Intervention Trials

Several large-scale prospective studies, involving intervention with selenium alone or in combination with other micronutrient supplements, have been carried out in China. Their results, though not providing absolute evidence of the protec-tive effects of selenium against cancer, do lend support to the hypothesis.

6.3.4.1 The Linxian Intervention Trials

One of the earliest of these intervention trials was carried out in Linxian, a remote rural county in north central China, whose inhabitants have one of the highest rates of esophageal cancer in the world. Incidence of gastric cancer is also very high (Anon, 1993). The trial involved nearly 30,000 adults, both male and female, and lasted for 5 years from 1985 to 1990. Subjects were randomly assigned to four groups, to receive one of four different combinations of different nutritional supplements or a placebo. The combinations were, vitamin A and zinc; riboflavin and niacin; vitamin C and molybdenum; and β-carotene, vitamin E, and nium. At the end of the trial it was found that the combinations containing sele-nium and the two other antioxidants, had had a significant beneficial effect on mortality, especially from stomach, but not from other forms of cancer. None of the other combinations had a significant effect on cancer mortality. The investi-gators concluded that while their findings pointed towards a protective role of the combined antioxidant group of supplements, it was not possible to say whether this was due to selenium alone or to one of the other antioxidants, either on its own, or in combination with the others (Blot et al., 1993).

A subsequent survey of the nutritional intake of Linxian residents found evi-dence of a generally poor nutritional status, with intakes below RDAs for sele-nium, zinc and calcium, as well as for vitamins A, C, B2, and E. Protein intake was also low (Zou et al., 2003). These findings suggest that the observed fall in esophageal cancer following supplementation was unlikely to have been due to the additional intake of selenium alone but to a general improvement of the diet.

A 15-year follow-up study of 1,103 subjects randomly selected from the larger trial cohort, did find, however, a significant inverse association between baseline serum selenium concentrations and death from esophageal squamous cell carci-noma and gastric cardia cancer (Wei et al., 2004).

6.3.4.2 Qidong Intervention Trial

A second Chinese intervention trial was carried out between 1984 and 1990 in Qidong, Jiansu Province. This is a high-risk area for HCC, a disease which accounts for more than a quarter of a million deaths each year in China, with an incidence rate in some areas approaching 150 cases/100,000/year (Huang et al., 2003). More than 130,000 people living in five townships in Qidong were involved in the trial. The inhabitants of one township were provided with salt (sodium chloride) fortified with 15 mg/kg of sodium selenite. The other four townships had unfortified salt. After 6 years, the incidence of liver cancer in the fortified group had fallen from an initial 52.84/100,000 to 34.49/100,000, whereas in the control townships there was no decrease in incidence. It was con-cluded by the investigators at the Qidong Liver Cancer Institute, that fortification of domestic salt with selenium is a cheap, effective, and simple way of decreas-ing the risk of cancer in this high-risk area (Li et al., 1992). This may have been an overoptimistic conclusion. It has been shown that exposure to hepatitis B virus and aflatoxin B1 (AFB1) contributes to the development of hepatocarcinogenesis (Lunn et al., 1997; Hsia et al., 1992). It is believed that high levels of aflatoxin may be associated with a genetic mutation identified in several human cancers (Niu et al., 2002). This mutation of the p53 tumor suppressor gene in which there is a serine substitution at codon 249, which may be a SNP (Huang et al., 2003), has been detected in a high percentage of tumors from Qidong. Up to 15% of the residents of the area are chronically infected with the hepatitis B virus and more than 90% of them have serum aflatoxin-albumin adducts, indicating a very high exposure rate to the toxin (Huang et al., 2003). Thus, exposure to aflatoxins and the presence of the 249^serp53 mutation in plasma appears to be strongly associ-ated with HCC in Qidong patients. It has been suggested that further trials are required to examine the interrelationship between HCC, viral hepatitis infection, and selenium in order to understand better the etiological mechanisms involved and evaluate the true chemopreventive potential of selenium compounds for liver cancer (Sakoda et al., 2005).

6.3.4.3 The Nutritional Prevention of Cancer with Selenium Trial

The Nutritional Prevention of Cancer (NPC) Trial, carried out by Clark et al.

(1996) in the USA, was the first double-blind, placebo-controlled intervention trial in a western population, designed to test the hypothesis that selenium sup-plementation could reduce the risk of cancer.

The primary purpose of the trial was to test the hypothesis that increased sele-nium status could reduce the risk of two forms of nonmelanoma carcinoma of the

skin: basal (BCC) and squamous (SCC). Secondary end points included total and cancer mortality and incidents of lung, colorectal, prostate, and other forms of cancer.

The carefully designed and extensive trial was carried out in seven dermato-logical clinics in the eastern USA. It involved 1,312 adult patients with a history of BCC or SCC. Recruitment was sex neutral and the patients were randomized from 1983 to the end of 1991. They were given 200 µg of selenium, in the form of a high-selenium brewer’s yeast tablet, or a placebo, daily for a mean of 4.5 years, with a follow-up of 6.4 years.

The results found that selenium supplementation did not significantly affect the incidence of either of the skin cancers. However, analysis of the secondary end points showed significant reductions in total cancer mortality, as well as incidents of lung (46% reduction), colorectal (58% reduction), and prostate (63% reduc-tion) cancers. It was concluded by Clark and his colleagues that, while selenium treatment did not protect against development of nonmelanoma skin cancers, their results did support the hypothesis that supplemental selenium may reduce the incidence of, and mortality from, prostate and several other types of cancer.

