Metformin could affect a woman’s risk of developing breast cancer, a USA study published today has suggested.
Writing in Annals of Oncology, the authors say their study of 44,541 women found no association between type 2 diabetes and developing breast cancer overall – but it found links that vary by breast cancer subtype and is affected by the use of metformin, a diabetes treatment.
Over the period of the study, which had an average of more than eight years of follow-up, the researchers found type 2 diabetes was associated with a 40% increased risk of triple negative breast cancer (TNBC) when compared with women who did not have diabetes.
However, when treatment options were examined, the researchers found women with type 2 diabetes who took metformin had 14% decrease in risk of developing ER-positive breast cancer among, but a 25% increased risk of developing ER-negative breast cancer when compared to non-diabetic women.
These results were not considered to be statistically significant, but the researchers established a 74% increased risk of developing TNBC among those treated with metformin.
Lead researcher Professor Dale Sandler, chief of the Epidemiology Branch at the US National Institute of Environmental Health Sciences (NIEHS), National Institutes of Health (USA), said: “We also found that having type 2 diabetes for 15 years or more seemed to be associated with a 39% reduced risk of ER-positive breast cancer, and we think this is most likely to be because of long-term use of metformin.
“We found that, compared to non-diabetic women, the risk of developing ER-positive breast cancer was reduced by 38% among women with type 2 diabetes who had used metformin for ten years or more.
“Taken together, these findings suggest that having type 2 diabetes may increase the risk of developing breast cancer, but that taking metformin may protect against developing ER-positive breast cancer, the most common type of breast cancer.”
The study analysed data from the Sister Study, which enrolled women from the USA and Puerto Rico between 2003 and 2009 using follow-up data through to the end of 2017. At enrolment, the women were 35-74 years old, had no previous diagnosis of breast cancer, but were sisters or half-sisters of women diagnosed with breast cancer. The women completed annual health updates and follow-up questionnaires every three years.
First author Dr Yong-Moon Mark Park, now an assistant professor at the University of Arkansas for Medical Sciences, USA, said: “Our study is the first to try to disentangle the effects of type 2 diabetes and the effects of metformin use.
“Having information about subtypes of breast cancer that may have different causes helped us to reach our conclusions. However, it’s important to note that some of our findings, especially for triple negative breast cancer, were based on a small number of cases and those results need replication.
“Further studies are needed to discover whether the apparent increased risk of triple negative breast cancer is caused by metformin or is due to the absence of protection from metformin.”
Park Y-M M, Bookwalter D B, O’Brien K M et al. A prospective study of type 2 diabetes, metformin use, and risk of breast cancer. Annals of Oncology 29 January 2021
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