The research comes from Kumamoto University in Japan and is published in Scientific Reports.

According to the team, a 2014 review from the World Health Organization (WHO) revealed that epithelial ovarian cancer (EOC) is the most common type of ovarian cancer. With a 5-year survival rate of approximately 40 percent, effective treatments for the illness are needed.

Although new cases of EOC ranks 10th among female malignancies, the team says the number of deaths due to this type of ovarian cancer ranks fifth in the United States.

About 80 percent of patients with EOC have a relapse after initial chemotherapy treatment. As such, the researchers looked into the effects that a natural compound in onions - called onionin A, or ONA - has on EOC.

After examining the effects of ONA on a preclinical model of EOC in cells, the researchers found that the growth of EOCs slowed down after the team introduced ONA.

They also discovered that ONA inhibited pro-tumor activities of myeloid-derived suppressor cells (MDSC), which the researchers say are linked with the suppression of the anti-tumor immune response of host lymphocytes.

Furthermore, they found that ONA enhanced anti-cancer drugs' effects by boosting their anti-proliferation ability.
First study to report anti-ovarian cancer effect of ONA

In further experiments on an ovarian cancer mouse model, the researchers used oral doses of ONA. Results showed that the mice had longer lifespans and showed diminished ovarian cancer tumor development.

The researchers say their study demonstrates that ONA slows progression of ovarian cancer tumors by interrupting myeloid cells' pro-tumor activity.

They add:

    "We found that ONA reduced the extent of ovarian cancer cell proliferation induced by co-culture with human macrophages. In addition, we found that ONA directly suppressed cancer cell proliferation.

    Thus, ONA is considered useful for the additional treatment of patients with ovarian cancer owing to its suppression of the pro-tumor activation of [tumor-associated macrophages] and direct cytotoxicity against cancer cells."

The investigators did not observe side effects in animals, and they say with more testing, an oral ONA supplement could help cancer patients.

They conclude their study by noting it is the first to report an anti-ovarian cancer effect of ONA.

In a previous study, the same research team found that ONA suppressed the pro-tumor activation of host myeloid cells.

Medical News Today previously investigated the health benefits of onions. Because they are a strong source of the antioxidant vitamin C, onions help to combat the formation of free radicals known to cause cancer.