-Carotene and Lung Cancer: A Lesson for Future Chemoprevention Investigations?

The search for chemoprevention agents to reduce the incidence and burden of cancer has become an increasingly important priority for medical researchers during the past two decades. Focusing on single nutrients, multiple dietary constituents, or complete diets has led researchers to a host of promising agents (1). Among these, β-carotene—an antioxidant that is present in many fruits and vegetables—has been the subject of some of the most intensive chemoprevention research on the basis of substantial evidence from epidemiologic studies that this nutrient has potential to reduce the risk of cancer at various sites, including lung and stomach and possibly esophagus, colorectum, breast, prostate, and cervix (2,3). β-carotene is one of the most commonly used dietary supplements and has been heavily marketed to consumers for protection against cancer, cataracts, and heart disease. The U.S. Food and Drug Administration (FDA) designates β-carotene as “Generally Recognized As Safe” and allows its use as a dietary supplement and as an additive in foods. However, questions are now being raised about the safety and health benefits of β-carotene, based in part on the surprising and disappointing results of the α-Tocopherol, β-Carotene Cancer Prevention (ATBC) Study (4) and the β-Carotene and Retinol Efficacy Trial (CARET) (5)—two large-scale trials designed to test the chemopreventive efficacy of β-carotene, as well as α-tocopherol and retinol, on lung cancer risk. Two classic papers published in 1996 in the Journal of the National Cancer Institute—Albanes et al. (6) and Omenn et al. (7)—reported results from the ATBC Study and CARET, repectively, that have been the subject of discussion on the benefits and risks of β-carotene supplementation. It is important that we do not lose sight of the lessons learned from these trials—lessons that can improve our approaches to chemoprevention research. The ATBC Study and CARET began pilot studies in 1985. The ATBC Study randomly assigned 29 133 male smokers to four study groups that received 50 mg α-tocopherol, 20 mg β-carotene, α-tocopherol plus β-carotene, or a placebo daily for 5–8 years. In CARET, 18 314 smokers, former smokers, and asbestos-exposed workers were randomly assigned to two study groups that received 30 mg β-carotene plus 25 000 IU of retinol or placebo daily for an average of 4 years. Contrary to expected results, the incidence of lung cancer increased in the ATBC Study (by 16%) and CARET (by 28%) among former and current smokers (4,5). Further analyses showed that the predominant adverse effects of supplemental β-carotene in the ATBC study were restricted to persons who smoked one pack of cigarettes or more per day or who drank above-average levels of alcohol (6); in the CARET study, there was no association between adverse effects of β-carotene supplementation and alcohol intake among smokers (7). One important point, however, is that the magnitude of increased risk in these trials represented approximately six cancers per 1000 participants in the intervention groups, compared with five cancers per 1000 participants in the control groups, a difference too small to be apparent in any observational epidemiologic study. If the randomized controlled ATBC Study and CARET had not been carried out, specific dietary guidelines based on epidemiologic evidence might have been considered, an action that would likely have caused harm to public health. The β-carotene story thus demonstrates clearly that although epidemiologic evidence can provide a basis for developing hypotheses of benefits of food constituents, these hypotheses must then be tested in randomized, large-scale clinical trials. The findings from the ATBC Study and CARET sparked an international debate on the benefits and safety of β-carotene supplementation among individuals at high risk of lung cancer, as well as a more general debate on the levels and types of scientific evidence required before large-scale clinical trials are implemented. Concerns within the research community included the choice of supplemental β-carotene as the study agent, the dosage of β-carotene, the possibility that β-carotene might inhibit the absorption of other dietary carotenoids or antioxidants, the decision to study subjects at high risk of lung cancer, and the duration of the studies [reviewed in (8)]. Of the other carotenoids available for investigation at that time (e.g., lutein and lycopene), β-carotene was considered representative of the class (8), although some investigators disagreed with this supposition, at least in retrospect (9). In addition, the synthetic β-carotene used in the trials was composed almost entirely of trans-isomeric β-carotene, which is thought to represent the most common isomer found in the body after metabolism; however, more than 272 geometrical isomers of β-carotene have been synthesized commercially, which may make it more difficult to attribute disease risk protection or reduction to any one isomeric form (10,11). The premise that the dosage of β-carotene used in the trials was not representative of dietary intake is based on the observation that the dose of β-carotene in the trials was five to 10 times the normal dietary intake (12). This dosing resulted in higher serum concentrations than those reported in observational studies (12,13). At these higher doses, it was suggested, β-carotene would inhibit the absorption of other dietary carotenoids or antioxidants with cancer-protective properties (14), although this hypothesis was subsequently questioned (15). Questions concerning the decision to study subjects