Publication of the NPC trial findings caused considerable excitement among researchers in cancer research as well as in the popular media. Its importance was underlined by editorials, not only in the journal of the AMA, but also in several other major medical journals worldwide. However, while the report was generally received with considerable interest, it was also subject to criticism. It was pointed out that the results were not definitive (Fleshner and Klotz, 1999). Several com-mentators noted that reductions in prostate, lung, and colon cancers had not been a priori hypotheses of the study, and that interpretation of secondary end points required caution (Pocock and Hughes, 1990; Colditz, 1996; Fleet, 1997).

Clark and his colleagues were not unaware of the shortcomings of their trial.

They did not claim that they had proved that selenium supplementation could pre-vent cancer, but only that selenium supplementation was found to be associated with significant reductions in secondary end points of total cancer incidence, lung, colorectal, and prostate cancer incidents, and lung cancer mortality. They were careful to conclude their report with a statement that these apparent benefi-cial effects of selenium supplementation require confirmation in independent tri-als of appropriate design before health recommendations regarding selenium supplementation could be made (Clark et al., 1996). The numerous follow-up and other trials, including those by Clark’s own group, that have been planned and undertaken since that time, show that their advice that further, independent and appropriately designed investigations should be undertaken, has been widely accepted in the scientific community. Unfortunately, that has not been the case in some areas of the pharmaceutical and health food industries, nor by some jour-nalists who have raised unrealistic expectations of a cure for cancer by selenium supplementation in their readers (Anon, 1998).

One of the first of the additional studies was that by Clark et al. (1998) which found further evidence of a decreased incidence of prostate cancer with selenium supplementation. It was a follow-up, over 10 years, of participants who had taken

part in the original NPC trial. Interestingly, while the results showed a significant treatment effect in participants in the lowest and middle tertiles of baseline plasma selenium, there was only a nonsignificant reduction in risk in the highest third.

6.3.4.4 The PRECISE Intervention Trial

Shortly before his untimely death in 2000 from prostate cancer, Clark with col-leagues from the USA and several other countries, designed a large-scale, inter-national trial as a definitive test for the hypothesis that selenium supplementation can prevent several different types of cancers in humans. The Prevention of Cancer with Selenium in Europe and America (PRECISE) trial was to be a 5-year, double-blind randomized, placebo-controlled cancer study, involving as many as 50,000 subjects in the USA and several European countries, including the UK, Denmark, the Netherlands, and Scandinavia. It was designed to replicate and extend the design of the NPC study by including participants in other coun-tries and by using supplementation doses of 100, 200, and 300 µg/day (Rayman and Clark, 1999). Two pilot studies following the PRECISE protocol, using sele-nium supplements up to 300 µg/day, but with relatively small numbers of sub-jects, are currently underway in the UK and in Denmark (Rayman, 2004). Trials, using daily selenium doses of up to 800 µg and even higher, have been carried out at the University of Arizona Cancer Center, as a follow-up to the NPC trial. They were designed to look at the effects of long-term selenium supplementation in men who are either at high risk of prostate cancer or are already diagnosed as hav-ing it (Stratton et al., 2003a). In one of the trials, known as the “Watchful Waithav-ing Trial,” it involved men with localized prostate cancer and life expectancy of <10 years. The majority of the participants were given 200 or 800 µg of selenium (as selenium-enriched yeast), daily for up to 3 years (Stratton et al., 2003b). No side effects of selenosis were observed in any of the subjects on this regime. In a small subgroup, eight subjects were given a daily selenium dose of 1,600 µg, and 16 others 3,200 µg for up to a year. Mean plasma selenium levels increased to 492 ± 188 and 640 ± 491 µg/l, respectively. Side effects, such as garlic breath, nail and hair damage, dizziness, and stomach upsets were experienced, especially by those taking the higher selenium dose. However, it was reported that blood chemistry and hematology results were within normal ranges and no severe or serious sele-nium-related toxicity was observed. However, because of the lack of safety infor-mation about such high doses in the literature, the trial was discontinued for the high intake subgroup at the end of its first year (Reid et al., 2004).

6.3.4.5 The SELECT Intervention Trial

The Selenium and Vitamin E Cancer Prevention Trial (SELECT) is another ongo-ing trial at the present time in the USA (Klein et al., 2003). It is a randomized, prospective, double-blind study designed to determine whether selenium and vitamin E, alone and in combination, can reduce the risk of prostate cancer among healthy men, with a PSA not greater than 4 ng/ml. The daily supplement

dose is 200 µg of selenium in the form of selenomethionine and/or 400 mg of racemic α-tocopherol or matched placebo. The primary end point is the clinical incidence of prostate cancer. Enrolment began in 2001 with the aim of recruiting more than 30,000 men. Results are expected in 2013.

6.3.4.6 The SU.VI.MAX Trial

A French study, Supplementation en Vitamines et Mineraux AntioXydants (SU.VI.MAX), designed to assess the effect of vitamin and mineral supplements on chronic disease risk, including prostate and other forms of cancer, begun in 1994, has followed more than 12,000 adult men and women over a period of 8 years (Hercberg et al., 1998a). Participants were given 100 µg selenium daily, and in addition, one to three times the RDA of vitamins E and C, as well as β-carotene and zinc (Hercberg et al., 1998b).

At the end of the 8 years of intervention, there was, overall, a moderate non significant reduction in prostate cancer rate in the men who were given the vita-min and vita-mineral supplements. However, the rate differed significantly between men with normal baseline prostate-specific antigen (PSA) levels (<3 µg/l), and those with an elevated PSA. Those with a normal PSA showed a significant reduction in the rate of prostate cancer, while in those with a higher PSA, the sup-plement was associated with an increased incidence of prostate cancer of border-line statistical significance. The results were interpreted as supporting the hypothesis that chemoprevention of prostate cancer can be achieved with nutritional doses of antioxidant vitamins and minerals (Meyer et al., 2005).

6.3.5 Evidence for an Association Between