A drug used to slow tumour growth may also prevent infertility caused by standard chemotherapy used for breast cancer treatment, according to a study.

The study, conducted on mice, found that the drug everolimus protects ovaries from cyclophosphamide -- a chemotherapy used often against breast cancer but known to deplete the supply of egg cells needed to achieve pregnancy.

Female mice treated with everolimus, along with chemotherapy, were found to have more than twice as many offspring afterward as mice treated with the chemotherapy alone. 

Such strong results with an available drug may speed the process of applying for permission to test it in premenopausal cancer patients, the researchers said.

"Our results argue that everolimus may represent a fertility-sparing drug treatment to complement the freezing of eggs and embryos, which are valued methods, but time-consuming, costly, less effective with age and not protective of long-term ovarian function," said lead author Kara Goldman, reproductive endocrinologist at New York University-Langone.

For the study, published in the Proceedings of the National Academy of Sciences, female mice were treated with cyclophosphamide weekly and then randomised to also receive either everolimus, an experimental drug called INK128 against several cancer types, or nothing. 

Everolimus and INK128 block the action of the enzyme mTOR, which is part of signalling mechanisms that encourage cell growth. 

Thus, everolimus is already approved to slow tumour growth in some forms of kidney cancer and breast cancer, but in a different way than chemotherapies. The results showed that mice treated with chemotherapy combined with either mTOR inhibitor had more mouse pups on average, than mice treated with chemotherapy alone.

In addition, mice treated with cyclophosphamide alone saw a 64 per cent reduction in their numbers of primordial follicles when compared to control mice, a pattern reversed by mTOR inhibitors.