents (diet-derived direct antioxidants and essential components of antioxidant enzymes) tested in those trials included β-carotene, vitamin E, vitamin C, selenium, retinol, zinc, riboflavin, and molybdenum. None of the completed trials produced convincing evidence to justify the use of traditional antioxidant-related vitamins or minerals for cancer prevention. Our search of ongoing trials identified six projects at various stages of completion. Five of those six trials use selenium as the intervention of interest delivered either alone or in combination with other agents. The lack of success to date can be explained by a variety of factors that need to be considered in the next generation research. These factors include lack of good biological rationale for selecting specific agents of interest; limited number of agents tested to date; use of pharmacological, rather than dietary, doses; and insufficient duration of intervention and follow-up. The latter consideration underscores the need for alternative endpoints that are associated with increased risk of neoplasia (i.e., biomarkers of risk), but are detectable prior to tumor occurrence. Although dietary antioxidants are a large and diverse group of compounds, only a small proportion of candidate agents have been tested. In summary, the strategy of focusing on large high-budget studies using cancer incidence as the endpoint and testing a relatively limited number of antioxidant agents has been largely unsuccessful. This lack of success in previous trials should not preclude us from seeking novel ways of preventing cancer by modulating oxidative balance. On the contrary, the well demonstrated mechanistic link between excessive oxidative stress and carcinogenesis underscores the need for new studies. It appears that future large-scale projects should be preceded by smaller, shorter, less expensive biomarker-based studies that can serve as a link from mechanistic and observational research to human cancer prevention trials. These relatively inexpensive studies would provide human experimental evidence for the likely efficacy, optimum dose, and long-term safety of the intervention of interest that would then guide the design of safe, more definitive large-scale trials.

Introduction

Cancer causes an estimated one in four deaths in the United States [1] and one in eight deaths worldwide [2]. The global burden of cancer more than doubled during the past 30 years with 2008 estimates of over 12 million new cases and 25 million persons alive with the diagnosis of cancer [3]. There is compelling, albeit indirect, evidence that a large proportion of cancers could be prevented through modifiable lifestyle-related risk factors such as smoking, obesity, physical activity, and diet [4]. Many of these lifestyle-related factors affect carcinogenesis through oxidative stress that occurs as a result of damage induced by reactive oxygen and nitrogen species (RONS), which produce potentially mutagenic DNA damage [5], [6], [7], [8]. Recently, the theory of oxidative stress was refined to account for an alternative mechanism—a disruption of thiol-redox circuits, which leads to aberrant cell signaling and dysfunctional redox control without involving RONS-induced macromolecular damage [9], [10].

Many of the lifestyle and dietary factors act as potent prooxidants. Inhaled tobacco smoke is considered a powerful exogenous prooxidant since high concentrations of RONS are present in both its tar and gas phases [11]. The direct increase in the oxidative burden of inhaled tobacco smoke can be further enhanced through the secondary oxidative stress due to inflammation [12]. Dietary fat is a well-documented contributor to oxidative stress through increased lipid peroxidation [13], [14]. Red meat is rich in fat, and its consumption is hypothesized to intensify oxidative stress via increased intake of heme iron, which catalyzes the oxidation of ascorbate and the production of highly reactive hydroxyl radicals via the Haber-Weiss reaction [15], [16]. It is also possible that heme iron may increase the risk of cancer via other mechanisms, such as the activation of redox-sensitive transcription factors including NF-kB, AP-1, and p53 [17], and the endogenous production of carcinogenic N-nitroso compounds [18], [19].

Lifestyle and especially diet can also serve as important sources of antioxidants. In vitro studies demonstrate that certain micronutrients counteract the effects of RONS and oxidative stress-inducing inflammation by various mechanisms, and may reduce DNA oxidation [20] as well as mutagenicity as reflected in the Ames test [21], [22], [23] or mutagen sensitivity assays [24]. Animal and in vitro studies also demonstrated the effects of dietary antioxidants on the cell cycle in a variety of tissues including the epithelium of the lung, colon, prostate, and breast—the four most common sites of carcinoma in humans [25], [26], [27], [28], [29], [30], [31]. These observations led to a conclusion that supplementation with antioxidant micronutrients may help prevent cancer. As a result, the use of antioxidant supplements in various forms and combinations has become widespread; it was reported that about 30% of healthy adults and up to 87% of cancer patients in the developed countries regularly take antioxidant supplements [32], [33].

In general, an antioxidant is defined as a compound that “when present at low concentrations compared to that of an oxidizable substrate significantly delays or inhibits oxidation of that substrate” [34]. With respect to their mechanism of action, antioxidants are divided into two major groups: enzymatic and nonenzymatic. For the purposes of cancer chemoprevention much of the emphasis has been on diet-derived compounds that act through nonenzymatic mechanisms [35]; however, enzymatic agents have also received considerable attention because the activity of antioxidant enzymes depends on the intake of trace metals (most notably selenium, molybdenum, copper, and zinc) [36], [37], [38], [39].

Despite the pervasive use of antioxidant supplements, most of the claims about their beneficial effects in humans are based on biochemical in vitro assays or animal experiments rather than human studies [40], [41]. It is important to emphasize, however, that definitive evidence about the effects of agents on human health (whether harmful or beneficial) can only be established from human studies [42]. When the effect in question is claimed to be beneficial (as in the case of antioxidants), the gold standard study is a randomized, controlled trial [43].

The purpose of this article is to describe the state-of-the-science on the preventive effects of various antioxidants in relation to cancer with an emphasis on randomized trials. This subject has been addressed in several recent comprehensive reviews and meta-analyses [33], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55] evaluating different aspects of research in this area, and, therefore, another review focusing solely on previously accumulated evidence would be somewhat redundant. Moreover, an exhaustive meta-analysis of all possible research questions addressed in previous trials would be difficult to carry out due to the heterogeneity of interventions and doses tested and the multitude of disease outcomes evaluated. For all of the above reasons, we chose an alternative approach that aims to summarize the most important and influential studies conducted in the past, provide an overview of currently ongoing trials, and discuss future steps needed to advance the science of oxidative stress and the use of antioxidants in relation to cancer prevention. As we report our observations, we expect that a summary of previous studies presented in chronological order will provide a contemporary view on how this field of science developed and matured, an appraisal of the ongoing research will offer a preview of the evidence expected in the next few years, and a discussion of the future steps will inform the planning and design of new trials.