In this large population of breast cancer patients from a state cancer registry, we demonstrate an increased odds for incident diabetes after breast cancer, and in particular metastatic breast cancer, compared to matched control women without breast cancer. While a relationship between pre- existing diabetes and increased risk of breast cancer has been established, this is the first study to our knowledge to explore the reverse relationship between pre- existing early stage versus metastatic breast cancer and subsequent diabetes incidence.
Including metastatic patients in this investigation is of particular importance. Breast cancer is an increasing survivable disease; prognosis for those with incurable metastatic disease is now measured in years and 30% of patients with bone- only disease are still living more than 10 years later[15]. The long- term sequelae of breast cancer and its treatment have been under- appreciated, particularly for those with metastatic disease. In early stage breast cancer, diabetes is associated with increased health care costs, treatment related side effects, worse quality of life, and inferior survival outcomes[16, 17]. While there is a notable lack of investigation of the impact of diabetes in the presence of metastatic breast cancer, similar impact likely exists. A small retrospective study indicated worse survival in patients with metastatic breast cancer who had poor glycemic control, compared to those who were normoglycemic[18].
There are complex biologic mechanisms linking insulin resistance to tumorigenesis[19]; however, it is likely that worse survival in patients with co- existing diabetes and breast cancer goes beyond biology and is further negatively impacted by less intensive diabetic monitoring and more diabetic complications. Prior work has found that patients undergoing cancer treatment perform less diabetes self- management behaviors, including exercise, diet control, blood glucose monitoring, and adherence with oral antidiabetic medications[20, 21]. Major barriers to the management of diabetes in the presence of cancer include lack of prioritization of diabetes by both patients and providers, and a lack of assigned responsibility for diabetes management[22]. This is consistent with our finding of significantly fewer hemoglobin A1C measurements after breast cancer diagnosis compared to the controls without cancer, even among those diagnosed with diabetes.
Contrary to our hypothesis, the rate of incident diabetes was lower among those receiving endocrine therapy for early stage breast cancer. In a case- control analysis by Hamood et al. of Isreali early stage breast cancer patients, endocrine therapy was associated with an increased risk of subsequent diabetes (HR 2.4, 95% CI 1.26- 4.55, p = 0.008)[9]. This was most significant for aromatase inhibitors, which more strongly increase bone turnover, compared with tamoxifen. This finding supports preclinical data linking bone turnover to impaired insulin secretion and insulin resistance28 through TGF-b mediated oxidation of RyR2 in pancreatic beta cells and alterations of osteocalcin, respectively. While the population used in our study is larger and represents a community based, diverse sample, important covariates are missing that were included in the Hamood analysis. Information such as body composition or rates of physical activity were not available for our analysis; these variables are important as obesity, adiposity, and chronic inflammation may worsen during endocrine therapy and impact both risk of diabetes and bone turnover.
Conversely, another case- control study[23] in early stage breast cancer patients over age 65 found tamoxifen to be associated with increased risk of diabetes (OR 1.24, 95% CI 1.08-1.42, p = 0.002), while no association was found for aromatase inhibitors. The authors theorized that tamoxifen may increase incidence of diabetes by influencing insulin resistance, while the lack of association with aromatase inhibitors was attributed to a small sample size. In both of these case-control analyses, cases and controls had breast cancer, thus no conclusions could be made about the influence of a cancer diagnosis on diabetes incidence.
In a study of Taiwanese survivors of early stage breast cancer matched to controls without cancer[24], breast cancer patients had a higher risk of developing diabetes than non- cancer controls (HR 1.14, 95% CI = 1.08 – 1.20), as seen in our study; however, this observation was only present in tamoxifen users. Consistent with our findings, aromatase inhibitor treatment was associated with a reduced risk of diabetes (HR 0.59, 95% CI 0.43- 0.80). Future work in prospective studies is needed to validate these findings, and to evaluate the influence of body composition and lifestyle habits on the development of comorbid conditions in women receiving endocrine therapy for early stage breast cancer.
Notably, we found an over 50% reduction in the incidence of diabetes in patients with bone metastases who received potent medication to inhibit bone turnover using zolendronic acid or denosumab. Preclinical work demonstrates that mice with increased bone resorption have impaired insulin secretion through effects of systemic bone-derived TGF-b mediated oxidation of RyR2 in pancreatic beta cells. Furthermore, insulin is a target gene of TGF-b; amplified TGF-b signaling to the effector protein Smad3 in these cells represses insulin transcription and reduces insulin secretion[25, 26]. Bisphosphonates block osteoclastic bone resorption and the release of TGF-b into the systemic circulation4. Thus, drugs that block bone resorption have the potential to block suppression of insulin release and will be the subject of future prospective studies. Risk of diabetes is important in this population with bone metastases, as hyperglycemia contributes to poor bone quality and increased fracture risk[13]. Further increases in bone turnover from diabetes may create a feed forward cycle resulting in progressively worsening bone frailty and metabolic impairments. Additionally, patients in this analysis with bone metastases had a higher rate of pre- existing diabetes. This supports data indicating that increases in bone turnover may start long before a clinical diagnosis of bone metastases and the subsequent complications. For example, the randomized AZURE trial of adjuvant zoledronic acid for early stage breast cancer found a clear association between elevated baseline bone turnover markers and later develop of in- bone recurrences, but not of metastases outside of bone[27].
Strengths of this study include the mechanistically- based hypothesis that is supported by preclinical models. Additionally, the study included very large sample size, validation of cancer cases within the state cancer registry, and exclusion of patients from both groups having other cancers. Range of age, race, and health care systems make these results generalizable across many populations and variables.
This study has several limitations. The INP includes only medical information that is supplied to the information exchange, thus is subject to selection bias of patient use and physicians’ testing and management choices. Additionally, there is potential detection bias as the codes in the EMR do not guarantee a condition is present or absent; the EMR only includes codes for what is interpreted and recorded by a provider. However, we did confirm cancer diagnoses in the state cancer registry. In addition, few patients (<4%) had their diabetes detected based on laboratory values alone without other corroborating evidence of codes and medications; additionally, the majority (73%) were detected by multiple criteria, assuring that the diagnosis of diabetes in these patients was more certain. Additionally, bone turnover markers and bone density results were not available for this analysis, preventing analysis of potential bone changes. Finally, patients may also refuse to start certain medications, or to stop them early due to side effects. Since our analysis focused on structured, coded data, we were not able to assess reasons for or against use of various medications in individual patients. This was not a randomized controlled trial of the effects of various medications. It is possible that patients receiving certain medications were otherwise less at risk for development of diabetes due to unknown confounders. For example, there may be covariates such as body composition, diet, and physical activity that were not possible to collect in this analysis but may influence outcomes.
As more patients are cured of breast cancer and those with metastatic breast cancer continue to live longer, identification and management of diabetes will be imperative given its impact on treatment delivery, quality of life, health care costs, and survival. While the mechanism linking breast cancer to diabetes is unknown, this analysis and that from other studies support preclinical data connecting increased bone turnover to impaired glucose hemostasis. This relationship warrants prospective clinical evaluation and investigation of the impact of skeletal protective therapy on systemic effects of bone destruction.