Selenium-enriched garlic inhibits the early stage but not the late stage of mammary carcinogenesis

Previous work has shown that the efficacy of cancer prevention by selenium-enriched garlic (Se-garlic) is primarily dependent on the action of selenium. Additionally, supplementation of Se-garlic inhibited the post-initiation phase of mammary carcinogenesis when it was given continuously to the animals. In this report, experiments were carried out in which treatment with the Se-garlic was started after carcinogen dosing (DMBA or MNU) but was restricted to either the early or late stage of neoplastic progression. The results from these two models showed that a short-term exposure to the Se-garlic for 1 month immediately following carcinogen administration was just as effective in cancer prevention as the continuous exposure regimen (5 months), suggesting that the Se-garlic may irreversibly alter the process of clonal expansion and/or selection of transformed cells during their early stage of development. Plasma and mammary tissue selenium levels essentially returned to basal levels at 1 month after withdrawal of supplementation. These observations imply that the outcome of cancer protection by short-term Se-garlic intervention was not due to a slow turnover, and therefore a lingering presence, of selenium in the target organ or in the circulation. The above finding was in contrast to that of a second study in which Se-garlic was supplemented starting at 13 weeks after carcinogen treatment. With this protocol, the number of new tumors and the number of new tumor-bearing rats found during the intervention period (weeks 13 to 22) were not statistically different between the control and supplemented groups, suggesting that Se-garlic had a minimal effect on the later stages of mammary carcinogenesis.