Methylselenocysteine is a naturally occurring organoselenium compound found in certain plants, such as garlic and broccoli. Its use in cancer prevention has been explored primarily due to its selenium content and the proposed chemopreventive properties of selenium compounds. Several preclinical studies (in vitro and animal models) have demonstrated that methylselenocysteine can inhibit tumor initiation and progression through mechanisms such as induction of apoptosis, inhibition of angiogenesis, and modulation of gene expression related to cell proliferation and detoxification. Notably, methylselenocysteine appears to have a higher efficacy and lower toxicity profile in these models compared to other forms of selenium.

However, clinical evidence in humans is limited. Large-scale trials of selenium supplementation for cancer prevention (such as the SELECT trial) have used other selenium forms (selenium methionine or selenized yeast) and have not shown a clear benefit for cancer prevention, with some even reporting potential risks. Specific studies on methylselenocysteine in humans are scarce, and there is insufficient direct evidence to recommend it for cancer prevention in clinical practice. Most of the scientific support comes from laboratory and animal research rather than human trials.

In summary, while there is a scientific rationale and supportive preclinical data for methylselenocysteine in cancer prevention, human data are lacking, and current evidence does not justify its routine use for this purpose.