Feasibility of using lower doses of chemopreventive agents in a combination regimen for cancer protection.

In experimental cancer chemoprevention studies, the doses of the agent used to produce an inhibitory response are generally very high, sometimes bordering on levels that will result in toxicity to the test animals. The present study was designed to test the hypothesis that low doses of two or more agents may be just as effective as high doses of a single agent. An earlier report from our laboratory showed that vitamin E, although ineffective by itself, potentiates the anti-carcinogenic potency of selenite. Our objective was to complete a comprehensive set of dose titration experiments in the quantitative analysis of the synergism between selenium (Se) and vitamin E. Results indicated that the minimal amount of vitamin E required was around 500 ppm, which when combined with as little as 1 ppm Se, produced a significant protective effect in the 7,12-dimethylbenz[a]anthracene (DMBA)-induced rat mammary tumor model. This level of Se is 5 times below the marginal toxicity level of 5 ppm observed in our previous experience. With a 3-agent combination protocol involving Se (1 ppm), vitamin E (500 ppm) and vitamin A (66 ppm), it was found that vitamin A at this particular dose did not contribute any further chemoprevention than that provided by the Se/vitamin E duo. Both Se and vitamin E were present at a concentration 10 times, while vitamin A was present at 30 times their respective nutritional requirement. Our findings thus suggest that the minimum effective dose has to be systematically established for each micronutrient, and that it is feasible to use lower doses of a combination of agents if the threshold level of each agent can be determined under a prescribed set of conditions.