Background: We assessed survival of breast cancer in women with type 2 diabetes (T2D) treated with metformin, other types of antidiabetic medication (ADM) and statins. Materials and Methods: The study cohort consisted of women with T2D and diagnosed with breast cancer in Finland in 1998─2011. Mortality rates from breast cancer and other causes were analysed by Cox models, and adjusted hazard ratios (HRs) with 95% confidence intervals (Cls) were estimated in relation to the use of different types of medication. Results: The final cohort consisted of 3,533 women. No clear evidence was found for breast cancer mortality being different in metformin users (HR 0.86, 95% Cl 0.63-1.17), but their other-cause mortality appeared to be lower (HR 0.73, 95% Cl 0.55-0.97) in comparison with women using other types of oral ADM. Other-cause mortality was higher among insulin users (HR 1.45, 95% Cl 1.16-1.80) compared with users of other oral ADMs, other than metformin. Prediagnostic statin use was observed to be associated with decreased mortality from both breast cancer (HR 0.76, 95% Cl 0.63-0.92) and other causes (HR 0.75, 95% Cl 0.64-0.87). Conclusions: We did not find any association between ADM use and disease-specific mortality among women with T2D diagnosed with breast cancer. However, interestingly, prediagnostic statin use was observed to predict reduced mortality from breast cancer and other causes. We hypothesise that treating treatment practices of T2D or hypercholesterolaemia of breast cancer patients might affect overall prognosis of women diagnosed with breast cancer and T2D.
Purpose To address the possible association between the use of metformin, other forms of antidiabetic medication (ADM) and statins with the incidence of breast cancer in women with type 2 diabetes (T2D). Methods Data were collected from a Finnish nationwide diabetes database (FinDM). The study cohort consisted of women diagnosed with T2D in 1996–2011 in Finland. In full-cohort analysis, Poisson regression was used to estimate hazard ratios (HRs) in relation to use of metformin, insulin, other forms of oral ADM and statins. In nested case–control analysis, up to 20 controls were matched for age and duration of diabetes to each case of breast cancer. Conditional logistic regression was used to estimate HRs in relation to medication use and cumulative use of different forms of ADM, and statins. Results 2300 women were diagnosed with breast cancer during follow-up. No difference in breast cancer incidence was observed between metformin users [HR 1.02, 95% confidence interval (CI) 0.93–1.11] or statin users (HR 0.97, 95% CI 0.89–1.05) compared with non-users. In nested case–control analysis the results were similar. Use of insulin (HR 1.18, 95% CI 1.03–1.36) was associated with a slightly increased incidence of breast cancer. Conclusions No evidence of an association between the use of metformin or statins and the incidence of breast cancer in women with T2D was found. Among insulin users, a slightly higher incidence of breast cancer was observed. Electronic supplementary material The online version of this article (10.1007/s10549-019-05185-0) contains supplementary material, which is available to authorized users.
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