A number of studies have suggested a link between diabetes, diabetes medicines, and breast cancer risk. Research strongly suggests that women diagnosed with diabetes are more likely to be diagnosed with breast cancer than women who aren’t diabetic. Other research suggests that diabetic women taking the medicine metformin (brand names: Fortamet, Glucophage, Glumetza, Riomet), which is commonly used to treat type 2 diabetes, have a lower-than-average risk of breast cancer.
An analysis of data from the ALTTO study suggests that diabetic women treated with metformin who have been diagnosed with HER2-positive, hormone-receptor-positive breast cancer have better outcomes, including overall survival, than similar women who were not treated with metformin.
The study was published online on March 13, 2017 by the Journal of Clinical Oncology. Read “Impact of Diabetes, Insulin, and Metformin Use on the Outcome of Patients With Human Epidermal Growth Factor Receptor 2-Positive Primary Breast Cancer: Analysis From the ALTTO Phase III Randomized Trial.”
Breastcancer.org Professional Advisory Board members Martine Piccart-Gebhart, M.D., Ph.D., associate professor of oncology at the Universite Libre de Bruxelles and Julie Gralow, M.D., associate professor of medical oncology at the University of Washington and director of breast medical oncology at the Seattle Cancer Care Alliance are two authors of the study.
In the ALTTO trial, 8,381 women from 44 countries diagnosed with early-stage, HER2-positive breast cancer that had been surgically removed were randomly assigned to get one of four anti-HER2 targeted therapy treatments for a year after surgery:
- Herceptin alone
- Tykerb alone
- Herceptin for 12 weeks, a 6-week break, then 34 weeks of Tykerb
- Herceptin and Tykerb given at the same time
The women’s doctors decided when they would get anti-HER2 therapies:
- after all chemotherapy was completed
- after anthracycline chemotherapy and at the same time as taxane chemotherapy
- at the same time as the chemotherapy combination of Taxotere (chemical name: docetaxel) and carboplatin (this chemotherapy regimen is anthracycline-free; heart problems can be a side effect of Herceptin, Tykerb, and anthracycline chemotherapy)
In this analysis, the researchers looked to see how many of the women were diabetic when diagnosed with breast cancer, as well as whether they had been treated with metformin or not:
- 7,935 women (94.7%) had no history of diabetes when diagnosed with breast cancer
- 186 women (2.2%) had diabetes when diagnosed with breast cancer, but had not been treated with metformin
- 260 women (3.1%) had diabetes when diagnosed with breast cancer and had been treated with metformin
During follow-up, which ranged from about 2 months to about 6.3 years:
- 1,205 women (14.38%) had the cancer come back in the breast area (recurrence)
- 929 women (11.08%) had the cancer come back in a part of the body away from the breast, such as the bones or liver (distant recurrence/metastasis)
- 528 women (6.3%) died from breast cancer
Compared to diabetic women diagnosed with HER2-positive, hormone-receptor-positive breast cancer who had not been treated with metformin, diabetic women diagnosed with the same type of breast cancer who had been treated with metformin were less likely to have a recurrence, a distant recurrence, or to die from breast cancer.
These differences were all statistically significant, which means they were likely because of the metformin treatment and not just due to chance.
The beneficial effects of metformin were seen only in diabetic women diagnosed with HER2-positive, hormone-receptor-positive breast cancer. Metformin did not seem to offer benefits to diabetic women diagnosed with HER2-positive, hormone-receptor-negative breast cancer.
The point of this analysis was to see if metformin was linked to better outcomes in diabetic women diagnosed with HER2-positive breast cancer. Still, the researchers also found that, overall, diabetic women diagnosed with HER2-positive breast cancer had worse outcomes than women without diabetes diagnosed with HER2-positive breast cancer. They also found that diabetic women diagnosed with HER2-postive, hormone-receptor-positive breast cancer who were treated with insulin had worse outcomes than diabetic women diagnosed with the same type of cancer who were treated with metformin or other antidiabetic medicines.
“…We believe that for patients with diabetes and HER2-positive and hormone receptor-positive disease, it is reasonable to recommend metformin treatment if patients have not already received treatment and to avoid as much as possible insulin use,” the researcher wrote. “From a prognostic point of view, patients with diabetes and HER2-positive and hormone receptor-positive disease who are treated with insulin should be considered at higher risk of recurrence.”
If you are a diabetic and have been diagnosed with HER2-positive, hormone-receptor-positive breast cancer, you may want to talk to both your oncologist and your endocrinologist about this study. If you use insulin to manage the diabetes, it makes sense to ask your doctors if metformin might be a better choice for you. Together, you and your doctors can figure out the best way to manage the diabetes and treat the breast cancer.
Stay tuned to Breastcancer.org Research News for the latest information on links between diabetes, metformin, insulin, and breast cancer.