at high risk of lung cancer have been addressed by analyses from observational studies reporting an inverse association between β-carotene and lung cancer that also reported that the majority of lung cancer cases in the studies occurred among smokers (8). Moreover, designing randomized clinical chemoprevention trials to test hypotheses in persons at high risk for site-specific cancer is both efficient and desirable. As to study duration, active treatment occurred for an average of 6 and 4 years in the ATBC Study and CARET, respectively. It has been suggested that lag-to-effect and occurrence of events may take substantially longer than 4–6 years, which raises the possibility that the ATBC Study and CARET underestimated the maximum achievable effects of treatment (16,17). The β-carotene experience provides researchers in the field of chemoprevention with lessons that should improve the approach and design of the second generation of clinical trials, which will remain the gold standard for elucidating causal relationships between chemopreventive agents and disease. Observational studies will remain an important method for assessing possible study agents, but it is clear that a full set of preclinical and phase I/II studies will be needed in the future before potential chemoprevention agents are tested in large, randomized clinical trials. The ATBC Study and CARET illustrate the importance both of understanding mechanisms of action and genetic determinants of prevention and etiology and of identifying intermediate endpoints or biomarkers before embarking on large-scale, randomized clinical trials. The next generation of clinical trials will benefit from a solid framework of both in vivo and in vitro mechanistic studies, development of new animal models, identification of genetic polymorphisms that are associated with increased or decreased disease risk, and scientifically sound epidemiologic studies to identify hypotheses for testing in large, well-designed, randomized, placebo-controlled investigations (18,19). Since the ATBC Study and CARET began, an impressive body of knowledge on β-carotene’s effects has become available that, had it been known in 1985, might have affected the decision to go forward with these trials. For example, as mentioned earlier, supplementation with high doses of β-carotene may affect absorption and plasma concentrations of other carotenoids (20). Moreover, the antioxidant activity of carotenoids can actually cause oxidative stress through pro-oxidant activity, depending on redox potential and biological environment (21). In addition, β-carotene has been shown to have co-carcinogenic effects, possibly by boosting the activity of the phase I enzymes that bioactivate tobacco smoke precarcinogens, such as polycyclic aromatic hydrocarbons (PAH). Individuals with the exon 7 polymorphism of CYP1A1, which encodes a phase I cytochrome P450 enzyme, have higher levels of PAH-DNA adducts and may be particularly susceptible to the co-carcinogenic effects of β-carotene (18). It is worth noting that the critiques of these trials exemplify a problem common to scientific review of results of completed clinical trials. For both the ATBC Study and CARET, results were reported more than 10 years after initiation of the trials. With results in hand, it is always easier to determine what should have been done. Experimental science today, both physical and theoretical, is far beyond what our capabilities were in 1985. Nevertheless, the results from the ATBC Study and CARET have provided information that may be useful in planning chemoprevention clinical trials of other dietary supplements and in bringing rigorous scientific methodology to the emerging field of complementary and alternative medicine. Results from these trials have also provided important new leads about other cancer risk factors. For example, secondary analyses in the ATBC Study indicated that vitamin E may reduce risk for colorectal and prostate cancer (22), which provided supporting evidence for initiating the Selenium and Vitamin E Cancer Prevention Trial (SELECT), a study designed to determine whether men who take selenium and vitamin E supplements have a lower risk of prostate cancer (23). After publication of results from the ATBC Study and CARET, some in the scientific community called for an end to further chemoprevention studies of β-carotene and expressed concern about the dangers and liabilities of its unregulated use (24). The FDA considered the results from these trials in disallowing health claims for β-carotene related to cancer prevention under the Dietary Supplement Health and Education Act of 1994 (25), in part because no recognized scientific body (such as the National Institutes of Health or the National Academy of Sciences) had provided “authoritative statements” supporting such claims. Nevertheless, the effect of the β-carotene experience on the dietary supplement industry, which has seen its sales double in the past 5 years, to more than $13 billion on more than 20 000 products (26), seems to have been minimal. Because the market for β-carotene and other antioxidants is growing exponentially each year, it is essential to have a better understanding of the efficacy and safety of these compounds. The next generation of chemoprevention trials will be helped by systematic application of microarray and proteomic methods for determining patterns of gene and protein expression. With these methods, it will be possible to investigate molecular interactions, influences of genetic polymorphisms, validated surrogate endpoints, epigenetic relationships, and other areas that are important in cancer prevention research.