Diabetes occurs in about one-third of patients with breast cancer (BC). These conditions share similar risk factors, such as increased body weight, lack of physical activity, and increased age.
Two recently published papers further examine the interaction between diabetes and BC. The first paper is a population-based cohort study published in the Journal of the American Medical Association that comprehensively examined the associations of preexisting diabetes with the use; time to treatment initiation; and treatment adherence, persistence, and continuation and/or completion of adjunctive therapies in Medicaid fee-for-service insured women diagnosed with BC in Missouri between January 2007 and December 2015.
Inclusion criteria included women with a first primary diagnosis of BC before age 65 years who were continuously enrolled in Medicaid (i.e., fewer than 60 consecutive days of nonenrollment in Medicaid per enrollment year). Excluded were women who were not continuously enrolled in Medicaid in the first year of diagnosis; those with stage IV or unknown stage tumors; those who did not have breast-conserving surgery or mastectomy; and those who died in the first year of diagnosis. Disease data were obtained from ICD-9 and ICD-10 billing codes, and lack of resources was determined by the composite socioeconomic deprivation index for each census tract. Adjuvant therapies studied included radiotherapy, chemotherapy, and endocrine therapy (ET).
For chemotherapy, analysis included whether any delay in administration occurred (i.e., if there was an interval >90 days postsurgery, which has been associated with increased mortality). The duration of chemotherapy, which typically consisted of four to eight cycles administered every 2 to 4 weeks, was assessed. Nine different guideline-recommended chemotherapy regimens were used by Medicaid patients with BC in this study. Initiation of ET was defined as at least one prescription for tamoxifen, toremifene, anastrozole, letrozole, or exemestane within 1 year of the BC diagnosis. Adherence was calculated using a medication possession ratio >80%. Persistence and continuation were defined as having no gaps in medication supply for at least 90 days in the first year of treatment.
The authors found that among the 3,704 patients (mean age 51.4 years), 28.1% were black, 70.1% were white, 20.7% had diabetes, 60.4% received chemotherapy, and 67.6% received ET. Women with diabetes tended to be older, were more likely to be black, lived in deprived neighborhoods, had pathologically favorable tumors, and underwent breast-conserving surgery. Those with diabetes had significantly lower odds (33%) of utilization (odds ratio [OR] 0.67; 95% CI, 0.48-0.93) and 29% reduced odds of completion of chemotherapy (OR 0.71; 95% CI, 0.50-0.99). Time to initiation of chemotherapy was similar between the groups. For ET, diabetes was associated with a 29% reduced odds of adherence (OR 0.71%; 95% CI, 0.56-0.91); however, there was no significant difference in ET initiation, persistence, or continuation.
The second paper is a retrospective cohort study that examined SEER (Surveillance, Epidemiology, End Results)—Medicare data of 133,324 women diagnosed with invasive, nonmetastatic first primary BC between 2000 and 2018 who were aged 66 to 84 years (median age at first primary BC diagnosis, 73.6 years). Additionally, to be included in the study, patients had to have survived ≥12 months without developing another cancer and had ≥12 months of continuous Part A/B, non–Health Maintenance Organization (HMO) Medicare coverage before and after the BC diagnosis.
The purpose of this study was to explore the relationship between diabetes and subsequent cancer risk among older women who were 1 year post BC diagnosis. In this study, 29.3% of patients had diabetes either before or following BC diagnosis. After a median follow-up of 4.3 years, investigators found that diabetes was associated with a 2.35-fold (hazard ratio [HR] 2.35; 95% CI, 1.48-3.74), 1.94-fold (HR 1.94; 95% CI, 1.26-2.96), and 1.38-fold (HR 1.38; 95% CI, 1.01-1.89), respectively, increased risk of developing liver, brain, and thyroid cancers. This study found that diabetes was more common in BC patients who were Asian, black, or Hispanic. Metformin use was associated with a reduced risk of developing colorectal cancer among women with BC and diabetes.
Both of these studies found an increased risk of diabetes in persons of color with BC. Diabetes impacted utilization and completion of chemotherapy and adherence to ET therapy. It also appeared to increase the risk of other solid tumors.
Pharmacists who are involved in the care of patients with BC, especially those belonging to a minority group, should monitor patients for an increased risk of diabetes and should be proactive in helping optimize the management of both comorbidities and their sequelae.
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