The use of selenium in cancer therapy is motivated, in part, by substantial evidence that good selenium nutrition can reduce cancer risk. Dr. Larry Clark, and colleagues, conducted a massive double blind clinical study that recruited over 1,300 American subjects known to be at high risk for skin cancer, but free of any serious cancers at the time of enrollment. For over a decade, these volunteers received either selenium (200 mcg daily) or a matching placebo. Although the supplemental selenium failed to reduce subsequent risk for skin cancer, the researchers were encouraged to find that the cancer death rate in the selenium supplemented group was only half as high as that in those receiving the placebo, 29 vs. 57. Indeed, the researchers were forced to terminate the study earlier than planned, as they considered it unethical to continue with the placebo supplementation. The lower cancer death rate in the selenium group was primarily attributable to a substantial reduction in the i incidence of new serious cancers in the lungs, colon, and prostate
Epidemiological studies have also pointed to decreased risks for certain cancers in people who have relatively high selenium intakes, or who live in regions of the world where soil selenium levels are relatively high.
One reason why people with poor selenium nutrition may be at increased cancer risk is that selenium is an important antioxidant nutrient that supports the production of enzymes that protect our cells against oxidant stress. Since oxidants can damage DNA, leading to potentially carcinogenic mutations, good selenium status clearly has anti-mutagenic potential.
Of course, preventing cancer and treating cancer are two different things. In animal studies, selenium isn’t as effective for controlling pre-existing cancers as it is for preventing cancer. But, the ability of selenium to prevent cancer in carcinogen-treated animals suggests that selenium administered in conjunction with chemotherapy may well reduce the chance that treatment with DNA-damaging cytotoxic agents could ultimately give rise to new cancers.
Furthermore, although selenium alone usually isn’t effective as a cancer therapy, there is exciting recent evidence that, as an adjuvant to high doses of intravenous vitamin C, chemotherapy or radiotherapy, supplemental selenium can render cancer cells more sensitive to these measures, while simultaneously protecting normal healthy tissues. By using selenium in conjunction with chemotherapy, scientists achieve higher cure rates in rodents with transplanted tumors. This is because the tumors become more sensitive to the chemotherapies, and because the researchers can use higher doses of the drugs without producing life-threatening toxicities. Clinical studies are now in progress at Roswell Park Memorial Hospital evaluating supplemental selenium as an adjuvant to chemotherapy regimens. Preliminary reports indicate that the selenium is helping to maintain effective white cell counts, reduce the need for transfusions, and decrease side effects such as nausea, vomiting, and hair loss. The Roswell researchers are giving their chemotherapy patients 2,000mcg selenium daily prior to and during chemotherapy.
One recent study demonstrates that selenium protects normal cells from cytotoxin-mediated DNA damage by boosting the ability of a protective protein known as p53 to trigger DNA repair mechanisms in cells. Since the p53 protein is absent in most advanced cancers, this might explain why the protective benefit of selenium is confined largely to normal cells.
A further reason for using selenium in cancer therapy is that high intakes of this mineral have been shown to boost immune responses. In particular, the types of immune cells involved in cancer control, cytotoxic T lymphocytes and NK cells, function more effectively with increased intakes of selenium. In summary, the likely benefits of selenium in clinical cancer therapy, especially when used as an adjuvant to chemotherapy, are: improved response of cancers to chemotherapy and high doses of intravenous vitamin C; a reduction in chemotherapy side effects; increased capacity of the immune system to fight cancer spread; and reduced risk that chemotherapy may eventually give rise to a new cancer.